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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 11, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Clearside Biomedical, Inc.
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	CLSD
Document Type	10-K
Entity Central Index Key	0001539029
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,039,245
Entity Public Float			$ 298,881,000

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 8,043	$ 9,224
Short-term investments	32,835	28,416
Prepaid expenses	2,049	1,445
Other current assets	17	116
Total current assets	42,944	39,201
Property and equipment, net	790	885
Restricted cash	360	360
Other assets	26	47
Total assets	44,120	40,493
Current liabilities:
Accounts payable	6,869	5,384
Accrued liabilities	2,923	4,716
Current portion of long-term debt	556	3,200
Current portion of deferred rent	128	199
Other current liabilities		20
Total current liabilities	10,476	13,519
Long-term debt	9,419	4,809
Deferred rent	605	610
Deferred revenue		140
Total liabilities	20,500	19,078
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized and no shares issued at December 31, 2018 and 2017
Common stock, $0.001 par value; 100,000,000 shares authorized at December 31, 2018 and 2017; 32,119,227 and 25,354,651 shares issued and outstanding at December 31, 2018 and 2017, respectively	32	25
Additional paid-in capital	230,475	145,618
Accumulated deficit	(206,887)	(124,220)
Accumulated other comprehensive loss		(8)
Total stockholders’ equity	23,620	21,415
Total liabilities and stockholders’ equity	$ 44,120	$ 40,493

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
License and collaboration revenue	$ 30	$ 345	$ 520
Type of Revenue [Extensible List]	clsd:LicenseAndCollaborationMember	clsd:LicenseAndCollaborationMember	clsd:LicenseAndCollaborationMember
Operating expenses:
Research and development	$ 68,291	$ 49,053	$ 19,455
General and administrative	14,684	9,700	6,263
Total operating expenses	82,975	58,753	25,718
Loss from operations	(82,945)	(58,408)	(25,198)
Other income (expense)	127	(567)	(684)
Net loss	$ (82,818)	$ (58,975)	$ (25,882)
Net loss per share of common stock — basic and diluted	$ (2.69)	$ (2.33)	$ (1.97)
Weighted average shares outstanding — basic and diluted	30,733,600	25,311,614	13,111,067
Net loss	$ (82,818)	$ (58,975)	$ (25,882)
Unrealized gain (loss) on available-for-sale investments		5	(13)
Comprehensive loss	$ (82,818)	$ (58,970)	$ (25,895)

Statement of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In-Capital	Additional Paid-In-Capital Redeemable Convertible Preferred Stock	Accumulated Deficit	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2015	$ (36,659)		$ 3	$ 2,701		$ (39,363)
Beginning balance, shares at Dec. 31, 2015			2,659,262
Exercise of stock options	31			31
Exercise of stock options, shares			10,724
Vesting of restricted stock, shares			6,485
Issuance of common stock upon exercise of warrants	410			410
Issuance of common stock upon exercise of warrants, Shares			133,560
Issuance of common stock in initial public offering	51,377		$ 8	51,369
Issuance of common stock in initial public offering, Shares			8,148,843
Issuance of common stock in follow-on offering	33,459		$ 4	33,455
Issuance of common stock in follow-on offering, Shares			4,000,000
Conversion of preferred stock to common stock	48,198		$ 10	48,188
Conversion of preferred stock to common stock, Shares			9,614,159
Issuance of warrants to purchase common stock	308			308
Other comprehensive gain (loss)	(13)						$ (13)
Accretion of stock issuance costs		$ (884)			$ (884)
Share-based compensation expense	1,314			1,314
Net loss	(25,882)					(25,882)
Ending balance at Dec. 31, 2016	71,659		$ 25	136,892		(65,245)	(13)
Ending balance, shares at Dec. 31, 2016			24,573,033
Issuance of common shares under employee stock purchase plan	66			66
Issuance of common shares under employee stock purchase plan, shares			9,692
Exercise of stock options	239			239
Exercise of stock options, shares			171,926
Issuance of common stock in follow-on offering	5,057			5,057
Issuance of common stock in follow-on offering, Shares			600,000
Other comprehensive gain (loss)	5						5
Share-based compensation expense	3,364			3,364
Net loss	(58,975)					(58,975)
Ending balance at Dec. 31, 2017	21,415		$ 25	145,618		(124,220)	(8)
Ending balance, shares at Dec. 31, 2017			25,354,651
Cumulative effect of accounting change	159					151	$ 8
Issuance of common shares under employee stock purchase plan	50			50
Issuance of common shares under employee stock purchase plan, shares			12,595
Exercise of stock options	465			465
Exercise of stock options, shares			203,269
Issuance of common stock in follow-on offering	79,564		$ 7	79,557
Issuance of common stock in follow-on offering, Shares			6,548,712
Share-based compensation expense	4,785			4,785
Net loss	(82,818)					(82,818)
Ending balance at Dec. 31, 2018	$ 23,620		$ 32	$ 230,475		$ (206,887)
Ending balance, shares at Dec. 31, 2018			32,119,227

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (82,818)	$ (58,975)	$ (25,882)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	189	182	65
Share-based compensation expense	4,785	3,364	1,314
Non-cash interest expense	154	211	283
Accretion of debt discount	112	212	108
Change in fair value of warrant liability			156
Amortization and accretion on available-for-sale investments, net	(748)	(31)	(43)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(505)	(833)	(487)
Other assets	21	(5)	882
Accounts payable and accrued liabilities	(314)	4,715	1,424
Deferred revenue		(20)	(520)
Deferred rent	(76)	97	(9)
Net cash used in operating activities	(79,200)	(51,083)	(22,709)
Investing activities
Purchase of available-for-sale investments	(80,142)	(48,116)	(54,485)
Maturities of available-for-sale-investments	76,470	68,543	5,708
Change in restricted cash			(360)
Acquisition of property and equipment	(88)	(306)	(3)
Net cash (used in) provided by investing activities	(3,760)	20,121	(49,140)
Financing activities
Proceeds from follow-on offering, net of issuance costs	79,564	5,057	33,456
Proceeds from initial public offering, net of issuance costs			51,376
Proceeds from exercise of stock options	465	239	31
Proceeds from shares issued under employee stock purchase plan	50	66
Proceeds from issuance of long-term debt	10,000		7,857
Principal payments made on long-term debt	(8,300)		(6,330)
Net cash provided by financing activities	81,779	5,362	86,390
Net (decrease) increase in cash, cash equivalents and restricted cash	(1,181)	(25,600)	14,541
Cash, cash equivalents and restricted cash, beginning of period	9,584	35,184	20,643
Cash, cash equivalents and restricted cash, end of period	8,403	9,584	35,184
Supplemental disclosure
Interest paid	$ 751	653	250
Tenant improvements paid by landlord		$ 637
Conversion of convertible preferred stock to common stock			48,198
Reclassification of deferred initial public offering costs			1,597
Issuance of warrants to purchase common stock			308
Accretion of redeemable convertible preferred stock to redemption value			884
Unpaid initial public offering costs in accounts payable and accrued expenses			$ 507

The Company

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

The Company

1. The Company

Clearside Biomedical, Inc. (the “Company”) is a late-stage clinical biopharmaceutical company developing first-in-class pharmacological therapies to restore and preserve vision for people with serious eye diseases. The Company’s current product candidates focus on treatments for diseases affecting the retina, choroid and sclera and are injected into the suprachoroidal space (“SCS”) using its proprietary SCS Microinjector. Incorporated in the State of Delaware on May 26, 2011, the Company has its corporate headquarters in Alpharetta, Georgia.

The Company’s activities since inception have primarily consisted of developing product and technology rights, raising capital and performing research and development activities. The Company has no current source of revenue to sustain present activities, and does not expect to generate meaningful revenue until and unless the Company receives regulatory approval of and successfully commercializes its product candidates. The Company is subject to a number of risks and uncertainties similar to those of other life science companies at a similar stage of development, including, among others, the need to obtain adequate additional financing, successful development efforts including regulatory approval of products, compliance with government regulations, successful commercialization of potential products, protection of proprietary technology and dependence on key individuals.

Liquidity

On June 1, 2016, the Company’s registration statement on Form S-1 relating to its initial public offering of its common stock (the “IPO”) was declared effective by the Securities and Exchange Commission (“SEC”). The IPO closed on June 7, 2016 and the Company received net proceeds of $45.3 million. On June 30, 2016, the underwriters of the IPO partially exercised their option to purchase additional shares, and on July 6, 2016, the Company received net proceeds of $6.1 million.

On December 9, 2016, the Company’s registration statement on Form S-1 relating to its follow-on public offering of its common stock was declared effective by the SEC. The follow-on public offering closed on December 14, 2016 and the Company received net proceeds of $33.4 million. On December 30, 2016, the underwriters of the follow-on public offering exercised their option to purchase additional shares and on January 6, 2017, the Company received net proceeds of $5.1 million.

On June 30, 2017, the Company entered into an at-the-market sales agreement with Cowen and Company LLC (“Cowen”), under which the Company may offer and sell, from time to time at its sole discretion, shares of its common stock having an aggregate offering price of up to $50.0 million through Cowen acting as its sales agent. Subsequent to December 31, 2018, the Company has sold 3.9 million shares of its common stock for net proceeds of $5.6 million under the at-the-market facility.

On March 12, 2018, the Company closed a follow-on public offering in which it sold 6,538,462 shares of common stock at a public offering price of $13.00 per share, resulting in net proceeds of $79.6 million after deducting underwriting discounts and commissions and estimated offering expenses.

On May 14, 2018, the Company entered into a second amended and restated loan and security agreement (the “2^nd A&R loan agreement”) with Silicon Valley Bank (“SVB”), MidCap Funding XII Trust and MidCap Financial Trust (together, “MidCap” and collectively with SVB, the “Lenders”), which amended and restated in its entirety the Company’s prior amended and restated loan and security agreement with the Lenders. The 2^nd A&R loan agreement provides for new term loans of up to $20.0 million, with a floating interest rate equal to 6.5% plus the greater of (i) the 30-day U.S. LIBOR, as reported in the Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue or (ii) 1.89%. The Company borrowed an initial tranche of $10.0 million on May 14, 2018, of which $7.0 million was used to repay all amounts outstanding under the prior amended and restated loan and security agreement, including the fees payable in connection with the final payment. The prepayment fees were waived. Of the remaining $10.0 million, $5.0 million is no longer available for draw as the Company’s SAPPHIRE clinical trial did not meet its primary endpoint. The other $5.0 million (the “Term C Loan”) is available for draw until March 31, 2019 or, if earlier, an event of default. The Company is required to pay accrued interest only on outstanding amounts through October 31, 2019, or if the Term C Loan is made during the applicable draw period, through December 31, 2019, followed by consecutive equal monthly payments of principal and interest in arrears continuing through the maturity date of October 1, 2022. The Company has the option to prepay the outstanding balance of the term loans in full, subject to a prepayment fee of 3% of the original principal amount of each term loan for any prepayment prior the first anniversary of the date such term loan is funded or 2% of the original principal amount of each term loan for any prepayment on or after the first anniversary of the date such term loan is funded but prior to October 1, 2022. A final payment of 5.50% of the aggregate borrowed amount is due at maturity of the loan on October 1, 2022, or upon the prepayment of the facility or the acceleration of amounts due under the facility as a result of an event of default.

The term loans under the 2^nd A&R loan agreement are secured by substantially all of the Company’s assets, except that the collateral does not include any of its intellectual property. However, pursuant to the terms of a negative pledge arrangement, the Company has agreed not to encumber any of its intellectual property.

The Company has funded its operations primarily through the sale of convertible preferred stock and common stock and the issuance of long-term debt. The Company will continue to need to obtain additional financing to fund future operations, including completing the development and commercialization of its primary product candidates. The Company will need to obtain additional financing to conduct additional trials for the regulatory approval of its product candidates if requested by regulatory bodies, and completing the development of any additional product candidates that might be acquired. If such products were to receive regulatory approval, the Company would need to prepare for the potential commercialization of its product candidates and fund the commercial launch of the products, if the Company decides to commercialize the products on its own. Moreover, the Company’s fixed expenses such as rent and other contractual commitments are substantial and are expected to increase in the future.

The Company had cash, cash equivalents and short-term investments of $40.9 million as of December 31, 2018. In the absence of product or other revenues, the amount, timing, nature or source of which cannot be predicted, the Company’s losses will continue as it conducts its research and development activities. Until the Company can generate sufficient revenue, the Company may finance future cash needs through public or private equity offerings, license agreements, debt financings, collaborations, strategic alliances and marketing or distribution arrangements.

The Company has suffered recurring losses, negative cash flows from operations since inception and anticipates incurring additional losses until such time, if ever, that it can obtain FDA approval to sell, and then generate significant sales of its lead product candidate, XIPERE. The Company will need additional financing to fund its operations and to develop and commercialize XIPERE.

Based on its current research and development plans, including the discontinuation of clinical development of XIPERE together with an anti-vascular endothelial growth factor agent for the treatment of retinal vein occlusion, its plans to reduce certain administrative expenses and its timing expectations related to the approval of its NDA submission, the Company expect that its existing cash, cash equivalents and short-term investments will enable it to fund its operating expenses and capital expenditure requirements into the first quarter of 2020. Accordingly, the Company has plans to mitigate this going concern risk, which primarily consist of raising additional capital, potentially in a combination of equity or debt financings, or entering into potential collaborations, partnering and other strategic arrangements. The Company has based this estimate on assumptions that may prove to be wrong, and the Company could exhaust its capital resources sooner than it expects. These conditions raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the financial statements are issued.

The Company’s financial statements have been prepared on a going concern basis, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business. The financial statements do not include any adjustments to reflect the possible future effects on the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result should the Company be unable to continue as a going concern.

Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Significant Accounting Policies

2. Significant Accounting Policies

Basis of Presentation

The Company’s financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Use of Estimates

The preparation of 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 reported amounts of income and expenses during the reporting periods. Significant items subject to such estimates and assumptions include the accounting for useful lives to calculate depreciation and amortization, clinical trial accruals, share-based compensation expense and income tax valuation allowance. Actual results could differ from these estimates.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment.

Property and Equipment, Net

Property and equipment is recorded at historical cost. Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets, or for leasehold improvement the lesser of the useful life or remaining lease term. Repairs and maintenance are expensed when incurred. Upon retirement or sale, the cost of the assets disposed of and the related accumulated depreciation are removed from the accounts, and any resulting gain or loss is included in the determination of net income.

Debt Discount

All debt discounts are recorded against the related debt obligation and are amortized using the effective interest rate method over the term of the underlying debt obligation and reflected in interest expense.

Fair Value Measurements

The Company records certain financial assets and liabilities at fair value in accordance with the provisions of Accounting Standards Codification (“ASC”) Topic 820, Fair Value Measurements and Disclosures, on fair value measurements. The Company’s material financial instruments at December 31, 2018 and 2017 consisted primarily of cash and cash equivalents, short-term investments and long-term debt. The fair value of cash and cash equivalents, other current assets, and accounts payable approximate their respective carrying values due to the short term nature of these instruments.

Income Taxes

Deferred tax assets or liabilities are recorded for temporary differences between financial statement and tax basis of assets and liabilities, using enacted rates in effect for the year in which the differences are expected to reverse. A valuation allowance is recorded if it is more likely than not that a deferred tax asset will not be realized. The Company has provided a full valuation allowance on its deferred tax assets, which primarily consist of cumulative net operating losses for the period from May 26, 2011 (inception) to December 31, 2018. Due to its history of operating losses since inception and losses expected to be incurred in the foreseeable future, a full valuation allowance was considered necessary.

Research and Development Costs

Research and development costs are charged to expense as incurred and include:

•

employee-related expenses, including salaries, benefits, travel and share-based compensation expense for research and development personnel;

•

expenses incurred under agreements with contract research organizations, contract manufacturing organizations and consultants that conduct clinical and preclinical studies;

•

costs associated with preclinical and development activities;

•

costs associated with submitting regulatory approval applications for the Company’s product candidates;

•

costs associated with technology and intellectual property licenses;

•

costs for the Company’s research and development facility; and

•

depreciation expense for assets used in research and development activities.

Costs for certain development activities, such as clinical studies, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by its vendors on their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the patterns of costs incurred, and are reflected in the financial statements as prepaid or accrued liabilities No material adjustments to these estimates have been recorded in these financial statements.

Share-Based Compensation

Compensation cost related to share-based awards granted to employees is measured based on the estimated fair value of the award at the grant date. The Company estimates the fair value of stock options using a Black-Scholes option pricing model. Compensation expense for options granted to non-employees is determined as the fair value of consideration received or the fair value of the equity instruments issued, whichever is more reliably measured. The fair value of awards granted to non-employees is re-measured each period until the related service is complete. Share-based compensation costs are expensed on a straight-line basis (net of estimated forfeitures) over the relevant vesting period.

Compensation cost related to shares purchased through the Company’s employee stock purchase plan, which is considered compensatory, is based on the estimated fair value of the shares on the offering date, including consideration of the discount and the look back period. The Company estimates the fair value of the shares using a Black-Scholes option pricing model. Compensation expense is recognized over the six month withholding period prior to the purchase date.

All share-based compensation costs are recorded in general and administrative or research and development costs in the statements of operations based upon the underlying employees’ roles within the Company.

Cash Equivalents

Cash equivalents consist of short-term, highly liquid investments with an original term of three months or less at the date of purchase.

Short-Term Investments

Short-term investments are investments with original maturities of between 90 and 365 days when purchased and are comprised of certificates of deposit, commercial paper, corporate and government bonds and treasury bills. The Company classifies its short-term investments as available-for-sale securities. Short-term investments are recorded at fair value and unrealized gains and losses are recorded within interest income. In addition, the Company evaluates the short-investments with unrealized losses to determine whether such losses are other-than-temporary.

Restricted Cash

The Company is required to maintain a stand-by letter of credit as a security deposit for its facility lease in Alpharetta, Georgia. The Company’s bank requires the Company to maintain a restricted cash balance to serve as collateral for the letter of credit issued to the landlord by the bank. As of December 31, 2018, the restricted cash balance was invested in a commercial money market account.

Concentration of Credit Risk Arising From Cash Deposits in Excess of Insured Limits

The Company maintains its cash in bank deposits that at times may exceed federally insured limits. The Company has not experienced any loss in such accounts. The Company believes it is not exposed to any significant risks with respect to its cash balances.

Recent Accounting Pronouncements

Accounting Pronouncements Recently Adopted

In May 2014, the Financial Accounting Standards Board (“FASB”) issued ASU 2014-09, Revenue from Contracts with Customers. Under ASU 2014-09, companies are required to recognize revenue to depict the transfer of goods or services to customers in amounts that reflect the consideration (that is, payment) to which the company expects to be entitled in exchange for those goods or services. The new standard also results in enhanced disclosures about revenue, provide guidance for transactions that were not previously addressed comprehensively (for example, service revenue and contract modifications) and modifies guidance for multiple-element arrangements.

The Company adopted the standard effective January 1, 2018 using the modified retrospective transition method. After evaluating its current and prior license agreements, as well as its other collaboration agreements, the Company recorded the remaining $160,000 of deferred revenue under those agreements as a cumulative adjustment to accumulated deficit. The adoption of the new standard did not have a material impact on the Company’s financial statements and related disclosures.

In January 2016, the FASB issued ASU 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which provides guidance related to the accounting for equity investments, financial liabilities under the fair value option and the presentation and disclosure requirements for financial instruments. The Company adopted the standard on January 1, 2018 and the adoption did not have a material impact on its consolidated financial statements and related disclosures.

In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows Classification of Certain Cash Receipts and Cash Payments. The update addresses eight specific cash flow matters with the objective of reducing diversity in practice in how certain cash receipts and payments are classified in the statement of cash flows. The update is effective for annual periods beginning after December 15, 2017, and interim periods within the period. The Company adopted the standard effective January 1, 2018, and the adoption did not have a material impact on its financial statements and related disclosures.

In November 2016, the FASB issued ASU 2016-18, Restricted Cash, which addresses diversity in practice in the classification and presentation of a change in restricted cash on the statement of cash flows. The amendments in this update require that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. The Company adopted the standard effective January 1, 2018, resulting in a change to the presentation of restricted cash on the statements of cash flows.

The following table is a reconciliation of cash and cash equivalents and restricted cash reported within the balance sheets that sum to the total amounts in the statements of cash flows (in thousands).

	December 31,
	2018		2017		2016
Cash and cash equivalents	$	8,043	$	9,224	$	34,824
Restricted cash		360		360		360
Cash, cash equivalents and restricted cash shown on the statements of cash flows	$	8,403	$	9,584	$	35,184

Restricted cash consists of amounts held by a financial institution under a contractual agreement.

In May 2017, the FASB issued ASU 2017-9, Compensation-Stock Compensation: Scope of Stock Compensation Modification Accounting. The ASU was issued to provide clarity and reduce both (1) diversity in practice and (2) cost and complexity when applying the guidance in Topic 718, Compensation—Stock Compensation, to a change to the terms or conditions of a share-based payment award. The amendments in this update provide guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The update is effective for annual periods beginning after December 15, 2017, and interim periods thereafter. The Company adopted ASU 2017-9 effective January 1, 2018, and the impact on its financial statements and related disclosure will depend on any future modifications to its share-based awards.

Recent Accounting Pronouncements Not Yet Adopted

In February 2016, the FASB issued ASU No. 2016-02, Leases (ASC 842), which requires lessees to recognize most leases on the balance sheet. This is expected to increase both reported assets and liabilities. The new lease standard does not substantially change lessor accounting. For public companies, the standard will be effective for the first interim reporting period within annual periods beginning after December 15, 2018, although early adoption is permitted. Lessees and lessors will be required to apply the new standard at the beginning of the earliest period presented in the financial statements in which they first apply the new guidance, using a modified retrospective transition method. The requirements of this standard include a significant increase in required disclosures. The Company will adopt the ASC 842 effective January 1, 2019 using the effective date method, will not restate any prior periods and will adopt the package of practical expedients. Under the package of practical expedients, the Company does not have to reassess whether contracts are or contain leases, the classification of leases or whether initial direct costs should be capitalized. Upon adoption, the recognition of the present value of lease assets and liabilities will impact the balance sheet, but will not have a material impact on the statements of operations or cash flows.

In June 2018, the FASB issued ASU No. 2018-07, Compensation-Stock Compensation: Improvements to Nonemployee Shared-Based Payment Accounting. The ASU update expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. The new standard is effective for fiscal years and interim periods beginning after December 15, 2018. Early adoption is permitted, but no earlier than an entity’s adoption date of ASU 2014-09, Revenue from Contracts with Customers. The Company is currently assessing the impact that adopting this new accounting standard will have on its financial statements and related disclosures.

Long-Term Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Long-Term Debt

5. Long-Term Debt

Loan and Security Agreements

In September 2016, the Company entered into an amended and restated loan and security agreement, which was subsequently amended on October 31, 2017 (as amended, the “1^st A&R loan agreement”) with the Lenders, which amended and restated in its entirety the Company’s prior loan agreement. The 1^st A&R loan agreement provided for new term loans of up to $15.0 million, with a floating interest rate equal to 7% plus the greater of (i) the 30-day U.S. LIBOR, reported in the Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, or (ii) 0.50%.

Under the terms of the 1^st A&R loan agreement, an initial tranche of $8.0 million was advanced on September 28, 2016. The draw period of the remaining $7.0 million available under the 1^st A&R loan agreement expired on March 31, 2018. The Company was required to pay accrued interest only on the outstanding $8.0 million balance through December 31, 2017, followed by 30 equal payments of principal and accrued interest. The Company had the option to prepay the outstanding balance of the term loans in full, subject to a prepayment fee of 2% of the original principal amount of the aggregate term loans for any prepayment through May 31, 2020. A final payment of $0.5 million was due at the maturity of the loan on June 1, 2020, or upon the prepayment of the facility or the acceleration of amounts due under the facility as a result of an event of default, and was being accreted in long-term debt over the life of the loan. Of the $8.0 million borrowed, $5.3 million was used to repay all amounts outstanding under the original loan agreement. Closing costs incurred in the refinancing portion of the loan were recorded as expense while the financing costs for the new portion of the loan are recorded in long-term debt and being accreted over the life of the loan. Upon repayment of the original loan agreement, all remaining closing costs associated with the original loan agreement are being accreted to long-term debt over the life of the 1^st A&R loan agreement.

On May 14, 2018, the Company entered into the 2^nd A&R loan agreement with the Lenders, which amended and restated in its entirety the 1^st A&R loan agreement. The 2^nd A&R loan agreement provides for new term loans of up to $20.0 million, with a floating interest rate equal to 6.50% plus the greater of (i) the 30-day U.S. LIBOR, as reported in the Wall Street Journal on the last business day of the month the immediately precedes the month in which the interest will accrue, or (ii) 1.89%.

The Company borrowed an initial tranche of $10.0 million on May 14, 2018, of which $7.0 million was used to repay all amounts outstanding under the 1^st A&R loan agreement, including fees associated with the final payment. The prepayment fees were waived. Of the remaining $10.0 million available under the 2^nd A&R loan agreement, $5.0 million is no longer available for draw as the SAPPHIRE clinical trial did not meet its primary endpoint. The other $5.0 million Term C Loan is available until the earlier of (i) March 31, 2019 and (ii) the occurrence of an event of default. The Company is required to pay accrued interest only on all amounts outstanding under the 2^nd A&R loan agreement through October 31, 2019 (or if the Term C Loan is made during the applicable draw period, through December 31, 2019), followed by consecutive equal monthly payments of principal and interest in arrears continuing through the maturity date of October 1, 2022. The Company has the option to prepay the outstanding balance of the term loans in full, subject to a prepayment fee of 3% of the original principal amount of each term loan for any prepayment prior the first anniversary of the date such term loan is funded or 2% of the original principal amount of each term loan for any prepayment on or after the first anniversary of the date such term loan is funded but prior to October 1, 2022. A final payment of 5.50% of the aggregate borrowed amount is due at maturity of the loan on October 1, 2022, or upon the prepayment of the facility or the acceleration of amounts due under the facility as a result of an event of default.

The Company accounted for the 2^nd A&R loan agreement as a modification in accordance with the guidance in ASC 470-50, Debt. Amounts paid to the Lenders and closing costs incurred in the refinancing portion of the loan were recorded as expense while the financing costs for the new portion of the loan, which consists of the final payment, are being accreted over the life of the loan and recorded in long-term debt. Upon repayment of the 1^st A&R loan agreement, all remaining closing costs associated with the 1^st A&R loan agreement are being accreted to long-term debt over the life of the 2^nd A&R loan agreement.

The term loans under the 2^nd A&R loan agreement are secured by substantially all of the Company’s assets, except that the collateral does not include any of the Company’s intellectual property. However, pursuant to the terms of a negative pledge arrangement, the Company has agreed not to encumber any of its intellectual property.

Interest expense on the borrowings under the original loan agreement, the 1^st A&R loan agreement and the 2^nd A&R loan agreement was $791,000, $653,000 and $287,000 for the years ended December 31, 2018, 2017 and 2016, respectively. Accretion of the scheduled final payment under the original loan agreement, the 1^st A&R loan agreement and the 2^nd A&R loan agreement was $154,000, $211,000 and $283,000 for the years ended December 31, 2018, 2017 and 2016, respectively. Accretion of the deferred closing costs under the original loan agreement, the 1^st A&R loan agreement and the 2^nd A&R loan agreement was $112,000, $212,000 and $261,000 for the years ended December 31, 2018, 2017 and 2016, respectively.

As of December 31, 2018, the scheduled payments for the loan agreement, including the scheduled final payment in 2020, were as follows (in thousands):

Year Ending December 31,	Principal		Interest and Final Payment		Total
2019	$	556	$	845	$	1,401
2020		3,333		651		3,984
2021		3,333		366		3,699
2022		2,778		638		3,416
	$	10,000	$	2,500	$	12,500

Share-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation

8. Share-Based Compensation

Stock Options

In January 2016, the Company’s board of directors adopted and approved the Clearside Biomedical, Inc. 2016 Equity Incentive Plan (the “2016 Plan”) which became effective on June 1, 2016. The 2016 Plan provides for the grant of share-based awards to employees, directors and consultants of the Company. The Company reserved 1,818,182 shares of common stock for issuance under the 2016 Plan. The 2016 Plan provides for the grant of incentive stock options to employees, and for the grant of nonqualified stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to the Company’s employees, directors, and non-employee third parties. The number of shares of common stock reserved for issuance under the 2016 Plan will automatically increase on January 1 each year, for a period of ten years, from January 1, 2017 through January 1, 2026, by 4% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Company’s board of directors. At December 31, 2018, under the 2016 Plan, options to purchase 2,606,231 shares of the Company’s common stock were outstanding at a weighted average price of $7.86 per share and 1,164,683 shares remained available for future grant. As of January 1, 2019, the number of shares of common stock that may be issued under the 2016 Plan was automatically increased by 1,284,769 shares, representing 4% of the total number of shares of common stock outstanding on December 31, 2018, increasing the number of shares of common stock available for issuance under the 2016 Plan as of that date to 2,449,452 shares.

As a result of the adoption of the 2016 Plan, no further grants may be made under the Company’s 2011 Stock Incentive Plan (the “2011 Plan”). The 2011 Plan provided for the grant of share-based awards to employees, directors and consultants of the Company. At December 31, 2018, options to purchase 839,629 shares of the Company’s common stock were outstanding under the 2011 Plan at a weighted average exercise price of $2.76 per share.

The Company has granted stock option awards to employees, directors and consultants. The total share-based compensation expense recognized is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016

Research and development	$	1,766	$	1,348	$	539
General and administrative		3,004		1,988		775
Total	$	4,770	$	3,336	$	1,314

Share-based compensation is accounted for in accordance with the provisions of ASC 718, Compensation-Stock Compensation. The estimated fair value of options granted is determined as of the date of grant using the Black-Scholes option pricing model. The resulting fair value is recognized ratably over the requisite service period, which is generally the vesting period of the awards.

The following table sets forth the weighted average assumptions utilized in the fair value calculation for the underlying common stock for the years ended December 31, 2018, 2017 and 2016.

	Year Ended December 31,
	2018			2017			2016

Expected term (years)		7.00			7.00			7.00
Expected stock price volatility		93.08	%		96.85	%		97.45	%
Risk-free interest rate		2.87	%		2.20	%		2.20	%
Expected dividend yield		0.00	%		0.00	%		0.00	%

Expected term (in years): The Company utilized the guidance set forth in ASC 718 to determine the expected term of options. The Company utilized the simplified method as prescribed by ASC 718, as the Company does not have sufficient historical exercise and post-vesting termination data to provide a reasonable basis upon which to estimate the expected term of stock options granted to employees.

Risk-free interest rate: The risk-free interest rate is based on the yields of U.S. Treasury securities with maturities similar to the expected time to a possible liquidity event.

Expected dividend yield: The Company has not paid and does not anticipate paying any dividends in the foreseeable future.

Expected stock price volatility: For the year ended December 31, 2018, the expected volatility is based on the Company’s historical volatility. For the years ended December 31, 2017 and 2016, the volatility assumption is based on the historical volatilities of the stock of several public entities that are similar to the Company, as the Company did not have sufficient historical transactions in its own shares on which to base expected volatility. The Company selected representative companies from the pharmaceutical industry with similar characteristics, including stage of product development and therapeutic focus.

Forfeitures: As of January 1, 2017, upon adopting ASU 2016-09, the Company began recording forfeitures as they occurred. In prior years, the Company had estimated its forfeiture rate to be zero for the periods presented and any expense true-ups for terminated employees have been immaterial.

The following table summarizes the activity related to stock options during the year ended December 31, 2018:

				Weighted
	Number of			Average
	Shares			Exercise Price

Options outstanding at January 1, 2018		3,075,349		$	6.17
Granted		889,500			7.44
Exercised		(203,269	)		2.29
Cancelled/Forfeited		(298,484	)		7.29
Options outstanding at December 31, 2018		3,463,096			6.62

Options exercisable at December 31, 2017		1,114,286			3.94

Options exercisable at December 31, 2018		1,583,749			5.63

The following table provides additional information about the Company’s stock options that were outstanding and exercisable at December 31, 2018 (aggregate intrinsic values in thousands):

							Weighted								Weighted
			Weighted				Average				Weighted				Average
			Average		Aggregate		Remaining				Average		Aggregate		Remaining
Exercise	Options		Exercise		Intrinsic		Contractual		Options		Exercise		Intrinsic		Contractual
Price	Outstanding		Price		Value		Life (Years)		Exercisable		Price		Value		Life (Years)

$0.02 - $2.22		364,949					5.02			319,949					4.33
$3.08 - $6.99		1,887,380					8.03			731,702					6.32
$7.00 - $8.99		868,989					7.68			464,384					7.22
$9.11 - $20.84		341,778						8.95		67,714					8.03
		3,463,096	$	6.62	$	218	7.72			1,583,749	$	5.63	$	218	6.26

As of December 31, 2018, the Company had $10.3 million of unrecognized compensation expense related to unvested stock options, which is expected to be recognized over a weighted average period of 2.7 years. The weighted-average fair values of all stock options granted for the years ended December 31, 2018, 2017 and 2016 was $5.98 per share, $5.25 per share and $7.89 per share, respectively. The intrinsic value is calculated as the difference between the fair market value and the exercise price per share of the stock options. The fair market value per share of common stock as of December 31, 2018 was $1.07, which was the closing sale price of the Company’s common stock on the Nasdaq Global Market on that date.

Employee Stock Purchase Plan

In January 2016, the Company’s board of directors adopted and approved the Clearside Biomedical, Inc. 2016 Employee Stock Purchase Plan (the “2016 ESPP”) which became effective on June 1, 2016. The 2016 ESPP permits employees to purchase shares of the Company’s common stock through payroll deductions up to 15% of their earnings. The Company reserved 181,818 shares of common stock for issuance under the 2016 ESPP. Additionally, the number of shares reserved for issuance under the 2016 ESPP will automatically increase for a period of ten years, from January 1, 2017 through January 1, 2026, by the lesser of (i) 1% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, (ii) 454,545 shares of common stock or (iii) a lesser number of shares as may be determined by the Company’s board of directors. The Company’s board of directors elected not to increase the shares reserved for issuance on January 1, 2019. The number of shares of common stock available for issuance under the 2016 ESPP as of December 31, 2018 was 658,807 shares.

The first offering period for the 2016 ESPP commenced on January 1, 2017. The 2016 ESPP is considered a compensatory plan and the fair value of the discount and the look-back period are estimated using the Black-Scholes option pricing model and expense is recognized over the six-month withholding period prior to the purchase date. During the years ended December 31, 2018 and 2017, the Company issued 12,595 and 9,692 shares, respectively, of common stock purchased under the 2016 ESPP.

The share-based compensation expense recognized for the 2016 ESPP is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2018		2017

Research and development	$	5	$	7
General and administrative		10		21
Total	$	15	$	28

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

9. Income Taxes

No provision for U.S. federal or state income taxes has been recorded as the Company has incurred net operating losses since inception. Significant components of the Company’s net deferred income tax assets consist of the following (in thousands):

	December 31,
	2018			2017			2016
Deferred tax asset (liability)
Net operating loss carryforwards	$	41,152		$	27,999		$	23,391
Non-deductible accrued expenses		245			248			358
Deferred rent		148			49			1
Deferred revenue		—			38			69
Stock compensation expense		899			387			101
Depreciation differences		(125	)		(18	)		(13	)
Federal tax credits		6,481			3,437			1,232
State tax credits		757			524			301
Charitable contributions		6			6			6
Valuation allowance		(49,563	)		(32,670	)		(25,446	)
Net deferred tax asset	$	—		$	—		$	—

A reconciliation of the statutory tax rates and the effective tax rates is as follows:

	Year Ended December 31,
	2018			2017			2016
U.S. federal tax rate		21.00	%		34.00	%		34.00	%
State tax rate		(0.73	)		2.42			3.79
Permanent difference and other		(0.33	)		(2.84	)		(1.98	)
Tax credit		3.96			3.97			1.62
Valuation allowance		(20.39	)		(12.42	)		(37.43	)
ASC 740-10		(3.51	)		—			—
Impact of federal rate change		—			(25.13	)		—
		0.00	%		0.00	%		0.00	%

In December 2017, the 2017 Tax Cuts and Jobs Act (the “2017 Tax Act”) was signed into law. The 2017 Tax Act significantly revises the U.S. corporate income tax by, among other things, lowering the statutory corporate tax rate from 34% to 21%, eliminating certain deductions, imposing a mandatory one-time tax on accumulated earnings of foreign subsidiaries, introducing new tax regimes, and changing how foreign earnings are subject to U.S. tax. The 2017 Tax Act also enhanced and extended through 2026 the option to claim accelerated depreciation deductions on qualified property. As of December 31, 2017, the Company recorded a provisional tax expense of the impact of the 2017 Tax Act of approximately $14.9 million. This amount was primarily comprised of the re-measurement of federal deferred tax liabilities resulting from the permanent reduction in the U.S. statutory corporate tax rate to 21% from 34%. As of December 31, 2018, the Company has completed the accounting for the effects of the 2017 Tax Act and recorded an insignificant adjustment to the provisional estimate recognized as of December 31, 2017.

Certain items of income and expense are not reported in tax returns and financial statements in the same year. The tax effect of such temporary differences is reported as deferred income taxes. The measurement of deferred tax assets is reduced, if necessary, by the amount of any tax benefit that, based on available evidence, is not expected to be realized. The Company establishes a valuation allowance for deferred tax assets for which realization is not likely. As of each reporting date, management considers new evidence, both positive and negative, that could affect its view of the future realization of deferred tax assets.

At December 31, 2018, the Company had a valuation allowance of $49.6 million recorded against the benefit of certain deferred tax assets. The valuation allowance was primarily related to federal and state net operating loss ("NOL") carryforwards that, in the judgment of management, are not more likely than not to be realized. In assessing the recoverability of the Company’s deferred tax assets, management considered, among other things, its deferred tax liabilities, its historical earnings and losses, projections of future income, and tax-planning strategies available to the Company in the relevant jurisdiction. The Company will release this valuation allowance when management determines that it is more likely than not that its deferred tax asset will be realized.

At December 31, 2018, the Company had income tax NOL carryforwards for federal and state purposes of $195.2 million and $7.6 million, respectively. The Company has recorded a deferred tax asset for both federal and state carryforwards of $41.0 million and $150,000, respectively. If not utilized, the federal NOL carryforwards will begin to expire beginning in 2031, and the state NOL carryforwards will begin to expire at various dates beginning in 2027. Additionally, under the 2017 Tax Act, federal net operating losses incurred in 2018 and beyond may be carried forward indefinitely. However, the deductibility of such federal net operating losses is limited. Certain states have also adopted the indefinite carryforward period beginning with the 2018 tax year, but state conformity varies state by state.

Ownership changes, as defined by Section 382 of the Internal Revenue Code of 1986, as amended (the “Code”), may limit the amount of net operating losses that a company may utilize to offset future taxable income and taxes payable. Pursuant to Section 382 of the Code, an ownership change occurs when the stock ownership of 5% stockholders increases by more than 50% over a testing period of three years. It is possible that the Company has undergone an ownership change as defined by Section 382 of the Code or that the Company may undergo such a change in the future. Any such ownership change may limit the Company’s ability to utilize net operating losses.

The Company is subject to taxation in the United States and certain state jurisdictions. The U.S. federal statute of limitations remains open for the periods from inception and forward. The Company has not been the subject of examination by the taxing authorities.

Liabilities for uncertain tax positions are recognized based on a two-step process. The first step is to evaluate the tax position for recognition by determining if the weight of available evidence indicates that it is more likely than not that the position will be sustained on audit, including resolution of related appeals or litigation processes, if any. Once it is determined that the position meets the recognition threshold, the second step requires the Company to estimate and measure the largest amount of tax benefit that is more likely than not to be realized upon ultimate settlement. The difference between the amount of recognizable tax benefit and the total amount of tax benefit from positions filed or to be filed with the tax authorities is recorded as a liability for uncertain tax benefits.

During 2018, the Company analyzed its uncertain tax positions and recorded an ASC 740-10 reserve in the amount of $2.9 million against its prior year Georgia NOLs. There are no other ASC 740-10 reserves as of December 31, 2018.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

13. Fair Value Measurements

The Company records certain financial assets and liabilities at fair value in accordance with the provisions of ASC Topic 820, Fair Value Measurements and Disclosures. As defined in the guidance, fair value, defined as an exit price, represents the amount that would be received to sell an asset or pay to transfer a liability in an orderly transaction between market participants. As a result, fair value is a market-based approach that should be determined based on assumptions that market participants would use in pricing an asset or a liability. As a basis for considering these assumptions, the guidance defines a three-tier value hierarchy that prioritizes the inputs used in the valuation methodologies in measuring fair value.

•

Level 1—Unadjusted quoted prices in active, accessible markets for identical assets or liabilities.

•

Level 2—Other inputs that are directly or indirectly observable in the marketplace.

•

Level 3—Unobservable inputs that are supported by little or no market activity.

The fair value hierarchy also requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value.

The Company’s material financial instruments at December 31, 2018 and 2017 consisted primarily of cash and cash equivalents, short-term investments and long-term debt. The fair value of cash and cash equivalents, government bonds, treasury bills, other current assets and accounts payable approximate their respective carrying values due to the short term nature of these instruments and are classified as Level 1 in the fair value hierarchy. The fair value of long-term debt approximates the carrying value due to variable interest rates that correspond to market rates. The Company has determined its short-term investments, comprised of certificates of deposit, corporate bonds and commercial paper, to be Level 2 in the fair value hierarchy. The fair value was determined using a market approach, based on prices and other relevant information generated by market transactions involving similar assets. The short-term investments consist of investments with original maturity dates from date of acquisition of 90 to 365 days and are classified as available-for-sale.

There were no significant transfers between Levels 1, 2 and 3 during the years ended December 31, 2018 and 2017.

The following tables summarize the fair value of financial assets and liabilities that are measured at fair value and the classification by level of input within the fair value hierarchy (in thousands):

	December 31, 2018
	Level 1		Level 2		Level 3		Recorded Value
Financial Assets:
Cash and money markets	$	8,042	$	—	$	—	$	8,042
Restricted cash money market		360		—		—		360
Treasury bills		7,490		—		—		7,490
Commercial paper		—		25,346		—		25,346
Total financial assets	$	15,892	$	25,346	$	—	$	41,238

	December 31, 2017
	Level 1		Level 2		Level 3		Recorded Value
Financial Assets:
Cash and money markets	$	9,224	$	—	$	—	$	9,224
Restricted cash money market		360		—		—		360
Government bonds and treasury bills		11,238		—		—		11,238
Certificates of deposit		—		1,960		—		1,960
Corporate bonds		—		5,064		—		5,064
Commercial paper		—		10,154		—		10,154
Total financial assets	$	20,822	$	17,178	$	—	$	38,000

- 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

Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Data [Abstract]

Quarterly Financial Information (Unaudited)

15. Quarterly Financial Information (unaudited)

Summarized quarterly financial information for each of the years ended December 31, 2018 and 2017 are as follows (in thousands except per share data):

	Quarter Ended
	March 31, 2018			June 30, 2018			September 30, 2018			December 31, 2018
Operating expenses	$	(16,453	)	$	(20,904	)	$	(23,956	)	$	(21,662	)
Net loss	$	(16,607	)	$	(20,701	)	$	(23,872	)	$	(21,638	)
Net loss per share of common stock — basic and diluted	$	(0.62	)	$	(0.65	)	$	(0.75	)	$	(0.68	)

	Quarter Ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Operating expenses	$	(10,261	)	$	(13,768	)	$	(18,348	)	$	(16,376	)
Net loss	$	(10,373	)	$	(13,773	)	$	(18,336	)	$	(16,493	)
Net loss per share of common stock — basic and diluted	$	(0.41	)	$	(0.54	)	$	(0.72	)	$	(0.65	)

Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

The Company’s financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Use of Estimates

Segment Information

Property and Equipment, Net

Debt Discount

Fair Value Measurements

Income Taxes

Research and Development Costs

Research and development costs are charged to expense as incurred and include:

•

employee-related expenses, including salaries, benefits, travel and share-based compensation expense for research and development personnel;

•

expenses incurred under agreements with contract research organizations, contract manufacturing organizations and consultants that conduct clinical and preclinical studies;

•

costs associated with preclinical and development activities;

•

costs associated with submitting regulatory approval applications for the Company’s product candidates;

•

costs associated with technology and intellectual property licenses;

•

costs for the Company’s research and development facility; and

•

depreciation expense for assets used in research and development activities.

Share-Based Compensation

Cash Equivalents

Cash equivalents consist of short-term, highly liquid investments with an original term of three months or less at the date of purchase.

Short-Term Investments

Restricted Cash

Concentration of Credit Risk Arising From Cash Deposits in Excess of Insured Limits

Recent Accounting Pronouncements

Accounting Pronouncements Recently Adopted

	December 31,
	2018		2017		2016
Cash and cash equivalents	$	8,043	$	9,224	$	34,824
Restricted cash		360		360		360
Cash, cash equivalents and restricted cash shown on the statements of cash flows	$	8,403	$	9,584	$	35,184

Restricted cash consists of amounts held by a financial institution under a contractual agreement.

Recent Accounting Pronouncements Not Yet Adopted

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Share-based Compensation Expense

The total share-based compensation expense recognized is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016

Research and development	$	1,766	$	1,348	$	539
General and administrative		3,004		1,988		775
Total	$	4,770	$	3,336	$	1,314

Weighted Average Assumptions Used in Fair Value Calculation for Underlying Common Stock

The following table sets forth the weighted average assumptions utilized in the fair value calculation for the underlying common stock for the years ended December 31, 2018, 2017 and 2016.

	Year Ended December 31,
	2018			2017			2016

Expected term (years)		7.00			7.00			7.00
Expected stock price volatility		93.08	%		96.85	%		97.45	%
Risk-free interest rate		2.87	%		2.20	%		2.20	%
Expected dividend yield		0.00	%		0.00	%		0.00	%

Summary of Activity Related to Stock Options

The following table summarizes the activity related to stock options during the year ended December 31, 2018:

				Weighted
	Number of			Average
	Shares			Exercise Price

Options outstanding at January 1, 2018		3,075,349		$	6.17
Granted		889,500			7.44
Exercised		(203,269	)		2.29
Cancelled/Forfeited		(298,484	)		7.29
Options outstanding at December 31, 2018		3,463,096			6.62

Options exercisable at December 31, 2017		1,114,286			3.94

Options exercisable at December 31, 2018		1,583,749			5.63

Additional Information about the Stock Option Outstanding and Exercisable

The following table provides additional information about the Company’s stock options that were outstanding and exercisable at December 31, 2018 (aggregate intrinsic values in thousands):

							Weighted								Weighted
			Weighted				Average				Weighted				Average
			Average		Aggregate		Remaining				Average		Aggregate		Remaining
Exercise	Options		Exercise		Intrinsic		Contractual		Options		Exercise		Intrinsic		Contractual
Price	Outstanding		Price		Value		Life (Years)		Exercisable		Price		Value		Life (Years)

$0.02 - $2.22		364,949					5.02			319,949					4.33
$3.08 - $6.99		1,887,380					8.03			731,702					6.32
$7.00 - $8.99		868,989					7.68			464,384					7.22
$9.11 - $20.84		341,778						8.95		67,714					8.03
		3,463,096	$	6.62	$	218	7.72			1,583,749	$	5.63	$	218	6.26

2016 Employee Stock Purchase Plan

Summary of Share-based Compensation Expense

The share-based compensation expense recognized for the 2016 ESPP is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2018		2017

Research and development	$	5	$	7
General and administrative		10		21
Total	$	15	$	28

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Net Deferred Tax Assets

Significant components of the Company’s net deferred income tax assets consist of the following (in thousands):

	December 31,
	2018			2017			2016
Deferred tax asset (liability)
Net operating loss carryforwards	$	41,152		$	27,999		$	23,391
Non-deductible accrued expenses		245			248			358
Deferred rent		148			49			1
Deferred revenue		—			38			69
Stock compensation expense		899			387			101
Depreciation differences		(125	)		(18	)		(13	)
Federal tax credits		6,481			3,437			1,232
State tax credits		757			524			301
Charitable contributions		6			6			6
Valuation allowance		(49,563	)		(32,670	)		(25,446	)
Net deferred tax asset	$	—		$	—		$	—

Schedule of Statutory and Effective Income Tax Rate Reconciliation

A reconciliation of the statutory tax rates and the effective tax rates is as follows:

	Year Ended December 31,
	2018			2017			2016
U.S. federal tax rate		21.00	%		34.00	%		34.00	%
State tax rate		(0.73	)		2.42			3.79
Permanent difference and other		(0.33	)		(2.84	)		(1.98	)
Tax credit		3.96			3.97			1.62
Valuation allowance		(20.39	)		(12.42	)		(37.43	)
ASC 740-10		(3.51	)		—			—
Impact of federal rate change		—			(25.13	)		—
		0.00	%		0.00	%		0.00	%

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Measured at Fair Value

	December 31, 2018
	Level 1		Level 2		Level 3		Recorded Value
Financial Assets:
Cash and money markets	$	8,042	$	—	$	—	$	8,042
Restricted cash money market		360		—		—		360
Treasury bills		7,490		—		—		7,490
Commercial paper		—		25,346		—		25,346
Total financial assets	$	15,892	$	25,346	$	—	$	41,238

	December 31, 2017
	Level 1		Level 2		Level 3		Recorded Value
Financial Assets:
Cash and money markets	$	9,224	$	—	$	—	$	9,224
Restricted cash money market		360		—		—		360
Government bonds and treasury bills		11,238		—		—		11,238
Certificates of deposit		—		1,960		—		1,960
Corporate bonds		—		5,064		—		5,064
Commercial paper		—		10,154		—		10,154
Total financial assets	$	20,822	$	17,178	$	—	$	38,000

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Data [Abstract]

Summary of Quarterly Financial Information

Summarized quarterly financial information for each of the years ended December 31, 2018 and 2017 are as follows (in thousands except per share data):

	Quarter Ended
	March 31, 2018			June 30, 2018			September 30, 2018			December 31, 2018
Operating expenses	$	(16,453	)	$	(20,904	)	$	(23,956	)	$	(21,662	)
Net loss	$	(16,607	)	$	(20,701	)	$	(23,872	)	$	(21,638	)
Net loss per share of common stock — basic and diluted	$	(0.62	)	$	(0.65	)	$	(0.75	)	$	(0.68	)

	Quarter Ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
Operating expenses	$	(10,261	)	$	(13,768	)	$	(18,348	)	$	(16,376	)
Net loss	$	(10,373	)	$	(13,773	)	$	(18,336	)	$	(16,493	)
Net loss per share of common stock — basic and diluted	$	(0.41	)	$	(0.54	)	$	(0.72	)	$	(0.65	)

The Company - Additional Information (Details) - USD ($)							1 Months Ended	12 Months Ended
	Jan. 01, 2019	May 14, 2018	Mar. 12, 2018	Jun. 30, 2017	Dec. 14, 2016	Sep. 28, 2016	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 06, 2017	Jul. 06, 2016	Jun. 07, 2016
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Entity incorporation date								May 26, 2011
Public offering closing date					Dec. 14, 2016
Issuance of common stock for follow-on public offering, value					$ 33,400,000			$ 79,564,000	$ 5,057,000	$ 33,459,000
Issuance of common stock in initial public offering										51,377,000
Issuance of common stock for follow-on public offering, shares			6,538,462
Common stock, Share offering price			$ 13.00
Net proceeds after deducting underwriting discounts and commissions and estimated offering expenses			$ 79,600,000
Long term debt								10,000,000
Cash, cash equivalents and short-term investments								$ 40,900,000
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Second Amended and Restated Loan and Security Agreement
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity		$ 20,000,000
Fixed interest rate		6.50%
Debt instrument, description of variable rate basis								30-day U.S. LIBOR
Variable interest rate		1.89%
Long term debt		$ 10,000,000
Line of credit facility remaining borrowing capacity not available for withdraw		5,000,000
Debt instrument, maturity date								Oct. 01, 2022
Final payment during maturity, percentage								5.50%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| First Amended and Restated Loan and Security Agreement, Initial Tranche
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Long term debt		10,000,000
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Prepayment Prior the First Anniversary \| Second Amended and Restated Loan and Security Agreement
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Prepayment fee percentage of the original principal amount of the each term loan								3.00%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Prepayment on or After First Anniversary but Prior to October 1, 2022 \| Second Amended and Restated Loan and Security Agreement
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Prepayment fee percentage of the original principal amount of the each term loan								2.00%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Term C Loan \| Second Amended and Restated Loan and Security Agreement
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Line of credit facility, remaining borrowing capacity		5,000,000
Silicon Valley Bank, MidCap Funding XII Trust and MidCap Financial Trust \| First Amended and Restated Loan and Security Agreement
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity							$ 15,000,000
Fixed interest rate							7.00%
Debt instrument, description of variable rate basis								30-day U.S. LIBOR
Variable interest rate							0.50%
Long term debt						$ 8,000,000
Repay all the amount outstanding under the amended and restated loan						$ 5,300,000
Line of credit facility, remaining borrowing capacity								$ 7,000,000
Debt instrument, maturity date								Jun. 01, 2020
Silicon Valley Bank, MidCap Funding XII Trust and MidCap Financial Trust \| Final Payment \| First Amended and Restated Loan and Security Agreement
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Repay all the amount outstanding under the amended and restated loan		$ 7,000,000
Common Stock
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Issuance of common stock for follow-on public offering, value								$ 7,000		4,000
Issuance of common stock in initial public offering										$ 8,000
Issuance of common stock for follow-on public offering, shares								6,548,712	600,000	4,000,000
IPO
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
IPO, closing date								Jun. 07, 2016
Net proceeds from issuance of stock													$ 45,300,000
Over-allotment option by underwriters
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Net proceeds from issuance of stock											$ 5,100,000	$ 6,100,000
At-the-market sales agreement \| Cowen and Company LLC \| Common Stock \| Subsequent Event
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Net proceeds from issuance of stock	$ 5,600,000
Number of shares sold under facility	3,900,000
At-the-market sales agreement \| Cowen and Company LLC \| Common Stock \| Maximum
Organization Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Issuance of common stock in initial public offering				$ 50,000,000

Accrued Liabilities - Schedule of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued research and development	$ 1,263	$ 3,360
Accrued bonuses	1,088	920
Accrued professional fees	63	62
Accrued vacation	103	113
Accrued interest payable	76	58
Accrued expense	330	203
Accrued liabilities, current	$ 2,923	$ 4,716

Long-Term Debt - Additional Information (Details) - USD ($)			1 Months Ended	12 Months Ended
	May 14, 2018	Sep. 28, 2016	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long term debt				$ 10,000,000
Interest expense on borrowings				791,000	$ 653,000	$ 287,000
Accretion of scheduled final payment				154,000	211,000	283,000
Accretion of deferred closing costs				$ 112,000	$ 212,000	$ 261,000
Silicon Valley Bank, MidCap Funding XII Trust and MidCap Financial Trust \| First Amended and Restated Loan and Security Agreement
Debt Instrument [Line Items]
Fixed interest rate			7.00%
Variable interest rate			0.50%
Debt instrument, description of variable rate basis				30-day U.S. LIBOR
Line of credit facility, maximum borrowing capacity			$ 15,000,000
Long term debt		$ 8,000,000
Line of credit facility, remaining borrowing capacity				$ 7,000,000
Debt Instrument, Payment Terms				The Company was required to pay accrued interest only on the outstanding $8.0 million balance through December 31, 2017, followed by 30 equal payments of principal and accrued interest.
Final payment during maturity				$ 500,000
Debt instrument, maturity date				Jun. 01, 2020
Repay all the amount outstanding under the loan agreement including fees		$ 5,300,000
Silicon Valley Bank, MidCap Funding XII Trust and MidCap Financial Trust \| First Amended and Restated Loan and Security Agreement \| Final Payment
Debt Instrument [Line Items]
Repay all the amount outstanding under the loan agreement including fees	$ 7,000,000
Silicon Valley Bank, MidCap Funding XII Trust and MidCap Financial Trust \| First Amended and Restated Loan and Security Agreement \| Prepayment Through May 31, 2020
Debt Instrument [Line Items]
Prepayment fee percentage of the original principal amount of the aggregate term loans			2.00%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Second Amended and Restated Loan and Security Agreement
Debt Instrument [Line Items]
Fixed interest rate	6.50%
Variable interest rate	1.89%
Debt instrument, description of variable rate basis				30-day U.S. LIBOR
Line of credit facility, maximum borrowing capacity	$ 20,000,000
Long term debt	10,000,000
Debt instrument, maturity date				Oct. 01, 2022
Line of credit facility remaining borrowing capacity not available for withdraw	5,000,000
Final payment during maturity, percentage				5.50%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Second Amended and Restated Loan and Security Agreement \| Term C Loan
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity	5,000,000
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Second Amended and Restated Loan and Security Agreement \| Prepayment Prior the First Anniversary
Debt Instrument [Line Items]
Prepayment fee percentage of the original principal amount of the each term loan				3.00%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| Second Amended and Restated Loan and Security Agreement \| Prepayment on or After First Anniversary but Prior to October 1, 2022
Debt Instrument [Line Items]
Prepayment fee percentage of the original principal amount of the each term loan				2.00%
Silicon Valley Bank, MidCap Funding III Trust and MidCap Financial Trust \| First Amended and Restated Loan and Security Agreement, Initial Tranche
Debt Instrument [Line Items]
Long term debt	$ 10,000,000

Long-Term Debt - Scheduled Payments for the Loan Agreement (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Debt Disclosure [Abstract]
Principal, 2019	$ 556
Principal, 2020	3,333
Principal, 2021	3,333
Principal, 2022	2,778
Principal	10,000
Interest and Final Payment, 2019	845
Interest and Final Payment, 2020	651
Interest and Final Payment, 2021	366
Interest and Final Payment, 2022	638
Interest and Final Payment	2,500
Total, 2019	1,401
Total, 2020	3,984
Total, 2021	3,699
Total, 2022	3,416
Total	$ 12,500

Share-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Jan. 01, 2019	Jan. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
2016 Stock Incentive Plan \| Subsequent Event
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares common stock reserved for issuance	2,449,452
Annual increase of common stock reserved for issuance	4.00%
Increase in common stock for future issuance, shares	1,284,769
2016 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares common stock reserved for issuance		181,818	658,807
Share based compensation arrangement description			the number of shares reserved for issuance under the 2016 ESPP will automatically increase for a period of ten years, from January 1, 2017 through January 1, 2026, by the lesser of (i) 1% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, (ii) 454,545 shares of common stock or (iii) a lesser number of shares as may be determined by the Company’s board of directors.
Annual increase of common stock reserved for issuance		1.00%
Employee payroll deductions, percent		15.00%
Number of shares reserved for issuance increase in years		10 years
Addition of common stock shares reserved for future issuance		454,545
Common stock purchased			12,595	9,692
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares grants			889,500
Option to purchase common stock shares outstanding			3,463,096	3,075,349
Weighted average exercise price			$ 6.62	$ 6.17
Unrecognized compensation expense related to unvested stock options			$ 10.3
Expected to be recognized over a weighted average period			2 years 8 months 12 days
Weighted-average fair values granted			$ 5.98	$ 5.25	$ 7.89
Fair market value per share of common stock			$ 1.07
Stock Options \| 2016 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares common stock reserved for issuance		1,818,182
Share based compensation arrangement description			The number of shares of common stock reserved for issuance under the 2016 Plan will automatically increase on January 1 each year, for a period of ten years, from January 1, 2017 through January 1, 2026, by 4% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Company’s board of directors.
Annual increase of common stock reserved for issuance			4.00%
Option to purchase common stock shares outstanding			2,606,231
Weighted average exercise price			$ 7.86
Shares available for future grant			1,164,683
Stock Options \| 2011 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares grants			0
Option to purchase common stock shares outstanding			839,629
Weighted average exercise price			$ 2.76

Share-Based Compensation - Summary of Activity Related to Stock Options (Details) - Stock Options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Options outstanding, Beginning balance	3,075,349
Number of Shares, Granted	889,500
Number of Shares, Exercised	(203,269)
Number of Shares, Cancelled/Forfeited	(298,484)
Number of Shares, Options outstanding, Ending balance	3,463,096	3,075,349
Number of Shares, Options exercisable	1,583,749	1,114,286
Weighted Average Exercise Price, Options outstanding, Beginning balance	$ 6.17
Weighted Average Exercise Price, Granted	7.44
Weighted Average Exercise Price, Exercised	2.29
Weighted Average Exercise Price, Cancelled/Forfeited	7.29
Weighted Average Exercise Price, Options outstanding, Ending balance	6.62	$ 6.17
Weighted Average Exercise Price, Options exercisable	$ 5.63	$ 3.94

Share-Based Compensation - Additional Information about the Stock Option Outstanding and Exercisable (Details) - Stock Options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding	3,463,096	3,075,349
Weighted Average Exercise Price	$ 6.62	$ 6.17
Aggregate Intrinsic Value	$ 218
Weighted Average Remaining Contractual Life (Years)	7 years 8 months 19 days
Options Exercisable	1,583,749	1,114,286
Weighted Average Exercise Price	$ 5.63	$ 3.94
Aggregate Intrinsic Value	$ 218
Weighted Average Remaining Contractual Life (Years)	6 years 3 months 3 days
$0.02 - $0.40
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	$ 0.02
Exercise Price Upper Range Limit	$ 2.22
Options Outstanding	364,949
Weighted Average Remaining Contractual Life (Years)	5 years 7 days
Options Exercisable	319,949
Weighted Average Remaining Contractual Life (Years)	4 years 3 months 29 days
$3.08 - $5.89
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	$ 3.08
Exercise Price Upper Range Limit	$ 6.99
Options Outstanding	1,887,380
Weighted Average Remaining Contractual Life (Years)	8 years 10 days
Options Exercisable	731,702
Weighted Average Remaining Contractual Life (Years)	6 years 3 months 25 days
$6.16 - $8.90
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	$ 7.00
Exercise Price Upper Range Limit	$ 8.99
Options Outstanding	868,989
Weighted Average Remaining Contractual Life (Years)	7 years 8 months 4 days
Options Exercisable	464,384
Weighted Average Remaining Contractual Life (Years)	7 years 2 months 19 days
$9.11 - $20.84
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Lower Range Limit	$ 9.11
Exercise Price Upper Range Limit	$ 20.84
Options Outstanding	341,778
Weighted Average Remaining Contractual Life (Years)	8 years 11 months 12 days
Options Exercisable	67,714
Weighted Average Remaining Contractual Life (Years)	8 years 10 days

Income Tax - Additional Information (Detail) - USD ($)	12 Months Ended
Income Tax - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
U.S. federal tax rate	21.00%	34.00%	34.00%
Tax Cuts and Jobs Act of 2017,Provisional Income Tax Expense		$ 14,900,000
Deferred tax assets, valuation allowance	$ 49,563,000	32,670,000	$ 25,446,000
Deferred tax assets, operating loss carryforwards	$ 41,152,000	$ 27,999,000	$ 23,391,000
Operating loss carryforwards, limitations on use	Ownership changes, as defined by Section 382 of the Internal Revenue Code of 1986, as amended (the “Code”), may limit the amount of net operating losses that a company may utilize to offset future taxable income and taxes payable. Pursuant to Section 382 of the Code, an ownership change occurs when the stock ownership of 5% stockholders increases by more than 50% over a testing period of three years. It is possible that the Company has undergone an ownership change as defined by Section 382 of the Code or that the Company may undergo such a change in the future. Any such ownership change may limit the Company’s ability to utilize net operating losses.
Reserve for tax positions of prior years	$ 2,900,000
U.S. Federal or State Income Taxes
Income Tax Disclosure [Line Items]
Provision for income taxes	0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	195,200,000
Deferred tax assets, operating loss carryforwards	$ 41,000,000
Federal \| Earliest Tax Year
Income Tax Disclosure [Line Items]
Operating loss carryforwards expiration period	2031
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 7,600,000
Deferred tax assets, operating loss carryforwards	$ 150,000
State \| Earliest Tax Year
Income Tax Disclosure [Line Items]
Operating loss carryforwards expiration period	2027

Schedule of Net Deferred Tax Assets (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax asset (liability)
Net operating loss carryforwards	$ 41,152,000	$ 27,999,000	$ 23,391,000
Deferred tax asset (liability) Non-deductible accrued expenses	245,000	248,000	358,000
Deferred rent	148,000	49,000	1,000
Deferred revenue		38,000	69,000
Stock compensation expense	899,000	387,000	101,000
Depreciation differences	(125,000)	(18,000)	(13,000)
Charitable contributions	6,000	6,000	6,000
Valuation allowance	(49,563,000)	(32,670,000)	(25,446,000)
Federal
Deferred tax asset (liability)
Net operating loss carryforwards	41,000,000
Tax credits	6,481,000	3,437,000	1,232,000
State
Deferred tax asset (liability)
Net operating loss carryforwards	150,000
Tax credits	$ 757,000	$ 524,000	$ 301,000

Schedule of Statutory and Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. federal tax rate	21.00%	34.00%	34.00%
State tax rate	(0.73%)	2.42%	3.79%
Permanent difference and other	(0.33%)	(2.84%)	(1.98%)
Tax credit	3.96%	3.97%	1.62%
Valuation allowance	(20.39%)	(12.42%)	(37.43%)
ASC 740-10	(3.51%)
Impact of federal rate change		(25.13%)
Total income tax provision (benefit)	0.00%	0.00%	0.00%

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares, issued	32,119,227	25,354,651
Common stock, shares outstanding	32,119,227	25,354,651

Significant Accounting Policies - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2018 USD ($) Segment
Significant Accounting Policies [Line Items]
Number of operating segments \| Segment		1
Short-term investments maturity start period		90 days
Short-term investments maturity end period		365 days
Cumulative adjustment to accumulated deficit		$ (159,000)
ASU 2014-09
Significant Accounting Policies [Line Items]
Cumulative adjustment to accumulated deficit	$ 160,000

Property and Equipment, Net - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 1,199	$ 1,132
Less: Accumulated depreciation	(409)	(247)
Property and equipment, net	790	885
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 382	303
Estimated Useful Lives (Years)	5 years
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 121	121
Estimated Useful Lives (Years)	5 years
Computer Equipment
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 19	41
Estimated Useful Lives (Years)	3 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and Equipment, gross	$ 677	$ 667
Estimated Useful Lives (Years)	Lesser of useful life or remaining lease term

Preferred and Common Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares authorized	100,000,000	100,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares outstanding	32,119,227	25,354,651

Common Stock Warrants - Additional Information (Details) - Convertible Stock Warrant - First Amended and Restated Loan and Security Agreement - $ / shares	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2018
Class Of Warrant Or Right [Line Items]
Class of warrant Exercise	$ 10.74
Warrants expiration term	2026-09
Weighted average remaining life of warrants		7 years 9 months
Maximum
Class Of Warrant Or Right [Line Items]
Number of stock that can be purchased by each warrant	29,796

Significant Accounting Policies - Schedule on Reconciliation of Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 8,043	$ 9,224	$ 34,824
Restricted cash	360	360	360
Cash, cash equivalents and restricted cash shown on the statements of cash flows	$ 8,403	$ 9,584	$ 35,184	$ 20,643

Commitment and Contingencies - Additional Information (Details)	1 Months Ended			12 Months Ended
	Aug. 31, 2018 USD ($) ft²	May 31, 2018	Nov. 30, 2016 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitment And Contingencies [Line Items]
Non-cancelable operating leases, expiration date				2017-03
Rent expense				$ 316,000	$ 214,000	$ 81,000
Manufacturing Supply Agreement \| Gerresheimer Regensburg GmbH
Commitment And Contingencies [Line Items]
Initial term of agreement		5 years
Renewal term of agreement		3 years
GEORGIA
Commitment And Contingencies [Line Items]
Area of office leased \| ft²			20,000
Operating lease agreement term			6 years 6 months
Operating lease agreement renewal option term			5 years
Minimum monthly lease payments			$ 35,145
Percentage of increase per year			3.00%
CALIFORNIA
Commitment And Contingencies [Line Items]
Area of office leased \| ft²	3,500
Operating lease agreement term	2 years
Operating lease agreement renewal option term	1 year
Minimum monthly lease payments	$ 12,775
Percentage of increase per year	3.00%

Commitment and Contingencies - Future Minimum Commitments Due Under Non-Cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 623
2020	574
2021	496
2022	511
2023	393
Total minimum lease payments	$ 2,597

License and Collaboration Agreements - Additional Information (Detail) - USD ($)		3 Months Ended	12 Months Ended
	Jan. 01, 2018	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2015	Aug. 31, 2014
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue			$ 30,000	$ 345,000	$ 520,000
Cumulative adjustment to accumulated deficit			(159,000)
Collaboration Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue			$ 30,000	325,000
ASU 2014-09
Research And Development Arrangement Contract To Perform For Others [Line Items]
Cumulative adjustment to accumulated deficit	$ 160,000
NovaMedica
Research And Development Arrangement Contract To Perform For Others [Line Items]
Patent expirations period			2027
NovaMedica \| License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue				$ 20,000	$ 20,000
NovaMedica \| ASU 2014-09
Research And Development Arrangement Contract To Perform For Others [Line Items]
Cumulative adjustment to accumulated deficit	$ 160,000
NovaMedica \| Upfront payment
Research And Development Arrangement Contract To Perform For Others [Line Items]
Deferred revenue		$ 200,000					$ 200,000
Spark Therapeutics, Inc. \| Upfront payment
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront, non-refundable payment						$ 500,000
Spark Therapeutics, Inc. \| Upfront payment \| License
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue		$ 500,000

Available-for-Sale Investments - Summary of Company's Available-For-Sale Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 32,836
Unrealized Loss	(1)
Fair Value	32,835	$ 28,416
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,346
Fair Value	25,346
Treasury Bills
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,490
Unrealized Loss	(1)
Fair Value	$ 7,489

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial Assets:
Total financial assets	$ 41,238	$ 38,000
Treasury Bills
Financial Assets:
Total financial assets	7,490
Government Bonds and Treasury Bills
Financial Assets:
Total financial assets		11,238
Corporate Bonds
Financial Assets:
Total financial assets		5,064
Commercial Paper
Financial Assets:
Total financial assets	25,346	10,154
Level 1
Financial Assets:
Total financial assets	15,892	20,822
Level 1 \| Treasury Bills
Financial Assets:
Total financial assets	7,490
Level 1 \| Government Bonds and Treasury Bills
Financial Assets:
Total financial assets		11,238
Level 2
Financial Assets:
Total financial assets	25,346	17,178
Level 2 \| Corporate Bonds
Financial Assets:
Total financial assets		5,064
Level 2 \| Commercial Paper
Financial Assets:
Total financial assets	25,346	10,154
Cash and Money Markets
Financial Assets:
Total financial assets	8,042	9,224
Cash and Money Markets \| Level 1
Financial Assets:
Total financial assets	8,042	9,224
Restricted Cash Money Market
Financial Assets:
Total financial assets	360	360
Restricted Cash Money Market \| Level 1
Financial Assets:
Total financial assets	$ 360	360
Certificates of Deposit
Financial Assets:
Total financial assets		1,960
Certificates of Deposit \| Level 2
Financial Assets:
Total financial assets		$ 1,960

Quarterly Financial Information (Unaudited) - Summary of Quarterly Financial Information - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Abstract]
Operating expenses	$ (21,662)	$ (23,956)	$ (20,904)	$ (16,453)	$ (16,376)	$ (18,348)	$ (13,768)	$ (10,261)	$ (82,975)	$ (58,753)	$ (25,718)
Net loss	$ (21,638)	$ (23,872)	$ (20,701)	$ (16,607)	$ (16,493)	$ (18,336)	$ (13,773)	$ (10,373)	$ (82,818)	$ (58,975)	$ (25,882)
Net loss per share of common stock — basic and diluted	$ (0.68)	$ (0.75)	$ (0.65)	$ (0.62)	$ (0.65)	$ (0.72)	$ (0.54)	$ (0.41)	$ (2.69)	$ (2.33)	$ (1.97)