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The Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland (M.K., G.L., S.A.D.); and Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania (S.H., S.M.B.)
Dantrolene was recently identified as a novel inhibitor of the plasmodial surface anion channel (PSAC), an unusual ion channel on Plasmodium falciparum-infected human red blood cells. Because dantrolene is used clinically, has a high therapeutic index, and has desirable chemical synthetic properties, it may be a lead compound for antimalarial development. However, dantrolene derivatives would need to preferentially interact with PSAC over the sarcoplasmic reticulum (SR) Ca2+ release channel to avoid unwanted side effects from antimalarial therapy. Furthermore, dantrolene's modest affinity for PSAC (Km of 1.2 µM) requires improvement. In this study, we tested 164 derivatives of dantrolene to examine whether these hurdles can be surmounted. A simple screen for PSAC block defined the minimal scaffold needed and identified compounds with 
5-fold higher affinity. Single-channel patch-clamp recordings on infected human red blood cells with two derivatives also revealed increased blocking affinity that resulted from slower unbinding from a site on the extracellular face of PSAC. We tested these derivatives in a frog skeletal muscle contractility assay and found that, in contrast to dantrolene, they had little or no effect on SR Ca2+ release. Finally, these blockers kill in vitro parasite cultures at lower concentrations than dantrolene, consistent with an essential role for PSAC. Because, as a class, these derivatives fulfil the requirements for drug leads and can be studied with simple screening technology, more extensive medicinal chemistry is warranted to explore antimalarial development.
Address correspondence to: Sanjay A. Desai, Laboratory of Malaria and Vector Research, NIAID, National Institutes of Health, Room 3W-01, 12735 Twinbrook Pkwy., Rockville, MD 20852. E-mail: sdesai{at}niaid.nih.gov
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