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Equity
 6 Months Ended
Jun. 30, 2023
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 June 30, 2023 and December 31, 2022. As of June 30, 2023 and December 31, 2022, the total number of shares of common stock issued and outstanding was 44,021,429 and 42,648,346, respectively.
As of June 30, 2023, the Company had pre-funded warrants outstanding to purchase an aggregate of 11,483,010 shares of common stock. The pre-funded warrants are exercisable at any time for an exercise price of $0.000001, except that the terms of the pre-funded warrants provide that such 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 (the "Exercise Cap"), subject to certain exceptions. However, any holder may increase or decrease the Exercise Cap to any other percentage (not in excess of 19.99%) upon at least 61 days’ prior notice from the holder to the Company. On July 18, 2023, the Company received notice from the holders of all of our outstanding pre-funded warrants to increase the Exercise Cap up to 19.99%, effective 61 days from the date of such notice. 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.
During the three months ended June 30, 2023, 1,180,000 shares of common stock were issued upon the exercise of pre-funded warrants. Proceeds of the exercise to the Company were immaterial.
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 June 30, 2023, no sales of common stock have been made pursuant to the Sales Agreement. As of March 20, 2023, the Company is subject to limitations on the amount of funds the Company can raise by selling shares of our common stock using our Form S-3, including sales under this ATM facility, to one-third of the aggregate market value of the shares of our common stock held by non-affiliates, or public float, due to the so-called "baby shelf" requirements set forth in the SEC general instructions of Form S-3. These restrictions will remain in place until such time as our public float exceeds $75 million.
(b)Stock-based compensation expense
Stock-based compensation expense is classified in the condensed statements of operations and comprehensive income (loss) as follows:
(in thousands)Three Months Ended
June 30,		Six Months Ended
June 30,
2023202220232022
Research and development expenses$— $1,051 $255 $2,315 
General and administrative expenses331 1,261 1,267 2,443 
Total stock-based compensation expense$331 $2,312 $1,522 $4,758 
Total unrecognized compensation expense for all stock-based compensation plans was $1.6 million as of June 30, 2023. This expense is expected to be recognized over a weighted average remaining vesting period of 1.88 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
June 30,		Six Months Ended
June 30,
 2023202220232022
Expected volatility87.37 %82.68 %86.77 %83.01 %
Expected dividends0 %0 %0 %0 %
Expected terms (years)5.275.735.605.96
Risk free rate3.82 %2.77 %3.67 %2.22 %

(c)Stock options
A summary of the Company’s stock option activity and related information for the six months ended June 30, 2023 is as follows:
 Number of SharesWeighted Average Exercise PriceWeighted Average Remaining Contractual Life
 (in Years)		Aggregate Intrinsic Value
(in Thousands)
Outstanding at December 31, 20228,514,381 $4.89 8.32$— 
Options granted260,500 $0.70 
Options exercised— $— 
Options cancelled/forfeited(4,138,321)$5.07 
Outstanding at June 30, 20234,636,560 $3.96 4.41$11 
Exercisable as of June 30, 20233,534,712 $4.51 3.01$— 
There were no exercises of options during the six months ended June 30, 2023. During the six months ended June 30, 2022, 36,500 shares of common stock were issued upon exercise of options with an aggregate intrinsic value of $0.1 million. The weighted-average grant date fair value of options granted during the six months ended June 30, 2023 and June 30, 2022 was $0.51 and $1.47 per share, respectively.
(d)Restricted stock units
A summary of the Company’s RSU activity and related information for the six months ended June 30, 2023 is as follows:
 Number of SharesWeighted Average Grant Date Fair Value
Non-vested at December 31, 2022378,500 $4.22 
Restricted stock units granted— $— 
Restricted stock units vested(180,000)$4.18 
Restricted stock units forfeited(152,250)$4.15 
Non-vested at June 30, 202346,250 $4.59 
(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.  
As of June 30, 2023 and December 31, 2022, employee contributions included in accounts payable and accrued liabilities in the accompanying condensed balance sheet were immaterial.