Repeated sequential administration of pegylated emulsion of SU5416 and liposomal paclitaxel enhances anti-tumor effect in 4T1 breast cancer-bearing mice

To improve vascular normalization strategy for intractable triple-negative breast cancer 4T1, we examined the anti-tumor effects of repeated sequential administration of polyethylene glycol (PEG)-modified emulsion of SU5416 (PE-SU5416), a vascular endothelial growth factor (VEGF) receptor-2 kinase inhibitor, and PEG-modified liposomal paclitaxel (PL-PTX) in mice bearing 4T1 cells. Three sequential administrations (Seq×3) of PE-SU5416 and PL-PTX exhibited significantly higher anti-tumor activity than a single sequential administration (Seq×1). The tumor vasculatures were structurally normalized until after two PE-SU5416 (PE-SU5416×2) or sequential (Seq×2) administrations, while the improvement in vascular function, such as oxygen supply, blood flow, and PEG-liposomal distribution, was evident until after three administrations of PE-SU5416 (PE-SU5416×3) and Seq×3. Although some discrepancies between the structural and functional improvement in tumor vasculatures were observed after PE-SU5416×3 and Seq×3, cancer-associated fibroblasts (CAFs) and collagen levels were significantly reduced after PE-SU5416×2, PE-SU5416×3, Seq×2, and Seq×3, suggesting that a possible decrease in interstitial fluid pressure due to the reduction in CAFs and collagen would have compensated for vascular function. Furthermore, PE-SU5416×2, PE-SU5416×3, Seq×2, and Seq×3 significantly decreased tumor growth factor-β (TGF-β), an activator of CAFs, in tumor tissues, suggesting that the reduction in TGF-β levels by PE-SU5416 suppresses CAF activation.