Abstract CT010: Pharmacodynamic (PD) changes in tumor RNA expression and the peripheral blood T cell receptor (TCR) repertoire in a phase I study of OX40 agonistic monoclonal antibody (mAb) PF-04518600 (PF-8600) (2018)

Abstract Introduction: PF-8600 is a fully human IgG2 agonistic mAb against the tumor necrosis factor superfamily receptor OX40. OX40 improves T cell survival, proliferation, and activation and may enhance anti-tumor immunity. In an ongoing phase I study (NCT02315066) of 52 patients (pts) with melanoma, hepatocellular carcinoma, head and neck carcinoma, or renal cell carcinoma treated with PF-8600 monotherapy in dose escalation, 2 pts had best overall response of partial response (PR) and 27 of stable disease. PF-8600 was well-tolerated at all doses. Peripheral blood flow cytometry had previously shown increased proliferation and activation of CD4 central memory T cells at certain dose levels, suggesting a PD effect. RNAseq analysis of tumor biopsy samples and TCR sequencing of peripheral blood were used to further support proof of mechanism. Methods: Biopsy samples at baseline and wk 6 were collected from 5 dose cohorts [0.1 (4), 0.3 (3), 1.5 (4), 3.0 (3) and 10.0 (2), dose in mg/kg (n)]. Biopsy tissue was analyzed by RNAseq and gene ranking-based gene set enrichment analysis (FGSEA) to identify immune pathways potentially up-regulated by OX40. CD4/CD8 cells were isolated from peripheral blood samples from 4 dose cohorts (0.1, 0.3, 1.5, and 3.0 mg/kg). DNA was extracted for sequencing of the TCR-β chain complementarity-determining region 3 (CDR3). Results: In a combined analysis of samples from pts dosed with ≥1.5 mg/kg, the top 3 gene sets showing enrichment at wk 6 of therapy were associated with inflammatory response, interferon-γ response and allograft rejection. These gene sets were identical to the top 3 most enriched in tumor from a syngeneic mouse tumor model exposed to a murine OX40 agonist. This pattern of enrichment was not observed if samples from lower doses were included in the analysis. TCR sequencing revealed clonal expansion of CD4/CD8 T cells at all dose levels at wk 6 [CD4: mean = 8 expanded clones per 100,000 clones (range = 1 - 80), CD8: mean = 56 (range = 1 - 500)]. The 2 patients with PR had among the lowest numbers of expanded CD4 and CD8 clones (CD4: 4 and 2; CD8: 7 and 4). Conclusion: Enrichment of gene sets associated with immune activation in tumor tissue from patients dosed with PF-8600 provides evidence supporting an active, immunomodulatory mechanism. Peripheral CD4/CD8 T cell populations exhibited clonal expansion in response to dosing with PF-8600 at all dose levels further suggesting a PD effect. However, clinical response did not necessarily correlate with a high number of expanded T cell clones, suggesting that clinical response to OX-40 agonism may be driven by the expansion of select anti-tumor T cell clones rather than a broad expansion of T cell clones in the peripheral blood. The phase I study will continue to evaluate PD changes in the tumor and peripheral blood in dose-expansion cohorts of PF-8600 ± utomilumab. Citation Format: Adi Diab, Omid Hamid, John A. Thompson, Willeke Ros, Fredericus A. L. M. Eskens, Toshihiko Doi, Siwen Hu-Lieskovan, Hua Long, Tenshang Joh, Shoba Potluri, Xiao Wang, Catherine Fleener, Carrie Turich Taylor, Bishu J. Ganguli, Jeffrey Chou, Anthony B. El-Khoueiry. Pharmacodynamic (PD) changes in tumor RNA expression and the peripheral blood T cell receptor (TCR) repertoire in a phase I study of OX40 agonistic monoclonal antibody (mAb) PF-04518600 (PF-8600) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT010.