3,3′-Disubstituted 5,5′-Bi(1,2,4-triazine) Derivatives with Potent in Vitro and in Vivo Antimalarial Activity

A series of 3,3′-disubstituted 5,5′-bi(1,2,4-triazine) derivatives was synthesized and screened against the erythrocytic stage of Plasmodium falciparum 3D7 line. The most potent dimer, 6k, with an IC50 (50% inhibitory concentration) of 0.008 μM, had high in vitro potency against P. falciparum lines resistant to chloroquine (W2, IC50 = 0.0047 ± 0.0011 μM) and artemisinin (MRA1240, IC50 = 0.0086 ± 0.0010 μM). Excellent ex vivo potency of 6k was shown against clinical field isolates of both P. falciparum (IC50 = 0.022–0.034 μM) and Plasmodium vivax (IC50 = 0.0093–0.031 μM) from the blood of outpatients with uncomplicated malaria. Despite 6k being cleared relatively rapidly in mice, it suppressed parasitemia in the Peters 4-day test, with a mean ED50 value (50% effective dose) of 1.47 mg kg–1 day–1 following oral administration. The disubstituted triazine dimer 6k represents a new class of orally available antimalarial compounds of considerable interest for further development.