Novel Plasmodial Surface Anion Channel Inhibitors as Antimalarial Drugs

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Summary/Abstract The overall objective of this project is to develop new, potent, selective antimalarials that act through a novel mechanism of blocking the plasmodial surface anion channel (PSAC), a previously unexploited and highly conserved plasmodial target. Human malaria is caused by five species of protozoan parasites in the genus Plasmodium. It is estimated that there are more than 200 million clinical cases of P. falciparum malaria and over 445,000 deaths annually, with the majority of the deaths occurring in sub-Saharan Africa. The malaria parasites, most importantly P. falciparum, require two hosts, which are humans and female Anopheles mosquitoes. Disease is transmitted to humans from the bite of an infected mosquito. There are no effective vaccines available to prevent malaria, but several small molecule treatment options exist, such as chloroquine (CQ) and artemisinin. CQ, once the mainstay of malaria treatment, has lost much of its efficacy because of mutations that confer resistance. Resistance to artemisinin-based therapy is now appearing in Southeast Asia. New small molecule drugs, especially those working on new targets that may be less susceptible to acquired resistance, are desperately needed. PSAC is a newly discovered essential antimalarial target which was validated by gene identification experiments. The channel is produced by the parasite and inserts into the infected erythrocyte membrane. It was demonstrated by Dr. Sanjay Desai, NIH, that PSAC inhibitors, discovered by high-throughput screening, kill parasites by direct action on this channel. In preliminary studies, Dr. Desai, developed and applied a screen for PSAC inhibitors using a sorbitol transport assay, that resulted in the identification of several chemotypes that displayed inhibitory potencies (K0.5 PSAC block) in the nanomolar range. Compounds also inhibited plasmodial growth with low nanomolar potencies (IC50). One of the “hit compound” chemical scaffolds were chosen for medicinal chemistry optimization based on their potency, low cytotoxicity, tractability of synthesis and overall favorable in vitro “drug-like” ADME results. The first, MBX 2366, was subjected to SAR evaluation in a Phase I SBIR project. Compounds in this series demonstrated efficacy, low toxicity and excellent in vitro ADME properties. The Phase II project focused on lead optimizing and scale-up chemistry as well as further mechanism of action studies and demonstrated good in vivo pharmacokinetics and toxicology studies and, notably, proof-of-concept efficacy in the humanized mouse model of P. falciparum infection. The proposed Phase IIB project will finalize compound optimization, including murine efficacy studies to be completed by Medicines for Malaria Venture (MMV), select a preclinical candidate and then conduct IND-enabling preclinical studies to advance a compound to the clinic. The preclinical candidate will be synthesized to a 1 Kg scale. The interdisciplinary approach, which will merge the antimalarial expertise of Dr. Desai and Dr. Jeremy Burrows of MMV with the anti-infective research and development capabilities of Microbiotix, will produce inhibitors for a novel, essential and conserved malarial target and provide new treatment options for resistant infections.