Table 4. Selectivity of CYP121 Ligands against Drug-Metabolizing Human P450 Isoformsa.
| IC50 (μM) |
KD (μM) | app SR | |||||
|---|---|---|---|---|---|---|---|
| CYP1A | CYP2C9 | CYP2C19 | CYP2D6 | CYP3A4 | CYP121 | (IC50/KD) | |
| 19a | 1.22 | >25 | >25 | >25 | >25 | 14.0 | 0.09 |
| 25a | 21.4 | >25 | >25 | >25 | 11.3 | 0.015 | 1427 |
| 26a | >25 | >25 | >25 | >25 | >25 | 6.3 | >4 |
The concentration of analogues 19a, 25a, and 26a required to inhibit the activity of human P450s by 50% (IC50, μM) was calculated from a seven-point dose–response curve. Compounds that did not achieve 50% inhibition over the tested concentration range 0–25 μM were considered non-inhibitors.60 Studies were performed in human liver microsomes. Turnover of human P450 substrates (CYP1A, ethoxyresorufin; CYP2C9, tolbutamide; CYP2C19, mephenytoin; CYP2D6, dextromethorphan; CYP3A4, midazolam) was detected by LC-MS/MS or fluorescence for CYP1A. Reported CYP1A activity represents the combined activities of both the CYP1A1 and CYP1A2 isoforms. The apparent selectivity ratio (app SR = off-target potency (IC50)/on-target potency (KD)) was calculated as a ratio of the IC50 (μM) of each compound for CYP1A versus their respective CYP121 KD (μM).57 The IC50 values of known inhibitors were determined as positive controls and are listed in Table S3 of the Supporting Information. Experiments were conducted by Cyprotex (Cheshire, UK).