Abstract 2882: Selective activation of CD8+ T cells by a CD8-targeted IL-21 results in enhanced anti-tumor efficacy and safety

Abstract Interleukin-21 (IL-21) is a pleiotropic cytokine that activates multiple lymphoid and myeloid immune cell subsets. IL-21 can have beneficial effects on anti-tumor immune responses due to its ability to activate STAT3 in CD8+ T cells and promote cytotoxicity, memory cell differentiation and survival. However, IL-21 can also induce immunosuppressive effects, such as suppression of antigen presentation, which may directly oppose its anti-tumor effects on CD8+ T cells. Although recombinant IL-21 did show monotherapy activity in the clinic, overall response rates were modest. In addition to the pharmacological sink created by the broad expression of IL-21R and potential immunosuppressive effects, clinical utility of IL-21 was further limited by its low bioavailability and dose-limiting toxicities. We hypothesized that maximizing IL-21’s activity on CD8+ T cells while avoiding non-CD8 cells would enhance efficacy and improve tolerability. We developed cis-targeted IL-21 (CD8-IL21) fusion proteins that exhibit improved bioavailability and selectively activate CD8+ T cells while exhibiting minimal activity on non-CD8 cells. In vitro phospho-STAT3 assays with CD8-IL21 in mouse splenocytes and human peripheral blood cells showed >1000-fold selective activation of CD8+ T cells over non-CD8 cells. A mouse CD8-IL21 surrogate demonstrated profound single agent efficacy in multiple syngeneic tumor models at considerably lower doses than an untargeted IL-21 control, despite similar in vitro potency. CD8-IL21 mediated anti-tumor efficacy without inducing body weight loss. Whereas prior data showed that repeated high doses of tumor-targeted IL-21 fusion protein (>3 mg/kg) had single agent efficacy in the MC38 tumor model overexpressing a tumor antigen [1], here we show that a single dose of CD8-targeted IL-21 as low as 0.1 mg/kg induced curative responses in mice bearing established MC38 tumors. We demonstrated using the lymphocyte migration inhibitor FTY720 that this enhanced potency of low dose CD8-IL21 is predominately driven by the activation of peripheral CD8 T cells. Furthermore, curative effects in the MC38 model were elicited in the absence of substantial CD8+ T cell expansion (<1.5-fold) suggesting that CD8-IL21 mediated anti-tumor efficacy through modulation of T cell phenotype. A CD8-IL21 selective for human and cynomolgus CD8+ T cells with improved bioavailability also exhibited improved tolerability over untargeted IL-21 in cynomolgus monkeys, showing that there is a tolerability benefit to CD8-selective activation. In summary, CD8-IL21 showed improved efficacy and safety over untargeted IL-21, demonstrating clear promise as a novel therapeutic agent which by activating STAT3 signaling in CD8+ T cells may offer complementary benefits to activation of STAT5 signaling by cytokines such as IL-2 and IL-15.Ref: 1. Deng et al., 2020 Citation Format: Renee Greer, David Liu, Henry Nguyen, Ruth Lan, Wei Wei Prior, Meghana Sukthankar, Paul Mesko, irene Ni, Mike Chin, Kelly Moynihan, Ton Schumacher, Ivana Djuretic, Andy Yeung. Selective activation of CD8+ T cells by a CD8-targeted IL-21 results in enhanced anti-tumor efficacy and safety [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 2882.