LAUSR

3753. P143 Rational combination of GITR agonism with PD-1 blockade in cancer patients Roberta Zappasodi, PhD, Cynthia Sirard, MD, Yanyun Li, PhD MD, Sadna Budhu, PhD, Moshen Abu-Akeel, Cailian Liu, MD, Xia Yang, Hong Zhong, BS, Walter Newman, PhD, Jingjing Qi, Phillip Wong, PhD, David Schaer, Henry Koon, MD, Vamsidhar Velcheti, MD FACP, Michael Postow, MD, Margaret K. Callahan, MD, PhD, Jedd Wolchok, MD, PhD, Taha Merghoub, PhD Memorial Sloan Kettering Institute, New York, NY, USA; MSKCC, New York, NY, USA; Leap Therapeutics, Walpole, MA, USA; Case Western Reserve University, Cleveland, OH, USA; Cleveland Clinic, Pepper Pike, OH, USA Correspondence: Taha Merghoub ([email protected]) Journal for ImmunoTherapy of Cancer 2018, 6(Suppl 1):P143 Background Despite the clinical successes of checkpoint blockade, many patients are refractory to these therapies, highlighting the need of more effective combination programs targeting alternative immune pathways. Engaging the T-cell costimulatory receptor glucocorticoidinduced TNFR-related protein (GITR) with agonist antibodies has shown promising activity in preclinical mouse models. Based on this rationale, we initiated the first in-human phase-I trial of GITR stimulation with the agonist antibody TRX518 in advanced solid cancer patients (NCT01239134). Treated patients showed frequent reductions in circulating regulatory T cells (Tregs) that correlated with intratumor Treg reductions. However, this was not sufficient to achieve a clinical benefit. Here, we investigate the mechanisms underlying resistance of advanced tumors to GITR agonism and provide the rationale to combine anti-GITR with checkpoint blockade in clinical trials. Methods Mice were implanted with B16F10 murine melanoma and treated with the anti-GITR DTA-1 alone or in combination with the anti-PD-1 RMP1-14. Composition, phenotype and function of tumor-infiltrating T cells were analyzed by flow cytometry, in vitro killing assays and molecular profiling. Results We modeled tumor sensitivity and refractoriness to anti-GITR therapy by treating B16F10-bearing mice on day 4 (responsive tumors) or day 7 (refractory tumors) after tumor implantation respectively. We found that intra-tumor Tregs were significantly reduced and effectorT-cell:Treg ratios increased in both responding and refractory tumors. However, time course analyses revealed complete lack of Treg accumulation in day-4-treated responding tumors, suggesting that Tregs may limit T-cell functionality during tumor development. Accordingly, in refractory compared to responding tumors, CD8+ T cells downregulated activation/memory T-cell markers and up-regulated exhaustion markers. To overcome resistance to anti-GITR, we coadministered anti-PD-1 with the day-7 anti-GITR suboptimal treatment. This combination controlled tumor growth similar to the day-4 curative anti-GITR monotherapy and achieved 50% long-lasting complete responses. These effects were associated with intra-tumor infiltration of highly activated and cytolytic CD8+ T cells. Based on these results and considering the biologic activity and safety profile of TRX518, we have initiated the clinical investigation of TRX518 in combination with anti-PD-1 in patients with advanced solid tumors (NCT02628574). Noteworthy, 3 of the first patients enrolled in this study have manifested clinical responses. Conclusions These findings indicate that Treg elimination from advanced tumors is not sufficient to activate cytotoxic CD8+ T-cell responses unless the T-cell exhaustion process is concurrently blocked, underscoring the need to combine Treginhibiting/depleting immunotherapies with strategies that counteract exhaustion to regress advanced tumors. This study highlights the importance of developing rational, evidence-based combination immunotherapies. Ethics Approval All mouse procedures were performed in accordance with institutional protocol guidelines at MSKCC. All procedures involving human subjects, human material, or human data, or involving animals were in compliance with the ethical regulations. P144 A fully-automated multiplex fluorescence IHC assay with whole slide multispectral imaging on mouse tissue: phenopticsTM quantitative pathology solutions translational workflow Yi Zheng, PhD, Carla Coltharp, PhD, Ryan Dilworth, PhD, Rachel Schaefer, Linying Liu, Victoria Duckworth, William Kennedy, Darryn Unfricht, PhD, Peter Miller, MS, Milind Rajopadhye, PhD PerkinElmer Inc., Hopkinton, MA, USA Correspondence: Milind Rajopadhye ([email protected]) Journal for ImmunoTherapy of Cancer 2018, 6(Suppl 1):P144