ABSTRACT
BACKGROUND: Pancreatic neuroendocrine tumors (PNETs) represent a distinct hypervascularized tumor entity, often diagnosed at metastatic stage. Therapeutic efficacy of anti-angiogenic multi-kinase inhibitors is frequently limited by primary or acquired resistance in vivo. This study aimed to characterize the molecular mode of action as well as resistance mechanisms to the anti-angiogenic multi-tyrosine kinase inhibitor (TKI) Regorafenib in vitro and in vivo. METHODS: In vitro, human and murine pancreatic neuroendocrine cell lines were comparatively treated with Regorafenib and other TKIs clinically used in PNETs. Effects on cell viability and proliferation were analyzed. In vivo, transgenic RIP1Tag2 mice were treated with Regorafenib at two different time periods during carcinogenesis and its impact on angiogenesis and tumor progression was evaluated. RESULTS: Compared to the established TKI therapies with Sunitinib and Everolimus, Regorafenib showed the strongest effects on cell viability and proliferation in vitro, but was unable to induce apoptosis. Unexpectedly and in contrast to these in vitro findings, Regorafenib enhanced proliferation during early tumor development in RIP1Tag2 mice and had no significant effect in late tumor progression. In addition, invasiveness was increased at both time points. Mechanistically, we could identify an upregulation of the pro-survival protein Bcl-2, the induction of the COX2-PGE2-pathway as well as the infiltration of CSF1R positive immune cells into the tumors as potential resistance mechanisms following Regorafenib treatment. DISCUSSION: Our data identify important tumor cell-autonomous and stroma-dependent mechanisms of resistance to antiangiogenic therapies.
