Chalcogen Derivatives for the Treatment of African Trypanosomiasis: Biological Evaluation of Thio- and Seleno-Semicarbazones and Their Azole Derivatives

Abstract

Human African trypanosomiasis (HAT) is caused by Trypanosoma brucei. Drug therapy remains challenging due to drug resistance and/or toxicity. New drugs are needed. Using thiosemicarbazones as a starting point, we employed a S to Se isosteric replacement strategy to design 44 analogs which were evaluated against T. brucei in vitro. Compounds were divided into 11 groups of four derivatives corresponding to thio-, selenosemicarbazones, and their cyclic counterparts, thio- and selenazoles. We selected three groups which contained a total of six derivatives that inhibited parasite growth by >70%. Then, we investigated the mechanism of action of these compounds, performing quantitative assays to measure their inhibition of the T. brucei cathepsin L-like protease (TbrCATL) and DPPH antioxidant activities. The lead compound (SeO3) showed antioxidant capacity and the best activity against T. brucei (EC₅₀ = 0.47 μM). Nevertheless, its toxicity should be improved. We also predicted the interactions of these compounds with TbrCATL utilizing molecular dynamics. We demonstrate that the Se derivatives are more active than their S analogues, and that the selenazole ring decreases Se-associated toxicity. Also, thio- and selenosemicarbazones are more potent against TbrCATL than the cyclic derivatives. We conclude that TbrCATL inhibition should be combined with antioxidant activity to obtain active compounds against T. brucei.

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Design strategy. (A) Compounds previously shown to have activity against T. brucei and TbrCATL ,, and (B) T. cruzi and cruzain. (C) General structures of the compounds studied: S-semicarbazones ( S O), Se-semicarbazones ( Se O), thiazoles ( S C) and selenazoles ( Se C).

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DR data for Se compounds against TbrCATL (A) and hCatL (B).

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(A) Plot of the evolution of RMSD values during MD simulations of TbrCATL complexed with the ligands Se O1 (blue), Se O3 (purple), Se O5 (red), Se C3 (green) and Se C5 (brown). (B) RMSF plot of protein residues in complex with the ligands Se O1 (blue), Se O3 (purple), Se O5 (red), Se C3 (green) and Se C5 (brown).

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Solvent accessible surface area (SASA) of unbound TbrCATL (A) and TbrCATL complexed with Se O1 (B), Se O3 (C), Se O5 (D), Se C3 (E) and Se C5 (F).

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Representative frames of the binding modes for Se O1 (A), Se O3 (B) and Se O5 (C) in the TbrCATL active site (PDB: 2P7U) obtained from MD simulations. The frames selected here show the predominant conformations of the ligand-protein complex throughout the simulation. Yellow dashed lines indicate all interactions between the ligands and the protein, including hydrogen bonds, CH-π interactions and π-hole interactions. Pink dashed lines highlight the potential covalent bond formed between the thiol group of Cys²⁵ and the selenocarbonyl group of the ligands. (D) Ligand atom labels involved in the recognition process.

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Representative frames of the binding modes of compounds Se C3 (A) and Se C5 (B) at the TbrCATL active site (PDB: 2P7U), obtained from MD simulations. The frames selected here show the predominant conformations of the ligand-protein complex throughout the simulation. Yellow dashed lines indicate all interactions between the ligands and the protein, including hydrogen bonds, CH-π interactions and π-hole interactions. (C) Ligand atoms labels involved in the recognition process.

7

Antioxidant activity of the compounds of interest at the three tested concentrations after 2 h. ASC, ascorbic acid and TRO, trolox, were used as positive controls.

8

BOILED-Egg model of the 12 top hits and positive drug controls (pentamidine and bortezomib). The white region contains those compounds that are prone to be passively absorbed from the gastrointestinal tract (HIA). The yellow region (yolk) contains the molecules predicted to cross the BBB. Compounds colored in blue are predicted to be P-glycoprotein substrates (PGP+), whereas those in red are not (PGP−).

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Superposition of the structures of Se O1 (pink), Se O3 (orange), Se O5 (green), Se C3 (red) and Se C5 (blue) in the TbrCATL active site (PDB 2P7U) obtained from MD simulations. The frames selected here show the predominant conformation of each ligand bound to the protein throughout the simulation.