STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue		$ 146
Operating expenses:
Research and development	$ 9,476	10,183
General and administrative	4,145	6,159
Total operating expenses	13,621	16,342
Loss from operations	(13,621)	(16,196)
Other income, net	187	148
Change in fair value of derivative liabilities		28
Net loss	(13,434)	(16,020)
Preferred stock dividends		(5,120)
Net loss applicable to common stockholders	$ (13,434)	$ (21,140)
Basic and diluted net loss per share	$ (0.08)	$ (0.15)
Weighted average common shares outstanding used to compute basic and diluted net loss per share	160,640,859	140,853,120