Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2019	Nov. 04, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	EIGR
Entity Registrant Name	Eiger BioPharmaceuticals, Inc.
Entity Central Index Key	0001305253
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		24,495,047
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-36183
Entity Tax Identification Number	33-0971591
Entity Address, Address Line One	2155 Park Boulevard
Entity Address, City or Town	Palo Alto
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94306
City Area Code	650
Local Phone Number	272-6138
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock (par value $0.001 per share)
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 19,406	$ 61,262
Debt securities, available-for-sale	90,545	39,091
Prepaid expenses and other current assets	4,459	1,492
Total current assets	114,410	101,845
Property and equipment, net	538	167
Operating lease right-of-use assets	1,659
Other assets	3,388	436
Total assets	119,995	102,448
Current liabilities:
Accounts payable	9,057	5,830
Accrued liabilities	7,222	4,194
Current portion of operating lease liabilities	466
Total current liabilities	16,745	10,024
Long-term debt, net	30,202	25,620
Operating lease liabilities	1,389
Other long-term liabilities		212
Total liabilities	48,336	35,856
Stockholders’ equity:
Common stock	24	19
Additional paid-in capital	296,121	237,795
Accumulated other comprehensive income (loss)	43	(25)
Accumulated deficit	(224,529)	(171,197)
Total stockholders’ equity	71,659	66,592
Total liabilities and stockholders’ equity	$ 119,995	$ 102,448

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 14,059	$ 13,196	$ 39,863	$ 25,080
General and administrative	4,247	3,643	12,529	9,874
Total operating expenses	18,306	16,839	52,392	34,954
Loss from operations	(18,306)	(16,839)	(52,392)	(34,954)
Interest expense	(884)	(681)	(2,518)	(1,574)
Interest income	585	371	1,598	654
Other (expense) income, net	(11)	5	(20)	(16)
Net loss	$ (18,616)	$ (17,144)	$ (53,332)	$ (35,890)
Net loss per common share, basic and diluted	$ (0.76)	$ (1.20)	$ (2.40)	$ (2.92)
Weighted-average common shares outstanding, basic and diluted	24,437,451	14,255,843	22,261,715	12,290,500

Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (18,616)	$ (17,144)	$ (53,332)	$ (35,890)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale debt securities, net	38	8	68	(3)
Comprehensive loss	$ (18,578)	$ (17,136)	$ (53,264)	$ (35,893)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 22,522	$ 11	$ 141,320	$ (3)	$ (118,806)
Balance, beginning of period, Shares at Dec. 31, 2017		10,526,599
Issuance of common stock upon ESPP purchase	34		34
Issuance of common stock upon ESPP purchase, Shares		9,522
Stock-based compensation expense	1,003		1,003
Unrealized gain (loss) on debt securities, net	3			3
Net loss	(8,831)				(8,831)
Balance at Mar. 31, 2018	14,731	$ 11	142,357		(127,637)
Balance, end of period, Shares at Mar. 31, 2018		10,536,121
Balance at Dec. 31, 2017	22,522	$ 11	141,320	(3)	(118,806)
Balance, beginning of period, Shares at Dec. 31, 2017		10,526,599
Unrealized gain (loss) on debt securities, net	(3)
Net loss	(35,890)
Balance at Sep. 30, 2018	33,767	$ 14	188,455	(6)	(154,696)
Balance, end of period, Shares at Sep. 30, 2018		14,379,413
Balance at Mar. 31, 2018	14,731	$ 11	142,357		(127,637)
Balance, beginning of period, Shares at Mar. 31, 2018		10,536,121
Issuance of common stock upon exercise of stock options	268		268
Issuance of common stock upon exercise of stock options, Shares		28,151
Issuance of common stock upon public offering, net of issuance costs	42,890	$ 3	42,887
Issuance of common stock upon public offering, net of issuance costs, shares		3,680,000
Stock-based compensation expense	1,321		1,321
Unrealized gain (loss) on debt securities, net	(14)			(14)
Net loss	(9,915)				(9,915)
Balance at Jun. 30, 2018	49,281	$ 14	186,833	(14)	(137,552)
Balance, end of period, Shares at Jun. 30, 2018		14,244,272
Issuance of common stock upon exercise of stock options	27		27
Issuance of common stock upon exercise of stock options, Shares		11,629
Vesting of common stock issued under Product Development Agreement	361		361
Issuance of common stock under Product Development Agreement, Shares		115,526
Issuance of common stock upon ESPP purchase	62		62
Issuance of common stock upon ESPP purchase, Shares		7,986
Stock-based compensation expense	1,172		1,172
Unrealized gain (loss) on debt securities, net	8			8
Net loss	(17,144)				(17,144)
Balance at Sep. 30, 2018	33,767	$ 14	188,455	(6)	(154,696)
Balance, end of period, Shares at Sep. 30, 2018		14,379,413
Balance at Dec. 31, 2018	66,592	$ 19	237,795	(25)	(171,197)
Balance, beginning of period, Shares at Dec. 31, 2018		19,211,759
Issuance of common stock upon exercise of stock options	65		65
Issuance of common stock upon exercise of stock options, Shares		41,546
Vesting of common stock issued under Product Development Agreement	56		56
Issuance of common stock upon ESPP purchase	59		59
Issuance of common stock upon ESPP purchase, Shares		7,138
Stock-based compensation expense	1,195		1,195
Unrealized gain (loss) on debt securities, net	30			30
Net loss	(17,189)				(17,189)
Balance at Mar. 31, 2019	50,808	$ 19	239,170	5	(188,386)
Balance, end of period, Shares at Mar. 31, 2019		19,260,443
Balance at Dec. 31, 2018	$ 66,592	$ 19	237,795	(25)	(171,197)
Balance, beginning of period, Shares at Dec. 31, 2018		19,211,759
Issuance of common stock upon exercise of stock options, Shares	91,725
Unrealized gain (loss) on debt securities, net	$ 68
Net loss	(53,332)
Balance at Sep. 30, 2019	71,659	$ 24	296,121	43	(224,529)
Balance, end of period, Shares at Sep. 30, 2019		24,494,185
Balance at Mar. 31, 2019	50,808	$ 19	239,170	5	(188,386)
Balance, beginning of period, Shares at Mar. 31, 2019		19,260,443
Issuance of common stock upon exercise of stock options	62		62
Issuance of common stock upon exercise of stock options, Shares		10,304
Vesting of common stock issued under Product Development Agreement	55		55
Issuance of common stock upon public offering, net of issuance costs	53,194	$ 5	53,189
Issuance of common stock upon public offering, net of issuance costs, shares		5,175,000
Stock-based compensation expense	1,487		1,487
Net loss	(17,527)				(17,527)
Balance at Jun. 30, 2019	88,079	$ 24	293,963	5	(205,913)
Balance, end of period, Shares at Jun. 30, 2019		24,445,747
Issuance of common stock upon exercise of stock options	379		379
Issuance of common stock upon exercise of stock options, Shares		39,875
Vesting of common stock issued under Product Development Agreement	59		59
Issuance of common stock upon ESPP purchase	71		71
Issuance of common stock upon ESPP purchase, Shares		8,563
Stock-based compensation expense	1,649		1,649
Unrealized gain (loss) on debt securities, net	38			38
Net loss	(18,616)				(18,616)
Balance at Sep. 30, 2019	$ 71,659	$ 24	$ 296,121	$ 43	$ (224,529)
Balance, end of period, Shares at Sep. 30, 2019		24,494,185

Condensed Consolidated Statements of Cash Flow (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net loss	$ (53,332)	$ (35,890)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	41	34
Amortization of debt securities discounts	(390)	(257)
Non-cash interest expense	535	393
Amortization of operating lease right-of-use assets	299
Common stock issued under Product Development Agreement	170	361
Stock-based compensation	4,331	3,496
Change in operating assets and liabilities:
Prepaid expenses and other current assets	(2,967)	(1,152)
Other assets	(2,952)	(77)
Accounts payable	3,178	3,334
Accrued liabilities	2,759	(625)
Operating lease liabilities	(315)
Other long-term liabilities		214
Net cash used in operating activities	(48,643)	(30,169)
Investing activities
Purchase of debt securities available-for-sale	(96,267)	(52,265)
Proceeds from maturities of debt securities available-for-sale	45,271	14,250
Purchase of property and equipment	(94)	(140)
Net cash used in investing activities	(51,090)	(38,155)
Financing activities
Proceeds from issuance of common stock upon public offering, net of issuance costs	53,194	42,890
Proceeds from borrowings in connection with term loan, net of issuance costs	6,627	9,935
Repayment of accrued exit fee and second amendment fee	(913)
Repayment of term loan	(1,667)
Proceeds from issuance of common stock upon stock option exercises	506	295
Proceeds from issuance of common stock upon ESPP purchase	130	96
Net cash provided by financing activities	57,877	53,216
Net decrease in cash and cash equivalents	(41,856)	(15,108)
Cash and cash equivalents at beginning of period	61,262	32,035
Cash and cash equivalents at end of period	19,406	16,927
Supplemental disclosure of cash flow information:
Interest paid	$ 1,944	$ 1,106

Description of Business

9 Months Ended

Sep. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Description of Business

1.	Description of Business

Eiger BioPharmaceuticals, Inc. (the “Company” or “Eiger”) was incorporated in the State of Delaware on November 6, 2008. Eiger is a late-stage biopharmaceutical company focused on the development and commercialization of well-characterized drugs for life-threatening, rare and ultra-rare diseases with high unmet medical needs and no approved therapies. Eiger has reported positive proof-of-concept clinical results in four programs: lonafarnib monotherapy, lonafarnib boosted with ritonavir, peginterferon lambda monotherapy and in combination with lonafarnib boosted with ritonavir, and avexitide, all with first-in-class drugs, now advancing into submission for regulatory approvals or Phase 3 clinical development.

The Company’s programs have several aspects in common: the disease targets represent conditions of high unmet medical need with no approved therapies; the therapeutic approaches are supported by an understanding of disease biology and mechanism as elucidated by academic research relationships; prior clinical experience with the product candidates guides an understanding of safety; and the development paths leverage the experience and capabilities of the Company’s experienced, commercially-focused management team.

Eiger’s lead program is in Phase 3, developing lonafarnib, a first-in-class prenylation inhibitor, boosted with ritonavir, for the treatment of Hepatitis Delta Virus (“HDV”) infection, the most severe form of human viral hepatitis for which there is currently no approved therapy. The Company is also developing lonafarnib monotherapy for treatment of Progeria and Progeroid Laminopathies. Progeria, also known as Hutchinson-Gilford Progeria Syndrome (“HGPS”), and Progeroid Laminopathies, are ultra-rare and rapidly fatal genetic conditions of accelerated aging in children. Peginterferon lambda (“lambda”) is the Company’s second program treating HDV. Lambda is a well-characterized, late-stage, first in class, type III interferon. The Company is developing avexitide, a well-characterized peptide, as a treatment for post-bariatric hypoglycemia (“PBH”). PBH is a debilitating and potentially life-threatening condition for which there is currently no approved therapy. Avexitide has also demonstrated clinical proof of concept for treatment of Congenital Hyperinsulinism (“CHI”), an ultra-rare pediatric metabolic disorder.

The Company’s principal operations are based in Palo Alto, California, and it operates in one segment.

Liquidity

As of September 30, 2019, the Company had $109.9 million of cash, cash equivalents and investments, comprised of $19.4 million of cash and cash equivalents and $90.5 million of debt securities available-for-sale. The Company had an accumulated deficit of $224.5 million and negative cash flows from operating activities as of September 30, 2019. The Company expects to continue to incur losses for the next several years.

On April 17, 2019, the Company completed an underwritten public offering of 5,175,000 shares of its common stock, including 675,000 shares sold upon full exercise of the underwriters’ option to purchase additional shares of common stock, at a price of $11.00 per share. The offering was made under Eiger’s effective shelf registration statement and resulted in net proceeds to the Company of $53.2 million, after deducting underwriting discounts and commissions and offering expenses.

Management believes that the currently available resources will be sufficient to fund its operations for at least the next 12 months following the issuance date of these unaudited condensed consolidated financial statements.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The unaudited condensed consolidated financial statements include the accounts of Eiger BioPharmaceuticals, Inc. and its wholly owned subsidiaries, EBPI Merger Inc., EB Pharma LLC, Eiger BioPharmaceuticals Europe Limited, and EigerBio Europe Limited have been prepared in accordance with accounting principles generally accepted in the United States of America, (“U.S. GAAP”) and following the requirements of the Securities and Exchange Commission (the “SEC”) for interim reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP can be condensed or omitted. These financial statements have been prepared on the same basis as the Company’s annual financial statements and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the Company’s financial information. These interim results are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other interim period or for any other future year. The balance sheet as of December 31, 2018, has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. All intercompany balances and transactions have been eliminated in consolidation.

The accompanying unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto contained in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission on March 14, 2019.

Use of Estimates

The preparation of unaudited condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates its estimates, including those related to clinical trial accrued liabilities, stock-based compensation and income taxes. The Company bases its estimates on historical experience and on various other market-specific and relevant assumptions that the Company believes to be reasonable under the circumstances. Actual results could differ from those estimates.

Debt Securities

Short-term securities consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than 365 days from the date of acquisition. All short-term securities are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale securities are excluded from earnings and are reported as a component of accumulated other comprehensive income (loss). The cost of available-for-sale securities sold is based on the specific-identification method. Realized gains and losses on the sale of debt securities are determined using the specific-identification method and recorded in other (expense) income, net on the accompanying unaudited condensed consolidated statements of operations.

Accrued Research and Development Costs

The Company accrues for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced and includes these costs in accrued liabilities in the unaudited condensed consolidated balance sheets and within research and development expense in the unaudited condensed consolidated statements of operations. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers. The Company makes judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities.

Leases

The Company has a real estate lease for its office space in Palo Alto, California. The Company determines the initial classification and measurement of its right-of-use assets and lease liabilities at the lease commencement date and thereafter if modified. The lease term includes any renewal options and termination options that the Company is reasonably assured to exercise. The present value of lease payments is determined by using the interest rate implicit in the lease, if that rate is readily determinable; otherwise, the Company uses its incremental borrowing rate. The incremental borrowing rate is determined by using the rate of interest that the Company would pay to borrow on a collateralized basis an amount equal to the lease payments for a similar term and in a similar economic environment.

Rent expense for operating leases is recognized on a straight-line basis, unless the operating lease right-of-use assets have been impaired, over the reasonably assured lease term based on the total lease payments and is included in operating expenses in the unaudited condensed consolidated statements of operations. For operating leases that reflect impairment, the Company will recognize the amortization of the operating lease right-of-use assets on a straight-line basis over the remaining lease term with rent expense still included in general and administrative expenses in the unaudited condensed consolidated statements of operations.

The Company has elected the practical expedient to not separate lease and non-lease components. The Company’s non-lease components are primarily related to property maintenance and insurance, which varies based on future outcomes, and thus is recognized in general and administrative expenses when incurred.

Net Loss per Share

Basic net loss per share of common stock is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration for potentially dilutive securities. Since the Company was in a loss position for all periods presented, diluted net loss per share is the same as basic net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive.

The following table sets forth the outstanding potentially dilutive securities which have been excluded in the calculation of diluted net loss per share because including such securities would be anti-dilutive (in common stock equivalent shares):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Options to purchase common stock		2,862,520		2,079,497		2,862,520		2,079,497
Warrants to purchase common stock		—		10,180		—		10,180
Total		2,862,520		2,089,677		2,862,520		2,089,677

Recently Adopted Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842), which requires lessees to recognize most leases on their balance sheet. The new standard is effective for fiscal years beginning after December 15, 2018. Early adoption is permitted. Originally, entities were required to adopt ASU 2016-02 using a modified retrospective transition method. However, in July 2018, the FASB issued ASU No. 2018-11, Targeted Improvements to Leases (Topic 842), which provides entities with an additional transition method. Under ASU No. 2018-11, entities have the option of recognizing the cumulative effect of applying the new standard as an adjustment to beginning retained earnings in the year of adoption while continuing to present all prior periods under previous lease accounting guidance. In July 2018, the FASB issued ASU No. 2018-10, Codification Improvements to Leases (Topic 842), which clarifies how to apply certain aspects of ASU 2016-02. Additionally, in March 2019, the FASB issued ASU No. 2019-01, Codification Improvements to Leases (Topic 842), which clarifies the transition disclosure requirements. The Company adopted this guidance on January 1, 2019 using the prospective transition method allowed per ASU 2018-11, and applied the standard only to leases that existed at that date. Under the prospective transition method, the Company does not need to restate the comparative period in transition and will continue to present financial information and disclosures for periods before January 1, 2019 in accordance with ASC Topic 840. The Company has elected the package of practical expedients allowed under ASC Topic 842, which permits the Company to account for its existing operating leases as operating leases under the new guidance, without reassessing the Company’s prior conclusions about lease identification, lease classification and initial direct cost. As a result of the adoption of the new lease accounting guidance, the Company recognized, on January 1, 2019, operating lease right–of–use assets of $2.0 million and operating lease liabilities of $2.2 million in the unaudited condensed consolidated balance sheet. The adoption of the standard did not have a material impact on the unaudited condensed consolidated statement of operations and the unaudited condensed consolidated statement of cash flows.

Accounting Pronouncements Not Yet Adopted

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326). The standard changes how entities will measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. Financial assets measured at amortized cost will be presented at the net amount expected to be collected by using an allowance for credit losses. In April 2019, the FASB issued ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, which clarifies and corrects certain unintended applications of the guidance contained in each of the amended Topics. Additionally, in May 2019, the FASB issued ASU No. 2019-05, Financial Instruments – Credit Losses (Topic 326), which provides an option to irrevocably elect to measure certain individual financial assets at fair value instead of amortized cost. The standard is effective for fiscal years and interim periods beginning after December 15, 2019. Early adoption is permitted for all periods beginning after December 15, 2018. In October 2019, the FASB proposed a deferral of the effective date for all entities, except public companies that are not smaller reporting companies, to fiscal years beginning after December 15, 2022, including interim periods within those fiscal years. This proposal has not been finalized as of the date of this report. When finalized, the Company plans to adopt the standard on January 1, 2023. The Company does not plan to early adopt and is currently in the process of evaluating the impact the standard will have on its consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820). The standard eliminates, modifies and adds disclosure requirements for fair value measurements. The pronouncement is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company is currently in the process of evaluating the impact the standard will have on its consolidated financial statements.

Fair Value Measurements

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

Fair value accounting is applied for all financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis (at least annually). At September 30, 2019 and December 31, 2018, the carrying amount of prepaid expenses and other current assets, accounts payable and accrued liabilities approximated their estimate fair value due to their relatively short maturities. Management believes the terms of long-term debt reflect current market conditions for an instrument with similar terms and maturity, therefore the carrying value of the Company’s debt approximated its fair value.

Assets and liabilities recorded at fair value on a recurring basis in the unaudited condensed consolidated balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

Level 1: Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 2: Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and

Level 3: Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.

The Company’s money market funds are classified as Level 1 because they are valued using quoted market prices. The Company’s debt securities consist of available-for-sale securities and are classified as Level 2 because their value is based on valuations using significant inputs derived from or corroborated by observable market data. There were no assets or liabilities classified as Level 3 as of September 30, 2019 and December 31, 2018.

There were no transfers between Level 1, Level 2 or Level 3 of the fair value hierarchy during the periods presented.

The following tables present the fair value hierarchy for assets and liabilities measured at fair value (in thousands):

	September 30, 2019
	Level 1		Level 2		Level 3		Total
Financial assets:
Money market funds	$	15,475	$	—	$	—	$	15,475
Corporate debt securities		—		17,639		—		17,639
Commercial paper		—		17,415		—		17,415
U.S. treasury bills		—		10,985		—		10,985
U.S. government bonds		—		44,506		—		44,506
Total	$	15,475	$	90,545	$	—	$	106,020

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Financial assets:
Money market funds	$	45,441	$	—	$	—	$	45,441
Corporate debt securities		—		23,474		—		23,474
Commercial paper		—		15,617		—		15,617
Total	$	45,441	$	39,091	$	—	$	84,532

There were no financial liabilities as of September 30, 2019 and December 31, 2018.

The following tables summarize the estimated value of the Company’s cash equivalents and debt securities and the gross unrealized holding gains and losses (in thousands):

	September 30, 2019
	Amortized cost		Unrealized gain		Unrealized loss			Estimated Fair Value
Cash equivalents:
Money market funds	$	15,475	$	—	$	—		$	15,475
Total cash equivalents	$	15,475	$	—	$	—		$	15,475
Debt securities:
Corporate debt securities	$	17,628	$	12	$	(1	)	$	17,639
Commercial paper		17,415		—		—			17,415
U.S. treasury bills		10,982		3		—			10,985
U.S. government bonds		44,477		29		—			44,506
Total debt securities	$	90,502	$	44	$	(1	)	$	90,545

	December 31, 2018
	Amortized cost		Unrealized gain		Unrealized loss			Estimated Fair Value
Cash equivalents:
Money market funds	$	45,441	$	—	$	—		$	45,441
Total cash equivalents	$	45,441	$	—	$	—		$	45,441
Debt securities:
Corporate debt securities	$	23,489	$	1	$	(16	)	$	23,474
Commercial paper		15,627		—		(10	)		15,617
Total debt securities	$	39,116	$	1	$	(26	)	$	39,091

As of December 31, 2018 and September 30, 2019, the contractual maturity of the available-for-sale debt securities is less than one year. The Company periodically reviews the available-for-sale investments for other-than-temporary impairment loss. The Company considers factors such as the duration, severity and the reason for the decline in value, the potential recovery period and its intent to sell. For debt securities, it also considers whether (i) it is more likely than not that the Company will be required to sell the debt securities before recovery of their amortized cost basis, and (ii) the amortized cost basis cannot be recovered as a result of credit losses. During the nine months ended September 30, 2019, the Company did not recognize any other-than-temporary impairment losses. All debt securities with unrealized losses have been in a loss position for less than twelve months.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Accrued Liabilities

9 Months Ended

Sep. 30, 2019

Payables And Accruals [Abstract]

Accrued Liabilities

4.	Accrued Liabilities

Accrued liabilities consist of the following (in thousands):

	September 30,		December 31,
	2019		2018
Contract research costs	$	3,992	$	2,191
Contract manufacturing costs		1,532		—
Compensation and related benefits		1,407		1,705
Consulting costs		184		258
Franchise tax		70		40
Other		37		—
Total accrued liabilities	$	7,222	$	4,194

License and Product Development Agreement

9 Months Ended

Sep. 30, 2019

Collaboration And License Agreements [Abstract]

License and Product Development Agreement

5.	License and Product Development Agreements

Product Development Agreement

On August 11, 2018, the Company entered into a Product Development Agreement and a First Project Agreement (the “Product Agreements”) with RRD International, LLC, pursuant to which the Company will receive development program support services for its HDV program. The services are to be provided from July 1, 2018 through the completion of the Phase 3 Clinical Study Reports and the subsequent NDA filing. As consideration, the Company has committed to pay fees of approximately $10.0 million in cash and stock over four years, including approximately $0.8 million for expert consultant fees and pass through costs to vendors, plus certain incentive-based regulatory milestone fees up to $1.0 million. In January 2019, the Company entered into the first amendment to the Product Agreements, which increased the Product Agreements’ fees by $1.0 million. As part of the aggregate payment, the Company issued 115,526 shares of common stock subject to quarterly vesting requirements related to successful performance of the services and achievement of a budget timeline set forth in the Product Agreements. The Product Agreements can be terminated by either party due to material breach or by the Company without cause with 90 days prior written notice. For the three and nine months ended September 30, 2019, the Company recognized research and development expense of $0.1 million and $0.2 million, respectively, related to the shares issued under the Product Agreements.

License Agreement with the Trustees of the University of Pennsylvania and the Children’s Hospital of Pennsylvania

In May 2019, the Company entered into a license agreement (the “UPenn/CHOP Agreement”) with the Trustees of the University of Pennsylvania (“UPenn”) and the Children’s Hospital of Pennsylvania (“CHOP”), under which the Company obtained an exclusive, royalty-bearing, worldwide license to develop, manufacture and sell certain Glucagon Like Peptide-1 (“GLP-1”) receptor antagonist(s) products to treat all human and animal conditions. The Company also obtained an exclusive, royalty-bearing, sublicenseable, worldwide license to certain data developed by CHOP. The Company is responsible for the development and commercialization of the licensed products at its sole cost and expense.

As part of the consideration for the rights granted to the Company under the UPenn/CHOP Agreement, the Company paid UPenn a one-time, non-refundable issue fee of $1.0 million, which is recorded in research and development expenses for the nine months ended September 30, 2019. In addition, the Company is obligated to pay UPenn a specified annual license maintenance fee up to $2.5 million in certain regulatory milestones, and up to $18.0 million in commercial milestones. The Company will also be required to pay UPenn a flat royalty in the low-single digits on net sales of all licensed products by the Company, subject to specified reductions and offsets, and specified minimum annual royalty payments. The Company’s obligation to pay royalties expires on a product-by-product and country-by-country basis, on the later of: (i) the expiration of the last valid claim in the licensed patents in any country, or (ii) the tenth anniversary of the first commercial sale of the product in such country. No milestones have been achieved as of September 30, 2019.

The Company may terminate the UPenn/CHOP Agreement in its entirety for any reason by providing prior written notice to UPenn and CHOP. UPenn or CHOP may terminate the UPenn/CHOP Agreement, upon a written notice, for the Company’s failure to achieve the specified diligence milestones within the specified periods, subject to the Company’s extension rights.

Debt

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Debt

In December 2016, the Company entered into an aggregate $25.0 million loan with Oxford Finance LLC (the “Oxford Loan”). The loan matures on July 1, 2021. The Company borrowed $15.0 million in December 2016 (“Tranche A”). In May 2018, the Company entered into an amendment to the Oxford Loan and borrowed $5.0 million (“Tranche B”). On August 3, 2018, the Company borrowed the remaining $5.0 million (“Tranche C”) under the Oxford Loan.

The Oxford Loan bears interest at a floating rate per annum equal to the greater of either the 30-day U.S. Dollar LIBOR reported in the Wall Street Journal plus 6.41% or 6.95%. The interest only period for borrowed funds is until February 1, 2019, followed by 30 equal monthly payments of principal plus accrued interest. At the time of final payment, the Company is required to pay an exit fee of 7.5% of the original principal balance of borrowed funds, or $1.9 million. In addition, at the time of final payment of Tranche B, the Company is required to pay an additional exit fee of $0.1 million. The Company recorded as a liability and debt discount the exit fee at the origination of the term loan. In addition, the Company incurred loan origination fees and debt issuance costs of $0.4 million which were recorded as a direct deduction from the carrying amount of the related debt liability as a debt discount.

On March 5, 2019, the Company entered into the third amendment to the Oxford Loan (the “Amended Oxford Loan”) to refinance the Oxford Loan. The Amended Oxford Loan increased the aggregate amount available to be borrowed to $35.0 million and extended the maturity date to March 1, 2024. On March 5, 2019, prior to entering into the Amended Oxford Loan, the outstanding balance of the Oxford Loan was $23.3 million. At the time of entering into the Amended Oxford Loan, the Company borrowed an additional $6.7 million in principal under the Amended Oxford Loan, which increased the total amount borrowed to $30.0 million (“Amended Tranche A”). The remaining $5.0 million (“Amended Tranche B”) will be available to the Company upon the latest to occur of (i) achievement of positive Lonafarnib Phase 3 HDV topline data sufficient to file new drug application (“Clinical Milestone”) and (ii) January 1, 2021.

The Amended Oxford Loan bears interest at a floating rate per annum equal to the greater of either the 30-day U.S. Dollar LIBOR reported in the Wall Street Journal plus 6.64% or 9.15%. The Amended Oxford Loan has an interest only period until April 1, 2021, followed by 36 equal monthly payments of principal and interest. Upon the receipt of Amended Tranche B, the interest only period for borrowed funds will be extended by one year until April 1, 2022, followed by 24 equal monthly payments of principal plus accrued interest. At the time of final payment, the Company is required to pay an exit fee of 7.5% of the original principal balance of borrowed funds, or $2.3 million. In addition, the Company is required to pay an additional exit fee of $1.0 million. The Company recorded as a liability and debt discount the exit fee for the Amended Oxford Loan. At the date of the Amended Oxford Loan, the Company paid $0.9 million for the accrued portion of the Oxford Loan exit fee and the Tranche B additional exit fee. The loan discount balance at the time of the Amended Oxford Loan was $0.2 million, which is being amortized over the remaining term of the Amended Oxford Loan.

The Company is also required to pay a 5.0% success fee of the total amount drawn under the Amended Oxford Loan within 30 days following the FDA’s approval of the Company’s first product, excluding lonafarnib in Progeria and Progeroid Laminopathies. This fee is enforceable within 10 years from the funding of Amended Tranche A. The Company determined that the success fee met the scope exemption from derivative accounting and should be accounted for under the guidance for contingencies. Accordingly, the Company will record a liability for the success fee upon receipt of approval from the FDA. The Amended Oxford Loan includes contingent interest features and mandatory prepayment features upon an event of default that meet the definition of a derivative but were not bifurcated from the debt instrument as their fair value was deemed to be insignificant. In connection with the execution of the Oxford Loan, the Company agreed to certain customary representations and warranties.

The refinancing of the term loan did not have a material impact on terms, conditions, representations, warranties, covenants or agreements set forth in the Oxford Loan. The loan is secured by the perfected first priority liens on the Company's assets, including a commitment by the Company to not allow any liens to be placed upon the Company’s intellectual property. The loan includes customary events of default, including failure to pay amounts due, breaches of covenants and warranties, material adverse effect events, certain cross defaults and judgments, and insolvency. If the Company is unable to comply with these covenants or if the Company defaults on any portion of the outstanding borrowings, the lenders can also impose a 5.0% penalty and restrict access to additional borrowings under the loan and security agreement. The Company was in compliance with the terms under the loan as of September 30, 2019 and December 31, 2018.

The Company is permitted to make voluntary prepayments of the Amended Oxford Loan with a prepayment fee, calculated as of the loan origination date, equal to (i) 2.0% of the loan prepaid during the first 12 months and (ii) 1.0% of the loan prepaid in months 13-24. The Company is required to make mandatory prepayments of the outstanding loan upon the acceleration by lender following the occurrence of an event of default, along with a payment of the final payment, the prepayment fee and any other obligations that are due and payable at the time of prepayment.

The Company accounts for the amortization of the debt discount utilizing the effective interest method. Long-term debt and unamortized discount balances are as follows (in thousands):

	September 30,			December 31,
	2019			2018
Face value of long-term debt	$	30,000		$	25,000
Exit fee		3,277			1,960
Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees		(3,075	)		(1,340	)
Long-term debt, net	$	30,202		$	25,620

Stock-Based Compensation

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

7.	Stock-Based Compensation

The following table summarizes stock option activity under the Company’s stock-based compensation plan during the nine months ended September 30, 2019 (in thousands, except option and share data):

	Shares Available for Grant			Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		755,337			1,996,211		$	11.80		7.54	$	2,347
Additional options authorized		960,588			—
Granted		(1,079,750	)		1,079,750		$	12.93
Exercised		—			(91,725	)	$	6.33
Canceled and forfeited		121,716			(121,716	)	$	20.46
Outstanding as of September 30, 2019		757,891			2,862,520		$	12.03		7.94	$	2,044
Vested and exercisable as of September 30, 2019					1,337,773		$	11.86		6.86	$	1,645

During the three and nine months ended September 30, 2019, the Company granted stock options for 217,000 and 1,079,750 shares, respectively. The weighted-average grant date fair value of these options was $7.18 and $9.01 for the three and nine months ended September 30, 2019, respectively. During the nine months ended September 30, 2018, the Company granted its employees stock options for 799,500 shares at a weighted-average grant date fair value of $7.50. There were no options granted during the three months ended September 30, 2018.

The Company records stock-based compensation of stock options granted by estimating the fair value of stock-based awards using the Black-Scholes option pricing model and amortizes the fair value of the stock-based awards granted over the applicable vesting period of the awards on a straight-line basis. The fair value of stock options was estimated using the following weighted-average assumptions:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2019	2018	2019		2018
Expected term (in years)	6.02-6.08	—	5.27-6.08		5.27-6.08
Contractual term (in years)	—	—		10.00		10.00
Volatility	74.19%-75.70%	—	74.19%-83.19%		84.00%-84.50%
Risk free interest rate	1.42%-1.90%	—	1.42%-2.57%		2.35%-2.68%
Dividend yield	—	—	—		—

Stock-Based Compensation Expense

Total stock-based compensation expense recognized was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	559	$	344	$	1,366	$	1,138
General and administrative		1,090		828		2,965		2,358
Total	$	1,649	$	1,172	$	4,331	$	3,496

As of September 30, 2019, the total unrecognized compensation expense related to unvested options was $12.5 million, which the Company expects to recognize over an estimated weighted average period of 2.8 years.

Commitments and Contingencies

9 Months Ended

Sep. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

9.	Commitments and Contingencies

Lease Agreements

In October 2017, the Company entered into a non-cancelable operating facility lease agreement for 8,029 square feet of office space located at 2155 Park Blvd. in Palo Alto, California 94306. The lease commenced on March 1, 2018 and expires in February 2023. The lease has a three-year renewal option prior to expiration; however, the Company is not reasonably assured to exercise this option. The lease includes rent escalation clauses throughout the lease term. In October 2017, the Company provided a security deposit of $0.3 million. The Company also has three additional operating leases that are included in its lease accounting but are not considered significant for disclosure.

The maturity of the Company’s operating lease liabilities as of September 30, 2019 and future minimum lease payments as of December 31, 2018 were as follows (in thousands):

Undiscounted lease payments	September 30, 2019		December 31, 2018
2019	$	151	$	593
2020		616		610
2021		633		629
2022		647		647
2023		109		109
Total undiscounted payments		2,156	$	2,588
Less: imputed interest		301
Present value of future lease payments		1,855
Less: current portion of operating lease liabilities		466
Operating lease liabilities	$	1,389

Rent expense recognized for the Company’s operating leases was $0.1 million and $0.4 million for the three months ended September 30, 2019 and 2018, respectively, and $0.4 million and $0.6 million for the nine months ended September 30, 2019 and 2018, respectively. Under the terms of the lease agreements, the Company is also responsible for certain variable lease payments that are not included in the measurement of the lease liability. Variable lease payments for the operating leases were $21,400 and $0.1 million for the three and nine months ended September 30, 2019, respectively.

The operating cash outflows for the operating lease liabilities were $0.5 million for the nine months ended September 30, 2019. As of September 30, 2019, the weighted-average remaining lease term and weighted-average discount rate were 3.4 years and 9.15%, respectively.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Debt Securities

Accrued Research and Development Costs

Leases

Net Loss per Share

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Options to purchase common stock		2,862,520		2,079,497		2,862,520		2,079,497
Warrants to purchase common stock		—		10,180		—		10,180
Total		2,862,520		2,089,677		2,862,520		2,089,677

Recent Accounting Pronouncements

Recently Adopted Accounting Pronouncements

Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Schedule of Antidilutive Securities Excluded from Computation of Earnings per Share

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Options to purchase common stock		2,862,520		2,079,497		2,862,520		2,079,497
Warrants to purchase common stock		—		10,180		—		10,180
Total		2,862,520		2,089,677		2,862,520		2,089,677

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Measured at Fair Value

The following tables present the fair value hierarchy for assets and liabilities measured at fair value (in thousands):

	September 30, 2019
	Level 1		Level 2		Level 3		Total
Financial assets:
Money market funds	$	15,475	$	—	$	—	$	15,475
Corporate debt securities		—		17,639		—		17,639
Commercial paper		—		17,415		—		17,415
U.S. treasury bills		—		10,985		—		10,985
U.S. government bonds		—		44,506		—		44,506
Total	$	15,475	$	90,545	$	—	$	106,020

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Financial assets:
Money market funds	$	45,441	$	—	$	—	$	45,441
Corporate debt securities		—		23,474		—		23,474
Commercial paper		—		15,617		—		15,617
Total	$	45,441	$	39,091	$	—	$	84,532

Summary of Estimated Value of Cash Equivalents and Debt Securities and Gross Unrealized Holding Gains and Losses

The following tables summarize the estimated value of the Company’s cash equivalents and debt securities and the gross unrealized holding gains and losses (in thousands):

	September 30, 2019
	Amortized cost		Unrealized gain		Unrealized loss			Estimated Fair Value
Cash equivalents:
Money market funds	$	15,475	$	—	$	—		$	15,475
Total cash equivalents	$	15,475	$	—	$	—		$	15,475
Debt securities:
Corporate debt securities	$	17,628	$	12	$	(1	)	$	17,639
Commercial paper		17,415		—		—			17,415
U.S. treasury bills		10,982		3		—			10,985
U.S. government bonds		44,477		29		—			44,506
Total debt securities	$	90,502	$	44	$	(1	)	$	90,545

	December 31, 2018
	Amortized cost		Unrealized gain		Unrealized loss			Estimated Fair Value
Cash equivalents:
Money market funds	$	45,441	$	—	$	—		$	45,441
Total cash equivalents	$	45,441	$	—	$	—		$	45,441
Debt securities:
Corporate debt securities	$	23,489	$	1	$	(16	)	$	23,474
Commercial paper		15,627		—		(10	)		15,617
Total debt securities	$	39,116	$	1	$	(26	)	$	39,091

Accrued Liabilities (Tables)

9 Months Ended

Sep. 30, 2019

Payables And Accruals [Abstract]

Schedule of Components of Accrued Liabilities

Accrued liabilities consist of the following (in thousands):

	September 30,		December 31,
	2019		2018
Contract research costs	$	3,992	$	2,191
Contract manufacturing costs		1,532		—
Compensation and related benefits		1,407		1,705
Consulting costs		184		258
Franchise tax		70		40
Other		37		—
Total accrued liabilities	$	7,222	$	4,194

Debt (Tables)

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Schedule of Long-Term Debt and Unamortized Discount Balances

Long-term debt and unamortized discount balances are as follows (in thousands):

	September 30,			December 31,
	2019			2018
Face value of long-term debt	$	30,000		$	25,000
Exit fee		3,277			1,960
Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees		(3,075	)		(1,340	)
Long-term debt, net	$	30,202		$	25,620

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2019

Stock Option Activity

The following table summarizes stock option activity under the Company’s stock-based compensation plan during the nine months ended September 30, 2019 (in thousands, except option and share data):

	Shares Available for Grant			Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		755,337			1,996,211		$	11.80		7.54	$	2,347
Additional options authorized		960,588			—
Granted		(1,079,750	)		1,079,750		$	12.93
Exercised		—			(91,725	)	$	6.33
Canceled and forfeited		121,716			(121,716	)	$	20.46
Outstanding as of September 30, 2019		757,891			2,862,520		$	12.03		7.94	$	2,044
Vested and exercisable as of September 30, 2019					1,337,773		$	11.86		6.86	$	1,645

Summary of Non-cash Stock Based Compensation Expense

Total stock-based compensation expense recognized was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	559	$	344	$	1,366	$	1,138
General and administrative		1,090		828		2,965		2,358
Total	$	1,649	$	1,172	$	4,331	$	3,496

Employees and Non Employees Stock Option [Member]

Weighted-average assumptions used in Black-Scholes Model to estimate fair value of stock options granted

The fair value of stock options was estimated using the following weighted-average assumptions:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2019	2018	2019		2018
Expected term (in years)	6.02-6.08	—	5.27-6.08		5.27-6.08
Contractual term (in years)	—	—		10.00		10.00
Volatility	74.19%-75.70%	—	74.19%-83.19%		84.00%-84.50%
Risk free interest rate	1.42%-1.90%	—	1.42%-2.57%		2.35%-2.68%
Dividend yield	—	—	—		—

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Maturity of Operating Leases Liabilities and Future Minimum Lease Payments

The maturity of the Company’s operating lease liabilities as of September 30, 2019 and future minimum lease payments as of December 31, 2018 were as follows (in thousands):

Undiscounted lease payments	September 30, 2019		December 31, 2018
2019	$	151	$	593
2020		616		610
2021		633		629
2022		647		647
2023		109		109
Total undiscounted payments		2,156	$	2,588
Less: imputed interest		301
Present value of future lease payments		1,855
Less: current portion of operating lease liabilities		466
Operating lease liabilities	$	1,389

Description of Business - Additional Information (Detail) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Apr. 17, 2019 USD ($) $ / shares shares	Jun. 30, 2019 shares	Jun. 30, 2018 shares	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Description Of Business [Line Items]
Entity incorporation, date of incorporation				Nov. 06, 2008
Number of operating segments \| Segment				1
Cash, Cash Equivalents, and Investments				$ 109,900
Cash and cash equivalents				19,406		$ 61,262
Debt securities, available-for-sale				90,545		39,091
Accumulated deficit				(224,529)		$ (171,197)
Net proceeds from issuance of common stock				$ 53,194	$ 42,890
Common Stock [Member]
Description Of Business [Line Items]
Issuance of common stock upon public offering, net of issuance costs, shares \| shares		5,175,000	3,680,000
Underwriting Agreement [Member]
Description Of Business [Line Items]
Net proceeds from issuance of common stock	$ 53,200
Underwriting Agreement [Member] \| Common Stock [Member]
Description Of Business [Line Items]
Issuance of common stock upon public offering, net of issuance costs, shares \| shares	5,175,000
Number of additional common stock issued \| shares	675,000
Common stock, offering price per share \| $ / shares	$ 11.00

Fair Value Measurements - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets	$ 106,020,000	$ 84,532,000
Financial liabilities	0	0
Transfers between levels	0	$ 0
Other-than-temporary impairment losses	$ 0
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available- for-sale of debt securities contractual maturity	1 year	1 year
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets	$ 0	$ 0
Financial liabilities	$ 0	$ 0

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value (Detail) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Financial assets:
Assets, Fair Value	$ 106,020,000	$ 84,532,000
U.S. Treasury Bills [Member]
Financial assets:
Assets, Fair Value	10,985,000
U.S. Government Bonds [Member]
Financial assets:
Assets, Fair Value	44,506,000
Corporate Debt Securities [Member]
Financial assets:
Assets, Fair Value	17,639,000	23,474,000
Money Market Funds [Member]
Financial assets:
Assets, Fair Value	15,475,000	45,441,000
Commercial Paper [Member]
Financial assets:
Assets, Fair Value	17,415,000	15,617,000
Level 1 [Member]
Financial assets:
Assets, Fair Value	15,475,000	45,441,000
Level 1 [Member] \| Money Market Funds [Member]
Financial assets:
Assets, Fair Value	15,475,000	45,441,000
Level 2 [Member]
Financial assets:
Assets, Fair Value	90,545,000	39,091,000
Level 2 [Member] \| U.S. Treasury Bills [Member]
Financial assets:
Assets, Fair Value	10,985,000
Level 2 [Member] \| U.S. Government Bonds [Member]
Financial assets:
Assets, Fair Value	44,506,000
Level 2 [Member] \| Corporate Debt Securities [Member]
Financial assets:
Assets, Fair Value	17,639,000	23,474,000
Level 2 [Member] \| Commercial Paper [Member]
Financial assets:
Assets, Fair Value	17,415,000	15,617,000
Level 3 [Member]
Financial assets:
Assets, Fair Value	$ 0	$ 0

Fair Value Measurements - Estimated Value of Cash Equivalents and Debt Securities and Gross Unrealized Holding Gains and Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash Equivalents and Investment Securities [Line Items]
Cash equivalents, Amortized cost	$ 15,475	$ 45,441
Cash equivalents, Estimated Fair Value	15,475	45,441
Debt securities, Amortized cost	90,502	39,116
Debt securities, Unrealized gain	44	1
Debt securities, Unrealized loss	(1)	(26)
Debt securities, Estimated Fair Value	90,545	39,091
U.S. Treasury Bills [Member]
Cash Equivalents and Investment Securities [Line Items]
Debt securities, Amortized cost	10,982
Debt securities, Unrealized gain	3
Debt securities, Estimated Fair Value	10,985
U.S. Government Bonds [Member]
Cash Equivalents and Investment Securities [Line Items]
Debt securities, Amortized cost	44,477
Debt securities, Unrealized gain	29
Debt securities, Estimated Fair Value	44,506
Money Market Funds [Member]
Cash Equivalents and Investment Securities [Line Items]
Cash equivalents, Amortized cost	15,475	45,441
Cash equivalents, Estimated Fair Value	15,475	45,441
Commercial Paper [Member]
Cash Equivalents and Investment Securities [Line Items]
Debt securities, Amortized cost	17,415	15,627
Debt securities, Unrealized loss		(10)
Debt securities, Estimated Fair Value	17,415	15,617
Corporate Debt Securities [Member]
Cash Equivalents and Investment Securities [Line Items]
Debt securities, Amortized cost	17,628	23,489
Debt securities, Unrealized gain	12	1
Debt securities, Unrealized loss	(1)	(16)
Debt securities, Estimated Fair Value	$ 17,639	$ 23,474

Accrued Liabilities - Schedule of Components of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Contract research costs	$ 3,992	$ 2,191
Contract manufacturing costs	1,532
Compensation and related benefits	1,407	1,705
Consulting costs	184	258
Franchise tax	70	40
Other	37
Total accrued liabilities	$ 7,222	$ 4,194

License and Product Development Agreement - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 11, 2018	Jan. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development			$ 14,059	$ 13,196	$ 39,863	$ 25,080
Product Development Agreement [Member] \| RRD International, LLC [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Product development fees	$ 10,000	$ 1,000
Expert consultant fees and pass through costs	800
Incentive-based regulatory milestone fees	$ 1,000
Number of common stock issued subject to quarterly vesting requirements	115,526
Product agreement termination notice period	90 days
Research and development			$ 100		200
UPenn/CHOP Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-Refundable Issue Fee					1,000
Milestone payment obligations					0
UPenn/CHOP Agreement [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License maintenance fee					2,500
UPenn/CHOP Agreement [Member] \| Commercial Sales [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment obligations					$ 18,000

Debt - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Sep. 30, 2019	Dec. 31, 2018	Mar. 05, 2019	Aug. 03, 2018	May 31, 2018	Dec. 31, 2016
Debt Instrument [Line Items]
Face value of term loan	$ 30,000,000	$ 25,000,000
Loan final repayment exit fees payable	$ 3,277,000	$ 1,960,000
Loan agreement, covenant noncompliance penalty	5.00%	5.00%
Loan agreement, covenant description	The loan includes customary events of default, including failure to pay amounts due, breaches of covenants and warranties, material adverse effect events, certain cross defaults and judgments, and insolvency. If the Company is unable to comply with these covenants or if the Company defaults on any portion of the outstanding borrowings, the lenders can also impose a 5.0% penalty and restrict access to additional borrowings under the loan and security agreement.
Loan agreement, covenant compliance	The Company was in compliance with the terms under the loan as of September 30, 2019 and December 31, 2018.
Oxford Loan [Member]
Debt Instrument [Line Items]
Loan agreement, aggregate borrowing capacity						$ 25,000,000
Loan, maturity period	Jul. 01, 2021
Loan agreement, floating rate terms	floating rate per annum equal to the greater of either the 30-day U.S. Dollar LIBOR reported in the Wall Street Journal plus 6.41% or 6.95%.
Principal and interest payment period	30 months
Percentage of exit fee on principal balance	7.50%
Loan final repayment exit fees payable	$ 1,900,000
Loan origination fees and debt issuance costs						400,000
Repayments of loan agreement exit fee	$ 900,000
Oxford Loan [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Loan agreement, interest rate	6.95%
Oxford Loan [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Loan agreement, percentage to be added to the interest rate under condition	6.41%
Oxford Loan [Member] \| Tranche A [Member]
Debt Instrument [Line Items]
Face value of term loan						$ 15,000,000
Oxford Loan [Member] \| Tranche B [Member]
Debt Instrument [Line Items]
Loan final repayment exit fees payable	$ 100,000
Amendment [Member] \| Tranche B [Member]
Debt Instrument [Line Items]
Face value of term loan					$ 5,000,000
Amendment [Member] \| Tranche C [Member]
Debt Instrument [Line Items]
Face value of term loan				$ 5,000,000
Amended Oxford Loan [Member]
Debt Instrument [Line Items]
Loan agreement, aggregate borrowing capacity			$ 35,000,000
Loan, maturity period	Mar. 01, 2024
Loan agreement, floating rate terms	floating rate per annum equal to the greater of either the 30-day U.S. Dollar LIBOR reported in the Wall Street Journal plus 6.64% or 9.15%
Principal and interest payment period	36 months
Percentage of exit fee on principal balance	7.50%
Loan final repayment exit fees payable	$ 2,300,000
Loan origination fees and debt issuance costs	200,000
Loan outstanding			23,300,000
Borrowings from loan agreement	6,700,000
Loan final repayment additional exit fees payable	$ 1,000,000
Prepayment of loan fee, description	Calculated as of the loan origination date, equal to (i) 2.0% of the loan prepaid during the first 12 months and (ii) 1.0% of the loan prepaid in months 13-24.
Amended Oxford Loan [Member] \| Loan Prepaid During First 12 Months [Member]
Debt Instrument [Line Items]
Debt instrument prepayment fee percentage	2.00%
Amended Oxford Loan [Member] \| Loan Prepaid During 13 to 24 Months [Member]
Debt Instrument [Line Items]
Debt instrument prepayment fee percentage	1.00%
Amended Oxford Loan [Member] \| Food and Drug Administration [Member]
Debt Instrument [Line Items]
Percentage of success fee	5.00%
Success fees payable maximum period after approval	30 days
Amended Oxford Loan [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Loan agreement, interest rate	9.15%
Amended Oxford Loan [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Loan agreement, percentage to be added to the interest rate under condition	6.64%
Amended Oxford Loan [Member] \| Amended Tranche A [Member]
Debt Instrument [Line Items]
Loan agreement, aggregate borrowing capacity			30,000,000
Amended Oxford Loan [Member] \| Amended Tranche B [Member]
Debt Instrument [Line Items]
Principal and interest payment period	24 months
Loan agreement, remaining borrowing capacity			$ 5,000,000
Extended term of interest only payment period	1 year
Amended Oxford Loan [Member] \| Amended Tranche B [Member] \| Food and Drug Administration [Member]
Debt Instrument [Line Items]
Fee payable maximum period from funding	10 years

Debt - Schedule of Long-Term Debt and Unamortized Discount Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Face value of long-term debt	$ 30,000	$ 25,000
Exit fee	3,277	1,960
Unamortized debt discount associated with exit fee, debt issuance costs and loan origination fees	(3,075)	(1,340)
Long-term debt, net	$ 30,202	$ 25,620

Stock-Based Compensation - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018
Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Exercise Price And Additional Disclosures [Abstract]
Shares Available for Grant, Outstanding, Beginning balance		755,337
Shares Available for Grant, Additional options authorized		960,588
Shares Available for Grant, Granted		(1,079,750)
Shares Available for Grant, Canceled and forfeited		121,716
Shares Available for Grant, Outstanding, Ending balance		757,891	755,337
Number of Options, Outstanding, Beginning balance		1,996,211
Number of Options, Granted	0	1,079,750
Number of Options, Exercised		(91,725)
Number of Options, Canceled and forfeited		(121,716)
Number of Options, Outstanding, Ending balance		2,862,520	1,996,211
Number of Options, Vested and exercisable		1,337,773
Weighted-Average Exercise Price, Outstanding, Beginning balance		$ 11.80
Weighted-Average Exercise Price, Granted		12.93
Weighted-Average Exercise Price, Exercised		6.33
Weighted-Average Exercise Price, Canceled and forfeited		20.46
Weighted-Average Exercise Price, Outstanding, Ending balance		12.03	$ 11.80
Weighted-Average Exercise Price, Vested and exercisable		$ 11.86
Weighted Average Remaining Contractual Life, Ending balance		7 years 11 months 8 days	7 years 6 months 14 days
Weighted-Average Remaining Contractual Life, Vested and exercisable		6 years 10 months 9 days
Aggregate Intrinsic Value		$ 2,044	$ 2,347
Aggregate Intrinsic Value, Vested and exercisable		$ 1,645

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted		0	1,079,750
Total unrecognized compensation expense	$ 12.5		$ 12.5
Weighted average period expected to recognized compensation expense (in years)			2 years 9 months 18 days
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted	217,000		1,079,750
Weighted-average grant date fair value of employee option grants	$ 7.18		$ 9.01
Employees Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted				799,500
Weighted-average grant date fair value of employee option grants				$ 7.50

Stock-Based Compensation - Weighted-average assumptions used in Black-Scholes Model to estimate fair value of stock options granted (Detail)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual term (in years)		7 years 11 months 8 days		7 years 6 months 14 days
Employees Stock Option [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 7 days	5 years 3 months 7 days	5 years 3 months 7 days
Employees Stock Option [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days
Non-Employees Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contractual term (in years)		10 years	10 years
Employees and Non Employees Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	74.19%	74.19%	84.00%
Volatility, maximum	75.70%	83.19%	84.50%
Risk free interest rate, minimum	1.42%	1.42%	2.35%
Risk free interest rate, maximum	1.90%	2.57%	2.68%

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2017 USD ($) ft²	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Other Commitments [Line Items]
Rent expense recognized for company's operating leases		$ 100,000	$ 400,000	$ 400,000	$ 600,000
Variable lease payments for operating leases		$ 21,400		100,000
Operating cash outflows for operating lease liabilities				$ 500,000
Weighted-average remaining lease term		3 years 4 months 24 days		3 years 4 months 24 days
Weighted-average discount rate		9.15%		9.15%
Palo Alto, California [Member]
Other Commitments [Line Items]
Total leased space \| ft²	8,029
Lease commencement date	Mar. 01, 2018
Long-term operating lease expiry period month and year	2023-02
Lease renewal term	3 years
Security deposit of collateral for lease	$ 300,000

Commitments and Contingencies - Maturity of Operating Leases Liabilities and Future Minimum Lease Payments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Operating leases minimum payments, 2019	$ 151
Operating leases minimum payments, 2019		$ 593
Operating leases minimum payments, 2020	616	610
Operating leases minimum payments, 2021	633	629
Operating leases minimum payments, 2022	647	647
Operating leases minimum payments, 2023	109	109
Total undiscounted payments	2,156	$ 2,588
Less: imputed interest	301
Present value of future lease payments	1,855
Less: current portion of operating lease liabilities	466
Operating lease liabilities	$ 1,389

Condensed Consolidated Statements of Stockholders' Equity (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Statement Of Stockholders Equity [Abstract]
Issuance of common stock upon public offering, issuance costs	$ 3,731	$ 3,110

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets	$ 1,659
Operating lease liabilities	$ 1,855
ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease right-of-use assets		$ 2,000
Operating lease liabilities		$ 2,200