Income Taxes - Significant Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Net operating loss carryforwards	$ 70,903	$ 51,398
Research and development and orphan drug credits	12,061	8,662
Accrued expenses	921	936
Intangibles	31	43
Operating lease liabilities	411	562
Derivative liability	30,491	16,700
Other, net	615	628
Total deferred tax assets	115,433	78,929
Deferred tax liabilities:
Operating lease right-of-use assets	(390)	(540)
Property and equipment, net	(694)	(324)
Total deferred tax liabilities	(1,084)	(864)
Net deferred tax assets before valuation allowance	114,349	78,065
Valuation allowance	(114,349)	(78,065)
Net deferred tax assets	$ 0	$ 0