Abstract P6-12-10: Bone seeking matrix metalloproteinase-2 inhibitors prevent bone metastatic breast cancer growth

Bone metastasis is a common event during breast cancer progression. The resultant lesions are painful and currently, despite medical advances, are incurable. The progression of bone metastatic breast cancer is critically dependent on interactions with the surrounding microenvironment. Therefore, identifying the underpinning molecular mechanisms is vital for the development of new therapies. Rationale: Gene expression analysis and validation in human and murine specimens of bone metastases revealed that matrix metalloproteinases, such as MMP-2, are highly expressed in the bone metastatic microenvironment and significantly associated with aggressive breast cancer and poorer overall survival. In bone, tumor or host derived MMP-2 contributes to breast cancer growth and does so by processing substrates including type I collagen and transforming growth factor beta (TGFβ) latency proteins. These data provide strong rationale for the application of MMP-2 inhibitors to treat the disease. However, in vivo, MMP-2 is systemically expressed. Therefore, to overcome potential toxicities noted with previous broad-spectrum MMP inhibitors (MMPIs), we used highly selective bisphosphonic based MMP-2 inhibitors (BMMPIs) that allowed for specific bone targeting. Methods: We utilized a novel chemical approach to synthesize bone seeking MMP inhibitors (BMMPIs) on a bisphosphonic backbone, with specificity for MMP-2 in the nanomolar range (IC50=140 nM). Results: In vitro, we tested the effect of BMMPIs at varying doses (1nM-100μM) on the viability of the major cellular components of the cancer-bone microenvironment, namely breast cancer cells, (PyMT, 4T1, MDA-MB-231, MCF-7), osteoblasts (MC3T3) and osteoclasts (primary monocytes and RAW 264.7). In vivo, we demonstrated using two bone metastatic models (PyMT-R221A-Luc and 4T1-Luc) that BMMPI treatment significantly reduced tumor growth and tumor associated bone destruction. Additionally, BMMPIs are superior in promoting tumor apoptosis compared to the standard of care bisphosphonate, zoledronate. MMP activity was also lower in the BMMPI treated groups (using tumor burden to normalize values). μCT/Xray/Histomorphometry analysis also illustrated the significant beneficial effects of the BMMPIs in reducing the size of osteolytic lesions (up to 80% by μCT; p<0.05). We demonstrated MMP-2 selective inhibition in the bone microenvironment using specific and broad spectrum MMP probes. Further, compared to zoledronate, BMMPI treated mice had significantly lower levels of TGFβ signaling and MMP generated type I collagen carboxy-terminal (ICTP) fragments. Taken together, our data show the feasibility of selective inhibition of MMPs in the bone metastatic breast cancer microenvironment. Conclusions. MMP-2 specific inhibition was achieved in the bone microenvironment. BMMPIs significantly inhibit breast cancer growth in bone, they are able to induce breast cancer cell apoptosis and prevent cancer induced bone destruction. Given that bisphosphonates are well tolerated in the clinical setting, we predict that BMMPIs could be translated to the clinical setting for the treatment and eradication of bone metastatic breast cancer.