Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net loss	$ (12,598)	$ (10,223)
Adjustments to reconcile net loss to net cash used in operating activities
Amortization of debt discount on notes payable	0	404
Amortization of investment premiums (accretion of investment discounts), net	(138)	149
Amortization of financing costs	60	60
Amortization of non-cash clinical trial costs	140	477
Change in fair value of debt conversion feature liability	0	(1,398)
Stock-based compensation	1,956	1,241
Realized loss on investments	2
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(535)	(1,396)
Accounts payable	910	(3)
Accrued expenses	592	116
Net cash used in operating activities	(9,611)	(10,573)
Cash flows from investing activities
Purchases of investments	(182,884)	(113,707)
Proceeds from maturities of investments	186,254	17,455
Net cash provided by (used in) investing activities	3,370	(96,252)
Cash flows from financing activities
Proceeds from issuances of common stock, net of underwriting discounts and commissions		131,402
Public offering and financing costs		(266)
Repayment of convertible notes payable		(3,833)
Value of shares withheld related to employee tax withholding	(122)	(99)
Proceeds from warrant and option exercises and stock issuance under employee stock purchase plan	707	5,243
Net cash provided by financing activities	585	132,447
Net increase (decrease) in cash and cash equivalents	(5,656)	25,622
Cash and cash equivalents beginning of period	24,779	8,988
Cash and cash equivalents end of period	$ 19,123	34,610
Supplemental disclosure of cash flow information:
Cash paid during the period for interest		81
Supplemental disclosure of non-cash investing and financing transactions
Unpaid deferred public offering and other financing costs		$ 309