Abstract B21: Synergistic antitumor immunity observed with combination FRα-targeting antibody-drug conjugate plus anti-PD-1 therapy is CD8+ cell dependent
Authors: Gavrilescu L. Cristina , Setiady Yulius , Dong Ling , Chicklas Sharon , Harris Luke , Pinkas Jan , Gregory Richard , Ponte Joe 
Published: June 2021

Abstract
Background: Immune checkpoint blockade, specifically targeting PD-1/PD-L1, is a promising approach for cancer therapy. However, for some common cancers, such as epithelial ovarian cancer (EOC), PD1/PDL1 blockade as monotherapy has a low response rate. In these less immunogenic cancer types, focus has turned to combining checkpoint inhibitors with cancer therapies that elicit immunologic cell death (ICD). Antibody-drug conjugates (ADCs), comprising a tumor-targeting antibody armed with highly potent small-molecule payloads, are a clinically validated anticancer therapy. Mirvetuximab soravtansine is an ADC comprised of a folate receptor alpha (FRα)-binding antibody linked to the potent tubulin-disrupting maytansinoid DM4 via a cleavable disulfide sulfo-SPDB linker and is currently being evaluated in combination with pembrolizumab (anti-PD1 antibody) in a phase 1b study (FORWARDII, NCT02606305). Here we report preclinical studies that provide a mechanistic understanding for the antitumor activity of combination therapy consisting of PD-1/PD-L1 blockade and an FRα targeting disulfide linked DM4 ADC.
Methods: A chimeric anti-mouse FRα antibody (rmFR1-12) was generated and its specificity for murine FRα was assessed by ELISA and FACS. rmFR1-12 was conjugated to DM4 via a s-SPDB linker to generate rmFR-1-12-s-SPDB-DM4 ADC, which was quality checked using standard analytical methods. In vitro assays were used to assess cytotoxicity against FRα-positive cell lines, as well as the ability of rmFR1-12-s-SPDB-DM4 to induce ICD and engage cells of the immune system. In vivo studies performed in albino C57/B6 mice evaluated the antitumor activity of rmFR1-12-s-SPDB-DM4, anti-PD-1 or the combination therapy against ID8 tumors, a translationally relevant murine model of EOC.
Results: rmFR1-12 binding was specific for FRα. The rmFR1-12-s-SPDB-DM4 ADC was cytotoxic against FRα-positive cell lines and induced ICD as measured by upregulation of ICD markers. In vivo studies using syngeneic ID8 tumors demonstrated that rmFR1-12-s-SPDB-DM4 or anti-PD-1 monotherapy treatment resulted in modest antitumor activity. In contrast, combination treatment demonstrated synergistic antitumor activity, inducing complete tumor regressions and increased survival over control or monotherapy regimens. Depletion of CD8+ cells abrogated the antitumor activity of the combination, whereas depletion of CD4+ cells had no impact. Finally, combination-treated mice generated a robust memory response when rechallenged with ID8 cells.
Conclusion: Anti-FRα-s-SPDB-DM4 induces ICD in FRα-positive cell lines and demonstrates synergistic antitumor activity when combined with anti-PD1 in a murine EOC model. The antitumor immunity is dependent upon CD8+ cells. These findings support the clinical assessment of mirvetuximab soravtansine and pembrolizumab.
Citation Format: L. Cristina Gavrilescu, Yulius Setiady, Ling Dong, Sharon Chicklas, Luke Harris, Jan Pinkas, Richard Gregory, Joe Ponte. Synergistic antitumor immunity observed with combination FRα-targeting antibody-drug conjugate plus anti-PD-1 therapy is CD8+ cell dependent [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B21.