Abstract PR01: Mechanistic rationale to combine GITR agonism with PD-1 blockade in cancer patients

Immune checkpoint blockade has evidenced the therapeutic activity of modulating T-cell co-inhibition/co-stimulation processes. However, many patients are refractory to these therapies, highlighting the need for developing additional forms of immunotherapy targeting alternative immune pathways. In this regard, the T-cell co-stimulatory receptor glucocorticoid-induced TNFR-related protein (GITR, TNFRSF18) is an attractive target for agonist antibodies (Abs). By promoting effector T-cell (Teff) function and hampering regulatory T-cell (Treg) suppression, GITR engagement may exert a dual positive effect on anti-tumor immune responses. We and others have reported potent antitumor effects of anti-GITR Abs in preclinical mouse models. Based on this rationale, we initiated the first-in-human phase-I trial of GITR stimulation with the GITR agonist monoclonal Ab (mAb) TRX518 (NCT01239134). TRX518 is a humanized aglycosylated IgG1κ mAb that binds and stimulates human GITR without engaging Fc effector functions. Here, we report the immune effects of a single ascending dose of TRX518 monotherapy in advanced cancer patients and provide mechanistic preclinical evidence to rationally combine GITR agonism with checkpoint blockade in future clinical trials. Analysis of peripheral blood mononuclear cells (PBMCs) from 37 advanced refractory solid cancer patients treated with >/= 0.005 mg/kg TRX518 (cohorts 3-9) revealed frequent reductions in circulating Tregs after treatment, with GITR+ Tregs and activated CD45RA-Foxp3hi effector Tregs (eTregs) being preferentially affected. In 8 patients for whom pre- and post-treatment PBMC samples and tumor biopsies were available, reductions in intratumor and circulating Tregs after TRX518 were positively correlated. However, coincident down-regulation of circulating and intratumor Tregs upon TRX518 was not sufficient to achieve a clinical benefit. To clarify the mechanisms underlying this outcome, we modeled tumor sensitivity and refractoriness to anti-GITR therapy by treating B16F10-melanoma-bearing mice with the mAb DTA-1 on day 4 (curative regimen, early tumors) or day 7 (refractory regimen, advanced/established tumors) after tumor implantation respectively. Time course analysis of T-cell infiltrates revealed that intratumor Tregs were significantly reduced and Teff:Treg ratios increased in both responding and refractory tumors. However, in responding tumors, Tregs completely failed to accumulate, suggesting that the presence of Tregs during tumor formation and progression could affect T-cell functionality. Gene expression analysis of intratumor CD8+ T-cells showed up-regulation of activation/memory T-cell markers and down-regulation of exhaustion markers in responding but not in refractory tumors. To overcome resistance to anti-GITR, we thus combined the anti-GITR refractory regimen (day 7 treatment) with PD-1 blockade starting on day 7 after tumor implantation. This combination controlled tumor growth similar to the curative anti-GITR monotherapy (day 4 treatment) and achieved 50% long-lasting complete response. This was associated with more activated and less exhausted profiles of intratumor CD8+ T-cells, which showed enhanced anti-tumor cytotoxicity compared to CD8+ T-cells from nonresponding tumors treated with each agent alone. These results indicate for the first time that Treg reduction may be a pharmacodynamic biomarker of anti-GITR therapy in patients. However, Treg elimination from advanced tumors is not sufficient to activate cytotoxic CD8+ T-cell responses unless the T-cell exhaustion process is concurrently blocked. This underscores the need to combine Treg-inhibiting/depleting immunotherapies with strategies able to counteract exhaustion, such as PD-1 blockade, to regress advanced tumors. Based on these observations, we have initiated a clinical trial exploring TRX518 in combination with PD-1 pathway blockade in patients with advanced solid tumor malignancies (NCT02628574). Citation Format: Roberta Zappasodi, Cynthia Sirard, Yanyun Li, Sadna Budhu, Moshen Abu-Akeel, Cailian Liu, Xia Yang, Hong Zhong, Walter Newman, Jinjin Qi, Phillip Wong, David Schaer, Henry Koon, Vamsidhar Velcheti, Michael Postow, Margaret K Callahan, Jedd D. Wolchok, Taha D. Merghoub. Mechanistic rationale to combine GITR agonism with PD-1 blockade in cancer patients [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr PR01.