Stock option plan (Tables)

12 Months Ended

Dec. 31, 2019

Stock option plan

Schedule of assumptions used in valuation of stock options

	Year ended December 31,
	2019	2018		2017
Expected volatility	73.5% ‑ 74.5%	71.5	%	71.9% ‑ 75.4%
Risk‑free rate	1.53% ‑ 2.63%	2.9	%	2.3% ‑ 2.5%
Expected dividend yield	0%	0	%	0%
Expected term in years	6.08 - 7.59	8.2		8.6 ‑ 9.7

Summary of stock option activity

			Weighted‑		Weighted
			average		average
			exercise		remaining	Aggregate
	Number of		price per		contractual	intrinsic
	options		share		term (in years)	value
Outstanding at January 1, 2017		1,237,292	$	1.85	9.2
Granted		663,224		0.73
Exercised		—		—
Cancelled/Forfeited		(3,310)		0.92
Outstanding at December 31, 2017		1,897,206	$	1.46	8.3
Granted		24,067		1.01
Exercised		(2,306)		0.92
Cancelled/Forfeited		(74,779)		17.31
Outstanding at December 31, 2018		1,844,188	$	0.82	7.6
Granted		2,018,796		8.82
Exercised		(322,267)		0.23
Cancelled/Forfeited		(40,783)		0.99
Outstanding at December 31, 2019		3,499,934	$	5.49	7.5	$	141,413,705
Exercisable at December 31, 2019		1,203,298	$	0.93	6.7	$	54,101,434
Vested or expected to vest at December 31, 2019		3,499,934	$	5.49	7.5	$	141,413,705