The
protozoan cysteine proteases cruzain in Trypanosoma
cruzi and rhodesain in Trypanosoma
brucei are therapeutic targets for Chagas disease
and Human African Trypanosomiasis (HAT), respectively. A benzimidazole
series was previously characterized as potent noncovalent competitive
cruzain and rhodesain inhibitors with activity against trypanosomes.
Common structure–activity relationships (SAR) trends and structural
modifications leading to selectivity against each enzyme were described.
However, some of these trends could not be understood based on the
reported binding mode of lead compound 1. Therefore,
we employed microsecond molecular dynamics simulations and free energy
calculations to understand qualitative SAR trends and to quantitatively
recapitulate them. Simulations revealed the most stable protein–ligand
interactions and provided insights concerning enzyme selectivity.
Calculated relative binding free energies of compound 1 analogs exhibited deviations of 1.1 and 2.2 kcal/mol from the experimental
values for cruzain and rhodesain, respectively. These data encourage
prospective thermodynamic integration (TI) studies to optimize this
series and facilitate the prioritization of compounds for synthesis.
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