Evaluation of the interaction between new trypanocide 1,2,4-triazolo-3-thiones with human serum albumin

Abstract

There is a strong demand for the development of new drugs for the treatment of Chagas disease, a neglected tropical disease. This work investigates human serum albumin (HSA) interaction with a new series of 1,2,4-triazolo-3-thiones derived from piperine (1b, 1c, 1d, and 1e), which were demonstrated by our group to present trypanocide activity in the low micromolar range. The study was carried out by spectroscopic techniques, including UV–vis absorption, circular dichroism, steady-state and time-resolved fluorescence combined with theoretical calculations (molecular docking). The binding is spontaneous (ΔG° < 0) and strong (Ka ≈ 105 M−1), resulting in a non-conventional ground state association (there is a combination of static and dynamic quenching mechanisms). The positive values of ΔH° and ΔS° are indicative that the binding is entropically driven and suggest that the interaction between the compounds and HSA is governed by hydrophobic interactions. Interestingly, competitive binding results show that compounds containing a cyclohexyl substituent at N4 (1c and 1e) bind preferentially to Sudlow’s site I, whereas compounds containing a phenylethyl substituent at N4 (1b and 1d) bind preferentially to Sudlow’s site II. Docking results indicate that the interaction profiles of 1c and 1e to Sudlow’s site I are quite similar, as are those of 1b and 1d in Sudlow’s site II. In all cases, the interactions are predominantly hydrophobic, in accordance with the thermodynamic data. The favorable interaction profiles with HSA should have a positive effect on the pharmacokinetics of the trypanocide piperine derivatives and probably a major influence on their in vivo efficacy.