The in vitro metabolism of (35S)chlorpromazine.

Abstract

Beckett & Hewick (1967) found that N-oxidation was the major metabolic route for the in vitro biotransformation of [35S]chlorpromazine using male rat liver microsomes. Further studies have been performed to determine the role of N-oxidation in other species and to ascertain the effect of various compounds on the metabolism of chlorpromazine. Incubation of chlorpromazine with suitably fortified liver homogenates is followed by extraction and concentration of the metabolites which are then separated by t.1.c. and quantitatively determined by liquid scintillation counting of the material removed from the t.1.c. plates. We have confirmed that N-oxidation is the major metabolic route in the rat, and now show that this route predominates in fresh liver microsomal preparations obtained from guinea-pig, mouse, rabbit and hamster. Guinea-pig tissue produces the most N-oxide followed by hamster, mouse, rat and rabbit in descending order. The effects of ' ageing ' microsomes from guinea-pig at 0 for a period of weeks caused demethylation and ring hydroxylation to decay at a rate similar to the destruction of cytochrome P-150, whereas N-oxidation increased slightly over the first few days before falling. Incorporation of low concn. (1 pmol/mg protein and less) of deoxycholate with guinea-pig microsomes before incubation damaged the demethylation and ring hydroxylation systems to the same extent as the cytochrome P-150, without affecting N-oxidation. Higher concn. of deoxycholate damaged all the oxidative pathways. The effect of incorporation of various substances into the incubation media on the in vitro metabolism of [35S]chlorpr~mazine has been determined. NEthylmaleimide and p-chloromercuribenzoate, compounds which react with the sulphydryl groups of proteins, inhibited demethylation and ring hydroxylation of chlorpromazine, whereas N-oxidation was not inhibited. SKF 525-A exerted an inhibitory effect on demethylation and ring hydroxylation only. This effect was also seen with 1,lO-phenanthroline and imidazole. Iproniazid showed a slight inhibitory effect on N-oxidation, but not on demethylation. Metyrapone had a greater inhibitory effect on demethylation and ring hydroxylation than on N-oxidation. A similar effect was also seen with 4-aminobiphenyl and 2,2-bipyridyl. M inhibited demethylation and ring hydroxylation but not N-oxidation. Pretreatment of female rats with phenobarbitone and methylcholanthrene produced an increase in the demethylation of chlorpromazine of about 300% and ZOOYO, respectively; an increase of 700% and l5Oy0 in ring hydroxylation; but had no effect on the N-oxidation of chlorpromazine. There is a sex difference in the metabolism of chlorpromazine, the male rat producing approximately 30% more N-oxide than the female rat.