Lead identification to clinical candidate selection: drugs for Chagas disease.

Chagas disease affects 8 million people worldwide and remains a main cause of death due to heart failure in Latin America. The number of cases in the United States is now estimated to be 300,000, but there are currently no Food and Drug Administration (FDA)-approved drugs available for patients with Chagas disease. To fill this gap, we have established a public-private partnership between the University of California, San Francisco and the Genomics Institute of the Novartis Research Foundation (GNF) with the goal of delivering clinical candidates to treat Chagas disease. The discovery phase, based on the screening of more than 160,000 compounds from the GNF Academic Collaboration Library, led to the identification of new anti-Chagas scaffolds. Part of the screening campaign used and compared two screening methods, including a colorimetric-based assay using Trypanosoma cruzi expressing ß-galactosidase and an image-based, high-content screening (HCS) assay using the CA-I/72 strain of T. cruzi. Comparing molecules tested in both assays, we found that ergosterol biosynthesis inhibitors had greater potency in the colorimetric assay than in the HCS assay. Both assays were used to inform structure-activity relationships for antiparasitic efficacy and pharmacokinetics. A new anti-T. cruzi scaffold derived from xanthine was identified, and we describe its development as lead series.

Selective and specific inhibition of the plasmodium falciparum lysyl-tRNA synthetase by the fungal secondary metabolite cladosporin.

With renewed calls for malaria eradication, next-generation antimalarials need be active against drug-resistant parasites and efficacious against both liver- and blood-stage infections. We screened a natural product library to identify inhibitors of Plasmodium falciparum blood- and liver-stage proliferation. Cladosporin, a fungal secondary metabolite whose target and mechanism of action are not known for any species, was identified as having potent, nanomolar, antiparasitic activity against both blood and liver stages. Using postgenomic methods, including a yeast deletion strains collection, we show that cladosporin specifically inhibits protein synthesis by directly targeting P. falciparum cytosolic lysyl-tRNA synthetase. Further, cladosporin is >100-fold more potent against parasite lysyl-tRNA synthetase relative to the human enzyme, which is conferred by the identity of two amino acids within the enzyme active site. Our data indicate that lysyl-tRNA synthetase is an attractive, druggable, antimalarial target that can be selectively inhibited.

A genome-wide overexpression screen in yeast for small-molecule target identification.

We describe a multicopy gene suppression screen of drug sensitivity in Saccharomyces cerevisiae that facilitates the identification of cellular targets of small molecules. An array of yeast transformants harboring a multicopy yeast genomic library was screened for resistance to growth inhibitors. Comparison of array growth patterns for several such inhibitors allowed the differentiation of general and molecule-specific genetic suppressors. Specific resistance to phenylaminopyrimidine (1), an inhibitor identified from a kinase-directed library, was associated with the overexpression of Pkc1 and a subset of downstream kinases. Components of two other pathways (pheromone response/filamentous growth and Pho85 kinase) that genetically interact with the PKC1 MAPK signaling cascade were also identified. Consistent with the suppression screen, inhibitor 1 bound to Pkc1 in yeast cell lysate and inhibited its activity in vitro. These results demonstrate the utility of this approach for the rapid deconvolution of small-molecule targets.