Product inhibition in orphenadrine metabolism as a result of a stable cytochrome P-450-metabolic intermediate complex formed during the disposition of mono-N-desmethylorphenadrine (tofenacine) in the rat.

Product inhibition is thought to be involved in unexpected accumulation of orphenadrine, which occurs during chronic medication with this anti-Parkinson drug in man. In previous studies (Biochem. Pharmacol. 31, 2745-2753 (1982) we established the formation of reactive metabolic intermediates (MI) during metabolism of orphenadrine and its mono-N-demethylated metabolite tofenacine, which may block cytochrome P-450 (MI-complex). In this study we investigated the role of MI-complexation in product inhibition. Three different assays were used to establish the amount of cytochrome P-450 involved in MI-complexation, which was induced by tofenacine (30 mg/kg i.p.) in phenobarbital pretreated rats. If liver microsomes were prepared 3 hours after tofenacine injection, both spectral titration of oxidized cytochrome P-450, determination of loss of metyrapone binding sites at reduced cytochrome P-450 as well as ferricyanide oxidation of the MI-complex revealed 8-13% complexation of cytochrome P-450. Our data also suggest that MI-complexation is generated on phenobarbital induced cytochrome P-450 species. Phenobarbital induction was also shown to activate orphenadrine metabolism in vitro. Moreover, with a newly developed capillary GLC method, using nitrogen detection, we showed inhibition of orphenadrine- and tofenacine metabolism in vitro, by MI-complexation. This study therefore showed that MI-complexation may produce product inhibition.