Selective Cytotoxic Activity of Derivatives of the Natural Alkaloid Tetrandrine Against Cancer Cell Lines: In Vitro and In Silico Studies

Tetrandrine (TET) is a bisbenzylisoquinoline alkaloid with promising anticancer activity, although its limited selectivity toward malignant over non-cancerous cells restricts its therapeutic potential. A series of mono- and bis-N-substituted tetrandrine derivatives was therefore investigated to determine whether N-functionalization could improve the cytotoxic selectivity of the parent scaffold. The series included the two new compounds MU1NT and DPBT. Following initial screening in non-cancerous ARPE-19 retinal epithelial cells, selected derivatives were evaluated against 22Rv1 prostate carcinoma, A549 lung adenocarcinoma, MDA-MB-231 triple-negative breast cancer, HCT-116 colorectal carcinoma, and HeLa cervical cancer cells. TET exhibited broad but poorly selective cytotoxicity, with IC50 values of 6.81 ± 1.79 and 6.00 ± 1.96 μM in ARPE-19 and 22Rv1 cells, respectively, corresponding to a selectivity index (SI) of 1.13. By contrast, DBT showed IC50 values of 45.44 ± 1.37 μM in ARPE-19 cells and 9.89 ± 1.08 μM in 22Rv1 cells, with an SI of 4.59, the highest experimentally determined value among the tetrandrine-based compounds evaluated. MU1NT also displayed improved selectivity toward 22Rv1 cells (SI = 2.57), although with lower cytotoxic potency than DBT. Fluorescence microscopy using DAPI and phalloidin revealed pronounced nuclear and cytoskeletal alterations in DBT-treated 22Rv1 cells, whereas ARPE-19 morphology was comparatively preserved. Annexin V-FITC/PI flow cytometry further demonstrated an apoptosis-associated response in 22Rv1 cells, with total apoptosis reaching 86.30 ± 1.27% for TET and 34.77 ± 5.13% for DBT after 24 h at their respective MTT-derived IC50 concentrations. Molecular docking suggested distinct predicted interaction profiles for TET and DBT across the apoptosis-related proteins evaluated. Targeted redocking of co-crystallized venetoclax validated the Bcl-2 site-specific protocol (RMSD = 0.55 Å); however, both alkaloids scored less favorably than venetoclax, and the small score difference between TET and DBT did not account for their distinct cellular selectivity profiles. Collectively, these findings identify DBT as a promising tetrandrine derivative with improved selectivity, warranting further mechanistic studies and in vivo evaluation in prostate cancer models.