Influence of steric factors on the interaction of isomeric phenyloxazoles and phenylthiazoles with microsomal oxidation

Inhibition of microsomal biotransformation by a series of nitrogen and oxygen heterocyclic histamine H2-antagonists

A homologous series of potent, long-lasting thiazolo-pyrimidone-pyridine histamine H2-antagonists were examined for their inhibitory effects on rat hepatic ethylmorphine N-demethylation. Inhibitory potency increased in the order: 2-pyridinyl less than 3-pyridinyl less than 4-pyridinyl histamine H2-antagonist. Substitution ortho to the pyridine nitrogen decreased inhibitory potency. Hydroxylation of the pyridine heterocycle decreased inhibitory potency, whereas substituent electronic effects did not appreciably alter the inhibitory potency of these compounds. Antagonists containing oxygen heterocycles were moderately potent inhibitors compared to those containing unsubstituted pyridine as the heterocycle. A 3-(6-methylpyridine) histamine H2-antagonist was shown to be a slightly more potent inhibitor of ethinamate metabolism than cimetidine in rats. However, unlike cimetidine, it did not inhibit the plasma half-life of antipyrine in dogs at doses that were equally efficacious in inhibiting gastric acid secretion.

Interactions of nitrogen heterocycles with cytochrome P-450 and monooxygenase activity

Three groups of isomeric nitrogen heterocycles, phenylpyridines, phenylimidazoles and pyridylimidazoles were studied in relation to the effect of steric factors on type II binding to cytochrome P-450 and inhibition of aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) activity in hepatic microsomes from phenobarbital(PB)- and beta-naphthoflavone(beta NF)-induced rats. Type II binding affinity was lower (higher Ks) in compounds with substituents on the carbon adjacent to the nitrogen undergoing ligand interaction than in those where steric hindrance near the nitrogen was minimal. Binding affinities of the compounds as measured by their Ks values, were quite similar in both PB- and beta NF-induced microsomes. In PB-induced microsomes, type II binding affinity was generally reflected by the ability of the compounds to inhibit AHH activity. In contrast, most of the compounds evaluated were inactive as AHH inhibitors in beta NF-induced microsomes.

The interaction of arylalkybenzimidazoles and related compounds with microsomal oxidation

A series of 2-arylalkyl- and 2-(4'-alkyl)phenoxymethylbenzimidazoles was synthesized and evaluated as inhibitors of mixed-function oxidase activity in phenobarbitone- and beta-naphthoflavone-induced rat liver microsomes. Higher homologues of the 2-arylalkyl series were more potent inhibitors than lower homologues against all mono-oxygenase activities except aniline p-hydroxylation. Smaller 2-substituents were associated with relatively low-affinity reverse type I spectral binding behaviour, whereas larger substituents were associated with type I binding of high affinity.