Synthesis and comparison of antileishmanial and cytotoxic activities of S-(−)-limonene benzaldehyde thiosemicarbazones with their R-(+)-analogues

In this study, we explore a series of novel potential antiprotozoal S-(−)-limonene-based benzaldehyde thiosemicarbazones were synthesised and their activity effective against the extracellular promastigote form of Leishmania amazonensis examined. Likewise, in parallel, a series of R-(+)-limonene-based thiosemicarbazones previously synthesised by our research group and thiosemicarbazones lacking the monoterpenic moiety, were also biologically evaluated. Here, we report the combination of theoretical and experimental approaches, as well as statistical analysis, to investigate the effect of the monoterpenic group and its stereochemistry in the biological activity of benzaldehyde thiosemicarbazone derivatives, for the identification of their structure-activity relationship. The terpenic thiosemicarbazones displayed the highest activities, confirming that the monoterpenic moiety is essential for activity. Notably, among the compounds tested, the S-(−)-enantiomer of the 4-nitro-derivative (8d) presented considerably lower cytotoxicity than its R-(+)-analogue, emphasizing the importance of the stereochemistry. The most active derivative (8d) exhibited a potent antiprotozoal activity (IC50 2.4 μM) and high selectivity (SI > 1147). Also, theoretical calculations were carried out at the density functional theory (DFT) level to show that the Gibbs free energy and LUMO orbitals present an excellent correlation with the experimental IC50 values. Finally, the combination of all these results may in principle be extremely advantageous to a deeper chemical understanding, as well as, allows a rational alternative for the future development of new drugs that act against leishmaniasis.