Targeted Screening Studies on Endemic Propolis Extracts for Selective Cytotoxicity Against Human Melanoma Cells Models

Melanoma is the most aggressive and life-threatening form of skin cancer. Propolis possesses diverse biological activities and is considered a promising source of compounds for anticancer therapy. This study evaluated the cytotoxic effects of propolis from endemic regions of Nepal, Greece, Kazakhstan, Ukraine, and Georgia on melanoma cell lines (A375, SK-MEL-3) and normal human skin fibroblasts (BJ). Chemical characterization was performed using UHPLC-UV-ESI-qTOF-MS/MS. Total flavonoid and polyphenol contents, as well as antioxidant activity, were also determined. Biological effects were assessed by measuring cell viability, apoptosis (fluorescent staining and confocal microscopy), and caspase-6 activity (fluorometric assay). Most analyzed extracts contained markers characteristic of black poplar-type propolis, including chrysin, pinocembrin, galangin, pinobanksin-3-acetate, and caffeic acid monoesters such as caffeic acid phenethyl ester (CAPE) and isomers of the 1,1-dimethylallyl ester. Three propolis samples from Georgia showed the strongest anticancer activity against melanoma cells while exhibiting low toxicity toward normal fibroblasts (SI > 1). These extracts also displayed high antioxidant activity and elevated polyphenol content. Apoptosis predominated in A375 cells, whereas necrosis was more evident in SK-MEL-3 cells, consistent with increased caspase-6 activity in A375 cells. These results suggest that Georgian propolis may be a promising candidate for further investigation in melanoma therapy.