Background: Alternative independent routes of dapsone metabolism include N-hydroxylation to the hydroxylamine, a potentially toxic metabolite, by cytochrome P450 enzymes and acetylation to a nontoxic metabolite by N-acetyltransferase. Potentially, therefore, the relative extents of these two routes in an individual could determine the occurrence of adverse reaction with dapsone therapy.
Methods: Phenotypic activity of these two routes of metabolism was assessed in 18 patients receiving longterm dapsone therapy for inflammatory dermatoses and was related to the development of dapsone toxicity. N-Hydroxylation was assessed by the dapsone recovery ratio, a ratio of dapsone hydroxylamine to the sum of hydroxylamine and dapsone in 8-hour urine, whereas N-acetylation was assessed by the acetylation ratio, a ratio of monoacetyldapsone to dapsone in 8-hour plasma sample after an oral dose of dapsone.
Results: There was wide intersubject variation in both the acetylation ratio and the dapsone recovery ratio, but both phenotypic measures remained stable within individuals. The dapsone recovery ratio showed a tendency toward being lower in fast than in slow acetylators, but this was not statistically significant. There was an inverse relationship between acetylation and hydroxylation (r = -0.69; P < .005) at steady state that was not apparent after the first dose. Neurotoxicity developed in two subjects and hemolytic anemia developed in two subjects. Plasma levels of dapsone in these four subjects were similar to those in subjects who showed no toxicity. All four were slow acetylators and three were rapid hydroxylators, consistent with the toxic nature of dapsone hydroxylamine.
Conclusions: These observations are consistent with what is known about the toxicity profile of dapsone metabolites and suggest that assessing N-acetylation and N-hydroxylation capacities can help to identify subjects at increased risk of a toxic response. This approach of assessing the phenotypic measures of drug-metabolizing activity to predict adverse reaction may also apply to other drugs with metabolic-based adverse effects.