Abstract

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 (K_m 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.