Purpose: Endoglin is a coreceptor for TGFb ligands that is highly expressed on proliferating endothelial cells and other cells in the tumor microenvironment. Clinical studies have noted increased... Show morePurpose: Endoglin is a coreceptor for TGFb ligands that is highly expressed on proliferating endothelial cells and other cells in the tumor microenvironment. Clinical studies have noted increased programmed cell death (PD)-1 expression on cytotoxic T cells in the peripheral blood of patients with cancer treated with TRC105, an endoglin-targeting antibody. In this study, we investigated the combination of endoglin antibodies (TRC105 and M1043) with an anti-PD1 antibody.Experimental Design: The combination anti-endoglin/anti-PD1 antibodies was tested in four preclinical mouse models representing different stages of cancer development. To investigate the underlying mechanism, Fc-receptor-knockout mice were used complemented with depletion of multiple immune subsets in mice. Tumor growth and the composition of immune infiltrate were analyzed by flow cytometry. Finally, human colorectal cancer specimens were analyzed for presence of endoglin-expressing regulatory T cells (Treg).Results: In all models, the combination of endoglin antibody and PD1 inhibition produced durable tumor responses, leading to complete regressions in 30% to 40% of the mice. These effects were dependent on the presence of Fcg receptors, indicating the involvement of antibody-dependent cytotoxic responses and the presence of CD8(+) cytotoxic T cells and CD4(+) Th cells. Interestingly, treatment with the endoglin antibody, TRC105, significantly decreased the number of intratumoral Tregs. Endoglin-expressing Tregs were also detected in human colorectal cancer specimens.Conclusions: Taken together, these data provide a rationale for combining TRC105 and anti-PD1 therapy and provide additional evidence of endoglin's immunomodulatory role. Show less
Endoglin, a transforming growth factor-beta co-receptor, is highly expressed on angiogenic endothelial cells in solid tumors. Therefore, targeting endoglin is currently being explored in clinical... Show moreEndoglin, a transforming growth factor-beta co-receptor, is highly expressed on angiogenic endothelial cells in solid tumors. Therefore, targeting endoglin is currently being explored in clinical trials for anti-angiogenic therapy. In this project, the redundancy between endoglin and vascular endothelial growth factor (VEGF) signaling in angiogenesis and the effects of targeting both pathways on breast cancer metastasis were explored. In patient samples, increased endoglin signaling after VEGF inhibition was observed. In vitro TRC105, an endoglin-neutralizing antibody, increased VEGF signaling in endothelial cells. Moreover, combined targeting of the endoglin and VEGF pathway, with the VEGF receptor kinase inhibitor SU5416, increased antiangiogenic effects in vitro and in a zebrafish angiogenesis model. Next, in a mouse model for invasive lobular breast cancer, the effects of TRC105 and SU5416 on tumor growth and metastasis were explored. Although TRC105 and SU5416 decreased tumor vascular density, tumor volume was unaffected. Strikingly, in mice treated with TRC105, or TRC105 and SU5416 combined, a strong inhibition in the number of metastases was seen. Moreover, upon resection of the primary tumor, strong inhibition of metastatic spread by TRC105 was observed in an adjuvant setting. To confirm these data, we assessed the effects of endoglin-Fc (an endoglin ligand trap) on metastasis formation. Similar to treatment with TRC105 in the resection model, endoglin-Fc-expressing tumors showed strong inhibition of distant metastases. These results show, for the first time, that targeting endoglin, either with neutralizing antibodies or a ligand trap, strongly inhibits metastatic spread of breast cancer in vivo. Show less
