A Novel Ex Vivo Drug Assay for Assessing the Transmission-Blocking Activity of Compounds on Field-Isolated Plasmodium falciparum Gametocytes

Plasmodium SEY1 is a novel druggable target that contributes to imidazolopiperazine mechanism of action

The precise mode of action of ganaplacide (KAF156), a phase III antimalarial candidate, remains elusive. Here we employ omics-based methods with the closely related chemical analog, GNF179, to search for potential Plasmodium targets. Ranking potential targets derived from chemical genetics and proteomic affinity chromatography methodologies identifies SEY1 , or Synthetic Enhancement of YOP1, which is predicted to encode an essential dynamin-like GTPase implicated in homotypic fusion of endoplasmic reticulum (ER) membranes. We demonstrate that GNF179 decreases Plasmodium SEY1 melting temperature. We further show that GNF179 binds to recombinant Plasmodium SEY1 and subsequently inhibits its GTPase activity, which is required for maintaining ER architecture. Using ultrastructure expansion microscopy, we find GNF179 treatment changes parasite ER and Golgi morphology. We also confirm that SEY1 is an essential gene in P. falciparum . These data suggest that SEY1 may contribute to the mechanism of action of imidazolopiperazines and is a new and attractive druggable target.

Imidazolopiperazine (IPZ)-Induced Differential Transcriptomic Responses on Plasmodium falciparum Wild-Type and IPZ-Resistant Mutant Parasites

Imidazolopiperazine (IPZ), KAF156, a close analogue of GNF179, is a promising antimalarial candidate. IPZ is effective against Plasmodium falciparum and Plasmodium vivax clinical malaria in human with transmission blocking property in animal models and effective against liver stage parasites. Despite these excellent drug efficacy properties, in vitro parasites have shown resistance to IPZ. However, the mechanism of action and resistance of IPZ remained not fully understood. Here, we used transcriptomic analysis to elucidate mode of action of IPZs. We report, in wild-type parasites GNF179 treatment down regulated lipase enzymes, two metabolic pathways: the hydrolysis of Phosphoinositol 4,5-bipohosphate (PIP2) that produce diacyglycerol (DAG) and the cytosolic calcium Ca2+ homeostasis which are known to be essential for P. falciparum survival and proliferation, as well for membrane permeability and protein trafficking. Furthermore, in wild-type parasites, GNF179 repressed expression of Acyl CoA Synthetase, export lipase 1 and esterase enzymes. Thus, in wild-type parasites only, GNF179 treatment affected enzymes leading lipid metabolism, transport, and synthesis. Lastly, our data revealed that IPZs did not perturb known IPZ resistance genes markers pfcarl, pfact, and pfugt regulations, which are all instead possibly involved in the drug resistance that disturb membrane transport targeted by IPZ.

Assessment of the transmission blocking activity of antimalarial compounds by membrane feeding assays using natural Plasmodium falciparum gametocyte isolates from West-Africa

Antimalarial drugs that can block the transmission of Plasmodium gametocytes to mosquito vectors would be highly beneficial for malaria elimination efforts. Identifying transmission-blocking drugs currently relies on evaluation of their activity against gametocyte-producing laboratory parasite strains and would benefit from a testing pipeline with genetically diverse field isolates. The aims of this study were to develop a pipeline to test drugs against P. falciparum gametocyte field isolates and to evaluate the transmission-blocking activity of a set of novel compounds. Two assays were designed so they could identify both the overall transmission-blocking activity of a number of marketed and experimental drugs by direct membrane feeding assays (DMFA), and then also discriminate between those that are active against the gametocytes (gametocyte killing or sterilizing) or those that block development in the mosquito (sporontocidal). These DMFA assays used venous blood samples from naturally infected Plasmodium falciparum gametocyte carriers and locally reared Anopheles gambiae s.s. mosquitoes. Overall transmission-blocking activity was assessed following a 24 hour incubation of compound with gametocyte infected blood (TB-DMFA). Sporontocidal activity was evaluated following addition of compound directly prior to feeding, without incubation (SPORO-DMFA); Gametocyte viability was retained during 24-hour incubation at 37°C when gametocyte infected red blood cells were reconstituted in RPMI/serum. Methylene-blue, MMV693183, DDD107498, atovaquone and P218 showed potent transmission-blocking activity in the TB-DMFA, and both atovaquone and the novel antifolate P218 were potent inhibitors of sporogonic development in the SPORO-DMA. This work establishes a pipeline for the integral use of field isolates to assess the transmission-blocking capacity of antimalarial drugs to block transmission that should be validated in future studies.