Polymorphic metabolism of opioid narcotic drugs: possible clinical implications

The oxidative metabolism of many drugs is under genetic control. The enzyme responsible for this reaction for this group of drugs is cytochrome P-450IID6. Humans can be classified either as extensive metabolisers or, if lacking this enzyme, poor metabolisers. The incidence of poor metabolisers in caucasian subjects is 7-10%. The hepatic O-demethylation of codeine to morphine, quantitatively a minor metabolic process, represents this type of genetic polymorphism and has been studied in human pharmacokinetic studies, human urinary recovery studies, and in human and rat liver microsome experiments. Based on the current understanding that the analgesic effect of codeine is mediated primarily through morphine, one might anticipate that poor metabolisers would not obtain pain relief from codeine. The clinical significance of this polymorphism to the antidiarrhoeal and antitussive properties of codeine is not known. Others opioids (dihydrocodeine, hydrocodone, oxycodone, and thebaine) are structurally similar to codeine and their metabolism (O-demethylation at the 3 position) might also be under genetic control. Pharmacogenetics may play an important role in explaining the wide variability of the clinical response to many opioid drugs.