Biotransformation of alprazolam by members of the human cytochrome P4503A subfamily

1. To aid in the prediction of drug interactions with alprazolam, the human CYP involved in the 1'- and 4-hydroxylation of alprazolam were characteriz ed using human liver microsomes, expressed enzymes and selective chemical i nhibitors. 2. The formation of 4-hydroxyalprazolam and 1'-hydroxyalprazolam at an alpr azolam concentration of 62.5 mu M were reduced by the prototypic CYP3A inhi bitor, troleandomycin (50 mu M), by 97 and 99% respectively. Only microsome s from B-lymphoblastoid cells expressing CYP3A4 were capable of catalysing the 1'- and 4-hydroxylation of alprazolam. 3. The formation rates of 1'-hydroxyalprazolam and 4-hydroxyalprazolam at a n alprazolam concentration of 1 mw were significantly correlated (n = 19, r = 0.95, p < 0.01) indicating that the same enzyme(s) mediated these biotra nsformations. A significant (p < 0.01) correlation was observed between alp razolam 4- and 1'-hydroxylase activity and CYP3A-mediated midazolam 4-hydro xylase, midazolam 1'-hydroxylase, dextromethorphan N-demethylase and erythr omycin N-demethylase activities. 4. In conclusion, in adult human liver the CYP3A. subfamily members are the principal enzymes involved in the 1'- and 4-hydroxylation of alprazolam. T hus, clinically significant drug-drug interactions between alprazolam and o ther CYP3A substrates are to be expected.