Reduction of dapsone hydroxylamine to dapsone during methaemoglobin formation in human erythrocytes in vitro

The fate of the toxic metabolite of dapsone, dapsone hydroxylamine, has been studied in the human red cell. Twice-washed red cells were incubated at 37 degrees with dapsone hydroxylamine: at 3 and 5 min, 27.0 +/- 2.2 and 33.2 +/- 2.7% of the haemoglobin had been converted to methaemoglobin, leading to a maximum at 45 min (45 +/- 1.8%). HPLC analysis revealed that parent amine was produced from dapsone hydroxylamine during methaemoglobin formation in the red cells. At 3 min, conversion of dapsone hydroxylamine to dapsone reached 7.0 +/- 3.9% leading to a maximum at 30 min (18.1 +/- 3.7%). There was a linear relationship between hydroxylamine-dependent methaemoglobin formation and conversion of hydroxylamine to dapsone (r = 0.97). At 4 degrees, methaemoglobin and dapsone formation was greatly retarded, and did not exceed 10%. Co-incubation of diethyl dithiocarbamate (DDC) with dapsone hydroxylamine and red cells led to a marked increase in methaemoglobin formation (61.4 +/- 3.4%) compared with hydroxylamine and red cells alone (45.0 +/- 1.8%, P < 0.001) at 45 min, and conversion of dapsone hydroxylamine to dapsone was almost doubled at 45 min (35.7 +/- 5.3%) compared with hydroxylamine and red cells (18.1 +/- 2.5%). A linear relationship between methaemoglobin formation and dapsone formation (r = 0.96) was also shown to occur in the presence of DDC. Incubation of red cells with DDC and dapsone hydroxylamine caused a significantly greater reduction in glutathione levels (98.3 +/- 1.6%) compared with red cells and dapsone hydroxylamine alone (84.8 +/- 2.7%) at 5 min (P < 0.001), although there was no significant difference between the groups at 15 min (96.9 +/- 2.6 vs 98.1 +/- 2.2%). Intra-erythrocytic glutathione was then depleted by 75 +/- 3.4%, by pretreatment with diethyl maleate (6 mM), and these cells in the presence of the hydroxylamine showed a significant fall in both methaemoglobin generation (29.7 +/- 1.2 vs 35.0 +/- 1.7%) and parent amine formation (11.1 +/- 0.2 vs 16.5 +/- 1.1%) compared with untreated red cells at 45 min. It is possible that a cycle exists between hepatic oxidation of dapsone to its hydroxylamine and reduction to the amine within the red cell, which may lead to re-oxidation by hepatic cytochrome P450. This process may contribute to the persistence of the drug in vivo.