A Multitargeted, Metronomic, and Maximum-Tolerated Dose “Chemo-Switch” Regimen is Antiangiogenic, Producing Objective Responses and Survival Benefit in a Mouse Model of Cancer

Abstract

Purpose: A transgenic mouse model has revealed parameters of the angiogenic switch during multistep tumorigenesis of pancreatic islets, and demonstrated efficacy of antiangiogenic therapies. Pericytes have been revealed as functionally important for tumor neovasculature, using kinase inhibitors targeting their platelet-derived growth factor receptors (PDGFRs). Additionally, vascular endothelial growth factor receptor (VEGFR) inhibitors and metronomic chemotherapy show modest benefit against early- but not late-stage disease. Materials and Methods: Seeking to improve efficacy against otherwise intractable end-stage pancreatic islet tumors, two receptor tyrosine kinase inhibitors, imatinib and SU11248, were used to disrupt PDGFR-mediated pericyte support of tumor endothelial cells in concert with maximum-tolerated dose (MTD) or metronomic chemotherapy and/or VEGFR inhibition. Results: Imatinib, despite equivocal efficacy as monotherapy, reduced pericyte coverage of tumor vessels and enhanced efficacy in combination with metronomic chemotherapy or VEGFR inhibition. A regimen involving all three was even better. MTD using cyclophosphamide caused transitory regression, but then rapid regrowth, in contrast to metronomic cyclophosphamide plus imatinib, which produced stable disease. The MTD regimen elicited apoptosis of tumor cells but not endothelial cells, whereas the other regimens increased endothelial cell apoptosis concordant with efficacy. A “chemo-switch” protocol, involving sequential MTD and then metronomic chemotherapy, overlaid with multitargeted inhibition of PDGFR and VEGFR, gave complete responses and unprecedented survival advantage in this model. Conclusion: This study demonstrates a potentially tractable clinical strategy in a stringent preclinical model, wherein standard-of-care chemotherapy is followed by a novel maintenance regimen: PDFGR is targeted to disrupt pericyte support, while metronomic chemotherapy and/or VEGFR inhibitors target consequently sensitized endothelial cells, collectively destabilizing pre-existing tumor vasculature and inhibiting ongoing angiogenesis.