In vitro metabolism of the antimalarial agent primaquine by mouse liver enzymes and identification of a methemoglobin-forming metabolite.

From a mouse liver microsomal system, we have isolated and identified a methemoglobin-forming metabolite of primaquine (PQ). Evidence has been found for both O-dealkylation and hydroxylation of PQ to form a metabolite, 5,6-dihydroxy-8-(4-amino-1-methylbutylamino)quinoline, which is highly active in forming methemoglobin in both normal and glucose-6-phosphate dehydrogenase-deficient erythrocytes. It also actively decreases glutathione levels in glucose-6-phosphate dehydrogenase-deficient erythrocytes. The inhibitor SKF 525-A prevented metabolite formation while iproniazid and carbon monoxide did not inhibit metabolism completely but may have resulted in formation of a different unidentified metabolite. Mass spectrometry, HPLC, NMR, and other more indirect methods were used to help identify the metabolite. It was identified indirectly via a blue compound which results from extracting the actual metabolite from the incubation mixture with organic solvents under alkaline conditions in the presence of light. The blue compound was identified as a quinonimine in which the 8-amino side chain of PQ cyclizes to produce a third ring system.