The Apicomplexan Molecular Physiology Section continued its studies of the plasmodial surface anion channel (PSAC) and made two significant contributions.? ? First, we identified and characterized a novel antimalarial drug resistance mechanism based on reduced drug uptake via PSAC (Antimicrob. Agents Chemother. 52:2346, 2008). This discovery entailed in vitro selection of malaria parasites resistant to leupeptin, a broad specificity cysteine and serine protease inhibitor. Because leupeptin has multiple targets within infected cells, workers assumed that resistance would not be acquired easily. We found that leupeptin resistance was not associated with upregulation of cysteine protease activity, reduced leupeptin sensitivity of this activity, or expression level changes for putative cysteine or serine proteases in the parasite genome. Instead, it was associated with marked changes in PSAC, as measured with patch-clamp and other transport assays. PSAC antagonists were found to antagonize parasite growth inhibition by leupeptin, suggesting that leupeptin can access its intracellular targets only after passive uptake via PSAC. The resistant parasite's channel mediated significantly reduced leupeptin uptake and could fully account for the acquired resistance. Reduced passive uptake of bulky hydrophilic antimalarial agents via selection of mutations in PSAC represents a novel antimalarial drug resistance mechanism that should be considered by drug discovery and development programs.? ? Second, we carried out additional high-throughput screens of small molecule libraries for PSAC antagonists. These screens have successfully identified novel antagonists with unique properties. These include compounds that have the highest known affinity (low nanomolar) and specificity for PSAC inhibition, novel agonists that increase transport via PSAC, compounds that alter solute selectivity, and compounds that provide new insights into the structure and function of PSAC.
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