Biochemical and Antiparasitic Properties of Inhibitors of the Plasmodium falciparum Calcium-Dependent Protein Kinase PfCDPK1

Abstract

ABSTRACTPfCDPK1 is aPlasmodium falciparumcalcium-dependent protein kinase, which has been identified as a potential target for novel antimalarial chemotherapeutics. In order to further investigate the role of PfCDPK1, we established a high-throughputin vitrobiochemical assay and used it to screen a library of over 35,000 small molecules. Five chemical series of inhibitors were initially identified from the screen, from which series 1 and 2 were selected for chemical optimization. Indicative of their mechanism of action, enzyme inhibition by these compounds was found to be sensitive to both the ATP concentration and substitution of the amino acid residue present at the “gatekeeper” position at the ATP-binding site of the enzyme. Medicinal chemistry efforts led to a series of PfCDPK1 inhibitors with 50% inhibitory concentrations (IC50s) below 10 nM against PfCDPK1 in a biochemical assay and 50% effective concentrations (EC50s) less than 100 nM for inhibition of parasite growthin vitro. Potent inhibition was combined with acceptable absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties and equipotent inhibition ofPlasmodium vivaxCDPK1. However, we were unable to correlate biochemical inhibition with parasite growth inhibition for this series overall. Inhibition ofPlasmodium bergheiCDPK1 correlated well with PfCDPK1 inhibition, enabling progression of a set of compounds toin vivoevaluation in theP. bergheirodent model for malaria. These chemical series have potential for further development as inhibitors of CDPK1.