IMMU-45. DOSE MODULATION OF TEMOZOLOMIDE HAVE DISTINCT EFFECTS ON HOST RESPONSE TO PD-1 BLOCKADE

Temozolomide is known to affect efficacy of immunotherapy due to effects on the tumor microenvironment and host immune cell function. The effects of temozolomide dosing on efficacy of immune checkpoint inhibition is unknown. HYPOTHESIS: Dose modification of temozolomide modulates host immunity to increase efficacy of immune checkpoint blockade in a murine syngeneic glioblastoma model. Experiments were performed utilizing GL261 tumor bearing mice treated with standard dose (SD) temozolomide (50mg/kg x 5 days), metronomic dose (MD) temozolomide (25mg/kg x 10 days) and/or anti-PD1 antibody. SD temozolomide treatment resulted in greater lymphopenia and a more immunosuppressive profile compared to MD temozolomide in GL261 tumor bearing mice. SD temozolomide caused an upregulation of Tim-3, Lag-3 and PD-1 on peripheral and splenic CD4 and CD8 T cells. MD temozolomide increased PD-1 expression without concomitant Tim-3 or Lag-3 expression on CD4 and CD8 T cells. SD temozolomide also resulted in an increase in myeloid derived suppressor cells which was not observed with MD temozolomide. Moreover, antigen specific CD8 T cells were less functional as measured by IFN-gamma secretion when treated with SD temozolomide as compared with MD temozolomide. Analysis of tumor infiltrating lymphocytes also demonstrated increased exhaustion when treated with SD temozolomide compared to MD temozolomide. Combination treatment with PD-1 blockade and either MD or SD temozolomide demonstrated higher expression of checkpoints and immune exhaustion profiles in the SD temozolomide group measured by RNA sequencing. Survival analysis revealed that PD-1 blockade resulted in survival benefit in tumor bearing animals. However, SD temozolomide abrogated this survival benefit when combined with PD-1 blockade. MD temozolomide preserved the survival advantage of PD-1 blockade. Dose modification of temozolomide impacts efficacy of immune checkpoint blockade by modulating immune effector cells. Strategies to reverse T cell exhaustion induced by SD temozolomide are underway.