Synthesis, Spectroscopic Characterization, Structural Studies, and In Vitro Antitumor Activities of Pyridine-3-carbaldehyde Thiosemicarbazone Derivatives

Eight new thiosemicarbazone derivatives, 6-(1-trifluoroethoxy)pyridine-3-carbaldehyde thiosemicarbazone (1), 6-(4′-fluorophenyl)
pyridine-3-carbaldehyde thiosemicarbazone (2), 5-chloro-pyridine-3-carbaldehyde thiosemicarbazone (3), 2-chloro-5-
bromo-pyridine-3-carbaldehyde thiosemicarbazone (4), 6-(3′,4′-dimethoxyphenyl)pyridine-3-carbaldehyde thiosemicarbazone
(5), 2-chloro-5-fluor-pyridine-3-carbaldehyde thiosemicarbazone, (6), 5-iodo-pyridine-3-carbaldehyde thiosemicarbazone (7),
and 6-(3′,5′-dichlorophenyl)pyridine-3-carbaldehyde thiosemicarbazone (8) were synthesized, from the reaction of the corresponding
pyridine-3-carbaldehyde with thiosemicarbazide. 5e synthesized compounds were characterized by ESI-Mass, UVVis,
IR, and NMR (1H, 13C, 19F) spectroscopic techniques. Molar mass values and spectroscopic data are consistent with the
proposed structural formulas. 5e molecular structure of 7 has been also confirmed by single crystal X-ray diffraction. In the solid
state 7 exists in the E conformation about the N2-N3 bond; 7 also presents the E conformation in solution, as evidenced by 1H
NMR spectroscopy.5ein vitro antitumor activity of the synthesized compounds was studied on six human tumor cell lines: H460
(lung large cell carcinoma), HuTu80 (duodenum adenocarcinoma), DU145 (prostate carcinoma), MCF-7 (breast adenocarcinoma),
M-14 (amelanotic melanoma), and HT-29 (colon adenocarcinoma). Furthermore, toxicity studies in 3T3 normal cells
were carried out for the prepared compounds. 5e results were expressed as IC50 and the selectivity index (SI) was calculated.
Biological studies revealed that 1 (IC50 = 3.36 to 21.35 μM) displayed the highest antiproliferative activity, as compared to the other
tested thiosemicarbazones (IC50 = 40.00 to >582.26 μM) against different types of human tumor cell lines. 1 was found to be about
twice as cytotoxic (SI = 1.82) than 5-fluorouracile (5-FU) against the M14 cell line, indicating its efficiency in inhibiting the cell
growth even at low concentrations. A slightly less efficient activity was shown by 1 towards the HuTu80 and MCF7 tumor cell
lines, as compared to that of 5-FU. 5erefore, 1 can be considered as a promising candidate to be used as a pharmacological agent,
since it presents significant activity and was found to be more innocuous than the 5-FU anticancer drug against the 3T3 mouse
embryo fibroblast cells.