A summary of warrants outstanding and exercisable as of December 31, 2021 is as follows (in thousands, except as noted): | | | | | | | | | | | | | | | Warrants Outstanding | | Warrants Exercisable | | | | | Weighted | | | | | | | | | | | | | Average | | Weighted | | | | Weighted | | | | | Remaining | | Average | | | | Average | | | Number | | Contractual | | Exercise | | Number | | Exercise | Year Issued | | Outstanding | | Life | | Price | | Exercisable | | Price | | | | | (in years) | | (per share) | | | | (per share) | 2016 | | 6 | | — | | $ | 466.26 | | 6 | | $ | 466.26 | 2017 | | 23 | | 0.4 | | | 118.95 | | 23 | | | 118.95 | 2018 | | 120 | | 2.2 | | | 19.20 | | 120 | | | 19.20 | 2019 | | 281 | | 2.8 | | | 13.76 | | 281 | | | 13.76 | | | 430 | | 2.5 | | $ | 27.61 | | 430 | | $ | 27.61 |
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