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Share-based compensation (Tables) | 6 Months Ended
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Jun. 30, 2023
Share-based compensation |
Schedule of stock-based compensation expense | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended June 30, |  | Six Months Ended June 30,
​ |  | 2023 |  | 2022 |  | 2023 |  | 2022
Research and development | ​ | $ | 427 | ​ | $ | 462 | ​ | $ | 857 | ​ | $ | 907
General and administrative | ​ |  | 606 | ​ |  | 865 | ​ |  | 1,400 | ​ |  | 1,730
​ | ​ | $ | 1,033 | ​ | $ | 1,327 | ​ | $ | 2,257 | ​ | $ | 2,637
Weighted average assumptions used in option-pricing | The weighted average assumptions used in the Black-Scholes option-pricing model for stock options granted were: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Six Months Ended June 30, |
​ |  | 2023 |  | 2022 |
Expected volatility |  | ​ | 88.5 | % | ​ | 85.6 | %
Risk-free interest rate |  | ​ | 3.8 | % | ​ | 2.7 | %
Expected term (in years) |  | ​ | 6.03 |  | ​ | 5.97 | ​
Expected dividend yield |  | ​ | — |  | ​ | — | ​
Fair value of common stock | ​ | $ | 4.87 | ​ | $ | 3.64 | ​
Summary of option activity | A summary of option activity under the 2020 Plan and prior Plans during the six months ended June 30, 2023 was as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Weighted | ​ | average
​ | ​ | ​ | ​ | average | ​ | remaining
​ | ​ | Number of | ​ | exercise price | ​ | contractual
​ | ​ | shares | ​ | per share | ​ | term (years)
Outstanding at January 1, 2023 |  | 2,519,405 | ​ | ​ | 9.60 | ​ | 7.90
Granted |  | 658,900 | ​ | ​ | 4.87 | ​ | 9.70
Forfeited |  | (98,216) | ​ | ​ | 11.56 | ​ | —
Expired | ​ | (42,009) | ​ | ​ | 16.25 | ​ | —
Exercised |  | — | ​ | ​ | — | ​ | —
Outstanding at June 30, 2023 |  | 3,038,080 | ​ | ​ | 8.42 | ​ | 7.88
Exercisable at June 30, 2023 |  | 1,563,646 | ​ | ​ | 8.66 | ​ | 7.02

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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