Accelerated bone loss in patients with cancer is a frequent problem that may result from invasion of the cancer to bone, paraneoplastic tumor proteins, and/or hormonal therapies utilized for cancer treatment.
- Invasion of the cancer to bone;
- Paraneoplastic tumor proteins;
- Hormonal therapies utilized for cancer treatmen.
Invasion of cancer to bone is common complication in patients. The proportion of cancer invading to bone is 20% to 25% in kidney cancers, 65% to 75% in breast and prostate cancers, and almost all patients (70% to 95%) with multiple myeloma.
Mechanism of Bone Metastases
- Metabolically active tumor cells invade and populate bone and secrete growth factors that affect bone resorption and formation by stimulation of osteoclasts, cells that destroy bone by attacking the mineralized bone matrix.
- On the other hand osteoclasts also secrete growth factors that induce tumor cells in the bone to grow, spread, and stimulate the activity of osteoblasts, cells responsible for the formation of bone. However, osteoblastic activity creates new bone formation away from the sites of osteolytic bone resorption. So weakened areas are not strengthened by osteoblastic activity.
- Also, osteoblasts release receptor activator of nuclear factor κB ligand (RANKL), a key mediator of osteoclast formation, function, and survival, which is one of the mechanisms of metastatic bone disease.
Patients with osteolytic bone disease from multiple myeloma and bone metastases from solid tumors may develop a vicious cycle of bone destruction involving both ostelytic and osteoblastic effects. Consequently, a variety of skeletal-related events (SREs) may occur, including pathological fractures, hypercalcemia, spinal cord compression, and the need for surgical intervention and radiation therapy.
- Pathological fractures;
- Spinal cord compression;
- The need for surgical intervention and radiation therapy.
Untreated patients with bone metastases are at risk for multiple SREs within a single year, ranging from 1.5 events for prostate cancer to 4.0 for breast cancer.
Now two types of agents are used to treat bone metastases – bisphosphonates and denosumab.
Bisphosphonates are unique drugs with an affinity for bone mineral matrix with the ability to inhibit bone resorption. Bisphosphonates decrease bone resorption and increase mineralization by entering osteoclasts and inhibiting farnesyl diphosphate synthase, a key enzyme in the biosynthetic mevalonate pathway.
Bisphosphonates may also affect bone resorption through the inhibition of osteoclast precursor maturation, induction of apoptosis in mature osteoclasts, inhibition of tumor cell adhesion to bone, and inhibition of inflammatory cytokine production.
Nitrogencontaining bisphosphonates (N-BPs) have the greatest antiresorptive activity. Based on in vitro studies, zoledronic acid is the most potent aminobisphosphonate and is the only intravenous bisphosphonate found to be effective in all types of metastatic bone lesions.
Bisphosphonates also have a potential antitumor effect. Data from multiple studies suggest that bisphosphonates may directly or indirectly impair multiple processes required for cancer growth and metastases. Bisphosphonates have demonstrated an ability to induce apoptosis in a variety of cancer cell lines. These agents may also inhibit metastases by decreasing tumor cell adhesion, migration, and invasion. Inhibition of angiogenesis is another property associated with bisphosphonates. Furthermore, these pharmacologic agents may modulate the immune system with subsequent antitumor activity. Recent research also found that zoledronic acid may exert its antitumor activity by inhibiting mesenchymal stem cell migration and blocking mesenchymal stem cell secretion of factors involved in breast cancer progression.
However data from the FDA and the United Kingdom showed the issue of potential risk of esophageal cancer with oral bisphosphonate use was raised. The FDA recently announced plans to continue review of the conflicting studies.
Safety and efficacy data of intravenous bisphosphonates in the metastatic setting are predominantly limited to 24 months of treatment. The most frequently reported side effects from intravenous bisphosphonates are fever and myalgias, which may occur in up to 55% of patients, typically within 12 hours of the initial infusion. Antiinflammatory agents may easily provide relief. Diarrhea and gastric irritation may develop with the oral bisphosphonates ibandronate and clodronate, which are not approved in the management of bone metastases in the United States. Electrolyte abnormalities, including hypophosphatemia, hypocalcemia, hypomagnesemia, and hypermagnesemia, are rarely reported with intravenous bisphosphonates. Other condition such as vitamin D deficiency, hypoparathyroidism, hypomagnesemia, or use of medication such as interferon, aminoglycosides, or loop diuretics may provoke these abnormalities. (more…)