Since the 1940s endochin and analogues thereof were known to be causal prophylactic and potent erythrocytic stage agents in avian models. Preliminary screening in a current in vitro assay identified several 4(1H)-quinolones with nanomolar EC50 against erythrocytic stages of multidrug resistant W2 and TM90-C2B isolates of Plasmodium falciparum. Follow-up structure?activity relationship (SAR) studies on 4(1H)-quinolone analogues identified several key features for biological activity. Nevertheless, structure?property relationship (SPR) studies conducted in parallel revealed that 4(1H)-quinolone analogues are limited by poor solubilities and rapid microsomal degradations. To improve the overall efficacy, multiple 4(1H)-quinolone series with varying substituents on the benzenoid quinolone ring and/or the 3-position were synthesized and tested for in vitro antimalarial activity. Several structurally diverse 6-chloro-2-methyl-7-methoxy-4(1H)-quinolones with EC50 in the low nanomolar range against the clinically relevant isolates W2 and TM90-C2B were identified with improved physicochemical properties while maintaining little to no cross-resistance with atovaquone.
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