Income taxes (Tables)
 12 Months Ended
Dec. 31, 2019
Income Tax Disclosure [Abstract]  
Schedule of the significant components of the deferred tax assets
The significant components of the Company’s deferred tax assets are comprised of the following:
 
December 31,
 
2019
 
2018
Deferred tax assets:
 
 
 
U.S. and state net operating loss carryforwards
$
56,906

 
$
49,614

Capitalized R&D
28,427

 
25,366

Research and development credits
11,717

 
10,445

Lease liability
1,779

 
—

Stock-based compensation
1,053

 
1,989

Depreciation and amortization
545

 
640

Accrued expenses
507

 
450

Other temporary differences
38

 
85

Total deferred tax assets
100,972

 
88,589

Less valuation allowance
(99,249
)
 
(88,589
)
Deferred tax assets less valuation allowance
$
1,723

 
$
—

 
 
 
 
Deferred tax liabilities:
 
 
 
ROU asset
$
(1,723
)
 
$
—

Total deferred tax liabilities
(1,723
)
 
—

Net deferred tax assets (deferred tax liabilities)
$
—

 
$
—
Schedule of a reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes
A reconciliation of income tax expense computed at the statutory federal income tax rate to income taxes as reflected in the consolidated financial statements is as follows:
 
 
Years ended December 31,
 
 
2019
 
2018
Federal income tax expense at statutory rate
 
21.0
 %
 
21.0
 %
State income tax, net of federal benefit
 
6.3
 %
 
8.9
 %
Permanent differences
 
0.0
 %
 
9.0
 %
Research and development credit
 
3.3
 %
 
6.7
 %
Change in valuation allowance
 
(27.4
)%
 
(43.1
)%
Other, net
 
(3.2
)%
 
(2.5
)%
Effective tax rate
 
0.0
 %
 
0.0
 %