Abstract 4184: Novel parathyroid hormone antagonist prevents breast cancer bone metastasis and cortical bone destruction

Abstract

Abstract The majority of patients (~70%) with advanced breast cancer will develop bone metastases and suffer from severe pain, bone fracture and eventually death. Current palliative treatments have only limited efficacy highlighting an urgent need for development of targeted agents. Bone metastatic breast cancer cells secrete parathyroid hormone related peptide (PTHrP) that promotes autocrine tumor growth, induces bone turnover that releases additional tumor stimulatory factors, and creates cavities in the bone that permit tumor outgrowth. Previous PTH/PTHrP antagonists failed clinically due to short half-life and inadequate targeting to bone. We created a novel bone-targeted PTH antagonist drug, PTH(7-33)-CBD, by fusing an N-terminally truncated analog of PTH (aa 7-33) with the collagen binding domain (CBD) from ColH collagenase (Clostridium histolyticum). Our previous preliminary study demonstrated in vivo efficacy of PTH(7-33)-CBD against tumor burden and cortical bone destruction using an established model of breast cancer bone metastasis by injecting a bone-trophic variant of estrogen receptor-negative MDA-MB-231 breast cancer cells expressing luciferase (MDA-MB-231-BM/luc+) into the tibia marrow of nude mice. PTH(7-33)-CBD treatment did not result in hypocalcemia or reduce animal body weight. We extended this previous study by using increased animal numbers, administration of a control drug, and assessment of apoptosis of breast cancer cell in vitro by PTH(7-33)-CBD. PTH(7-33)-CBD reduced PTH agonist-stimulated cAMP accumulation in SaOS-2 osteosarcoma cells by 71% confirming the PTH antagonist activity of the compound. PTH(7-33)-CBD exhibited a direct cytotoxic effect towards MDA-MB-231 breast cancer cells in vitro by increasing apoptosis as assessed by the Caspase Glow Assay. To assess in vivo efficacy, on Day 0 nude mice were treated IP with either vehicle, 1000 µg/kg control drug PTH-7-34 (without CBD domain), or 1000 µg/kg PTH(7-33)-CBD (N=8/treatment group). On Day 1 mice were injected with 2x106 MDA-MB-231-BM/luc+ cells into the tibia marrow and tumor burden and bone density were assessed weekly by bioluminescence imaging and X-ray imaging, respectively. PTH(7-33)-CBD significantly reduced tumor burden in bone from weeks 4-8 post-tumor cell inoculation compared to vehicle (P<0.05), and from weeks 6-7 compared to PTH-7-34 control drug (P<0.05). PTH(7-33)-CBD reduced cortical bone destruction from weeks 3-7 weeks compared to both vehicle and PTH-7-34 control drug (P<0.05). Taken together, these data demonstrate that PTH(7-33)-CBD reduces both bone metastatic breast tumor burden and osteolytic lesions in nude mice, and induces apoptosis of MDA-MB-231 breast cancer cells in vitro. Support: RCG and BGR are co-principal investigators for Breast Cancer Research Program (BCRP) Breakthrough Award, BC150685. Citation Format: Murali Anbalagan, Tulasi Ponnapakkam, Yibin Kang, Robert C. Gensure, Brian G. Rowan. Novel parathyroid hormone antagonist prevents breast cancer bone metastasis and cortical bone destruction [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4184.