Clinical and Immunomodulatory Effects of Toceranib Combined with Low-Dose Cyclophosphamide in Dogs with Cancer

Background: Tyrosine kinase inhibitors (TKIs) and metronomic dosing of cyclophosphamide (CYC) can improve tumor control by suppression of regulatory T cells (Treg) and restoration of T cell-mediated immune responses in mice and humans. The immunomodulatory effects of the TKI toceranib, as a single agent or in combination with metronomic CYC, have not been previously investigated in dogs. Hypothesis: The primary objectives of this study were to determine the effects of toceranib and metronomic CYC treatment on lymphocyte subsets including Treg and on interferon-gamma (IFN-gamma) secretion in dogs with cancer. We hypothesized that toceranib would selectively decrease Treg numbers and increase IFN-gamma production and that addition of CYC would further enhance these effects. Animals: Fifteen client-owned dogs with advanced tumors were entered into a prospective clinical trial. Methods: Dogs received toceranib at 2.75 mg/kg once every other day. After 2 weeks, oral CYC was added at 15 mg/m(2) daily. Numbers of Treg and lymphocyte subsets were measured in blood by flow cytometry during the 8-week study period. Serum concentrations of IFN-gamma were measured by ELISA. Results: Administration of toceranib significantly decreased the number and percentage of Treg in the peripheral blood of dogs with cancer. Dogs receiving toceranib and CYC demonstrated a significant increase in serum concentrations of IFN-gamma, which was inversely correlated with Treg numbers after 6 weeks of combination treatment. Conclusions: In addition to antitumor effects, these data support further investigations into the immunomodulatory effects of toceranib, administered alone or in combination with CYC in dogs with cancer.