Abstract 398: Specific skeletal targeting of MMP-2 inhibitors for the treatment of bone metastatic breast cancer

Background. Bone metastatic breast cancer promotes extensive bone destruction/osteolysis and is currently incurable. Progression of the disease is critically dependent on cancer-bone interaction. Defining the molecular mechanisms underlying this communication can lead to the identification of new therapeutic targets that will eradicate the disease. Rationale. Gene expression analysis and validation in human and murine specimens of bone metastases revealed that matrix metalloproteinases (MMPs) such as MMP-2 are highly expressed in the bone metastatic microenvironment. Genetic ablation of MMP-2 highlighted the importance of this MMP in driving the growth of the osteolytic breast cancer lesions. We subsequently found that MMP-2 regulation of transforming growth factor β (TGF β) bioavailability was a major mechanism through which MMP-2 mediated this effect. These data support the rationale for the development of selective MMP inhibitors and imply that MMP-2 inhibition would be a successful strategy for the eradication of active bone metastatic breast cancer. Methods. To address systemic dose limiting side effects noted in previous broad spectrum MMP inhibitor trials, we utilized a novel chemical approach to generate bone-targeting, highly selective MMP-2 inhibitors grafted onto a bisphosphonic backbone. 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), osteoblasts (MC3T3) and osteoclasts (primary monocytes and RAW 264.7). In vivo, mice were inoculated with either luciferase expressing 4T1 or PyMT (100,000) cells. Mice (n = 10/group) then received vehicle, zoledronate (1 mg/kg) or BMMPIs (1 mg/kg). Tumor growth was determined via luminescence quantitation. Cancer induced bone disease was measured ex vivo by μCT, Xray and histomorphometry. MMP activity in vivo and ex vivo was determined via an activatable MMP probe. Results. BMMPIs significantly impacted the viability of breast cancer cells and osteoclasts in vitro (p<0.05) compared to control. In vivo BMMPIs significantly reduced the growth of bone metastatic breast cancer compared to control and 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). Conclusions. MMP-2 specific BMMPIs prevent bone metastatic breast cancer growth by impacting cancer cell viability and cancer induced osteolysis. 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.