Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating activities
Net loss	$ (64,616)	$ (62,873)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,093	6,150
Depreciation expense	1,493	1,264
Net accretion of discount marketable securities	0	(164)
Non-cash interest expense	250	263
Non-cash lease expense	1,541
Gain on sale of fixed assets, net	(5)	(2)
Changes in assets and liabilities:
Prepaid expenses and other assets	267	(633)
Accounts payable	2,105	90
Accrued expenses and other current liabilities	263	2,907
Operating lease liabilities	(1,771)	0
Other liabilities	270	12
Net cash used in operating activities	(54,110)	(52,986)
Investing activities
Proceeds from sales and maturities of investments	0	55,000
Purchases of property and equipment	(912)	(2,569)
Proceeds from sale of fixed assets	6	2
Net cash (used in)/provided by investing activities	(906)	52,433
Financing activities
Net proceeds from the issuance of long-term debt	10,000	19,481
Repayment of long-term debt	0	(15,000)
Proceeds from issuance of common stock, net of issuance cost	48,424	0
Proceeds from issuance of common stock under employee stock purchase plan and exercise of stock options	339	282
Net cash provided by financing activities	58,763	4,763
Net increase in cash, cash equivalents and restricted cash	3,747	4,210
Cash, cash equivalents and restricted cash – beginning of period	79,333	94,351
Cash, cash equivalents and restricted cash – end of period	83,080	98,561
Supplemental disclosure of cash flow information
Cash paid for interest	1,509	728
Cash paid for taxes	20	0
Noncash investing and financing activities
Deferred financing and public offering costs in accounts payable and accrued expenses	19	0
Property and equipment additions in accounts payable and accrued expenses	$ 33	$ 290