The purine transferase from Trypanosoma cruzi as a potential target for bisphosphonate-based chemotherapeutic compounds

Structural differences between hypoxanthine phosphoribosyltransferase family members highlight opportunities for antiparasitic drug design in neglected diseases

Approaches to identify novel druggable targets for treating neglected diseases include computational studies that predict possible interactions of drugs and their molecular targets. Hypoxanthine phosphoribosyltransferase (HPRT) plays a central role in the purine salvage pathway. This enzyme is essential for the survival of the protozoan parasite T. cruzi, the causal agent of Chagas disease, and other parasites related to neglected diseases. Here we found dissimilar functional behaviours between TcHPRT and the human homologue, HsHPRT, in the presence of substrate analogues that can lie in differences in their oligomeric assemblies and structural features. To shed light on this issue, we carried out a comparative structural analysis between both enzymes. Our results show that HsHPRT is considerably more resistant to controlled proteolysis than TcHPRT. Moreover, we observed a variation in the length of two key loops depending on the structural arrangement of each protein (groups D1T1 and D1T1'). Such variations might be involved in inter-subunit communication or influencing the oligomeric state. Besides, to understand the molecular basis that govern D1T1 and D1T1' folding groups, we explored the distribution of charges on the interaction surfaces of TcHPRT and HsHPRT, respectively. To know whether the rigidity degree bears effect on the active site, we studied the flexibility of both proteins. The analysis performed here illuminates the underlying reasons and significance behind each protein's preference for one or the other quaternary arrangement that can be exploited for therapeutic approaches.

Novel Purine Chemotypes with Activity against Plasmodium falciparum and Trypanosoma cruzi

Malaria and Chagas disease, caused by Plasmodium spp. and Trypanosoma cruzi parasites, remain important global health problems. Available treatments for those diseases present several limitations, such as lack of efficacy, toxic side effects, and drug resistance. Thus, new drugs are urgently needed. The discovery of new drugs may be benefited by considering the significant biological differences between hosts and parasites. One of the most striking differences is found in the purine metabolism, because most of the parasites are incapable of de novo purine biosynthesis. Herein, we have analyzed the in vitro anti-P. falciparum and anti-T. cruzi activity of a collection of 81 purine derivatives and pyrimidine analogs. We firstly used a primary screening at three fixed concentrations (100, 10, and 1 µM) and progressed those compounds that kept the growth of the parasites < 30% at 100 µM to dose–response assays. Then, we performed two different cytotoxicity assays on Vero cells and human HepG2 cells. Finally, compounds specifically active against T. cruzi were tested against intracellular amastigote forms. Purines 33 (IC₅₀ = 19.19 µM) and 76 (IC₅₀ = 18.27 µM) were the most potent against P. falciparum. On the other hand, 6D (IC₅₀ = 3.78 µM) and 34 (IC₅₀ = 4.24 µM) were identified as hit purines against T. cruzi amastigotes. Moreover, an in silico docking study revealed that P. falciparum and T. cruzi hypoxanthine guanine phosphoribosyltransferase enzymes could be the potential targets of those compounds. Our study identified two novel, purine-based chemotypes that could be further optimized to generate potent and diversified anti-parasitic drugs against both parasites.

Zoledronate repositioning as a potential trypanocidal drug. Trypanosoma cruzi HPRT an alternative target to be considered

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi and affects 7 million people worldwide. Considering the side effects and drug resistance shown by current treatments, the development of new anti-Chagas therapies is an urgent need. T. cruzi hypoxanthine phosphoribosyltransferase (TcHPRT), the key enzyme of the purine salvage pathway, is essential for the survival of trypanosomatids. Previously, we assessed the inhibitory effect of different bisphosphonates (BPs), HPRT substrate analogues, on the activity of the isolated enzyme. BPs are used as a treatment for bone diseases and growth inhibition studies on T. cruzi have associated BPs action with the farnesyl diphosphate synthase inhibition. Here, we demonstrated significant growth inhibition of epimastigotes in the presence of BPs and a strong correlation with our previous results on the isolated TcHPRT, suggesting this enzyme as a possible and important target for these drugs. We also found that the parasites exhibited a delay at S phase in the presence of zoledronate pointing out enzymes involved in the cell cycle, such as TcHPRT, as intracellular targets. Moreover, we validated that micromolar concentrations of zoledronate are capable to interfere with the progression of cell infection by this parasite. Altogether, our findings allow us to propose the repositioning of zoledronate as a promising candidate against Chagas disease and TcHPRT as a new target for future rational design of antiparasitic drugs.

Acyclic nucleoside phosphonates as possible chemotherapeutics against Trypanosoma brucei

Human African trypanosomiasis is a life-threatening illness caused by Trypanosoma brucei. Owing to the toxic side effects of the available therapeutics, new medications for this disease are needed. One potential drug target is the 6-oxopurine phosphoribosyltransferases (PRTs), the activity of which is crucial to produce purine nucleotide monophosphates required for DNA and RNA synthesis. Inhibitors of the 6-oxopurine PRTs that show promising results as drug leads are the acyclic nucleoside phosphonates (ANPs). ANPs are very flexible in their structure, enabling important conformational changes to facilitate the binding of this class of compounds in the active site of the 6-oxopurine PRTs.

Evaluation of the Trypanosoma brucei 6-oxopurine salvage pathway as a potential target for drug discovery

Due to toxicity and compliance issues and the emergence of resistance to current medications new drugs for the treatment of Human African Trypanosomiasis are needed. A potential approach to developing novel anti-trypanosomal drugs is by inhibition of the 6-oxopurine salvage pathways which synthesise the nucleoside monophosphates required for DNA/RNA production. This is in view of the fact that trypanosomes lack the machinery for de novo synthesis of the purine ring. To provide validation for this approach as a drug target, we have RNAi silenced the three 6-oxopurine phosphoribosyltransferase (PRTase) isoforms in the infectious stage of Trypanosoma brucei demonstrating that the combined activity of these enzymes is critical for the parasites’ viability. Furthermore, we have determined crystal structures of two of these isoforms in complex with several acyclic nucleoside phosphonates (ANPs), a class of compound previously shown to inhibit 6-oxopurine PRTases from several species including Plasmodium falciparum. The most potent of these compounds have K_i values as low as 60 nM, and IC₅₀ values in cell based assays as low as 4 μM. This data provides a solid platform for further investigations into the use of this pathway as a target for anti-trypanosomal drug discovery.

Human African Trypanosomiasis (HAT) is a life-threatening infectious disease caused by the protozoan parasite, Trypanosoma brucei. Current treatments suffer from low efficacy, toxicity issues and complex medication regimens. Moreover, an alarming number of these parasites are demonstrating resistance to current drugs. For these reasons, there is a renewed effort to develop new classes of modern therapeutics based upon the unique T. brucei cellular processes. One potential new drug target is 6-oxopurine phosphoribosyltransferase (PRTase), an enzyme central to the purine salvage pathway and whose activity is critical for the production of the nucleotides (GMP and IMP) required for DNA/RNA synthesis within this protozoan parasite. We demonstrated that T. brucei encodes two isoforms of hypoxanthine-guanine PRTases (HGPRT) and one hypoxanthine-guanine-xanthine PRTase (HGXPRT). The concurrent activity of these enzymes is required for the normal cell growth in vitro. Moreover, acyclic nucleoside phosphonates represent a promising class of potent and selective compounds as they inhibit the enzymes with K_i values in nanomolar range and exert cytotoxic effects on T. brucei cells grown in vitro with EC₅₀ values in the single digit micromolar range. Our results provide a new foundation for further investigations of these compounds in vivo and suggest that 6-oxopurine salvage pathway represents a possible target for future drug discovery efforts directed at eliminating HAT.

Risedronate in the treatment of Murine Chagas' disease

Risedronate, a bisphosphonate, was used to treat CD-1 mice infected with the Brazil strain of Trypanosoma cruzi. When given by subcutaneous injection 3 times/week, there was a significant reduction in mortality, however, the myocardial pathology and right ventricular dilation was unchanged in these mice compared to control animals. In C57BL/6 mice infected with the Tulahuen strain, there was no change in mortality in response to risedronate treatment. These data suggest that this class of compounds has activity against T. cruzi in vivo and illustrate the utility of imaging and pathologic studies as adjuncts in the evaluation of therapeutic compounds as treatments for experimental Chagas' disease. In addition, it underscores the need to use different strains of T. cruzi.