ISOFORM-SELECTIVE MECHANISM-BASED INHIBITION OF HUMAN CYTOCHROME-P450 1A2 BY FURAFYLLINE

Biotransformation reactions catalyzed by human cytochrome P450 1A2 (P450 1A2) appear to play a significant role in both the metabolic clearance of drugs and the activation of environmental contaminants and drugs to toxic or carcinogenic species. Furafylline is a potent and selective inhibitor of P450 1A2 activity in human liver microsomes [Sesardic, D., Boobis, A., Murray, B., Murray, S., Segura, J., De La Torre, R., and Davies, D. (1990) Br. J. Clin. Pharmacol. 29, 651-663] which may be of great utility in defining the role of P450 1A2 in metabolic processes. We have investigated the hypothesis that furafylline is a mechanism-based inhibitor of P450 1A2. Key findings consistent with this hypothesis are the following: (1) Furafylline causes a time- and cofactor-dependent loss of P450 1A2 activity which does not return upon dialysis. (2) The loss of activity is associated with a reduction of P450 spectral content which is in turn proportional in amount to P450 1A2-associated catalytic activity in uninhibited microsomes from 7 individual livers. (3) The inactivation of P450 1A2 is characterized by a K(i) of 23 muM, a k(inact) of 0.87 min-1 and a furafylline depletion-based partition ratio of approximately 3-6 metabolic events per inactivating event. (4) The processing of the C-8 methyl group of furafylline is involved in inactivation as demonstrated by the observation of a deuterium isotope effect of approximately 2.0 on k(inact) and no effect on K(i) when the C-8 methyl group protons of furafylline are replaced with deuterium atoms. (5) Under conditions in which all P450 1A2 activity is inhibited, the activities of the P450 enzymes 3A4 and 2C9 are unaffected. Furafylline's potency, persistence of effect, and selectivity of action suggest that it may be an ideal and unique probe for determination of the role of P450 1A2 in the processing of drugs and environmental contaminants.