Strop, Pavel

Abstract Bispecific T cell engagers (TCE) are novel immunotherapies for cancer that redirect a synthetic immune response against tumors. Early clinical trial data show that BCMA-targeting TCEs are highly effective immunotherapies for treating relapsed and refractory multiple myeloma (MM). Thus, it is important to define combinatorial approaches that increase the efficacy and durability of TCE. We recently investigated a murine anti-BCMA TCE in the immunocompetent IMiD-sensitive Vk*MYChCRBN model of MM. Our data show that anti-BCMA TCE was safe and efficacious in most mice, but failed in those with high-tumor burden, consistent with clinical reports of TCE in leukemia. We tested the combination of anti-BCMA TCE and IMiDs to provide an additive anti-tumor effect and stimulatory cytokines, such as IL-2 in the tumor environment. This combination expanded cytolytic T cells and improved activity even in IMiD-resistant high-tumor burden cases. Yet, survival was only marginally extended due to T cell exhaustion. We hypothesized that blockade of the inhibitory immune checkpoint PD-1 would synergize with TCE and IMiDs to potentiate the response directed by the TCE by reinvigorating exhausted T cells. We evaluated the efficacy of the triple therapy combination in Vk*MYChCRBN: the anti-BCMA TCE, pomalidomide, and anti-PD1. Checkpoint blockade alone, or in combination with TCE did not improve tumor control or survival compared to control treatments. The triple combination proved most effective at controlling tumor growth, inducing a complete response after 2 weeks of treatment, and was curative in 40% of the mice. Longitudinal analysis of the immune response showed that the addition of anti-PD1 to TCE-IMiDs rescued T cell expansion at tumor sites and sustained expression of activation markers after repeated doses. Longitudinal analysis of blood and bone marrow (BM) cytokines showed the typical profile associated with cytokine release syndrome (CRS) during T cell immunotherapy. Of note, we observed IL-2, IFN-γ, and GM-CSF production was sustained in the BM tumor environment in the triple combination. We suspect that these cytokines, especially IL-2, are key to sustaining non-exhausted effector T cell proliferation in response to TCE therapy. Expectedly, we observed toxicity with the elevated cytokine release in the triple combination. that we anticipate could be controlled with step-up dosing or CRS mitigation treatment. We conclude that combination of PD1 blockade, IMiDs and TCE provides superior efficacy in the immunocompetent preclinical model of MM, Vk*MYChCRBN, while increasing inflammatory pathways that can lead to CRS. Finally, as we found this combination to be curative in some mice, we will investigate whether this combination immunotherapy also enhanced an endogenous anti-tumor immune response leading to protective immunosurveillance. Citation Format: Erin W. Meermeier, Seth J. Welsh, Meaghen Sharik, Megan T. Du, Chang-Xin Shi, Bryant Chau, Feng Wang, Matthew Wheeler, Natalie Bezman, Pavel Strop, P. Leif Bergsagel, Marta Chesi. PD-1 blockade synergizes with IMiDs to enhance bispecific T cell engager immune responses to Vk*MYChCRBN multiple myeloma by preventing T cell exhaustion [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 5588.

Abstract BCMA/CD3-targeting bispecific antibodies (BsAb) are a recently developed immunotherapy class that shows potent tumor killing activity in multiple myeloma. Here, we investigated a murine BCMA/CD3-targeting BsAb in the immunocompetent Vk*MYC model and its immunomodulatory imide drug (IMiD)–sensitive derivative Vk*MYChCRBN model of multiple myeloma. The BCMA/CD3 BsAb was safe and efficacious in a subset of mice but failed in those with high tumor burden, consistent with clinical reports of BsAb in leukemia. The combination of BCMA/CD3 BsAb with pomalidomide expanded lytic T cells and improved activity even in IMiD-resistant high–tumor burden cases. Yet, survival was only marginally extended due to acute toxicity and T-cell exhaustion, which impaired T-cell persistence. In contrast, the combination with cyclophosphamide was safe and allowed for a tempered proinflammatory response associated with long-lasting complete remission. Concurrent cytotoxic therapy with BsAb actually improved T-cell persistence and function, offering a promising approach to patients with a large tumor burden. Significance: BCMA-targeted therapy induces deep but transient clinical responses. We developed an immunocompetent, IMiD-sensitive genetically engineered mouse model and show that IMiDs potentiate T-cell activation, increasing short-term efficacy of anti-BCMA/CD3 BsAb, but exacerbate T-cell exhaustion. Surprisingly, by reducing tumor burden and depleting regulatory T cells, cyclophosphamide prevents BsAb-induced T-cell exhaustion and promotes long-term multiple myeloma control. See related commentary by Louvet et al., p. 297.