Ocugen (OCGN) S-4 14 Jun 19 Registration of securities issued in business combination transactions Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 2019
Document And Entity Information [Abstract]
Document Type	S-4
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Trading Symbol	HSGX
Entity Registrant Name	Histogenics Corporation
Entity Central Index Key	0001372299
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 7,376	$ 15,542	$ 7,081
Marketable securities			900
Prepaid expenses and other current assets	764	858	194
Total current assets	8,140	16,400	8,175
Property and equipment, net	135	141	2,723
Other assets	6,426	750
Restricted cash	137	137	137
Total assets	14,838	17,428	11,035
Current liabilities:
Accounts payable	1,100	1,590	776
Accrued expenses	99	1,000	2,705
Accrued lease liability-current portion	1,118
Current portion of deferred rent		45	35
Current portion of deferred lease incentive		238	111
Current portion of equipment loan			178
Total current liabilities	2,317	2,873	3,805
Accrued expenses due to Intrexon Corporation	1,125	1,125	3,040
Accrued lease liability-long-term	6,918
Deferred revenue	10,000	10,000
Deferred rent		351	280
Deferred lease incentive		1,025	499
Warrant liability	10	2,512	14,679
Total liabilities	20,370	17,886	22,303
Commitments and contingencies
Convertible preferred stock and stockholders' deficit:
Convertible preferred stock
Common stock	839	513	159
Additional paid-in capital	219,874	215,859	196,760
Accumulated deficit	(226,245)	(216,830)	(208,187)
Total stockholders' deficit	(5,532)	(458)	(11,268)
Total liabilities and stockholders' deficit	$ 14,838	$ 17,428	$ 11,035

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Convertible preferred stock, par value	$ 0.01	$ 0.01
Convertible preferred stock, shares authorized	30,000	30,000
Convertible preferred stock, shares issued	400	400
Convertible preferred stock, shares outstanding	400	400
Common stock, par value	$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000	100,000,000
Common stock, shares issued	94,599,601	62,025,398	24,571,029
Common stock, shares outstanding	94,599,601	62,025,398	24,571,029
Series A Convertible Preferred Stock [Member]
Convertible preferred stock, par value		$ 0.01	$ 0.01
Convertible preferred stock, shares authorized		30,000	30,000
Convertible preferred stock, shares issued		400.4910	4,605.6533
Convertible preferred stock, shares outstanding		400.4910	4,605.6533

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	1,583	3,286	15,634	15,566
General and administrative	2,929	2,807	10,204	9,384
Restructuring	2,789
Loss due to asset impairment	750		4,270
Total operating expenses	8,051	6,093	30,108	24,950
Loss from operations	(8,051)	(6,093)	(30,108)	(24,950)
Other income (expense):
Interest income (expense), net	48	37	163	134
Other expense, net	(5)	(24)	(106)	(116)
Gain due to extinguishment of liability			1,540
Warrant expense			(733)
Change in fair value of warrant liability	(1,407)	(8,753)	20,601	(1,482)
Total other income (expense), net	(1,364)	(8,740)	21,465	(1,464)
Net loss	(9,415)	(14,833)	(8,643)	(26,414)
Loss attributable to common stockholders-basic			(8,522)	(22,499)
Comprehensive loss	(9,415)	(14,833)
Loss attributable to common stockholders-diluted			$ (29,123)	$ (22,499)
Net loss attributable to common stockholders-basic and diluted	$ (9,394)	$ (14,370)
Loss per common share-basic			$ (0.23)	$ (0.99)
Net loss per common share-basic and diluted	$ (0.12)	$ (0.52)
Loss per common share-diluted			$ (0.79)	$ (0.99)
Weighted-average shares used to compute net loss per common share-basic and diluted	80,484,113	27,670,118
Weighted-average shares used to compute loss per common share-basic			36,398,450	22,669,819
Weighted-average shares used to compute loss per common share-diluted			37,090,197	22,669,819

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Series A Convertible Preferred Stock [Member]	Restricted Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 13,567	$ 159	$ 195,181	$ (181,773)
Beginning Balance at Dec. 31, 2016					$ 0
Beginning Balance, Shares at Dec. 31, 2016		20,645,723				1,889
Beginning Balance, Shares at Dec. 31, 2016					13,416
Stock-based compensation expense	1,573		1,573
Exercise of common stock options	6		6
Exercise of common stock options, Shares		7,497
Vesting of restricted stock		1,889				(1,889)
Conversion of convertible preferred stock, Shares		3,915,920			(8,811)
Net loss	(26,414)			(26,414)
Ending Balance at Dec. 31, 2017	(11,268)	$ 159	196,760	(208,187)
Balance at December 31, 2018 at Dec. 31, 2017					$ 0
Ending Balance, Shares at Dec. 31, 2017		24,571,029
Ending Balance, Shares at Dec. 31, 2017 at Dec. 31, 2017					4,605
Stock-based compensation expense	403		403
Exercise of common stock options	2		2
Exercise of common stock options, Shares		919
Issuance of common stock -net	5,869	$ 27	5,842
Issuance of common stock -net, Shares		2,691,494
Fees related to issuance of common stock	(243)		(243)
Conversion of convertible preferred stock, Shares		1,423,970			(3,204)
Net loss	(14,833)			(14,833)
Ending Balance at Mar. 31, 2018	(20,070)	$ 186	202,764	(223,020)
Balance at December 31, 2018 at Mar. 31, 2018					$ 0
Ending Balance, Shares at Mar. 31, 2018		28,687,412
Ending Balance, Shares at Dec. 31, 2017 at Mar. 31, 2018					1,401
Beginning Balance at Dec. 31, 2017	(11,268)	$ 159	196,760	(208,187)
Beginning Balance at Dec. 31, 2017					$ 0
Beginning Balance, Shares at Dec. 31, 2017		24,571,029
Beginning Balance, Shares at Dec. 31, 2017					4,605
Stock-based compensation expense	1,625		1,625
Exercise of common stock options	$ 2		2
Exercise of common stock options, Shares	919	919
Exercise of warrants	$ 1		1
Exercise of warrants, Shares		104,092
Issuance of common stock -net	17,825	$ 354	17,471
Issuance of common stock -net, Shares		35,480,397
Conversion of convertible preferred stock, Shares		1,868,961			(4,205)
Net loss	(8,643)			(8,643)
Ending Balance at Dec. 31, 2018	(458)	$ 513	215,859	(216,830)
Balance at December 31, 2018 at Dec. 31, 2018					$ 0
Ending Balance, Shares at Dec. 31, 2018		62,025,398
Ending Balance, Shares at Dec. 31, 2017 at Dec. 31, 2018					400
Stock-based compensation expense	107		107
Exercise of warrants	4,234	$ 326	3,908
Exercise of warrants, Shares		32,574,203
Net loss	(9,415)			(9,415)
Ending Balance at Mar. 31, 2019	$ (5,532)	$ 839	$ 219,874	$ (226,245)
Balance at December 31, 2018 at Mar. 31, 2019					$ 0
Ending Balance, Shares at Mar. 31, 2019		94,599,601
Ending Balance, Shares at Dec. 31, 2017 at Mar. 31, 2019					400

Consolidated Statements of Co_2

Consolidated Statements of Convertible Preferred Stock and Stockholders' Deficit (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Common stock, par value	$ 0.01	$ 0.01
Common Stock [Member]
Common stock, par value	0.01	0.01
Series A Convertible Preferred Stock [Member]
Temporary equity, par value	0.01	0.01
Restricted Stock [Member]
Common stock, par value	$ 0.01	$ 0.01

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (9,415)	$ (14,833)	$ (8,643)	$ (26,414)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation	6	172	450	1,534
Amortization of discount of investments				54
Deferred revenue			10,000
Deferred rent and lease incentive		853	734	(543)
Stock-based compensation	107	403	1,625	1,573
Warrant expense			733
Change in fair value of warrant liability	1,407	8,753	(20,601)	1,482
Loss due to asset impairment	750		4,270
Gain on extinguishment of liability due to Intrexon Corporation			(1,540)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	94	(664)	(664)	(21)
Other long term assets			(750)
Other assets and accrued lease obligations, net	(49)	(375)
Accounts payable	(491)	586	689	(849)
Accounts payable due to Intrexon Corporation				(360)
Accrued expenses due to Intrexon Corporation			(375)
Accrued expenses	(901)	(1,609)	(1,705)	524
Deferred revenue		10,000
Net cash provided by (used in) operating activities	(8,492)	3,286	(15,777)	(23,020)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment		(1,255)	(2,013)	(276)
Proceeds from maturities of marketable securities		900	900	7,050
Purchases of marketable securities				(8,004)
Net cash used in investing activities		(355)	(1,113)	(1,230)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net proceeds from issuance of common stock		5,732	25,526
Repayments on equipment term loan		(133)	(178)	(583)
Proceeds from exercise of stock options and warrants			3	6
Expenses incurred for at-the-market sales agreement of common stock		(106)
Proceeds from exercise of warrants	326
Proceeds from exercise of stock options		2
Net cash provided by (used in) financing activities	326	5,495	25,351	(577)
Net increase (decrease) in cash and cash equivalents	(8,166)	8,426	8,461	(24,827)
Cash and cash equivalents and restricted cash -Beginning of period	15,679	7,218	7,218	32,045
Cash and cash equivalents and restricted cash -End of period	7,513	15,644	15,679	7,218
Supplemental Disclosure of Non-Cash Items:
Purchases of property and equipment in accounts payable		905	125
Public offering costs in accounts payable			99
Reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets:
Cash and cash equivalents	7,376	15,507	15,542	7,081
Restricted cash	137	137	100
Total cash, cash equivalents, and restricted cash at the end of period	7,513	15,644
Supplemental Disclosure of Cash Flow information:
Cash paid for taxes			105	197
Cash paid for interest			1	$ 30
Supplemental cash flow disclosures from investing and financing activities:
Right-of-use assets and lease liability recorded upon adoption of ASC 842	$ 6,635
Purchases of property and equipment in accounts payable and accrued expenses		905	$ 125
Public offering costs in accounts payable		$ 106

Nature of Business

Nature of Business	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business	1. NATURE OF BUSINESS Organization Histogenics Corporation (the “Company”) was incorporated under the laws of the Commonwealth of Massachusetts on June 28, 2000 and has its principal operations in Waltham, Massachusetts. In 2006, the Company’s board of directors approved a corporate reorganization pursuant to which the Company incorporated as a Delaware corporation. The Company historically focused on the development of restorative cell therapies (“RCTs”). RCTs refer to a new class of products that are designed to offer patients rapid-onset pain relief and restored function through the repair of damaged or worn tissue. The Company’s lead product, NeoCart ® In connection with the decision to discontinue the development of NeoCart, the Company’s board of directors engaged a financial advisory firm to help explore its available strategic alternatives, including possible mergers and business combinations, a sale of part or all of its assets, and collaboration and licensing arrangements. On April 8, 2019, the Company and Ocugen, Inc. (“Ocugen”) announced entering into the Merger Agreement. Upon the terms and subject to the satisfaction of the conditions described in the Merger Agreement, including approval of the transaction by our stockholders and Ocugen’s stockholders, a wholly-owned subsidiary of the Company will be merged with and into Ocugen, with Ocugen surviving the Merger as a wholly-owned subsidiary of the Company. The proposed Merger is structured as a stock-for-stock At the effective time of the Merger (the “Effective Time”), the Company’s board of directors is expected to consist solely of members designated by Ocugen. Following the Closing, Shankar Musunuri is expected to serve as Histogenics’ Chairman of the Board and Chief Executive Officer and Susan L. Drexler is expected to serve as the Company’s Interim Chief Financial Officer. Also at the Effective Time, the Company will effect a name change to “Ocugen, Inc.” and it is anticipated that trading for Ocugen’s securities will be listed on The Nasdaq Capital Market under the symbol “OCGN.” The Merger Agreement contains customary representations, warranties and covenants made by the Company and Ocugen, including covenants relating to obtaining the requisite approvals of the stockholders of the Company and Ocugen, indemnification of directors and officers, and the Company’s and Ocugen’s conduct of their respective businesses between the date of signing of the Merger Agreement and the Closing. In connection with the Merger, the Company will prepare and file with the U.S. Securities and Exchange Commission (“SEC”) a registration statement on Form S-4 • the Merger Agreement, including the issuance of shares of Histogenics common stock to the Ocugen’s stockholders in connection with the transactions contemplated by the Merger Agreement; • the amendment of Histogenics’ restated certificate of incorporation to effect a reverse split of all outstanding shares of the Company’s common stock at a reverse stock split ratio as mutually agreed to by Histogenics and Ocugen; and • the change of control of the Company resulting from the Merger pursuant to pertinent Nasdaq rules. The Closing is subject to satisfaction or waiver of certain conditions including, among other things, (i) the required approvals by the parties’ stockholders, (ii) the accuracy of the representations and warranties, subject to certain materiality qualifications, (iii) compliance by the parties with their respective covenants, (iv) no law or order preventing the Merger and related transactions, and (v) the listing of the Shares on The Nasdaq Capital Market. The Merger Agreement contains certain termination rights for both Histogenics and Ocugen, and further provides that, upon termination of the Merger Agreement under specified circumstances, Histogenics may be required to pay to Ocugen a termination fee of $0.6 million or Ocugen may be required to pay to Histogenics a termination fee of $0.7 million, and in other circumstances each party may be required to reimburse the other party’s expenses incurred, up to a maximum of $0.3 million. In connection with the execution of the Merger Agreement, the executive officers and directors of the Company entered into voting agreements with Ocugen and Histogenics relating to the Merger covering less than one percent of the outstanding capital stock of Histogenics, as of date of the Merger Agreement (the “Histogenics Voting Agreements”). The Histogenics Voting Agreements provide, among other things, that the stockholders who are parties to the Histogenics Voting Agreements will vote all of the shares held by them in favor of Histogenics Stockholder Matters and against any competing acquisition proposals. The Histogenics Voting Agreements also place certain restrictions on the transfer of the shares of Histogenics held by the respective signatories thereto. In connection with the execution of the Merger Agreement, certain officers, directors, stockholders and noteholders of Ocugen entered into voting agreements with Histogenics and Ocugen covering approximately 68% of the outstanding capital stock of Ocugen as of date of the Merger Agreement (the “Ocugen Voting Agreements”). The Ocugen Voting Agreements provide, among other things, that the officers, stockholders and investors party to the Ocugen Voting Agreements will vote all of the shares of Ocugen held by them in favor of the adoption of the Merger Agreement, the approval of the Merger and the other transactions contemplated by the Merger Agreement and against any competing acquisition proposals. The Ocugen Voting Agreements also place certain restrictions on the transfer of the shares of Ocugen held by the respective signatories thereto. Concurrently with the execution of the Merger Agreement, the officers and directors of Histogenics and the officers, directors and certain stockholders of Ocugen entered into lock-up 180-day Although the Company has entered into the Merger Agreement and intends to consummate the proposed Merger, there is no assurance that it will be able to successfully consummate the proposed Merger on a timely basis, or at all. If, for any reason, the proposed Merger is not completed, Histogenics will reconsider its strategic alternatives and could pursue one or more of the following courses of action: • Pursue potential collaborative, partnering or other strategic arrangements for our NeoCart assets, including a sale or other divestiture. • Pursue another strategic transaction like the proposed Merger. • Dissolve and liquidate the Company’s assets. Since its inception, the Company has devoted substantially all of its efforts to product development, recruiting management and technical staff, raising capital, starting up production and building infrastructure and has not yet generated product revenues. Expenses have primarily been for research and development and related administrative costs. The Company is subject to a number of risks including the successful development of therapeutics, the ability to obtain adequate financing, the ability to obtain FDA approval and reimbursement for any products we may develop, protection of intellectual property, fluctuations in operating results, dependence on key personnel and collaborative partners, rapid technological changes inherent in the target markets of any products the Company may develop, the introduction of substitute products and competition from larger companies. Liquidity The consolidated financial statements have been prepared on a going-concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. As shown in the accompanying consolidated financial statements, the Company has incurred losses and cash flow deficits from operations since inception, resulting in an accumulated deficit at March 31, 2019 of $226.2 million. The Company has financed operations to date primarily through public and private placements of equity securities, and borrowings under debt agreements. The Company anticipates that it will continue to incur net losses for the foreseeable future. The Company believes that its existing cash, cash equivalents and marketable securities will only be sufficient to fund its projected cash needs into the middle of 2019. Accordingly, these factors, among others, raise substantial doubt about the Company’s ability to continue as a going concern and its ability to complete the proposed merger with Ocugen. If Histogenics needs to raise additional capital to complete the merger, the Company would need to raise additional capital through debt or equity financing or other strategic transactions. However, any such financing may not be on favorable terms or available to the Company. The failure of the Company to obtain sufficient funds on commercially acceptable terms when needed will have a material adverse effect on the Company’s business, results of operations and financial condition. The forecast of cash resources is forward-looking information that involves risks and uncertainties, and the actual amount of our expenses could vary materially and adversely as a result of a number of factors. The Company has based its estimates on assumptions that may prove to be wrong, and the Company’s expenses could prove to be significantly higher than it currently anticipates. Basis of Accounting The consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). However, they do not include all of the information and footnotes required by GAAP for complete financial statements. These interim consolidated financial statements, in the opinion of the Company’s management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended March 31, 2019 and 2018. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K, The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, ProChon and Histogenics Securities Corporation. All significant intercompany accounts and transactions are eliminated in consolidation.	1. NATURE OF BUSINESS Organization Histogenics Corporation (the “Company”) was incorporated under the laws of the Commonwealth of Massachusetts on June 28, 2000 and has its principal operations in Waltham, Massachusetts. In 2006, the Company’s board of directors approved a corporate reorganization pursuant to which the Company incorporated as a Delaware corporation. The Company historically focused on the development of restorative cell therapies (RCTs). RCTs refer to a new class of products that are designed to offer patients rapid-onset pain relief and restored function through the repair of damaged or worn tissue. The Company’s lead product, NeoCart ® On May 13, 2011, the Company completed the acquisition of ProChon Biotech Ltd. (“ProChon”), a privately-held biotechnology company focused on modulating the fibroblast growth factor system for consideration of $2.2 million to enable it to create more effective solutions for tissue regeneration. ProChon’s products combine cell regeneration technologies with proprietary growth factors and biocompatible scaffolds to restore injured or chronically damaged tissues. The acquisition led to the initial recognition of goodwill, which was subsequently written off in 2011, and intangible assets including IPR&D and a licensing agreement which were fully impaired in 2016 as discussed in Note 2. On September 29, 2016, the Company closed a private placement of common stock, preferred stock and warrants, contemplated by a securities purchase agreement dated September 15, 2016, with certain institutional and accredited investors. The net proceeds after deducting placement agent fees and other transaction-related expenses were $27.6 million. See Note 8, Capital Stock, for further discussion of the private placement. In January 2018, the Company closed a registered direct offering where the Company issued 2,691,494 shares of common stock at a price of $2.35 per share. The underwriter option to purchase additional shares of 351,064 were fully exercised. The total net proceeds of the offering were approximately $5.9 million after deducting underwriting discounts and commissions. In March 2018, the Company entered into an equity distribution agreement (“ATM Agreement”) with Canaccord Genuity Inc. (“Canaccord”), pursuant to which the Company may, from time to time, sell shares of its common stock having an aggregate offering price of up to $10.0 million (the “Shares”) through Canaccord, as sales agent. During the year ended December 31, 2018, the Company sold an aggregate of 6,633,903 shares of common stock and received $4.5 million after deducting commissions related to the ATM Agreement and other offering costs. In October 2018, the Company closed an underwritten public offering of 26,155,000 shares of its common stock and warrants to purchase up to 19,616,250 shares of common stock, at a combined purchase price of $0.65 per share of common stock and accompanying warrant. The gross proceeds to Histogenics from this offering were $17.0 million, before deducting underwriting discounts and commissions, and offering expenses payable by the Company. The warrants are exercisable immediately upon issuance at a price of $0.70 per share of common stock and have a term of five years commencing on the date of issuance. Since its inception, the Company has devoted substantially all of its efforts to product development, recruiting management and technical staff, raising capital, starting up production and building infrastructure and has not yet generated product revenues. Expenses have primarily been for research and development and related administrative costs. The Company is subject to a number of risks including the successful development of therapeutics, the ability to obtain adequate financing, the ability to obtain FDA approval and reimbursement for any products we may develop, protection of intellectual property, fluctuations in operating results, dependence on key personnel and collaborative partners, rapid technological changes inherent in the target markets of any products the Company may develop, the introduction of substitute products and competition from larger companies. Liquidity The consolidated financial statements have been prepared on a going-concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. As shown in the accompanying consolidated financial statements, the Company has incurred losses and cash flow deficits from operations since inception, resulting in an accumulated deficit at December 31, 2018 of $216.8 million. The Company has financed operations to date primarily through public and private placements of equity securities, and borrowings under debt agreements. The Company anticipates that it will continue to incur net losses for the foreseeable future. The Company believes that its existing cash, cash equivalents and marketable securities will only be sufficient to fund its projected cash needs into the middle of 2019. Accordingly, these factors, among others, raise substantial doubt about the Company’s ability to continue as a going concern. To meet future capital needs, the Company would need to raise additional capital through debt or equity financing or other strategic transactions. However, any such financing may not be on favorable terms or available to the Company. The failure of the Company to obtain sufficient funds on commercially acceptable terms when needed will have a material adverse effect on the Company’s business, results of operations and financial condition. The forecast of cash resources is forward-looking information that involves risks and uncertainties, and the actual amount of our expenses could vary materially and adversely as a result of a number of factors. The Company has based its estimates on assumptions that may prove to be wrong, and the Company’s expenses could prove to be significantly higher than it currently anticipates.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES During the three months ended March 31, 2019, there have been no material changes to the significant accounting policies described in the Company’s audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Annual Report on Form 10-K, Recently Adopted Accounting Standards - Leases The Company adopted Financial Accounting Standards Board Accounting Standards Update No. 2016-02, The Company elected the following practical expedients when assessing the transition impact of the new standard from both the lessee and lessor perspective and did not: (i) reassess whether any expired or existing contracts as of January 1, 2019, are or contain leases; (ii) reassess the lease classification for any expired or existing leases as of January 1, 2019; (iii) reassess initial direct costs for any existing leases as of January 1, 2019; and (iv) reassess whether land easements meet the definition of a lease. The primary impact was the balance sheet recognition of right-of-use Fair Value Measurements The carrying amounts reported in the Company’s consolidated financial statements for cash and cash equivalents, accounts payable, and accrued liabilities approximate their respective fair values because of the short-term nature of these accounts. Fair value is defined as the price that would be received if selling an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Additionally, from time to time, the Company may be required to record at fair value other assets on a nonrecurring basis, such as assets held for sale and certain other assets. These nonrecurring fair value adjustments typically involve the application of lower-of-cost-or-market The fair value hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets (Level 1), and the lowest priority to unobservable inputs (Level 3). The Company’s financial assets are classified within the fair value hierarchy based on the lowest level of inputs that is significant to the fair value measurement. The three levels of the fair value hierarchy, and their applicability to the Company’s financial assets, are described below. Level 1 Level 2 Level 3 non-transferability, An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. The Company had no material re-measurements The fair value of the warrants issued in connection with the September 2016 private placement was determined using a Monte Carlo simulation model. This model incorporated several assumptions at each valuation date including: the price of the Company’s common stock on the date of valuation, the historical volatility of the price of the Company’s common stock, the remaining contractual term of the warrant and estimates of the probability of a fundamental transaction occurring. The fair value of the warrants issued in connection with the October 2018 underwritten public offering was determined using the Black Scholes model. See Note 6, Capital Stock, for further discussion of the private placement and underwritten public offering. The Company’s financial instruments as of March 31, 2019 and December 31, 2018 consisted primarily of cash and cash equivalents and warrant liability. As of March 31, 2019, and December 31, 2018, the Company’s financial assets recognized at fair value consisted of the following: Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) (in thousands) March 31, 2019 Assets: Cash Equivalents Money market funds $ 4,747 $ 4,747 $ — $ — Liabilities: Warrant liability $ 10 $ — $ — $ 10 December 31, 2018 Assets: Cash Equivalents Money market funds $ 9,711 $ 9,711 $ — $ — Liabilities: Warrant liability $ 2,512 $ — $ — $ 2,512 The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs: March 31, 2019 (in thousands) Beginning balance, December 31, 2018 $ 2,512 Exercise of warrants (1,095 ) Change in fair value of warrant liability (1,407 ) Ending balance $ 10 Cash and Cash Equivalents The Company considers all highly liquid securities with original maturities of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents are comprised of funds in money market accounts. In addition, the Company has recorded restricted cash of $0.1 million as of March 31, 2019 and December 31, 2018. Restricted cash consists of a security deposit related to a lease obligation. Revenue Recognition In May 2014, the Financial Accounting Standards Board (the “FASB”) issued a new standard related to revenue recognition, Accounting Standard Update (“ASU”) No. 2014-09, No. 2015-14, Revenue is recognized when, or as, performance obligations are satisfied, which occurs when control of the promised products or services is transferred to customers. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring products or services to a customer (“transaction price”). To the extent that the transaction price includes variable consideration, the Company estimates the amount of variable consideration that should be included in the transaction price utilizing the most likely amount method. Variable consideration is included in the transaction price if, in the Company’s judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Estimates of variable consideration and determination of whether to include estimated amounts in the transaction price are based largely on an assessment of the Company’s anticipated performance and all information (historical, current and forecasted) that is reasonably available. If the contract contains a single performance obligation, the entire transaction price is allocated to the single performance obligation. Contracts that contain multiple performance obligations require an allocation of the transaction price to each performance obligation based on a relative standalone selling price basis unless the transaction price is variable and meets the criteria to be allocated entirely to a performance obligation or to a distinct service that forms part of a single performance obligation. The Company currently generates revenue primarily through collaborative research, development and commercialization agreements. The terms of these agreements may contain multiple promises which may include: (i) licenses to the Company’s technology; (ii) services related to the transfer and update of know-how; non-refundable The Company assesses the promises to determine if they are distinct performance obligations. Once the performance obligations are determined, the transaction price is allocated based on a relative standalone selling price basis. Milestone payments and royalties are typically considered variable consideration at the outset of the contract and are recognized in the transaction price either upon occurrence or when the constraint of a probable reversal is no longer applicable. Collaboration Revenue While no revenue has been recognized as of March 31, 2018, the Company has collaboration and license agreements with strategic partners for the development and commercialization of product candidates. The collaboration and license agreements are within the scope of Accounting Standards Codification (ASC 606) Revenue from Contracts with Customers. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. Licenses of intellectual property: non-refundable, up-front non-refundable, up-front Manufacturing Supply Services: non-refundable, up-front Milestone Payments: re-evaluates catch-up Royalties: License and Collaboration Arrangements MEDINET Co., Ltd. In December 2017, the Company entered into a License and Commercialization Agreement (the “License Agreement”) with MEDINET Co., Ltd. (“MEDINET”) to grant MEDINET a license under certain patents, patent applications, know-how, non-human In exchange for the license, MEDINET agreed to pay the Company an upfront cash payment of $10.0 million which the Company received in January 2018. As of March 31, 2019, the contract with MEDINET was wholly unperformed and all revenue under the License Agreement has been deferred and has not been recognized. As of March 31, 2019, the aggregate amount of the transaction price allocated to remaining performance obligations was $10.0 million. Because the License Agreement was not terminated as of March 31, 2019, the authoritative accounting literature requires that the $10.0 million of deferred revenue remain a liability on the Company’s balance sheet. The Company anticipates that MEDINET will terminate the License Agreement in 2019 resulting in the recognition of the $10 million of deferred revenue in the period of termination. MEDINET also agreed to pay the Company tiered royalties, at percentages ranging from the low single digits to low double digits, of net sales of MEDINET products governed by the License Agreement. Over the life of the License Agreement, the Company is eligible to receive up to ¥330 million ($2.9 million as of March 31, 2019) in development milestone payments, $1.0 million and ¥720 million ($7.4 million as of March 31, 2019) in regulatory payments and up to an aggregate of ¥7,300 million ($64.1 million as of March 31, 2019) for the achievement of certain commercial milestones related to the sales of MEDINET products governed by the License Agreement. As a condition of the License Agreement, the Company agreed to supply NeoCart for MEDINET’s planned Phase 3 clinical trial in Japan. The Company assessed its promised goods and services under the License Agreement to determine if they are distinct. Due to the unique nature of the clinical manufacturing services to be provided by the Company, there are currently no other third-party vendors from which MEDINET can obtain such supply. The Company expected to be the only vendor capable of providing the manufacturing services for a period of at least one to two years, which is approximately the estimated length of time for the Japanese clinical trial period. After this point, if the Company were to transfer to a third-party its technology and know-how Based on the assessment of the combined performance obligation, the Company determined that the predominant promise in the arrangement is the transfer of the license and associated manufacturing know-how In determining the correct measure of progress to use when recognizing revenue over time, the Company assessed whether an input- or output- based measure of progress would be appropriate. The Company determined that an output-based measure of progress would be appropriate to use when recognizing revenue associated with the combined performance obligation and intended to recognize revenue under the License Agreement as the clinical manufacturing services were performed. Revenue was to be recognized using the output method over the length of the clinical trial enrollment, as the clinical manufacturing services were delivered, over the estimated service period. Upon the conclusion of the clinical manufacturing period, the Company would have expected other third-party vendors to have the capabilities to provide similar services. At this point, the license would effectively become a distinct performance obligation, with no remaining undelivered obligations. Therefore, the Company determined that the up-front At contract inception, the Company determined that the $10.0 million non-refundable re-evaluates per-patient The Company incurred cost of $0.9 million related to the License Agreement with MEDINET. $0.8 million was recorded as an asset that was to be expensed proportionally over the performance service period. However, given the Company’s decision to discontinue the development of NeoCart and terminate its manufacturing operations, the Company concluded that the asset was impaired and a decision was made to write down the value of this asset to $0 in the first quarter of 2019. This impairment resulted in a charge to the income statement of $0.8 million. Stock-Based Compensation The Company accounts for stock options and restricted stock based on their grant date fair value and recognizes compensation expense on a straight-line basis over their vesting period. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model, with the exception of stock options that include a market condition, and restricted stock based on the fair value of the underlying common stock as of the date of grant or the value of the services provided, whichever is more readily determinable. The Company, in conjunction with adoption of ASU 2016-09- pre-vesting For stock option grants with vesting triggered by the achievement of performance-based milestones, the expense is recorded over the remaining service period after the point when the achievement of the milestone is probable or the performance condition has been achieved. For stock option grants with both performance-based milestones and market conditions, expense is recorded over the derived service period after the point when the achievement of the performance-based milestone is probable or the performance condition has been achieved. For stock option grants with market conditions, the expense is calculated using the Monte Carlo model based on the grant date fair value of the option and is recorded on a straight line basis over the requisite service period, which represents the derived service period and accelerated when the market condition is satisfied. The Company did not issue awards with market conditions during the three months ended March 31, 2019. The Company accounts for stock options and restricted stock awards to non-employees non-employees Warrant Accounting As noted in Note 6, Capital Stock, the Company classifies a warrant to purchase shares of its common stock as a liability on its consolidated balance sheet if the warrant is a free-standing financial instrument that may require the Company to transfer consideration upon exercise. Each warrant of this type is initially recorded at fair value on date of grant using the Monte Carlo simulation model net of issuance costs, and is subsequently re-measured Recent Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, In August 2018, the FASB issued ASU No. 2018-13, In August 2018, the SEC adopted the final rule under SEC Release No. 33-10532, 10-Q In June 2018, the FASB issued ASU No. 2018-07,	2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Accounting The consolidated financial statements are prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The consolidated financial statements include the accounts of Histogenics Corporation and its wholly-owned subsidiaries, ProChon and Histogenics Securities Corporation. All significant intercompany accounts and transactions are eliminated in consolidation. Use of Estimates The preparation of the Company’s consolidated financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in the Company’s consolidated financial statements and accompanying notes. The most significant estimates in the Company’s consolidated financial statements relate to the valuation of equity awards, warrant liability, recoverability of deferred tax assets, estimated useful lives of fixed assets. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. The Company evaluates its estimates and assumptions on an ongoing basis. The Company recorded in 2018 a loss on impairment of fixed assets. See Note 5 Property and Equipment. The Company’s actual results may differ from these estimates under different assumptions or conditions. Foreign Currency Translation The Company’s consolidated financial statements are prepared in U.S. dollars. The Company’s foreign subsidiary uses the U.S. dollar as its functional and reporting currency, as management determined that the U.S. dollar is the primary currency of the economic environment in which the subsidiary operates. When transactions are required to be paid in the local currency of the foreign subsidiary, any resulting foreign currency transaction gain or loss is recorded as a component of “Other income (expense), net” in the consolidated statements of operations. Segment and Geographic Information Operating segments are defined as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker (“CODM”) or decision-making group in making decisions regarding resource allocation and assessing performance. The Company operates in two geographic regions: the United States (Massachusetts) and Israel (Tel Aviv) and views its operations as two operating segments: Histogenics Corporation (United States) and ProChon (Israel) as the CODM reviews separate discrete financial information in making decisions regarding resource allocations and assessing performance. Operating segments that have similar economic characteristics can be aggregated. As the nature of the products, customers, and methods to distribute products are the same and the nature of the regulatory environment, the production processes and historical and estimated future margins are similar, the two operating segments have been aggregated into one reporting segment as they have similar economic characteristics. Fair Value Measurements The carrying amounts reported in the Company’s consolidated financial statements for cash and cash equivalents, accounts payable, equipment loan, and accrued liabilities approximate their respective fair values because of the short-term nature of these accounts. Fair value is defined as the price that would be received if selling an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Fair value should be based on the assumptions that market participants would use when pricing an asset or liability and is based on a fair value hierarchy that prioritizes the information used to develop those assumptions. Fair value measurements should be disclosed separately by level within the fair value hierarchy. For assets and liabilities recorded at fair value, it is the Company’s policy to maximize the use of observable inputs (quoted prices in active markets) and minimize the use of unobservable inputs (Company assumptions) when developing fair value measurements, in accordance with established fair value hierarchy. Fair value measurements for assets and liabilities where there exists limited or no observable market data are based primarily upon estimates, and often are calculated based on the economic and competitive environment, the characteristics of the asset or liability and other factors. Therefore, the results cannot be determined with precision and may not be realized in an actual sale or immediate settlement of the asset or liability. Additionally, there may be inherent weaknesses in any valuation technique, and changes in the underlying assumptions used, including discount rates and estimates of future cash flows, could significantly affect the results of current or future values. Additionally, from time to time, the Company may be required to record at fair value other assets on a nonrecurring basis, such as assets held for sale and certain other assets. These nonrecurring fair value adjustments typically involve application of lower-of-cost-or-market The fair value hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets (Level 1), and the lowest priority to unobservable inputs (Level 3). The Company’s financial assets are classified within the fair value hierarchy based on the lowest level of inputs that is significant to the fair value measurement. The three levels of the fair value hierarchy, and its applicability to the Company’s financial assets, are described below. Level 1 Level 2 Level 3 Level 3 valuations are for instruments that are not traded in active markets or are subject to transfer restrictions and may be adjusted to reflect illiquidity and/or non-transferability, An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. The Company had no assets or liabilities classified as Level 1 or Level 2 as of December 31, 2018 and 2017 other than the money market fund described in the “Cash and Cash Equivalents” section and the asset-backed securities in the “Marketable securities” section below. There were no material re-measurements The fair value of the warrants issued in connection with the September 2016 private placement was determined using a Monte Carlo simulation model. This model incorporated several assumptions at each valuation date including: the price of the Company’s common stock on the date of valuation, the historical volatility of the price of the Company’s common stock, the remaining contractual term of the warrant and estimates of the probability of a fundamental transaction occurring. The fair value of the warrants issued in connection with the October 2018 underwritten public offering was determined using the Black Scholes model. See Note 8, Capital Stock, for further discussion of the private placement and underwritten public offering. Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) (in thousands) December 31, 2018 Assets: Cash Equivalents Money market funds 9,711 9,711 — — Liabilities: Warrant liability 2,512 — — 2,512 December 31, 2017 Assets: Cash Equivalents Money market funds 5,547 5,547 — — Marketable securities: Asset-backed securities 900 — 900 — Liabilities: Warrant liability 14,679 — — 14,679 The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs: As of December 31, (in thousands) Beginning balance, January 1, 2018 $ 14,679 Issuance of warrants 8,434 Change in fair value of warrant liability (20,601 ) Ending balance $ 2,512 Cash and Cash Equivalents Cash and cash equivalents include cash in readily available checking and savings accounts and money market funds. The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Marketable Securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available for sale. The Company considers all available for sale securities, including those with maturity dates beyond 12 months, as available to support current operational liquidity needs and therefore classifies all securities including those with maturity dates beyond 90 days at the date of purchase as current assets within the consolidated balance sheets. Available for sale securities are maintained by the Company’s investment managers and may consist of commercial paper, high-grade corporate notes, U.S. Treasury securities, U.S. government agency securities, and certificates of deposit. Available for sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity (deficit) until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s consolidated statement of operations and comprehensive loss. The Company did not hold any available for sale securities prior to the first quarter of 2017. The amortized cost of available for sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2018, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available for sale marketable securities. The aggregate fair value of available for sale securities held by the Company for less than twelve months as of December 31, 2017 was $0.9 million which matured in 2018. The Company did not hold any available for sale securities and there were no sales of such during the year ended December 31, 2018. The Company determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2017. The weighted average maturity of the Company’s portfolio was less than one month at December 31, 2017. Concentration of Credit Risk Financial instruments, which potentially subject the Company to significant concentration of credit risk, consist primarily of cash and cash equivalents. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no financial instruments with off-balance Property and Equipment Property and equipment are recorded at historical cost. Costs for capital assets not yet placed into service are capitalized as construction in progress, and are depreciated in accordance with the below guidelines once placed into service. Maintenance and repair costs are expensed as incurred. Costs which materially improve or extend the lives of existing assets are capitalized. The Company provides for depreciation and amortization using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Category Estimated Useful Lives Office equipment 3 to 5 years Laboratory equipment 3 to 5 years Leasehold improvements Shorter of the remaining lease term or useful life Upon retirement or sale, the cost of assets disposed and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is recorded in the consolidated statements of operations. Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment and identifiable intangible assets. When impairment indicators exist, the Company’s management evaluates long-lived assets for potential impairment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. In 2018 a triggering event caused an impairment of the Company’s property and equipment. The triggering event related to the Company’s announcement that the NeoCart program was discontinued and the subsequent 73% reduction in stock price. An impairment loss of $4.3 million was recognized in earnings. See Note 5 Property and Equipment. Restricted Cash Restricted cash represents cash held in a depository account at a financial institution to collateralize a conditional stand-by non-current Deferred Rent Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the facilities the Company occupies. The Company’s leases for its Waltham, Massachusetts and Lexington, Massachusetts facilities provide for fixed increases in minimum annual rental payments. The total amount of rental payments due over each lease term is being charged to rent expense ratably over the life of each lease, respectively. Financial Instruments Indexed to and Potentially Settled in the Company’s Common Stock The Company evaluates all financial instruments issued in connection with its equity offerings when determining the proper accounting treatment for such instruments in the Company’s financial statements. The Company considers a number of generally accepted accounting principles under U.S. GAAP to determine such treatment and evaluates the features of the instrument to determine the appropriate accounting treatment. The Company utilizes the Probability Weighted Expected Return Method (“PWERM”), Option Pricing Model (“OM”) or other appropriate methods to determine the fair value of its derivative financial instruments, such as the warrant liability. For financial instruments indexed to and potentially settled in the Company’s common stock that are determined to be classified as liabilities on the consolidated balance sheet, changes in fair value are recorded as a gain or loss in the Company’s consolidated statement of operations with the corresponding amount recorded as an adjustment to the liability on its consolidated balance sheet. Revenue Recognition In May 2014, the Financial Accounting Standards Board (the “FASB”) issued a new standard related to revenue recognition, Accounting Standard Update (“ASU”) No. 2014-09, No. 2015-14, Revenue is recognized when, or as, performance obligations are satisfied, which occurs when control of the promised products or services is transferred to customers. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring products or services to a customer (“transaction price”). To the extent that the transaction price includes variable consideration, the Company estimates the amount of variable consideration that should be included in the transaction price utilizing the most likely amount method. Variable consideration is included in the transaction price if, in the Company’s judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Estimates of variable consideration and determination of whether to include estimated amounts in the transaction price are based largely on an assessment of the Company’s anticipated performance and all information (historical, current and forecasted) that is reasonably available. If the contract contains a single performance obligation, the entire transaction price is allocated to the single performance obligation. Contracts that contain multiple performance obligations require an allocation of the transaction price to each performance obligation based on a relative standalone selling price basis unless the transaction price is variable and meets the criteria to be allocated entirely to a performance obligation or to a distinct service that forms part of a single performance obligation. The Company’s revenues are generated primarily through collaborative research, development and commercialization agreements. The terms of these agreements may contain multiple promises which may include: (i) licenses to the Company’s technology; (ii) services related to the transfer and update of know-how; non-refundable The Company assesses the promises to determine if they are distinct performance obligations. Once the performance obligations are determined, the transaction price is allocated based on a relative standalone selling price basis. Milestone payments and royalties are typically considered variable consideration at the outset of the contract and are recognized in the transaction price either upon occurrence or when the constraint of a probable reversal is no longer applicable. Collaboration Revenue While no revenue has been recognized as of December 31, 2018, the Company has collaboration and license agreements with strategic partners for the development and commercialization of product candidates. The collaboration and license agreements are within the scope of Accounting Standards Codification (ASC 606) Revenue from Contracts with Customers. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. Licenses of intellectual property: non-refundable, up-front non-refundable, up-front Manufacturing Supply Services: non-refundable, up-front Milestone Payments: re-evaluates catch-up Royalties: License and Collaboration Arrangements MEDINET Co., Ltd. In December 2017 the Company entered into the License and Commercialization Agreement (the “License Agreement”) with MEDINET Co., Ltd. (“MEDINET”) to grant MEDINET a license under certain patents, patent applications, know-how, In exchange for the license, MEDINET agreed to pay the Company an non-refundable The Company assessed the promised goods and services to determine if they are distinct. Due to the unique nature of the clinical manufacturing services, there are no third-party vendors from which MEDINET can obtain such services from currently. The Company expects to be the only vendor capable of providing the commercial manufacturing services for a period of at least one to two years, which is approximately the estimated length of the clinical trial period in Japan. After this point, if the Company were to transfer to a third-party its technology and know-how Based on the assessment of the combined performance obligation, the Company determined that the predominant promise in the arrangement is the transfer of the license and associated knowhow which are expected to occur over the length of the clinical trial. The Company determined that MEDINET will be simultaneously receiving and consuming the benefits of the Company’s performance of the clinical trial. Therefore, the revenue associated with the combined performance obligation will be recognized over time. In determining the correct measure of progress to use when recognizing revenue over time, the Company assessed whether an input or output based measure of progress would be appropriate. The Company determined that an output based measure of progress would be appropriate to use when recognizing revenue associated with the combined performance obligation. The Company will recognize revenue based on the clinical manufacturing services completed to date. At the outset of the clinical trial in Japan to be conducted by MEDINET, the Company will have quantifiable estimates of total clinical candidates, and therefore, of total estimated performance. The Company will recognize revenue based on performance completed to date, as evidenced by the estimated number of clinical trial enrollees. The Company expects to provide the clinical manufacturing services to MEDINET over an estimated period of two years. Therefore, the estimated two-year two-year up-front At contract inception, the Company determined that the $10.0 million non-refundable re-evaluate per-patient The upfront transaction price of $10.0 million will be recognized over a period of approximately two-years, non-current Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represents the transaction price of contracts for which work has not been performed (or has been partially performed) and excludes unexercised contract options. As of December 31, 2018, the aggregate amount of the transaction price allocated to the remaining performance obligations was $10.0 million. The contract with MEDINET is wholly unperformed, and the Company recognized no revenue associated with the agreement during the years ended December 31, 2018 and 2017, respectively. Research and Development Costs Research and development costs are charged to expense as incurred. These costs include, but are not limited to: license fees related to the acquisition of in-licensed Costs for certain development activities, such as clinical trials, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by its vendors with respect to their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid or accrued research and development expense. Collaboration Arrangements Costs reimbursed to a collaborator for work that it performs are recorded as research and development expenses. These reimbursements can include payments for work performed, or a milestone for which a payment is due, the reimbursements or development milestone achievement are recorded as research and development expense. In September 2014, the Company entered into a collaboration agreement with Intrexon Corporation (“Intrexon”) for the development and commercialization of allogeneic cell therapeutics for the treatment or repair of damaged articular hyaline cartilage in humans, utilizing Intrexon’s proprietary technology (the “Collaboration Agreement”). Under the terms of the Collaboration Agreement, the Company is responsible for the costs of development and commercialization, with some exceptions. This agreement was terminated in December 2018. In connection with the Mutual Termination Agreement, in lieu of payment of the Accrued Expenses, the Company agreed to pay Intrexon an aggregate of up to $1.5 million, with $0.375 million paid at the time of entering into the Mutual Termination Agreement and $1.125 million payable within one year following any submission of a BLA to the FDA for NeoCart. We adjusted the accrued expenses to reflect $1.125 million balance as of December 31, 2018 and gain on extinguishment of liability of $1.5 million. License Agreements Costs associated with licenses of technology are expensed as incurred and are included in research and development expenses. Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense as incurred since the recoverability of such expenditures is uncertain. Stock-Based Compensation The Company accounts for grants of stock options and restricted stock based on their grant date fair value and recognizes compensation expense over their vesting period. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model and restricted stock based on the fair value of the underlying common stock as determined by management or the value of the services provided, whichever is more readily determinable. Stock-based compensation expense represents the cost of the grant date fair value of employee stock option grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis, net of estimated forfeitures. The expense is adjusted for actual forfeitures at year end. Stock-based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest. For stock option grants with performance-based milestones, the expense is recorded over the remaining service period after the point when the achievement of the milestone is probable or the performance condition has been achieved. For stock option grants with both performance-based milestones and market conditions, expense is recorded over the derived service period after the point when the achievement of the performance-based milestone is probable or the performance condition has been achieved. The Company did not issue performance-based awards in 2018 or 2017. The Company accounts for stock options and restricted stock awards to non-employees non-employees On October 1, 2018, the Compensation Committee of the Board of Directors approved a repricing (the “Repricing”) of 3,807,779 stock options (the “Options”) granted prior to September 1, 2018 pursuant to the 2013 Equity Incentive Plan and the 2012 Equity Incentive Plan to executive officers, employees and consultants of the Company. The Options had exercise prices between $0.75628 and $9.97 per share, which were reduced to $0.568 per share (the closing price of the Company’s common stock on The Nasdaq Capital Market on October 1, 2018). The number of shares, vesting schedules and expiration period of the Options were not altered. The impact to the Company’s financial statements in 2018 was immaterial. Income Taxes The Company accounts for income taxes under the liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning The Company records uncertain tax positions on the basis of a two-step Loss per Common Share Loss per common share is calculated using the two-class as-converted Diluted earnings per share is computed using the more dilutive of (a) the two-class if-converted Warrant Accounting As noted in Note 9, the Company classifies warrants to purchase shares of its common stock as a liability on its consolidated balance sheet if the warrant is a free-standing financial instrument that may require the Company to transfer consideration upon exercise. Each warrant of this type is initially recorded at fair value on date of grant using a Monte Carlo simulation or Black Scholes model and net of issuance costs, and is subsequently re-measured In the first quarter of 2019 the Company reduced the exercise price of all but 508,714 warrants issued in connection with the 2016 private placement and all of the warrants issued in connection with the October 2018 underwritten public offering. Refer to Note 15, Subsequent Events Recent Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, In August 2018, the FASB issued ASU No. 2018-13, In August 2018, the SEC adopted the final rule under SEC Release No. 33-10532, 10-Q No. 2018-07, In July 2017, the FASB issued ASU No. 2017-11, In May 2017, the FASB issued ASU No. 2017-09, In November 2016, the FASB issued ASU 2016-18, Statement 2016-18”). beginning-of-period end-of-period 2016-18 2016-18, As of December 31, 2018 2017 (in thousands) Cash and cash equivalents $ 15,542 $ 7,081 Restricted cash 137 137 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 15,679 $ 7,218 In February 2016, the FASB issued ASU No. 2016-02- Leases 2016-02 right-of-use In May 2014, the FASB issued ASU No. 2014-09,

Loss Per Common Share

Loss Per Common Share	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Loss Per Common Share	3. LOSS PER COMMON SHARE The Company computes basic and diluted loss per share using a methodology that gives effect to the impact of outstanding participating securities (the “two-class Three Months Ended March 31, 2019 2018 (In thousands, except Numerator: Net loss $ (9,415 ) $ (14,833 ) Net loss attributable to Series A Preferred Stock (a) (21 ) (463 ) Income attributable to common stockholders—basic and diluted $ (9,394 ) $ (14,370 ) Denominator: Weighted-average number of common shares used in loss per share—basic and diluted 80,484,113 27,670,118 Loss per share - basic and diluted $ (0.12 ) $ (0.52 ) (a) The Series A Preferred Stock participates in income and losses. The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in common stock equivalent shares): Three Months Ended March 31, 2019 2018 Unvested restricted stock and options to purchase common stock 1,803,167 3,101,143 Series A Preferred Stock unconverted 177,996 622,987 Warrants exercisable into common stock 571,025 13,633,070	3. LOSS PER COMMON SHARE Basic and diluted loss per common share are calculated as follows: For the Year Ended 2018 2017 (In thousands, except share and per share data) Numerator: Net loss $ (8,643 ) $ (26,414 ) Net Loss attributable to Series A Preferred Stock (a) (121 ) (3,915 ) Numerator for basic EPS—loss attributable to common stockholders $ (8,522 ) $ (22,499 ) Effect of dilutive securities: Deduct change in fair value of warrant liability $ (20,601 ) $ — Numerator for diluted EPS—loss attributable to common stockholders after assumed conversions $ (29,123 ) $ (22,499 ) Denominator: Weighted-average number of common shares used in loss per share—basic 36,398,450 22,669,819 Effect of dilutive securities: Nonparticipating warrants 691,747 — Denominator for diluted EPS—adjusted weighted average shares 37,090,197 22,669,819 Loss per share—basic $ (0.23 ) $ (0.99 ) Loss per share—diluted $ (0.79 ) $ (0.99 ) (a) The Series A Preferred Stock participates in income and losses The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in common stock equivalent shares): As of December 31, 2018 2017 Unvested restricted stock and options to purchase common stock 3,339,471 2,158,348 Series A preferred stock unconverted 177,996 2,046,957 Warrants exercisable into common stock — 13,633,070

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure Abstract
Prepaid Expenses and Other Current Assets	4. PREPAID EXPENSES AND OTHER CURRENT ASSETS Prepaid expenses and other current assets consisted of the following: As of December 31, 2018 2017 (in thousands) Insurance 686 72 Other current assets 172 122 Prepaid expenses and other current assets $ 858 $ 194

Property and Equipment

Property and Equipment	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment	4. PROPERTY AND EQUIPMENT Property and equipment consisted of the following: March 31, December 31, (in thousands) Office equipment $ 233 $ 266 Laboratory equipment 4,536 4,561 Leasehold improvements 5,383 5,504 Software 96 96 Total property and equipment 10,248 10,427 Less: accumulated depreciation (10,113 ) (10,286 ) Property and equipment, net $ 135 $ 141 Depreciation expense related to property and equipment amounted to less than $0.1 million and $0.2 million for the three months ended March 31, 2019 and 2018, respectively.	5. PROPERTY AND EQUIPMENT Property and equipment consisted of the following: December 31, December 31, (in thousands) Office equipment $ 266 $ 279 Laboratory equipment 4,561 4,565 Leasehold improvements 5,504 7,712 Construction in progress — 990 Software 96 96 Total property and equipment 10,427 13,642 Less: accumulated depreciation (10,286 ) (10,919 ) Property and equipment, net $ 141 $ 2,723 Depreciation expense related to property and equipment amounted to $0.5 million and $1.5 million for the years ended December 31, 2018 and 2017, respectively. For year ended December 31, 2018 the company deemed the value of its property and equipment to be impaired based on the triggering event that occurred in December 2018. In connection with the impairment, the Company reduced the net book value of its property and equipment to an estimated fair value which was calculated based on the present value of expected future cash flows from these assets. As a result, the Company incurred a charge of $4.3 million that was recorded in the Statement of Operations.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued Expenses	6. ACCRUED EXPENSES Accrued expenses consisted of the following: As of December 31, 2018 2017 (in thousands) Accrued compensation $ 514 $ 1,671 Accrued audit fees 159 133 Accrued license fees 90 70 Accrued clinical expenses 86 199 Accrued other 151 632 Total accrued expenses $ 1,000 $ 2,705

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. COMMITMENTS AND CONTINGENCIES Leases The Company leases its office and research facilities in Waltham and Lexington, Massachusetts under non-cancellable pro-rata In evaluating any potential accounting for leases, the Company determines if an arrangement contains a lease at inception. For arrangements where the Company is the lessee, operating leases are included in other assets, net; other accrued liabilities; and other long-term liabilities on the Consolidated Balance Sheet. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at the commencement date. ROU assets also include any initial direct costs incurred and any lease payments made at or before the lease commencement date, less lease incentives received. The Company uses its incremental borrowing rate based on the information available at the commencement date in determining the lease liabilities as the Company’s leases generally do not provide an implicit rate. Lease terms may include options to extend or terminate when the Company is reasonably certain that the option will be exercised. Lease expense is recognized on a straight-line basis over the lease term. The Company also has lease arrangements with lease and non-lease non-lease Operating lease expense was $0.4 million for the three months ended March 31, 2019. Variable executory cost was $0.3 million for the three months ended March 31, 2019. The following table contains supplemental cash flow information related to operating leases as of March 31, 2019. (in thousands) Cash paid within operating cash flows $ 459 Right-of-use — The following table contains supplemental balance sheet information related to operating leases, as of March 31, 2019. (in thousands) Other assets, net (Right-of-use $ 6,426 Accrued lease liability, current 1,118 Accrued lease liability, long-term 6,918 Total lease liabilities $ 8,036 Weighted average remaining lease term 5.3 years Weighted average discount rate 10 % The Company deemed restructurings in the first quarter of 2019 to be a triggering event that necessitated an evaluation of a potential impairment to the ROU asset related to the Company’s operating leases. Based on the analysis performed no impairment was necessary. Maturities of the Company’s operating lease liabilities as of March 31, 2019 are summarized in the table below. Year Amount (in thousands) 2019 $ 1,385 2020 1,892 2021 1,941 2022 1,991 2023 1,729 2024 and after 1,460 Total lease payments 10,398 Less: imputed interest (2,362 ) Total operating lease liabilities $ 8,036 License Agreements From time to time, the Company enters into various licensing agreements whereby the Company may use certain technologies in conjunction with its product research and development. Licensing agreements and the Company’s commitments under the agreements are as follows: Hydrogel License In May 2005, the Company entered into an exclusive license agreement with Angiotech Pharmaceuticals (US), Inc. for the use of certain patents, patent applications, and knowledge related to the manufacture and use of a hydrogel material in conjunction with NeoCart and certain other products (“Hydrogel License Agreement”). As of March 31, 2019, the Company has paid an aggregate $3.2 million in commercialization milestones under the terms of the Hydrogel License Agreement, which have been expensed to research and development. Under the terms of the Hydrogel License Agreement, the Company’s future commitments include: • A one-time • Single digit royalties on the net sales of NeoCart and certain other future products. Tissue Regeneration License In April 2001, the Company entered into an exclusive license agreement with The Board of Trustees of the Leland Stanford Junior University (“Stanford University”) for the use of certain technology to develop, manufacture and sell licensed products in the field of growth and regeneration of cartilage (“Tissue Regeneration License Agreement”). The term of the Tissue Regeneration License Agreement extends to the expiration date of Stanford University’s last to expire domestic or foreign patents. As of March 31, 2019, the Company has paid an aggregate $0.8 million in patent reimbursement costs, royalty fees, and commercialization milestone payments under the terms of the Tissue Regeneration License Agreement, which have been recorded to research and development expense. Under the terms of the Tissue Regeneration License Agreement, the Company’s future commitments include: • A one-time • An annual minimum non-refundable • Low single digit royalties on net sales. Tissue Processor Sub-License In December 2005, the Company entered into an exclusive agreement to sub-license (“Sub-License Sub-License sub-license Sub-License The Sub-License In May 2016, the Original Agreement was amended whereby the Company acquired the development and commercialization rights to NeoCart for the Japanese market from Purpose. Under the terms of the amended agreement, the Company assumes sole responsibility for and rights to the development and commercialization of NeoCart in Japan. In exchange for the transfer of development and commercialization rights, the Company will pay a success-based milestone to Purpose upon conditional approval of NeoCart in Japan, as well as commercial milestones and a low single digit royalty on Japanese sales of NeoCart, upon full approval, if any, in Japan. In addition to the above, the Company’s future commitments under the terms of the Original Agreement and Sub-License • A minimum non-refundable • An additional, non-refundable • $10.2 million in potential milestone payments; and • Low single digit royalties on net sales of a licensed product.	7. COMMITMENTS AND CONTINGENCIES Operating Leases The Company leases its office and research facilities in Waltham and Lexington, Massachusetts under non-cancellable pro-rata Aggregate minimum annual lease commitments of the Company under its non-cancellable For the Year Ended December 31, (in thousands) 2019 $ 1,845 2020 1,892 2021 1,941 2022 1,991 2023 1,728 Thereafter 1,460 Total minimum lease payments $ 10,857 Rent expense under operating lease agreements amounted to $1.6 million and $1.0 million for the years ended December 31, 2018 and 2017, respectively. As an inducement to enter into the Waltham facility lease extension, the lessor agreed to provide the Company with a construction allowance of up to $0.9 million towards the total cost of tenant improvements. As an inducement to enter into its Lexington facility lease, the lessor agreed to provide the Company with a construction allowance of up to $1.0 million towards the total cost of tenant improvements. The Company has recorded these costs in the consolidated balance sheet as leasehold improvements, with the corresponding liability as deferred lease incentive. These liabilities are amortized on a straight-line basis over the term of the lease as a reduction of rent expense. License Agreements From time to time, the Company enters into various licensing agreements whereby the Company may use certain technologies in conjunction with its product research and development. Licensing agreements and the Company’s commitments under the agreements are as follows: Hydrogel License In May 2005, the Company entered into an exclusive license agreement with Angiotech Pharmaceuticals (US), Inc. for the use of certain patents, patent applications, and knowledge related to the manufacture and use of a hydrogel material in conjunction with NeoCart and certain other products (“Hydrogel License Agreement”). As of December 31, 2018, the Company has paid an aggregate $3.2 million in commercialization milestones under the terms of the Hydrogel License Agreement, which has been expensed to research and development. Under the terms of the Hydrogel License Agreement, the Company’s future commitments include: • A one-time • Single digit royalties on the net sales of NeoCart and certain other future products. Tissue Regeneration License In April 2001, the Company entered into an exclusive license agreement with The Board of Trustees of the Leland Stanford Junior University (“Stanford University”) for the use of certain technology to develop, manufacture and sell licensed products in the field of growth and regeneration of cartilage (“Tissue Regeneration License Agreement”). The term of the Tissue Regeneration License Agreement extends to the expiration date of Stanford University’s last to expire domestic or foreign patents. As of December 31, 2018, the Company has paid an aggregate $0.8 million in patent reimbursement costs, royalty fees, and commercialization milestone payments under the terms of the Tissue Regeneration License Agreement, which have been recorded to research and development expense. Under the terms of the Tissue Regeneration License Agreement, the Company’s future commitments include: • A one-time • An annual minimum non-refundable • Low single digit royalties on net sales. Tissue Processor Sub-License In December 2005, the Company entered into an exclusive agreement to sub-license (“Sub-License Sub-License sub-license Sub-License The Sub-License In May 2016, the Original Agreement was amended whereby the Company acquired the development and commercialization rights to NeoCart for the Japanese market from Purpose. Under the terms of the amended agreement, the Company assumes sole responsibility for and rights to the development and commercialization of NeoCart in Japan. In exchange for the transfer of development and commercialization rights, the Company will pay a success-based milestone to Purpose upon conditional approval of NeoCart in Japan, as well as commercial milestones and a low single digit royalty on Japanese sales of NeoCart, upon full approval, if any, in Japan In addition to the above, the Company’s future commitments under the terms of the Original Agreement and Sub-License • A minimum non-refundable • An additional, non-refundable • $10.2 million in potential milestone payments; and • Low single digit royalties on net sales of a licensed product. Collagen Supply Agreement In September 2015, the Company entered into an agreement with Collagen Solutions (UK) Limited (the “Supplier”) to purchase soluble collagen that meets specifications provided by the Company. The initial term of the agreement is three years and will automatically renew from year to year thereafter unless otherwise terminated with at least 180 days’ notice by either party. In February 2017, the Company entered into an amendment with the Supplier. Pursuant to the amendment, the Company agreed to pay the Supplier approximately $0.1 million in exchange for eliminating the minimum annual order of material and/or services and any other amounts due Supplier. The payment of $0.1 million will be made over the 18 months following the date of the amendment. As of December 31, 2018, the Company has paid all of the required payments totaling $0.1 million under the terms of the amendment.

Capital Stock

Capital Stock	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Capital Stock	6. CAPITAL STOCK In October 2018, the Company closed an underwritten public offering of 26,155,000 shares of its common stock and warrants to purchase up to 19,616,250 shares of common stock, at a combined purchase price of $0.65 per share of common stock and accompanying warrant. The gross proceeds to the Company from this offering were $17.0 million, before deducting underwriting discounts and commissions, and offering expenses payable by the Company. The warrants were exercisable immediately upon issuance at a price of $0.70 per share of common stock and have a term of five years commencing on the date of issuance. The exercise price of the warrants was subject to adjustment upon the occurrence of specific events, including stock dividends, stock splits, combinations and reclassifications of the Company’s Common Stock. In the event of certain fundamental transactions of the Company, a warrant holder may have demanded redemption of its warrant for cash in accordance with a Black-Scholes option pricing model. A fundamental transaction was defined as a merger, sale of assets, sale of the Company, recapitalization of stock and a sale of stock whereby any owner after the transaction would own greater than 50% of the outstanding common stock in the Company. The Company determined the warrants were classified as a liability on the consolidated balance sheet because of the provision whereby in a fundamental transaction (as described above), the holder could have elected to receive either the amount they were entitled to on an as-if-exercised In three months ended March 31, 2019, the Company reduced the exercise price of the warrants issued in 2018 from $0.70 to $0.01 per share (the “2018 Reduced Exercise Price”) and all of the holders of these warrants (the “Participating 2018 Holders”) entered into a Warrant Exercise Agreement (the “2018 Exercise Agreement”) pursuant to which, in consideration for the 2018 Reduced Exercise Price, the Participating 2018 Holders agreed to exercise the warrants held by such Participating 2018 Holders in full at the 2018 Reduced Exercise Price for cash. In connection with the exercise of the warrants by the Participating 2018 Holders, the Company received aggregate gross proceeds of approximately $0.2 million. In March 2018, the Company entered into an equity distribution agreement (“ATM Agreement”) with Canaccord Genuity Inc. (“Canaccord”), pursuant to which the Company may, from time to time, sell shares of its common stock having an aggregate offering price of up to up to $10.0 million (the “Shares”) through Canaccord, as sales agent. The Shares will be offered and sold by the Company pursuant to its previously filed and currently effective Registration Statement on Form S-3 No. 333-216741) In January 2018, the Company completed an underwritten registered direct offering of 2,691,494 shares of common stock at a price of $2.35 per share. The total net proceeds of the offering were $5.7 million after deducting underwriter’s discounts and commissions, and expenses related to the offering. In September 2016, the Company closed the private placement contemplated by the securities purchase agreement (the “Purchase Agreement”), dated September 15, 2016, between the Company and certain institutional and accredited investors in which the Company received gross proceeds of $30.0 million (the “Private Placement”). The net proceeds after deducting placement agent fees and other transaction-related expenses was $27.6 million. At the closing, the Company issued 2,596,059 shares of the Company’s common stock at a per share price of $2.25 and 24,158.8693 shares of the Company’s newly-created Series A Convertible Preferred Stock (“Series A Preferred Stock”), which are convertible into approximately 10,737,275 shares of common stock. As of March 31, 2019, there were 400.4910 shares of Series A Preferred Stock outstanding, which remain convertible into 177,996 shares of the Company’s common stock. As part of the Private Placement, the investors received warrants to purchase up to 13,333,334 shares of the Company’s common stock at an exercise price of $2.25 per share. The placement agent for the Private Placement, H.C. Wainwright & Co. LLC (“HCW”), and certain of its affiliates were also granted warrants to purchase 133,333 shares of the Company’s common stock at an exercise price of $2.25 per share in exchange for the services provided by HCW. The placement agent warrants were considered a financing cost of the Company and included in warrant expense within the consolidated statements of operations. The warrants include a cashless-exercise feature that may be exercised solely in the event there is no effective registration statement, or no current prospectus available for, the resale of the shares of common stock underlying the warrants as of the six-month as-if-exercised Concurrent with the closing of the Private Placement, the Company’s Certificate of Incorporation was amended by the filing of a Certificate of Designation to create the Series A Preferred Stock. The Series A Preferred Stock has a par value of $0.01 and each share is convertible into 444.44 shares of common stock, at a conversion price of $2.25 per share, at the option of the holder. The Series A Preferred Stock has no voting rights and is only entitled to dividends as declared on an as-converted as-converted As part of the Private Placement, affiliates of certain members of the Company’s Board of Directors purchased an aggregate of 283,046 shares of common stock, an aggregate of 2,563.1439 shares of Series A Preferred Stock and received warrants to purchase up to 1,422,221 shares of common stock at an exercise price of $2.25 per share in the Private Placement. These amounts are included in the amounts noted above. In the three months ended March 31, 2019, the Company and certain holders of the warrants issued in 2016 (the “Participating 2016 Holders”) entered into a Warrant Amendment and Exercise Agreement (the “2016 Exercise Agreement”) pursuant to which the Company agreed to reduce the exercise price of the warrants held by such Participating 2016 Holders from $2.25 to $0.01 per share (the “2016 Reduced Exercise Price”) in consideration for the exercise of the warrants held by such Participating 2016 Holders in full at the 2016 Reduced Exercise Price for cash. In connection with the exercise of the warrants by the Participating 2016 Holders, the Company received aggregate gross proceeds of approximately $0.1 million. After the full exercise of the warrants held by the Participating 2016 Holders, warrants issued in 2016 to purchase approximately 508,714 shares of the Company’s Common Stock are outstanding.	8. CAPITAL STOCK. In October 2018, the Company closed a underwritten public offering of 26,155,000 shares of its common stock and warrants to purchase up to 19,616,250 shares of common stock, at a combined purchase price of $0.65 per share of common stock and accompanying warrant. The gross proceeds to Histogenics from this offering were $17.0 million, before deducting underwriting discounts and commissions, and offering expenses payable by the Company. The warrants are exercisable immediately upon issuance at a price of $0.70 per share of common stock and have a term of five years commencing on the date of issuance. The exercise price of the Warrants is subject to adjustment upon the occurrence of specific events, including stock dividends, stock splits, combinations and reclassifications of the Company’s Common Stock. In the event of certain fundamental transactions of the Company, a warrant holder may demand redemption of its warrant for cash in accordance with a Black-Scholes option pricing model. A fundamental transaction is defined as a merger, sale of assets, sale of the Company, recapitalization of stock and a sale of stock whereby any owner after the transaction would own greater than 50% of the outstanding common stock in the Company. The Company determined the warrants are classified as a liability on the consolidated balance sheet because of the provision whereby in a fundamental transaction (as described above), the holder can elect to receive either the amount they are entitled to on an as-if-exercised In March 2018, the Company entered into an equity distribution agreement (“the Equity Distribution Agreement”) with Canaccord Genuity Inc. (“Canaccord”), pursuant to which the Company may, from time to time, sell shares of its common stock having an aggregate offering price of up to $10.0 million (the “Shares”) through Canaccord, as sales agent. During the year ended December 31, 2018, the Company sold an aggregate of 6,633,903 shares of common stock and received $4.5 million after deducting commissions related to the Equity Distribution Agreement and other offering costs. In January 2018, the Company closed a registered direct offering where the Company issued 2,691,494 shares ofcommon stock at a price of $2.35 per share. The underwriter option to purchase additional shares of 351,064 were fully exercised. The total net proceeds of the offering were approximately $5.9 million after deducting underwriting discounts and commissions. In September 2016, the Company closed the private placement contemplated by the securities purchase agreement (the “Purchase Agreement”), dated September 15, 2016, between the Company and certain institutional and accredited investors in which the Company received gross proceeds of $30.0 million (the “Private Placement”). The net proceeds after deducting placement agent fees and other transaction-related expenses was $27.6 million. At the closing, the Company issued 2,596,059 shares of the Company’s common stock at a per share price of $2.25 and 24,158.8693 shares of the Company’s newly-created Series A Convertible Preferred Stock (“Series A Preferred Stock”), which are convertible into approximately 10,737,275 shares of common stock. As of December 31, 2018, there were 400.4910 shares of Series A Preferred Stock outstanding, which remain convertible into 177,996 shares of the Company’s common stock. As part of the Private Placement, the investors received warrants to purchase up to 13,333,334 shares of the Company’s common stock at an exercise price of $2.25 per share. The placement agent for the Private Placement, H.C. Wainwright & Co. LLC (“HCW”), and certain of its affiliates were also granted warrants to purchase 133,333 shares of the Company’s common stock at an exercise price of $2.25 per share in exchange for the services provided by HCW. The placement agent warrants were considered a financing cost of the Company and included in warrant expense within the consolidated statements of operations. The warrants include a cashless-exercise feature that may be exercised solely in the event there is no effective registration statement, or no current prospectus available for, the resale of the shares of common stock underlying the warrants as of the six-month as-if-exercised Concurrent with the closing of the Private Placement, the Company’s Certificate of Incorporation was amended by the filing of a Certificate of Designation to create the Series A Preferred Stock. The Series A Preferred Stock has a par value of $0.01 and each share is convertible into 444.44 shares of common stock, at a conversion price of $2.25 per share, at the option of the holder. The Series A Preferred Stock has no voting rights and is only entitled to dividends as declared on an as-converted as-converted As part of the Private Placement, affiliates of certain members of the Company’s Board of Directors purchased an aggregate of 283,046 shares of common stock, an aggregate of 2,563.1439 shares of Series A Preferred Stock and received warrants to purchase up to 1,422,221 shares of common stock at an exercise price of $2.25 per share in the Private Placement. These amounts are included in the amounts noted above. Common Stock -100,000,000 The holders of shares of common stock are entitled to one vote per share. The holders of shares of common stock are not entitled to receive dividends, unless declared by the Company’s board of directors out of legally available funds, if ever. Reserved for future issuance The Company has reserved for future issuance the following number of shares of common stock: As of December 31, 2018 2017 Options to purchase common stock 3,339,471 2,158,348 Common stock warrants 33,145,228 13,633,070 Total 36,484,699 15,791,418 Preferred Stock -30,000 Series A Convertible Preferred Stock On September 29, 2016, the Company issued 24,158.8693 shares of newly-created Series A Convertible Preferred Stock, which were convertible into approximately 10,737,275 shares of common stock at an initial conversion price of $2.25. The Series A Preferred Stock has a par value of $0.01 and each share is convertible into 444.44 shares of common stock at the option of the holder. The holders of Series A Preferred Stock have no voting rights, share in both income and losses and are only entitled to dividends as declared on an as-converted as-converted

Warrants

Warrants	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Warrants and Rights Note Disclosure [Abstract]
Warrants	7. WARRANTS The Company has warrants to purchase its common stock outstanding as of March 31, 2019, as follows: Issued Classification Warrants Exercise Expiration September 2016 Liability 508,714 $ 2.25 November 2021 March 2015 Equity 3,699 9.75 March 2025 July 2014 Equity 6,566 7.99 July 2024 July 2012 Equity 52,046 0.01 July 2022	9. WARRANTS Issued Classification Warrants Outstanding Exercise Price Expiration October 2018 Liability 19,616,250 $ 0.70 October 2028 September 2016 Liability 13,466,667 2.25 November 2022 March 2015 Equity 3,699 9.75 March 2025 July 2014 Equity 6,566 7.99 July 2024 July 2012 Equity 52,046 0.01 July 2022 In the first quarter of 2019 the Company reduced the exercise price for all but 508,714 of the warrants issued in September 2016 and all of the warrants issued in October 2018. Refer to Note 15, Subsequent Events

Equipment Loan Payable

Equipment Loan Payable	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Equipment Loan Payable	10. EQUIPMENT LOAN PAYABLE As of December 31, 2018 and 2017, the Company had the following outstanding borrowing obligations: December 31, December 31, (in thousands) Silicon Valley Bank Equipment Loan Payable $ — $ 178 Less: current portion — (178 ) Long-term debt, net $ — $ — In July 2014, the Company entered into a loan and security agreement with Silicon Valley Bank, which provided for a line of credit to finance certain equipment purchases up to an aggregate of $1.8 million through March 31, 2015. The line has been fully drawn and is payable in 36 monthly installments of principal and interest, with an annual interest rate of 2.75% plus the greater of 3.25% or the prime rate in effect at the time of each draw, as published in the Wall Street Journal. The outstanding balance on the line of credit is secured by a first priority lien over all equipment purchased using the line of credit. In accordance with the terms of the equipment line of credit, the Company issued a warrant to Silicon Valley Bank in July 2014 to purchase 6,566 shares of our common stock at an exercise price per share of $7.99. The equipment line of credit includes customary operating but non-financial

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	8. STOCK-BASED COMPENSATION Stock option activity under the Company’s 2012 Equity Incentive Plan (the “2012 Plan”) and 2013 Equity Incentive Plan (the “2013 Plan”) for the three months ended March 31, 2019 is summarized as follows: Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2018 3,339,471 $ 1.03 8.1 $ — Cancelled (1,536,304 ) 0.57 — — Outstanding at March 31, 2019 1,803,167 $ 1.03 7.2 $ — Vested and expected to vest at March 31, 2019 1,766,323 $ 1.41 7.2 $ — Exercisable at March 31, 2019 1,723,797 $ 1.37 7.1 $ — As of March 31, 2019, the weighted average grant date fair value of vested options and options outstanding was $1.82. Stock-Based Compensation Expense The Company did not grant any stock options to employees during the three ended March 31, 2019 but did grant stock options to employees during the three months ended March 31, 2018. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model and restricted stock based on the stock price, with the exception of those stock options that included a market condition. The Company estimates the fair value of stock options that include a market condition using a Monte-Carlo model. Stock options and restricted stock issued to non-board non-employees Stock-based compensation expense amounted to $0.1 million and $0.4 million for the three months ended March 31, 2019 and 2018, respectively. The allocation of stock-based compensation for all options granted and restricted stock awards is as follows: Three Months Ended March 31, 2019 2018 (in thousands) Research and development $ (38 ) $ 96 General and administrative 145 307 Total stock-based compensation expense $ 107 $ 403 The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants and non-employee Three Months Ended March 31, 2018 Employees Non-employees Risk-free interest rate 2.70 % 1.97 % Expected volatility 88.2 % 74.0 % Expected term (in years) 6.08 6.08 Expected dividend yield 0.0 % 0.0 %	11. STOCK-BASED COMPENSATION Restricted Stock Awards and Stock Options The Company adopted the 2012 Equity Incentive Plan, as amended (“2012 Plan”) in July 2012 pursuant to which 609,389 shares of common stock were authorized for issuance to employees, officers, directors, consultants and advisors of the Company as of December 31, 2014. Upon the closing of the IPO on December 3, 2014, no further grants were made under the 2012 Plan as the 2013 Equity Incentive Plan (“2013 Plan”) replaced the 2012 Plan on this date. The 2012 Plan provided for the grant of incentive stock options, non-statutory In determining the exercise prices for options granted, the board of directors considered the fair value of the common stock as of the measurement date. The fair value of the common stock was determined by the board of directors based on a variety of different factors, including valuations prepared by third party valuation specialists, the Company’s financial position, the status of development efforts within the Company, the composition and ability of the current scientific and management teams, the current climate in the marketplace, the illiquid nature of the Company’s common stock, any arm’s length sale of the Company’s preferred stock, the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event, among others. 2013 Equity Incentive Plan The Company’s board of directors adopted the 2013 Plan in November 2013 which the stockholders approved in October 2014. The 2013 Plan provides for the grant of incentive stock options, non-statutory 2013 Employee Stock Purchase Plan The Company’s board of directors adopted the 2013 Employee Stock Purchase Plan (“2013 ESPP”) in November 2013 which the stockholders approved in October 2014. The 2013 ESPP became effective upon the closing of the IPO on December 3, 2014. The Company’s 2013 ESPP qualifies under Section 423 of the Internal Revenue Code of 1986, as amended (the “Code”). Under the 2013 ESPP, 103,665 shares of the Company’s common stock are authorized for issuance to eligible employees. The number of shares reserved for issuance under the 2013 ESPP is automatically increased on the first business day of each of the Company’s fiscal years, commencing in 2015, by a number equal to the lowest of (a) 51,832 shares of common stock, (b) 1% of the shares of common stock outstanding on the last business day of the prior fiscal year; or (c) the number of shares determined by the Company’s Board of Directors. Accordingly, the number of authorized shares of the Company’s common stock authorized for issuance to eligible employees under the 2013 ESPP was increased by 206,476 shares on January 1, 2017 and an additional 51,832 shares on January 1, 2018. The number of shares reserved under the 2013 ESPP will automatically be adjusted in the event of a stock split, stock dividend or a reverse stock split (including an adjustment to the per-purchase Stock option activity under the 2012 and 2013 plans is summarized as follows: Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2017 2,158,348 $ 4.40 8.1 $ 436 Granted 4,311,090 1.21 Exercised (919 ) 2.56 Cancelled (3,129,048 ) 3.60 Outstanding at December 31, 2018 3,339,471 $ 1.03 8.1 $ — Vested and expected to vest at December 31, 2018 3,314,542 $ 1.03 8.1 $ — Exercisable at December 31, 2018 1,603,725 $ 1.37 7.1 $ — As of December 31, 2018 and 2017, the unrecognized compensation cost related to outstanding options was $2.3 million and $1.4. million, respectively, and is expected to be recognized as expense over approximately 2.50 years and 1.70 years, respectively. The intrinsic value of options exercised during the years ended December 31, 2018 and 2017 was $0 and $8 thousand, respectively. As of December 31, 2018, the weighted average grant date fair value of vested options was $0.76 and the weighted average grant date fair value of options outstanding was $0.52. Additional information about the Company’s stock option activity is as follows: Year Ended December 31, 2018 2017 Weighted-average grant date fair value per share of employee option grants within the year $ 0.59 $ 1.02 Cash received upon exercise of options 2 6 As of December 31, 2016, the unrecognized compensation cost related to restricted stock awards was $1 thousand which was recognized as expense during the year ended December 31, 2017. Stock-Based Compensation Expense The Company granted stock options to employees for the years ended December 31, 2018 and 2017. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model and restricted stock based on the fair value of the award. Stock options and restricted stock issued to non-board non-employees For all periods from inception to date, stock-based compensation for all options granted and restricted stock awards are classified as research and development expense and general and administrative expense. Stock compensation expense amounted to approximately $1.6 million and $1.6 million for the years ended December 31, 2018 and 2017, respectively. Stock-based compensation is as follows: Year Ended December 31, 2018 2017 (in thousands) Research and development $ 488 $ 411 General and administrative 1,137 1,162 Total stock-based compensation expense $ 1,625 $ 1,573 The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Year Ended December 31, 2018 2017 Risk-free interest rate 2.84 % 2.03 % Expected volatility 84.4 % 62.9 % Expected term (in years) 5.31 6.08 Expected dividend yield 0.0 % 0.0 % The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the non-employee Year Ended December 31, 2018 2017 Risk-free interest rate 2.57 % 1.29 % Expected volatility 88.3 % 62.3 % Expected term (in years) 6.08 6.08 Expected dividend yield 0.0 % 0.0 % Risk-free Interest Rate Expected Volatility Expected Term Expected Dividend Yield On October 1, 2018, the Compensation Committee of the Board of Directors approved a repricing (the “Repricing”) of 3,807,779 stock options (the “Options”) granted prior to September 1, 2018 pursuant to our 2013 Equity Incentive Plan and our 2012 Equity Incentive Plan to executive officers, employees and consultants of the Company. The Options had exercise prices between $0.75628 and $9.97 per share, which were reduced to $0.568 per share (the closing price of the Company’s common stock on The Nasdaq Capital Market on October 1, 2018). The number of shares, vesting schedules and expiration period of the Options were not altered. The impact to the Company’s financial statements in 2018 was immaterial.

Income Taxes

Income Taxes	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	9. INCOME TAXES Deferred tax assets and deferred tax liabilities are recognized based on temporary differences between the financial reporting and tax basis of assets and liabilities using statutory rates. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. The Company recorded no income tax expense or benefit during the three months ended March 31, 2019 and 2018, due to a full valuation allowance recognized against its deferred tax assets. TAX REFORM On December 22, 2017, the Tax Cuts and Jobs Act (the “TCJA”) was signed into United States law. The TCJA includes a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 34% to 21%, effective as of January 1, 2018, as well as limitation of the deduction for net operating losses to 80% of annual taxable income and elimination of net operating loss carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such net operating losses may be carried forward indefinitely). The tax rate change resulted in (i) a reduction in the gross amount of the Company’s deferred tax assets recorded as of December 31, 2017, without an impact on the net amount of its deferred tax assets, which are recorded with a full valuation allowance, and (ii) no income tax expense or benefit being recognized as of the enactment date of the TCJA. The staff of the Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the TCJA. In connection with the initial analysis of the impact of the TCJA, the Company remeasured its deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21%.	12. INCOME TAXES For the years ended December 31, 2018 and 2017, the Company did not record a current or deferred income tax expense or benefit due to current and historical losses incurred by the Company. The components of loss before income taxes were as follows: As of December 31, 2018 2017 (in thousands) U.S. $ (8,503 ) $ (26,275 ) Foreign (140 ) (140 ) Total $ (8,643 ) $ (26,415 ) A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: As of December 31, 2018 2017 Federal income tax (benefit) at statutory rate 21.1 % 33.7 % (Increase) decrease income tax benefit resulting from: State income tax benefit, net of federal benefit 22.5 % 0.0 % Permanent differences 54.4 % (2.6 )% Net Operating Loss Limitation 0.0 % 0.0 % Federal Tax Rate change 0 % (47.2 )% R&D Credit Limitation 0.9 % 0.0 % Change in valuation allowance (98.5 )% 14.6 % Other -0.4 % 1.5 % Income tax expense (benefit) 0.0 % 0.0 % Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets and liabilities are comprised of the following: As of December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 24,941 $ 18,540 Depreciation and amortization 4,878 4,121 Accrued expenses 141 1,484 Capitalized start-up 9,143 7,942 R&D credits 312 243 Other 1,155 808 Deferred tax assets before valuation allowance 40,570 33,138 Valuation allowance (40,570 ) (33,138 ) — — Deferred tax liabilities IPR&D — — Change in accounting method — — Net deferred tax assets $ — $ — The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. As of December 31, 2018 and 2017, based on the Company’s history of operating losses, the Company has concluded that it is not more likely than not that the benefit of its deferred tax assets will be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of December 31, 2018 and 2017. The valuation allowance decreased by $7.4 million during the year ended December 31, 2018, due primarily to net operating losses generated and capitalized expenses. The valuation allowance decreased $1.0 million during the year ended December 31, 2017, due primarily to net operating losses generated, net of the impact of a federal tax rate change of $11.5 million. In addition, the reduction in net operating losses were related to Section 382 limits as a result in a change in ownership. As of December 31, 2018 and 2017, the Company had U.S. federal NOL carryforwards of $67 million and $43.9 million respectively, which may be available to offset future income tax liabilities and expire at various dates through 2037. As of December 31, 2018 and 2017, the Company also had U.S. state NOL carryforwards of $66.9 million and $43.6 million respectively, which may be available to offset future income tax liabilities and expire at various dates through 2037. At December 31, 2018 and 2017, the Company also had $26.2 and $26.1 respectively, of foreign NOL carryforwards which may be available to offset future income tax liabilities, which carryforwards do not expire. Utilization of the NOL and research and development credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that have occurred or that could occur in the future, as required by Section 382 and Section 383 of the Code, as well as similar state and foreign provisions. These ownership changes may limit the amount of NOL and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an “ownership change” as defined by Section 382 of the Code results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percentage points of the outstanding stock of a company by certain stockholders. The Company has completed a study to assess whether an ownership change has occurred or whether there have been multiple ownership changes since its formation. The results of this study indicated that the Company experienced ownership changes as defined by Section 382 of the Code. The Company has not recorded NOLs that, as a result of these restrictions, from the 2016 ownership change, will expire unused. Accordingly, the Company has recorded NOL carryforwards net of these limitations, which are approximately $52.9 million. TAX REFORM On December 22, 2017 the Tax Cuts and Jobs Act (the “TCJA”) was signed into United States law. The TCJA includes a number of changes to existing tax law, including, among other things, a permanent reduction in the federal corporate income tax rate from 34% to 21%, effective as of January 1, 2018, as well as limitation of the deduction for net operating losses to 80% of annual taxable income and elimination of net operating loss carrybacks, in each case, for losses arising in taxable years beginning after December 31, 2017 (though any such net operating losses may be carried forward indefinitely). The tax rate change resulted in (i) a reduction in the gross amount of the Company’s deferred tax assets recorded as of December 31, 2017, without an impact on the net amount of its deferred tax assets, which are recorded with a full valuation allowance, and (ii) no income tax expense or benefit being recognized as of the enactment date of the TCJA. The staff of the Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the TCJA. In connection with the initial analysis of the impact of the TCJA, the Company remeasured its deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21%. The remeasurement of the Company’s deferred tax assets and liabilities was offset by a corresponding change in the valuation allowance for the year ended December 31, 2017. As a result, there was no impact to the Company’s consolidated statements of operations and comprehensive loss as a result of the reduction in tax rates. The other provisions of the TCJA did not have a material impact on the Company’s consolidated financial statements. The Company’s final determination of the TCJA impact and the remeasurement of its deferred assets and liabilities was completed prior to the deadline of one year from the enactment of the TCJA. For the year ended December 31, 2018, there were no material changes to the analysis originally performed as of December 31, 2017. The changes in the Company’s unrecognized tax benefits are summarized as follows: As of December 31, 2018 2017 (in thousands) Unrecognized tax benefit, beginning of year $ 303 $ 562 Increase (decrease) related to current year positions — (123 ) Federal rate revision — (136 ) Unrecognized tax benefit, end of year $ 303 $ 303 As of December 31, 2018 and 2017, the total amount of unrecognized tax benefits was $0.3 million and $0.3 million, respectively which, if recognized, would favorably affect the effective income tax rate in future periods. Note that liabilities for unrecognized tax benefits have been recorded to the extent that they do not exceed the Company’s available losses that are not limited as a result of ownership changes that have occurred under Section 382 of the Code. Reductions to unrecognized tax benefits for limitations on the utilization of net operating losses due to ownership changes occurring during the year has been reflected in the table as reductions based on tax positions related to the current year. The Company accrues interest and penalties related to unrecognized tax benefits as a component of its provision for income taxes. No accrued interest and penalties related to the Company’s unrecognized tax benefits has been accrued as of December 31, 2018 and 2017. The Company believes that it is reasonably possible that none of its unrecognized tax benefits, may be recognized at the end of 2018. The Company or one of its subsidiaries files income tax returns in the United States and various states and Israel. The Company is subject to U.S. federal, state and local income tax examinations by tax authorities for years 2001 through present. Carryforward attributes that were generated in earlier periods remain subject to examination to the extent the year in which they were used or will be used remains open for examination. The tax years which remain subject to examination by tax authorities in Israel, as of December 31, 2018, include years 2014 through the present.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Employee Benefits	13. EMPLOYEE BENEFITS The Company has a defined contribution 401(k) plan for employees who are at least 21 years of age. Employees are eligible to participate in the plan beginning on the first day of the calendar quarter following their date of hire. Under the terms of the plan, employees may make voluntary contributions as a percent of compensation. No matching contributions have been made by the Company since the adoption of the 401(k) plan.

Related Parties

Related Parties	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Parties	10. RELATED PARTIES Purpose, Co. In June 2012, the Company entered into an agreement with Purpose to amend its previous agreements. In the previous agreements, Purpose granted the Company a perpetual license to its patents related to its exogenous tissue processor which is used in the development of the Company’s products. In exchange, the Company granted Purpose a perpetual license to all of the Company’s biotechnology and biomaterial for use in Japan. The agreement provided for Purpose to manufacture and sell machinery to the Company for cost until the Company’s products become commercially viable. The Company also agreed to pay royalties on any third-party revenue generated using Purpose’s licensed technology. Under the June 2012 amendment, the Company received exclusive rights to all of Purpose’s technology related to the exogenous tissue processor, continued supply of exogenous tissue processors during the Company’s clinical trials, and rights to manufacture the exogenous tissue processors at any location the Company chooses. In exchange for such consideration, the Company named Purpose the sole manufacturer of equipment and also clarified the geographic territories of the exclusive license that Purpose was granted for use of the Company’s technology. In addition, the Company agreed to reimburse Purpose for $0.3 million of development costs on a next generation tissue processer. Refer to the discussion under Note 5, Commitments and Contingencies—License Agreements —Tissue Processor Sub-License In May 2016, the Company acquired the development and commercialization rights to NeoCart for the Japanese market from Purpose. Under the terms of the amended agreement, the Company assumes sole responsibility for and rights to the development and commercialization of NeoCart in Japan. In exchange for the transfer of development and commercialization rights, the Company will pay a success-based milestone to Purpose upon conditional approval of NeoCart in Japan, as well as commercial milestones and a low single digit royalty on Japanese sales of NeoCart, upon full approval, if any, in Japan. The Company paid Purpose $0.1 million in each of the three months ended March 31, 2019 and 2018.	14. RELATED PARTIES Purpose, Co. In June 2012, the Company entered into an agreement with Purpose, Co. to amend its previous agreements. In the previous agreements, Purpose, Co. granted the Company a perpetual license to its patents related to its exogenous tissue processor which is used in the development of the Company’s products. In exchange, the Company granted Purpose, Co. a perpetual license to all of the Company’s biotechnology and biomaterial for use in Japan. The agreement provides for Purpose, Co. to manufacture and sell machinery to the Company for cost until the Company’s products become commercially viable. The Company has also agreed to pay royalties on any third-party revenue generated using Purpose, Co.’s licensed technology. Under the June 2012 amendment, the Company received exclusive rights to all of Purpose, Co.’s technology related to the exogenous tissue processor, continued supply of exogenous tissue processors during the Company’s clinical trials, and rights to manufacture the exogenous tissue processors at any location the Company chooses. In exchange for such consideration, the Company named Purpose, Co. the sole manufacturer of equipment and also clarified the geographic territories of the exclusive license that Purpose Co. was granted for use of the Company’s technology. Also, the Company agreed to reimburse Purpose, Co. for $0.3 million of development costs on a next generation tissue processer. Refer to the discussion under Note 7, Tissue Processor Sub-License In May 2016, the Company acquired the development and commercialization rights to NeoCart for the Japanese market from Purpose, Co. Under the terms of the amended agreement, the Company assumes sole responsibility for and rights to the development and commercialization of NeoCart in Japan. In exchange for the transfer of development and commercialization rights, the Company will pay a success-based milestone to Purpose upon conditional approval of NeoCart in Japan, as well as commercial milestones and a low single digit royalty on Japanese sales of NeoCart, upon full approval, if any, in Japan. The amounts that have been paid to Purpose, Co. under this agreement were $0.1 and $0.1 million for the years ended December 31, 2018 and 2017, respectively. Board of Director Affiliates Affiliates of certain members of the Company’s Board of Directors participated in the Private Placement as described in Note 8.

Subsequent Events

Subsequent Events	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	12. SUBSEQUENT EVENTS On April 8, 2019, the Company and Ocugen announced entering into the Merger Agreement. Upon the terms and subject to the satisfaction of the conditions described in the Merger Agreement, including approval of the transaction by the Company’s stockholders and Ocugen’s stockholders, a wholly-owned subsidiary of the Company will be merged with and into Ocugen, with Ocugen surviving the Merger as a wholly-owned subsidiary of the Company. The proposed Merger is structured as a stock-for-stock On April 30, 2019, the Company terminated its property lease in Waltham, Massachusetts and made a payment of $1.3 million to the landlord in connection with the termination. The Company expects to record a $0.5 million loss in connection with the settlement of the lease obligations in the second quarter of 2019. On May 8, 2019, the Company entered into an agreement with the landlord to terminate its property lease Lexington, Massachusetts effective as of June 30, 2019. In connection with the termination of this lease, the Company paid the landlord $0.3 million and agreed that the landlord shall retain the full amount of the security deposit of $0.1 million. In the event these premises are leased to a new tenant prior to June 30, 2019, the Company’s obligation to pay base rent, operating expenses and any other obligations due under the lease shall terminate as of the commencement of such new lease and such date shall be deemed to be the termination date. On May 8, 2019, the Company entered into an asset purchase agreement with Medavate Corp., a Colorado corporation (the “Asset Purchase Agreement”), pursuant to which the Company has agreed to sell substantially all of its assets relating to its NeoCart program, including, without limitation, intellectual property, business and license agreements and clinical trial data (the “Assets”) in return for a cash payment of $6.5 million. The closing of the sale of the Assets is subject to and conditioned upon the consummation of the planned merger with Ocugen following a vote of the Company’s stockholders approving such transaction as contemplated by the Merger Agreement made and entered as of April 5, 2019, by and among the Company, Restore Merger Sub, Inc. and Ocugen, Inc.	15. SUBSEQUENT EVENTS Restructuring— one-time Additional Restructuring one-time Warrant Amendments Also in the first quarter of 2019, the Company reduced the exercise price of the warrants issued in 2018 from $0.70 to $0.01 per share (the “2018 Reduced Exercise Price”) and all of the holders of these warrants (the “Participating 2018 Holders”) entered into a Warrant Exercise Agreement (the “2018 Exercise Agreement”) pursuant to which in consideration for the 2018 Reduced Exercise Price, the Participating 2018 Holders agreed to exercise the warrants held by such Participating 2018 Holders in full at the 2018 Reduced Exercise Price for cash. In connection with the exercise of the warrants by the Participating 2018 Holders, the Company received aggregate gross proceeds of approximately $0.2 million.

Restructuring

Restructuring	3 Months Ended
	Mar. 31, 2019
Restructuring and Related Activities [Abstract]
Restructuring	11. RESTRUCTURING In connection with the aforementioned restructuring plans implemented in January 2019 and March 2019, the Company severed substantially all of its employees before March 31, 2019. The Company incurred a restructuring charge of approximately $2.8 million, of which $0.1 million remains unpaid at March 31, 2019. The restructuring charges were comprised solely of employee severance packages which included salary, healthcare benefits and guaranteed bonuses.

Nature of Business (Policies)

Nature of Business (Policies)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Accounting	Basis of Accounting The consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). However, they do not include all of the information and footnotes required by GAAP for complete financial statements. These interim consolidated financial statements, in the opinion of the Company’s management, reflect all normal recurring adjustments necessary for a fair presentation of the Company’s financial position and results of operations for the interim periods ended March 31, 2019 and 2018. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year. These interim financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2018, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K, The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, ProChon and Histogenics Securities Corporation. All significant intercompany accounts and transactions are eliminated in consolidation.	Basis of Accounting The consolidated financial statements are prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The consolidated financial statements include the accounts of Histogenics Corporation and its wholly-owned subsidiaries, ProChon and Histogenics Securities Corporation. All significant intercompany accounts and transactions are eliminated in consolidation.
Use of Estimates		Use of Estimates The preparation of the Company’s consolidated financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in the Company’s consolidated financial statements and accompanying notes. The most significant estimates in the Company’s consolidated financial statements relate to the valuation of equity awards, warrant liability, recoverability of deferred tax assets, estimated useful lives of fixed assets. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. The Company evaluates its estimates and assumptions on an ongoing basis. The Company recorded in 2018 a loss on impairment of fixed assets. See Note 5 Property and Equipment. The Company’s actual results may differ from these estimates under different assumptions or conditions.
Foreign Currency Translation		Foreign Currency Translation The Company’s consolidated financial statements are prepared in U.S. dollars. The Company’s foreign subsidiary uses the U.S. dollar as its functional and reporting currency, as management determined that the U.S. dollar is the primary currency of the economic environment in which the subsidiary operates. When transactions are required to be paid in the local currency of the foreign subsidiary, any resulting foreign currency transaction gain or loss is recorded as a component of “Other income (expense), net” in the consolidated statements of operations.
Segment and Geographic Information		Segment and Geographic Information Operating segments are defined as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker (“CODM”) or decision-making group in making decisions regarding resource allocation and assessing performance. The Company operates in two geographic regions: the United States (Massachusetts) and Israel (Tel Aviv) and views its operations as two operating segments: Histogenics Corporation (United States) and ProChon (Israel) as the CODM reviews separate discrete financial information in making decisions regarding resource allocations and assessing performance. Operating segments that have similar economic characteristics can be aggregated. As the nature of the products, customers, and methods to distribute products are the same and the nature of the regulatory environment, the production processes and historical and estimated future margins are similar, the two operating segments have been aggregated into one reporting segment as they have similar economic characteristics.
Fair Value Measurements	Fair Value Measurements The carrying amounts reported in the Company’s consolidated financial statements for cash and cash equivalents, accounts payable, and accrued liabilities approximate their respective fair values because of the short-term nature of these accounts. Fair value is defined as the price that would be received if selling an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Additionally, from time to time, the Company may be required to record at fair value other assets on a nonrecurring basis, such as assets held for sale and certain other assets. These nonrecurring fair value adjustments typically involve the application of lower-of-cost-or-market The fair value hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets (Level 1), and the lowest priority to unobservable inputs (Level 3). The Company’s financial assets are classified within the fair value hierarchy based on the lowest level of inputs that is significant to the fair value measurement. The three levels of the fair value hierarchy, and their applicability to the Company’s financial assets, are described below. Level 1 Level 2 Level 3 non-transferability, An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. The Company had no material re-measurements The fair value of the warrants issued in connection with the September 2016 private placement was determined using a Monte Carlo simulation model. This model incorporated several assumptions at each valuation date including: the price of the Company’s common stock on the date of valuation, the historical volatility of the price of the Company’s common stock, the remaining contractual term of the warrant and estimates of the probability of a fundamental transaction occurring. The fair value of the warrants issued in connection with the October 2018 underwritten public offering was determined using the Black Scholes model. See Note 6, Capital Stock, for further discussion of the private placement and underwritten public offering. The Company’s financial instruments as of March 31, 2019 and December 31, 2018 consisted primarily of cash and cash equivalents and warrant liability. As of March 31, 2019, and December 31, 2018, the Company’s financial assets recognized at fair value consisted of the following: Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) (in thousands) March 31, 2019 Assets: Cash Equivalents Money market funds $ 4,747 $ 4,747 $ — $ — Liabilities: Warrant liability $ 10 $ — $ — $ 10 December 31, 2018 Assets: Cash Equivalents Money market funds $ 9,711 $ 9,711 $ — $ — Liabilities: Warrant liability $ 2,512 $ — $ — $ 2,512 The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs: March 31, 2019 (in thousands) Beginning balance, December 31, 2018 $ 2,512 Exercise of warrants (1,095 ) Change in fair value of warrant liability (1,407 ) Ending balance $ 10	Fair Value Measurements The carrying amounts reported in the Company’s consolidated financial statements for cash and cash equivalents, accounts payable, equipment loan, and accrued liabilities approximate their respective fair values because of the short-term nature of these accounts. Fair value is defined as the price that would be received if selling an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Fair value should be based on the assumptions that market participants would use when pricing an asset or liability and is based on a fair value hierarchy that prioritizes the information used to develop those assumptions. Fair value measurements should be disclosed separately by level within the fair value hierarchy. For assets and liabilities recorded at fair value, it is the Company’s policy to maximize the use of observable inputs (quoted prices in active markets) and minimize the use of unobservable inputs (Company assumptions) when developing fair value measurements, in accordance with established fair value hierarchy. Fair value measurements for assets and liabilities where there exists limited or no observable market data are based primarily upon estimates, and often are calculated based on the economic and competitive environment, the characteristics of the asset or liability and other factors. Therefore, the results cannot be determined with precision and may not be realized in an actual sale or immediate settlement of the asset or liability. Additionally, there may be inherent weaknesses in any valuation technique, and changes in the underlying assumptions used, including discount rates and estimates of future cash flows, could significantly affect the results of current or future values. Additionally, from time to time, the Company may be required to record at fair value other assets on a nonrecurring basis, such as assets held for sale and certain other assets. These nonrecurring fair value adjustments typically involve application of lower-of-cost-or-market The fair value hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets (Level 1), and the lowest priority to unobservable inputs (Level 3). The Company’s financial assets are classified within the fair value hierarchy based on the lowest level of inputs that is significant to the fair value measurement. The three levels of the fair value hierarchy, and its applicability to the Company’s financial assets, are described below. Level 1 Level 2 Level 3 Level 3 valuations are for instruments that are not traded in active markets or are subject to transfer restrictions and may be adjusted to reflect illiquidity and/or non-transferability, An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. The Company had no assets or liabilities classified as Level 1 or Level 2 as of December 31, 2018 and 2017 other than the money market fund described in the “Cash and Cash Equivalents” section and the asset-backed securities in the “Marketable securities” section below. There were no material re-measurements The fair value of the warrants issued in connection with the September 2016 private placement was determined using a Monte Carlo simulation model. This model incorporated several assumptions at each valuation date including: the price of the Company’s common stock on the date of valuation, the historical volatility of the price of the Company’s common stock, the remaining contractual term of the warrant and estimates of the probability of a fundamental transaction occurring. The fair value of the warrants issued in connection with the October 2018 underwritten public offering was determined using the Black Scholes model. See Note 8, Capital Stock, for further discussion of the private placement and underwritten public offering. Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) (in thousands) December 31, 2018 Assets: Cash Equivalents Money market funds 9,711 9,711 — — Liabilities: Warrant liability 2,512 — — 2,512 December 31, 2017 Assets: Cash Equivalents Money market funds 5,547 5,547 — — Marketable securities: Asset-backed securities 900 — 900 — Liabilities: Warrant liability 14,679 — — 14,679 The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs: As of December 31, (in thousands) Beginning balance, January 1, 2018 $ 14,679 Issuance of warrants 8,434 Change in fair value of warrant liability (20,601 ) Ending balance $ 2,512
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid securities with original maturities of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents are comprised of funds in money market accounts. In addition, the Company has recorded restricted cash of $0.1 million as of March 31, 2019 and December 31, 2018. Restricted cash consists of a security deposit related to a lease obligation.	Cash and Cash Equivalents Cash and cash equivalents include cash in readily available checking and savings accounts and money market funds. The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents.
Marketable Securities		Marketable Securities The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available for sale. The Company considers all available for sale securities, including those with maturity dates beyond 12 months, as available to support current operational liquidity needs and therefore classifies all securities including those with maturity dates beyond 90 days at the date of purchase as current assets within the consolidated balance sheets. Available for sale securities are maintained by the Company’s investment managers and may consist of commercial paper, high-grade corporate notes, U.S. Treasury securities, U.S. government agency securities, and certificates of deposit. Available for sale securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity (deficit) until realized. Any premium or discount arising at purchase is amortized and/or accreted to interest income and/or expense over the life of the instrument. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other-than-temporary” and, if so, marks the investment to market through a charge to the Company’s consolidated statement of operations and comprehensive loss. The Company did not hold any available for sale securities prior to the first quarter of 2017. The amortized cost of available for sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. At December 31, 2018, the balance in the Company’s accumulated other comprehensive loss was composed solely of activity related to the Company’s available for sale marketable securities. The aggregate fair value of available for sale securities held by the Company for less than twelve months as of December 31, 2017 was $0.9 million which matured in 2018. The Company did not hold any available for sale securities and there were no sales of such during the year ended December 31, 2018. The Company determined that there was no material change in the credit risk of any of its investments. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2017. The weighted average maturity of the Company’s portfolio was less than one month at December 31, 2017.
Concentration of Credit Risk		Concentration of Credit Risk Financial instruments, which potentially subject the Company to significant concentration of credit risk, consist primarily of cash and cash equivalents. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no financial instruments with off-balance
Property and Equipment		Property and Equipment Property and equipment are recorded at historical cost. Costs for capital assets not yet placed into service are capitalized as construction in progress, and are depreciated in accordance with the below guidelines once placed into service. Maintenance and repair costs are expensed as incurred. Costs which materially improve or extend the lives of existing assets are capitalized. The Company provides for depreciation and amortization using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Category Estimated Useful Lives Office equipment 3 to 5 years Laboratory equipment 3 to 5 years Leasehold improvements Shorter of the remaining lease term or useful life Upon retirement or sale, the cost of assets disposed and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is recorded in the consolidated statements of operations.
Impairment of Long-Lived Assets		Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment and identifiable intangible assets. When impairment indicators exist, the Company’s management evaluates long-lived assets for potential impairment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. In 2018 a triggering event caused an impairment of the Company’s property and equipment. The triggering event related to the Company’s announcement that the NeoCart program was discontinued and the subsequent 73% reduction in stock price. An impairment loss of $4.3 million was recognized in earnings. See Note 5 Property and Equipment.
Restricted Cash		Restricted Cash Restricted cash represents cash held in a depository account at a financial institution to collateralize a conditional stand-by non-current
Deferred Rent		Deferred Rent Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the facilities the Company occupies. The Company’s leases for its Waltham, Massachusetts and Lexington, Massachusetts facilities provide for fixed increases in minimum annual rental payments. The total amount of rental payments due over each lease term is being charged to rent expense ratably over the life of each lease, respectively.
Financial Instruments Indexed to and Potentially Settled in the Company's Common Stock		Financial Instruments Indexed to and Potentially Settled in the Company’s Common Stock The Company evaluates all financial instruments issued in connection with its equity offerings when determining the proper accounting treatment for such instruments in the Company’s financial statements. The Company considers a number of generally accepted accounting principles under U.S. GAAP to determine such treatment and evaluates the features of the instrument to determine the appropriate accounting treatment. The Company utilizes the Probability Weighted Expected Return Method (“PWERM”), Option Pricing Model (“OM”) or other appropriate methods to determine the fair value of its derivative financial instruments, such as the warrant liability. For financial instruments indexed to and potentially settled in the Company’s common stock that are determined to be classified as liabilities on the consolidated balance sheet, changes in fair value are recorded as a gain or loss in the Company’s consolidated statement of operations with the corresponding amount recorded as an adjustment to the liability on its consolidated balance sheet.
Revenue Recognition	Revenue Recognition In May 2014, the Financial Accounting Standards Board (the “FASB”) issued a new standard related to revenue recognition, Accounting Standard Update (“ASU”) No. 2014-09, No. 2015-14, Revenue is recognized when, or as, performance obligations are satisfied, which occurs when control of the promised products or services is transferred to customers. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring products or services to a customer (“transaction price”). To the extent that the transaction price includes variable consideration, the Company estimates the amount of variable consideration that should be included in the transaction price utilizing the most likely amount method. Variable consideration is included in the transaction price if, in the Company’s judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Estimates of variable consideration and determination of whether to include estimated amounts in the transaction price are based largely on an assessment of the Company’s anticipated performance and all information (historical, current and forecasted) that is reasonably available. If the contract contains a single performance obligation, the entire transaction price is allocated to the single performance obligation. Contracts that contain multiple performance obligations require an allocation of the transaction price to each performance obligation based on a relative standalone selling price basis unless the transaction price is variable and meets the criteria to be allocated entirely to a performance obligation or to a distinct service that forms part of a single performance obligation. The Company currently generates revenue primarily through collaborative research, development and commercialization agreements. The terms of these agreements may contain multiple promises which may include: (i) licenses to the Company’s technology; (ii) services related to the transfer and update of know-how; non-refundable The Company assesses the promises to determine if they are distinct performance obligations. Once the performance obligations are determined, the transaction price is allocated based on a relative standalone selling price basis. Milestone payments and royalties are typically considered variable consideration at the outset of the contract and are recognized in the transaction price either upon occurrence or when the constraint of a probable reversal is no longer applicable. Collaboration Revenue While no revenue has been recognized as of March 31, 2018, the Company has collaboration and license agreements with strategic partners for the development and commercialization of product candidates. The collaboration and license agreements are within the scope of Accounting Standards Codification (ASC 606) Revenue from Contracts with Customers. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. Licenses of intellectual property: non-refundable, up-front non-refundable, up-front Manufacturing Supply Services: non-refundable, up-front Milestone Payments: re-evaluates catch-up Royalties:	Revenue Recognition In May 2014, the Financial Accounting Standards Board (the “FASB”) issued a new standard related to revenue recognition, Accounting Standard Update (“ASU”) No. 2014-09, No. 2015-14, Revenue is recognized when, or as, performance obligations are satisfied, which occurs when control of the promised products or services is transferred to customers. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring products or services to a customer (“transaction price”). To the extent that the transaction price includes variable consideration, the Company estimates the amount of variable consideration that should be included in the transaction price utilizing the most likely amount method. Variable consideration is included in the transaction price if, in the Company’s judgment, it is probable that a significant future reversal of cumulative revenue under the contract will not occur. Estimates of variable consideration and determination of whether to include estimated amounts in the transaction price are based largely on an assessment of the Company’s anticipated performance and all information (historical, current and forecasted) that is reasonably available. If the contract contains a single performance obligation, the entire transaction price is allocated to the single performance obligation. Contracts that contain multiple performance obligations require an allocation of the transaction price to each performance obligation based on a relative standalone selling price basis unless the transaction price is variable and meets the criteria to be allocated entirely to a performance obligation or to a distinct service that forms part of a single performance obligation. The Company’s revenues are generated primarily through collaborative research, development and commercialization agreements. The terms of these agreements may contain multiple promises which may include: (i) licenses to the Company’s technology; (ii) services related to the transfer and update of know-how; non-refundable The Company assesses the promises to determine if they are distinct performance obligations. Once the performance obligations are determined, the transaction price is allocated based on a relative standalone selling price basis. Milestone payments and royalties are typically considered variable consideration at the outset of the contract and are recognized in the transaction price either upon occurrence or when the constraint of a probable reversal is no longer applicable. Collaboration Revenue While no revenue has been recognized as of December 31, 2018, the Company has collaboration and license agreements with strategic partners for the development and commercialization of product candidates. The collaboration and license agreements are within the scope of Accounting Standards Codification (ASC 606) Revenue from Contracts with Customers. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. Licenses of intellectual property: non-refundable, up-front non-refundable, up-front Manufacturing Supply Services: non-refundable, up-front Milestone Payments: re-evaluates catch-up Royalties:
License and Collaboration Arrangements	License and Collaboration Arrangements MEDINET Co., Ltd. In December 2017, the Company entered into a License and Commercialization Agreement (the “License Agreement”) with MEDINET Co., Ltd. (“MEDINET”) to grant MEDINET a license under certain patents, patent applications, know-how, non-human In exchange for the license, MEDINET agreed to pay the Company an upfront cash payment of $10.0 million which the Company received in January 2018. As of March 31, 2019, the contract with MEDINET was wholly unperformed and all revenue under the License Agreement has been deferred and has not been recognized. As of March 31, 2019, the aggregate amount of the transaction price allocated to remaining performance obligations was $10.0 million. Because the License Agreement was not terminated as of March 31, 2019, the authoritative accounting literature requires that the $10.0 million of deferred revenue remain a liability on the Company’s balance sheet. The Company anticipates that MEDINET will terminate the License Agreement in 2019 resulting in the recognition of the $10 million of deferred revenue in the period of termination. MEDINET also agreed to pay the Company tiered royalties, at percentages ranging from the low single digits to low double digits, of net sales of MEDINET products governed by the License Agreement. Over the life of the License Agreement, the Company is eligible to receive up to ¥330 million ($2.9 million as of March 31, 2019) in development milestone payments, $1.0 million and ¥720 million ($7.4 million as of March 31, 2019) in regulatory payments and up to an aggregate of ¥7,300 million ($64.1 million as of March 31, 2019) for the achievement of certain commercial milestones related to the sales of MEDINET products governed by the License Agreement. As a condition of the License Agreement, the Company agreed to supply NeoCart for MEDINET’s planned Phase 3 clinical trial in Japan. The Company assessed its promised goods and services under the License Agreement to determine if they are distinct. Due to the unique nature of the clinical manufacturing services to be provided by the Company, there are currently no other third-party vendors from which MEDINET can obtain such supply. The Company expected to be the only vendor capable of providing the manufacturing services for a period of at least one to two years, which is approximately the estimated length of time for the Japanese clinical trial period. After this point, if the Company were to transfer to a third-party its technology and know-how Based on the assessment of the combined performance obligation, the Company determined that the predominant promise in the arrangement is the transfer of the license and associated manufacturing know-how In determining the correct measure of progress to use when recognizing revenue over time, the Company assessed whether an input- or output- based measure of progress would be appropriate. The Company determined that an output-based measure of progress would be appropriate to use when recognizing revenue associated with the combined performance obligation and intended to recognize revenue under the License Agreement as the clinical manufacturing services were performed. Revenue was to be recognized using the output method over the length of the clinical trial enrollment, as the clinical manufacturing services were delivered, over the estimated service period. Upon the conclusion of the clinical manufacturing period, the Company would have expected other third-party vendors to have the capabilities to provide similar services. At this point, the license would effectively become a distinct performance obligation, with no remaining undelivered obligations. Therefore, the Company determined that the up-front At contract inception, the Company determined that the $10.0 million non-refundable re-evaluates per-patient The Company incurred cost of $0.9 million related to the License Agreement with MEDINET. $0.8 million was recorded as an asset that was to be expensed proportionally over the performance service period. However, given the Company’s decision to discontinue the development of NeoCart and terminate its manufacturing operations, the Company concluded that the asset was impaired and a decision was made to write down the value of this asset to $0 in the first quarter of 2019. This impairment resulted in a charge to the income statement of $0.8 million.	License and Collaboration Arrangements MEDINET Co., Ltd. In December 2017 the Company entered into the License and Commercialization Agreement (the “License Agreement”) with MEDINET Co., Ltd. (“MEDINET”) to grant MEDINET a license under certain patents, patent applications, know-how, In exchange for the license, MEDINET agreed to pay the Company an non-refundable The Company assessed the promised goods and services to determine if they are distinct. Due to the unique nature of the clinical manufacturing services, there are no third-party vendors from which MEDINET can obtain such services from currently. The Company expects to be the only vendor capable of providing the commercial manufacturing services for a period of at least one to two years, which is approximately the estimated length of the clinical trial period in Japan. After this point, if the Company were to transfer to a third-party its technology and know-how Based on the assessment of the combined performance obligation, the Company determined that the predominant promise in the arrangement is the transfer of the license and associated knowhow which are expected to occur over the length of the clinical trial. The Company determined that MEDINET will be simultaneously receiving and consuming the benefits of the Company’s performance of the clinical trial. Therefore, the revenue associated with the combined performance obligation will be recognized over time. In determining the correct measure of progress to use when recognizing revenue over time, the Company assessed whether an input or output based measure of progress would be appropriate. The Company determined that an output based measure of progress would be appropriate to use when recognizing revenue associated with the combined performance obligation. The Company will recognize revenue based on the clinical manufacturing services completed to date. At the outset of the clinical trial in Japan to be conducted by MEDINET, the Company will have quantifiable estimates of total clinical candidates, and therefore, of total estimated performance. The Company will recognize revenue based on performance completed to date, as evidenced by the estimated number of clinical trial enrollees. The Company expects to provide the clinical manufacturing services to MEDINET over an estimated period of two years. Therefore, the estimated two-year two-year up-front At contract inception, the Company determined that the $10.0 million non-refundable re-evaluate per-patient The upfront transaction price of $10.0 million will be recognized over a period of approximately two-years, non-current Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represents the transaction price of contracts for which work has not been performed (or has been partially performed) and excludes unexercised contract options. As of December 31, 2018, the aggregate amount of the transaction price allocated to the remaining performance obligations was $10.0 million. The contract with MEDINET is wholly unperformed, and the Company recognized no revenue associated with the agreement during the years ended December 31, 2018 and 2017, respectively.
Research and Development Costs		Research and Development Costs Research and development costs are charged to expense as incurred. These costs include, but are not limited to: license fees related to the acquisition of in-licensed Costs for certain development activities, such as clinical trials, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by its vendors with respect to their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid or accrued research and development expense. Collaboration Arrangements Costs reimbursed to a collaborator for work that it performs are recorded as research and development expenses. These reimbursements can include payments for work performed, or a milestone for which a payment is due, the reimbursements or development milestone achievement are recorded as research and development expense. In September 2014, the Company entered into a collaboration agreement with Intrexon Corporation (“Intrexon”) for the development and commercialization of allogeneic cell therapeutics for the treatment or repair of damaged articular hyaline cartilage in humans, utilizing Intrexon’s proprietary technology (the “Collaboration Agreement”). Under the terms of the Collaboration Agreement, the Company is responsible for the costs of development and commercialization, with some exceptions. This agreement was terminated in December 2018. In connection with the Mutual Termination Agreement, in lieu of payment of the Accrued Expenses, the Company agreed to pay Intrexon an aggregate of up to $1.5 million, with $0.375 million paid at the time of entering into the Mutual Termination Agreement and $1.125 million payable within one year following any submission of a BLA to the FDA for NeoCart. We adjusted the accrued expenses to reflect $1.125 million balance as of December 31, 2018 and gain on extinguishment of liability of $1.5 million.
License Agreements		License Agreements Costs associated with licenses of technology are expensed as incurred and are included in research and development expenses.
Patent Costs		Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense as incurred since the recoverability of such expenditures is uncertain.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock options and restricted stock based on their grant date fair value and recognizes compensation expense on a straight-line basis over their vesting period. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model, with the exception of stock options that include a market condition, and restricted stock based on the fair value of the underlying common stock as of the date of grant or the value of the services provided, whichever is more readily determinable. The Company, in conjunction with adoption of ASU 2016-09- pre-vesting For stock option grants with vesting triggered by the achievement of performance-based milestones, the expense is recorded over the remaining service period after the point when the achievement of the milestone is probable or the performance condition has been achieved. For stock option grants with both performance-based milestones and market conditions, expense is recorded over the derived service period after the point when the achievement of the performance-based milestone is probable or the performance condition has been achieved. For stock option grants with market conditions, the expense is calculated using the Monte Carlo model based on the grant date fair value of the option and is recorded on a straight line basis over the requisite service period, which represents the derived service period and accelerated when the market condition is satisfied. The Company did not issue awards with market conditions during the three months ended March 31, 2019. The Company accounts for stock options and restricted stock awards to non-employees non-employees	Stock-Based Compensation The Company accounts for grants of stock options and restricted stock based on their grant date fair value and recognizes compensation expense over their vesting period. The Company estimates the fair value of stock options as of the date of grant using the Black-Scholes option pricing model and restricted stock based on the fair value of the underlying common stock as determined by management or the value of the services provided, whichever is more readily determinable. Stock-based compensation expense represents the cost of the grant date fair value of employee stock option grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis, net of estimated forfeitures. The expense is adjusted for actual forfeitures at year end. Stock-based compensation expense recognized in the consolidated financial statements is based on awards that are ultimately expected to vest. For stock option grants with performance-based milestones, the expense is recorded over the remaining service period after the point when the achievement of the milestone is probable or the performance condition has been achieved. For stock option grants with both performance-based milestones and market conditions, expense is recorded over the derived service period after the point when the achievement of the performance-based milestone is probable or the performance condition has been achieved. The Company did not issue performance-based awards in 2018 or 2017. The Company accounts for stock options and restricted stock awards to non-employees non-employees On October 1, 2018, the Compensation Committee of the Board of Directors approved a repricing (the “Repricing”) of 3,807,779 stock options (the “Options”) granted prior to September 1, 2018 pursuant to the 2013 Equity Incentive Plan and the 2012 Equity Incentive Plan to executive officers, employees and consultants of the Company. The Options had exercise prices between $0.75628 and $9.97 per share, which were reduced to $0.568 per share (the closing price of the Company’s common stock on The Nasdaq Capital Market on October 1, 2018). The number of shares, vesting schedules and expiration period of the Options were not altered. The impact to the Company’s financial statements in 2018 was immaterial.
Income Taxes		Income Taxes The Company accounts for income taxes under the liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning The Company records uncertain tax positions on the basis of a two-step
Loss per Common Share		Loss per Common Share Loss per common share is calculated using the two-class as-converted Diluted earnings per share is computed using the more dilutive of (a) the two-class if-converted
Warrant Accounting	Warrant Accounting As noted in Note 6, Capital Stock, the Company classifies a warrant to purchase shares of its common stock as a liability on its consolidated balance sheet if the warrant is a free-standing financial instrument that may require the Company to transfer consideration upon exercise. Each warrant of this type is initially recorded at fair value on date of grant using the Monte Carlo simulation model net of issuance costs, and is subsequently re-measured	Warrant Accounting As noted in Note 9, the Company classifies warrants to purchase shares of its common stock as a liability on its consolidated balance sheet if the warrant is a free-standing financial instrument that may require the Company to transfer consideration upon exercise. Each warrant of this type is initially recorded at fair value on date of grant using a Monte Carlo simulation or Black Scholes model and net of issuance costs, and is subsequently re-measured In the first quarter of 2019 the Company reduced the exercise price of all but 508,714 warrants issued in connection with the 2016 private placement and all of the warrants issued in connection with the October 2018 underwritten public offering. Refer to Note 15, Subsequent Events
Recent Accounting Pronouncements	Recent Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, In August 2018, the FASB issued ASU No. 2018-13, In August 2018, the SEC adopted the final rule under SEC Release No. 33-10532, 10-Q In June 2018, the FASB issued ASU No. 2018-07,	Recent Accounting Pronouncements In November 2018, the FASB issued ASU No. 2018-18, In August 2018, the FASB issued ASU No. 2018-13, In August 2018, the SEC adopted the final rule under SEC Release No. 33-10532, 10-Q No. 2018-07, In July 2017, the FASB issued ASU No. 2017-11, In May 2017, the FASB issued ASU No. 2017-09, In November 2016, the FASB issued ASU 2016-18, Statement 2016-18”). beginning-of-period end-of-period 2016-18 2016-18, As of December 31, 2018 2017 (in thousands) Cash and cash equivalents $ 15,542 $ 7,081 Restricted cash 137 137 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 15,679 $ 7,218 In February 2016, the FASB issued ASU No. 2016-02- Leases 2016-02 right-of-use In May 2014, the FASB issued ASU No. 2014-09,
Recently Adopted Accounting Standards - Leases	Recently Adopted Accounting Standards - Leases The Company adopted Financial Accounting Standards Board Accounting Standards Update No. 2016-02, The Company elected the following practical expedients when assessing the transition impact of the new standard from both the lessee and lessor perspective and did not: (i) reassess whether any expired or existing contracts as of January 1, 2019, are or contain leases; (ii) reassess the lease classification for any expired or existing leases as of January 1, 2019; (iii) reassess initial direct costs for any existing leases as of January 1, 2019; and (iv) reassess whether land easements meet the definition of a lease. The primary impact was the balance sheet recognition of right-of-use

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The Company’s financial instruments as of March 31, 2019 and December 31, 2018 consisted primarily of cash and cash equivalents and warrant liability. As of March 31, 2019, and December 31, 2018, the Company’s financial assets recognized at fair value consisted of the following: Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) (in thousands) March 31, 2019 Assets: Cash Equivalents Money market funds $ 4,747 $ 4,747 $ — $ — Liabilities: Warrant liability $ 10 $ — $ — $ 10 December 31, 2018 Assets: Cash Equivalents Money market funds $ 9,711 $ 9,711 $ — $ — Liabilities: Warrant liability $ 2,512 $ — $ — $ 2,512	The fair value of the warrants issued in connection with the September 2016 private placement was determined using a Monte Carlo simulation model. This model incorporated several assumptions at each valuation date including: the price of the Company’s common stock on the date of valuation, the historical volatility of the price of the Company’s common stock, the remaining contractual term of the warrant and estimates of the probability of a fundamental transaction occurring. The fair value of the warrants issued in connection with the October 2018 underwritten public offering was determined using the Black Scholes model. See Note 8, Capital Stock, for further discussion of the private placement and underwritten public offering. Description Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) (in thousands) December 31, 2018 Assets: Cash Equivalents Money market funds 9,711 9,711 — — Liabilities: Warrant liability 2,512 — — 2,512 December 31, 2017 Assets: Cash Equivalents Money market funds 5,547 5,547 — — Marketable securities: Asset-backed securities 900 — 900 — Liabilities: Warrant liability 14,679 — — 14,679
Schedule of Reconciliation of Liabilities Measured at Fair Value	The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs: March 31, 2019 (in thousands) Beginning balance, December 31, 2018 $ 2,512 Exercise of warrants (1,095 ) Change in fair value of warrant liability (1,407 ) Ending balance $ 10	The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs: As of December 31, (in thousands) Beginning balance, January 1, 2018 $ 14,679 Issuance of warrants 8,434 Change in fair value of warrant liability (20,601 ) Ending balance $ 2,512
Schedule of Depreciation and Amortization using Straight-line Method over Estimated Useful Lives of Assets		The Company provides for depreciation and amortization using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Category Estimated Useful Lives Office equipment 3 to 5 years Laboratory equipment 3 to 5 years Leasehold improvements Shorter of the remaining lease term or useful life
Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash		The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the statement of financial position that sum to the total of the same such amounts shown in the statement of cash flows. As of December 31, 2018 2017 (in thousands) Cash and cash equivalents $ 15,542 $ 7,081 Restricted cash 137 137 Total cash, cash equivalents, and restricted cash shown in the statement of cash flows $ 15,679 $ 7,218

Loss Per Common Share (Tables)

Loss Per Common Share (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Loss Per Common Share	The Company computes basic and diluted loss per share using a methodology that gives effect to the impact of outstanding participating securities (the “two-class Three Months Ended March 31, 2019 2018 (In thousands, except Numerator: Net loss $ (9,415 ) $ (14,833 ) Net loss attributable to Series A Preferred Stock (a) (21 ) (463 ) Income attributable to common stockholders—basic and diluted $ (9,394 ) $ (14,370 ) Denominator: Weighted-average number of common shares used in loss per share—basic and diluted 80,484,113 27,670,118 Loss per share - basic and diluted $ (0.12 ) $ (0.52 ) (a) The Series A Preferred Stock participates in income and losses.	Basic and diluted loss per common share are calculated as follows: For the Year Ended 2018 2017 (In thousands, except share and per share data) Numerator: Net loss $ (8,643 ) $ (26,414 ) Net Loss attributable to Series A Preferred Stock (a) (121 ) (3,915 ) Numerator for basic EPS—loss attributable to common stockholders $ (8,522 ) $ (22,499 ) Effect of dilutive securities: Deduct change in fair value of warrant liability $ (20,601 ) $ — Numerator for diluted EPS—loss attributable to common stockholders after assumed conversions $ (29,123 ) $ (22,499 ) Denominator: Weighted-average number of common shares used in loss per share—basic 36,398,450 22,669,819 Effect of dilutive securities: Nonparticipating warrants 691,747 — Denominator for diluted EPS—adjusted weighted average shares 37,090,197 22,669,819 Loss per share—basic $ (0.23 ) $ (0.99 ) Loss per share—diluted $ (0.79 ) $ (0.99 ) (a) The Series A Preferred Stock participates in income and losses
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in common stock equivalent shares): Three Months Ended March 31, 2019 2018 Unvested restricted stock and options to purchase common stock 1,803,167 3,101,143 Series A Preferred Stock unconverted 177,996 622,987 Warrants exercisable into common stock 571,025 13,633,070	The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive (in common stock equivalent shares): As of December 31, 2018 2017 Unvested restricted stock and options to purchase common stock 3,339,471 2,158,348 Series A preferred stock unconverted 177,996 2,046,957 Warrants exercisable into common stock — 13,633,070

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Deferred Costs Capitalized Prepaid And Other Assets Disclosure Abstract
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following: As of December 31, 2018 2017 (in thousands) Insurance 686 72 Other current assets 172 122 Prepaid expenses and other current assets $ 858 $ 194

Property and Equipment (Tables)

Property and Equipment (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following: March 31, December 31, (in thousands) Office equipment $ 233 $ 266 Laboratory equipment 4,536 4,561 Leasehold improvements 5,383 5,504 Software 96 96 Total property and equipment 10,248 10,427 Less: accumulated depreciation (10,113 ) (10,286 ) Property and equipment, net $ 135 $ 141	Property and equipment consisted of the following: December 31, December 31, (in thousands) Office equipment $ 266 $ 279 Laboratory equipment 4,561 4,565 Leasehold improvements 5,504 7,712 Construction in progress — 990 Software 96 96 Total property and equipment 10,427 13,642 Less: accumulated depreciation (10,286 ) (10,919 ) Property and equipment, net $ 141 $ 2,723

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consisted of the following: As of December 31, 2018 2017 (in thousands) Accrued compensation $ 514 $ 1,671 Accrued audit fees 159 133 Accrued license fees 90 70 Accrued clinical expenses 86 199 Accrued other 151 632 Total accrued expenses $ 1,000 $ 2,705

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Minimum Annual Lease Commitments Under Non-Cancellable Operating Leases		Aggregate minimum annual lease commitments of the Company under its non-cancellable For the Year Ended December 31, (in thousands) 2019 $ 1,845 2020 1,892 2021 1,941 2022 1,991 2023 1,728 Thereafter 1,460 Total minimum lease payments $ 10,857
Schedule of Supplemental Cash Flow Information Related to Leases	The following table contains supplemental cash flow information related to operating leases as of March 31, 2019. (in thousands) Cash paid within operating cash flows $ 459 Right-of-use —
Schedule of Supplemental Balance Sheet Information Related to Leases	The following table contains supplemental balance sheet information related to operating leases, as of March 31, 2019. (in thousands) Other assets, net (Right-of-use $ 6,426 Accrued lease liability, current 1,118 Accrued lease liability, long-term 6,918 Total lease liabilities $ 8,036 Weighted average remaining lease term 5.3 years Weighted average discount rate 10 %
Schedule Of Maturities Of Operating Lease Liabilities	Maturities of the Company’s operating lease liabilities as of March 31, 2019 are summarized in the table below. Year Amount (in thousands) 2019 $ 1,385 2020 1,892 2021 1,941 2022 1,991 2023 1,729 2024 and after 1,460 Total lease payments 10,398 Less: imputed interest (2,362 ) Total operating lease liabilities $ 8,036

Capital Stock (Tables)

Capital Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	The Company has reserved for future issuance the following number of shares of common stock: As of December 31, 2018 2017 Options to purchase common stock 3,339,471 2,158,348 Common stock warrants 33,145,228 13,633,070 Total 36,484,699 15,791,418

Warrants (Tables)

Warrants (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Warrants and Rights Note Disclosure [Abstract]
Schedule of Warrants to Purchase Common Stock Outstanding	The Company has warrants to purchase its common stock outstanding as of March 31, 2019, as follows: Issued Classification Warrants Exercise Expiration September 2016 Liability 508,714 $ 2.25 November 2021 March 2015 Equity 3,699 9.75 March 2025 July 2014 Equity 6,566 7.99 July 2024 July 2012 Equity 52,046 0.01 July 2022	Issued Classification Warrants Outstanding Exercise Price Expiration October 2018 Liability 19,616,250 $ 0.70 October 2028 September 2016 Liability 13,466,667 2.25 November 2022 March 2015 Equity 3,699 9.75 March 2025 July 2014 Equity 6,566 7.99 July 2024 July 2012 Equity 52,046 0.01 July 2022

Equipment Loan Payable (Tables)

Equipment Loan Payable (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Outstanding Borrowing Obligations	As of December 31, 2018 and 2017, the Company had the following outstanding borrowing obligations: December 31, December 31, (in thousands) Silicon Valley Bank Equipment Loan Payable $ — $ 178 Less: current portion — (178 ) Long-term debt, net $ — $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity Under the 2012 and 2013 Plans	Stock option activity under the Company’s 2012 Equity Incentive Plan (the “2012 Plan”) and 2013 Equity Incentive Plan (the “2013 Plan”) for the three months ended March 31, 2019 is summarized as follows: Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2018 3,339,471 $ 1.03 8.1 $ — Cancelled (1,536,304 ) 0.57 — — Outstanding at March 31, 2019 1,803,167 $ 1.03 7.2 $ — Vested and expected to vest at March 31, 2019 1,766,323 $ 1.41 7.2 $ — Exercisable at March 31, 2019 1,723,797 $ 1.37 7.1 $ —	Stock option activity under the 2012 and 2013 plans is summarized as follows: Number of Options Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2017 2,158,348 $ 4.40 8.1 $ 436 Granted 4,311,090 1.21 Exercised (919 ) 2.56 Cancelled (3,129,048 ) 3.60 Outstanding at December 31, 2018 3,339,471 $ 1.03 8.1 $ — Vested and expected to vest at December 31, 2018 3,314,542 $ 1.03 8.1 $ — Exercisable at December 31, 2018 1,603,725 $ 1.37 7.1 $ —
Additional Information about Stock Option Activity		Additional information about the Company’s stock option activity is as follows: Year Ended December 31, 2018 2017 Weighted-average grant date fair value per share of employee option grants within the year $ 0.59 $ 1.02 Cash received upon exercise of options 2 6
Summary of Stock-Based Compensation	The allocation of stock-based compensation for all options granted and restricted stock awards is as follows: Three Months Ended March 31, 2019 2018 (in thousands) Research and development $ (38 ) $ 96 General and administrative 145 307 Total stock-based compensation expense $ 107 $ 403	Stock-based compensation is as follows: Year Ended December 31, 2018 2017 (in thousands) Research and development $ 488 $ 411 General and administrative 1,137 1,162 Total stock-based compensation expense $ 1,625 $ 1,573
Summary of Weighted-Average Assumptions Used in the Black-Scholes Option Pricing Model to Determine the Fair Value of the Employee Stock Option Grants	The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants and non-employee Three Months Ended March 31, 2018 Employees Non-employees Risk-free interest rate 2.70 % 1.97 % Expected volatility 88.2 % 74.0 % Expected term (in years) 6.08 6.08 Expected dividend yield 0.0 % 0.0 %	The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Year Ended December 31, 2018 2017 Risk-free interest rate 2.84 % 2.03 % Expected volatility 84.4 % 62.9 % Expected term (in years) 5.31 6.08 Expected dividend yield 0.0 % 0.0 %
Summary of Weighted-Average Assumptions Used in the Black-Scholes Option Pricing Model to Determine the Fair Value of the Non-Employee Stock Option Grants		The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the non-employee Year Ended December 31, 2018 2017 Risk-free interest rate 2.57 % 1.29 % Expected volatility 88.3 % 62.3 % Expected term (in years) 6.08 6.08 Expected dividend yield 0.0 % 0.0 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Components of Loss Before Income Taxes	The components of loss before income taxes were as follows: As of December 31, 2018 2017 (in thousands) U.S. $ (8,503 ) $ (26,275 ) Foreign (140 ) (140 ) Total $ (8,643 ) $ (26,415 )
Schedule of Reconciliation of Income Tax Expense (Benefit) Computed at Statutory Federal Income Tax Rate	A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: As of December 31, 2018 2017 Federal income tax (benefit) at statutory rate 21.1 % 33.7 % (Increase) decrease income tax benefit resulting from: State income tax benefit, net of federal benefit 22.5 % 0.0 % Permanent differences 54.4 % (2.6 )% Net Operating Loss Limitation 0.0 % 0.0 % Federal Tax Rate change 0 % (47.2 )% R&D Credit Limitation 0.9 % 0.0 % Change in valuation allowance (98.5 )% 14.6 % Other -0.4 % 1.5 % Income tax expense (benefit) 0.0 % 0.0 %
Components of Deferred Tax Assets and Liabilities	The significant components of the Company’s deferred tax assets and liabilities are comprised of the following: As of December 31, 2018 2017 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 24,941 $ 18,540 Depreciation and amortization 4,878 4,121 Accrued expenses 141 1,484 Capitalized start-up 9,143 7,942 R&D credits 312 243 Other 1,155 808 Deferred tax assets before valuation allowance 40,570 33,138 Valuation allowance (40,570 ) (33,138 ) — — Deferred tax liabilities IPR&D — — Change in accounting method — — Net deferred tax assets $ — $ —
Summary of Changes In Unrecognized Tax Benefits	The changes in the Company’s unrecognized tax benefits are summarized as follows: As of December 31, 2018 2017 (in thousands) Unrecognized tax benefit, beginning of year $ 303 $ 562 Increase (decrease) related to current year positions — (123 ) Federal rate revision — (136 ) Unrecognized tax benefit, end of year $ 303 $ 303

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($)	Sep. 29, 2016	May 13, 2011	Oct. 31, 2018	Jan. 31, 2018	Sep. 30, 2016	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Nature Of Business And Basis Of Presentation [Line Items]
Net proceeds of offering after deducting underwriting discounts and commissions							$ 5,732,000	$ 25,526,000
Warrant exercise price								$ 2.25
Accumulated deficit						$ (226,245,000)		$ (216,830,000)	$ (208,187,000)
Percentage of Ownership before transaction						10.00%
Termination fees						$ 700,000
Liabilities obligation, termination fee						$ 300,000
Transfer of shares restriction period						180 days
Voting Agreement [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Percentage of Voting Interests Acquired						68.00%
ProChon [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Business acquisition date								May 13, 2011
Consideration paid to acquisition		$ 2,200,000
Ocugen [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Percentage of Ownership before transaction						90.00%
Termination fees						$ 600,000
Maximum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Percentage of Ownership before transaction						5.00%
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Shares issued				2,691,494			2,691,494	35,480,397
Common stock price per share				$ 2.35
Option to purchase additional shares				351,064
Net proceeds of offering after deducting underwriting discounts and commissions				$ 5,700,000
Common Stock [Member] | Previously Reported [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Net proceeds of offering after deducting underwriting discounts and commissions				$ 5,900,000
Private Placement [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Proceeds from issuance of common stock, preferred stock and warrants, net of issuance costs	$ 27,600,000				$ 27,600,000
Common stock price per share					$ 2.25
Class of Warrants or right to purchase common stock					13,333,334
Warrant exercise price					$ 2.25
Private Placement [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Shares issued					2,596,059
At-The-Market Sales Agreement [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Shares issued				6,633,903
Net proceeds of offering after deducting underwriting discounts and commissions				$ 4,500,000
At-The-Market Sales Agreement [Member] | Common Stock [Member] | Maximum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Aggregate offering price of shares				$ 10,000,000
Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Class of Warrants or right to purchase common stock			19,616,250
Gross proceeds from offering, before deducting underwriting discounts and commissions and offering expenses			$ 17,000,000
Warrant exercise price			$ 0.70
Warrants exercisable term			5 years
Underwritten Public Offering [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Shares issued			26,155,000
Underwritten Public Offering [Member] | Common Stock And Warrants [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock price per share			$ 0.65

Summary of Significant Accoun_3

Summary of Significant Accounting Policy - Additional Information (Detail) $ / shares in Units, ¥ in Millions	Oct. 01, 2018$ / sharesshares	Mar. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)	Jan. 31, 2018USD ($)	Jan. 31, 2018CNY (¥)	Mar. 31, 2019USD ($)shares	Mar. 31, 2019CNY (¥)	Dec. 31, 2018USD ($)Segment$ / sharesshares	Dec. 31, 2017USD ($)shares	Mar. 31, 2019CNY (¥)shares	Jan. 01, 2019USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment								2
Number of reporting segment | Segment								1
Transfers between Levels 1, 2 and 3			$ 0					$ 0	$ 0
Available for sale securities			$ 0					0					$ 0
Marketable securities other-than-temporary impairment									0
Aggregate fair value of available for sale securities held for less than twelve months									900,000
Sale of available for sale securities								$ 0
Percentage of reduction in stock price			73.00%					73.00%
Asset Impairment Charges						$ 750,000		$ 4,270,000
Remaining performance obligations			$ 10,000,000					10,000,000
Accrued expenses		$ 1,125,000	1,125,000			1,125,000		1,125,000	$ 3,040,000
Gain on extinguishment of liability								$ 1,540,000
Stock options granted | shares								4,311,090
Stock options reduced exercise price | $ / shares	$ 0.568
Stock options exercise price, lower range | $ / shares								$ 0.75628
Stock options exercise price, upper range | $ / shares								$ 9.97
Maximum percentage of recognition of tax benefits from uncertain tax positions								50.00%
Right-of-use assets		6,426,000				6,426,000
Lease liability		8,036,000				8,036,000
Restricted cash		137,000	100,000			137,000		$ 100,000				$ 137,000
Deferred license agreement cost		$ 800,000				800,000
Cost incurred related to license agreement						900,000
Deductions for aggregate write-downs						$ 0
Scenario, Forecast [Member] | 2016 Private Placement [Member]
Summary Of Significant Accounting Policies [Line Items]
Warrants issued | shares		508,714				508,714				508,714
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Weighted average of maturity portfolio									1 month
Maximum [Member] | ASU 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets											$ 10,000,000
Lease liability											10,000,000
Minimum [Member] | ASU 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets											8,000,000
Lease liability											$ 8,000,000
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Summary Of Significant Accounting Policies [Line Items]
Assets, fair value disclosure, recurring			0					0	$ 0
Liabilities, fair value disclosure, recurring			0					0	$ 0
Quoted Prices in Active Markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Summary Of Significant Accounting Policies [Line Items]
Assets, fair value disclosure, recurring			0					0
Liabilities, fair value disclosure, recurring			0					0
Intrexon [Member]
Summary Of Significant Accounting Policies [Line Items]
Aggregate agreement terimation amount			1,500,000
Payment of termination amount			375,000
Termination amount payable subject to certain condition			1,125,000
Accrued expenses			1,125,000					$ 1,125,000
Gain on extinguishment of liability			1,500,000
Performance-based Awards [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of shares issued | shares								0	0
Stock Option Repricing [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock options reduced exercise price | $ / shares	$ 0.568
Stock Option Repricing [Member] | 2013 Equity Incentive Plan and 2012 Equity Incentive Plan [Member]
Summary Of Significant Accounting Policies [Line Items]
Stock options granted | shares	3,807,779
MEDINET Co Ltd [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-refundable upfront cash payment under license agreement				$ 10,000,000
Royalties at percentage rates ranging								Low single digits to low double digits
Development milestone payments					¥ 330			$ 3,000,000
Regulatory payments		$ 1,000,000		1,000,000	720	$ 7,400,000	¥ 720	6,500,000
Commercial Milestones Payment					¥ 7,100	64,100,000	¥ 7,300	64,300,000
Non-refundable upfront amount				10,000,000
Upfront transaction price				$ 10,000,000
Upfront transaction price recognized period				2 years	2 years
Upfront transaction price noncurrent				$ 10,000,000
Remaining performance obligations		10,000,000	$ 0			10,000,000		$ 0	$ 0
Deferred revenue						10,000,000
Development milestone payments		$ 2,900,000				$ 2,900,000				¥ 330

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets:
Money market funds, fair value	$ 4,747,000	$ 9,711,000	$ 5,547,000
Marketable securities, fair value		0		$ 0
Liabilities:
Warrant liability, fair value	10,000	2,512,000	14,679,000
Asset Backed Securities [Member]
Assets:
Marketable securities, fair value			900,000
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Money market funds, fair value	4,747,000	9,711,000	5,547,000
Significant Other Observable Inputs (Level 2) [Member] | Asset Backed Securities [Member]
Assets:
Marketable securities, fair value			900,000
Significant Unobservable Inputs (Level 3) [Member]
Liabilities:
Warrant liability, fair value	$ 10,000	$ 2,512,000	$ 14,679,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in fair value of warrant liability	$ (1,407)	$ (8,753)	$ 20,601	$ (1,482)
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	2,512	$ 14,679	14,679
Exercise of warrants	(1,095)
Issuance of warrants			8,434
Change in fair value of warrant liability	(1,407)		(20,601)
Ending balance	$ 10		$ 2,512	$ 14,679

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Depreciation and Amortization using Straight-line Method over Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2018
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	Shorter of the remaining lease term or useful life
Minimum [Member] | Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Minimum [Member] | Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	3 years
Maximum [Member] | Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years
Maximum [Member] | Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives	5 years

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents	$ 7,376	$ 15,542	$ 15,507	$ 7,081
Restricted cash	137	100	137
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 7,513		$ 15,644
ASU 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash and cash equivalents		15,542		7,081
Restricted cash		137		137
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows		$ 15,679		$ 7,218

Loss Per Common Share - Schedul

Loss Per Common Share - Schedule of Basic and Diluted Loss Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss		$ (9,415)	$ (14,833)	$ (8,643)	$ (26,414)
Net Loss attributable to Series A Preferred Stock	[1]	(21)	(463)	(121)	(3,915)
Numerator for basic EPS - loss attributable to common stockholders				(8,522)	(22,499)
Income attributable to common stockholders - basic and diluted		$ (9,394)	$ (14,370)
Effect of dilutive securities:
Deduct change in fair value of warrant liability				(20,601)
Numerator for diluted EPS - loss attributable to common stockholders after assumed conversions				$ (29,123)	$ (22,499)
Denominator:
Weighted-average number of common shares used in loss per share-basic				36,398,450	22,669,819
Weighted-average number of common shares used in loss per share - basic and diluted		80,484,113	27,670,118
Loss per share - basic and diluted		$ (0.12)	$ (0.52)
Effect of dilutive securities:
Nonparticipating warrants				691,747
Denominator for diluted EPS - adjusted weighted average shares				37,090,197	22,669,819
Loss per share-basic				$ (0.23)	$ (0.99)
Loss per share-diluted				$ (0.79)	$ (0.99)
[1]	The Series A Preferred Stock participates in income and losses.

Loss Per Common Share - Sched_2

Loss Per Common Share - Schedule of Antidilutive Securities Excluded from computation of Earnings Per Share (Detail) - shares	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Series A Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	177,996	622,987	177,996	2,046,957
Unvested Restricted Stock and Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	1,803,167	3,101,143	3,339,471	2,158,348
Warrants Exercisable into Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	571,025	13,633,070		13,633,070

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense and Other Assets, Current [Abstract]
Insurance		$ 686	$ 72
Other current assets		172	122
Prepaid expenses and other current assets	$ 764	$ 858	$ 194

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 10,248	$ 10,427	$ 13,642
Less: accumulated depreciation	(10,113)	(10,286)	(10,919)
Property and equipment, net	135	141	2,723
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	233	266	279
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	4,536	4,561	4,565
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	5,383	5,504	7,712
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment			990
Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 96	$ 96	$ 96

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 6,000	$ 172,000	$ 450,000	$ 1,534,000
Impairment charge	750,000		$ 4,270,000
Maximum [Member]
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 100,000	$ 200,000

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued compensation		$ 514	$ 1,671
Accrued audit fees		159	133
Accrued license fees		90	70
Accrued clinical expenses		86	199
Accrued other		151	632
Total accrued expenses	$ 99	$ 1,000	$ 2,705

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended		12 Months Ended		23 Months Ended
	Feb. 28, 2017USD ($)	Sep. 30, 2015	Jun. 30, 2012USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2018USD ($)Option	Dec. 31, 2017USD ($)	Dec. 31, 2018USD ($)
Loss Contingencies [Line Items]
Rent expense under operating lease agreements				$ 400,000	$ 400,000	$ 1,600,000	$ 1,000,000
Research and development expense				1,583,000	$ 3,286,000	$ 15,634,000	$ 15,566,000
Operating Lease expense				400,000
Variable executory cost				300,000
Collagen Supply Agreement [Member]
Loss Contingencies [Line Items]
Supplier agreement description						The initial term of the agreement is three years and will automatically renew from year to year thereafter unless otherwise terminated with at least 180 days' notice by either party.
Initial term of the agreement		3 years
Minimum amount of material and/or services	$ 100,000
Amount paid during period for amending the agreement								$ 100,000
Amount agreed for amending minimum annual order	$ 100,000
Tissue Processor Sub License Agreement [Member]
Loss Contingencies [Line Items]
Research and development expense				700,000		$ 1,000,000
Reimbursement for development cost			$ 300,000
Additional non-refundable royalty fee				$ 30,000		$ 30,000
Additional non-refundable royalty fee payment description				2016 through 2019		2016 through 2019
Potential milestone payment				$ 10,200,000		$ 10,200,000		10,200,000
Minimum [Member] | Tissue Processor Sub License Agreement [Member]
Loss Contingencies [Line Items]
Additional non-refundable royalty fee				20,000		20,000
Hydrogel License Agreement [Member]
Loss Contingencies [Line Items]
Research and development expense				3,200,000		3,200,000
Amount to be paid upon FDA approval				3,000,000		3,000,000		3,000,000
Tissue Regeneration License Agreement [Member]
Loss Contingencies [Line Items]
Research and development expense				800,000		800,000
Amount to be paid upon FDA approval				$ 300,000		$ 300,000		$ 300,000
Percentage of royalty offsetting				50.00%		50.00%
Tissue Regeneration License Agreement [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Non-refundable royalty fee				$ 10,000		$ 10,000
Waltham [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Construction allowances to total cost of tenant improvements						900,000
Lexington [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Construction allowances to total cost of tenant improvements						$ 1,000,000
Massachusetts [Member] | Waltham [Member]
Loss Contingencies [Line Items]
Number of additional renewal terms | Option						1
Additional operating lease term						5 years		5 years
Effective date of amended lease						Jan. 31, 2018
Lease termination date						Dec. 31, 2024
Massachusetts [Member] | Lexington [Member]
Loss Contingencies [Line Items]
Lease expiration period						2023
Number of additional renewal terms | Option						1
Additional operating lease term						5 years		5 years

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Annual Lease Commitments Under Non-Cancellable Operating Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	$ 1,845
2020	1,892
2021	1,941
2022	1,991
2023	1,728
Thereafter	1,460
Total minimum lease payments	$ 10,857

Capital Stock - Additional Info

Capital Stock - Additional Information (Detail) - USD ($)	Sep. 29, 2016	Oct. 31, 2018	Jan. 31, 2018	Sep. 30, 2016	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Class of Warrants or right to purchase common stock, exercise price							$ 2.25
Minimum percentage of outstanding common stock					50.00%		50.00%
Fair value of warrants					$ 8,400,000		$ 8,400,000
Proceeds from issuance of common stock						$ 5,732,000	$ 25,526,000
Convertible preferred stock, shares outstanding					400		400
Preferred stock par value					$ 0.01		$ 0.01
Common stock, shares authorized					100,000,000		100,000,000	100,000,000
Preferred stock, shares authorized					30,000		30,000
Minimum threshold percentage of beneficial common stock interest	4.99%
Maximum threshold percentage of beneficial common stock interest on holder's discretion	9.99%
Convertible preferred stock, terms of conversion							The Series A Preferred Stock shall not be converted if, after giving effect to the conversion, the holder and its affiliated persons would own beneficially more than 4.99% of our common stock (subject to adjustment up to 9.99% solely at the holder's discretion upon 61 days' prior notice to us or, solely as to a holder, if such limitation is waived by such holder upon execution of the private placement agreement).
Proceeds from Warrant Exercises					$ 326,000
Two Thousand Eighteen Warrants [Member]
Class of Stock [Line Items]
Class of Warrants or right to purchase common stock, exercise price					$ 0.70
Warrant reduced exercise price					$ 0.01
Proceeds from Warrant Exercises					$ 200,000
Two Thousand Sixteen Warrants [Member]
Class of Stock [Line Items]
Class of Warrants or right to purchase common stock					508,714
Class of Warrants or right to purchase common stock, exercise price					$ 2.25
Warrant reduced exercise price					$ 0.01
Proceeds from Warrant Exercises					$ 100,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Number of shares issued	24,158.8693
Conversion of preferred stock in to common stock, preferred stock converted	10,737,275						444.44
Convertible preferred stock, shares outstanding							400.4910
Preferred stock par value							$ 0.01
Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued			2,691,494			2,691,494	35,480,397
Common stock price per share			$ 2.35
Proceeds from issuance of common stock			$ 5,700,000
Common Stock [Member] | Series A Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of preferred stock in to common stock, preferred stock converted							177,996
Common Stock [Member] | Previously Reported [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock			$ 5,900,000
Underwritten Public Offering [Member]
Class of Stock [Line Items]
Class of Warrants or right to purchase common stock		19,616,250
Gross proceeds from offering, before deducting underwriting discounts and commissions and offering expenses		$ 17,000,000
Class of Warrants or right to purchase common stock, exercise price		$ 0.70
Warrants exercisable term		5 years
Underwritten Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued		26,155,000
Underwritten Public Offering [Member] | Common Stock And Warrants [Member]
Class of Stock [Line Items]
Common stock price per share		$ 0.65
The Equity Distribution Agreement [Member] | Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued					0	0	6,633,903
Proceeds from issuance of common stock							$ 4,500,000
Underwriter Option [Member]
Class of Stock [Line Items]
Number of shares issued			351,064
Private Placement [Member]
Class of Stock [Line Items]
Class of Warrants or right to purchase common stock				13,333,334
Common stock price per share				$ 2.25
Class of Warrants or right to purchase common stock, exercise price				$ 2.25
Minimum percentage of outstanding common stock					50.00%		50.00%
Fair value of warrants					$ 30,700,000		$ 30,700,000
Gross proceeds from issuance of common stock, preferred stock and warrants				$ 30,000,000
Proceeds from issuance of common stock, preferred stock and warrants, net of issuance costs	$ 27,600,000			$ 27,600,000
Conversion of preferred stock in to common stock, preferred stock converted				10,737,275	444.44		444.44
Warrants granted to placement agent				133,333
Warrants granted to placement agent,exercise price				$ 2.25
Warrants expiry period					5 years		5 years
Warrants exercisable period					6 months		6 months
Preferred stock par value					$ 0.01		$ 0.01
Private Placement [Member] | Members of Board of Directors [Member]
Class of Stock [Line Items]
Class of Warrants or right to purchase common stock					1,422,221		1,422,221
Class of Warrants or right to purchase common stock, exercise price					$ 2.25		$ 2.25
Private Placement [Member] | Series A Preferred Stock [Member]
Class of Stock [Line Items]
Number of shares issued				24,158.8693
Convertible preferred stock, shares outstanding					400.4910		400.4910
Private Placement [Member] | Series A Preferred Stock [Member] | Members of Board of Directors [Member]
Class of Stock [Line Items]
Number of shares issued					2,563.1439		2,563.1439
Private Placement [Member] | Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued				2,596,059
Private Placement [Member] | Common Stock [Member] | Members of Board of Directors [Member]
Class of Stock [Line Items]
Number of shares issued					283,046		283,046
Private Placement [Member] | Common Stock [Member] | Series A Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of preferred stock in to common stock, preferred stock converted					177,996		177,996
Maximum [Member] | The Equity Distribution Agreement [Member] | Common Stock [Member]
Class of Stock [Line Items]
Aggregate offering price of shares						$ 10,000,000	$ 10,000,000

Capital Stock - Schedule of Com

Capital Stock - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock reserved for future issuance	36,484,699	15,791,418
Warrants Exercisable into Common Stock [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance	33,145,228	13,633,070
Stock Options [Member]
Class of Stock [Line Items]
Common stock reserved for future issuance	3,339,471	2,158,348

Warrants - Schedule of Warrants

Warrants - Schedule of Warrants to Purchase Common Stock Outstanding (Detail) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Exercise Price		$ 2.25
Warrants Issued October 2018 [Member]
Class of Warrant or Right [Line Items]
Issued		2018-10
Warrants Outstanding		19,616,250
Exercise Price		$ 0.70
Expiration		2028-10
Warrants Issued September 2016 [Member]
Class of Warrant or Right [Line Items]
Issued	2016-09	2016-09
Warrants Outstanding	508,714	13,466,667
Exercise Price	$ 2.25	$ 2.25
Expiration	2021-11	2022-11
Warrants Issued March 2015 [Member]
Class of Warrant or Right [Line Items]
Issued	2015-03	2015-03
Warrants Outstanding	3,699	3,699
Exercise Price	$ 9.75	$ 9.75
Expiration	2025-03	2025-03
Warrants Issued July 2014 [Member]
Class of Warrant or Right [Line Items]
Issued	2014-07	2014-07
Warrants Outstanding	6,566	6,566
Exercise Price	$ 7.99	$ 7.99
Expiration	2024-07	2024-07
Warrants Issued July 2012 [Member]
Class of Warrant or Right [Line Items]
Issued	2012-07	2012-07
Warrants Outstanding	52,046	52,046
Exercise Price	$ 0.01	$ 0.01
Expiration	2022-07	2022-07

Warrants - Additional Informati

Warrants - Additional Information (Detail)	Mar. 31, 2019shares
Scenario, Forecast [Member] | 2016 Private Placement [Member]
Class of Warrant or Right [Line Items]
Class of Warrants or right to purchase common stock	508,714

Equipment Loan Payable - Schedu

Equipment Loan Payable - Schedule of Outstanding Borrowing Obligations (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Less: current portion	$ (178)
Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Total debt	$ 178

Equipment Loan Payable - Additi

Equipment Loan Payable - Additional Information (Detail) - USD ($)	1 Months Ended
	Jul. 31, 2014	Dec. 31, 2018
Class of Warrant or Right [Line Items]
Exercise Price		$ 2.25
Silicon Valley Bank [Member]
Class of Warrant or Right [Line Items]
Amount of loan to purchase equipment	$ 1,800,000
Line of credit facility, interest rate, stated percentage	3.25%
Bank loan and security agreement, repayment period	36 months
Warrants Outstanding	6,566
Exercise Price	$ 7.99
Silicon Valley Bank [Member] | Prime Rate [Member]
Class of Warrant or Right [Line Items]
Amount of loan bears interest basis spread on variable rate	2.75%

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands

Oct. 01, 2018

Jan. 01, 2018

Jan. 01, 2017

Jun. 16, 2016

Dec. 03, 2014

Oct. 31, 2014

Jul. 31, 2012

Mar. 31, 2019

Mar. 31, 2018

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2014

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Unrecognized compensation cost related to outstanding options

$ 2,300

$ 1,400

Intrinsic value of options exercised

$ 0

8

Weighted average grant date fair value of vested options

$ 1.82

$ 0.76

Weighted average grant date fair value of options outstanding

$ 1.82

$ 0.52

Stock-based compensation expense

$ 107

$ 403

$ 1,625

$ 1,573

Stock options granted

4,311,090

Stock options reduced exercise price

$ 0.568

Stock options exercise price, lower range

$ 0.75628

Stock options exercise price, upper range

$ 9.97

Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Unrecognized compensation cost, recognition period

2 years 6 months

1 year 8 months 12 days

Restricted Stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Unrecognized compensation cost related to restricted stock awards

$ 1

Stock-based compensation expense

$ 1

Stock Option Repricing [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options reduced exercise price

$ 0.568

2012 Equity Incentive Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of shares authorized for issuance

609,389

Stock options vesting period

3 years

Term of stock options granted

10 years

2012 Equity Incentive Plan [Member] | First Anniversary of Original Vesting Date [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options vesting percentage

25.00%

2013 Equity Incentive Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options vesting period

3 years

Term of stock options granted

10 years

2013 Equity Incentive Plan [Member] | EIP Amendment [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Increase in authorized shares of common stock amount

982,841

825,904

300,000

Increase in authorized shares, percentage of common stock outstanding

4.00%

2013 Equity Incentive Plan [Member] | First Anniversary of Original Vesting Date [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options vesting percentage

25.00%

2013 Employee Stock Purchase Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of shares authorized for issuance

103,665

Increase in authorized shares of common stock amount

51,832

206,476

51,832

Increase in authorized shares, percentage of common stock outstanding

1.00%

2013 Equity Incentive Plan and 2012 Equity Incentive Plan [Member] | Stock Option Repricing [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options granted

3,807,779

Employees [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock options granted

0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity Under the 2012 and 2013 Plans (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Options, Outstanding Beginning Balance	3,339,471	2,158,348
Number of Options, Granted		4,311,090
Number of Options, Exercised		(919)
Number of Options, Cancelled	(1,536,304)	(3,129,048)
Number of Options, Outstanding Ending Balance	1,803,167	3,339,471	2,158,348
Number of Options, Vested and expected to vest outstanding	1,766,323	3,314,542
Number of Options, Exercisable	1,723,797	1,603,725
Weighted Average Exercise Price, Outstanding Beginning Balance	$ 1.03	$ 4.40
Weighted Average Exercise Price, Granted		1.21
Weighted Average Exercise Price, Exercised		2.56
Weighted Average Exercise Price, Cancelled	0.57	3.60
Weighted Average Exercise Price, Outstanding Ending Balance	1.03	1.03	$ 4.40
Weighted Average Exercise Price, Vested and expected to vest outstanding	1.41	1.03
Weighted Average Exercise Price, Exercisable	$ 1.37	$ 1.37
Weighted Average Remaining Contractual Term, Outstanding	7 years 2 months 12 days	8 years 1 month 6 days	8 years 1 month 6 days
Weighted Average Remaining Contractual Term, Vested and expected to vest outstanding	7 years 2 months 12 days	8 years 1 month 6 days
Weighted Average Remaining Contractual Term, Exercisable	7 years 1 month 6 days	7 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding	$ 0	$ 0	$ 436
Aggregate Intrinsic Value ,Vested and expected to vest outstanding	0	0
Aggregate Intrinsic Value, Exercisable	$ 0	$ 0

Stock-Based Compensation - Ad_2

Stock-Based Compensation - Additional Information about Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Weighted-average grant date fair value per share of employee option grants within the year		$ 0.59	$ 1.02
Cash received upon exercise of options	$ 2	$ 2	$ 6

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-Based Compensation (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 107	$ 403	$ 1,625	$ 1,573
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	(38)	96	488	411
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Share-based compensation expense	$ 145	$ 307	$ 1,137	$ 1,162

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-Based Compensation - Summary of Weighted-Average Assumptions Used in the Black-Scholes Option Pricing Model to Determine the Fair Value of the Employee Stock Option Grants (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Risk-free interest rate	2.70%	2.84%	2.03%
Expected volatility	88.20%	84.40%	62.90%
Expected term (in years)	6 years 29 days	5 years 3 months 21 days	6 years 29 days
Expected dividend yield	0.00%	0.00%	0.00%
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Risk-free interest rate	1.97%	2.57%	1.29%
Expected volatility	74.00%	88.30%	62.30%
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days
Expected dividend yield	0.00%	0.00%	0.00%

Income Taxes - Components of Lo

Income Taxes - Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S.	$ (8,503)	$ (26,275)
Foreign	(140)	(140)
Total	$ (8,643)	$ (26,415)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax Expense (Benefit) Computed at Statutory Federal Income Tax Rate (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax (benefit) at statutory rate	21.10%	21.10%	33.70%
(Increase) decrease income tax benefit resulting from:
State income tax benefit, net of federal benefit		22.50%	0.00%
Permanent differences		54.40%	(2.60%)
Net Operating Loss Limitation		0.00%	0.00%
Federal Tax Rate change		0.00%	(47.20%)
R&D Credit Limitation		0.90%	0.00%
Change in valuation allowance		(98.50%)	14.60%
Other		(0.40%)	1.50%
Income tax expense (benefit)		0.00%	0.00%

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 24,941	$ 18,540
Depreciation and amortization	4,878	4,121
Accrued expenses	141	1,484
Capitalized start-up costs	9,143	7,942
R&D credits	312	243
Other	1,155	808
Deferred tax assets before valuation allowance	40,570	33,138
Valuation allowance	$ (40,570)	$ (33,138)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes Disclosure [Line Items]
Decrease in valuation allowance			$ (7,400,000)	$ (1,000,000)
Impact of a federal tax rate change				$ 11,500,000
NOL carry forwards, net of limitations			$ 52,900,000
Federal corporate income tax rate	21.10%		21.10%	33.70%
Tax cuts and jobs act of 2017, deduction for net operating losses percentage of taxable income			80.00%
Income tax expense or benefit			$ 0
Unrecognized tax benefits			303,000	$ 303,000	$ 562,000
Accrued interest and penalties related to unrecognized tax benefits			0	0
Interest and penalties related to uncertain tax benefits			$ 0	0
Unrecognized tax benefits may be recognized by the end of 2018				0
Income tax examination description			The Company is subject to U.S. federal, state and local income tax examinations by tax authorities for years 2001 through present. Carryforward attributes that were generated in earlier periods remain subject to examination to the extent the year in which they were used or will be used remains open for examination. The tax years which remain subject to examination by tax authorities in Israel, as of December 31, 2018, include years 2014 through the present.
Income tax expense or benefit	$ 0	$ 0
U.S. Federal [Member]
Income Taxes Disclosure [Line Items]
NOL carryforwards			$ 67,000,000	43,900,000
NOL carryforwards, expiration date			Dec. 31, 2037
U.S. State [Member]
Income Taxes Disclosure [Line Items]
NOL carryforwards			$ 66,900,000	43,600,000
NOL carryforwards, expiration date			Dec. 31, 2037
Foreign [Member]
Income Taxes Disclosure [Line Items]
NOL carryforwards			$ 26,200,000	$ 26,100,000

Income Taxes - Summary of Chang

Income Taxes - Summary of Changes In Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Unrecognized tax benefit, beginning of year	$ 303	$ 562
Increase (decrease) related to current year positions	0	(123)
Federal rate revision	0	(136)
Unrecognized tax benefit, end of year	$ 303	$ 303

Employee Benefits - Additional

Employee Benefits - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018USD ($)
Defined Contribution Plan Disclosure [Line Items]
Defined benefit plan contribution by employer	$ 0
Minimum [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan eligibility years of age for employees	21 years

Related Parties - Additional In

Related Parties - Additional Information (Detail) - Purpose, Co. [Member] - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2012
Reimbursement for development cost, amount					$ 0.3
Reimbursement for development cost, paid	$ 0.1	$ 0.1	$ 0.1	$ 0.1

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail)	May 08, 2019USD ($)	Apr. 08, 2019USD ($)	Mar. 14, 2019	Jan. 23, 2019	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)$ / sharesshares	Apr. 30, 2019USD ($)	Mar. 22, 2019Employee	Dec. 31, 2018$ / shares
Subsequent Event [Line Items]
One-time charge for severance and related expenses						$ 2,789,000
Warrant exercise price | $ / shares									$ 2.25
Proceeds from Warrant Exercises						326,000
Termination fees						$ 700,000
Warrants Issued September 2016 [Member]
Subsequent Event [Line Items]
Warrant exercise price | $ / shares						$ 2.25			2.25
Warrants Issued September 2016 [Member] | 2016 Exercise Agreement [Member]
Subsequent Event [Line Items]
Warrant exercise price | $ / shares									2.25
Warrants Issued October 2018 [Member]
Subsequent Event [Line Items]
Warrant exercise price | $ / shares									0.70
Warrants Issued October 2018 [Member] | 2018 Exercise Agreement [Member]
Subsequent Event [Line Items]
Warrant exercise price | $ / shares									$ 0.70
Scenario, Forecast [Member]
Subsequent Event [Line Items]
One-time charge for severance and related expenses						$ 1,400,000
Maximum number of additional consultants employees to assist with continuing evaluation of strategic alternative | Employee								4
Loss on settlement of lease obligation					$ (500,000)
Scenario, Forecast [Member] | Warrants Issued September 2016 [Member] | 2016 Exercise Agreement [Member]
Subsequent Event [Line Items]
Warrant exercise price | $ / shares						$ 0.01
Proceeds from Warrant Exercises						$ 100,000
Class of Warrants or right to purchase common stock | shares						508,714
Scenario, Forecast [Member] | Warrants Issued October 2018 [Member] | 2018 Exercise Agreement [Member]
Subsequent Event [Line Items]
Warrant exercise price | $ / shares						$ 0.01
Proceeds from Warrant Exercises						$ 200,000
Scenario, Forecast [Member] | Additional Second Restructuring [Member]
Subsequent Event [Line Items]
One-time charge for severance and related expenses						2,200,000
Ocugen [Member]
Subsequent Event [Line Items]
Termination fees						$ 600,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Percentage of workforce positions eliminated				65.00%
Effective date of restructuring			Mar. 22, 2019
Payment on lease termination	$ 300,000						$ 1,300,000
Security deposit	100,000
Subsequent Event [Member] | Medavate Corp [Member]
Subsequent Event [Line Items]
Cash paid to purchase assets	$ 6,500,000
Subsequent Event [Member] | Ocugen [Member] | Before Merger [Member]
Subsequent Event [Line Items]
Percentage of ownership prior to merger		90.00%
Subsequent Event [Member] | Ocugen [Member] | Before Merger [Member] | Maximum [Member]
Subsequent Event [Line Items]
Termination fees		$ 600,000
Subsequent Event [Member] | Ocugen [Member] | After Merger [Member]
Subsequent Event [Line Items]
Stock percentage of ownership prior merger		10.00%
Subsequent Event [Member] | Ocugen [Member] | After Merger [Member] | Maximum [Member]
Subsequent Event [Line Items]
Termination fees		$ 700,000

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Supplemental Cash Flow Information Related to Leases (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
Cash paid within operating cash flows	$ 459
Right-of-use assets recognized in exchange for new lease obligations	$ 0

Commitment and Contingencies -

Commitment and Contingencies - Schedule Of Supplemental Balance Sheet Information Related To Leases (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
Other assets, net (Right-of-use assets)	$ 6,426
Accrued lease liability, current	1,118
Accrued lease liability, long-term	6,918
Total operating lease liabilities	$ 8,036
Weighted average remaining lease term	5 years 3 months 18 days
Weighted average discount rate	10.00%

Commitment and Contingencies _2

Commitment and Contingencies - Schedule of Operating Lease Liabilities (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 1,385
2020	1,892
2021	1,941
2022	1,991
2023	1,729
2024 and after	1,460
Total lease payments	10,398
Total lease payments	10,398
Less: imputed interest	(2,362)
Total operating lease liabilities	$ 8,036

Restructuring - Additional Info

Restructuring - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Restructuring and Related Activities [Abstract]
Restructuring Charges	$ 2,789
Restructuring Reserve	$ 100