Abstract 3278: Novel knock-in mice expressing human/mouse chimeric endoglin (ENG; CD105) develop normally and will be valuable for evaluating clinical application of anti-human ENG antibodies

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL The major objective of this study is to develop a novel animal model to facilitate optimal clinical application of anti-human ENG (huENG) monoclonal antibodies (mAbs) to treat cancer patients. ENG is a proliferation-associated homodimeric cell membrane antigen of endothelial cells and leukemia cells. It is strongly expressed on angiogenic vascular endothelium in tumors and also expressed in lymphatic vessels in tumors. Furthermore, ENG is a TGF-β coreceptor and essential for angiogenesis/vascular development. Mice lacking ENG die from defective vascular development. Mutations in the huENG gene are associated with hereditary hemorrhagic telangiectasia type 1 (HHT1) which is characterized by vascular malformations and recurrent bleeding. huENG shows a relatively restricted expression pattern in tissues and cells. We hypothesized that huENG is a safe and effective target in vascular-targeting antiangiogenic therapy (VTAT). Clinical trials of a chimeric anti-huENG mAb c-SN6j (also known as TRC105) are in progress (ClinicalTrials.gov Identifier: [NCT00582985][1] and [NCT01090765][2]). The major obstacle in animal model studies of anti-huENG mAbs is that most anti-huENG mAbs do not react with mouse ENG or mouse endothelial cells. To overcome this obstacle, we generated knock-in mice expressing human/mouse chimeric ENG. We designed the chimeric vectors to contain huENG gene segments of desired location and length so that the expressed huENG protein contains desired epitopes. Nevertheless, the major concern was that the generated knock-in mice might suffer from defective vascular development or HHT-like symptoms if the chimeric ENG does not function properly in mice. We generated two types of knock-in mice containing different sizes of huENG gene. In both cases, mice develop normally and show no sign of HHT-like symptoms in the homozygous mice. Tumors of 4T1 breast cancer cells and Colon26 colon cancer cells were generated in the knock-in mice to test expression of chimeric ENG protein in the tumor microvessels. Immunohistochemical staining of the tumors using several anti-huENG mAbs showed that the tumor microvessels express desired epitopes of huENG protein which are candidate targets for VTAT. The epitopes defined by these mAbs have been mapped. This novel knock-in mouse model will be useful for evaluating antitumor efficacy of many anti-huENG mAbs. In addition, this model will be useful for evaluating appropriate combination of an anti-huENG mAb with other anticancer agents to achieve optimal antitumor efficacy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3278. doi:10.1158/1538-7445.AM2011-3278 [1]: /lookup/external-ref?link_type=CLINTRIALGOV&access_num=NCT00582985&atom=%2Fcanres%2F71%2F8_Supplement%2F3278.atom [2]: /lookup/external-ref?link_type=CLINTRIALGOV&access_num=NCT01090765&atom=%2Fcanres%2F71%2F8_Supplement%2F3278.atom