Extensive Metabolizers of Debrisoquine Become Poor Metabolizers during Quinidine Treatment

Seven male patients (age 55–75 years) were debrisoquine‐tested immediately before and after one week of treatment with quinidine sulphate (200 mg x 3‐4 day^‐1). Before quinidine all patients were classified as extensive metabolizers (metabolic ratio 0.1‐3.6). During quinidine the formation of 4‐OH‐debrisoquine was practically abolished and the phenotype of the patients was altered to PM (metabolic ratio 15‐51). This is probably due to an inhibition of debrisoquine hydroxylation by quinidine. Inhibition of 4‐OH‐debrisoquine formation was associated with a disproportionate increase in debrisoquine excretion.