Antitumor Efficacy of Apatinib and Etoposide-Loaded D-α-Tocopheryl Polyethylene Glycol Succinate Mixed Micelles Against Multidrug-Resistant Ovarian Cancer Cells

Background: Ovarian cancer is frequently diagnosed at an advanced stage and remains one of the most lethal gynecologic malignancies. The development of multidrug resistance (MDR) during repeated chemotherapy is a major cause of treatment failure and is often associated with increased drug efflux mediated by transporters such as P-glycoprotein (P-gp). In this study, D-α-tocopheryl polyethylene glycol succinate (TPGS) and Soluplus® (SOL) mixed micelles, abbreviated as TS, were developed to co-deliver apatinib (APA), a VEGFR-2 inhibitor, and etoposide (ETP), a topoisomerase II inhibitor, for MDR ovarian cancer models. The formulation was designed to improve the aqueous dispersion, cellular accumulation, and antitumor activity of APA and ETP. Methods: APA/ETP-loaded TS micelles (APA/ETP-mTS) were prepared and characterized in terms of particle size, polydispersity index (PDI), zeta potential, encapsulation efficiency (EE), storage stability, and in vitro drug release. Anticancer efficacy was assessed using MTT assays, cellular uptake assays, and 3D tumor spheroid studies employing HeyA8-MDR cells, followed by in vivo toxicity and antitumor efficacy studies. Micellar formulations were denoted using the cargo–carrier format, where APA/ETP indicates co-loaded APA and ETP, C6 indicates coumarin-6 (C6) used as a fluorescent probe, mTS indicates TPGS/SOL mixed micelles, and mSOL indicates SOL-only micelles. Results: The selected APA/ETP-mTS formulation showed a particle size of 20.0 ± 5.1 nm, a PDI of 0.16 ± 0.05, near-neutral zeta potential, and encapsulation efficiencies exceeding 60% for both drugs. The micelles maintained colloidal stability at 4 °C for 4 weeks, although partial decreases in encapsulation efficiency were observed. Compared with APA/ETP solution, APA/ETP-mTS delayed the release of both drugs. C6-mTS showed higher intracellular fluorescence intensity than C6-mSOL, suggesting enhanced cellular accumulation associated with TPGS incorporation. APA/ETP-mTS showed greater cytotoxicity than free drugs in HeyA8-MDR monolayer cells and produced the strongest spheroid growth inhibition among the tested micellar formulations. In the HeyA8-MDR xenograft model, APA/ETP-mTS suppressed tumor growth and resulted in the lowest final tumor weight without apparent overt toxicity based on body weight and survival observations. Conclusions: These results suggest that APA/ETP-mTS is a promising micellar co-delivery platform for hydrophobic anticancer drugs in MDR ovarian cancer.