In vitro antiproliferative and apoptotic effects of thiosemicarbazones based on (-)-camphene and R-(+)-limonene in human melanoma cells

A series of 38 thiosemicarbazone derivatives based on camphene and limonene were evaluated for their antiproliferative activity. Among them, 19 were synthesized and characterized using proton and carbon-13 nuclear magnetic resonance ((1)H and (13)C NMR). For initial compound selection, human melanoma cells (SK-MEL-37) were exposed to a single concentration of a compound (100 μM) for 24, 48, and 72 hours, and cell detachment was visually observed. Cell viability was determined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. Nineteen compounds (4, 6, 8, 11, 13, 14, 15, 16, 17, 18, 20, 22, 25, 26, 31, 3’, 4’, 6’, and 9’) yielded cell viability below 20%. Subsequently, IC(50) values for these compounds were determined, ranging from 11.56 to 55.38 μM, after 72 hours of treatment. Compound 17 (o-hydroxybenzaldehyde (-)-camphene-based thiosemicarbazone) demonstrated the lowest IC(50) value, followed by compound 4 (benzaldehyde (-) camphene-based thiosemicarbazone) at 12.84 μM. Regarding compound 4, we observed the induction of a characteristic ladder pattern of DNA fragmentation through gel electrophoresis. Furthermore, fluorescence, flow cytometry and scanning microscopy assays revealed morphological changes consistent with apoptosis induction. Additionally, the measurement of caspase 6 and 8 activity in cellular extracts after treatment for 2, 4, 6, and 24 hours suggested the potential involvement of the extrinsic apoptosis pathway in the mechanism of action of compound 4. Further investigations, including molecular docking studies, are required to fully explore the potential of compound 4 and the other selected compounds, highlighting their promising role in future melanoma therapy research.