Evaluation of cytotoxic potential of Callistemon leaf extracts against breast and colon cancer: Metabolomic, flow cytometry and in silico studies

This study explores the phytochemical and the cytotoxic prospective of leaf ethanol extracts from four Callistemon species ( C. citrinus, C. macropunctatus, C. viminalis, and C. subulatus ) against breast (MCF-7) and colon (Caco-2) cancer cell lines. Metabolomic profiling of the ethanolic extracts of the studied Callistemon species was performed using UPLC-ESI-MS/MS. In-vitro cytotoxicity effects were evaluated using 3-(4,5-dimethylthiazol-2- yl)-2,5-diphenyltetrazolium bromide (MTT) assay followed by flow cytometric analysis. In-silico docking studies of predominant compounds against cell cycle regulatory enzymes were conducted, followed by molecular dynamics simulations for top binding interactions. UPLC-ESI-MS/MS tentatively identified 20 compounds in the extracts, with lignans, and flavonoids being prominent. C. macropunctatus extract showed the strongest cytotoxicity against both cancer lines. Notably, this extract induced cell cycle arrest at S and G1 phases in both MCF-7 and Caco-2 cells, as well as promoted apoptosis. In-silico docking simulations further investigated that the evaluated compounds exhibited promising binding affinities (-6.6 to -11.2 kcal/mol) particularly cyanidin 3,5-diglucoside (-11.1), nilocitin (-10.9), and quercetin 3- O -(2′'-galloyl)- β -D-galactopyranoside (-11.2), exhibited the most favourable docking scores towards cyclin-dependent kinase-2 CDK2, a key cell cycle regulator, compared to the co-crystal ligand roniciclib (-9.5). These findings suggest strong interactions with crucial CDK2 amino acid residues. Collectively, this study highlights the cytotoxic potential of Callistemon leaf extracts, particularly C. macropunctatus , warranting further exploration of their anti-cancer properties, with a focus on CDK2 inhibition mechanisms.