Abstract 4167: Combination of cyclophosphamide with immune checkpoint blockade inhibition elicits potent tumor control in a preclinical melanoma model
Authors: George Mariam M. , Warner Allison Betof , Hamadene Linda , Hirschhorn Daniel , Houghton Alan N. , Wolchok Jedd D. , Merghoub Taha 
Published: June 2022

Abstract
Immune Checkpoint Blockade (ICB) with anti- programmed cell death protein (PD)-1 and anti-cytotoxic T lymphocyte associated protein (CTLA)-4 have had immense success in cancer treatment. However, primary and acquired resistance to these therapies limits their clinical benefit. Accumulating evidence indicates that chemotherapeutic agents such as Cyclophosphamide (CTX) that can induce anticancer immunity stand out as particularly promising partners for use in combination with ICBs. A combination approach to reorchestrate the anti-tumor immune response could magnify the clinical benefit of ICBs. CTX is an alkylating chemotherapeutic which is directly tumoricidal that can also modulate immune cells. CTX is known to preferentially deplete certain T cell subsets (such as T regulatory cells, T regs) and lead to homeostatic proliferation of antigen-specific T cells in addition to the modulation of myeloid cells. Here, we hypothesized that the addition of ICBs will augment the immunomodulatory changes induced by CTX including but not limited to the homeostatic proliferation of T cells which will reset the T cell receptor (TCR) repertoire to favor tumor antigen-specific T cells. The murine melanoma tumor model, B16-F10, is known to be refractory to treatment with ICBs, particularly anti-PD-1, as these only modestly slow the growth of small tumors. Here, we show that a single dose of CTX one day prior to starting an ICB (anti-PD-1 and anti-CTLA-4) regimen slows tumor progression of B16-F10 compared to CTX or ICB alone. Similarly, the triple combination (CTX+ anti- PD-1 + anti-CTLA-4) also significantly prolongs the survival of tumor bearing mice. Further, the combination of CTX with anti- PD-1 and anti-CTLA-4 increased the number of activated and proliferating CD8+ tumor infiltrating lymphocytes (TILs). This increase in cytotoxic T cells was accompanied by a decrease in Tregs, which further augments the tumor control. Additionally, we also observed a significant increase in a highly cytolytic population of CD4+ CD8+ double positive TILs. The depletion of CD8+ cells but not CD4+ cells abrogates the therapeutic effect of the triple combination suggesting that the anti-tumor effect is CD8+ T cell dependent. Overall, our results suggest that the combination of CTX with the ICBs, anti- PD-1 and anti-CTLA-4, is a potent combinatorial approach that can prime an anti-tumor response and promote robust control of tumor growth. Thus, these findings form the basis for further investigations in understanding the mechanisms of combinatorial cancer therapies in tumor models that are refractory to ICB therapies and can inform the design of future therapeutic interventions that combine ICB with chemotherapy.
Note: M.M.G, A.B.W, and L.H contributed equally to this work
Citation Format: Mariam M. George, Allison Betof Warner, Linda Hamadene, Daniel Hirschhorn, Alan N. Houghton, Jedd D. Wolchok, Taha Merghoub. Combination of cyclophosphamide with immune checkpoint blockade inhibition elicits potent tumor control in a preclinical melanoma model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4167.