Abstract 291: Synergistic inhibition of cancer invasion and metastasis by combined anti-PD1-TRC105-mediated Endoglin targeting on cancer-associated fibroblasts and endothelial cells
Authors: Schoonderwoerd Mark JA , Paauwe Madelon , Angela Ricardo , Koops Maaike , Harryvan Tom , Sier Cornelis , Fransen Marieke , Theuer Charles , Hawinkels Lukas JAC 
Published: August 2019

Abstract
Cancer-associated fibroblasts (CAFs) are the major component of the tumor microenvironment in colorectal cancer (CRC) and play an important role in tumor progression and metastasis, partly through the transforming growth factor-β (TGF-β) signaling pathway. Recently, we showed that a subset of CAFs characterized by the expression of the TGF-β family co-receptor endoglin, selectively localized at the invasive borders of CRC tissues. Moreover, CAF-specific endoglin expression increased with higher tumor stage and predicted metastasis-free survival of stage-II CRC patients (n= 140, p&lt;0.05). Targeting endoglin on CAFs using TRC105 (carotuximab), an endoglin neutralizing antibody currently in clinical development, inhibits endoglin dependent signaling and metastatic colonization when CRC cells and CAFs are co-injected into mice. We further investigated these anti-tumor effects in four different models of CRC (chemically-induced model of early CRC, subcutaneous MC38 and CT26 syngeneic mouse CRC models, and orthotopic MC38 model). TRC105 treatment inhibited tumor initiation (early stage CRC), growth, and progression. Therapeutic effects of TRC105 therapy were dependent on antibody dependent cytotoxicity (ADCC) and CD8+ T-cells, as no therapeutic benefit from TRC105 was observed in Fc-y I-IV receptor knockout mice or CD8+ cell depleted mice. Since therapeutic TRC105 activity was immune cell dependent, we combined TRC105 with anti-programmed cell death (PD-1 immunotherapy). We observed synergistic inhibition of tumor initiation and growth in all four tumor models, leading to delayed tumor outgrowth, increased survival and cure rates up to 60% in the combination treated mice bearing palpable tumors. Combined treatment also induced memory responses, as shown by resistance to repeated tumor challenge. Anti-tumor responses were accompanied by enhanced immune cell activation including an increased number of tumor-specific T-cells, NK cells, decreased number of tumor-infiltrating neutrophils and increased granzyme B production. CD8+ cytotoxic T-cells were instrumental in these synergistic therapeutic responses, as their depletion abolished the effects. Taken together, our data indicate significant therapeutic effects of targeting endoglin by affecting signaling and multiple immune related components in the tumor microenvironment. These results are currently being translated in a clinical trial combining TRC105 with nivolumab in patients with non-small cell lung cancer (NCT03181308).
Citation Format: Mark JA Schoonderwoerd, Madelon Paauwe, Ricardo Angela, Maaike Koops, Tom Harryvan, Cornelis Sier, Marieke Fransen, Charles Theuer, Lukas JAC Hawinkels. Synergistic inhibition of cancer invasion and metastasis by combined anti-PD1-TRC105-mediated Endoglin targeting on cancer-associated fibroblasts and endothelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 291.