Abstract 2604: Evaluating the potency of antibody-drug conjugate (ADC) free-drugs on T cells

Abstract Antibody-drug conjugates (ADCs) combine the targeting specificity of antibodies with powerful cytotoxic payloads to direct antigen-specific tumor cell killing. Multiple ADCs have shown single-agent clinical activity against solid and hematological cancers and are currently being evaluated in combination with immune checkpoint inhibitors (CPIs). Therapeutic benefit from CPIs relies on the presence and quality of responding antitumor T cells, both in the tumor and from new T cell priming and expansion outside the tumor. Select cytotoxic ADC payloads, like monomethyl auristatin E (MMAE), have the potential to amplify T cell responses through immunogenic cell death (ICD) and, as such, may be optimal combination partners with CPIs. However, ADC payloads released in the tumor (bystander drug) or released systemically also have the potential to negatively impact T cell viability, proliferation, and priming. The compatibility of different ADC payload chemotypes (Auristatins, camptothecins (CPTs), Pyrrolobenzodiazepine (PBDs), Tubulysins) with CPIs may depend on both the direct and indirect effects of payloads and their associated free-drugs on immune cells. Here, we determined the relative potencies of ADC payload free-drugs MMAE and DXd on activated human and murine T cells in vitro and in a model of xeno-graft versus host disease (xeno-GVHD) to evaluate the impact of ADC bystander effects on human immune cell expansion and function in vivo. In vitro, ADC payloads showed a range of potencies on activated T cells, with IC50 values skewing lower for CD4 compared to CD8 T cells. For each payload, murine T cells were significantly less sensitive than human T cells, which may have implications for modeling combinations of ADCs and CPIs. Mouse CD8 T cells were ~10-fold less sensitive to MMAE and 20-25-fold less sensitive to clinical CPT payloads compared to human T cells. In an in vivo model of human immune cell activation, xeno-GVHD, bystander effects from a non-targeted MMAE-ADC did not impair the expansion of human immune cells, however, a non-targeted DXd-ADC reduced CD4 T cells and robustly reduced populations of non-T cells in spleens. Consistent with reduced immune cell expansion, the DXd-ADC significantly slowed the xeno-GVHD disease course compared to the MMAE-ADC treated and control groups. These data suggest that the most commonly used ADC payload classes, MMAE and DXd, may have different direct effects on proliferating human immune cells as bystander free-drugs, which may have clinical relevance for combinations of ADCs with CPIs. Continued research exploring the balance between ADC potency on tumor cells, direct and indirect effects of payloads on immune cells, and dose timing and sequencing could help maximize the potential of ADCs in combination with immunotherapies. Citation Format: Reice D. James, Michelle L. Ulrich, J Hartsuyker, Alyson J. Smith, Ryan A. Heiser. Evaluating the potency of antibody-drug conjugate (ADC) free-drugs on T cells abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 2604.