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Equity
 9 Months Ended
Sep. 30, 2022
Equity [Abstract]  
Equity Equity 
(a)Common stock and pre-funded warrants
The Company is authorized to issue 100,000,000 shares of common stock with a par value of $0.000001 as of September 30, 2022 and December 31, 2021. As of September 30, 2022 and December 31, 2021, the total number of shares of common stock issued and outstanding was 42,594,602 and 42,457,471, respectively.
As of September 30, 2022, the Company had pre-funded warrants outstanding to purchase an aggregate of 12,663,010 shares of common stock. The pre-funded warrants are exercisable at any time for an exercise price of $0.000001, except that the pre-funded warrants cannot be exercised by the holders if, after giving effect thereto, the holders would beneficially own more than 9.99% of the outstanding common stock, subject to certain exceptions. However, any holder may increase or decrease such percentage to any other percentage (not in excess of 19.99%) upon at least 61 days’ prior notice from the holder to the Company. The holders of the pre-funded warrants will not have the right to vote on any matter except to the extent required by Delaware law.
On November 4, 2021, the Company entered into an ATM or “at-the-market” Equity Offering Sales Agreement (the “Sales Agreement”) with BofA Securities, Inc., as agent (“BofA”), pursuant to which the Company may offer and sell, from time to time through BofA, shares of the Company’s common stock, having an aggregate offering price of up to $40.0 million. The offer and sale of the shares will be made pursuant to a shelf registration statement on Form S-3 and the related prospectus filed on December 11, 2020, and declared effective by the SEC on December 21, 2020, as supplemented by a prospectus supplement dated November 4, 2021. The Company has no obligation to sell any such shares under the Sales Agreement. Through September 30, 2022, no sales of common stock have been made pursuant to the Sales Agreement.
(b)Stock-based compensation expense
Stock-based compensation expense is classified in the condensed statements of operations as follows:
(in thousands)Three Months Ended
September 30,		Nine Months Ended
September 30,
2022202120222021
Research and development expenses$973 $1,423 $3,288 $3,850 
General and administrative expenses1,058 1,787 3,501 4,698 
Total stock-based compensation expense$2,031 $3,210 $6,789 $8,548 
Total unrecognized compensation expense for all stock-based compensation plans was $17.8 million as of September 30, 2022. This expense is expected to be recognized over a weighted average remaining vesting period of 2.33 years.
The fair values of stock options granted are estimated using the Black-Scholes option pricing model with the following weighted average assumptions:
 Three Months Ended
September 30,		Nine Months Ended
September 30,
 2022202120222021
Expected volatility84.18 %89.17 %83.80 %89.18 %
Expected dividends0 %0 %0 %0 %
Expected terms (years)6.086.076.046.03
Risk free rate2.83 %0.83 %2.63 %0.89 %
(c)Stock options
A summary of the Company’s stock option activity and related information for the nine months ended September 30, 2022 is as follows:
 Number of SharesWeighted Average Exercise PriceWeighted Average Remaining Contractual Life
 (in Years)		Aggregate Intrinsic Value
(in Thousands)
Outstanding at December 31, 20218,963,945 $7.20 8.32$6,912 
Options granted3,360,450 $1.34 
Options exercised(36,500)$3.67 
Options cancelled/forfeited(2,871,996)$7.86 
Outstanding at September 30, 20229,415,899 $4.92 8.30$— 
Exercisable as of September 30, 20223,729,902 $6.04 7.16$— 
During the nine months ended September 30, 2022, 36,500 shares of common stock were issued upon exercise of options with an aggregate intrinsic value of $0.1 million. During the nine months ended September 30, 2021, 124,366 shares of common stock were issued upon exercise of options with an aggregate intrinsic value of $1.3 million. The weighted-average grant date fair value of options granted during the nine months ended September 30, 2022 and September 30, 2021 was $0.96 and $6.49 per share, respectively.
(d)Restricted stock units
A summary of the Company’s RSU activity and related information for the nine months ended September 30, 2022 is as follows:
 Number of SharesWeighted Average Grant Date Fair Value
Non-vested at December 31, 2021132,000 $9.97 
Restricted stock units granted700,000 $3.69 
Restricted stock units vested(24,750)$12.20 
Restricted stock units forfeited(353,000)$4.86 
Non-vested at September 30, 2022454,250 $4.14 
(e)Employee stock purchase plan
The Company’s 2020 ESPP was adopted by the Company’s Board of Directors in March 2020 and approved by the Company’s stockholders in May 2020. A total of 759,936 shares of common stock have been reserved for issuance under the 2020 ESPP.
Subject to share and dollar limits as described in the plan, the 2020 ESPP allows eligible employees to contribute, through payroll deductions, up to 15% of their earnings for the purchase of shares of the Company’s common stock at the lower of 85% of the closing price of the Company’s common stock on the first trading day of the offering period or 85% of the closing price of the Company’s common stock on the last trading day of the offering period. There are two six-month offering periods during each fiscal year, ending on May 15 and November 15.  
For the nine months ended September 30, 2022, the Company issued 75,881 shares of common stock at a purchase price of $0.83 per share under the 2020 ESPP. Cash received from the purchases under the 2020 ESPP for the nine months ended September 30, 2022 was $0.1 million. As of September 30, 2022, $0.1 million of employee contributions are included in accounts payable and accrued liabilities in the accompanying condensed balance sheet.