Dipeptide derivatives of primaquine as transmission-blocking antimalarials: effect of aliphatic side-chain acylation on the gametocytocidal activity and on the formation of carboxyprimaquine in rat liver homogenates

Purpose. Dipeptide derivatives of primaquine (PQ) with reduced oxidative deamination to the inactive metabolite carboxyprimaquine were synthesized and evaluated as a novel class of transmission-blocking antimalarials. Methods. Antimalarial activity was studied using a model consisting of mefloquine- resistant Plasmodium berghei ANKA 25R/10, Balb C mice, and Anopheles stephensi mosquitoes. Metabolic studies were performed with rat liver homogenates, and the incubates were analyzed by HPLC. Results. All dipeptide derivatives and glycyl-PQ completely inhibited the appearance of oocysts in the midguts of the mosquitoes at 15 mg/kg, while N-acetylprimaquine was not active at this dose. However, none of the title compounds were able to block oocyst production at 3.75 mg/kg, in contrast with primaquine. Exception for sarc-gly-PQ, all remaining compounds prevented sporozoite formation in the salivary glands of mosquitoes at a dose of 3.75 mg/kg. Simultaneous hydrolysis to primaquine and gly-PQ occurred with the following order of V(max)/K(m): for primaquine formation, L-ala-gly-PQ > L-phe-gly-PQ > gly- gly-PQ; and for gly-PQ formation, L-phe-gly-PQ > L-ala-gly-PQ > gly-gly-PQ. In contrast, primaquine was not released from D-phe-gly-PQ, sarc-gly-PQ, and N-acetylprimaquine. Neither carboxyprimaquine nor 8-amino-6-methoxy-quinoline were detected in any of the incubation mixtures. Conclusions. The title compounds prevent the development of the sporogonic cycle of Plasmodium berghei. Gametocytocidal activity is independent of the rate and pathway of primaquine formation. Acylation of the aliphatic side-chain effectively prevents the formation of carboxyprimaquine, but the presence of a terminal amino group appears to be essential for the gametocytocidal activity.