Sienna Biopharmaceuticals (SNNAQ) 10-Q 8 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 05, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	SNNA
Entity Registrant Name	Sienna Biopharmaceuticals, Inc.
Entity Central Index Key	1,656,328
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Smaller Reporting Company	true
Emerging Growth Company	true
Entity Extended Transition Period	true
Entity Common Stock, Shares Outstanding		21,140,101

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 63,975	$ 74,467
Restricted cash	181	181
Prepaid expenses and other current assets	2,903	2,698
Total current assets	67,059	77,346
Property and equipment, net	345	432
In-process research and development	46,112	47,597
Goodwill	11,114	11,472
Total assets	124,630	136,847
Current liabilities:
Accounts payable	3,479	2,357
Accrued personnel costs	3,025	2,646
Other accrued expenses	7,092	3,007
Early exercise liability, current portion	213	231
Total current liabilities	13,809	8,241
Early exercise liability-net of current portion	98	258
Contingent consideration	24,500	22,900
Success payment liability	688	3,285
Long-term debt, net	29,983
Deferred tax liability	10,622	10,964
Total liabilities	79,700	45,648
Commitments and contingencies (Note 11)
Stockholders' equity:
Preferred stock, $0.0001 par value, 10,000 shares authorized, no shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively
Common stock, $0.0001 par value, 300,000 shares authorized, 21,140 and 20,740 shares issued and 20,756 and 20,194 shares outstanding at September 30, 2018 and December 31, 2017, respectively	0	0
Additional paid in capital	181,108	171,726
Accumulated other comprehensive income	3,855	5,370
Accumulated deficit	(140,033)	(85,897)
Total stockholders' equity	44,930	91,199
Total liabilities and stockholders' equity	$ 124,630	$ 136,847

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	21,140,000	20,740,000
Common stock, shares outstanding	20,756,000	20,194,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating expenses:
Research and development	$ 12,146	$ 9,398	$ 40,819	$ 21,020
General and administrative	4,438	4,924	15,910	13,562
Total operating expenses	16,584	14,322	56,729	34,582
Loss from operations	(16,584)	(14,322)	(56,729)	(34,582)
Other income (expense), net	(210)	(2,154)	2,593	(4,986)
Net loss before taxes	(16,794)	(16,476)	(54,136)	(39,568)
Income tax benefit	0	85	0	212
Net loss	(16,794)	(16,391)	(54,136)	(39,356)
Other comprehensive income (loss):
Cumulative translation adjustment	(396)	1,588	(1,515)	5,065
Comprehensive loss	$ (17,190)	$ (14,803)	$ (55,651)	$ (34,291)
Per share information:
Net loss, basic and diluted	$ (0.82)	$ (1.12)	$ (2.66)	$ (6.31)
Basic and diluted weighted average shares outstanding	20,473	14,674	20,331	6,233

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (54,136,000)	$ (39,356,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	117,000	80,000
Amortization of debt discount and issuance costs	130,000	729,000
Stock-based compensation	3,391,000	1,055,000
Fair value adjustment of success payment liability	(2,597,000)	4,532,000
Fair value adjustment of contingent consideration	1,600,000	3,190,000
Non-cash interest expense	330,000	34,000
Loss on disposal of property and equipment	3,000	5,000
Changes in assets and liabilities:
Prepaid expenses and other current assets	(210,000)	(701,000)
Accounts payable and other accrued liabilities	5,278,000	3,501,000
Net cash used in operating activities	(46,094,000)	(26,931,000)
Investing activities
Investment in property and equipment	(33,000)	(170,000)
Proceeds from sale of property and equipment		8,000
Net cash used in investing activities	(33,000)	(162,000)
Financing activities
Proceeds from issuance of common stock, net of issuance costs and early exercise liability	5,444,000	188,000
Repurchase of unvested early exercise stock options	(18,000)
Net proceeds from issuance of long-term debt	29,853,000
Proceeds from issuance of common stock for ESPP purchase	388,000
Proceeds from issuance of convertible promissory notes		3,906,000
Proceeds from initial public offering, net of issuance costs		66,378,000
Net cash provided by financing activities	35,667,000	106,778,000
Effect of exchange rate changes on cash	(32,000)	(103,000)
Net (decrease)/increase in cash, cash equivalents and restricted cash	(10,492,000)	79,582,000
Cash, cash equivalents and restricted cash at beginning of period	74,648,000	9,200,000
Cash, cash equivalents and restricted cash at end of period	$ 64,156,000	88,782,000
Series B Preferred Stock [Member]
Financing activities
Net proceeds from issuance of preferred stock		$ 36,306,000

Organization and Description of

Organization and Description of Business	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Organization and Description of Business	1. Organization and Description of Business In these notes to the unaudited condensed consolidated financial statements, the “Company,” “Sienna,” “we,” “us,’” and “our” refers to Sienna Biopharmaceuticals, Inc. (formerly Sienna Labs, Inc.) and its subsidiaries on a consolidated basis. Sienna Biopharmaceuticals, Inc. was incorporated on July 27, 2010, under the laws of the State of Delaware and is headquartered in Westlake Village, California. The Company is a clinical-stage biopharmaceutical company focused on bringing innovations in biotechnology to the discovery, development and commercialization of first-in-class, On July 20, 2017, the Company amended and restated its certificate of incorporation, giving effect to a 1-for-5.87 On August 1, 2017, the Company completed its initial public offering, or IPO, of 4,983,333 shares of common stock, which included the exercise in full by the underwriters of their option to purchase up to 650,000 additional shares of common stock, at an offering price to the public of $15.00 per share. The Company received net proceeds of approximately $66.4 million after deducting underwriting discounts, commissions and offering related transaction costs. In connection with the IPO, the Company’s outstanding shares of convertible preferred stock were automatically converted into 12.8 million shares of common stock. As of September 30, 2018, the Company had 21.1 million shares of common stock outstanding. See Note 13, “Stockholders’ Equity.” In connection with the completion of its IPO, on August 1, 2017, the Company’s certificate of incorporation was amended and restated to provide for 300.0 million authorized shares of common stock with a par value of $0.0001 per share and 10.0 million authorized shares of preferred stock with a par value of $0.0001 per share. On June 29, 2018, the Company entered into a new loan and security agreement (the “SVB Loan Agreement”) with Silicon Valley Bank (“SVB”), pursuant to which SVB agreed to make available to the Company term loans with an aggregate principal amount of up to $40.0 million, $30.0 million of which was funded on June 29, 2018 and $10.0 million of which remains available for borrowing, subject to the satisfaction of certain conditions set forth in the SVB Loan Agreement. See Note 10, “Long-Term Debt.” On August 3, 2018 the Company entered into a sales agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”), pursuant to which the Company may sell from time to time, at its option, up to $75.0 million of the Company’s common stock through an “at-the-market” On August 3, 2018, the Company also filed a Registration Statement on Form S-3

Liquidity Risks

Liquidity Risks	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Liquidity Risks	2. Liquidity Risks The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop product candidates, including conducting preclinical and clinical trials and providing general and administrative support for these operations. The Company had an accumulated deficit of $140.0 million and $85.9 million as of September 30, 2018 and December 31, 2017, respectively. The Company had net losses of $16.8 million and $54.1 million for the three and nine months ended September 30, 2018 and $16.4 million and $39.4 million for the three and nine months ended September 30, 2017, respectively, and net cash used in operating activities of $46.1 million and $26.9 million for the nine months ended September 30, 2018 and 2017, respectively. The Company has historically financed its operations primarily through private and public equity issuances and debt offerings, and more recently through term loans under the SVB Loan Agreement and proceeds from the Company’s ATM Offering Program. Pursuant to the SVB Loan Agreement, if the Company fails to receive positive Phase 2b data for SNA-120 by for SNA-125 in The Company had cash and cash equivalents of $64.0 million and $74.5 million at September 30, 2018 and December 31, 2017, respectively. The Company believes that its current capital resources will be sufficient to fund operations through at least the next twelve months based on the expected cash burn rate. The Company will be required to raise additional capital to fund future operations through the sale of its equity securities, incurring additional debt, entering into licensing or collaboration agreements with partners, grants or other sources of financing. There can be no assurance that sufficient funds will be available to the Company at all or on attractive terms when needed from equity or debt financings. If the Company is unable to obtain additional funding from these or other sources when needed, or to the extent needed, it may be necessary to significantly reduce its current rate of spending through reductions in staff and delaying, scaling back, or stopping certain research and development programs. Insufficient liquidity may also require the Company to relinquish rights to product candidates at an earlier stage of development or on less favorable terms to it or its stockholders than the Company would otherwise choose.

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	3. Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States, or GAAP, and the applicable rules and regulations of the Securities and Exchange Commission, or the SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by GAAP have been condensed or omitted. Unaudited Condensed Consolidated Financial Statements The accompanying financial information for the three and nine months ended September 30, 2018 and 2017 are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of September 30, 2018 and its results of operations for the three and nine months ended September 30, 2018 and 2017 and cash flows for the nine months ended September 30, 2018 and 2017. The results for interim periods are not necessarily indicative of the results expected for the full fiscal year or any other period(s). Principles of Consolidation The unaudited condensed consolidated financial statements include the accounts of Sienna Biopharmaceuticals, Inc. and results of its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the condensed consolidated financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. The most significant estimates in the Company’s consolidated financial statements relate to the valuation of equity awards and the success payment liability, clinical trial accruals and the valuation of the contingent consideration obligations incurred in connection with the acquisition of Creabilis plc, or Creabilis. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment and one reportable segment, primarily in the United States. Cash and Cash Equivalents The Company’s investment policy limits investments to certain types of instruments such as certificates of deposit, money market instruments, and obligations issued by U.S. government and U.S. government agencies, and places restrictions on maturities and concentration by type and issuer. The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. As of September 30, 2018, cash and cash equivalents are comprised of funds in cash and U.S. Treasury money market funds. From time to time, the Company maintains cash balances in excess of amounts insured by the Federal Deposit Insurance Corporation. The accounts are monitored by management to mitigate the risk. Restricted Cash At September 30, 2018 and December 31, 2017, the Company held $0.2 million of restricted cash related to cash collateralized standby letters of credit in connection with obligations under the Company’s facility lease. Fair Value Measurements The Company’s financial instruments, in addition to those presented in Note 7, “Fair Value Measurements”, include cash and cash equivalents, restricted cash, accounts payable, accrued liabilities and long-term debt. The carrying amount of cash and cash equivalents, restricted cash, accounts payable and accrued liabilities approximate fair value because of the short-term nature of these instruments. Further, based on the borrowing rates currently available to the Company for loans with similar terms, the Company believes the carrying amount of the long-term debt approximates its fair value. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation on property and equipment is calculated using the straight-line method over the estimated useful lives of the assets which range from three to five years. Maintenance and repairs are expensed as incurred. The Company reviews the carrying values of its property and equipment for possible impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. There were no impairments recognized during the nine months ended September 30, 2018 and the year ended December 31, 2017. In-process Intangible assets acquired in a business combination are recognized separately from goodwill and are initially recognized at their fair value at the acquisition date (which is regarded as their cost). Intangible assets related to in-process Goodwill represents the excess of the purchase price over the estimated fair value of the identifiable assets acquired and liabilities assumed in a business combination. Goodwill will not be amortized but will be evaluated for impairment annually upon the occurrence of triggering events or substantive changes in circumstances that could indicate a potential impairment. An impairment loss is recognized when the fair value of the reporting unit to which the goodwill relates is below its carrying value for the difference between the fair value and its carrying amounts. There was no impairment of goodwill for the nine months ended September 30, 2018 and the year ended December 31, 2017. Research and Development Costs Research and development costs are expensed as incurred. These costs include direct program expenses, which are payments made to third parties that specifically relate to the Company’s research and development, such as payments to clinical research organizations, clinical investigators, manufacturing of clinical material, pre-clinical Stock-Based Compensation The Company measures employee and director stock-based compensation expense for all stock-based awards at the grant date based on the fair value measurement of the award. The expense is recorded on a straight-line basis over the requisite service period, which is generally the vesting period, for the entire award. Expense is adjusted for actual forfeitures of unvested awards as they occur. The Company calculates the fair value measurement of stock options using the Black-Scholes valuation model. Stock options issued to non-employees Clinical Trial Accruals As part of the process of preparing its consolidated financial statements, the Company is required to estimate its expenses resulting from its obligations under contracts with vendors and consultants and clinical site agreements in connection with conducting clinical trials. The financial terms of these contracts are subject to negotiations, which vary from contract to contract and may result in payment flows that do not match the periods over which materials or services are provided to the Company under such contracts. The Company’s objective is to reflect the appropriate trial expenses in its consolidated financial statements by matching those expenses with the period in which services and efforts are expended. The Company accounts for these expenses according to the progress of the trial as measured by patient progression and the timing of various aspects of the trial. The Company determines accrual estimates through financial models taking into account discussion with applicable personnel and outside service providers as to the progress or state of consummation of trials, or the services completed. During the course of a clinical trial, the Company adjusts its rate of clinical expense recognition if actual results differ from its estimates. The Company makes estimates of its accrued expenses as of each balance sheet date in its consolidated financial statements based on the facts and circumstances known at that time. Although the Company does not expect its estimates to be materially different from amounts actually incurred, its understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and may result in the Company reporting amounts that are too high or too low for any particular period. Through September 30, 2018, there have been no material adjustments to the Company’s prior period estimates of accrued expenses for clinical trials. The Company’s clinical trial accrual is dependent in part upon the timely and accurate reporting of contract research organizations and other third-party vendors. Other accrued expenses include accrued clinical trial costs of $5.0 million and $0.9 million as of September 30, 2018 and December 31, 2017, respectively. At December 31, 2017, prepaid expenses and other current assets include prepaid clinical trial costs of $1.1 million. Basic and Diluted Net Loss Per Common Share Basic net loss per common share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, excluding the effects of converting preferred stock, stock options and unvested restricted stock outstanding. Diluted net loss per common share is computed by dividing the net loss by the sum of the weighted average number of shares of common stock outstanding during the period plus the potential dilutive effects of convertible preferred stock, convertible notes, stock options and unvested restricted stock outstanding during the period calculated in accordance with the treasury stock method but are excluded if their impact is anti-dilutive. Because the impact of these items is anti-dilutive during periods of net loss, there was no difference between the weighted average number of shares used to calculate basic and diluted net loss per common share for the three and nine months ended September 30, 2018 and 2017. Shares excluded from the calculation were 2.6 million and 1.7 million at September 30, 2018 and 2017, respectively. Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred taxes are recognized based on the differences between financial statement and income tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are established, when necessary, to reduce deferred tax assets to the amount expected to be realized. The Company has provided a full valuation allowance on its deferred tax assets. The provision for income taxes represents the current tax payable for the period and the change during the period in deferred tax assets and liabilities. The Company recognizes the effect of an income tax position only if, based on its merits, the position is more likely than not to be sustained on audit by the taxing authorities. Interest and penalties related to uncertain tax positions are recorded as income tax expense. Foreign currency translation The net assets of international subsidiaries where the local currencies have been determined to be the functional currencies are translated into U.S. dollars using current exchange rates. These include the entities acquired as part of the Creabilis acquisition. See Note 4, “Creabilis Acquisition”. As part of this transaction, the Company acquired entities in the United Kingdom, denominated in British pounds, and Italy and Luxembourg, denominated in euros. The U.S. dollar effects that arise from translating net assets of these subsidiaries at changing rates are recognized in other comprehensive loss in the condensed consolidated balance sheet. The earnings or loss of these subsidiaries are translated into U.S. dollars using average exchange rates for the periods. Recently Issued Accounting Standards Accounting Pronouncements Adopted In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740)—Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118”, No. 115-97, In January 2017, the FASB issued ASU 2017-01 “Business Combinations (Topic 805): Clarifying the Definition of a Business”, In November 2016, the FASB issued ASU 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash” (“ASU 2016-18”). No. 2016-18 beginning-of-period end-of-period 2016-18, In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230)” (“ASU 2016-15”), In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) 2014-09”) ASU 2014-09 Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In June 2018 the FASB issued ASU 2018-07, Compensation —Stock Compensation (Topic 718) —Improvements to Nonemployee Share-Based Payment Accounting ” 2018-07”), Compensation—Stock Compensation 2018-07 505-50, Equity—Equity-Based Payments to Non-Employees. 2018-07 Revenue from Contracts with Customers. In February 2018, the FASB issued ASU 2018-02, “ Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income”, In May 2017, the FASB issued ASU 2017-09, Compensation Stock Compensation (Topic 718): Scope of Modification Accounting In February 2016, the FASB issued ASU No. 2016-02, Leases (ASC 842), right-of-use The FASB subsequently issued the following amendments to ASU 2016-02, • ASU No. 2018-10, Codification Improvements to Topic 842, Leases 2016-02; • ASU No. 2018-11, Leases (Topic 842): Targeted Improvements 2016-02 non-lease The Company is in the process of identifying and analyzing all agreements which may be affected by the new guidance and is currently evaluating the update and assessing the impact it may have on its consolidated financial statements and disclosures. The Company will adopt the new leasing standards using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019 and prior periods will not be adjusted.

Creabilis Acquisition

Creabilis Acquisition	9 Months Ended
	Sep. 30, 2018
Business Combinations [Abstract]
Creabilis Acquisition	4. Creabilis Acquisition Pursuant to the acquisition of Creabilis in December 2016, the Company obtained SNA-120, SNA-125 SNA-120 first-in-class SNA-125 Upon closing, the Company became obligated to make certain contingent payments up to an aggregate of $58.0 million in a combination of cash and stock upon the achievement of certain development and approval milestones. In addition, the Company became obligated to make certain contingent payments up to an aggregate of $80.0 million in cash upon the achievement of certain annual net sales thresholds and one-time On October 19, 2017, the Company achieved the first dosing of a human subject in a Phase 2b clinical trial for SNA-120, The agreement to pay the future milestones and potential one-time As a result of the acquisition of Creabilis, the Company recorded a deferred tax liability of $9.4 million for the non-deductible in-process

Identifiable Intangible Assets

Identifiable Intangible Assets	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Identifiable Intangible Assets	5. Identifiable Intangible Assets The Company’s only identifiable intangible assets as of September 30, 2018 and December 31, 2017 are indefinite-lived IPR&D assets related to SNA-120 SNA-125. Indefinite-lived intangible assets are initially measured at their respective fair values and will not be amortized until commercialization. If commercialization occurs, intangible assets will be amortized over their estimated useful lives. In-process

Property and Equipment

Property and Equipment	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment	6. Property and Equipment Property and equipment consisted of the following as of September 30, 2018 and December 31, 2017 (in thousands): Estimated Useful Life (in years) September 30, 2018 December 31, 2017 Lab equipment 5 $ 307 $ 307 Computer hardware 3 136 116 Capital lease equipment 3 46 46 Furniture and fixtures 5 87 91 Software 3 9 — Leasehold improvements 105 105 Total 690 665 Less accumulated depreciation (345 ) (233 ) Property and equipment, net $ 345 $ 432 Leasehold improvements are depreciated over the lease term. Depreciation expense was $39,000 and $0.1 million for the three and nine months ended September 30, 2018, and $27,000 and $80,000 for the three and nine months ended September 30, 2017, respectively.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	7. Fair Value Measurements We determine the fair value of financial and nonfinancial assets and liabilities using the fair value hierarchy, which describes three levels of inputs that may be used to measure fair value, as follows: Level 1 Level 2 Level 3 In certain cases where there is limited activity or less transparency around inputs to valuation, assets are classified as Level 3 within the valuation hierarchy. The following tables set forth the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis based on the three-tier fair value hierarchy as of September 30, 2018 and December 31, 2017 (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 63,975 $ — $ — Total $ 63,975 $ — $ — Liabilities: Success payment liability $ — $ — $ 688 Contingent consideration — — 24,500 Total $ — $ — $ 25,188 December 31, 2017 Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 74,467 $ — $ — Total $ 74,467 $ — $ — Liabilities: Success payment liability $ — $ — $ 3,285 Contingent consideration — — 22,900 Total $ — $ — $ 26,185 The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial liabilities (in thousands): Success Payment Liability Contingent Consideration Balance at December 31, 2017 $ 3,285 $ 22,900 Change in fair value due to remeasurement (2,597 ) 1,600 Balance at September 30, 2018 $ 688 $ 24,500 Cash equivalents At September 30, 2018, the Company’s cash equivalents are comprised of U.S. Treasury money market funds whose value is based upon quoted market prices in active markets for identical assets or liabilities with no adjustments applied. Accordingly, these investments are classified as Level 1 of the fair value measurements and disclosure guidance. Intangible assets In connection with the acquisition of Creabilis, the Company acquired intangible in-process Contingent consideration The Company also agreed to pay additional amounts based on the achievement of certain development, approval and sales milestones. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going The fair value of the contingent consideration was determined by an independent third-party valuation firm applying the income approach. This approach calculates fair value by estimating future cash flows attributable to the related IPR&D assets using several significant unobservable inputs, including risk adjusted discount rates ranging from 5.1% to 16.7%, projected future revenues and expenses based on the cumulative probabilities of multiple scenarios with individual probabilities ranging from 0.1% to 25.0%, and estimates of the timing of the achievement of the various product development, regulatory approval and sales milestones. Significant increases or decreases in any of the probabilities of success and other inputs would result in a significantly higher or lower fair value measurement, respectively. Changes in the fair values of the contingent consideration obligations are recorded in general and administrative expense in the condensed consolidated statement of operations. In October 2017, the Company commenced the additional Phase 2b clinical trial for SNA-120, 20-day Success payment liability In October 2015, as a result of an agreement in which the Company agreed to pay certain stockholders success payments if the stock price of the Company’s common stock reached certain thresholds, the Company recorded a success payment liability. The success payment liability was recorded at fair value based on significant unobservable inputs, as discussed in Note 9, “Success Payment Liability”. The valuation of the fair value of the success payment liability uses assumptions the Company believes would be made by a market participant and the Company assesses these estimates on an on-going There were no transfers of assets or liabilities between the fair value measurement levels during the nine months ended September 30, 2018 or the year ended December 31, 2017.

Convertible Notes

Convertible Notes	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Convertible Notes	8. Convertible Notes In January 2017, the Company entered into a note purchase agreement pursuant to which the Company issued, in two tranches, subordinated convertible promissory notes (the “Series B Bridge Notes” and together with the note purchase agreement, the “Series B Bridge Note Agreements”) in an aggregate principal amount of $3.9 million. The Series B Bridge Notes provided for an annual interest rate of 6.0% and a maturity date of January 27, 2018. Under the terms of the Series B Bridge Note Agreements, under certain circumstances, the unpaid principal of the Series B Bridge Notes, including any accrued but unpaid interest thereon, would convert into shares of convertible preferred stock upon the closing of a future preferred stock financing that met specified criteria. Such conversion would be at a 15% discount to the per share price of the convertible preferred stock sold in the financing. In addition, the notes were voluntarily convertible into shares upon the occurrence of a non-qualified The conversion feature included a 15% discount in the convertible notes, which constituted a beneficial conversion feature that was bifurcated and allocated to additional paid in capital. The intrinsic value of the beneficial conversion feature due to the 15% discount on conversion of the principal and accrued interest was calculated to be $0.7 million. This resulted in a discount of $0.7 million being allocated to the Series B Bridge Notes which was being amortized to interest expense in the condensed consolidated statement of operations on an effective interest method from the date of issuance of each Series B Bridge Notes through the maturity date of January 27, 2018. In April 2017, in connection with the Company’s Series B convertible preferred stock financing, the outstanding principal under the Series B Bridge Notes of $3.9 million, plus $34,000 of accrued interest, converted into an aggregate of 0.4 million shares of Series B convertible preferred stock at a rate of $10.40 per share and the Series B Bridge Notes were cancelled. In connection with the IPO, the Series B convertible preferred stock was automatically converted into 0.4 million shares of common stock. The remaining unamortized discount was recognized in interest expense in the condensed consolidated statement of operations. Included in other income (expense), net for the three and nine months ended September 30, 2017 is amortization of the debt discount of $0 and $0.7 million and accrued interest expense of $0 and $34,000, respectively.

Success Payment Liability

Success Payment Liability	9 Months Ended
	Sep. 30, 2018
Text Block [Abstract]
Success Payment Liability	9. Success Payment Liability In October 2015, the Company entered into a letter agreement with certain stockholders pursuant to which the Company agreed to make success payments to such stockholders (the “Success Payment Agreement”). The agreement ends on its fifth anniversary in October 2020. Success payments are payable in cash or common stock at the Company’s sole discretion and will be owed in the event that the value of its common stock meets or exceeds certain specified share price thresholds on any of the following dates during the success payment period: (1) any date after the 90th day after the date on which the Company completes an initial public offering of its common stock; (2) the date on which the Company sells, leases, transfers, or exclusively licenses all or substantially all of its assets to another company; and (3) the date on which the Company merges or consolidates with or into another entity (other than a merger in which the pre-merger 90-day The amount of the success payment is determined based on whether the value of the common stock of the Company meets or exceeds certain specified share price thresholds, subject to adjustment for any stock dividend, stock split, combination of shares, or other similar events. Each success payment and the associated share price threshold is ascending from $10.0 million payable at a share price threshold of $53.71 per share to $35.0 million payable at $71.61 per share and with a maximum payment of $60.0 million at a share price threshold of $107.42 per share. Each success payment is inclusive of any preceding payments, if previously made, such that the success payments to stockholders will not exceed $60.0 million in the aggregate. Upon their issuance, the success payments did not require any future service to be provided by the recipients and as such, the success payments were accounted for under accounting guidance for derivatives and hedging. Accordingly, the Company recorded an initial liability at fair value and remeasures the liability each reporting period, with changes being recognized in the consolidated statements of operations. The fair value of the success payments liability was estimated based on a third-party valuation using a model which simulates the future movement of stock prices based on several key variables. The following variables were incorporated in the estimated fair value of the success payment liability: estimated term of the success payments, fair value of the Company’s common stock, expected volatility, and risk-free interest rate. The computation of expected volatility was estimated using a combination of available information about the historical volatility of stocks of similar publicly-traded companies blended with historical volatility of the Company’s own stock price, for a period matching the expected term assumption. During the three and nine months ended September 30, 2018, the Company recorded other income of $0.2 million and $2.6 million, respectively, and during the three and nine months ended September 30, 2017, the Company recorded other expense of $2.5 million and $4.5 million, respectively, due to re-measurement

Long-Term Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Long-Term Debt	10. Long-Term Debt On June 29, 2018 the Company entered into a Loan and Security Agreement (the “SVB Loan Agreement”) with Silicon Valley Bank (“SVB”). Under the SVB Loan Agreement, SVB will provide the Company with access to term loans in an aggregate principal amount of up to $40.0 million. The first credit extension, of a principal amount of $30.0 million, was funded on June 29, 2018, and is repayable in monthly installments until July 1, 2023, including an initial interest-only period through July 31, 2020. Interest on the term loans accrue at a per annum rate of the greater of (i) the Wall Street Journal prime rate plus 2.50% and (ii) 7.25%. On September 30, 2018, the rate was 7.75%. If the Company fails to receive positive Phase 2b data for SNA-120 by for SNA-125 in An additional term loan, of a principal amount of up to $10.0 million, can be drawn down at the Company’s option during the period of time commencing on the date the Company has received positive pivotal data for SNA-001 and for SNA-120 and for SNA-001 or for SNA-120, an The Company may prepay the outstanding principal balance of the term loans advanced by SVB in whole but not in part, subject to a prepayment fee ranging from 1.0% to 3.0% of any amount prepaid, depending upon when the prepayment occurs. The Company will also pay a final payment fee equal to 5.50% of the total term loans advanced, due upon the earliest of maturity, acceleration, prepayment or termination of the SVB Loan Agreement. Under the terms of the SVB Loan Agreement, the Company granted first priority liens and security interests in substantially all of the Company’s assets (excluding all of its intellectual property, which is subject to a negative pledge) and a pledge of the shares of one of its wholly-owned subsidiaries as collateral for the obligations thereunder. The SVB Loan Agreement also contains representations and warranties by the Company and SVB and indemnification provisions in favor of SVB and customary covenants (including limitations on other indebtedness, liens, acquisitions, investments and dividends, but no financial covenants), and events of default (including payment defaults, breaches of covenants following any applicable cure period, a material impairment in the perfection or priority of SVB’s security interest in the collateral, and events relating to bankruptcy or insolvency). As of September 30, 2018, the carrying value of the term loan consists of $30.0 million principal outstanding less the debt issuance costs of approximately $0.1 million. The debt issuance costs have been recorded as a debt discount which are being accreted to interest expense through the maturity date of the term loan. Interest expense relating to the term loan for the three and nine months ended September 30, 2018 was $0.7 million. Interest expense is calculated using the effective interest method, and is inclusive of non-cash Future principal payments for the long-term debt are as follows (in thousands): September 30, 2018 2018 $ — 2019 — 2020 4,515 2021 9,599 2022 10,382 2023 5,504 Total principal payments 30,000 Final fee due at maturity in 2023 1,650 Total principal and final fee payments 31,650 Unamortized discount and debt issuance costs (1,667 ) Long-term debt, net $ 29,983

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Operating Lease In May 2016, the Company entered into a 40-month During the three and nine months ended September 30, 2018, the Company incurred $0.1 million and $0.3 million, respectively, for rent expense and $0.2 million and $0.4 million, for the three and nine months ended September 30, 2017, respectively. License and Supply Agreement The Company has an exclusive license and supply agreement with nanoComposix, pursuant to which the Company owes minimum annual royalties of $50,000 or low single digit royalties on net sales of licensed products. Indemnifications The Company has indemnification obligations to its directors and executive officers for specified events or occurrences, subject to some limits, while the directors and executive officers are serving at the Company’s request in such capacities. There have been no claims to date and the Company did not accrue any liabilities related to these agreements as of September 30, 2018 and December 31, 2017. Contingencies From time to time, the Company may be subject to various litigation and related matters arising in the ordinary course of business, including those set forth in Part II, Item 1 “Legal Proceedings”. As of September 30, 2018, there are no matters where there is at least a reasonable probability that a material loss has been or will be incurred.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions Venvest Biotech, LLC Dr. Beddingfield, the Company’s President and Chief Executive Officer and a member of the Company’s board of directors, is an advisor to Venvest Biotech, LLC, or Venvest, and is considered a non-managing Series A-3 Stock Purchase Rights In January 2016, in connection with his commencement of employment with the Company, the Company’s board of directors granted Dr. Beddingfield, the Company’s President and Chief Executive Officer, the right to purchase 0.6 million shares of the Company’s common stock for a purchase price of $2.35 per share, which the board of directors determined was the fair market value on the date of grant. With respect to 0.5 million shares subject to the stock purchase right, 25% of the shares vest on the first anniversary of the grant, and 1/48th of the shares vest monthly thereafter, subject to Dr. Beddingfield continuing to provide services to the Company through each such vesting date. With respect to 49,000 shares subject to the stock purchase right, 50% of the shares vest on the first date the volume-weighted average trading price of the Company’s common stock equals or exceeds $71.03 per share, and 1/24th of the shares vest monthly thereafter, subject to Dr. Beddingfield continuing to provide services to the Company through each such vesting date. With respect to the remaining 49,000 shares subject to the stock purchase right, 50% of the shares vest upon achievement of a milestone related to clinical development, and 1/24th of the shares vest monthly thereafter, subject to Dr. Beddingfield continuing to provide services to the Company through each such vesting date. The Company determined that the stock purchase rights effectively represented an option and the fair value of the option was $1.3 million which is being amortized as compensation expense over the performance period of the award, with $0.2 million and $0.4 million recognized as compensation expense for the three and nine months ended September 30, 2018, respectively and $36,000 and $0.1 million recognized during the three and nine months ended September 30, 2017, respectively. In May 2016, Dr. Beddingfield exercised his stock purchase rights in full and purchased restricted stock that vests on the same schedule as the stock purchase rights by providing a promissory note to the Company in the principal amount of $1.3 million, with an interest rate of 1.43% per annum. The promissory note was considered to be substantively non-recourse non-recourse, non-recourse Success Payments Todd Harris, the Company’s Head of Corporate Development and member of the Company’s board of directors, is a beneficiary of the Success Payments Agreement, as described in Note 9 “Success Payment Liability” and will receive 25.22% of any related payouts.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2018
Federal Home Loan Banks [Abstract]
Stockholders' Equity	13. Stockholders’ Equity As of September 30, 2018, the authorized stock of the Company was 300.0 million shares of common stock, $0.0001 par value per share, and 10.0 million shares of preferred stock, $0.0001 par value per share. Common Stock Holders of common stock are entitled to one vote per share and, upon liquidation, dissolution, or winding up of the Company, are entitled to receive all assets available for distribution to stockholders. The holders have no preemptive or other subscription rights and there are no redemption or sinking fund provisions with respect to such shares. Pursuant to the automatic increase provisions of our 2017 Incentive Award Plan (“the 2017 Plan”) and 2017 Employee Stock Purchase Plan (“ESPP”), 829,606 additional shares were reserved for future issuance under the 2017 Plan and 207,401 additional shares under the ESPP on January 1, 2018. Shares of common stock reserved for future issuance were as follows (in thousands): September 30, 2018 Common stock awards outstanding 2,171 Common stock awards available for grant under the ESPP 376 Common stock awards available for grant under the 2017 Plan 1,123 Total shares of common stock reserved for future issuance 3,670 Convertible Preferred Stock As of September 30, 2018, there was no convertible preferred stock outstanding. In connection with the Company’s IPO, all outstanding shares of convertible preferred stock were automatically converted into 12.8 million shares of common stock. In April 2017, the Company issued an aggregate of 3.0 million shares of Series B convertible preferred stock at a price per share of $12.24 for aggregate proceeds of $36.5 million, exclusive of 0.4 million shares of Series B convertible preferred stock issued upon conversion of the Series B Bridge Notes. See Note 8 “Convertible Notes”. All of the Company’s outstanding Series B convertible preferred stock was automatically converted into common stock in connection with the IPO. ATM Offering Program In August 2018, the Company entered into a sales agreement with Cowen pursuant to which the Company may sell from time to time, at its option, up to $75.0 million of the Company’s common stock through ATM Offering Program, under which Cowen will act as sales agent. The aggregate compensation payable to Cowen shall be 3% of the gross sales price of the common stock sold by Cowen pursuant to the Sales Agreement. During the three months ended September 30, 2018, the Company issued 340,307 shares of its common stock through its ATM Offering Program and received net proceeds of approximately $5.0 million, after deducting commissions of $0.2 million and other offering expenses of $0.4 million. Stock Awards and Stock-Based Compensation In July 2017, the Company’s board of directors approved the 2017 Plan, which became effective upon the completion of the IPO on August 1, 2017. The 2017 Plan serves as the successor incentive award plan to the Company’s 2010 Equity Incentive Plan, or the 2010 Plan, and has 1.2 million shares of common stock available for issuance pursuant to a variety of stock-based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock-based awards, plus shares of common stock that were reserved for issuance pursuant to future awards under the 2010 Plan at the time the 2017 Plan became effective, plus shares represented by awards outstanding under the 2010 Plan that are forfeited or lapse unexercised and which following the effective date of the 2017 Plan are not issued under the 2010 Plan. In addition, the 2017 Plan reserve increased on January 1, 2018 and will increase on each subsequent anniversary through 2027, by an amount equal to the lesser of (a) four percent of the shares of stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (b) such smaller number of shares of stock as determined by our board of directors; provided, however, that no more than 12.0 million shares of stock may be issued upon the exercise of incentive stock options. The terms of awards pursuant to the 2017 Plan are determined by the administrator of the 2017 Plan. The 2017 Plan is administered by the compensation committee of the Company’s board of directors unless the Company’s board of directors assumes authority for administration. In addition, the Company’s board of directors has delegated authority to grant awards to employees other than executive officers and certain senior executives of the Company to a committee consisting of the Company’s chief executive officer. Stock options granted pursuant to the 2017 Plan must have an exercise price of not less than the fair market value of the Company’s common stock on the date of grant, except that incentive stock options granted to an individual who owns (or is deemed to own) at least 10% of the total combined voting power of all classes of the Company’s capital stock (a “10% Holder”), must have an exercise price of at least 110% of the fair market value of a share of common stock on the date of grant. Stock options granted under the 2017 Plan generally expire ten years from the date of the grant, except that incentive stock options granted to a 10% Holder must not be exercisable after five years from the date of grant. The Company’s stock awards under the 2017 Plan vest based on terms in the stock award agreements and generally vest over four years. Following the Company’s IPO and in connection with the effectiveness of the Company’s 2017 Plan, the 2010 Plan terminated and no further awards will be granted under that plan. However, all outstanding awards under the 2010 Plan will continue to be governed by their existing terms. The fair value of each employee award granted during 2018 and 2017 was estimated on the grant date using the Black-Scholes option-pricing model. The fair value of each non-employee Nine Months Ended September 30, 2018 Year Ended December 31, 2017 Expected stock price volatility 65.40%–74.21% 59.13%–64.09% Expected dividend yield —% —% Expected term (in years) 5.0–6.1 5.3–10.0 Risk-free interest rate 2.55%–2.96% 1.77%–2.60% Due to limited historical data, the Company estimates stock price volatility based on the actual volatility of comparable publicly traded companies over the expected life of the award. The Company has never paid and does not expect to pay dividends in the foreseeable future. The expected term represents the average time that awards that vest are expected to be outstanding. For employee awards that have an early exercise provision, the Company has sufficient information to utilize four years as an expected term. For awards without an early exercise provision, the Company does not have sufficient history of stock option exercises to estimate the expected term and, thus, calculates expected term using the simplified method, based on the midpoint between the average vesting date and the contractual term. For all non-employees, The table below summarizes the stock option activity for the nine months ended September 30, 2018: Number of Shares (in thousands) Weighted Average Exercise Price Per Share Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2017 1,159 $ 8.19 $ 11,795 Granted 1,235 16.49 Exercised (37 ) 11.04 Cancelled (186 ) 11.74 Outstanding at September 30, 2018 2,171 $ 12.56 $ 7,497 Exercisable at September 30, 2018 455 $ 6.53 $ 3,925 The aggregate intrinsic value of the options outstanding is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock as of September 30, 2018. The Company did not grant any non-employee Total compensation cost recorded in the condensed consolidated statements of operations and comprehensive loss, which includes non-cash non-cash Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 Research and development $ 368 $ 201 $ 1,099 $ 279 General and administrative 1,127 540 2,292 776 $ 1,495 $ 741 $ 3,391 $ 1,055 As of September 30, 2018, there was $15.0 million of unrecognized compensation expense related to unvested employee stock award agreements, which is expected to be recognized over a weighted-average period of approximately 3.39 years. For stock option awards subject to graded vesting, we recognize compensation cost on a straight-line basis over the service period for the entire award. The weighted-average grant date fair value of all stock options granted during the nine months ended September 30, 2018 was $10.55. The weighted-average remaining contractual life of options outstanding at September 30, 2018 is 9.0 years. The total fair value of the shares vested during the nine months ended September 30, 2018 was $2.2 million. Additionally, stock compensation expense includes $3,000 and $0.3 million, $0.4 million and $0.5 million related to non-employee Prior to its termination in connection with the effectiveness of the 2017 Plan, the 2010 Plan allowed the Company to grant to employees the right to exercise stock options in exchange for cash before the requisite service was provided (e.g., before the award is vested under its original terms); however, such arrangements permit the Company to subsequently repurchase such shares at the exercise price if the employee ceases to be a service provider. Such an exercise is not substantive for accounting purposes. Therefore, the payment received for the exercise price is recognized as an early exercise liability in the condensed consolidated balance sheets and will be transferred to common stock and additional paid in capital as such shares vest. As of September 30, 2018 and December 31, 2017, 0.4 million and 0.5 million unvested shares, respectively, were legally issued but are not considered outstanding for accounting purposes and are therefore excluded from basic and diluted net loss per share until the repurchase right lapses and the shares are no longer subject to the repurchase feature. In connection with these unvested shares, the Company has recorded an early exercise liability as of September 30, 2018 and December 31, 2017, of $0.3 million and $0.5 million, respectively, of which $0.2 million and $0.2 million is included in current liabilities, and $0.1 million and $0.3 million is included in non-current 2017 Employee Stock Purchase Plan The Company adopted the ESPP, which became effective upon the completion of the IPO on August 1, 2017. The ESPP is designed to allow the Company’s eligible employees to purchase shares of the Company’s common stock, at semi-annual intervals, with their accumulated payroll deductions. Under the ESPP, participants are offered the option to purchase shares of the Company’s common stock at a discount during a series of successive offering periods. The option purchase price will be the lower of 85% of the closing trading price per share of the Company’s common stock on the first trading date of an offering period in which a participant is enrolled or 85% of the closing trading price per share on the purchase date, which will occur on the last trading day of each offering period. The Company began the first offering period on December 31, 2017. The ESPP is structured to qualify under Section 423 of the U.S. Internal Revenue Service Code of 1986, as amended. The maximum number of the Company’s common stock which will be authorized for sale under the ESPP is equal to the sum of (a) 198,883 shares of common stock and (b) an annual increase on the first day of each year beginning in 2018 and ending in 2027, equal to the lesser of (i) 1% of the shares of common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company’s board of directors; provided, however, no more than 3.0 million shares of the Company’s common stock may be issued under the ESPP. The Company recognized $0.1 million and $0.2 million in compensation expense related to the ESPP for the three and nine months ended September 30, 2018. During the nine months ended September 30, 2018, 30,486 shares of common stock were issued under the ESPP.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes There is no provision for income taxes for the three and nine months ended September 30, 2018 as the Company has incurred operating losses since inception. Intraperiod tax allocation rules require the Company to allocate the provision for income taxes between continuing operations and other categories of earnings. As a result, for the three and nine months ended September 30, 2017 the Company recorded an income tax benefit $85,000 and $0.2 million, respectively in the consolidated statement of operations. The Company has evaluated the available evidence supporting the realization of its deferred tax assets, including the amount and timing of future taxable income, and has determined that it is more likely than not that its net deferred tax assets will not be realized in the U.S. and certain foreign jurisdictions. Due to uncertainties surrounding the realization of the deferred tax assets, the Company maintains a full valuation allowance against substantially all deferred tax assets. When the Company determines that it will be able to realize some portion or all of its deferred tax assets, an adjustment to its valuation allowance on its deferred tax assets would have the effect of increasing net income in the period such determination is made. As of September 30, 2018, the Company does not have any accrued interest or penalties related to uncertain tax positions. The Company’s policy is to recognize interest and penalties related to uncertain tax positions in income tax expense. The Company is subject to U.S. federal tax authority and U.S. state tax authority examinations for all years with the net operating loss and credit carryforwards. On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act, or the Act. The Act amends the Internal Revenue Code of 1986, as amended, or the Code, to reduce tax rates and modify policies, credits and deductions for individuals and businesses. For businesses, the Act reduces the corporate tax rate from a maximum of 35% to a flat 21% rate. The rate reduction became effective on January 1, 2018. Due to the uncertainties which currently exist in the interpretation of the provisions of the Act regarding Code Section 162(m), the Company has not evaluated all of the potential impacts of the Act, as amended on its consolidated financial statements. In accordance with SAB 118, the Company has determined that there is no deferred tax benefit or expense with respect to the remeasurement of certain deferred tax assets and liabilities due to the full valuation allowance against net deferred tax assets. Additional analysis of the law and the impact to the Company will be performed and any impact will be recorded in the respective quarters, within the available measurement period in 2018.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States, or GAAP, and the applicable rules and regulations of the Securities and Exchange Commission, or the SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by GAAP have been condensed or omitted.
Unaudited Condensed Consolidated Financial Statements	Unaudited Condensed Consolidated Financial Statements The accompanying financial information for the three and nine months ended September 30, 2018 and 2017 are unaudited. The unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of September 30, 2018 and its results of operations for the three and nine months ended September 30, 2018 and 2017 and cash flows for the nine months ended September 30, 2018 and 2017. The results for interim periods are not necessarily indicative of the results expected for the full fiscal year or any other period(s).
Principles of Consolidation	Principles of Consolidation The unaudited condensed consolidated financial statements include the accounts of Sienna Biopharmaceuticals, Inc. and results of its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the condensed consolidated financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. The most significant estimates in the Company’s consolidated financial statements relate to the valuation of equity awards and the success payment liability, clinical trial accruals and the valuation of the contingent consideration obligations incurred in connection with the acquisition of Creabilis plc, or Creabilis. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions.
Segment Reporting	Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment and one reportable segment, primarily in the United States.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company’s investment policy limits investments to certain types of instruments such as certificates of deposit, money market instruments, and obligations issued by U.S. government and U.S. government agencies, and places restrictions on maturities and concentration by type and issuer. The Company considers all highly liquid securities with original final maturities of three months or less from the date of purchase to be cash equivalents. As of September 30, 2018, cash and cash equivalents are comprised of funds in cash and U.S. Treasury money market funds. From time to time, the Company maintains cash balances in excess of amounts insured by the Federal Deposit Insurance Corporation. The accounts are monitored by management to mitigate the risk.
Restricted Cash	Restricted Cash At September 30, 2018 and December 31, 2017, the Company held $0.2 million of restricted cash related to cash collateralized standby letters of credit in connection with obligations under the Company’s facility lease.
Fair Value Measurements	Fair Value Measurements The Company’s financial instruments, in addition to those presented in Note 7, “Fair Value Measurements”, include cash and cash equivalents, restricted cash, accounts payable, accrued liabilities and long-term debt. The carrying amount of cash and cash equivalents, restricted cash, accounts payable and accrued liabilities approximate fair value because of the short-term nature of these instruments. Further, based on the borrowing rates currently available to the Company for loans with similar terms, the Company believes the carrying amount of the long-term debt approximates its fair value.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation on property and equipment is calculated using the straight-line method over the estimated useful lives of the assets which range from three to five years. Maintenance and repairs are expensed as incurred. The Company reviews the carrying values of its property and equipment for possible impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. There were no impairments recognized during the nine months ended September 30, 2018 and the year ended December 31, 2017.
In-process Research and Development and Goodwill	In-process Intangible assets acquired in a business combination are recognized separately from goodwill and are initially recognized at their fair value at the acquisition date (which is regarded as their cost). Intangible assets related to in-process Goodwill represents the excess of the purchase price over the estimated fair value of the identifiable assets acquired and liabilities assumed in a business combination. Goodwill will not be amortized but will be evaluated for impairment annually upon the occurrence of triggering events or substantive changes in circumstances that could indicate a potential impairment. An impairment loss is recognized when the fair value of the reporting unit to which the goodwill relates is below its carrying value for the difference between the fair value and its carrying amounts. There was no impairment of goodwill for the nine months ended September 30, 2018 and the year ended December 31, 2017.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. These costs include direct program expenses, which are payments made to third parties that specifically relate to the Company’s research and development, such as payments to clinical research organizations, clinical investigators, manufacturing of clinical material, pre-clinical
Stock-Based Compensation	Stock-Based Compensation The Company measures employee and director stock-based compensation expense for all stock-based awards at the grant date based on the fair value measurement of the award. The expense is recorded on a straight-line basis over the requisite service period, which is generally the vesting period, for the entire award. Expense is adjusted for actual forfeitures of unvested awards as they occur. The Company calculates the fair value measurement of stock options using the Black-Scholes valuation model. Stock options issued to non-employees
Clinical Trial Accruals	Clinical Trial Accruals As part of the process of preparing its consolidated financial statements, the Company is required to estimate its expenses resulting from its obligations under contracts with vendors and consultants and clinical site agreements in connection with conducting clinical trials. The financial terms of these contracts are subject to negotiations, which vary from contract to contract and may result in payment flows that do not match the periods over which materials or services are provided to the Company under such contracts. The Company’s objective is to reflect the appropriate trial expenses in its consolidated financial statements by matching those expenses with the period in which services and efforts are expended. The Company accounts for these expenses according to the progress of the trial as measured by patient progression and the timing of various aspects of the trial. The Company determines accrual estimates through financial models taking into account discussion with applicable personnel and outside service providers as to the progress or state of consummation of trials, or the services completed. During the course of a clinical trial, the Company adjusts its rate of clinical expense recognition if actual results differ from its estimates. The Company makes estimates of its accrued expenses as of each balance sheet date in its consolidated financial statements based on the facts and circumstances known at that time. Although the Company does not expect its estimates to be materially different from amounts actually incurred, its understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and may result in the Company reporting amounts that are too high or too low for any particular period. Through September 30, 2018, there have been no material adjustments to the Company’s prior period estimates of accrued expenses for clinical trials. The Company’s clinical trial accrual is dependent in part upon the timely and accurate reporting of contract research organizations and other third-party vendors. Other accrued expenses include accrued clinical trial costs of $5.0 million and $0.9 million as of September 30, 2018 and December 31, 2017, respectively. At December 31, 2017, prepaid expenses and other current assets include prepaid clinical trial costs of $1.1 million.
Basic and Diluted Net Loss Per Common Share	Basic and Diluted Net Loss Per Common Share Basic net loss per common share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, excluding the effects of converting preferred stock, stock options and unvested restricted stock outstanding. Diluted net loss per common share is computed by dividing the net loss by the sum of the weighted average number of shares of common stock outstanding during the period plus the potential dilutive effects of convertible preferred stock, convertible notes, stock options and unvested restricted stock outstanding during the period calculated in accordance with the treasury stock method but are excluded if their impact is anti-dilutive. Because the impact of these items is anti-dilutive during periods of net loss, there was no difference between the weighted average number of shares used to calculate basic and diluted net loss per common share for the three and nine months ended September 30, 2018 and 2017. Shares excluded from the calculation were 2.6 million and 1.7 million at September 30, 2018 and 2017, respectively.
Income Taxes	Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred taxes are recognized based on the differences between financial statement and income tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are established, when necessary, to reduce deferred tax assets to the amount expected to be realized. The Company has provided a full valuation allowance on its deferred tax assets. The provision for income taxes represents the current tax payable for the period and the change during the period in deferred tax assets and liabilities. The Company recognizes the effect of an income tax position only if, based on its merits, the position is more likely than not to be sustained on audit by the taxing authorities. Interest and penalties related to uncertain tax positions are recorded as income tax expense.
Foreign currency translation	Foreign currency translation The net assets of international subsidiaries where the local currencies have been determined to be the functional currencies are translated into U.S. dollars using current exchange rates. These include the entities acquired as part of the Creabilis acquisition. See Note 4, “Creabilis Acquisition”. As part of this transaction, the Company acquired entities in the United Kingdom, denominated in British pounds, and Italy and Luxembourg, denominated in euros. The U.S. dollar effects that arise from translating net assets of these subsidiaries at changing rates are recognized in other comprehensive loss in the condensed consolidated balance sheet. The earnings or loss of these subsidiaries are translated into U.S. dollars using average exchange rates for the periods.
Recently Issued Accounting Standards	Recently Issued Accounting Standards Accounting Pronouncements Adopted In March 2018, the FASB issued ASU 2018-05, Income Taxes (Topic 740)—Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118”, No. 115-97, In January 2017, the FASB issued ASU 2017-01 “Business Combinations (Topic 805): Clarifying the Definition of a Business”, In November 2016, the FASB issued ASU 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash” (“ASU 2016-18”). No. 2016-18 beginning-of-period end-of-period 2016-18, In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230)” (“ASU 2016-15”), In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) 2014-09”) ASU 2014-09 Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In June 2018 the FASB issued ASU 2018-07, Compensation —Stock Compensation (Topic 718) —Improvements to Nonemployee Share-Based Payment Accounting ” 2018-07”), Compensation—Stock Compensation 2018-07 505-50, Equity—Equity-Based Payments to Non-Employees. 2018-07 Revenue from Contracts with Customers. In February 2018, the FASB issued ASU 2018-02, “ Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income”, In May 2017, the FASB issued ASU 2017-09, Compensation Stock Compensation (Topic 718): Scope of Modification Accounting In February 2016, the FASB issued ASU No. 2016-02, Leases (ASC 842), right-of-use The FASB subsequently issued the following amendments to ASU 2016-02, • ASU No. 2018-10, Codification Improvements to Topic 842, Leases 2016-02; • ASU No. 2018-11, Leases (Topic 842): Targeted Improvements 2016-02 non-lease The Company is in the process of identifying and analyzing all agreements which may be affected by the new guidance and is currently evaluating the update and assessing the impact it may have on its consolidated financial statements and disclosures. The Company will adopt the new leasing standards using a modified retrospective transition approach to be applied to leases existing as of, or entered into after, January 1, 2019 and prior periods will not be adjusted.

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2018
Property, Plant and Equipment [Abstract]
Components of Property and Equipment	Property and equipment consisted of the following as of September 30, 2018 and December 31, 2017 (in thousands): Estimated Useful Life (in years) September 30, 2018 December 31, 2017 Lab equipment 5 $ 307 $ 307 Computer hardware 3 136 116 Capital lease equipment 3 46 46 Furniture and fixtures 5 87 91 Software 3 9 — Leasehold improvements 105 105 Total 690 665 Less accumulated depreciation (345 ) (233 ) Property and equipment, net $ 345 $ 432

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Financial Assets and Liabilities Measured on Recurring Basis	The following tables set forth the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis based on the three-tier fair value hierarchy as of September 30, 2018 and December 31, 2017 (in thousands): September 30, 2018 Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 63,975 $ — $ — Total $ 63,975 $ — $ — Liabilities: Success payment liability $ — $ — $ 688 Contingent consideration — — 24,500 Total $ — $ — $ 25,188 December 31, 2017 Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 74,467 $ — $ — Total $ 74,467 $ — $ — Liabilities: Success payment liability $ — $ — $ 3,285 Contingent consideration — — 22,900 Total $ — $ — $ 26,185
Summary of Changes in Fair Value of Level 3 Financial Liabilities	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial liabilities (in thousands): Success Payment Liability Contingent Consideration Balance at December 31, 2017 $ 3,285 $ 22,900 Change in fair value due to remeasurement (2,597 ) 1,600 Balance at September 30, 2018 $ 688 $ 24,500

Long-Term Debt (Tables)

Long-Term Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Future Principal Payments for Long-term Debt	Future principal payments for the long-term debt are as follows (in thousands): September 30, 2018 2018 $ — 2019 — 2020 4,515 2021 9,599 2022 10,382 2023 5,504 Total principal payments 30,000 Final fee due at maturity in 2023 1,650 Total principal and final fee payments 31,650 Unamortized discount and debt issuance costs (1,667 ) Long-term debt, net $ 29,983

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Federal Home Loan Banks [Abstract]
Summary of Shares of Common Stock Reserved for Future Issuance	Shares of common stock reserved for future issuance were as follows (in thousands): September 30, 2018 Common stock awards outstanding 2,171 Common stock awards available for grant under the ESPP 376 Common stock awards available for grant under the 2017 Plan 1,123 Total shares of common stock reserved for future issuance 3,670
Summary of Assumptions Used for Grant Date Fair Value	The following assumptions were used for grant date fair value for the nine months ended September 30, 2018 and the year ended December 31, 2017: Nine Months Ended September 30, 2018 Year Ended December 31, 2017 Expected stock price volatility 65.40%–74.21% 59.13%–64.09% Expected dividend yield —% —% Expected term (in years) 5.0–6.1 5.3–10.0 Risk-free interest rate 2.55%–2.96% 1.77%–2.60%
Summary of Stock Option Activity	The table below summarizes the stock option activity for the nine months ended September 30, 2018: Number of Shares (in thousands) Weighted Average Exercise Price Per Share Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2017 1,159 $ 8.19 $ 11,795 Granted 1,235 16.49 Exercised (37 ) 11.04 Cancelled (186 ) 11.74 Outstanding at September 30, 2018 2,171 $ 12.56 $ 7,497 Exercisable at September 30, 2018 455 $ 6.53 $ 3,925
Summary of Non-cash Stock-based Compensation Expense	Total compensation cost recorded in the condensed consolidated statements of operations and comprehensive loss, which includes non-cash non-cash Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 Research and development $ 368 $ 201 $ 1,099 $ 279 General and administrative 1,127 540 2,292 776 $ 1,495 $ 741 $ 3,391 $ 1,055

Organization and Description _2

Organization and Description of Business - Additional Information (Detail)	Aug. 01, 2017USD ($)$ / sharesshares	Jul. 20, 2017	Sep. 30, 2018USD ($)$ / sharesshares	Sep. 30, 2017USD ($)	Aug. 31, 2018USD ($)	Aug. 03, 2018USD ($)	Jun. 29, 2018USD ($)	Dec. 31, 2017$ / sharesshares
Organization And Description Of Business [Line Items]
Reverse stock split		0.17036
Net proceeds from initial public offering	$ 66,400,000			$ 66,378,000
Common stock, shares outstanding | shares			21,100,000
Common stock, shares authorized | shares	300,000,000		300,000,000					300,000,000
Common stock, par value | $ / shares	$ 0.0001		$ 0.0001					$ 0.0001
Preferred stock, shares authorized | shares	10,000,000		10,000,000					10,000,000
Preferred stock, par value | $ / shares	$ 0.0001		$ 0.0001					$ 0.0001
Principal outstanding			$ 30,000,000
Term Loan [Member] | Silicon Valley Bank [Member]
Organization And Description Of Business [Line Items]
Debt instrument aggregate principal amount							$ 40,000,000
Debt instrument remains available for borrowing							10,000,000
Principal outstanding							$ 30,000,000
Common Stock [Member]
Organization And Description Of Business [Line Items]
Convertible preferred stock, as converted into common stock | shares	12,800,000		12,800,000
Initial Public Offering [Member] | Common Stock [Member]
Organization And Description Of Business [Line Items]
Common stock issued through initial public offering, shares | shares	4,983,333
Public offering price per share | $ / shares	$ 15
Over-Allotment Option [Member] | Common Stock [Member]
Organization And Description Of Business [Line Items]
Common stock issued through initial public offering, shares | shares	650,000
ATM Offering Program [Member]
Organization And Description Of Business [Line Items]
Common stock issued through initial public offering, shares | shares			340,307
Proceeds from issuance of common stock, net			$ 5,000,000
Issuance of common stock, commissions paid			200,000
Issuance of common stock, other offering expenses			$ 400,000
ATM Offering Program [Member] | Cowen [Member]
Organization And Description Of Business [Line Items]
Amount which can be raised under ATM offering program					$ 75,000,000	$ 75,000,000
Shelf Registration Statement [Member] | Maximum [Member]
Organization And Description Of Business [Line Items]
Aggregate registered amount						$ 250,000,000

Liquidity Risks - Additional In

Liquidity Risks - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Liquidity Risks [Line Items]
Accumulated deficit	$ (140,033,000)	$ (140,033,000)		$ (140,033,000)		$ (85,897,000)
Net losses		(16,794,000)	$ (16,391,000)	(54,136,000)	$ (39,356,000)
Net cash used in operating activities				$ (46,094,000)	$ (26,931,000)
Substantial doubt about going concern, within one year				false
Cash and cash equivalents	63,975,000	$ 63,975,000		$ 63,975,000		$ 74,467,000
Silicon Valley Bank [Member] | Minimum [Member]
Liquidity Risks [Line Items]
Cash security required if positive clinical trial data is not received	15,000,000
Equity financing requirement	50,000,000
Silicon Valley Bank [Member] | Term Loan [Member]
Liquidity Risks [Line Items]
Term loan, covenant description				If the Company fails to receive positive Phase 2b data for SNA-120 by March 31, 2019, the Company must immediately cash secure at least the lesser of the outstanding principal balance of the term loans or $15.0 million, until such time that the Company raises at least $50.0 million in equity financing and has received positive Phase 1 data for SNA-125 in atopic dermatitis on or prior to April 30, 2019.
Silicon Valley Bank [Member] | Term Loan [Member] | Minimum [Member]
Liquidity Risks [Line Items]
Cash security required if positive clinical trial data is not received	15,000,000
Equity financing requirement	$ 50,000,000

Significant Accounting Polici_3

Significant Accounting Policies - Additional Information (Detail) shares in Millions	Dec. 31, 2017USD ($)	Sep. 30, 2018USD ($)Segmentshares	Sep. 30, 2017shares	Dec. 31, 2017USD ($)
Significant Accounting Policies [Line Items]
Number of operating segment | Segment		1
Number of reportable segment | Segment		1
Impairments charge		$ 0		$ 0
Impairments of intangible assets		0		0
Impairment of goodwill		0		0
Accrued clinical trial costs	$ 900,000	$ 5,000,000		900,000
Prepaid clinical trial costs	1,100,000
Shares excluded from calculation of earnings per share | shares		2.6	1.7
Revenue		$ 0
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of asset		3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of asset		5 years
Standby Letters of Credit [Member]
Significant Accounting Policies [Line Items]
Cash collateral for borrowed securities	$ 200,000	$ 200,000		$ 200,000

Creabilis Acquisition - Additio

Creabilis Acquisition - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Oct. 19, 2017	Sep. 30, 2018	Dec. 31, 2017	Dec. 06, 2016
Business Acquisition [Line Items]
Deferred tax liability		$ 10,622	$ 10,964
Goodwill		11,114	11,472
Creabilis [Member]
Business Acquisition [Line Items]
Payment for contingent consideration liability	$ 5,000
Acquired tangible assets, deferred tax liability				$ 9,400
Deferred tax liability		10,600	11,000
Translation adjustments of deferred tax liability		400
Goodwill		11,100	11,500	9,800
Translation adjustments of goodwill		$ 400
Creabilis [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Business combination shares issued	201,268
Business combination shares issued, value	$ 4,200
Share price per share	$ 20.86
Maximum [Member] | Creabilis [Member]
Business Acquisition [Line Items]
Royalty percentage		1.00%
Contingent Consideration [Member] | Creabilis [Member]
Business Acquisition [Line Items]
Fair value of contingent consideration in acquisition		$ 24,500	$ 22,900
Net Sales Thresholds and One Time Royalties [Member] | Creabilis [Member]
Business Acquisition [Line Items]
Business combination, aggregate contingent consideration		$ 80,000
Development and Approval Milestones [Member] | Creabilis [Member]
Business Acquisition [Line Items]
Business combination, aggregate contingent consideration				$ 58,000

Identifiable Intangible Assets

Identifiable Intangible Assets - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 06, 2016
Indefinite-lived Intangible Assets [Line Items]
In-process research and development intangible assets, carrying value	$ 46,112	$ 47,597
In-process Research and Development Intangible Assets [Member]
Indefinite-lived Intangible Assets [Line Items]
Identifiable intangible assets, initial value recorded			$ 42,300

Property and Equipment - Compon

Property and Equipment - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 690	$ 665
Less accumulated depreciation	(345)	(233)
Property and equipment, net	$ 345	$ 432
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (in years)	5 years	5 years
Property, plant and equipment, gross	$ 307	$ 307
Computer Hardware [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (in years)	3 years	3 years
Property, plant and equipment, gross	$ 136	$ 116
Capital Lease Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (in years)	3 years	3 years
Property, plant and equipment, gross	$ 46	$ 46
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (in years)	5 years	5 years
Property, plant and equipment, gross	$ 87	$ 91
Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (in years)	3 years	3 years
Property, plant and equipment, gross	$ 9
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 105	$ 105

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 39,000	$ 27,000	$ 117,000	$ 80,000

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Level 1 [Member]
Assets:
Cash and cash equivalents	$ 63,975	$ 74,467
Total	63,975	74,467
Level 3 [Member]
Liabilities:
Financial liabilities fair value	25,188	26,185
Success Payment Liabilities [Member] | Level 3 [Member]
Liabilities:
Financial liabilities fair value	688	3,285
Contingent Consideration [Member] | Level 3 [Member]
Liabilities:
Financial liabilities fair value	$ 24,500	$ 22,900

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Changes in Fair Value of Level 3 Financial Liabilities (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Success Payment Liabilities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 3,285
Change in fair value due to remeasurement	(2,597)
Ending balance	688
Contingent Consideration [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	22,900
Change in fair value due to remeasurement	1,600
Ending balance	$ 24,500

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	Oct. 19, 2017USD ($)shares	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value transfer amount				$ 0		$ 0
Creabilis [Member] | Discount Rate [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input		0.051		0.051
Creabilis [Member] | Discount Rate [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input		0.167		0.167
Creabilis [Member] | Probability of Multiple Scenarios [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input		0.001		0.001
Creabilis [Member] | Probability of Multiple Scenarios [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input		0.250		0.250
In-process Research and Development Intangible Assets [Member] | Creabilis [Member] | Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Intangible asset measurement input		0.205		0.205
In-process Research and Development Intangible Assets [Member] | Creabilis [Member] | Probability of Multiple Scenarios [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Intangible asset measurement input		0.001		0.001
In-process Research and Development Intangible Assets [Member] | Creabilis [Member] | Probability of Multiple Scenarios [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Intangible asset measurement input		0.225		0.225
Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value		$ 700,000	$ 1,100,000	$ 1,600,000	$ 3,200,000
Fair value of contingent payment settled in stock				4,200,000
Change in fair value due to re-measurement recorded income (expense)				(1,600,000)
Contingent Consideration [Member] | Creabilis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent milestone payment	$ 5,000,000
Offsets of the contingent consideration	$ 300,000
Success Payment Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value due to re-measurement recorded income (expense)				2,597,000
Other Expense [Member] | Success Payment Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value due to re-measurement recorded income (expense)			$ 2,500,000		$ 4,500,000
Other Income [Member] | Success Payment Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value due to re-measurement recorded income (expense)		$ 200,000		$ 2,600,000
Common Stock [Member] | Creabilis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business combination shares issued | shares	201,268

Convertible Notes - Additional

Convertible Notes - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Apr. 30, 2017	Jan. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Conversion [Line Items]
Accrued interest expense				$ 330,000	$ 34,000
Series B Bridge Notes [Member]
Debt Conversion [Line Items]
Debt instrument face amount	$ 3,900,000	$ 3,900,000
Debt instrument face amount Debt instrument interest percentage		6.00%
Debt instrument maturity date		Jan. 27, 2018
Debt conversion discount percentage		15.00%
Debt discount		$ 700,000
Series B Bridge Notes [Member] | Series B Convertible Preferred Stock [Member]
Debt Conversion [Line Items]
Number of shares issued on conversion of debt	0.4
Per share price of shares issued on conversion of debt	$ 10.40
Amortization of debt discount			$ 0		700,000
Accrued interest payable	$ 34,000
Accrued interest expense			$ 0		$ 34,000

Success Payment Liability - Add

Success Payment Liability - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Success Payment Liability [Line Items]
Success payment liability threshold one			$ 10,000,000
Success payment liability threshold one per share			$ 53.71
Success payment liability threshold two			$ 35,000,000
Success payment liability threshold two per share			$ 71.61
Success payment liability threshold maximum			$ 60,000,000
Success payment liability threshold maximum per share			$ 107.42
Success payment liability	$ 60,000,000		$ 60,000,000
Success Payment Liabilities [Member]
Success Payment Liability [Line Items]
Change in fair value due to re-measurement recorded income (expense)			2,597,000
Other Income [Member] | Success Payment Liabilities [Member]
Success Payment Liability [Line Items]
Change in fair value due to re-measurement recorded income (expense)	$ 200,000		$ 2,600,000
Other Expense [Member] | Success Payment Liabilities [Member]
Success Payment Liability [Line Items]
Change in fair value due to re-measurement recorded income (expense)		$ 2,500,000		$ 4,500,000

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	Sep. 30, 2018	Jun. 29, 2018	Sep. 30, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Principal outstanding	$ 30,000,000		$ 30,000,000	$ 30,000,000
Final maturity payment	1,650,000		1,650,000	$ 1,650,000
Silicon Valley Bank [Member] | Minimum [Member]
Debt Instrument [Line Items]
Cash security required if positive clinical trial data is not received	15,000,000
Equity financing requirement	50,000,000
Silicon Valley Bank [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Term loan, aggregate principal amount		$ 40,000,000
Term loan, first credit extension		$ 30,000,000
Term loan, initiation date				Jun. 29, 2018
Term loan, maturity date				Jul. 1, 2023
Term loan, interest-only period end date				Jul. 31, 2020
Term loan, covenant description				If the Company fails to receive positive Phase 2b data for SNA-120 by March 31, 2019, the Company must immediately cash secure at least the lesser of the outstanding principal balance of the term loans or $15.0 million, until such time that the Company raises at least $50.0 million in equity financing and has received positive Phase 1 data for SNA-125 in atopic dermatitis on or prior to April 30, 2019.
Term loan, remaining principal amount	10,000,000		10,000,000	$ 10,000,000
Term loan, borrowing capacity description				An additional term loan, of a principal amount of up to $10.0 million, can be drawn down at the Company’s option during the period of time commencing on the date the Company has received positive pivotal data for SNA-001 and positive Phase 2b data for SNA-120 and ending on the earlier of June 30, 2019 or the occurrence of an event of default that continues; provided, however, that such period of time shall not commence if, on the date of the occurrence of either Company’s receipt of positive pivotal data for SNA-001 or receipt of positive Phase 2b data for SNA-120, an event of default has occurred and is continuing.
Final payment fee percentage		5.50%
Principal outstanding	30,000,000		30,000,000	$ 30,000,000
Debt issuance costs	100,000		100,000	100,000
Interest expense			0.7	0.7
Final maturity payment	$ 1,700,000		$ 1,700,000	$ 1,700,000
Effective interest rate	9.42%		9.42%	9.42%
Silicon Valley Bank [Member] | Term Loan [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate percentage	7.25%		7.25%	7.25%
Cash security required if positive clinical trial data is not received	$ 15,000,000
Equity financing requirement	$ 50,000,000
Prepayment fee percentage		1.00%
Silicon Valley Bank [Member] | Term Loan [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate percentage	7.75%		7.75%	7.75%
Prepayment fee percentage		3.00%
Silicon Valley Bank [Member] | Term Loan [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Interest rate percentage, spread on variable rate	2.50%

Long-Term Debt - Schedule of Fu

Long-Term Debt - Schedule of Future Principal Payments for Long-term Debt (Detail) $ in Thousands	Sep. 30, 2018USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2,018	$ 0
2,019	0
2,020	4,515
2,021	9,599
2,022	10,382
2,023	5,504
Total principal payments	30,000
Final fee due at maturity in 2023	1,650
Total principal and final fee payments	31,650
Unamortized discount and debt issuance costs	(1,667)
Long-term debt, net	$ 29,983

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2016	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)ft²
Other Commitments [Line Items]
Rent expense		$ 100,000	$ 200,000	$ 300,000	$ 400,000
Directors And Officers Liability Insurance [Member]
Other Commitments [Line Items]
Indemnification claims		0		0
Accrued indemnification liabilities		0		0		$ 0
Lease Arrangement - Office Space in Carlsbad, California [Member]
Other Commitments [Line Items]
Operating lease obligation period	40 months
Operating lease expiry date	Feb. 29, 2020
Amended Lease Arrangement - Office Space in Westlake Village, California [Member]
Other Commitments [Line Items]
Operating lease renewal option term			3 years		3 years
Additional area of office space leased | ft²							5,973
Maximum [Member] | Amended Lease Arrangement - Office Space in Westlake Village, California [Member]
Other Commitments [Line Items]
Additional allowance for leasehold improvements							$ 100,000
Minimum [Member]
Other Commitments [Line Items]
Annual royalties		$ 50,000		$ 50,000

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($)	Sep. 30, 2018	May 17, 2018	Dec. 31, 2017	Jan. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	May 31, 2016
Related Party Transaction [Line Items]
Number of nonvested shares	400,000		500,000		400,000		400,000
Recognized compensation expense							$ 3,391,000	$ 1,055,000
Notes Receivable [Member]
Related Party Transaction [Line Items]
Principal balance and interest forgiven on promissory note									$ 1,300,000
Dr.Beddingfield [Member]
Related Party Transaction [Line Items]
Stock issued during period shares acquisitions		47,594
President and Chief Executive Officer [Member] | Stock Purchase Rights [Member]
Related Party Transaction [Line Items]
Number of shares granted under stock purchase rights				600,000
Purchase price of common stock				$ 2.35
Fair value of the option granted				$ 1,300,000
Recognized compensation expense					$ 200,000	$ 36,000	$ 400,000	$ 100,000
Number of shares vested	200,000		200,000				85,000
President and Chief Executive Officer [Member] | Stock Purchase Rights [Member] | Notes Receivable [Member]
Related Party Transaction [Line Items]
Principal amount of promissory note										$ 1,300,000
Interest rate percentage										1.43%
President and Chief Executive Officer [Member] | Share-based Compensation Award, Tranche One [Member] | Stock Purchase Rights [Member]
Related Party Transaction [Line Items]
Number of nonvested shares				500,000
Vesting condition description							25% of the shares vest on the first anniversary of the grant, and 1/48th of the shares vest monthly thereafter, subject to Dr. Beddingfield continuing to provide services to the Company through each such vesting date.
Percentage of shares vest on first anniversary				25.00%
Percentage of shares vest monthly thereafter				2.00%
President and Chief Executive Officer [Member] | Share-based Compensation Award, Tranche Two [Member] | Stock Purchase Rights [Member]
Related Party Transaction [Line Items]
Number of nonvested shares				49,000
Vesting condition description							50% of the shares vest on the first date the volume-weighted average trading price of the Company's common stock equals or exceeds $71.03 per share, and 1/24th of the shares vest monthly thereafter, subject to Dr. Beddingfield continuing to provide services to the Company through each such vesting date.
Percentage of shares vest monthly thereafter				4.00%
Percentage of shares vesting				50.00%
Minimum vesting share price				$ 71.03
President and Chief Executive Officer [Member] | Share-based Compensation Award, Tranche Three [Member] | Stock Purchase Rights [Member]
Related Party Transaction [Line Items]
Number of nonvested shares				49,000
Vesting condition description							50% of the shares vest upon achievement of a milestone related to clinical development, and 1/24th of the shares vest monthly thereafter, subject to Dr. Beddingfield continuing to provide services to the Company through each such vesting date.
Percentage of shares vest monthly thereafter				4.00%
Percentage of shares vesting				50.00%
Todd Harris [Member]
Related Party Transaction [Line Items]
Success payments liability related payouts percentage							25.22%

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - $ / shares	Sep. 30, 2018	Dec. 31, 2017	Aug. 01, 2017
Class of Stock Disclosures [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000	300,000,000
Preferred stock, shares authorized	10,000,000	10,000,000	10,000,000
Preferred stock, par value	$ 0.0001	$ 0.0001	$ 0.0001

Stockholders' Equity - Common S

Stockholders' Equity - Common Stock - Additional Information (Detail) - shares	Sep. 30, 2018	Jan. 01, 2018
Class of Stock [Line Items]
Shares reserved for future issuance	3,670,000
Two Thousand Seventeen Plan [Member]
Class of Stock [Line Items]
Shares reserved for future issuance		829,606
2017 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Shares reserved for future issuance		207,401

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Shares of Common Stock Reserved for Future Issuance (Detail) - shares	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock awards outstanding	2,171,000		1,159,000
Total shares of common stock reserved for future issuance	3,670,000
Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock awards available for grant	376,000
Two Thousand Seventeen Plan [Member]
Class of Stock [Line Items]
Common stock awards available for grant	1,123,000
Total shares of common stock reserved for future issuance		829,606

Stockholders' Equity - Converti

Stockholders' Equity - Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Apr. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Aug. 01, 2017
Class of Stock [Line Items]
Preferred stock, shares outstanding		0	0
Common Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock, as converted into common stock		12,800,000		12,800,000
Series B Convertible Preferred Stock Financing [Member]
Class of Stock [Line Items]
Aggregate proceeds from issuance of preferred stock	$ 36.5
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding		0
Series B Convertible Preferred Stock [Member] | Series B Convertible Preferred Stock Financing [Member]
Class of Stock [Line Items]
Per share price of shares issued on conversion of debt	$ 12.24
Number of shares issued on conversion of debt	3,000,000
Series B Convertible Preferred Stock [Member] | Series B Bridge Notes [Member]
Class of Stock [Line Items]
Per share price of shares issued on conversion of debt	$ 10.40
Number of shares issued on conversion of debt	400,000

Stockholders' Equity - ATM Offe

Stockholders' Equity - ATM Offering Program - Additional Information (Detail) - ATM Offering Program [Member] - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Aug. 03, 2018
Class of Stock [Line Items]
Issuance of common stock	340,307
Proceeds from issuance of common stock, net	$ 5
Issuance of common stock, commissions paid	0.2
Issuance of common stock, other offering expenses	$ 0.4
Cowen [Member]
Class of Stock [Line Items]
Amount which can be raised under ATM offering program		$ 75	$ 75
Stock issuance costs under sales agreement as a percent of gross proceeds		3.00%

Stockholders' Equity - Stock Aw

Stockholders' Equity - Stock Awards and Stock-Based Compensation - Additional Information (Detail) - USD ($)	Aug. 01, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Jul. 31, 2017
Class of Stock [Line Items]
Common stock available for issuance under plan		3,670,000		3,670,000
Number of shares granted to non-employee				1,235,000
Unrecognized compensation expense related to unvested employee stock award		$ 15,000,000		$ 15,000,000
Unrecognized compensation expense related to unvested employee stock award, recognized period				3 years 4 months 20 days
Weighted-average grant date fair value of stock options granted				$ 10.55
Weighted-average remaining contractual life of options outstanding				9 years
Fair value of shares vested				$ 2,200,000
Stock compensation expense related to non-employee option grants		$ 3,000	$ 400,000	$ 300,000	$ 500,000
Unvested shares		400,000		400,000			500,000
Early exercise liability related to unvested shares		$ 300,000		$ 300,000			$ 500,000
Early exercise liability related to unvested shares, current		213,000		213,000			231,000
Early exercise liability related to unvested shares, non-current		$ 98,000		$ 98,000			$ 258,000
Stock repurchase				7,775
Shares repurchased, price per share		$ 2.35		$ 2.35
Recognized compensation expense		$ 1,495,000	$ 741,000	$ 3,391,000	$ 1,055,000
2017 Incentive Award Plan [Member]
Class of Stock [Line Items]
Common stock available for issuance under plan								1,200,000
Incentive award plan, description				The 2017 Plan serves as the successor incentive award plan to the Company’s 2010 Equity Incentive Plan, or the 2010 Plan, and has 1.2 million shares of common stock available for issuance pursuant to a variety of stock-based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards and other stock-based awards, plus shares of common stock that were reserved for issuance pursuant to future awards under the 2010 Plan at the time the 2017 Plan became effective, plus shares represented by awards outstanding under the 2010 Plan that are forfeited or lapse unexercised and which following the effective date of the 2017 Plan are not issued under the 2010 Plan. In addition, the 2017 Plan reserve increased on January 1, 2018 and will each subsequent anniversary through 2027, by an amount equal to the lesser of (a) four percent of the shares of stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (b) such smaller number of shares of stock as determined by our board of directors; provided, however, that no more than 12.0 million shares of stock may be issued upon the exercise of incentive stock options.
2010 Equity Incentive Plan [Member]
Class of Stock [Line Items]
Stock awards expiration period				10 years
Stock awards vesting period				4 years
2017 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock available for issuance under plan						207,401
Number of authorized shares of common stock under ESPP	198,883
Common stock shares outstanding percentage	1.00%
Employee stock purchase price closing trading price	85.00%
Employee stock purchase plan, description				The maximum number of the Company’s common stock which will be authorized for sale under the ESPP is equal to the sum of (a) 198,883 shares of common stock and (b) an annual increase on the first day of each year beginning in 2018 and ending in 2027, equal to the lesser of (i) 1% of the shares of common stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the Company’s board of directors; provided, however, no more than 3.0 million shares of the Company’s common stock may be issued under the ESPP.
Recognized compensation expense		$ 100,000		$ 200,000
Number of shares issued from ESPP		30,486
Non-employee [Member]
Class of Stock [Line Items]
Contractual term				10 years
Number of shares granted to non-employee				0
Maximum [Member]
Class of Stock [Line Items]
Fair value of a common stock percentage				110.00%
Maximum [Member] | 2017 Incentive Award Plan [Member]
Class of Stock [Line Items]
Number of authorized shares of common stock under ESPP								12,000,000
Maximum [Member] | 2010 Equity Incentive Plan [Member]
Class of Stock [Line Items]
Stock awards exercisable period				5 years
Maximum [Member] | 2017 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Number of authorized shares of common stock under ESPP	3,000,000
Minimum [Member]
Class of Stock [Line Items]
Common stock voting rights percentage				10.00%
Minimum [Member] | 2017 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Employee stock purchase price lower of the closing trading price	85.00%

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Assumptions Used for Grant Date Fair Value (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected stock price volatility, minimum	65.40%	59.13%
Expected stock price volatility, maximum	74.21%	64.09%
Expected dividend yield	0.00%	0.00%
Risk-free interest rate, minimum	2.55%	1.77%
Risk-free interest rate, maximum	2.96%	2.60%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Expected term (in years)	5 years	5 years 3 months 18 days
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years 1 month 6 days	10 years

Stockholders' Equity - Summar_3

Stockholders' Equity - Summary of Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)$ / sharesshares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of Shares, Outstanding, beginning balance | shares	1,159
Number of Shares, Granted | shares	1,235
Number of Shares, Exercised | shares	(37)
Number of Shares, Cancelled | shares	(186)
Number of Shares, Outstanding, ending balance | shares	2,171
Number of Shares, Exercisable, ending balance | shares	455
Weighted Average Exercise Price Per Share, Outstanding, beginning balance	$ 8.19
Weighted Average Exercise Price Per Share, Granted	16.49
Weighted Average Exercise Price Per Share, Exercised	11.04
Weighted Average Exercise Price Per Share, Cancelled	11.74
Weighted Average Exercise Price Per Share, Outstanding, ending balance	12.56
Weighted Average Exercise Price Per Share, Exercisable, ending balance	$ 6.53
Aggregate Intrinsic Value, Outstanding, beginning balance | $	$ 11,795
Aggregate Intrinsic Value, Granted	$ 0
Aggregate Intrinsic Value, Exercised | $	$ 0
Aggregate Intrinsic Value, Cancelled | $	0
Aggregate Intrinsic Value, Outstanding, ending balance | $	7,497
Aggregate Intrinsic Value, Exercisable, ending balance | $	$ 3,925

Stockholders' Equity - Summar_4

Stockholders' Equity - Summary of Non-cash Stock-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total non-cash stock-based compensation expense	$ 1,495	$ 741	$ 3,391	$ 1,055
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total non-cash stock-based compensation expense	368	201	1,099	279
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total non-cash stock-based compensation expense	$ 1,127	$ 540	$ 2,292	$ 776

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax benefit	$ 0	$ 85,000	$ 0	$ 212,000
Accrual for unrecognized tax benefits and interest	$ 0		$ 0
Statutory tax rate			21.00%		35.00%