Omeprazole and the cytochrome P450 system

SUMMARY Enzymes of the cytochrome P450 superfamily play a key role in xenobiotic metabolism. Their properties and significance are discussed with particular reference to interactions with the H + , K + ‐ATPase blocker, omeprazole. Such interactions include both inhibitory (subfamily 2C) and inducing effects (subfamily 1A). Delayed metabolic elimination of diazepam, warfarin, carbamazepin and phenytoin is probably due to omeprazole competition for the concerned isoform of subfamily 2 C ; however, these effects are modest to negligible in magnitude and, for phenytoin, not consistently reproducible. Also, induction of subfamily 1A is only minor as assessed from the resultant changes in N ‐3‐demethylation of caffeine, a reaction specific to this subfamily. Concerns about a possible activation of procarcinogens that might arise from subfamily 1A induction appear ill‐founded given the fact that cruciferous vegetables such as broccoli and Brussels sprouts are potent inducers, but rather seem to lower the incidence of certain types of cancer. Likewise, the idea that the toxicity of acetaminophen might increase upon subfamily 1A induction appears far‐fetched, mainly because much stronger inducers of subfamily 1A (cigarette smoke and charcoaled beef) are unable to alter acetaminophen metabolism.