Targeting a Large Active Site: Structure‐Based Design of Nanomolar Inhibitors of Trypanosoma brucei Trypanothione Reductase

Abstract

Trypanothione reductase (TR) plays a key role in the unique redox metabolism of trypanosomatids, the causative agents of human African trypanosomiasis (HAT), Chagas’ disease, and leishmaniases. Introduction of a new, lean propargylic vector to a known class of TR inhibitors resulted in the strongest reported competitive inhibitor of Trypanosoma (T.) brucei TR, with an inhibition constant Ki of 73 nm, which is fully selective against human glutathione reductase (hGR). The best ligands exhibited in vitro IC50 values (half‐maximal inhibitory concentration) against the HAT pathogen, T. brucei rhodesiense, in the mid‐nanomolar range, reaching down to 50 nm. X‐Ray co‐crystal structures confirmed the binding mode of the ligands and revealed the presence of a HEPES buffer molecule in the large active site. Extension of the propargylic vector, guided by structure‐based design, to replace the HEPES buffer molecule should give inhibitors with low nanomolar Ki and IC50 values for in vivo studies.

Recent Advancement in the Search of Innovative Antiprotozoal Agents Targeting Trypanothione Metabolism

“…Noteworthy, showing anti‐TR and anti‐ T. brucei activities at nanomolar concentrations, the cyclobutyl derivative 46 (Figure 9, Table 4) resulted not only the most active TRI of the series, but also the most potent Tb TRI reported so far, for which the assessment of in vivo efficacy in further studies is strongly expected. Moreover, besides maintaining previously observed contacts within MBS, X‐ray structure (PDB IDs: 6OEZ) showed extension of propargylamino moiety towards a hydrophobic sub‐pocket near the catalytic cysteines of Tb TR, while the alkylamino chain interacts with Asp116 [48f] …”

5-Vinylquinoline-substituted nitrofurans as inhibitors of trypanothione reductase and antitrypanosomal agents

“…To the best of our knowledge, this is the first demonstration of the semiquantitative relationship between the antitrypanosomal in vitro activity of a series of homologous compounds, and their efficacy as TR inhibitors. Interestingly, this type of parallelism has not been observed in the recent studies of hybrids of indole and 1,3-thiazole [24,25], and amides of 5-nitrofuran-2-carbonic acid [9]. In our opinion, this may be attributed to the presence of a great variety of functional groups in the investigated compounds, and/or to the differences in their lipophilicity.…”

Targeting Trypanothione Reductase, a Key Enzyme in the Redox Trypanosomatid Metabolism, to Develop New Drugs against Leishmaniasis and Trypanosomiases

“…Structural studies on TR, intensified over the past 10 years, strongly improved the understanding of the molecular basis of ligand binding, allowing to identify hot-spots for interaction with substrates and inhibitors. This knowledge has been exploited in few structure-based design approaches which, in some cases, have led to a significant improvement in the performances of lead molecules [27,37,40].…”

“…Further efforts for improving properties and potency of this class have been recently undertaken by De Gasparo and colleagues [27,46]. They explored the possibility to combine the 2 different binding modes observed for compound 10a by introducing other substituents on the thiazole to be able to increase water solubility and binding affinity.…”

“…Later, new substitutions resulted in a significant improvement of potency and selectivity. Indeed, compound (+)-2 (M9J in PDB), claimed to be the most effective non-covalent inhibitor of TR ever reported, inhibits T. brucei TR with an inhibition constant Ki of 73 nM and is fully ineffective against human GR, even if its toxicity against mammalian cells is relatively high [27]. Two major structural changes led to this result: the modification of the substituent on the indole moiety, combined with the introduction onto position 4 of the central thiazole moiety of a propargylic substituent, designed to target a hydrophobic sub-pocket near the catalytic cysteines in the TR active In 2013, Ilari and collaborators [35] described the binding of a diarylpyrrole to TR from L. infantum.…”