Functional characterization of a putative Plasmodium falciparum calcium/hydrogen antiporter pfcha

SUMMARY Despite substantial efforts to control the spread of malaria, this infectious disease is still a major global health problem as chemotherapy of malaria parasites is limited by established drug resistance and lack of novel affordable treatment options. To combat malaria, identification and characterization of essential genes are needed. Although the calcium metabolism is very important in the life of P. falciparum, little is known about the genes which regulate the transport of calcium. With the release of the P. falciparum genome, a putative calcium hydrogen antiporter has been identified. To understand the role of this gene in the calcium metabolism of P. falciparum and to evaluate it as a drug target, we describe here the functional characterization by the heterologuous expression of this putative calcium hydrogen antiporter in Xenopus oocytes and in a S. cerevisiae strain which is deficient for the yeast vacuolar calcium hydrogen antiporter VCX1. The putative 441-residue protein, which we designate PfCHA, is predicted to contain 11 membrane-spanning segments and is related to the calcium exchanger family. Expression of pfcha in Xenopus oocytes resulted in a higher and pHdependent uptake of pfcha expressing oocytes, in comparison with control oocytes. External pH estimation showed that the calcium uptake was coupled to extrusion of protons out of the oocytes. In other experiments, we found that PfCHA has an antiport activity which is linear with time, unidirectional and electrogenic. Moreover PfCHA was inhibitable by La3+, a calcium transporter competitor, and was moderately inhibited by a sodium calcium exchanger inhibitor KB-R7943. This inhibitor showed also a moderate inhibition on parasite growth in culture. In contrast to the Xenopus oocytes, expression of pfcha in yeasts mutants defective in the Ca2+ transport resulted in a weakly complementat ion of the calcium tolerance. Our subcellular experiments suggested that PfCHA was localized to the yeast vacuole. These results confirm that PfCHA is a calcium hydrogen antiporter and we speculate that PfCHA could be a potential drug target. Further studies are needed to localize it in the parasite and to evaluate it as a drug target

Chemical genetics of \u3ci\u3ePlasmodium falciparum\u3c/i\u3e

Malaria caused by Plasmodium falciparum is a disease that is responsible for 880,000 deaths per year worldwide. Vaccine development has proved difficult and resistance has emerged for most antimalarial drugs. To discover new antimalarial chemotypes, we have used a phenotypic forward chemical genetic approach to assay 309,474 chemicals. Here we disclose structures and biological activity of the entire library—many of which showed potent in vitro activity against drug-resistant P. falciparum strains—and detailed profiling of 172 representative candidates. A reverse chemical genetic study identified 19 new inhibitors of 4 validated drug targets and 15 novel binders among 61 malarial proteins. Phylochemogenetic profiling in several organisms revealed similarities between Toxoplasma gondii and mammalian cell lines and dissimilarities between P. falciparum and related protozoans. One exemplar compound displayed efficacy in a murine model. Our findings provide the scientific community with new starting points for malaria drug discovery

Discovery of Novel, Orally Bioavailable, Antileishmanial Compounds Using Phenotypic Screening

Leishmaniasis is a parasitic infection that afflicts approximately 12 million people worldwide. There are several limitations to the approved drug therapies for leishmaniasis, including moderate to severe toxicity, growing drug resistance, and the need for extended dosing. Moreover, miltefosine is currently the only orally available drug therapy for this infection. We addressed the pressing need for new therapies by pursuing a two-step phenotypic screen to discover novel, potent, and orally bioavailable antileishmanials. First, we conducted a high-throughput screen (HTS) of roughly 600,000 small molecules for growth inhibition against the promastigote form of the parasite life cycle using the nucleic acid binding dye SYBR Green I. This screen identified approximately 2,700 compounds that inhibited growth by over 65% at a single point concentration of 10 μM. We next used this 2700 compound focused library to identify compounds that were highly potent against the disease-causing intra-macrophage amastigote form and exhibited limited toxicity toward the host macrophages. This two-step screening strategy uncovered nine unique chemical scaffolds within our collection, including two previously described antileishmanials. We further profiled two of the novel compounds for in vitro absorption, distribution, metabolism, excretion, and in vivo pharmacokinetics. Both compounds proved orally bioavailable, affording plasma exposures above the half-maximal effective concentration (EC50) concentration for at least 12 hours. Both compounds were efficacious when administered orally in a murine model of cutaneous leishmaniasis. One of the two compounds exerted potent activity against trypanosomes, which are kinetoplastid parasites related to Leishmania species. Therefore, this compound could help control multiple parasitic diseases. The promising pharmacokinetic profile and significant in vivo efficacy observed from our HTS hits highlight the utility of our two-step phenotypic screening strategy and strongly suggest that medicinal chemistry optimization of these newly identified scaffolds will lead to promising candidates for an orally available anti-parasitic drug

A conserved metabolic signature associated with response to fast-acting anti-malarial agents

Characterizing the mode of action of anti-malarial compounds that emerge from high-throughput phenotypic screens is central to understanding how parasite resistance to these drugs can emerge. Here, we have employed untargeted metabolomics to inform on the mechanism of action of anti-malarial leads with different speed of kill profiles being developed by the Novartis Institute of Tropical Diseases (NITD). Time-resolved global changes in malaria parasite metabolite profiles upon drug treatment were quantified using liquid chromatography-based mass spectrometry and compared to untreated controls. Using this approach, we confirmed previously reported metabolomics profiles of the fast-killing (2.5 h) drug dihydroartemisinin (DHA) and the slower killing atovaquone. A slow-acting anti-malarial lead from NITD of imidazolopiperazine (IZP) class, GNF179, elicited little or no discernable metabolic change in malaria parasites in the same 2.5-h window of drug exposure. In contrast, fast-killing drugs, DHA and the spiroindolone (NITD246), elicited similar metabolomic profiles both in terms of kinetics and content. DHA and NITD246 induced peptide losses consistent with disruption of hemoglobin catabolism and also interfered with the pyrimidine biosynthesis pathway. Two members of the recently described class of anti-malarial agents of the 5-aryl-2-amino-imidazothiadiazole class also exhibited a fast-acting profile that featured peptide losses indicative of disrupted hemoglobin catabolism. Our screen demonstrates that structurally unrelated, fast-acting anti-malarial compounds generate similar biochemical signatures in Plasmodium pointing to a common mechanism associated with rapid parasite death. These profiles may be used to identify and possibly predict the mode of action of other fast-acting drug candidates

Discovery of Novel Antimalarial Compounds Enabled by QSAR-Based Virtual Screening

Quantitative structure–activity relationship (QSAR) models have been developed for a dataset of 3133 compounds defined as either active or inactive against P. falciparum. Since the dataset was strongly biased towards inactive compounds, different sampling approaches were employed to balance the ratio of actives vs. inactives, and models were rigorously validated using both internal and external validation approaches. The balanced accuracy for assessing the antimalarial activities of 70 external compounds was between 87% and 100% depending on the approach used to balance the dataset. Virtual screening of the ChemBridge database using QSAR models identified 176 putative antimalarial compounds that were submitted for experimental validation, along with 42 putative inactives as negative controls. Twenty five (14.2%) computational hits were found to have antimalarial activities with minimal cytotoxicity to mammalian cells, while all 42 putative inactives were confirmed experimentally. Structural inspection of confirmed active hits revealed novel chemical scaffolds, which could be employed as starting points to discover novel antimalarial agents

Discovery and Preclinical Pharmacology of INE963, a Potent and Fast-Acting Blood-Stage Antimalarial with a High Barrier to Resistance and Potential for Single-Dose Cures in Uncomplicated Malaria.

A series of 5-aryl-2-amino-imidazothiadiazole (ITD) derivatives were identified by a phenotype-based high-throughput screening using a blood stage Plasmodium falciparum (Pf) growth inhibition assay. A lead optimization program focused on improving antiplasmodium potency, selectivity against human kinases, and absorption, distribution, metabolism, excretion, and toxicity properties and extended pharmacological profiles culminated in the identification of INE963 (1), which demonstrates potent cellular activity against Pf 3D7 (EC50 = 0.006 μM) and achieves artemisinin-like kill kinetics in vitro with a parasite clearance time of \u3c24 h. A single dose of 30 mg/kg is fully curative in the Pf-humanized severe combined immunodeficient mouse model. INE963 (1) also exhibits a high barrier to resistance in drug selection studies and a long half-life (T1/2) across species. These properties suggest the significant potential for INE963 (1) to provide a curative therapy for uncomplicated malaria with short dosing regimens. For these reasons, INE963 (1) was progressed through GLP toxicology studies and is now undergoing Ph1 clinical trials

JAEGER – Hunting for Antimalarials with Generative Chemistry

Recent advances in generative modeling allow designing novel compounds through deep neural networks. One such neural network model, the Junction Tree Variational Auto- Encoder (JT-VAE), excels at proposing chemically valid structures. Based on JT-VAE, we built a generative modeling approach (JAEGER) for finding novel chemical matter with desired bioactivity. Using JAEGER, we designed compounds to inhibit malaria. To prioritize the compounds for synthesis, we used the in-house Profile-QSAR (pQSAR) program, a massively-multitask bioactivity model based on 12,000 Novartis assays. Based on the pQSAR activity predictions, we selected, synthesized, and experimentally profiled two compounds. Both compounds exhibited low nanomolar activity in a malaria proliferation assay as well as a biochemical assay measuring activity against PI(4)K, which is an essential kinase that regulates intracellular development in malaria. The compounds also showed low activity in a cytotoxicity assay. Our findings show that JAEGER is a viable approach for finding novel active compounds for drug discovery

Design of potent antimalarials with generative chemistry

Recent advances in generative modelling allow designing novel compounds through deep neural networks. One such neural network model, JT-VAE (the Junction Tree Variational Auto-Encoder), excels at proposing chemically valid structures. Here, on the basis of JT-VAE, we built a generative modelling approach, JAEGER, for finding novel chemical matter with desired bioactivity. Using JAEGER, we designed compounds to inhibit malaria. To prioritize the compounds for synthesis, we used the in-house pQSAR (Profile-QSAR) program, a massively multitask bioactivity model based on 12,000 Novartis assays. On the basis of pQSAR activity predictions, we selected, synthesized and experimentally profiled two compounds. Both compounds exhibited low nanomolar activity in a malaria proliferation assay as well as a biochemical assay measuring activity against PI(4)K, which is an essential kinase that regulates intracellular development in malaria. The compounds also showed low activity in a cytotoxicity assay. Our findings show that JAEGER is a viable approach for finding novel active compounds for drug discovery