Abstract 2474: A rat monoclonal antibody against bone sialoprotein II shows differential activity in MDA-MB-231 cells growing in vitro or in vivo

Abstract

Abstract Elevated serum levels of the small integrin binding ligand N-linked glycoprotein (SIBLING) family member bone sialoprotein II (BSP) have been related to breast cancer skeletal metastasis. Also, knockdown of BSP was associated with significant anti-proliferative, anti-migratory and anti-clonogenic effects in MDA-MB-231 human breast cancer cells, and rats presenting with osteolytic metastases following femoral artery injection of these cells underwent complete remission following BSP knockdown, thus validating BSP as target. In order to follow a translational perspective, we used the IDK-1 rat monoclonal antibody against BSP (Immundiagnostik, Bensheim, Germany) for treating MDA-MB-231 cells in vitro and in vivo. MDA-MB-231 cells were exposed to IDK-1 for assessing its effect on proliferation and migration. The presence of BSP in the cytosol and on cell membranes was checked by FACS analysis. In addition, the location of BSP was investigated by immunocytochemistry. For in vivo experiments, 1×105 MDA-MB-231luc breast cancer cells were injected into the femoral artery of male nude rats with skeletal lesions developing subsequently in the respective hind leg. A preventive arm based on pretreated rats and / or MDA-MB-231 cells was compared with a treatment arm, in which antibody administration (10 mg/kg/week) started when tumor bearing rats had shown stable tumor growth. The appearance and growth of soft tissue tumors was monitored by luciferin induced light emission and recorded by a Xenogen IVIS 100 imaging system. Concomitant skeletal lesions were detected by CT scans. Tumors as well as skeletal lesions were subjected to pathohistological evaluation by hematoxylin and eosin staining as well as immunohistochemical staining for BSP. There was no effect of the anti-BSP antibody IDK-1 on the proliferation or migration of MDA-MB-231 cells. In line with this, FACS analysis revealed only low concentrations of BSP in MDA-MB-231 cells growing in vitro. Immunocytochemical staining for BSP showed that this SIBLING protein could be detected only in a minority of the MDA-MB-231 cells. However, when treating nude rats bearing fully established MDA-MB-231 tumors, administration of the IDK-1 antibody caused complete remissions in 80% of treated rats (10mg/kg/week for 6 weeks). When pre-treating the nude rats, however, and / or pre-exposing the MDA-MB-231 cells to IDK-1 in addition to treating the established tumors, the above mentioned effect could not be increased. Interestingly, histological evaluation of serial sections of MDA-MB-231 tumors growing in nude rats showed a very robust expression and vesicular secretion of BSP. It is concluded, that the in vivo growth of MDA-MB-231 cells is associated with dramatically increased expression and secretion of BSP, which then is a valid target for the anti-BSP antibody, leading to complete remissions of MDA-MB-231 tumors in nude rats. Citation Format: Michael Zepp, Irina Berger, Heidegard Hilbig, Franz-Paul Armbruster, Martin R. Berger. A rat monoclonal antibody against bone sialoprotein II shows differential activity in MDA-MB-231 cells growing in vitro or in vivo. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2474. doi:10.1158/1538-7445.AM2015-2474