IDENTIFICATION OF THE PHARMACOGENETIC DETERMINANTS OF ALFENTANIL METABOLISM - CYTOCHROME P-450-3A4 - AN EXPLANATION OF THE VARIABLE ELIMINATION CLEARANCE

There is considerable variability in the elimination clearance of the opioid analgesic alfentanil. It has been shown previously that alfentanil clearance is independent of the polymorphic debrisoquine hydroxylase (P-450 2D6), and it is therefore of interest to identify the human cytochrome P-450 enzymes involved in noralfentanil formation, the primary reaction involved in the oxidative N-dealkylation at the piperidine nitrogen. Purified human P-450 3A4 showed appreciable catalytic activity, and yeast recombinant P-450 3A4 also showed alfentanil oxidation activity. When microsomes prepared from different human liver samples were compared, noralfentanil formation activity was well correlated (r = 0.95, P < 0.005) with nifedipine oxidation (a P-450 3A4 marker) hut not with markers of other P-450s, including phenacetin O-deethylation (P-450 1A2), chlorzoxazone 6-hydroxylation (P-450 2E1), and (S)-mephenytoin 4'-hydroxylation (a P-450 2C enzyme). Using antibodies that recognize specific human P-450 enzymes (immunoinhibition techniques), it was possible to demonstrate that anti-P-450 3A4 nearly completely inhibited alfentanil oxidation activity in the human liver microsomes, but no other antibodies showed a measurable inhibitory effect. Selective chemical inhibitors of P-450 3A4, gestodene and troleandomycin, inhibited as much as 90% of the microsomal noralfentanil formation activity, but other chemical inhibitors did not show a detectable inhibitory effect. 7,8-Benzoflavone inhibited as much as 90% of the alfentanil oxidation activity of the microsomal or reconstituted P450 3A4 system. This work indicates that P450 3A4 contributes significantly to human liver microsomal alfentanil oxidation, whereas P450 2D6 does not contribute. Human P450 3A4 metabolizes many clinically important substrates, including nifedipine, cyclosporin, midazolam, lidocaine, and quinidine, and is induced by barbiturates and some antibiotics. Therefore, the possibility exists for pharmacokinetic interactions between alfentanil and other concomitantly administered drugs that are also substrates for, or inducers of, P450 3A4.