In vitro inhibition of a polymorphic human liver P-450 isozyme by narcotic analgesics

Genetically determined differences in the ability of individuals to oxidize certain drugs by hepatic P-450 processes has gained increasing attention. The so-called debrisoquin hydroxylase, which is defective in approximately 5-10% of whites, is known to be of major importance for the metabolism of several drugs. The possible relation of this isozyme to the metabolism of narcotics eliminated by oxidative biotransformation has not been studied. Several narcotics were screened for in vitro interaction with this P-450 isozyme. This was accomplished by testing for competitive inhibition by narcotics of the 2-hydroxylation of desmethylimipramine in a human liver microsomal preparation. Alfentanil, fentanyl, and dextropropoxyphene were found to competitively inhibit this pathway demonstrating an interaction with this polymorphic isozyme. No interaction was found for codeine, meperidine, methadone, morphine, or nalbuphine. These results suggest that a genetic defect may be important for elimination clearance by metabolism for dextropropoxyphene, alfentanil, and fentanyl and that in vivo investigation is warranted.