The development of cycloguanil-based derivatives for the treatment of parasitic diseases

Parasitic diseases still represent a plague that provokes a major impact to public health, and overall social and economic well-being of affected countries, mainly of developing countries. Parasites belonging to Trypanosomatidae family are the etiological agents of human and animal vector-borne diseases, including human African trypanosomiasis (HAT), which is caused by Trypanosoma brucei, and Leishmaniasis, which is caused by several Leishmania spp. Although treatment options for these infections exist, their use is limited by several factors, including toxicity, suboptimal efficacy, difficult route of administration, and cost. Moreover, the emergence of drug resistance threatens the positive therapy outcome. This scenario claims the need of addressing more adequate therapies. Targeting the enzymes of the folate metabolism has demonstrated to be a successful approach in the treatment of bacterial infections and malaria, and recently it has been proposed also for the development of novel antiparasitic treatments. Trypanosomatids are auxotrophic for folates and pterins that are crucial cofactors for biosynthesis of nucleic acids and proteins. The inhibition of the key enzymes involved in the folate pathways, namely dihydrofolate reductase (DHFR) and pteridine reductase-1 (PTR1) should provide an effective treatment for these trypanosomatidic infections. The antimalarial drug cycloguanil (CYC), a known DHFR inhibitor, was shown to be also an inhibitor of TbPTR1. Considering the structural analysis of CYC binding modes to TbPTR1 and TbDHFR, the efforts of my PhD work have concerned the development of four novel series of antiprotozoan agents exploring the chemical space around the amino 1,6-dihydrotriazine core structure of CYC. Series 1 includes 2-aminotriazino[1,2-a]benzimidazole derivatives and their chemical precursors, the 2-guanidino benzimidazoles, while series 2 is characterised by the azaspiro-2,4-diamino-1,6-dihydrotriazine scaffold, decorated at C(6) of CYC with the benzyl-piperidine spirane moiety. CYC-like 2,4-diamino-1,6-dihydrotriazines form series 3, whilst series 4 has explored the N-benzyl benzamide chain replacing the 4-Cl atom of CYC. Each series of compounds has reached a different stage of research, so it is dealt with separately. In general, the compounds have been investigated for their on-target activity, human DHFR inhibition to ascertain their selectivity for the protozoan enzymes, cytotoxicity and antiparasitic effect. In the case of series 2, I have also performed docking studies to better understand the binding mode of these compounds to parasite and human enzymes of the folate pathway. Overall, the SAR analysis derived from this study has allowed to obtain key insights for the future design of more promising antifolates for the treatment of protozoan diseases.