In vitro and in vivo activities of formycin B against Trypanosoma cruzi

Rate-of-Kill (RoK) assays to triage large compound sets for Chagas disease drug discovery: Application to GSK Chagas Box

Chagas disease (CD) is a human disease caused by Trypanosoma cruzi. Whilst endemic in Latin America, the disease is spread around the world due to migration flows, being estimated that 8 million people are infected worldwide and over 10,000 people die yearly of complications linked to CD. Current chemotherapeutics is restricted to only two drugs, i.e. benznidazole (BNZ) and nifurtimox (NIF), both being nitroaromatic compounds sharing mechanism of action and exerting suboptimal efficacy and serious adverse effects. Recent clinical trials conducted to reposition antifungal azoles have turned out disappointing due to poor efficacy outcomes despite their promising preclinical profile. This apparent lack of translation from bench models to the clinic raises the question of whether we are using the right in vitro tools for compound selection. We propose that speed of action and cidality, rather than potency, are properties that can differentiate those compounds with better prospect of success to show efficacy in animal models of CD. Here we investigate the use of in vitro assays looking at the kinetics of parasite kill as a valuable surrogate to tell apart slow- (i.e. azoles targeting CYP51) and fast-acting (i.e. nitroaromatic) compounds. Data analysis and experimental design have been optimised to make it amenable for high-throughput compound profiling. Automated data reduction of experimental kinetic points to tabulated curve descriptors in conjunction with PCA, k-means and hierarchical clustering provide drug discoverers with a roadmap to guide navigation from hit qualification of a screening campaign to compound optimisation programs and assessment of combo therapy potential. As an example, we have studied compounds belonging to the GSK Chagas Box stemmed from the HTS campaign run against the full GSK 1.8 million compounds collection [1].

One of the challenges in early drug discovery of small molecules is the improvement of the poor success rate in the translation from in vitro biological profile into efficacy in disease models, and ultimately in the clinic. Reductionist in vitro models on the bench may not properly recapitulate disease biology, thus overlooking critical properties of candidate compounds. Chagas Disease is a neglected tropical disease caused by Trypanosoma cruzi, a protozoan parasite with a complex life cycle. Despite the promising prospect based on in vitro and in vivo preclinical studies, efforts to reposition antifungal azoles turned out to be disappointing in clinical trials, with treatment failure in Chagas patients. This raises the question of whether we are using the right preclinical tools for decision-making about moving compounds forward for the treatment of this disease. We hypothesise that in vitro potency and efficacy values alone might be distorting the translational power of preclinical compounds, and we propose the use of rate-of-kill (RoK) assays in high-throughput mode. Herewith we disclose a simple, systematic, and automated methodology of analysis of the otherwise complex kinetic patterns, which provides drug discoverers with a navigation guide along a compound optimisation program or prioritisation of best exemplars across different chemical series.

Purine and pyrimidine transporters of pathogenic protozoa - conduits for therapeutic agents

Purines and pyrimidines are essential nutrients for any cell. Most organisms are able to synthesize their own purines and pyrimidines, but this ability was lost in protozoans that adapted to parasitism, leading to a great diversification in transporter activities in these organisms, especially for the acquisition of amino acids and nucleosides from their hosts throughout their life cycles. Many of these transporters have been shown to have sufficiently different substrate affinities from mammalian transporters, making them good carriers for therapeutic agents. In this review, we summarize the knowledge obtained on purine and pyrimidine activities identified in protozoan parasites to date and discuss their importance for the survival of these parasites and as drug carriers, as well as the perspectives of developments in the field.

Discovery of Novel 7-Aryl 7-Deazapurine 3'-Deoxy-ribofuranosyl Nucleosides with Potent Activity against Trypanosoma cruzi

Chagas disease is the leading cause of cardiac-related mortality in Latin American countries where it is endemic. Trypanosoma cruzi, the disease-causing pathogen, is unable to synthesize purines de novo, necessitating salvage of preformed host purines. Therefore, purine and purine-nucleoside analogues might constitute an attractive source for identifying antitrypanosomal hits. In this study, structural elements of two purine-nucleoside analogues (i.e., cordycepin and a recently discovered 7-substituted 7-deazaadenosine) led to the identification of novel nucleoside analogues with potent in vitro activity. The structure-activity relationships of substituents at C-7 were investigated, ultimately leading to the selection of compound 5, with a C-7 para-chlorophenyl group, for in vivo evaluation. This derivative showed complete suppression of T. cruzi Y-strain blood parasitemia when orally administered twice daily for 5 days at 25 mg/kg and was able to protect infected mice from parasite-induced mortality. However, sterile cure by immunosuppression could not be demonstrated.

Biological activity of analogs of guanine and guanosine against American Trypanosoma and Leishmania spp

The growth inhibitory effects of six guanine and guanosine analogs, 3-deazaguanine (compound 1); 3-deazaguanosine (compound 2); 6-aminoallopurinol (compound 3); 9-beta-xylofuranosyl guanine (compound 4); a ribosylated derivative of compound 3, 6-aminopyrazolo(3,4-d)pyrimidin-4-one (compound 5); and 5-aminoformycin B (compound 6), were tested against some pathogenic members of the family of American Trypanosomatidae. Compounds 1 and 2 were highly active against Trypanosoma cruzi, Trypanosoma rangeli, and American Leishmania spp. in in vitro culture forms. Both compounds also showed antiprotozoal activity in T. cruzi-infected mice, with the optimal dose being about 30 mg/kg of body weight per day given as 10 consecutive doses. Compound 3 was the most active compound in vitro, inhibiting all of the American Trypanosomatidae culture forms tested. It was also highly inhibitory in mice that were acutely infected with T. cruzi, with the optimal dose being about 10 mg/kg of body weight per day. Ribosylation of compound 3 resulted in a derivative that showed decreased inhibitory activity on Trypanosomatidae multiplication. Compound 6 was highly inhibitory of in vitro multiplication of American Leishmania and T. rangeli but had no effect on T. cruzi epimastigotes and on mice that were acutely infected with T. cruzi. Compound 4 showed only a slight effect on T. cruzi epimastigotes.

Novobiocin antagonism of amastigotes of Trypanosoma cruzi growing in cell-free medium

Inhibitors of the enzyme bacterial topoisomerase II (DNA gyrase) were evaluated for activity against Trypanosoma cruzi (Brazil strain), based on the theoretical need for a topoisomerase II in the replication of the kinetoplast DNA network. Novobiocin (500 micrograms/ml) antagonized amastigotes of T. cruzi growing in a cell-free medium at 37 degrees C, as manifested by inhibition of multiplication, abnormal morphology of Giemsa-stained organisms, and delayed or absent growth of cells upon subculturing in a drug-free medium. In contrast, novobiocin (1,000 micrograms/ml) essentially had no effect on the multiplication and motility of epimastigotes growing in a cell-free medium at 27 degrees C. This resistance of epimastigotes represented a difference in the physiology of this morphologic stage and not in the temperature of experimentation, because novobiocin inhibited multiplication of amastigotes at 27 degrees C as well and accelerated transformation to epimastigotes. With T. cruzi growing within cultured human fibroblasts, novobiocin (200 micrograms/ml) markedly inhibited transformation of intracellular amastigotes to trypomastigotes. Clorobiocin, a structural analog of novobiocin and likewise an inhibitor of the B subunit of bacterial topoisomerase II, was five times more potent on a molar basis than novobiocin was in antagonism of amastigotes growing in a cell-free medium and did not antagonize epimastigotes. Coumermycin A1, another analog of novobiocin, and five 4-quinolone antibacterial agents, antagonists of the A subunit of bacterial topoisomerase II, inhibited neither amastigotes nor epimastigotes. These experiments indicate that novobiocin and clorobiocin represent a new structural class of drugs with activity against T. cruzi. Whether the mechanism of action of these drugs involves antagonism of a T. cruzi topoisomerase II or an unrelated target is yet to be determined.

In vitro screens in the experimental chemotherapy of leishmaniasis and trypanosomiasis

The search for more effective drugs for the treatment of leishmaniasis and trypanosomiasis has increasingly involved the use of in vitro screens. These possess the immediate advantages of requiring only a few mg of compound for tests, providing for a large through-put of compounds with rapid results at lower costs, and requiring fewer animals. Models for the cultivation or maintenance in vitro of 'mammalian stages' of Leishmania and Trypanosoma cruzi have been available for many years but only in the last decade have satisfactory techniques for the cultivation of bloodstream forms of T. brucei been developed.