Vol. 297, Issue 2, 509-515, May 2001

Primaquine-Induced Hemolytic Anemia: Formation and Hemotoxicity of the Arylhydroxylamine Metabolite 6-Methoxy-8-hydroxylaminoquinoline

Laura J. C. Bolchoz, Robert A. Budinsky¹, David C. McMillan and David J. Jollow

Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina

Primaquine is an important antimalarial agent because of its activity against exoerythrocytic forms of Plasmodium spp. However, methemoglobinemia and hemolytic anemia are dose-limiting side effects of primaquine therapy that limit its efficacy. These hemotoxicities are thought to be mediated by metabolites; however, the identity of the toxic species has remained unclear. Since N-hydroxy metabolites are known to mediate the hemotoxicity of several arylamines, the present studies were undertaken to determine whether 6-methoxy-8-aminoquinoline (6-MAQ), a known human metabolite of primaquine, could undergo N-hydroxylation to form a hemotoxic metabolite. When 6-MAQ was incubated with rat and human liver microsomes, a single metabolite was detected by high performance liquid chromatography (HPLC) with electrochemical detection. This metabolite was identified as 6-methoxy-8-hydroxylaminoquinoline (MAQ-NOH) by HPLC and mass spectral analyses. As measured by decreased survival of ⁵¹Cr-labeled erythrocytes in rats, MAQ-NOH was hemolytic in vivo. Furthermore, in vitro exposure of ⁵¹Cr-labeled erythrocytes to MAQ-NOH caused a concentration-dependent decrease in erythrocyte survival (EC₅₀ of 350 µM) when the exposed cells were returned to the circulation of isologous rats. MAQ-NOH also induced the formation of methemoglobin when incubated with suspensions of rat erythrocytes. These data indicate that 6-MAQ can be metabolized to MAQ-NOH by both rat and human liver microsomes and that MAQ-NOH has the requisite properties to be a hemotoxic metabolite of primaquine. The contribution of MAQ-NOH to the hemotoxicity of primaquine in vivo remains to be assessed.