AN INVESTIGATION OF THE ROLE OF METABOLISM IN DAPSONE-INDUCED METHEMOGLOBINEMIA USING A 2 COMPARTMENT INVITRO TEST SYSTEM

1. We have utilized a two compartment system in which two teflon chambers are separated by a semi‐permeable membrane in order to investigate the role of metabolism in dapsone‐induced methaemoglobinaemia. Compartment A contained a drug metabolizing system (microsomes prepared from human liver +/‐ NADPH), whilst compartment B contained target cells (human red cells). 2. Incubation of dapsone (1‐ 100 microM) with human liver microsomes (2 mg protein) and NADPH (1 mM) in compartment A (final volume 500 microliters) led to a concentration‐ dependent increase in the methaemoglobinaemia (15.4‐18.9% at 100 microM) compared with control (2.3 +/‐ 0.4%) detected in the red cells within compartment B. In the absence of NADPH dapsone had no effect. 3. Of the putative dapsone metabolites investigated, only dapsone‐ hydroxylamine caused methaemoglobin formation in the absence of NADPH (40.6 +/‐ 6.3% with 100 microM). However, methaemoglobin was also detected when monoacetyl‐dapsone, 4‐amino‐4′‐nitro‐diphenylsulphone and 4‐aminoacetyl‐4′‐nitro‐diphenylsulphone were incubated with human liver microsomes in the presence of NADPH. 4 Dapsone‐dependent methaemoglobin formation was inhibited by addition of ketoconazole (1‐1000 microM) to compartment A, with IC50 values of 285 and 806 microM for the two liver microsomal samples studied. In contrast, methaemoglobin formation was not inhibited by cimetidine or a number of drugs pharmacologically‐ related to dapsone. The presence of glutathione or ascorbate (500 microM) did not alter the level of methaemoglobin observed. 1990 The British Pharmacological Society