The role of bisphosphonates in metastatic breast cancer

Tumor dormancy in bone

Background

Bone marrow is a common site of metastasis for a number of tumor types, including breast, prostate, and lung cancer, but the mechanisms controlling tumor dormancy in bone are poorly understood. In breast cancer, while advances in drug development, screening practices, and surgical techniques have dramatically improved survival rates in recent decades, metastatic recurrence in the bone remains common and can develop years or decades after elimination of the primary tumor.

Recent Findings

It is now understood that tumor cells disseminate to distant metastatic sites at early stages of tumor progression, leaving cancer survivors at a high risk of recurrence. This review will discuss mechanisms of bone lesion development and current theories of how dormant cancer cells behave in bone, as well as a number of processes suspected to be involved in the maintenance of and exit from dormancy in the bone microenvironment.

Conclusions

The bone is a complex microenvironment with a multitude of cell types and processes. Many of these factors, including angiogenesis, immune surveillance, and hypoxia, are thought to regulate tumor cell entry and exit from dormancy in different bone marrow niches.

Multiple actions of parathyroid hormone-related protein in breast cancer bone metastasis

The sequence similarity within the amino-terminal regions of parathyroid hormone (PTH) and PTH-related protein (PTHrP) allows the two to share actions at a common site, the PTH1 receptor. A number of biological activities have been ascribed to actions of other domains within PTHrP. PTHrP production by late stage breast cancer has been shown to contribute to bone metastasis formation through promotion of osteoclast formation and bone resorption by action through PTH1 receptors. There is evidence also for a role for PTHrP early in breast cancer that is protective against tumour progression. No signalling pathway has been identified for this effect. PTHrP has also been identified as a factor promoting the emergence of breast cancer cells from dormancy in bone. In that case, PTHrP does not function through activation of PTH1 receptors, despite having very substantial effects on transcriptional activity of the breast cancer cells. This indicates actions of PTHrP that are non-canonical, that is, mediated through domains other than the amino-terminal. It is concluded that PTHrP has several distinct paracrine, autocrine, and intracrine actions in the course of breast cancer pathophysiology. Some are mediated through action at PTH1 receptors and others are controlled by other domains within PTHrP. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Enhanced anti-tumor activity and safety profile of targeted nano-scaled HPMA copolymer-alendronate-TNP-470 conjugate in the treatment of bone malignances

Bone neoplasms, such as osteosarcoma, exhibit a propensity for systemic metastases resulting in adverse clinical outcome. Traditional treatment consisting of aggressive chemotherapy combined with surgical resection, has been the mainstay of these malignances. Therefore, bone-targeted non-toxic therapies are required. We previously conjugated the aminobisphosphonate alendronate (ALN), and the potent anti-angiogenic agent TNP-470 with N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer. HPMA copolymer-ALN-TNP-470 conjugate exhibited improved anti-angiogenic and anti-tumor activity compared with the combination of free ALN and TNP-470 when evaluated in a xenogeneic model of human osteosarcoma. The immune system has major effect on toxicology studies and on tumor progression. Therefore, in this manuscript we examined the safety and efficacy profiles of the conjugate using murine osteosarcoma syngeneic model. Toxicity and efficacy evaluation revealed superior anti-tumor activity and decreased organ-related toxicities of the conjugate compared with the combination of free ALN plus TNP-470. Finally, comparative anti-angiogenic activity and specificity studies, using surrogate biomarkers of circulating endothelial cells (CEC), highlighted the advantage of the conjugate over the free agents. The therapeutic platform described here may have clinical translational relevance for the treatment of bone-related angiogenesis-dependent malignances.

Advances in the biology of bone metastasis: how the skeleton affects tumor behavior

It is increasingly evident that the microenvironment of bone can influence the cancer phenotype in many ways that favor growth in bone. The ability of cancer cells to adhere to bone matrix and to promote osteoclast formation are key requirements for the establishment and growth of bone metastases. Several cytokine products of breast cancers (e.g. PTHrP, IL-11, IL-8) have been shown to act upon host cells of the bone microenvironment to promote osteoclast formation, allowing for excessive bone resorption. The increased release of matrix-derived growth factors, especially TGF-β, acts back upon the tumor to facilitate further tumor expansion and enhance cytokine production, and also upon osteoblasts to suppress bone formation. This provides a self-perpetuating cycle of bone loss and tumor growth within the skeleton. Other contributing factors favoring tumor metastasis and colonization in bone include the unique structure and stiffness of skeletal tissue, along with the diverse cellular composition of the marrow environment (e.g. bone cells, stromal fibroblasts, immune cells), any of which can contribute to the phenotypic changes that can take place in metastatic deposits that favor their survival. Additionally, it is also apparent that breast cancer cells begin to express different bone specific proteins as well as proteins important for normal breast development and lactation that allow them to grow in bone and stimulate bone destruction. Taken together, these continually emerging areas of study suggest new potential pathways important in the pathogenesis of bone metastasis and potential areas for targeting therapeutics.

Targeting angiogenesis-dependent calcified neoplasms using combined polymer therapeutics

BACKGROUND

There is an immense clinical need for novel therapeutics for the treatment of angiogenesis-dependent calcified neoplasms such as osteosarcomas and bone metastases. We developed a new therapeutic strategy to target bone metastases and calcified neoplasms using combined polymer-bound angiogenesis inhibitors. Using an advanced "living polymerization" technique, the reversible addition-fragmentation chain transfer (RAFT), we conjugated the aminobisphosphonate alendronate (ALN), and the potent anti-angiogenic agent TNP-470 with N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer through a Glycine-Glycine-Proline-Norleucine linker, cleaved by cathepsin K, a cysteine protease overexpressed at resorption sites in bone tissues. In this approach, dual targeting is achieved. Passive accumulation is possible due to the increase in molecular weight following polymer conjugation of the drugs, thus extravasating from the tumor leaky vessels and not from normal healthy vessels. Active targeting to the calcified tissues is achieved by ALN's affinity to bone mineral.

METHODS AND FINDING

The anti-angiogenic and antitumor potency of HPMA copolymer-ALN-TNP-470 conjugate was evaluated both in vitro and in vivo. We show that free and conjugated ALN-TNP-470 have synergistic anti-angiogenic and antitumor activity by inhibiting proliferation, migration and capillary-like tube formation of endothelial and human osteosarcoma cells in vitro. Evaluation of anti-angiogenic, antitumor activity and body distribution of HPMA copolymer-ALN-TNP-470 conjugate was performed on severe combined immunodeficiency (SCID) male mice inoculated with mCherry-labeled MG-63-Ras human osteosarcoma and by modified Miles permeability assay. Our targeted bi-specific conjugate reduced VEGF-induced vascular hyperpermeability by 92% and remarkably inhibited osteosarcoma growth in mice by 96%.

CONCLUSIONS

This is the first report to describe a new concept of a narrowly-dispersed combined polymer therapeutic designed to target both tumor and endothelial compartments of bone metastases and calcified neoplasms at a single administration. This new approach of co-delivery of two synergistic drugs may have clinical utility as a potential therapy for angiogenesis-dependent cancers such as osteosarcoma and bone metastases.

Palliation and supportive care in radiation medicine

The treatment of bone metastases represents a paradigm for evaluating palliative care in terms of symptom relief, toxicities of therapy, and the financial burden to the patient, caregivers, and society. Despite enormous expenditures to treat metastases, patients continue to sustain symptoms of the disease, and uninterrupted aggressive therapies are pursued until death that incur toxicity in approximately 25% of patients. This approach is inconsistent with the goals of palliative care, which should efficiently provide comfort using antineoplastic therapies or supportive care approaches to the patient with the fewest treatment-related side effects, recognizing that the patient will die of the disease.The development of therapies such as bisphosphonates is important in advancing options for palliative care; however, clinical trials demonstrating the efficacy of bisphosphonates have not addressed important issues for clinical practice. The primary study endpoints should primarily address pertinent patient outcomes such as pain relief rather than asymptomatic radiographic findings. These studies should define clear indications of when to start and stop the therapy, the appropriate patient populations to receive the therapy, and the cost effectiveness of the treatment relative to other available therapies such as radiation. Cost-utility analyses, which account for a broader domain of cost effectiveness, need to be performed as part of clinical trials, especially for palliative care endpoints. Clinical trials that include these criteria are critical to future practice guideline development. As health care resources continue to become more limited, the criteria for care must be better defined to avoid administration of therapy with limited benefit. Leadership must come from the specialty as clinical trials and clinical practice increasingly interface with health care policy. Goals of therapy must remain clear for the benefit of the individual and all patients.

Temporal changes in bone mass and mechanical properties in a murine model of tumor osteolysis

Pathological fracture is a devastating complication of osteolytic bone metastases. The progression of osteolysis and its effect on bone fracture risk are poorly understood. The goal of this study was to determine the temporal changes in bone strength following tumor inoculation in a preclinical model of tumor osteolysis. In addition, a predictive model was developed between non-invasive radiographic measures and bone strength. The right femora of female nude mice were injected with breast cancer cells; the left limb served as a sham-operated control. Radiographs and DEXA scans were obtained at the time of surgery and at 3, 6, and 9 weeks. Groups of mice were euthanized at each time point for mechanical assessment. Micro-CT analysis was performed on a sub-set of mice with advanced state disease to quantify bone loss. Radiographs documented an increase in tumor osteolysis over time, with 58% of the mice showing signs of osteolysis at 3 weeks, 75% at 6 weeks, and 81% at 9 weeks. BMD measurements revealed a 21.6% increase from baseline in the controls whereas tumor-injected femora failed to increase in BMD over the same time course. Tumor-bearing limbs exhibited statistically significant decreases in torque at failure (86%), energy to failure (88%), and initial stiffness (94%) compared to the controls. Both lysis scores and BMD measurements proved to be modest predictors of mechanical strength, accounting for approximately 73% and 41% of variation in torque at failure, respectively. Micro-CT analysis revealed decreases in both total bone volume in the distal femur (31%) and metaphyseal fractional trabecular bone (89%). We have shown that non-invasive radiographic techniques provide a useful tool for monitoring the progression of tumor osteolysis and for predicting the mechanical strength of tumor-bearing bones in this model. By integrating non-invasive measures of tumor osteolysis and fracture risk, we have validated a clinically relevant platform for evaluating new therapeutic approaches for preserving and/or restoring bone affected by metastatic disease.