Plasmodium falciparum infections leading to malaria have severe clinical manifestations and high mortality rates. Chloroquine (CQ), a former mainstay of malaria chemotherapy, has been rendered ineffective due to the emergence of widespread resistance. Recent studies, however, have unveiled a novel mode of action in which low-micromolar levels of CQ permeabilized the parasite's digestive vacuole (DV) membrane, leading to calcium efflux, mitochondrial depolarization, and DNA degradation. These phenotypes implicate the DV as an alternative target of CQ and suggest that DV disruption is an attractive target for exploitation by DV-disruptive antimalarials. In the current study, high-content screening of the Medicines for Malaria Venture (MMV) Pathogen Box (2015) was performed to select compounds which disrupt the DV membrane, as measured by the leakage of intravacuolar Ca2+ using the calcium probe Fluo-4 AM. The hits were further characterized by hemozoin biocrystallization inhibition assays and dose-response half-maximal (50%) inhibitory concentration (IC50) assays across resistant and sensitive strains.
Three hits, MMV676380, MMV085071, and MMV687812, were shown to demonstrate a lack of CQ cross-resistance in parasite strains and field isolates. Through systematic analyses, MMV085071 emerged as the top hit due to its rapid parasiticidal effect, low-nanomolar IC50, and good efficacy in triggering DV disruption, mitochondrial degradation, and DNA fragmentation in P. falciparum These programmed cell death (PCD)-like phenotypes following permeabilization of the DV suggests that these compounds kill the parasite by a PCD-like mechanism. From the drug development perspective, MMV085071, which was identified to be a potent DV disruptor, offers a promising starting point for subsequent hit-to-lead generation and optimization through structure-activity relationships.
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