Intratumoral temozolomide synergizes with immunotherapy in a T cell-dependent fashion

Despite temozolomide (TMZ) treatment, the prognosis for patients with glioblastoma multiforme is still dismal. As dose escalation of TMZ is limited by systemic toxicity, intratumoral delivery emerges as an attractive treatment modality, which may sustain cytotoxic drug concentrations intratumorally and induce immunogenic cell death. Both clinical and experimental gliomas have responded to immunotherapy, but the benefit of simultaneous chemo- and immunotherapy is inadequately studied. Here, we monitored survival of GL261-bearing C57BL/6 mice following a 3-day treatment with either intratumoral TMZ (micro-osmotic pump, 4.2 mg/kg/day) or systemic TMZ (i.p. injections, 50 mg/kg/day) alone, or combined with immunization using GM-CSF secreting GL261 cells. Peripheral and intratumoral leukocytes were analyzed by flow cytometry and immunohistochemistry. Intratumoral TMZ induced higher survival rate than systemic TMZ (45 vs. 8 %). When T cells were depleted following intratumoral TMZ, the therapeutic effect was completely abrogated (0 % survival). Intratumoral TMZ synergistically increased survival rate of immunized mice (from 25 to 83 %), while systemic TMZ failed (0 %). While systemic TMZ induced a transient leukopenia, intratumoral TMZ and immunotherapy sustained the proliferation of CD8⁺ T cells and decreased the number of intratumoral immunosuppressive cells. In conclusion, intratumoral TMZ alone or in combination with immunotherapy could cure glioma-bearing mice, due to attenuation of local immunosuppression and increase in potential effector immune cells.