In millions, except per share items | Dec-31-21 | Dec-31-20 | Dec-31-19 | Dec-31-18 | Dec-31-17 | Dec-31-16 | Dec-31-15 | Mar-31-15 |
| 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K |
Assets | | | | | | | | |
Current assets: | | | | | | | | |
Cash and cash equivalents | 53.8 | 31.7 | 23.4 | 2.6 | 3.6 | 4.2 | 18.9 | 10.4 |
Accounts receivable | 0.1 | 0.1 | 7.2 | | | | | |
Prepaid expenses and other current assets | 0.9 | 0.7 | 0.8 | 0.7 | 0.7 | 0.6 | 1.6 | 0.2 |
Total current assets | 54.8 | 32.5 | 31.4 | 3.3 | 4.2 | 4.8 | 20.5 | 10.5 |
Property and equipment, net | 0.3 | 0.4 | 0.3 | 0.4 | 0.6 | 0.8 | 0.7 | 0.2 |
Operating lease right-of-use asset | 2.1 | 1.5 | 0.6 | | | | | |
Long-term restricted cash | 1.6 | 0.2 | 0.2 | | | | | |
Total assets | 58.8 | 38.1 | 36.1 | 14.7 | 16.0 | 16.7 | 44.9 | 10.7 |
|
Liabilities and stockholders’ equity | | | | | | | | |
Current liabilities: | | | | | | | | |
Accounts payable | 0.8 | 0.9 | 0.6 | 1.2 | 0.5 | 0.7 | 1.1 | |
Accrued expenses and other current liabilities | 1.2 | 2.0 | 2.5 | 1.7 | 2.2 | 1.3 | 1.5 | 0.1 |
Operating lease liability | 1.4 | 1.1 | 0.7 | | | | | |
Deferred revenue | 0.9 | 4.2 | 13.4 | | | | | |
Total current liabilities | 4.4 | 8.2 | 17.2 | 2.9 | 5.8 | 4.7 | 3.6 | 0.6 |
Deferred revenue, net of current portion | 6.1 | 6.2 | 7.9 | | | | | |
Total liabilities | 11.4 | 15.0 | 25.1 | 2.9 | 5.8 | 7.9 | 12.3 | 0.6 |
|
Commitments and contingencies (Note 11) | | | | | | | | |
Stockholders’ equity: | | | | | | | | |
Preferred stock, $0.0001 par value — 500,000 shares authorized; 6,746 shares designated Series A convertible preferred stock; no shares issued and outstanding at June 30, 2023 and December 31, 2022 | | | | | | | | |
Common stock, $0.0001 par value — 200,000,000 shares authorized; 3,652,285 and 3,639,185 shares issued and outstanding at June 30, 2023 and December 31, 2022, respectively | | | | | | | | |
Additional paid-in capital | 301.0 | 257.6 | 226.2 | 206.4 | 184.1 | 164.7 | 160.7 | 20.4 |
Accumulated deficit | -254.6 | -234.5 | -215.2 | -194.6 | -174.0 | -155.9 | -128.1 | -10.4 |
Total stockholders’ equity | 47.4 | 23.1 | 11.0 | 11.8 | 10.1 | 8.8 | 32.6 | 10.0 |
Total liabilities and stockholders’ equity | 58.8 | 38.1 | 36.1 | 14.7 | 16.0 | 16.7 | 44.9 | 10.7 |