Versatile Imidazole Scaffold with Potent Activity against Multiple Apicomplexan Parasites

Malaria, toxoplasmosis, and cryptosporidiosis are caused by apicomplexan parasites Plasmodium spp., Toxoplasma gondii, and Cryptosporidium parvum, respectively, and pose major health challenges. Their therapies are inadequate, ineffective or threatened by drug resistance. The development of novel drugs against them requires innovative and resource-efficient strategies. We exploited the kinome conservation of these parasites to determine the cellular targets and effects of two Plasmodium falciparum inhibitors in T. gondii and C. parvum. The imidazoles, (R)-RY-1-165 and (R)-RY-1-185, were developed to target the cGMP dependent protein kinase of P. falciparum (PfPKG), orthologs of which are present in T. gondii and C. parvum. Using structural and modeling approaches we determined that the molecules bind stereospecifically and interact with PfPKG in a manner unique among described inhibitors. We used enzymatic assays and mutant P. falciparum expressing PfPKG with a substituted “gatekeeper” residue to determine that cellular activity of the molecules is mediated through targets additional to PfPKG. These likely include P. falciparum calcium dependent protein kinase 1 and 4 (PfCDPK-1, -4), kinases that, like PfPKG, have small amino acids at the “gatekeeper” position. The molecules are active against T. gondii and C. parvum, with T. gondii tachyzoites being particularly sensitive. Using mutant parasites, enzyme assays and modeling studies we demonstrate that targets in T. gondii include TgPKG, TgCDPK1, TgCDPK4 and the mitogen activated kinase-like 1 (MAPKL-1). Our results suggest that this scaffold holds promise for the development of new toxoplasmosis drugs.