Folate-hapten-mediated immunotherapy synergizes with vascular endothelial growth factor receptor inhibitors in treating murine models of cancer

The overexpression of folate receptors (FR) on many human cancers has led to the development of folate-linked drugs for the imaging and therapy of FR-expressing cancers. In a recent phase I clinical trial of late-stage renal cell carcinoma patients, folate was exploited to deliver an immunogenic hapten, fluorescein, to FR tumor cells in an effort to render the cancer cells more immunogenic. Although >50% of the patients showed prolonged stable disease, all patients eventually progressed, suggesting that the folate-hapten immunotherapy was insufficient by itself to treat the cancer. In an effort to identify a companion therapy that might augment the folate-hapten immunotherapy, we explored coadministration of two approved cancer drugs that had been previously shown to also stimulate the immune system. We report that sunitinib and axitinib (VEGF receptor inhibitors that simultaneously mitigate immune suppression) synergize with the folate-hapten-Targeted immunotherapy to reduce tumor growth in three different syngeneic murine tumor models. We further demonstrate that the combination therapy not only enhances tumor infiltration of CD4 and CD8 effector cells, but surprisingly reduces tumor neovasculogenesis more than predicted. Subsequent investigation of the mechanism for this unexpected suppression of neovasculogenesis revealed that it is independent of elimination of any tumor cells, but instead likely derives from a reduction in the numbers of FR tumor-Associated macrophages and myeloid-derived suppressor cells, that is, immunosuppressive cells that release significant quantities of VEGF. These data suggest that a reduction in stromal cells of myeloid origin can inhibit tumor growth by suppressing neovasculogenesis.