Abstract P6-14-01: Enhanced efficacy of redirected T-cell therapy of TNBC with a Trop-2/CD3 bispecific antibody in combination with a checkpoint inhibitor
Authors: Rossi EA , Chang K , Cardillo TM , Rossi DL , Li R , Mostafa A , Sharkey RM , Goldenberg DM 
Published: March 2017

Abstract
Bispecific antibodies (bsAbs) for redirecting T cells to cancers have shown promise in both pre-clinical and clinical studies. However, clinical success has been minimal for solid cancers to-date. Previously, we reported highly effective T-cell redirected therapy of pancreatic and gastric tumors in xenograft models using a trivalent bsAb, designated (E1)-3s, which comprises an anti-CD3 scFv covalently conjugated to a stabilized dimer of a Trop-2-targeting humanized Fab. Trop-2 is highly expressed in diverse epithelial cancers, including breast, lung, gastric, colorectal, pancreatic, bladder, ovarian, uterine, and prostate carcinomas, with limited presence on normal human tissues. Compared to first generation bsAbs (e.g., BiTE), which induce high-level cytokine production that can lead to serious side effects, efficient T-cell killing is mediated by (E1)-3s with minimal cytokine release. Herein, we report the potential utility of (E1)-3s for therapy of breast cancers, including TNBC. Additionally, we show that addition of a checkpoint inhibitor can enhance bsAb-redirected T-cell therapy. IMMU-cPD1 is a chimeric mAb that binds with high affinity to human PD-1 and efficiently blocks binding to its ligand, PD-L1. The MDA-MB-231 human TNBC cell line has relatively low levels of surface Trop-2 (36,000/cell) and expresses PD-L1 constitutively. In ex-vivo assays, where MDA-MB-231 cells were mixed with purified human T cells, (E1)-3s mediated potent T-cell redirected killing (IC50&lt; 10 pM). Addition of IMMU-cPD1 enhanced (E1)-3s-mediated T-cell killing. The advantage of combining the checkpoint inhibitor with redirected T-cell therapy was supported with in-vivo xenograft studies. NOD-SCID mice were co-injected with purified human T cells (2.5 x 106) and MDA-MB-231 (5 x 106). Groups were administered: 1) five daily injections of (E1)-3s; 2) cPD-1 twice weekly for 4 weeks; or 3) a combination of (E1)-3s and cPD1 treatments. An untreated control group comprising T cells and tumor cells reached the endpoint (tumors &gt;1 cm3) with a median survival time (MST) of 35 days. The group treated with (E1)-3s had significantly smaller tumors (P=0.0023, AUC) and improved MST (42 days, P=0.0019). The group treated with the combination of (E1)-3s and IMMU-cPD1 had significantly smaller tumors (P=0.0121, AUC) and longer MST (49 days, P=0.008), compared to those treated with (E1)-3s alone. Treatment with IMMU-cPD1 alone did not retard tumor growth or improve MST, compared to the untreated control group. In conclusion, (E1)-3s is an attractive candidate for T-cell redirected therapy of breast cancer due to its potent activity with potentially reduced side effects and the prevalence of Trop-2 expression associated with this disease. Tumor microarrays representing 117 breast cancer patients showed &gt;85% positivity for Trop-2. Our immunohistochemical analysis of more than 50 individual TNBC patient specimens demonstrated 92% positivity, with 80% having moderate to strong Trop-2-staining. Combining checkpoint inhibitors with redirected T-cell therapy may represent a new paradigm for the management of solid cancers, including breast, and is worthy of further investigation.
Citation Format: Rossi EA, Chang K, Cardillo TM, Rossi DL, Li R, Mostafa A, Sharkey RM, Goldenberg DM. Enhanced efficacy of redirected T-cell therapy of TNBC with a Trop-2/CD3 bispecific antibody in combination with a checkpoint inhibitor [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-14-01.