A Comparative Assessment of Smart Nanoselenium Hybrid Plurosomes for Augmenting Repaglinide Delivery Against Breast and Liver Cancer Treatment: D-Optimal Statistical Design, In Vitro Cytotoxicity, Biomarkers Assessment, and In Silico Studies

Background/Objectives: Nanotherapeutics integrated with drug repurposing represent an emerging approach to overcome ongoing cancer treatment resistance, particularly in breast cancer (BC) and hepatoblastoma (HB). This study comparatively evaluated an optimally designed smart anticancer nanoselenium hybrid repaglinide (RG)-loaded plurosomes (SHPs) against MDA-MB-231 and HepG2 cell lines. Methods: SHPs were fabricated by the thin-film hydration method and optimized using D-optimal design to investigate the impact of formulation parameters, including Pluronic® amount, sonication time, and hydrophilic–lipophilic balance of Pluronic® on entrapment efficacy (EE%), particle size (PS), and zeta potential (ZP). Results: The optimum formulation (OP.SHP) exhibited high EE% (70.50 ± 1.86%), desired nanosize (242.90 ± 13.77 nm), and excellent stability (ZP = −66.40 ± 2.12 mV). The thermal profile of OP.SHP confirmed successful RG entrapment. The in vitro drug release profile demonstrated a pH-dependent behavior with enhanced drug release at pH 5.5 (91.29 ± 5.67% over 10 h) and a slower release rate at pH 7.4 than the RG suspension. The MTT assay showed cell line-dependent cytotoxicity, with OP.SHP showing stronger antiproliferative activity against MDA-MB-231 (IC50 = 107.50 µg/mL) than HepG2 cells (IC50 = 215.40 µg/mL), compared with RG suspension. Additionally, cell-cycle analysis revealed S and G2/M phase accumulation in MDA-MB-231 cells and S phase accumulation in HepG2 cells, indicating cell-cycle disruption. Furthermore, the Annexin-V/PI assay showed apoptotic-necrotic cell death in BC and predominant apoptotic cell death in HB cells. Notably, OP.SHP treatment markedly reduced Bcl-2 levels and significantly increased caspase-3 levels. Molecular docking analysis showed good binding affinity of RG toward AMPK and Bcl-2 (−8.7 and −8.0 kcal/mol, respectively). Conclusions: Lastly, OP.SHP demonstrated significant antitumor activity against both cell lines, highlighting its potential as a nanotherapeutic strategy for cancer treatment.