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Stock-based compensation
 12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Stock-based compensation Stock-based Compensation
Stock incentive plans
As of June 30, 2017, there were 3,141,625 stock options outstanding under the Company’s 2007 Stock Incentive Plan (the "2007 Plan") The 2007 Plan expired in June 2017. Any cancellations or forfeitures of options granted under the 2007 Plan will increase the options available under the 2017 Stock Incentive Plan (the "2017 Plan"), as described below.
In June 2017 the Company’s stockholders approved the 2017 Plan. Under the 2017 Plan initially, up to 2,255,000 shares of common stock could be granted to the Company's employees, officers, directors, consultants and advisors in the form of options, RSUs or other stock-based awards. The number of shares of common stock issuable under the 2017 Plan will be cumulatively increased annually on January 1 by the lesser of (a) 4% of the outstanding shares on the immediately preceding December 31 or (b) such other amount specified by the Board. The terms of the awards are determined by the Board, subject to the provisions of the 2017 Plan. Any cancellations or forfeitures of options granted under the 2007 Plan, which expired in June 2017, would increase the number of shares that could be granted under the 2017 Plan. On January 1, 2022, the number of shares of common stock issuable under the 2017 Plan was increased by 2,948,362 shares. During the year ended December 31, 2022, the Company granted 3,541,558 RSUs and options to purchase shares of common stock to employees and non-employee directors under the 2017 Plan. As of December 31, 2022 there were 1,501,829 shares available for future issuance under the 2017 Plan.
Under the 2017 Plan, both with respect to incentive stock options and nonqualified stock options, the exercise price per share will not be less than the fair market value of the common stock on the date of grant, and the vesting period for options granted to employees is generally four years. Options granted under the 2017 Plan expire no later than 10 years from the date of grant. Options under the 2007 Plan were granted at an exercise price established by the Board (or an authorized committee thereof) that was not less than the fair market value of the underlying common stock on the date of grant and subject to such vesting provisions determined by the Board (or an authorized committee thereof). The Board may accelerate vesting or otherwise adjust the terms of granted options in the case of a merger, consolidation, dissolution, or liquidation of the Company.
Inducement awards
From time to time, the Company grants to its employees, upon approval by the Board or an authorized committee thereof, options to purchase shares of common stock and/or RSUs as an inducement to employment in accordance with Nasdaq Listing Rule 5635(c)(4). Prior to February 2022, only options were granted, and they were granted outside of an existing equity incentive plan. These options are subject to terms substantially the same as the 2017 Plan.
In February 2022, the Board adopted the Company's 2022 Inducement Stock Incentive Plan (the "Inducement Plan"), which provides for the grant of nonstatutory options, stock appreciation rights, restricted stock, RSUs and other stock-based awards, with respect to an aggregate of 2,000,000 shares of the Company's common stock (subject to adjustment as provided in the Inducement Plan). During the year ended December 31, 2022, the Company granted 713,025 RSUs and options to purchase shares of common stock to newly hired employees under the Inducement Plan. As of December 31, 2022, there were 1,342,175 shares available for future issuance under the Inducement Plan.
As of December 31, 2022 there were options to purchase 757,500 shares of common stock outstanding which were granted as inducement awards prior to establishment of the Inducement Plan.
Stock option activity
A summary of stock option activity is as follows:
Number
of Shares
Weighted-
Average
Exercise Price
Weighted Average
Remaining
Contractual Term
Aggregate
Intrinsic Value
Outstanding at January 1, 20228,342,429 $11.25 7.2$8,458 
Granted2,793,659 $5.43 
Exercised(414,914)$3.21 
Cancelled(669,891)$13.09 
Outstanding at December 31, 202210,051,283 $9.84 7.2$8,197 
Exercisable at December 31, 20225,410,541 $9.45 6.1$6,076 
The weighted-average grant date fair value of options granted during the years ended December 31, 2022, 2021 and 2020, was $3.97, $11.71 and $7.99 per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2022, 2021 and 2020, was $1.5 million, $4.3 million, and $11.1 million, respectively. The aggregate intrinsic value represents the difference between the exercise price and the selling price received by option holders upon the exercise of stock options during the period.
Cash received from the exercise of stock options was $1.3 million, $1.8 million and $3.1 million for the years ended December 31, 2022, 2021 and 2020, respectively.
Restricted stock units
The Company periodically issues RSUs with a service condition to certain officers and other employees that typically vest between one year and four years from the grant date.

A summary of the RSU activity is as follows:
Number
of Shares		Weighted-Average
Remaining
Contractual Term		Aggregate
Intrinsic Value		Weighted-Average
Grant Date
Fair Value
Unvested at January 1, 2022817,609 1.5$5,086 $15.68 
Granted1,460,924 — $5.70 
Vested(235,591)— $15.62 
Forfeited(172,151)— $8.54 
Unvested at December 31, 20221,870,791 1.5$10,963 $8.55 
The total fair value of RSUs vested during the years ended December 31, 2022, 2021 was $1.5 million and $5.8 million, respectively. No RSUs vested during the year ended December 31, 2020.
Employee stock purchase plan
During the year ended December 31, 2017, the Board adopted, and the Company's stockholders approved the 2017 employee stock purchase plan (the "2017 ESPP"). The Company initially reserved 225,000 shares of common stock for issuance under the 2017 ESPP, plus an annual increase, to be added as of January 1st of each year, equal to the least of (i) 450,000 shares of common stock; (ii) one percent of the number of shares of common stock outstanding as of the close of business on the immediately preceding December 31st; and (iii) the number of shares of common stock determined by the Board on or prior to such date for such year, up to maximum of 4,725,000 shares of common stock in the aggregate. On January 1, 2022, no shares were added to the 2017 ESPP. During the years ended December 31, 2022 and 2021 the Company issued 270,774 and 78,253 shares, respectively, under the 2017 ESPP. As of December 31, 2022, there were 295,791 shares available for issuance under the 2017 ESPP.
Stock-based compensation expense
The Company uses the provisions of ASC 718 to account for all stock-based awards to employees and non-employees.
Stock-based compensation expense is recognized over the requisite service period, which is generally the vesting period, using the straight-line method.
The following table presents stock-based compensation expense by award type included within the Company’s consolidated statements of operations and comprehensive loss:
Year ended December 31,
(in thousands)202220212020
Stock options
$15,814 $14,528 $5,725 
Restricted stock units
5,175 3,522 1,187 
Employee stock purchase plan
533 359 260 
Stock-based compensation expense included in total operating expenses$21,522 $18,409 $7,172 
The following table presents stock-based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss:
Year ended December 31,
(in thousands)202220212020
Research and development$11,386 $9,984 $3,841 
General and administrative10,136 8,425 3,331 
Stock-based compensation expense included in total operating expenses$21,522 $18,409 $7,172 
As of December 31, 2022, there was $30.4 million and $11.6 million of unrecognized stock-based compensation expense related to unvested stock options and unvested RSUs, respectively, that is expected to be recognized over a weighted average period of 2.0 years and 2.6 years, respectively.
The fair value of each option award is estimated on the date of grant using the Black–Scholes option pricing model with the following weighted average assumptions:
December 31,
202220212020
Risk-free interest rate
2.1 %0.9 %1.2 %
Expected dividend yield
— %— %— %
Expected term (years)
5.996.066.05
Expected stock price volatility
88 %82 %74 %
Expected volatility for the Company’s common stock is determined based on a blended rate of its historical volatility combined with the historical volatility of comparable publicly traded companies. The risk-free interest rate is based on the yield of U.S. Treasury securities consistent with the expected term of the option. No dividend yield was assumed as the Company has not historically and does not expect to pay dividends on its common stock. The expected term of the options granted is based on the use of the simplified method, in which the expected term is presumed to be the mid-point between the vesting date and the end of the contractual term.
The fair value of RSUs is determined based on the closing price of the Company’s common stock on the date of grant.