title: In vitro inhibition of hepatic drug oxidation by thioridazine. Kinetic analysis of the inhibition of cytochrome P-450 isoform-specific reactions.
abstract: The phenothiazine tranquilizer thioridazine has been associated with drug interactions in man. This study investigated the capacity of the drug to inhibit hepatic drug oxidations mediated by cytochromes P-450 (P-450) in microsomes in vitro. Thioridazine was a potent linear mixed-type inhibitor of P-450b-dependent 7-pentoxyresorufin O-depentylase activity in phenobarbital-induced rat liver. The kinetic analysis revealed the enzyme-substrate dissociation constant (Ks) to be 1.6 microM whereas the dissociation constant of the enzyme-inhibitor complex (Ki) was 0.11 microM. In contrast, 7-ethoxyresorufin O-deethylase activity (mediated by P-450c) in beta-naphthoflavone-induced rat hepatic microsomes was inhibited to a lesser extent (Ki = 2.4 microM) in relation to the Ks value (0.5 microM). Spectral studies indicated that the efficiency of thioridazine binding in phenobarbital-induced microsomes was about 25-fold greater than in microsomes from beta-naphthoflavone-induced rat liver. This finding is consistent with the relative capacity of thioridazine to inhibit oxidase activities catalyzed by P-450b and P-450c. Mixed-function oxidase activities catalysed by other P-450s were also inhibited by thioridazine, although to a lesser extent than those catalysed by forms b and c. Thus, the 6 beta- and 16 beta-hydroxylations of androst-4-ene-3,17-dione in hepatic microsomes from untreated rats were inhibited to a similar extent (I50S = 52 and 43 microM, respectively). The 7 alpha- and 16 alpha-hydroxylase pathways were approximately only half as susceptible to inhibition by thioridazine. These findings demonstrate the capacity of thioridazine to inhibit a range of P-450-dependent drug oxidations, with those catalysed by forms b and c most susceptible. The present study strongly suggests that drug interactions elicited by thioridazine are most likely a consequence of inhibitory interactions with P-450 enzymes.
authors.affiliation: Department of Medicine, University of Sydney, Westmead Hospital, NSW, Australia.
authors.forename: M
authors.initials: M
authors.lastname: Murray
authors.forename: G F
authors.initials: GF
authors.lastname: Reidy
mesh_terms: D000818:Animals; D019363:Cytochrome P-450 CYP1A1; D019362:Cytochrome P-450 CYP2B1; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D007527:Isoenzymes / Q000037:antagonists & inhibitors* / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D010088:Oxidoreductases / Q000037:antagonists & inhibitors*; D051381:Rats; D011919:Rats, Inbred Strains; D013881:Thioridazine / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007527:Isoenzymes; D003577:Cytochrome P-450 Enzyme System; D010088:Oxidoreductases; D019363:Cytochrome P-450 CYP1A1; D019362:Cytochrome P-450 CYP2B1; D013881:Thioridazine