The use of cimetidine as a selective inhibitor of dapsone N-hydroxylation in man.

1. The N-hydroxylation of dapsone is thought to be responsible for the methaemoglobinaemia and haemolysis associated with this drug. We wished to investigate the effect of concurrent administration of cimetidine (400 mg three times per day) on the disposition of a single dose (100 mg) of dapsone in seven healthy volunteers in order to inhibit selectively N-hydroxylation. 2. The AUC of dapsone (31.0 +/- 7.2 micrograms ml-1 h) was significantly increased (P less than 0.001) in the presence of cimetidine (43.3 +/- 8.8 micrograms ml-1 h). 3. Peak methaemoglobin levels observed after dapsone administration (2.5 +/- 0.6%) were significantly (P less than 0.05) reduced in the presence of cimetidine (0.98 +/- 0.35%). 4. The percentage of the dose excreted in urine as the glucuronide of dapsone hydroxylamine was significantly (P less than 0.05) reduced in the presence of cimetidine (34.2 +/- 9.3 vs 23.1 +/- 4.2%). 5. Concurrent cimetidine therapy might reduce some of the haematological side-effects of dapsone.