Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Taxes

Reconciliation of Federal Income Tax Rate

	Year Ended December 31,
	2019		2018		2017
Federal statutory income tax rate	21.0	%	21.0	%	34.0	%
Tax reform change	—		—		(43.0)
Research and development tax credits	2.1		3.6		3.3
State taxes, net of federal benefit	2.7		4.5		3.8
Stock-based compensation	(1.3)		(1.3)		(2.2)
Other	(0.5)		(0.1)		0.2
Change in deferred tax asset valuation allowance	(24.0)		(27.7)		3.9
Effective income tax rate	—	%	—	%	—	%

Net Deferred Tax Assets

	December 31,
	2019		2018
Deferred tax assets:
Net operating loss carryforwards	$	70,317	$	49,699
Tax credit carryforwards		11,966		10,160
Capitalized start-up costs		695		870
Capitalized research and development expenses, net		14,521		18,418
Operating lease liabilities		2,505		—
Derivative liability		3,028		—
Accrued expenses and other temporary differences		6,987		5,196
Total deferred tax assets		110,019		84,343
Valuation allowance		(105,062)		(84,343)
Net deferred tax assets		4,957		—
Deferred tax liabilities:
Operating lease right of use assets		(1,662)		—
2026 Convertible Notes		(3,295)		—
Total deferred tax liabilities		(4,957)		—
Net deferred tax assets	$	—	$	—

Changes in Valuation Allowance for Deferred Tax Assets

	Year Ended December 31,
	2019		2018		2017
Valuation allowance as of beginning of year	$	84,343	$	67,726	$	70,236
Increases recorded to income tax provision		20,719		16,617		24,773
Decreases recorded to income tax provision		—		—		(27,283)
Valuation allowance as of end of year	$	105,062	$	84,343	$	67,726