Highly selective inhibition of human CYP3A in vitro by azamulin and evidence that inhibition is irreversible

Azamulin [14-O-(5-(2-amino-1,3,4-triazolyl)thioacetyl)-dihydromutilin] is an azole derivative of the pleuromutilin class of anti-infectives. We tested the inhibition potency of azamulin toward 18 cytochromes P450 using human liver microsomes or microsomes from insect cells expressing single isoforms. In a competitive inhibition model, IC50 values for CYP3A (0.03-0.24 muM) were at least 100-fold lower than all other non-CYP3A enzymes except CYP2J2 (similar to50-fold lower). The IC50 value with heterologously expressed CYP3A4 was 15-fold and 13-fold less than those of CYP3A5 and CYP3A7, respectively. The reference inhibitor ketoconazole was less selective and exhibited potent inhibition (IC50 values < 10 mu M) for CYP1A1, CYP1B1, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP4F2, and CYP4F12. Inhibition of CYP3A by azamulin appeared sigmoidal and well behaved with the substrates 7-benzyloxy-4-trifluoromethylcoumarin, testosterone, and midazolam. Preincubation of 4.8 mu M azamulin in the presence of NADPH for 10 min inhibited similar to 95% of testosterone 6 beta-hydroxylase activity compared with preincubation in the absence of NADPH. Catalytic activities of CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1 were unaffected by similar experiments. Incubation of azamulin with heterologously expressed CYP3A4 yielded a type I binding spectrum with a spectral dissociation constant of 3.5 mu M, whereas no interaction was found with CYP2D6. Azamulin exhibited good chemical stability when stored in acetonitrile for up to 12 days. Aqueous solubility was found to be >300 muM. Azamulin represents an important new chemical tool for use in characterizing the contribution of CYP3A to the metabolism of xenobiotics.