Omeprazole and aryl hydrocarbon hydroxylases: should we be worried?

Diaz and colleagues have carefully studied the effects of omeprazole on the expression of cytochrome P-450 in primary cultures of human hepatocytes. When omeprazole was added to the culture medium in varying concentrations, there was an increase in P-450IA2 protein and mRNA concentrations, an increase in de novo synthesis of P-450IA2 protein, and an increase in microsomal catalytic activities characteristic of P-450IA2 (phenacetin deethylase and acetanilide hydroxylase). Omeprazole treatment also resulted in an increase in both enzymatic activity characteristic of P-450IA1 (ethoxyresorufin deethylase and benzpyrene hydroxylase) and concentration of P-450IA1 mRNA. In contrast, omeprazole appeared to have no significant effect on expression of other P-450s within the P-450II or P-450III families in the hepatocytes. To validate these in vitro observations, liver biopsy specimens were obtained from five patients before and after a 4-day course of pharmacological doses of omeprazole. In each patient, omeprazole treatment appeared to result in a two- to eightfold increase in P-450IA2 immunoreactive protein and P-450IA1 and P-450IA2 enzymatic activities. The authors conclude that omeprazole is an inducer of P-450IA2 and probably P-450IA1 in human liver. Induction of these enzymes could potentiate the bioactivation of carcinogens or the hepatotoxicity of some drugs such as acetaminophen.