Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Operating expenses:
Research and development	$ 7,182	$ 5,525	$ 14,687	$ 11,683
General and administrative	3,707	2,315	7,098	6,066
Total operating expenses	10,889	7,840	21,785	17,749
Operating loss	(10,889)	(7,840)	(21,785)	(17,749)
Other income (expense):
Interest income (expense), net	116	51	212	70
Reimbursements from Aerpio	84	171	189	364
Net loss	(10,689)	(7,618)	(21,384)	(17,315)
Reconciliation of net loss to net loss applicable to common stockholders:
Net loss	(10,689)	(7,618)	(21,384)	(17,315)
Accretion on preferred stock				(86,900)
Net loss applicable to common stockholders	$ (10,689)	$ (7,618)	$ (21,384)	$ (104,215)
Net loss per share applicable to common stockholders—basic and diluted	$ (0.40)	$ (0.39)	$ (0.92)	$ (9.48)
Weighted-average number of common shares used in net loss per share applicable to common stockholders—basic and diluted	26,614,671	19,652,056	23,340,590	10,987,692
Comprehensive loss:
Net loss	$ (10,689)	$ (7,618)	$ (21,384)	$ (17,315)
Other comprehensive loss:
Unrealized loss on securities	(19)	(8)	(27)	(8)
Comprehensive loss	$ (10,708)	$ (7,626)	$ (21,411)	$ (17,323)