Recent reports of increased tolerance to artemisinin derivatives-the last widely effective class of antimalarials -bolster the medical need for new treatments. The spirotetrahydro-β-carbolines, or spiroindolones, are a new class of fast-acting and potent schizonticidal drugs displaying low nanomolar potency against Plasmodium falciparum and Plasmodium vivax clinical isolates. Spiroindolones rapidly diminish protein synthesis in P. falciparum, an effect that is ablated in parasites bearing non-synonymous mutations in the gene encoding the P-type cation-transporter ATPase4 (PfATP4). The optimized spiroindolone NITD609 shows an acceptable safety profile and pharmacokinetic properties compatible with once-daily oral dosing; and demonstrates singledose efficacy in a rodent malaria model. Collectively, these data demonstrate that NITD609 possesses a pharmacological profile suitable for a new drug candidate for the treatment of malaria.Globally, 3.3 billion people are exposed to malaria, a devastating disease that causes over 800,000 deaths each year and kills more under five-year-olds than any other infectious agent (1). Fifty years ago, malaria had been eliminated from many areas of the world through effective antimalarial drug treatments, vector control interventions and disease prevention # Corresponding authors (Winzeler@scripps.edu and Thierry.diagana@novartis.com). * These authors equally contributed to this work One-sentence summary We describe the pharmacological profile of a new antimalarial drug candidate-the spiroindolone NITD609-which through a novel mechanism of action rapidly clears a Plasmodium infection upon administration of a single oral dose in a malaria mouse model. NIH Public Access Author ManuscriptScience. Author manuscript; available in PMC 2011 September 3. (2). However, the global spread of drug resistance resulted, by the 1980s, in a substantial increase in disease incidence and mortality. Today, some encouraging epidemiological data suggest that the introduction of new drugs (notably the artemisinin-based combination therapies or ACTs) may have reversed that trend (3). Derivatives of the endoperoxide artemisinin constitute the only antimalarial drugs that remain effective in all malariaendemic regions, but recent reports suggest that decades of continuous use as monotherapies might have fostered the emergence of resistance (4-6). This realization has triggered a concerted search for new drugs that could be deployed if artemisinin resistance were to spread.Many of the therapies currently in development utilize known antimalarial pharmacophores (e.g. aminoquinolines and/or peroxides) chemically modified to overcome the liabilities of their predecessors (7). While these compounds may prove to be important in the treatment of malaria, it would be preferable to discover novel chemotypes with a distinct mechanism of action (8). However, despite significant advances in our understanding of Plasmodium genome biology, the identification and validation of new drug targets has proven challengi...

SummaryAchieving the goal of malaria elimination will depend on targeting Plasmodium pathways essential across all life stages. Here, we identify a lipid kinase, phosphatidylinositol 4-kinase (PI4K), as the target of imidazopyrazines, a novel antimalarial compound class that inhibits the intracellular development of multiple Plasmodium species at each stage of infection in the vertebrate host. Imidazopyrazines demonstrate potent preventive, therapeutic, and transmission-blocking activity in rodent malaria models, are active against blood-stage field isolates of the major human pathogens, P. falciparum and P. vivax, and inhibit liver stage hypnozoites in the simian parasite P. cynomolgi. We show that imidazopyrazines exert their effect through inhibitory interaction with the ATP-binding pocket of PI4K, altering the intracellular distribution of phosphatidylinositol 4-phosphate. Collectively, our data define PI4K as a key Plasmodium vulnerability, opening up new avenues of target-based discovery to identify drugs with an ideal activity profile for the prevention, treatment and elimination of malaria.

Most malaria drug development focuses on parasite stages detected in red-blood cells even though to achieve eradication next-generation drugs active against both erythrocytic and exo-erythrocytic forms would be preferable. We applied a multifactorial approach to a set of >4,000 commercially available compounds with previously demonstrated blood stage activity (IC50 < 1 μM), and identified chemical scaffolds with potent activity against both forms. From this screen, we identified an imidazolopiperazine scaffold series that was highly enriched among compounds active against Plasmodium liver stages. Our orally bioavailable lead imidazolopiperazine confers complete causal prophylactic protection (15 mg/kg) in rodent models of malaria and shows potent in vivo blood-stage therapeutic activity. The open source chemical tools resulting from our effort provide starting points for future drug discovery programs, as well as opportunities for researchers to investigate the biology of exo-erythrocytic forms.

SummaryThe malaria parasite Plasmodium falciparum establishes in the host erythrocyte plasma membrane new permeability pathways that mediate nutrient uptake into the infected cell. These pathways simultaneously allow Na+ influx, causing [Na+] in the infected erythrocyte cytosol to increase to high levels. The intraerythrocytic parasite itself maintains a low cytosolic [Na+] via unknown mechanisms. Here we present evidence that the intraerythrocytic parasite actively extrudes Na+ against an inward gradient via PfATP4, a parasite plasma membrane protein with sequence similarities to Na+-ATPases of lower eukaryotes. Mutations in PfATP4 confer resistance to a potent class of antimalarials, the spiroindolones. Consistent with this, the spiroindolones cause a profound disruption in parasite Na+ homeostasis, which is attenuated in parasites bearing resistance-conferring mutations in PfATP4. The mutant parasites also show some impairment of Na+ regulation. Taken together, our results are consistent with PfATP4 being a Na+ efflux ATPase and a target of the spiroindolones.

SummaryWith renewed calls for malaria eradication, next-generation antimalarials need be active against drug-resistant parasites and efficacious against both liver- and blood-stage infections. We screened a natural product library to identify inhibitors of Plasmodium falciparum blood- and liver-stage proliferation. Cladosporin, a fungal secondary metabolite whose target and mechanism of action are not known for any species, was identified as having potent, nanomolar, antiparasitic activity against both blood and liver stages. Using postgenomic methods, including a yeast deletion strains collection, we show that cladosporin specifically inhibits protein synthesis by directly targeting P. falciparum cytosolic lysyl-tRNA synthetase. Further, cladosporin is >100-fold more potent against parasite lysyl-tRNA synthetase relative to the human enzyme, which is conferred by the identity of two amino acids within the enzyme active site. Our data indicate that lysyl-tRNA synthetase is an attractive, druggable, antimalarial target that can be selectively inhibited.