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Received for publication December 27, 2004.
Revised April 20, 2005.
Accepted for publication April 20, 2005.
Dantrolene was recently identified as a novel inhibitor of the plasmodial surface anion channel (PSAC), an unusual ion channel on P. 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) Ca++ release channel to avoid unwanted side effects from antimalarial therapy. Furthermore, dantrolene's modest affinity for PSAC (Km of 1.2 µM) requires improvement. Here, we tested 164 derivatives of dantrolene to examine if 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 (RBCs) with two derivatives also revealed increased affinity and indicated that it results from slower unbinding from a site on PSAC's extracellular face. 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 Ca++ 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.
Key words:
Calcium (Votage-Gated Channels), Chloride, Structure-activity relationships and modeling, Combinatorial chemistry, Single channel kinetics, Antiprotozoal drugs
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