Preliminary In Vitro and In Ovo Evaluation of the Anticancer Potential of Selected Pentacyclic Triterpenoids in A375 Human Melanoma Cells

Natural compounds serve as an important source of novel drug development. Among them, pentacyclic triterpenoids constitute a widely studied class of plant-derived secondary metabolites that display a broad spectrum of biological activities. The most reported activities in the literature include antioxidant, anti-inflammatory, antiangiogenic, antitumor, antiviral, antidiabetic, and hepatoprotective. Moreover, this class of phytochemicals is currently being intensively studied for its promising anticancer potential. In this study, seven pentacyclic triterpenoid compounds were evaluated for their activity against the A375 human melanoma cell line. The investigated compounds comprise four triterpenic acids—betulonic (BoA), ursonic (UoA), oleanolic (OA) and oleanonic (OoA) acids—and their three semisynthetic methylphosphonate derivatives—oleanolic acid-derived compounds with C(17)-C(O)OCH₂P(O)(OMe)₂ (OPm) and C(17)-C(O)OCH₂P(O)(ONa)₂ (OP) moieties and oleanonic acid-derived compound with C(17)-C(O)OCH₂P(O)(OMe)₂ side chain (OoPm). Among them, BoA, UoA, OoA, and OoPm significantly decreased cell viability and determined higher cytotoxicity against melanoma cells (OoPm—47.5% LDH release at 75 µM) in the MTT and LDH assays. At the same tested concentrations, OA, OoA, and UoA produced different MTT response profiles in A375 and HaCaT cells, with the mean response in HaCaT cells remaining above 50% of the untreated control throughout the tested concentration range. Based on the initial in vitro screening, UoA and OoA were selected for further exploratory assessment of H₂O₂-associated signals and caspase-3/7 activity. The H₂O₂ luminescent assay revealed that OoA determined a maximum increase in intracellular H₂O₂ levels up to 126% at 10 µM, while the Caspase 3/7 test showed that both compounds increased the caspase 3/7 levels at intermediate concentrations (OoA—172.4% and UoA—194.8% at 10 µM). Moreover, qualitative CAM assessment suggested differences in vascular morphology in A375-bearing CAMs treated with OoA and UoA. The results of this preliminary study suggest the promising potential of OoA and UoA against melanoma, underscoring the need for further research using 2D, 3D, and animal models.