Synthesis and in vitro evaluation of antimycobacterial activity of two palladium(II) complexes based on 5-alkyl-1,3,4-oxadiazol-2(3H)-thione derivatives

•Two palladium(II) complexes containing mixed ligands were prepared.•One of them showed good activity against Mycobacterium tuberculosis.•Antitubercular activity decreased with increasing alkyl chain length.•Both were not cytotoxic against prostate and breast cell lines. In this work, two palladium(II) complexes, namely, [Pd(L1)2(phen)] 1 and [Pd(L2)2(phen)] 2 (L1 = 5-heptyl-1,3,4-oxadiazole-2-(3H)-thione, L2 = 5-nonyl-1,3,4-oxadiazole-2-(3H)-thione and phen = 1,10-phenanthroline), were prepared and charactherized by conventional techniques. In both complexes, the spectral data indicated that L1 and L2 are coordinated to the metal in a monodentate manner through the sulfur atom (S−), while a phenanthroline molecule acting as a bidentate ligand completes the coordination sphere. This proposal was supported by DFT calculation of structures and NMR spectra. Subsequently, the free ligands and their metal complexes were tested in vitro against the human prostate cell lines PNT-2 (non-tumorigenic), LNCaP (androgen-sensitive prostate cancer) and PC-3 (castration-resistant prostate cancer), as well as in two human breast cancer cell lines, MCF7 (luminal) and MDA-MB231 (triple-negative). Regarding cytotoxicity, all compounds were considered inactive, since the IC50 values found were higher than 80 µM. The complexes were also evaluated for activity against Mycobacterium tuberculosis, and 1 showed good activity (MIC = 8.94 µg/mL), while 2 exhibited moderate to poor activity (MIC > 25.0 µg/mL), revealing that an increase in the aliphatic chain makes it less active. A conformation sampling was conducted for the complexes 1 and 2 using molecular dynamics simulation. The analysis provided a clear picture of the steric and electrostatic flexibility of these compounds, which is an important feature for multi-target drugs. [Display omitted]