Efficacy of pro-apoptotic receptor agonists in the treatment of primary breast cancer and bone metastasis.

Abstract

Breast cancer is the most common malignancy among women which frequently metastasises to the bone. Despite the significant improvements in detecting and treating primary breast cancer, metastatic breast cancer remains a challenging condition to treat. The studies presented in this thesis were aimed to exploit the therapeutic potential of Pro-Apoptotic Receptor Agonists (PARAs) including the recombinant TNF-related apoptosis-inducing ligand, Apo2L/TRAIL, and its agonistic monoclonal antibody, drozitumab, for the treatment of primary breast cancer and bone metastases in vitro and in vivo. Drozitumab is a fully human agonistic monoclonal antibody which binds to Apo2L/TRAIL death receptor DR5 and triggers apoptosis. The anticancer efficacy of drozitumab was evaluated using murine models of breast cancer xenografted at the orthotopic site and in bone. In vitro, drozitumab induced apoptosis in various human breast cancer cell lines, without being toxic to normal cells. In vivo, drozitumab exerted remarkable tumour suppressive activity as a single agent and co-operated with chemotherapeutic drugs, for increased efficacy against mammary tumours. In addition, drozitumab treatment completely inhibited tumour growth in bone, even in animal having well-advanced tibial tumours, leading to complete resolution of osteolytic lesions. Osteoprotegerin (OPG) is a soluble member of the TNF receptor superfamily, which binds the receptor activator of NF-kB (RANKL) and inhibits bone resorption. OPG can also bind and inhibit the activity of Apo2L/TRAIL, raising the possibility that the anticancer efficacy of Apo2L/TRAIL may be abrogated in the bone microenvironment, where OPG expression is high. In vitro, breast cancer cells engineered to overexpress OPG were protected from Apo2L/TRAIL-induced apoptosis. However, when mice were injected intratibially with cells overexpressing OPG, Apo2L/TRAIL treatment resulted in strong growth inhibition of OPG overexpressing intratibial tumours indicating that OPG levels in bone, even in the face of supra-physiological concentrations, are unlikely to play a significant role in modulatingApo2L/TRAIL therapeutic potential. Previous preclinical studies have shown that systemic administration of recombinant OPG inhibited tumour growth in bone and prevented cancer-induced osteolysis. However, the data presented in this thesis have demonstrated that while overexpression of OPG by breast cancer cells protected the bone from cancer-induced osteolysis, it was without effect on overall tumour burden. Despite the OPG-mediated bone protection, OPG overexpression led to a significant increase in the incidence of pulmonary metastasis. These results suggest that OPG-mediated inhibition of bone resorption modulates the bone microenvironment and it may affect the likelihood of cancer cells spreading elsewhere in the body. This also suggests that other anti-resorptive therapeutic agents including bisphosphonates (BPs), which have been the standard care for patients with skeletal malignancies, have the potential to harm by promoting cancer metastasis to other non-skeletal sites. In conclusion, the data presented in this thesis demonstrate that drozitumab and Apo2L/TRAIL represent potent immunotherapeutic agents with strong activity as single agents and in combination with conventional chemotherapy against the development and progression of breast cancer. In addition, these studies provide important preclinical evidence that modulating the bone microenvironment by inhibiting osteoclastic bone resorption may not always be beneficial, a phenomenon which needs further investigation.