Synthetic Analogs of the Alkaloid Cassiarin A with Enhanced Antimalarial Activity

Background: Among the alkaloids from Cassia siamea, cassiarin A has outstanding antiprotozoal activity, but structure-activity relationships for this chemotype were only poorly understood until now. Methods: We worked out efficient approaches to hitherto underexplored analogs (12 examples) on three synthesis routes which mainly comprised variations in the methyl groups at C-2 and C-5. The new compounds were tested for antiprotozoal and cytotoxic activities. Results: Introduction of a (substituted) benzene ring at C-2 led to a significant enhancement of activity against Plasmodium falciparum, while modifications of the methyl group at C-5 and the phenolic group had detrimental effects. Two of the 2-phenyl analogs further showed a resistance index comparable to the one of the reference drug chloroquine. Although the novel derivatives did not show hemolytic effects, investigation on human endothelial (HUVEC) cells at relevant concentrations indicated strong cytotoxic effects on human cells. Conclusions: Systematic structure modifications of cassiarin A led to a significant enhancement of antiplasmodial activity, but the observed strong cytotoxicity to human cells renders this library of cassiarin A derivatives inadequate for drug development.