Metronomic cyclophosphamide given on an intermittent, 6-day repeating schedule, but not on an exposure dose-equivalent daily schedule, activates an anti-tumor innate immune response that leads to major regression of large implanted gliomas, without anti-angiogenesis.

Methods and Approach: Mice bearing implanted 9L gliomas were used to investigate the effects of this 6-day repeating, immunogenic cyclophosphamide schedule on myeloid-derived suppressor cells, which are pro-angiogenic and can inhibit anti-tumor immunity, and to elucidate the mechanism whereby the innate immune cell-dependent tumor regression response to metronomic cyclophosphamide treatment is blocked by several anti-angiogenic receptor tyrosine kinase inhibitors.

Intermittent metronomic cyclophosphamide scheduling strongly increased glioma-associated CD11b+ immune cells but not CD11b+Gr1+ myeloid-derived suppressor cells, while bone marrow and spleen reservoirs of the suppressor cells were decreased. The inhibition of immune cell recruitment and tumor regression by anti-angiogenic receptor tyrosine kinase inhibitors, previously observed in several brain tumor models, was recapitulated in the 9L tumor model with the VEGFR2-specific inhibitory monoclonal antibody DC101 (p < 0.01), implicating VEGFR2 signaling as an essential step in metronomic cyclophosphamide-stimulated immune cell recruitment. In contrast, sorafenib, a multi-receptor tyrosine kinase inhibitor with comparatively weak VEGF receptor phosphorylation inhibitory activity, was strongly anti-angiogenic but did not block metronomic cyclophosphamide-induced innate immunity or tumor regression (p > 0.05).

The interference by receptor tyrosine kinase inhibitors in the immunogenic actions of intermittent metronomic chemotherapy is not a consequence of anti-angiogenesis per se, as demonstrated in an implanted 9L tumor model. Furthermore, this undesirable interaction with tyrosine kinase inhibitors can be avoided by using anti-angiogenic drugs that spare the VEGFR2 pathway.