Metabolism-dependent neutrophil cytotoxicity of amodiaquine: A comparison with pyronaridine and related antimalarial drugs

Life-threatening agranulocytosis and hepatotoxicity during prophylactic adm inistration of amodiaquine have led to its withdrawal, Agranulocytosis is t hought to involve bioactivation to a protein-reactive quinoneimine metaboli te. The toxicity of amodiaquine and the lack of cheap drugs have prompted a search for alternative antimalarial agent. The aim of this study was to de termine the metabolism and neutrophil toxicity of amodiaquine, pyronaridine , and other related antimalarial agents. Horseradish peroxidase and hydroge n peroxide were used to activate drugs to their respective quinoneimine met abolites. Metabolites were trapped as stable glutathione conjugates, prior to analysis by LC/MS. Amodiaquine was metabolized to a polar metabolite (m/ z 661), identified as a glutathione adduct. Tebuquine was converted. to two polar metabolites. The principal metabolite (m/z 686) was derived from glu tathione conjugation and side chain elimination, while the minor metabolite gave a protonated molecule m/z 496). Only parent ions were identified when chloroquine, cycloquine, or pyronaridine was incubated with the activating system and glutathione. Calculation of the heat of formation of the drugs, however, demonstrated that amodiaquine, tebuquine, cycloquine, and pyronar idine readily under go oxidation to their quinoneimine. None of the antimal arial compounds depleted the level of intracellular glutathione (1-300 mu M ) when incubated with neutrophils alone, Additionally, with the exception o f tebuquine, no cytotoxicity below 100 mu M was observed. In the presence o f the full activating system, however, all compounds except. chloroquine re sulted in depletion of the level of glutathione and were cytotoxic. Pretrea ting the cells with glutathione and other antioxidants inhibited metabolism -dependent cytotoxicity. In summary, our data show that amodiaquine and rel ated antimalarials containing ap-aminophenol moiety undergo bioactivation i n vitro to chemically reactive and cytotoxic intermediates. In particular, pyonaridine, which is currently being investigated in humans, was metaboliz ed to a compound which was toxic to neutrophils, Thus, the possibility that it will cause agranulocytosis in clinical practice cannot be excluded, and will require careful monitoring.