CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities:
Net loss	$ (74,114)	$ (37,897)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	230	172
Amortization of operating lease right-of-use assets	320	204
Accretion (amortization) of premiums and discounts on investments	(341)	(756)
Amortization of Term Loan discount and issuance costs	751	488
Stock-based compensation	8,374	2,658
Changes in fair value of compound derivative liability	846	174
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(777)	243
Accounts payable	4,892	(3,390)
Accrued expenses and other liabilities	8,910	14,479
Operating lease liabilities	(125)	(217)
Net cash used in operating activities	(51,034)	(23,842)
Investing activities:
Purchases of investments	(46,045)	(44,769)
Maturities of investments	88,867	54,981
Purchases of property and equipment	(360)	(193)
Net cash provided by investing activities	42,462	10,019
Financing activities:
Payments of equity offering costs	0	(321)
Proceeds from issuance of common stock under equity incentive plans	567	251
Repayment of leasehold improvement loan	(14)	(32)
Proceeds (payments) under Term Loan, net	0	(1,319)
Net cash provided by (used in) financing activities	553	(1,421)
Net increase (decrease) in cash and cash equivalents	(8,019)	(15,244)
Cash and cash equivalents at beginning of period	18,574	37,172
Cash and cash equivalents at end of period	10,555	21,928
Supplemental disclosures
Cash paid for interest	1,211	782
Supplemental disclosures of non-cash investing and financing activities
Deferred offering costs incurred but not paid	0	396
Warrants and compound derivative liability related to Term Loan	0	284
Purchase of property and equipment in accounts payable and accrued expenses	$ 206	$ 0