PhaseBio Pharmaceuticals (PHASQ) 10-Q 29 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 26, 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	PHAS
Entity Registrant Name	PhaseBio Pharmaceuticals Inc
Entity Central Index Key	1,169,245
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock Shares Outstanding		24,498,275

CONDENSED BALANCE SHEETS (unaud

CONDENSED BALANCE SHEETS (unaudited) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 24,341,000	$ 13,406,000
Other receivable	411,000
Prepaid expenses and other assets	216,000	340,000
Total current assets	24,968,000	13,746,000
Property and equipment, net	251,000	302,000
Deferred offering costs	2,218,000
Other assets	51,000	51,000
Total assets	27,488,000	14,099,000
Current liabilities:
Convertible promissory notes, net of discount		12,095,000
Derivative liability		3,028,000
Current portion of long-term debt	3,125,000	761,000
Accounts payable	1,576,000	430,000
Accrued expenses	2,987,000	1,281,000
Total current liabilities	7,688,000	17,595,000
Preferred stock warrant liability	5,990,000	1,656,000
Deferred rent		5,000
Long-term debt	4,375,000	2,625,000
Total liabilities	18,053,000	21,881,000
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock, $0.001 par value; 18,292,696 and 13,321,350 shares authorized at September 30, 2018 and December 31, 2017, respectively; 13,200,115 and 9,131,999 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively; liquidation preference of $129,071,361 and $89,776,055 at September 30, 2018 and December 31, 2017, respectively	125,609,295	89,633,543
Stockholders’ deficit:
Common stock, $0.001 par value; 20,789,273 shares authorized; 795,640 shares issued and 765,673 shares outstanding at September 30, 2018; 775,755 shares issued and 745,788 shares outstanding at December 31, 2017	1,000	1,000
Treasury stock, at cost, 29,967 shares as of September 30, 2018 and December 31, 2017	(24,000)	(24,000)
Additional paid-in capital	1,892,000	1,672,000
Accumulated deficit	(118,043,000)	(99,065,000)
Total stockholders’ deficit	(116,174,000)	(97,416,000)
Total liabilities, redeemable convertible preferred stock, and stockholders' deficit	$ 27,488,000	$ 14,099,000

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) (unaudited) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Redeemable convertible preferred stock, par value	$ 0.001	$ 0.001
Redeemable convertible preferred stock, shares authorized	18,292,696	13,321,350
Redeemable convertible preferred stock, shares issued	13,200,115	9,131,999
Redeemable convertible preferred stock, shares outstanding	13,200,115	9,131,999
Redeemable convertible preferred stock, liquidation preference	$ 129,071,361	$ 89,776,055
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	20,789,273	20,789,273
Common stock, shares issued	795,640	775,755
Common stock, shares outstanding	765,673	745,788
Treasury stock, shares	29,967	29,967

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Grant revenues	$ 411		$ 411
Type of Revenue [Extensible List]	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember
Operating expenses:
Research and development	$ 4,354	$ 1,330	$ 9,779	$ 4,387
General and administrative	1,056	565	2,616	1,700
Total operating expenses	5,410	1,895	12,395	6,087
Loss from operations	(4,999)	(1,895)	(11,984)	(6,087)
Other income (expense):
Interest income	60	14	132	29
Interest expense	(890)	(618)	(3,741)	(1,605)
Change in fair value of warrant liability	(1,713)	5	(2,709)	130
Change in fair value of derivative liability	(359)	(66)	(676)	(179)
Total other income (expense)	(2,902)	(665)	(6,994)	(1,625)
Net loss	$ (7,901)	$ (2,560)	$ (18,978)	$ (7,712)
Net loss per common share, basic and diluted	$ (10.45)	$ (3.44)	$ (25.33)	$ (10.38)
Weighted average common shares outstanding, basic and diluted	755,908	743,241	749,198	743,241

CONDENSED STATEMENTS OF REDEEMA

CONDENSED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' DEFICIT (unaudited) - 9 months ended Sep. 30, 2018 - USD ($)	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit	Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2017	$ 89,633,543					$ 89,634,000
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2017	9,131,999					9,131,999
Issuance of redeemable preferred stock						$ 14,890,000
Issuance of redeemable preferred stock, shares						1,842,959
Issuance of redeemable preferred stock upon conversion of promissory notes						$ 19,778,000
Issuance of redeemable preferred stock upon conversion of promissory notes, shares						2,080,209
Exercises of warrants						$ 1,212,000
Exercises of warrants, shares						144,948
Temporary equity, Accretion of redeemable preferred stock to redemption value						$ 95,000
Redeemable convertible preferred stock, ending balance at Sep. 30, 2018	$ 125,609,295					$ 125,609,000
Redeemable convertible preferred stock, ending balance, shares at Sep. 30, 2018	13,200,115					13,200,115
Beginning balance at Dec. 31, 2017	$ (97,416,000)	$ 1,000	$ (24,000)	$ 1,672,000	$ (99,065,000)
Beginning balance, shares at Dec. 31, 2017	745,788	775,755	(29,967)
Exercises of stock options	$ 53,000			53,000
Exercises of stock options, shares		19,885
Stock-based compensation	262,000			262,000
Accretion of redeemable preferred stock to redemption value	(95,000)			(95,000)
Net loss	(18,978,000)				(18,978,000)
Ending balance at Sep. 30, 2018	$ (116,174,000)	$ 1,000	$ (24,000)	$ 1,892,000	$ (118,043,000)
Ending balance, shares at Sep. 30, 2018	765,673	795,640	(29,967)

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net loss	$ (18,978)	$ (7,712)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	82	68
Stock-based compensation	262	58
Non-cash interest expense	3,580	1,605
Change in fair value warrant liability	2,709	(130)
Change in fair value derivative liability	676	179
Changes in operating assets and liabilities:
Other receivable	(411)
Prepaid expenses and other assets	130	(67)
Accounts payable	558	(39)
Accrued expenses	1,182	204
Deferred rent	(5)	(1)
Net cash used in operating activities	(10,215)	(5,835)
Investing activities
Purchases of property and equipment	(31)	(204)
Net cash used in investing activities	(31)	(204)
Financing activities
Proceeds from issuance of redeemable convertible preferred stock, net	17,712
Proceeds from term loan, net	3,995
Payments of offering costs	(595)
Proceeds from exercise of stock options	53
Proceeds from exercise of warrants	16
Proceeds from convertible promissory notes, net		6,601
Net cash provided by financing activities	21,181	6,601
Net increase in cash and cash equivalents	10,935	562
Cash and cash equivalents at the beginning of the period	13,406	3,715
Cash and cash equivalents at the end of the period	24,341	4,277
Supplemental disclosure for cash flow
Cash paid for interest	161
Supplemental disclosure of cash flow information
Conversion of convertible promissory notes into redeemable convertible preferred stock	19,778
Issuance of warrants in conjunction with debt	2,822	1,046
Deferred offering costs included in accounts payable and accrued expenses	1,623
Warrant liability converted to redeemable convertible preferred stock upon the exercise of warrants	1,196
Accretion of redeemable convertible preferred stock	$ 95	50
Issuance of derivative in conjunction with debt		$ 1,309

Organization and Description of

Organization and Description of Business	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Description of Business	1. Organization and Description of Business Description of Business PhaseBio Pharmaceuticals, Inc. (the “Company”) was incorporated as a Delaware corporation on January 10, 2002. The Company is a clinical-stage biopharmaceutical company focused on the development and commercialization of novel therapies to treat orphan diseases, with an initial focus on cardiopulmonary indications. The Company’s lead product candidate, PB2452, is a first-in-class reversal agent for the antiplatelet drug ticagrelor, which the company is developing for the treatment of patients on ticagrelor who are experiencing a major bleeding event or those who require urgent surgery. The Company recently completed a Phase 1 clinical trial of PB2452 in healthy subjects. The Company’s second product candidate, PB1046, is a once-weekly fusion protein currently in a Phase 2b clinical trial for the treatment of pulmonary arterial hypertension. PB1046 utilizes the Company’s proprietary half-life extending elastin-like polypeptide (“ELP”), technology, which also serves as the engine for future product pipeline Reverse Stock Split In October 2018, the Company effected a 11.0634-for-1 reverse split of its outstanding common stock and redeemable convertible preferred stock. All share and per share amounts, including stock options, have been retroactively adjusted in these condensed financial statements for all periods presented to reflect the reverse stock split. Further, exercise prices of stock options have been retroactively adjusted in these condensed financial statements for all periods presented to reflect the reverse stock split. Liquidity The Company has experienced net losses and negative cash flows from operations since its inception and, as of September 30, 2018, had an accumulated deficit of $118.0 million. The Company expects to continue to incur net losses for at least the next several years. In August 2018, the Company received $17.7 million in net proceeds from the sale of Series D redeemable convertible preferred stock (“Series D”). Additionally, in August 2018, the Company borrowed $2.0 million under its term loan with Silicon Valley Bank. On October 22, 2018, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the issuance and sale of 9,864,666 shares of common stock at a public offering price of $5.00 per share, generating net proceeds of approximately $43.0 million after deducting underwriting discounts and commissions and other offering costs. Upon closing of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock were converted into an aggregate of 13,225,114 shares of common stock. The Company expects to use the net proceeds from the IPO, together with its existing cash, to advance PB2452 and PB1046, to fund development of its ELP technology and preclinical programs and for working capital and general corporate purposes. Management believes that its cash and cash equivalents, including the net proceeds from the IPO, are sufficient to fund the Company’s operating expenses and capital requirements into the second quarter of 2020. Basis of Presentation The accompanying condensed financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial reporting and the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and note disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to those rules and regulations. All adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the accompanying condensed financial statements. Although these interim financial statements do not include all of the information and footnotes required for complete annual financial statements, management believes the disclosures are adequate to make the information presented not misleading. The unaudited interim results of operations and cash flows for the three and nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the full year. The unaudited interim condensed financial statements and footnotes should be read in conjunction with the audited financial statements and footnotes included in the Company’s IPO prospectus filed with the SEC on October 17, 2018, wherein a more complete discussion of significant accounting policies and certain other information can be found. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) promulgated by the Financial Accounting Standards Board (“FASB”). The Company manages its operations as a single reportable segment for the purposes of assessing performance and making operating decisions.

Significant Accounting Policies

Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Use of Estimates The preparation of the Company’s condensed financial statements requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in the Company’s condensed financial statements and accompanying notes. The most significant estimates in the Company’s condensed financial statements relate to the valuation of redeemable convertible preferred stock warrants, equity awards and clinical trial accruals. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results could differ materially from those estimates and assumptions. Concentrations of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains certain deposits in federally insured financial institutions in excess of federally insured limits. The Company could experience losses on the money market funds in the future. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity from the date of purchase of three months or less to be cash equivalents. Cash and cash equivalents include cash in readily available checking and money market Restricted Cash The Company had restricted cash of $20,000 as of September 30, 2018 and December 31, 2017 , Fair Value of Financial Instruments The carrying amounts of accounts receivable, prepaid expenses and other current assets, accounts payable and accrued expenses are reasonable estimates of their fair value because of the short maturity of these items. The carrying amount of the convertible promissory notes approximates fair value because the interest rates on these instruments are reflective of rates that the Company could obtain on unaffiliated third party debt with similar terms and conditions. Based on the borrowing rates currently available to the Company for loans with similar terms, the Company believes the fair value of the term loan approximates its carrying value (see Note Property and Equipment Property and equipment are recorded at cost and depreciated over the estimated useful lives of the assets (three to five years) using the straight-line method. Leasehold improvements are amortized over the shorter of their estimated useful lives or the lease term. Long-Lived Assets The Company regularly reviews the carrying value and estimated lives of all of its long-lived assets, including property and equipment to determine whether indicators of impairment may exist which warrant adjustments to carrying values or estimated useful lives. The determinants used for this evaluation include management’s estimate of the asset’s ability to generate net positive cash flow in future periods as well as the strategic significance of the assets to the Company’s business objective. Should an impairment exist, the impairment loss would be measured based on the extent that the estimated fair value is less than its carrying value. The Company has not recognized any impairment Preferred Stock Warrant Liability The Company has issued freestanding warrants to purchase shares of its redeemable convertible preferred stock. Since the underlying redeemable convertible preferred stock is classified outside of permanent equity, these warrants are classified as liabilities in the accompanying condensed balance sheet. Warrants classified as liabilities are recorded at their estimated fair value on the date of issuance and are revalued at each subsequent balance sheet date, with fair value changes recognized as increases or reductions to other income (expense) in the accompanying condensed statements of operations. The Company estimates the fair value of these warrants using the Black-Scholes option pricing Preclinical and Clinical Trial Accruals The Company accrues and expenses preclinical studies and clinical trial activities performed by third parties based upon estimates of the proportion of work completed over the life of the individual trial and subject enrollment rates in accordance with agreements with clinical research organizations and clinical trial sites. The Company determines the estimates by reviewing contracts, vendor agreements and purchase orders, and through discussions with internal clinical personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services. However, actual costs and timing of clinical trials are highly uncertain, subject to risks and may change depending upon a number of factors, including the Company’s clinical development Management makes estimates of the Company’s accrued expenses as of each balance sheet date in the Company’s condensed financial statements based on facts and circumstances known to the Company at that time. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. Nonrefundable advance payments for goods and services, including fees for process development or manufacturing and distribution of clinical supplies that will be used in future research and development activities, are deferred and recognized as expense in the period that the related goods are consumed or services are Research and Development Expenses Research and development costs are expensed as incurred. Stock-Based Compensation The Company measures and recognizes compensation expense for all stock-based compensation for employees and directors based on the estimated fair value at the date of grant, and to consultants based on the ongoing estimated fair value. Currently, the Company’s stock-based awards consist only of stock options; however, future grants under the Company’s equity compensation plan may also consist of shares of restricted stock, restricted stock units, stock appreciation rights, performance awards and performance units. The Company estimates the fair value of stock options using the Black-Scholes option-pricing model, which requires the use of estimates. The Company recognizes stock-based compensation cost for employees and directors for ratably vesting stock options on a straight-line basis over the requisite service period of the award. The fair value of options granted to consultants is estimated using the Black-Scholes option-pricing model and is re-measured at each reporting date with changes in fair value prior to vesting recognized as expense in the condensed consolidated statements of operations across the applicable vesting period. The Black-Scholes option-pricing model requires the input of subjective assumptions, including the risk-free interest rate, the fair value of the underlying common stock, the expected dividend yield of the Company’s common stock, the expected volatility of the price of the Company’s common stock, and the expected term of the option. These estimates involve inherent uncertainties and the application of management’s judgment. If factors change and different assumptions are used, the Company’s stock-based compensation expense could be materially different in the future. Income Taxes The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the condensed financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the condensed financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies, and results of recent operations. If management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. The Company records uncertain tax positions on the basis of a two-step process whereby (1) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits, if any, within income tax expense, and any accrued interest and penalties are included within the related tax liability line. Grant Revenue Grant revenues are derived from government grants that support the Company’s efforts on specific research projects. The Company has determined that the government agencies providing grants to the Company are not customers. The Company recognizes grant revenue when there is reasonable assurance of compliance with the conditions of the grant and reasonable assurance that the grant revenue will be received. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed by dividing the net loss by The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive. As of September 30, 2018 2017 Redeemable convertible preferred stock 13,200,115 9,131,999 Common stock options 1,315,625 1,084,243 Warrants to purchase redeemable convertible preferred stock 708,484 269,225 Total 15,224,224 10,485,467 Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases The Company will adopt ASU 2016-02 in 2019. The adoption of this guidance is expected to result in a significant increase in the total assets and liabilities reported on the condensed balance sheets. In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments (Topic 230) In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting classified share-based payment awards issued to nonemployees will be measured at the grant date, instead of the previous requirement to remeasure the awards through the performance completion date, (ii) for performance conditions, compensation cost associated with the award will be recognized when the achievement of the performance condition is probable, rather than upon achievement of the performance condition, and (iii) the current requirement to reassess the classification (equity or liability) for nonemployee awards upon vesting will be eliminated, except for awards in the form of convertible instruments. This new guidance will be effective for public companies for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. The Company does not believe the adoption of this guidance will have a material impact on its financial statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement - Disclosure Framework (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement

Fair Value Measurement

Fair Value Measurement	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurement	3. Fair Value Measurement Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a The Company classifies fair value measurements in one of the following three categories for disclosure purposes: Level 1: Quoted prices in active markets for identical assets or Level 2: Inputs other than Level 1 prices for similar assets or liabilities that are directly or indirectly observable in the marketplace. Level 3: Unobservable inputs which are supported by little or no market activity and values determined using pricing models, discounted cash flow methodologies, or similar techniques, as well as instruments for which the determination of fair value requires significant judgment or estimation. The Company’s cash equivalents are classified using Level 1 inputs within the fair value hierarchy because they are valued using quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. None of the Company’s non-financial assets or liabilities are recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented. The Company estimates the fair value of redeemable convertible preferred stock warrants at the time of issuance and subsequent remeasurement using the Black-Scholes option pricing model at each reporting date, based on the following inputs: the risk-free interest rates; the expected dividend rates; the remaining contractual life of the warrants; the fair value of the underlying stock; and the expected volatility of the price of the underlying common stock. The estimates are based, in part, on subjective assumptions and could differ materially in the future. Changes to these assumptions as well as the fair value of the Company’s stock on the reporting date can have a significant impact on the fair value of the redeemable convertible preferred stock warrant liability. The following table summarizes the Company’s assets and liabilities that require fair value measurements on a recurring basis and their respective input levels based on the fair value hierarchy (in thousands): Fair Value Measurements at Reporting Date Total Level 1 Level 2 Level 3 As of September 30, 2018: Assets Cash equivalents $ 23,103 $ 23,103 $ — $ — Liabilities Preferred stock warrant liability $ 5,990 $ — $ — $ 5,990 As of December 31, 2017: Assets Cash equivalents $ 12,472 $ 12,472 $ — $ — Liabilities Derivative liability $ 3,028 $ — $ — $ 3,028 Preferred stock warrant liability 1,656 — — 1,656 Total liabilities $ 4,684 $ — $ — $ 4,684 The following weighted-average assumptions were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option pricing model as of September 30, 2018 and December 31, 2017: September 30, December 31, 2018 2017 Expected volatility 68.7 % 68.0 % Risk-free interest rate 3.0 % 2.2 % Contractual term (in years) 5.7 5.9 Expected dividend yield — — The following estimated fair values per share of the Company’s underlying redeemable convertible preferred stock were used to determine the estimated fair value of the preferred stock warrant liability: September 30, December 31, 2018 2017 Series AA $ — $ 5.75 Series B $ 8.95 $ 3.76 Series C-1 $ 8.95 $ 3.76 The following tables present activity for the preferred stock warrant liability and the derivative liability measured at fair value using significant unobservable Level 3 inputs during the nine months ended September 30, 2018 (in thousands): Preferred Stock Warrant Liability Balance at December 31, 2017 $ 1,656 Issuance of warrants 2,822 Exercise of warrants (1,197 ) Changes in fair value reflected as change in fair value of warrant liability 2,709 Balance at September 30, 2018 $ 5,990 Derivative Liability Balance at December 31, 2017 $ 3,028 Changes in fair value reflected as change in fair value of derivative liability 676 Extinguishment of derivative upon conversion of convertible promissory notes (3,704 ) Balance at September 30, 2018 $ —

Property and Equipment

Property and Equipment	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	4. Property and Equipment The following table presents the composition of property and equipment, net as of September 30, 2018 and December 31, 2017 (in thousands): As of As of September 30, December 31, 2018 2017 Lab equipment $ 1,684 $ 1,681 Computer hardware, software and telephone 200 174 Furniture and fixtures 70 70 Leasehold improvements 24 22 1,978 1,947 Less accumulated depreciation (1,727 ) (1,645 ) Property and equipment, net $ 251 $ 302

Accrued Expenses

Accrued Expenses	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Accrued Expenses	5. Accrued Expenses The following table presents the composition of accrued expenses as of September 30, 2018 and December 31, 2017 (in thousands): As of As of September 30, December 31, 2018 2017 Accrued clinical and related costs $ 1,335 $ 197 Accrued stock offering costs 1,041 — Accrued compensation and related costs 261 346 Accrued interest 110 628 Accrued other 240 110 Accrued expenses $ 2,987 $ 1,281

Debt

Debt	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt	6. Debt Convertible Promissory Notes In January 2017 and October 2017, the Company issued $14.7 million of convertible promissory notes (the “2017 Notes”) to holders of Series C-1 redeemable convertible preferred stock (“Series C-1”). The 2017 Notes bore interest at the rate of 8% per annum. Upon a subsequent equity financing of at least $10.0 million prior to the stated maturity date, the 2017 Notes plus accrued interest would automatically convert into shares of the stock issued by the Company in such financing at a price equal to 80% of the lowest issue price. The 2017 Notes could have converted into a variable number of shares of preferred stock, and accordingly, the Company determined the conversion provision to be a redemption feature. The redemption feature was evaluated as an embedded derivative and bifurcated from the convertible promissory notes due to the substantial premium paid upon redemption and accounted for as a derivative instrument. Upon bifurcating the redemption feature, the Company recorded a debt discount of $3.0 million that was recognized in interest expense over the term of the 2017 Notes. In connection with the 2017 Notes, the Company issued warrants to the noteholders to purchase 304,397 shares of Series C-1. The warrants were exercisable for $0.12 per share and expire upon the earlier of (1) the date of the initial closing of a liquidation event, as defined, (2) the closing of a firm commitment underwritten initial public offering, or (3) January 2024. All warrants were exercised in connection with the closing of the Company’s IPO. The Company recorded a debt discount of $1.7 million, which represents the estimated fair value of the warrants, upon issuance of the 2017 Notes, which was being amortized to interest expense over the term of the 2017 Notes using the effective-interest method. In August 2018, the Company sold 1,842,959 shares of Series D redeemable preferred stock (“Series D”) to new and existing investors at a price of $9.659 per share for net proceeds of $17.7 million and issued warrants to purchase 368,582 shares of Series C-1 at an exercise price of $0.12 (the “Series D Financing”). Concurrent with the Series D Financing, all of the Company’s previously outstanding 2017 Notes, including accrued interest thereon, were converted into 2,080,209 shares of Series D. Interest expense, including the debt discount related to the 2017 Notes, was $0.7 million and $0.6 million for the three months ended September 30, 2018 and 2017, respectively. Interest expense was $3.4 million and $1.6 million for the nine months ended September 30, 2018 and 2017, respectively. Term Loan In October 2017, the Company entered into a Loan and Security Agreement (“SVB Loan”) with Silicon Valley Bank (“SVB”), pursuant to which the Company may borrow up to $7.5 million, issuable in three separate tranches (“Growth Capital Advances”) of $3.5 million (“Tranche A”), $2.0 million (“Tranche B”) and $2.0 million (“Tranche C”). Each of the Growth Capital Advances would become available upon the achievement of certain clinical and regulatory milestones. Under the original terms of the SVB Loan, the Company was to make interest-only payments through June 30, 2018 at a rate equal to the Prime Rate as defined per the SVB Loan. The interest-only period would be extended to December 31, 2018 if the Company borrowed the remaining tranches, followed by an amortization period of 24 months of equal monthly payments of principal plus interest amounts until paid in full. In connection with the SVB Loan, the Company issued to SVB a warrant to purchase 49,713 shares of Series C-1 at an exercise price of $9.659 per share. The warrant is immediately exercisable and expires on October 18, 2027. The Company is required to make a final payment equal to 7% of the original aggregate principal amount of the Growth Capital Advances at maturity. In November 2017, the Company drew $3.5 million from Tranche A The Company has the option to prepay all, but not less than all, of the borrowed amounts, provided that the Company will be obligated to pay a prepayment fee equal to (a) 3.0% of the outstanding principal balance of the applicable Growth Capital Advances if prepayment is made prior to the first anniversary of the effective date of the SVB Loan, (b) 2.0% of the outstanding principal balance of the applicable Growth Capital Advances if prepayment is made by the second anniversary of the effective date of the SVB Loan or (c) 1.0% of the outstanding principal balance of the applicable Growth Capital Advances if prepayment is made after the second anniversary of the effective date of the SVB Loan. In April 2018, the SVB Loan was amended to extend the draw period of Tranche B and Tranche C to April 30, 2018 and July 31, 2018, respectively, as well as to extend the interest-only period through July 31, 2018, which would be extended to December 31, 2018 if the Company borrowed Tranche B and Tranche C. Additionally, all Capital Growth Advances would mature on June 1, 2020; however, if the Company were to draw Tranche B and Tranche C, the maturity date would be December 31, 2020. On April 30, 2018, the Company borrowed $2.0 million under Tranche B. In July 2018, the SVB Loan was amended to further extend the draw period of Tranche C to August 31, 2018, as well as to extend the interest-only period of the SVB Loan through August 31, 2018, which would be extended to December 31, 2018 if the Company were to draw Tranche C. In August 2018, the Company borrowed $2.0 million under Tranche C. The Company’s obligations under the SVB Loan are secured by a first priority security interest in substantially all of its current and future assets, other than its owned intellectual property. The Company is also obligated to comply with various other customary covenants, including restrictions on its ability to encumber intellectual property assets. The Company recorded a debt discount of $0.4 million for the estimated fair value of warrants and debt issuance costs upon the borrowing of Tranche A and Tranche B, which is being amortized to interest expense over the term of the SVB Loan using the effective-interest method. As of September 30, 2018, and December 31, 2017, the Company had $7.5 million and $3.5 million, respectively, of outstanding principal under the SVB Loan and $7.5 million and $3.4 million, respectively, is reflected on the balance sheet net of debt discounts. Interest expense, including amortization of debt discount related to the term debt, totaled $0.2 million for the three months ended September 30, 2018 and $0.4 million for the nine months ended September 30, 2018. The Company is in compliance with all covenants under the SVB Loan as of September 30, 2018. Based on a 24-month amortization of the outstanding principal amounts for the Company’s Growth Capital Advances beginning on January 1, 2019 as discussed above, the following table sets forth by year the Company’s required future principal payments (in thousands): Years Ending December 31, 2018 (remaining three months) $ — 2019 3,750 2020 3,750 Thereafter — $ 7,500

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Operating Leases The Company leases office and research and development facilities under various non-cancellable operating lease agreements with expiration dates into 2023. The Company’s facility leases generally provide for periodic rent increases. In January 2010, the Company entered into a lease for office and laboratory space in Malvern, Pennsylvania (the “Malvern Lease”). The Malvern Lease commenced in March 2010 and was originally to expire on July 31, 2015. In December 2014, the Malvern Lease was amended to extend its term to July 31, 2018. In February 2018, the Malvern lease was further amended to extend its term to September 30, 2023. The Company’s future minimum commitments under its non-cancelable operating leases were as follows (in thousands): Year Ending December 31, 2018 (remaining three months) $ 63 2019 251 2020 256 2021 262 2022 267 Thereafter 203 Total $ 1,302 The Company recognizes rent expense for the facility operating leases on a straight-line basis. The Company accounts for the difference between the minimum lease payments and the straight-line amount as deferred rent. Rent expense was $0.1 million for each of the three months ended September 30, 2018 and 2017, and $0.3 million for each of the nine months ended September 30, 2018 and 2017. Legal Proceedings The Company is not currently a party to any litigation, nor is management aware of any pending or threatened litigation against the Company, that it believes would materially affect the Company’s business, operating results, financial condition or cash flows. However, the Company’s industry is characterized by frequent claims and litigation including securities litigation, claims regarding patent and other intellectual property rights and claims for product liability. As a result, in the future, the Company may be involved in various legal proceedings from time to time.

Preferred Stock Warrants

Preferred Stock Warrants	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Preferred Stock Warrants	8. Preferred Stock Warrants The Company accounts for its warrants to purchase shares of redeemable convertible preferred stock as liabilities as they are exercisable for a redeemable instrument. The Company will continue to adjust the liability for changes in fair value of these warrants until the exercise of warrants or the consummation of the Company’s IPO, at which time the liability will be reclassified to stockholders’ equity. The following table summarizes the outstanding redeemable convertible preferred stock warrants and the corresponding exercise price as of September 30, 2018 and December 31, 2017: Number of Shares Outstanding September 30, December 31, 2018 2017 Exercise Price Expiration Date Series AA warrants — 1,506 $ 13.28 March 7, 2018 2009 Series B warrants 25,884 25,884 9.659 December 22, 2019 2014 Series B warrants 104,856 104,856 0.12 May 14, 2021 Convertible debt Series C-1 warrants 304,397 304,397 0.12 January 17, 2024 Term loan Series C-1 warrants 49,713 49,713 9.659 October 18, 2027 2018 Series C-1 warrants 223,634 — 0.12 August 25, 2025 708,484 486,356 In August 2018, as part of the Series D Financing, the Company issued warrants to purchase 368,582 shares of Series C-1 redeemable convertible preferred stock (the “2018 Series C-1 warrants”) at an exercise price of $0.12. Of those 2018 Series C-1 warrants, 144,948 were exercised in August and September 2018. In connection with the IPO in October 2018, stockholders exercised the outstanding 2014 Series B warrants, convertible debt Series C-1 warrants, and the 2018 Series C-1 warrants to purchase an aggregate of 632,887 shares of the Company’s redeemable convertible preferred stock, at a weighted-average exercise price of $0.12 per share, and all such shares were automatically converted into an aggregate of 632,888 shares of common stock upon the closing of the IPO on October 22, 2018.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock and Stockholders’ Deficit	9 Months Ended
	Sep. 30, 2018
Stockholders Equity Note [Abstract]
Redeemable Convertible Preferred Stock and Stockholders’ Deficit	9. Redeemable Convertible Preferred Stock and Stockholders’ Deficit Preferred Stock The Company has issued and outstanding Series 1 redeemable convertible preferred stock (“Series 1”), Series 2 redeemable preferred stock (“Series 2”), Series AA redeemable convertible preferred stock (“Series AA”), Series B redeemable convertible preferred stock (“Series B”), Series C-1, Series C-2 redeemable convertible preferred stock (“Series C-2”), Series C-3 redeemable convertible preferred stock (“Series C-3”), and Series D (collectively, “Preferred In August 2018, the Company sold 1,842,959 shares of Series D to new and existing investors at a price of $9.659 per share for net proceeds of $17.7 million. As of September 30, 2018, the authorized, issued, and outstanding shares of Preferred Stock and their carrying amounts and liquidation values were as follows: Shares Issued Shares and Carrying Liquidation Authorized Outstanding Amount Value Series 1 132,255 132,255 $ 523,838 $ 526,778 Series 2 1 1 243,640 250,000 Series AA 575,470 573,961 7,617,120 7,619,998 Series B 6,382,259 6,251,502 60,371,358 60,379,282 Series C-1 4,524,375 2,319,228 22,140,786 22,401,423 Series C-2 916,095 — — — Series C-3 790,895 — — — Series D 4,971,346 3,923,168 34,712,552 37,893,880 Total 18,292,696 $ 13,200,115 $ 125,609,295 $ 129,071,361 As of December 31, 2017, the authorized, issued, and outstanding shares of Preferred Stock and their carrying amounts and liquidation values were as follows: Shares Issued Shares and Carrying Liquidation Authorized Outstanding Amount Value Series 1 132,255 132,255 $ 522,266 $ 526,778 Series 2 1 1 240,242 250,000 Series AA 575,470 573,961 7,615,583 7,619,998 Series B 6,382,259 6,251,502 60,366,480 60,379,282 Series C-1 4,524,375 2,174,280 20,888,972 20,999,997 Series C-2 916,095 — — — Series C-3 790,895 — — — Series D — — — — Total 13,321,350 9,131,999 $ 89,633,543 $ 89,776,055 Dividends The holders of the Series AA, Series B, Series C-1, and Series D (together, the “Senior Stock”) and the holders of the Series C-2 and Series C-3 (together, the “Senior Series C Stock”) were entitled to receive noncumulative cash dividends, on a pari passu basis among the Senior Stock and the Senior Series C Stock, at the rate of 8% when and if declared by the board of directors. After payment of dividends on the Senior Series C Stock and the Senior Stock, the holders of the Series 1 were entitled to receive noncumulative dividends at the rate of 8% when and if declared by the board of directors. The holder of the Series 2 was not entitled to receive any dividends. Any declared but unpaid dividends were payable upon any liquidation, dissolution, or winding up of the Company by another entity, whether voluntary or involuntary, or conversion of the applicable shares of Preferred Stock to common stock. No dividends were declared on the Preferred Stock. Liquidation Preference In the event of a liquidation, dissolution, or winding up of the Company, or in the event the Company merges with or was acquired by another entity, the holders of the Senior Stock and the Senior Series C Stock were entitled to be paid an amount equal to $13.28 per share of Series AA and Series C-3, $9.659 per share of Series B, Series C-1 and Series D, and $11.462 per share of Series C-2, plus any declared but unpaid dividends. After payment in full of the Senior Stock and the Senior Series C Stock liquidation preference, the holder of the Series 2 was entitled to be paid a liquidation preference of $250,000. After payment in full of the Series 2 liquidation preference, the holders of the Series 1 were entitled to receive an amount equal to $3.99 per share, plus any declared but unpaid dividends. Once the preceding liquidation preferences had been paid, any remaining assets would have been distributed pro rata among the holders of the Series D, Series C-1, Series C-2, Series C-3, Series B, Series AA, Series 1 and common stock. Voting Rights The holders of the Series 1, Series AA, Series B, Series C-1, Series C-2, Series C-3, and Series D were entitled to a number of votes equal to the number of shares of common stock into which their shares could have been converted. The holder of the Series 2 did not have voting rights. Conversion The shares of Series 1, Series AA, Series B, Series C-1, Series C-2, Series C-3 and Series D were convertible into an equal number of shares of common stock, at the option of the holder, subject to certain anti-dilution adjustments. The holder of the Series 2 did not have any voluntary conversion rights. All shares of Preferred Stock were automatically converted into an aggregate of 13,225,114 shares of common stock upon the closing of the IPO on October 22, 2018. Redemption The holders of 60% of the Preferred Stock, voting together as a single class on an as-converted basis, were entitled to require the Company to redeem all of the then-outstanding shares of Preferred Stock at any time on or after February 26, 2020. The redemption price was equal to the greater of (a) the liquidation preference, including any declared but unpaid dividends, of the applicable class of Preferred Stock or (b) the then fair market value of such Preferred Stock. If funds were insufficient to redeem all shares of Preferred Stock, all shares of Series D would have been redeemed first, then all remaining shares of Senior Stock and Senior Series C Stock would have been redeemed, and all shares of Series 1 would have been redeemed before the share of Series 2. The carrying values of the Preferred Stock were being accreted to their redemption value by a charge to additional paid-in capital, if any, then to accumulated deficit. Upon election of the holders of a majority of the outstanding Series 2 and approval by the holders of at least 60% of the remaining Preferred Stock, the Company would have redeemed all outstanding shares of Series 2 preferred stock for an aggregate amount of $125,000. Common Stock Reserved for Future Issuance Number of Shares as of September 30, 2018 December 31, 2017 Conversion of redeemable convertible preferred stock 13,200,115 9,131,999 Redeemable convertible preferred stock warrants 708,484 486,356 Stock options issued and outstanding 1,315,625 1,075,209 Authorized for future options 37,830 45,062 15,262,054 10,738,626

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	10. Stock-Based Compensation In July 2018, the Company amended the Company’s Amended and Restated 2002 Stock Plan (the “2002 Plan”) to reserve an additional 800,000 shares of common stock for issuance under the 2002 Plan. In October 2018, the Company’s board of directors and stockholders adopted and approved the 2018 Equity Incentive Plan (the “2018 Plan”), which is a successor to and continuation of the Company’s Amended and Restated 2002 Stock Plan (the “2002 Plan), which the Company’s board of directors and stockholders approved in July 2002. The 2018 Plan became effective upon the execution of the underwriting agreement related to the IPO on October 17, 2018. No further grants will be made under the 2002 Plan. In 2002, the Company adopted the 2002 Stock Plan, which was later amended and restated, pursuant to which 37,830 shares were available for future grants as of September 30, 2018. The 2002 Plan provided for the grant of incentive stock options, nonstatutory stock options, stock awards, and stock purchase rights to employees, directors, and consultants. The terms of the grant agreements were determined by the Company’s board of directors. The Company’s stock options vest based on the terms in each grant agreement and generally vest over four years and have a term of 10 years. In October 2018, the Company’s board of directors and stockholders adopted and approved the 2018 Equity Incentive Plan (the “2018 Plan”), which is a successor to and continuation of the 2002 Plan, which the Company’s board of directors and stockholders approved in July 2002. The 2018 Plan became effective upon the execution of the underwriting agreement related to the IPO on October 17, 2018. No further grants will be made under the 2002 Plan. The following table summarizes stock option activity for the 2002 Plan for the nine months ended September 30, 2018: Weighted- Weighted- Average Average Exercise Remaining Aggregate Total Price Per Contractual Intrinsic Options Share Term (in years) Value Outstanding at December 31, 2017 1,075,209 $ 1.60 7.3 $ 711,328 Granted 260,301 $ 3.40 Exercised (19,885 ) $ 2.68 Outstanding at September 30, 2018 1,315,625 $ 1.94 7.2 $ 8,105,669 Vested and expected to vest at September 30, 2018 1,217,885 $ 1.90 7.1 $ 7,553,919 Vested and exercisable at September 30, 2018 807,177 $ 1.60 6.2 $ 5,243,664 The weighted-average grant date fair value per share of options granted was $2.30 for the nine months ended September 30, 2018. The aggregate intrinsic value of options exercised was $27,000 for the nine months ended September 30, 2018. As of September 30, 2018, the total unrecognized compensation expense related to unvested employee and non-employee stock option awards was $0.6 million, which was expected to be recognized in expense over a weighted-average period of approximately 2.9 years. Determining Fair Value of Stock Options The fair value of each grant of stock options was determined by the Company using the methods and assumptions discussed below. Certain of these inputs are subjective and generally require judgment to determine. Expected Term —The expected term of stock options represents the weighted average period the stock options are expected to be outstanding. The Company uses the simplified method for estimating the expected term. The simplified method calculates the expected term as the average time-to-vesting and the contractual life of the options. Expected Volatility —Due to the Company’s limited operating history and lack of company-specific historical or implied volatility, the expected volatility assumption was determined by examining the historical volatilities of a group of industry peers whose share prices are publicly available. Risk-Free Interest Rate —The risk-free rate assumption is based on the U.S. Treasury instruments, the terms of which were consistent with the expected term of the Company’s stock options. Expected Dividend —The Company has not paid and does not intend to pay dividends. Common Stock Valuation —Due to the absence of a public market trading the Company’s common stock, it was necessary to estimate the fair value of the common stock underlying the stock-based grants when performing fair value calculations using the Black-Scholes option pricing model. The fair value of the common stock underlying the stock-based grants was assessed for each grant date by the board of directors. All options to purchase shares of common stock have been granted with an exercise price per share no less than the fair value per share of the common stock underlying those options on the date of grant. In the absence of a public trading market for the Company’s common stock, the Company determined the estimated fair value of its common stock using methodologies, approaches, and assumptions consistent with the American Institute of Certified Public Accountants (“AICPA”) Audit and Accounting Practice Aid Series: Valuation of Privately Held Company Equity Securities Issued as Compensation (“AICPA Practice Aid”). The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below: Nine Months Ended September 30, 2018 2017 Risk-free interest rate 2.87% 2.28% Expected term (in years) 7.0 7.1 Expected volatility 69% 68% Expected dividend yield — — Estimated fair value of common stock $ 2.32 $ 1.00 Stock-based compensation expense has been reported in the Company’s statements of operations for the three months ended September 30, 2018 and 2017 and for the nine months ended September 30, 2018 and 2017 as follows (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 General and administrative $ 66 $ 10 $ 119 $ 30 Research and development 31 9 143 28 Total stock-based compensation $ 97 $ 19 $ 262 $ 58

License Agreements

License Agreements	9 Months Ended
	Sep. 30, 2018
License Agreements [Abstract]
License Agreements	11. License Agreements MedImmune Limited In November 2017, the Company entered into a license agreement (“MedImmune License”) with MedImmune Limited (“MedImmune”). MedImmune is a wholly owned subsidiary of AstraZeneca plc. Pursuant to the terms of the MedImmune License, MedImmune granted the Company exclusive global rights for the purpose of developing and commercializing products under the MedImmune License (“MedImmune licensed product”). In consideration of the license and other rights granted by MedImmune, the Company made an upfront payment of $0.1 million, which was included as research and development expense for the year ended December 31, 2017. The Company is also obligated to make a series of contingent milestone payments totaling up to an aggregate of $18.0 million upon the achievement of clinical development and regulatory milestones. As of September 30, 2018, none of the clinical development or regulatory filing milestones had been met. In addition, the Company will pay MedImmune tiered royalties ranging from mid-single-digit to low-teen percentages of net sales of any MedImmune licensed products and additional payments of up to $50.0 million in aggregate commercial milestones. The Company also must pay quarterly fees relating to technical services provided by MedImmune. The MedImmune License requires the Company to cooperate with MedImmune on commercial messaging of PB2452 and provides MedImmune with the return of rights to PB2452 if certain commercial diligence requirements are not achieved by the Company. In addition, the MedImmune License offers an option for third party product storage costs. As of December 31, 2017, the Company had incurred and reimbursed MedImmune $0.5 million for such third party product storage costs. The Company did not incur any third party product storage costs in the nine months ended September 30, 2018. Duke University In October 2006, the Company entered into a license agreement with Duke University (“Duke”) (as amended, the “Duke License”). Pursuant to the Duke License, Duke granted to the Company an exclusive, worldwide license under certain patent rights and a non-exclusive license to know-how owned or controlled by Duke to develop and commercialize any products or processes covered under the Duke License (the “Duke licensed products”). The Duke License was amended in February 2016 to allow Duke to use the Company’s technology in the area of small-molecule oncologics. The Duke License is a worldwide, sublicensable agreement and remains in full effect for the life of the last-to-expire patents included in the patent rights, which is approximately 20 years. The Company is obligated to pay up to $2.2 million upon the achievement of clinical development and regulatory milestones and up to $0.4 million upon the achievement of commercial milestones. The Duke License may be terminated by Duke if the Company fails to meet certain clinical development and regulatory milestones within specified timeframes. As of September 30, 2018, the Company was in compliance with its development obligations. The Company is required to use commercially reasonable efforts to develop one or more products or processes and introduce them into commercial markets. Duke will receive low single-digit royalty percentages on net sales by the Company or its sublicensee, with minimum aggregate royalties of $0.2 million payable following the Company’s achievement of certain commercial milestones. No sales of Duke licensed products or services have occurred since the effective date through September 30, 2018. Certain alliance fee payments up to the greater of $0.3 million or a low double-digit percentage of the fees the Company receives from a third party in consideration of forming a strategic alliance, may be required depending upon how the patent rights are commercialized. The Company will pay Duke the first $1.0 million of nonroyalty payments it receives from a sublicensee, and thereafter a specified percentage of any additional nonroyalty payments it receives. If Duke receives revenue as a result of a license or sublicense to a third party in the field of small-molecule oncologics, it will pay the Company a specified percentage of the amount of such revenue in excess of $1.0 million. Duke is also a stockholder of the Company.

Grant Revenue

Grant Revenue	9 Months Ended
	Sep. 30, 2018
Revenue From Contract With Customer [Abstract]
Grant Revenue	12. Grant Revenue In February 2018, the Company received Small Business Innovation Research, or SBIR, grants from the National Institutes of Health in an aggregate amount of $2.8 million to support the clinical development of PB1046 for the treatment of pulmonary arterial hypertension (“PAH”) for the period from February 17, 2018 to July 31, 2020. In connection with the SBIR grants, the U.S. government will receive a non-exclusive, royalty-free license to use any technology the Company develops under such grants. The Company recognized $0.4 million under the SBIR grants in the three months and nine months ended September 30, 2018.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	13. Related Party Transactions As described above in Note 11, the Company is party to the MedImmune License. MedImmune is a related party of the Company.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events Seventh Amended and Restated Certificate of Incorporation and Reverse Stock Split In October 2018, the Company implemented a 11.0634-for-1 reverse stock split of its outstanding common stock and redeemable convertible preferred stock, which became effective upon filing of the Company’s Seventh Amended and Restated Certificate of Incorporation on October 4, 2018. The accompanying condensed financial statements give retroactive effect to the stock split for all periods presented. Initial Public Offering, Conversion of Preferred Stock, Eighth Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws In October 2018, the Company completed its IPO of 9,864,666 shares of common stock at an offering price of $5.00 per share. The Company received net proceeds of approximately $43.0 million, after deducting underwriting discounts, commissions and estimated offering-related transaction costs. Upon the closing of the IPO on October 22, 2018, all shares of Preferred Stock were automatically converted into an aggregate of 13,225,114 shares of common stock. In connection with the closing of the IPO, the Company filed an Eighth Amended and Restated Certificate of Incorporation (the “Post-IPO Certificate”) with the Secretary of State of the State of Delaware on October 22, 2018. T The Post-IPO Certificate amends and restates in its entirety the Company’s Seventh Amended and Restated Certificate of Incorporation and the Post-IPO Bylaws amend and restate, in their entirety, the Company’s Bylaws. Collectively, the Post-IPO Certificate and the Post-IPO Bylaws, among other things: (i) authorize 200,000,000 shares of common stock; (ii) eliminate all references to the previously existing series of Preferred Stock; (iii) authorize 10,000,000 shares of undesignated preferred stock that may be issued from time to time by the Company’s board of directors in one or more series; (iv) provide that the Company’s board of directors be divided into three classes with staggered three-year terms, with one class of directors to be elected at each annual meeting of the Company’s stockholders; (v) provide that directors may only be removed with cause and only upon the affirmative vote of holders of at least two-thirds of the voting power of all then-outstanding shares of capital stock of the Company entitled to vote generally in the election of directors; (vi) provide that, subject to limited exceptions, vacancies on the Company’s board of directors shall only be filled by the remaining members of the Board; (vii) eliminate the ability of the Company’s stockholders to take action by written consent in lieu of a meeting; and (viii) provide that only the Company’s board of directors, the chairman of the board or the chief executive officer may call a special meeting of stockholders. Approval of 2018 Plan The Company’s board of directors and stockholders approved the 2018 Plan in October 2018. The 2018 Plan became effective on October 17, 2018 (the “2018 Plan Effective Date”). On and after the 2018 Plan Effective Date, no additional stock awards will be granted under the 2002 Plan. The 2018 Plan for the grant of incentive stock options (“ISOs”), nonstatutory stock options (“NSOs”), stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards, performance cash awards and other forms of stock awards to employees, directors and consultants, including employees and consultants of the Company’s affiliates, Initially, the maximum number of shares of the Company’s common stock that may be issued under the 2018 Plan will be 3,231,626 shares. The number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each calendar year, starting on January 1, 2019 through January 1, 2028, in an amount equal to 3% of the total number of shares of the Company’s capital stock outstanding on the last day of the calendar month before the date of each automatic increase, or a lesser number of shares determined by the Company’s board of directors. The maximum number of shares of the Company’s common stock that may be issued on the exercise of incentive stock options under the 2018 Plan is 9,694,878. Approval of 2018 Employee Stock Purchase Plan The Company’s board of directors and stockholders approved the 2018 Employee Stock Purchase Plan (the “ESPP”) in October 2018. The ESPP became effective on October 17, 2018. The purpose of the ESPP is to secure the services of new employees, to retain the services of existing employees and to provide incentives for such individuals to exert maximum efforts toward the Company’s success and that of the Company’s affiliates. The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423 of the Code for U.S. employees. The ESPP authorizes the issuance of 196,000 shares of the Company’s common stock under purchase rights granted to the Company’s employees or to employees of any of the Company’s designated affiliates. The number of shares of common stock reserved for issuance will automatically increase on January 1 of each calendar year, beginning on January 1, 2019 through January 1, 2028, by the lesser of (1) 1% of the total number of shares of the Company’s common stock outstanding on the last day of the calendar month before the date of the automatic increase, and (2) 490,000 shares; provided that before the date of any such increase, the Company’s board of directors may determine that such increase will be less than the amount set forth in clauses (1) and (2). As of the date hereof, no shares of common stock have been purchased under the ESPP. Exercises of Warrants In connection with the IPO, stockholders exercised warrants to purchase an aggregate of 632,887 shares of the Company’s redeemable convertible preferred stock, at a weighted-average exercise price of $0.12 per share, and all such shares were automatically converted into an aggregate of 632,887 shares of common stock.

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 condensed financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial reporting and the rules and regulations of the Securities and Exchange Commission (“SEC”). Certain information and note disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to those rules and regulations. All adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the accompanying condensed financial statements. Although these interim financial statements do not include all of the information and footnotes required for complete annual financial statements, management believes the disclosures are adequate to make the information presented not misleading. The unaudited interim results of operations and cash flows for the three and nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the full year. The unaudited interim condensed financial statements and footnotes should be read in conjunction with the audited financial statements and footnotes included in the Company’s IPO prospectus filed with the SEC on October 17, 2018, wherein a more complete discussion of significant accounting policies and certain other information can be found. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) promulgated by the Financial Accounting Standards Board (“FASB”). The Company manages its operations as a single reportable segment for the purposes of assessing performance and making operating decisions.
Use of Estimates	Use of Estimates The preparation of the Company’s condensed financial statements requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in the Company’s condensed financial statements and accompanying notes. The most significant estimates in the Company’s condensed financial statements relate to the valuation of redeemable convertible preferred stock warrants, equity awards and clinical trial accruals. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results could differ materially from those estimates and assumptions.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains certain deposits in federally insured financial institutions in excess of federally insured limits. The Company could experience losses on the money market funds in the future.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity from the date of purchase of three months or less to be cash equivalents. Cash and cash equivalents include cash in readily available checking and money market
Restricted Cash	Restricted Cash The Company had restricted cash of $20,000 as of September 30, 2018 and December 31, 2017 ,
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amounts of accounts receivable, prepaid expenses and other current assets, accounts payable and accrued expenses are reasonable estimates of their fair value because of the short maturity of these items. The carrying amount of the convertible promissory notes approximates fair value because the interest rates on these instruments are reflective of rates that the Company could obtain on unaffiliated third party debt with similar terms and conditions. Based on the borrowing rates currently available to the Company for loans with similar terms, the Company believes the fair value of the term loan approximates its carrying value (see Note
Property and Equipment	Property and Equipment Property and equipment are recorded at cost and depreciated over the estimated useful lives of the assets (three to five years) using the straight-line method. Leasehold improvements are amortized over the shorter of their estimated useful lives or the lease term.
Long-Lived Assets	Long-Lived Assets The Company regularly reviews the carrying value and estimated lives of all of its long-lived assets, including property and equipment to determine whether indicators of impairment may exist which warrant adjustments to carrying values or estimated useful lives. The determinants used for this evaluation include management’s estimate of the asset’s ability to generate net positive cash flow in future periods as well as the strategic significance of the assets to the Company’s business objective. Should an impairment exist, the impairment loss would be measured based on the extent that the estimated fair value is less than its carrying value. The Company has not recognized any impairment
Preferred Stock Warrant Liability	Preferred Stock Warrant Liability The Company has issued freestanding warrants to purchase shares of its redeemable convertible preferred stock. Since the underlying redeemable convertible preferred stock is classified outside of permanent equity, these warrants are classified as liabilities in the accompanying condensed balance sheet. Warrants classified as liabilities are recorded at their estimated fair value on the date of issuance and are revalued at each subsequent balance sheet date, with fair value changes recognized as increases or reductions to other income (expense) in the accompanying condensed statements of operations. The Company estimates the fair value of these warrants using the Black-Scholes option pricing
Preclinical and Clinical Trial Accruals	Preclinical and Clinical Trial Accruals The Company accrues and expenses preclinical studies and clinical trial activities performed by third parties based upon estimates of the proportion of work completed over the life of the individual trial and subject enrollment rates in accordance with agreements with clinical research organizations and clinical trial sites. The Company determines the estimates by reviewing contracts, vendor agreements and purchase orders, and through discussions with internal clinical personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services. However, actual costs and timing of clinical trials are highly uncertain, subject to risks and may change depending upon a number of factors, including the Company’s clinical development Management makes estimates of the Company’s accrued expenses as of each balance sheet date in the Company’s condensed financial statements based on facts and circumstances known to the Company at that time. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. Nonrefundable advance payments for goods and services, including fees for process development or manufacturing and distribution of clinical supplies that will be used in future research and development activities, are deferred and recognized as expense in the period that the related goods are consumed or services are
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred.
Stock-Based Compensation	Stock-Based Compensation The Company measures and recognizes compensation expense for all stock-based compensation for employees and directors based on the estimated fair value at the date of grant, and to consultants based on the ongoing estimated fair value. Currently, the Company’s stock-based awards consist only of stock options; however, future grants under the Company’s equity compensation plan may also consist of shares of restricted stock, restricted stock units, stock appreciation rights, performance awards and performance units. The Company estimates the fair value of stock options using the Black-Scholes option-pricing model, which requires the use of estimates. The Company recognizes stock-based compensation cost for employees and directors for ratably vesting stock options on a straight-line basis over the requisite service period of the award. The fair value of options granted to consultants is estimated using the Black-Scholes option-pricing model and is re-measured at each reporting date with changes in fair value prior to vesting recognized as expense in the condensed consolidated statements of operations across the applicable vesting period. The Black-Scholes option-pricing model requires the input of subjective assumptions, including the risk-free interest rate, the fair value of the underlying common stock, the expected dividend yield of the Company’s common stock, the expected volatility of the price of the Company’s common stock, and the expected term of the option. These estimates involve inherent uncertainties and the application of management’s judgment. If factors change and different assumptions are used, the Company’s stock-based compensation expense could be materially different in the future.
Income Taxes	Income Taxes The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the condensed financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the condensed financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies, and results of recent operations. If management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. The Company records uncertain tax positions on the basis of a two-step process whereby (1) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits, if any, within income tax expense, and any accrued interest and penalties are included within the related tax liability line.
Grant Revenue	Grant Revenue Grant revenues are derived from government grants that support the Company’s efforts on specific research projects. The Company has determined that the government agencies providing grants to the Company are not customers. The Company recognizes grant revenue when there is reasonable assurance of compliance with the conditions of the grant and reasonable assurance that the grant revenue will be received.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed by dividing the net loss by The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive. As of September 30, 2018 2017 Redeemable convertible preferred stock 13,200,115 9,131,999 Common stock options 1,315,625 1,084,243 Warrants to purchase redeemable convertible preferred stock 708,484 269,225 Total 15,224,224 10,485,467
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases The Company will adopt ASU 2016-02 in 2019. The adoption of this guidance is expected to result in a significant increase in the total assets and liabilities reported on the condensed balance sheets. In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments (Topic 230) In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting classified share-based payment awards issued to nonemployees will be measured at the grant date, instead of the previous requirement to remeasure the awards through the performance completion date, (ii) for performance conditions, compensation cost associated with the award will be recognized when the achievement of the performance condition is probable, rather than upon achievement of the performance condition, and (iii) the current requirement to reassess the classification (equity or liability) for nonemployee awards upon vesting will be eliminated, except for awards in the form of convertible instruments. This new guidance will be effective for public companies for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. The Company does not believe the adoption of this guidance will have a material impact on its financial statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement - Disclosure Framework (Topic 820): Changes to the Disclosure Requirements for Fair Value Measurement

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Schedule Of Outstanding Potentially Dilutive Securities Excluded In Calculation Of Diluted Net Loss Per Share	The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive. As of September 30, 2018 2017 Redeemable convertible preferred stock 13,200,115 9,131,999 Common stock options 1,315,625 1,084,243 Warrants to purchase redeemable convertible preferred stock 708,484 269,225 Total 15,224,224 10,485,467

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities that Require Fair Value Measurements on a Recurring Basis	The following table summarizes the Company’s assets and liabilities that require fair value measurements on a recurring basis and their respective input levels based on the fair value hierarchy (in thousands): Fair Value Measurements at Reporting Date Total Level 1 Level 2 Level 3 As of September 30, 2018: Assets Cash equivalents $ 23,103 $ 23,103 $ — $ — Liabilities Preferred stock warrant liability $ 5,990 $ — $ — $ 5,990 As of December 31, 2017: Assets Cash equivalents $ 12,472 $ 12,472 $ — $ — Liabilities Derivative liability $ 3,028 $ — $ — $ 3,028 Preferred stock warrant liability 1,656 — — 1,656 Total liabilities $ 4,684 $ — $ — $ 4,684
Weighted-Average Assumptions Used In Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option Pricing Model	The following weighted-average assumptions were used in determining the fair value of the preferred stock warrant liability valued using the Black-Scholes option pricing model as of September 30, 2018 and December 31, 2017: September 30, December 31, 2018 2017 Expected volatility 68.7 % 68.0 % Risk-free interest rate 3.0 % 2.2 % Contractual term (in years) 5.7 5.9 Expected dividend yield — —
Estimated Fair Values Per Share of Underlying Redeemable Convertible Preferred Stock	The following estimated fair values per share of the Company’s underlying redeemable convertible preferred stock were used to determine the estimated fair value of the preferred stock warrant liability: September 30, December 31, 2018 2017 Series AA $ — $ 5.75 Series B $ 8.95 $ 3.76 Series C-1 $ 8.95 $ 3.76

Property and Equipment (Tables)

Property and Equipment (Tables)	9 Months Ended
	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Composition of Property and Equipment, Net	The following table presents the composition of property and equipment, net as of September 30, 2018 and December 31, 2017 (in thousands): As of As of September 30, December 31, 2018 2017 Lab equipment $ 1,684 $ 1,681 Computer hardware, software and telephone 200 174 Furniture and fixtures 70 70 Leasehold improvements 24 22 1,978 1,947 Less accumulated depreciation (1,727 ) (1,645 ) Property and equipment, net $ 251 $ 302

Accrued Expenses (Tables)

Accrued Expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Payables And Accruals [Abstract]
Schedule of Composition of Accrued Expenses	The following table presents the composition of accrued expenses as of September 30, 2018 and December 31, 2017 (in thousands): As of As of September 30, December 31, 2018 2017 Accrued clinical and related costs $ 1,335 $ 197 Accrued stock offering costs 1,041 — Accrued compensation and related costs 261 346 Accrued interest 110 628 Accrued other 240 110 Accrued expenses $ 2,987 $ 1,281

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Summary of Future Principal Payments	Based on a 24-month amortization of the outstanding principal amounts for the Company’s Growth Capital Advances beginning on January 1, 2019 as discussed above, the following table sets forth by year the Company’s required future principal payments (in thousands): Years Ending December 31, 2018 (remaining three months) $ — 2019 3,750 2020 3,750 Thereafter — $ 7,500

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Commitments Under Non-cancelable Operating Leases	The Company’s future minimum commitments under its non-cancelable operating leases were as follows (in thousands): Year Ending December 31, 2018 (remaining three months) $ 63 2019 251 2020 256 2021 262 2022 267 Thereafter 203 Total $ 1,302

Preferred Stock Warrants (Table

Preferred Stock Warrants (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Summary of Outstanding Redeemable Convertible Preferred Stock Warrants and Corresponding Exercise Price	The following table summarizes the outstanding redeemable convertible preferred stock warrants and the corresponding exercise price as of September 30, 2018 and December 31, 2017: Number of Shares Outstanding September 30, December 31, 2018 2017 Exercise Price Expiration Date Series AA warrants — 1,506 $ 13.28 March 7, 2018 2009 Series B warrants 25,884 25,884 9.659 December 22, 2019 2014 Series B warrants 104,856 104,856 0.12 May 14, 2021 Convertible debt Series C-1 warrants 304,397 304,397 0.12 January 17, 2024 Term loan Series C-1 warrants 49,713 49,713 9.659 October 18, 2027 2018 Series C-1 warrants 223,634 — 0.12 August 25, 2025 708,484 486,356

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock and Stockholders’ Deficit (Tables)	9 Months Ended
	Sep. 30, 2018
Stockholders Equity Note [Abstract]
Summary of Authorized, Issued, and Outstanding Shares of Preferred Stock and Carrying Amounts and Liquidation Values	As of September 30, 2018, the authorized, issued, and outstanding shares of Preferred Stock and their carrying amounts and liquidation values were as follows: Shares Issued Shares and Carrying Liquidation Authorized Outstanding Amount Value Series 1 132,255 132,255 $ 523,838 $ 526,778 Series 2 1 1 243,640 250,000 Series AA 575,470 573,961 7,617,120 7,619,998 Series B 6,382,259 6,251,502 60,371,358 60,379,282 Series C-1 4,524,375 2,319,228 22,140,786 22,401,423 Series C-2 916,095 — — — Series C-3 790,895 — — — Series D 4,971,346 3,923,168 34,712,552 37,893,880 Total 18,292,696 $ 13,200,115 $ 125,609,295 $ 129,071,361 As of December 31, 2017, the authorized, issued, and outstanding shares of Preferred Stock and their carrying amounts and liquidation values were as follows: Shares Issued Shares and Carrying Liquidation Authorized Outstanding Amount Value Series 1 132,255 132,255 $ 522,266 $ 526,778 Series 2 1 1 240,242 250,000 Series AA 575,470 573,961 7,615,583 7,619,998 Series B 6,382,259 6,251,502 60,366,480 60,379,282 Series C-1 4,524,375 2,174,280 20,888,972 20,999,997 Series C-2 916,095 — — — Series C-3 790,895 — — — Series D — — — — Total 13,321,350 9,131,999 $ 89,633,543 $ 89,776,055
Summary of Common Stock Reserved for Future Issuance	Common Stock Reserved for Future Issuance Number of Shares as of September 30, 2018 December 31, 2017 Conversion of redeemable convertible preferred stock 13,200,115 9,131,999 Redeemable convertible preferred stock warrants 708,484 486,356 Stock options issued and outstanding 1,315,625 1,075,209 Authorized for future options 37,830 45,062 15,262,054 10,738,626

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes stock option activity for the 2002 Plan for the nine months ended September 30, 2018: Weighted- Weighted- Average Average Exercise Remaining Aggregate Total Price Per Contractual Intrinsic Options Share Term (in years) Value Outstanding at December 31, 2017 1,075,209 $ 1.60 7.3 $ 711,328 Granted 260,301 $ 3.40 Exercised (19,885 ) $ 2.68 Outstanding at September 30, 2018 1,315,625 $ 1.94 7.2 $ 8,105,669 Vested and expected to vest at September 30, 2018 1,217,885 $ 1.90 7.1 $ 7,553,919 Vested and exercisable at September 30, 2018 807,177 $ 1.60 6.2 $ 5,243,664
Estimated Fair Value of Option on Grant Date using Weighted Average Assumptions	The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below: Nine Months Ended September 30, 2018 2017 Risk-free interest rate 2.87% 2.28% Expected term (in years) 7.0 7.1 Expected volatility 69% 68% Expected dividend yield — — Estimated fair value of common stock $ 2.32 $ 1.00
Stock-based Compensation Expense	Stock-based compensation expense has been reported in the Company’s statements of operations for the three months ended September 30, 2018 and 2017 and for the nine months ended September 30, 2018 and 2017 as follows (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2018 2017 2018 2017 General and administrative $ 66 $ 10 $ 119 $ 30 Research and development 31 9 143 28 Total stock-based compensation $ 97 $ 19 $ 262 $ 58

Organization and Description _2

Organization and Description of Business - Additional Information (Details) $ / shares in Units, $ in Thousands	Oct. 22, 2018USD ($)$ / sharesshares	Oct. 31, 2018	Aug. 31, 2018USD ($)$ / shares	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Organization and Description of Business [Line Item]
Reverse stock split description				The Company effected a 11.0634-for-1 reverse split of its outstanding common stock and redeemable convertible preferred stock.
Accumulated deficit				$ (118,043)	$ (99,065)
Net proceeds from the sale of redeemable convertible preferred stock				17,712
Cash and cash equivalents				24,300
Working capital				$ 17,300
Silicon Valley Bank
Organization and Description of Business [Line Item]
Term loan			$ 2,000
Series D
Organization and Description of Business [Line Item]
Net proceeds from the sale of redeemable convertible preferred stock			$ 17,700
Common stock,public offering price | $ / shares			$ 9.659
Subsequent Event | Initial Public Offering
Organization and Description of Business [Line Item]
Common stock, shares, issued and sold | shares	9,864,666
Common stock,public offering price | $ / shares	$ 5
Proceeds from issuance of initial public offering	$ 43,000
Subsequent Event | Common Stock
Organization and Description of Business [Line Item]
Reverse stock split conversion ratio		11.0634
Subsequent Event | Common Stock | Initial Public Offering
Organization and Description of Business [Line Item]
Redeemable convertible preferred stock | shares	13,225,114

Significant Accounting Polici_4

Significant Accounting Policies - Additiona Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Restricted cash	$ 20,000	$ 20,000
Minimum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful lives	three
Maximum
Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful lives	five

Significant Accounting Polici_5

Significant Accounting Policies - Schedule Of Outstanding Potentially Dilutive Securities Excluded In Calculation Of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Significant Accounting Policies [Line Items]
Outstanding potentially dilutive securities excluded in calculation of diluted net loss per share	15,224,224	10,485,467
Redeemable Convertible Preferred Stock
Significant Accounting Policies [Line Items]
Outstanding potentially dilutive securities excluded in calculation of diluted net loss per share	13,200,115	9,131,999
Common Stock Options
Significant Accounting Policies [Line Items]
Outstanding potentially dilutive securities excluded in calculation of diluted net loss per share	1,315,625	1,084,243
Warrants to Purchase Redeemable Convertible Preferred Stock
Significant Accounting Policies [Line Items]
Outstanding potentially dilutive securities excluded in calculation of diluted net loss per share	708,484	269,225

Fair Value Measurement - Summar

Fair Value Measurement - Summary of Assets and Liabilities that Require Fair Value Measurements on a Recurring Basis (Details) - Fair Value Measurements on Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 23,103	$ 12,472
Liabilities
Derivative liability		3,028
Preferred stock warrant liability	5,990	1,656
Total liabilities		4,684
Level 1
Assets
Cash equivalents	23,103	12,472
Level 3
Liabilities
Derivative liability		3,028
Preferred stock warrant liability	$ 5,990	1,656
Total liabilities		$ 4,684

Fair Value Measurement - Weight

Fair Value Measurement - Weighted-Average Assumptions Used In Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option Pricing Model (Details) - Weighted-Average Assumptions - Preferred Stock Warrant Liability	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Contractual term (in years)	5 years 8 months 12 days	5 years 10 months 24 days
Expected Volatility
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Expected volatility	0.687	0.680
Risk-Free Interest Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Expected volatility	0.030	0.022

Fair Value Measurement - Estima

Fair Value Measurement - Estimated Fair Values Per Share of Underlying Redeemable Convertible Preferred Stock (Details) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Series AA
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair values preferred stock		$ 5.75
Series B
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair values preferred stock	$ 8.95	3.76
Series C-1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair values preferred stock	$ 8.95	$ 3.76

Fair Value Measurement - Activi

Fair Value Measurement - Activity for Preferred Stock Warrant Liability and Derivative Liability Measured At Fair Value Using Significant Unobservable Level 3 Inputs (Details) - Level 3 $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Derivative Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2017	$ 3,028
Changes in fair value reflected as change in fair value of warrant liability	676
Extinguishment of derivative upon conversion of convertible promissory notes	(3,704)
Preferred Stock Warrant Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2017	1,656
Issuance of warrants	2,822
Exercise of warrants	(1,197)
Changes in fair value reflected as change in fair value of warrant liability	2,709
Balance at September 30, 2018	$ 5,990

Property and Equipment - Compos

Property and Equipment - Composition of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,978	$ 1,947
Less accumulated depreciation	(1,727)	(1,645)
Property and equipment, net	251	302
Lab Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,684	1,681
Computer Hardware, Software and Telephone
Property Plant And Equipment [Line Items]
Property and equipment, gross	200	174
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	70	70
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 24	$ 22

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Composition of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued clinical and related costs	$ 1,335	$ 197
Accrued stock offering costs	1,041
Accrued compensation and related costs	261	346
Accrued interest	110	628
Accrued other	240	110
Accrued expenses	$ 2,987	$ 1,281

Debt - Additional Information (

Debt - Additional Information (Details) - USD ($)	1 Months Ended						3 Months Ended		9 Months Ended
	Aug. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Nov. 30, 2017	Oct. 31, 2017	Jan. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Sale of stock							13,200,115		13,200,115		9,131,999
Net proceeds from the sale of redeemable convertible preferred stock									$ 17,712,000
Long-term debt, net of discount							$ 7,500,000		7,500,000
Term Loan | Silicon Valley Bank
Debt Instrument [Line Items]
Number of warrants issued					49,713
Exercise price of warrant					$ 9.659
Warrants exercisable date					Oct. 18, 2027
Interest expense							200,000		400,000
Line of credit facility, maximum borrowing capacity					$ 7,500,000
Interest only period extended					Dec. 31, 2018
Amortization period of payments of principal plus interest amounts					24 months
Percentage of aggregate principal amount					7.00%
Debt instrument, maturity date			Jun. 1, 2020
Long-term Debt, gross							7,500,000		7,500,000		$ 3,500,000
Long-term debt, net of discount							7,500,000		7,500,000		$ 3,400,000
Term Loan | Silicon Valley Bank | Tranche A
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 3,500,000
Proceeds from lines of credit				$ 3,500,000
Term Loan | Silicon Valley Bank | Tranche B
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 2,000,000		2,000,000
Line of credit facility, extended period			Apr. 30, 2018
Term Loan | Silicon Valley Bank | Tranche C
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 2,000,000			$ 2,000,000
Interest only period extended		Aug. 31, 2018
Line of credit facility, extended period		Dec. 31, 2018	Jul. 31, 2018
Term Loan | Silicon Valley Bank | Prepayment Made Prior To First Anniversary | Loan and Security Agreement
Debt Instrument [Line Items]
Prepayment fee percentage					3.00%
Term Loan | Silicon Valley Bank | Prepayment Made By Second Anniversary | Loan and Security Agreement
Debt Instrument [Line Items]
Prepayment fee percentage					2.00%
Term Loan | Silicon Valley Bank | Prepayment Made After Second Anniversary | Loan and Security Agreement
Debt Instrument [Line Items]
Prepayment fee percentage					1.00%
Term Loan | Silicon Valley Bank | Tranche B and C
Debt Instrument [Line Items]
Debt discount							$ 400,000		$ 400,000
Interest only period extended			Dec. 31, 2018
Debt instrument, maturity date			Dec. 31, 2020
Series D
Debt Instrument [Line Items]
Sale of stock	1,842,959						3,923,168		3,923,168
Sale of stock price per share	$ 9.659
Net proceeds from the sale of redeemable convertible preferred stock	$ 17,700,000
Debt converted into shares	2,080,209
Series C-1
Debt Instrument [Line Items]
Sale of stock							2,319,228		2,319,228		2,174,280
Warrants | Series C-1
Debt Instrument [Line Items]
Number of warrants issued	368,582
Exercise price of warrant	$ 0.12
2017 Notes
Debt Instrument [Line Items]
Debt instrument principal amount						$ 14,700,000
Interest rate						8.00%
Proceeds from sale of equity						$ 10,000,000
Debt instrument, Redemption price, percentage						80.00%
Debt discount							$ 3,000,000		$ 3,000,000
Interest expense							700,000	$ 600,000	3,400,000	$ 1,600,000
2017 Notes | Warrants
Debt Instrument [Line Items]
Debt discount							$ 1,700,000		$ 1,700,000
Number of warrants issued							304,397		304,397
Exercise price of warrant							$ 0.12		$ 0.12
Warrants exercisable date									Jan. 31, 2024

Debt - Summary of Future Princi

Debt - Summary of Future Principal Payments (Details) $ in Thousands	Sep. 30, 2018USD ($)
Debt Disclosure [Abstract]
2,019	$ 3,750
2,020	3,750
Total	$ 7,500

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
	Feb. 28, 2018	Dec. 31, 2014	Jan. 31, 2010	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies [Line Items]
Rent expense				$ 0.1	$ 0.1	$ 0.3	$ 0.3
Malvern, Pennsylvania
Commitments And Contingencies [Line Items]
Lease commencement date			Mar. 31, 2010
Lease expiration date			Jul. 31, 2015
Lease extended expiration date	Sep. 30, 2023	Jul. 31, 2018
Lessee, operating lease, option to extend description						In December 2014, the Malvern Lease was amended to extend its term to July 31, 2018. In February 2018, the Malvern lease was further amended to extend its term to September 30, 2023.
Lessee, operating lease, existence of option to extend [true false]						true

Commitment and Contingencies -

Commitment and Contingencies - Schedule of Future Minimum Commitments Under Non-cancelable Operating Leases (Details) $ in Thousands	Sep. 30, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018 (remaining three months)	$ 63
2,019	251
2,020	256
2,021	262
2,022	267
Thereafter	203
Total	$ 1,302

Preferred Stock Warrants - Summ

Preferred Stock Warrants - Summary of Outstanding Redeemable Convertible Preferred Stock Warrants and Corresponding Exercise Price (Details) - Redeemable Convertible Preferred Stock - $ / shares	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	708,484		486,356
Series AA Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding			1,506
Exercise Price	$ 13.28
Expiration Date	Mar. 7, 2018
2009 Series B Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	25,884		25,884
Exercise Price	$ 9.659
Expiration Date	Dec. 22, 2019
2014 Series B Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	104,856		104,856
Exercise Price	$ 0.12
Expiration Date	May 14, 2021
Convertible Debt Series C-1 Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	304,397		304,397
Exercise Price	$ 0.12
Expiration Date	Jan. 17, 2024
Term Loan Series C-1 Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	49,713		49,713
Exercise Price	$ 9.659
Expiration Date	Oct. 18, 2027
2018 Series C-1 Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	223,634
Exercise Price	$ 0.12	$ 0.12
Expiration Date	Aug. 25, 2025

Preferred Stock Warrants - Addi

Preferred Stock Warrants - Additional Information (Details) - $ / shares	Oct. 22, 2018	Sep. 30, 2018	Aug. 31, 2018
Subsequent Event | Common Stock
Class Of Warrant Or Right [Line Items]
Warrants converted into shares	632,887
Subsequent Event | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrants converted into shares	13,225,114
Subsequent Event | Initial Public Offering | Common Stock
Class Of Warrant Or Right [Line Items]
Warrants converted into shares	632,888
Subsequent Event | Weighted-average
Class Of Warrant Or Right [Line Items]
Exercise price of warrant	$ 0.12
Redeemable Convertible Preferred Stock | Subsequent Event
Class Of Warrant Or Right [Line Items]
Warrants issued to purchase shares	632,887
2018 Series C-1 Warrants | Redeemable Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants issued to purchase shares			368,582
Exercise price of warrant		$ 0.12	$ 0.12
Class of warrants exercised		144,948
2014 Series B, Convertible Debt Series C-1, and 2018 Series C-1 Warrants | Redeemable Convertible Preferred Stock | Subsequent Event | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Warrants issued to purchase shares	632,887
2014 Series B, Convertible Debt Series C-1, and 2018 Series C-1 Warrants | Redeemable Convertible Preferred Stock | Subsequent Event | Weighted-average | Initial Public Offering
Class Of Warrant Or Right [Line Items]
Exercise price of warrant	$ 0.12

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock and Stockholders' Deficit - Additional Information (Details) - USD ($)	Oct. 22, 2018	Aug. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Sale of stock			13,200,115	9,131,999
Net proceeds from the sale of redeemable convertible preferred stock			$ 17,712,000
Liquidation preference value			$ 129,071,361	$ 89,776,055
Initial Public Offering | Subsequent Event
Temporary Equity [Line Items]
Sale of stock price per share	$ 5
Redeemable convertible preferred stock converted to aggregate of common stock	13,225,114
Series D
Temporary Equity [Line Items]
Sale of stock		1,842,959	3,923,168
Sale of stock price per share		$ 9.659
Net proceeds from the sale of redeemable convertible preferred stock		$ 17,700,000
Liquidation preference per share			$ 9.659
Liquidation preference value			$ 37,893,880
Senior Stock
Temporary Equity [Line Items]
Entitled to receive noncumulative cash dividends, percentage			8.00%
Senior Series C Stock
Temporary Equity [Line Items]
Entitled to receive noncumulative cash dividends, percentage			8.00%
Series 1
Temporary Equity [Line Items]
Sale of stock			132,255	132,255
Entitled to receive noncumulative cash dividends, percentage			8.00%
Liquidation preference per share			$ 3.99
Liquidation preference value			$ 526,778	$ 526,778
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Dividends declared			$ 0
Series AA
Temporary Equity [Line Items]
Liquidation preference per share			$ 13.28
Series C-3
Temporary Equity [Line Items]
Liquidation preference per share			$ 13.28
Series B
Temporary Equity [Line Items]
Sale of stock			6,251,502	6,251,502
Liquidation preference per share			$ 9.659
Liquidation preference value			$ 60,379,282	$ 60,379,282
Series C-1
Temporary Equity [Line Items]
Sale of stock			2,319,228	2,174,280
Liquidation preference per share			$ 9.659
Liquidation preference value			$ 22,401,423	$ 20,999,997
Series C-2
Temporary Equity [Line Items]
Liquidation preference per share			$ 11.462
Series 2
Temporary Equity [Line Items]
Sale of stock			1	1
Liquidation preference value			$ 250,000	$ 250,000
Aggregate amount of redeemed outstanding preferred stock			$ 125,000

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock and Stockholders' Deficit - Summary of Authorized, Issued, and Outstanding Shares of Preferred Stock and Carrying Amounts and Liquidation Values (Details) - USD ($)	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares Authorized	18,292,696		13,321,350
Shares Issued	13,200,115		9,131,999
Shares Outstanding	13,200,115		9,131,999
Carrying Amount	$ 125,609,295		$ 89,633,543
Liquidation Value	$ 129,071,361		$ 89,776,055
Series 1
Temporary Equity [Line Items]
Shares Authorized	132,255		132,255
Shares Issued	132,255		132,255
Shares Outstanding	132,255		132,255
Carrying Amount	$ 523,838		$ 522,266
Liquidation Value	$ 526,778		$ 526,778
Series 2
Temporary Equity [Line Items]
Shares Authorized	1		1
Shares Issued	1		1
Shares Outstanding	1		1
Carrying Amount	$ 243,640		$ 240,242
Liquidation Value	$ 250,000		$ 250,000
Series AA
Temporary Equity [Line Items]
Shares Authorized	575,470		575,470
Shares Issued	573,961		573,961
Shares Outstanding	573,961		573,961
Carrying Amount	$ 7,617,120		$ 7,615,583
Liquidation Value	$ 7,619,998		$ 7,619,998
Series B
Temporary Equity [Line Items]
Shares Authorized	6,382,259		6,382,259
Shares Issued	6,251,502		6,251,502
Shares Outstanding	6,251,502		6,251,502
Carrying Amount	$ 60,371,358		$ 60,366,480
Liquidation Value	$ 60,379,282		$ 60,379,282
Series C-1
Temporary Equity [Line Items]
Shares Authorized	4,524,375		4,524,375
Shares Issued	2,319,228		2,174,280
Shares Outstanding	2,319,228		2,174,280
Carrying Amount	$ 22,140,786		$ 20,888,972
Liquidation Value	$ 22,401,423		$ 20,999,997
Series C-2
Temporary Equity [Line Items]
Shares Authorized	916,095		916,095
Series C-3
Temporary Equity [Line Items]
Shares Authorized	790,895		790,895
Series D
Temporary Equity [Line Items]
Shares Authorized	4,971,346
Shares Issued	3,923,168	1,842,959
Shares Outstanding	3,923,168
Carrying Amount	$ 34,712,552
Liquidation Value	$ 37,893,880

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock and Stockholders' Deficit - Summary of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock reserved for future issuance	15,262,054	10,738,626
Conversion of Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Common stock reserved for future issuance	13,200,115	9,131,999
Redeemable Convertible Preferred Stock Warrants
Class Of Stock [Line Items]
Common stock reserved for future issuance	708,484	486,356
Stock Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock reserved for future issuance	1,315,625	1,075,209
Authorized for Future Options
Class Of Stock [Line Items]
Common stock reserved for future issuance	37,830	45,062

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Sep. 30, 2018	Jul. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance		15,262,054	15,262,054		10,738,626
Weighted average fair value of options granted			$ 2.30
Intrinsic value of share option awards exercised			$ 27,000
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance		1,315,625	1,315,625		1,075,209
Unrecognized compensation expense		$ 600,000	$ 600,000
Weighted-average period expected to be recognized			2 years 10 months 24 days
2002 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance				800,000
Number of share grants made under the plan			260,301
Number of options or stock awards available for grant under the Plan		37,830	37,830
2002 Plan | Employee Stock Option | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, stock option vesting period		4 years
2002 Plan | Employee Stock Option | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share based compensation, stock option vesting period		10 years
2002 Plan | Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of share grants made under the plan	0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - 2002 Plan - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Options, Outstanding - Beginning balance	1,075,209
Number of Options, Granted	260,301
Number of Options, Exercised	(19,885)
Number of Options, Outstanding - Ending balance	1,315,625	1,075,209
Number of Options, Vested and expected to vest at September 30, 2018	1,217,885
Number of Options, Vested and exercisable at September 30, 2018	807,177
Weighted- Average Exercise Price, Beginning balance	$ 1.60
Weighted- Average Exercise Price, Granted	3.40
Weighted- Average Exercise Price, Exercised	2.68
Weighted- Average Exercise Price, Ending balance	1.94	$ 1.60
Weighted- Average Exercise Price, Vested and expected to vest at September 30, 2018	1.90
Weighted- Average Exercise Price, Vested and exercisable at September 30, 2018	$ 1.60
Weighted- Average Remaining Contractual Term, Balance	7 years 2 months 12 days	7 years 3 months 18 days
Weighted- Average Remaining Contractual Term, Vested and expected to vest at September 30, 2018	7 years 1 month 6 days
Weighted- Average Remaining Contractual Term, Vested and exercisable at September 30, 2018	6 years 2 months 12 days
Aggregate Intrinsic Value, Outstanding	$ 8,105,669	$ 711,328
Aggregate Intrinsic Value, Vested and expected to vest at September 30, 2018	7,553,919
Aggregate Intrinsic Value, Vested and exercisable at September 30, 2018	$ 5,243,664

Stock-Based Compensation - Esti

Stock-Based Compensation - Estimated Fair Value of Option on Grant Date using Weighted Average Assumptions (Details) - Employee Stock Option - $ / shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.87%	2.28%
Expected term (in years)	7 years	7 years 1 month 6 days
Expected volatility	69.00%	68.00%
Estimated fair value of common stock	$ 2.32	$ 1

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 97	$ 19	$ 262	$ 58
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	66	10	119	30
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 31	$ 9	$ 143	$ 28

License Agreements - Additional

License Agreements - Additional Information (Details)	1 Months Ended		3 Months Ended	9 Months Ended
	Nov. 30, 2017USD ($)	Feb. 01, 2016	Sep. 30, 2018USD ($)Milestone	Sep. 30, 2018USD ($)Milestone	Dec. 31, 2017USD ($)
License Agreements [Line Items]
Revenue			$ 411,000	$ 411,000
Duke University
License Agreements [Line Items]
Sublicensable agreement term		20 years
Sale of licensed products or services				0
Fees payment				300,000
Nonroyalty payment received				1,000,000
Duke University | License
License Agreements [Line Items]
Revenue				1,000,000
Maximum | Achievement Of Clinical Development And Regulatory Milestones | Duke University
License Agreements [Line Items]
Potential milestones payment				2,200,000
Maximum | Commercial Milestones | Duke University
License Agreements [Line Items]
Potential milestones payment				400,000
Minimum | Duke University
License Agreements [Line Items]
Aggregate royalties payment				$ 200,000
MedImmune Limited
License Agreements [Line Items]
License agreement upfront payment	$ 100,000
Number of milestones met | Milestone			0	0
Reimbursement of costs incurred to product storage costs					$ 500,000
MedImmune Limited | Maximum | Achievement Of Clinical Development And Regulatory Milestones
License Agreements [Line Items]
Potential milestones payment	18,000,000
MedImmune Limited | Maximum | Commercial Milestones
License Agreements [Line Items]
Potential milestones payment	$ 50,000,000

Grant Revenue - Additional Info

Grant Revenue - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation Of Revenue [Line Items]
Revenue		$ 411		$ 411
Type of Revenue [Extensible List]		us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember
SBIR Grants
Disaggregation Of Revenue [Line Items]
Revenue		$ 400		$ 400
Grants from the National Institutes of Health	$ 2,800
Type of Revenue [Extensible List]		us-gaap:GrantMember		us-gaap:GrantMember

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) $ / shares in Units, $ in Millions	Oct. 22, 2018USD ($)$ / sharesshares	Oct. 17, 2018shares	Oct. 31, 2018	Dec. 31, 2027shares	Nov. 29, 2018shares	Sep. 30, 2018shares	Dec. 31, 2017shares
Subsequent Event [Line Items]
Common stock, shares authorized						20,789,273	20,789,273
2018 Plan | Scenario, Forecast
Subsequent Event [Line Items]
Number of shares of common stock that may be issued				9,694,878
2018 Employee Stock Purchase Plan | Scenario, Forecast
Subsequent Event [Line Items]
Number of shares of common stock that may be issued				490,000
Subsequent Event
Subsequent Event [Line Items]
Common stock, shares authorized					200,000,000
Preferred stock authorized					10,000,000
Subsequent Event | Redeemable Convertible Preferred Stock
Subsequent Event [Line Items]
Number of warrants issued	632,887
Subsequent Event | Weighted-average
Subsequent Event [Line Items]
Exercise price of warrant | $ / shares	$ 0.12
Subsequent Event | 2018 Plan
Subsequent Event [Line Items]
Number of share grants made under the plan		0
Number of shares of common stock that may be issued		3,231,626
Plan description		The number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each calendar year, starting on January 1, 2019 through January 1, 2028, in an amount equal to 3% of the total number of shares of the Company’s capital stock outstanding on the last day of the calendar month before the date of each automatic increase, or a lesser number of shares determined by the Company’s board of directors.
Share-based payment award, percentage of outstanding stock Maximum		3.00%
Subsequent Event | Initial Public Offering
Subsequent Event [Line Items]
Common stock issued during initial public offering	9,864,666
Sale of stock price per share | $ / shares	$ 5
Proceeds from issuance of initial public offering | $	$ 43
Warrants converted into shares	13,225,114
Subsequent Event | 2018 Employee Stock Purchase Plan
Subsequent Event [Line Items]
Number of shares of common stock that may be issued		196,000
Plan description		The number of shares of common stock reserved for issuance will automatically increase on January 1 of each calendar year, beginning on January 1, 2019 through January 1, 2028, by the lesser of (1) 1% of the total number of shares of the Company’s common stock outstanding on the last day of the calendar month before the date of the automatic increase, and (2) 490,000 shares; provided that before the date of any such increase, the Company’s board of directors may determine that such increase will be less than the amount set forth in clauses (1) and (2).
Share-based payment award, percentage of outstanding stock Maximum		1.00%
Common stock purchased		0
Subsequent Event | Common Stock
Subsequent Event [Line Items]
Reverse stock split conversion ratio			11.0634
Warrants converted into shares	632,887
Subsequent Event | Common Stock | Initial Public Offering
Subsequent Event [Line Items]
Warrants converted into shares	632,888