From polyphosphates to bisphosphonates and their role in bone and calcium metabolism

Inorganic polyphosphates: biologically active biopolymers for biomedical applications

Inorganic polyphosphate (polyP) is a widely occurring but only rarely investigated biopolymer which exists in both prokaryotic and eukaryotic organisms. Only in the last few years, this polymer has been identified to cause morphogenetic activity on cells involved in human bone formation. The calcium complex of polyP was found to display a dual effect on bone-forming osteoblasts and bone-resorbing osteoclasts. Exposure of these cells to polyP (Ca(2+) complex) elicits the expression of cytokines that promote the mineralization process by osteoblasts and suppress the differentiation of osteoclast precursor cells to the functionally active mature osteoclasts dissolving bone minerals. The effect of polyP on bone formation is associated with an increased release of the bone morphogenetic protein 2 (BMP-2), a key mediator that activates the anabolic processes leading to bone formation. In addition, polyP has been shown to act as a hemostatic regulator that displays various effects on blood coagulation and fibrinolysis and might play an important role in platelet-dependent proinflammatory and procoagulant disorders.

Hydroxy-alkyl bisphosphonic acid partial esters (HABPA-PE): Structures of two symmetrical and non-symmetrical members of a new class of prodrugs in bone disease treatments

Two new hydroxy alkyl bisphosphonate partial esters (HABPA-PE) have been crystallised. The first compound C6H15NaO7P2C2H5O, crystallises as an ethanol solvate and forms rigid infinite β-sheet-like ribbons. The second, C6H16O7P2, displays self-association in dimers. In these new structures, packing analyses confirm the enhanced hydrophobic character due to partial esterification compared to naked hydroxybisphosphonic acids and the simplification of their H bonding patterns.

The clinical and cost considerations of bisphosphonates in preventing bone complications in patients with metastatic breast cancer or multiple myeloma

The bisphosphonates are potent inhibitors of osteoclast-mediated bone resorption and are now the treatment of choice for the management of hypercalcaemia of malignancy. The incidences of hypercalcaemia and other skeletal complications (bone pain, pathological fracture) remain high despite apparent responses to systemic therapy, with particularly high event rates in women with advanced skeletal metastases of breast cancer. This review focuses on studies addressing the long-term efficacy of bisphosphonates to reduce skeletal complications in breast cancer (5 studies) and multiple myeloma (4 studies), with particular reference to controlled studies of sufficient magnitude and duration to allow confidence in the estimation of efficacy. Bearing in mind the limitations of differences in trial design and the lack of direct studies comparing drugs, adequate exposure to a bisphosphonate reduces the incidence of skeletal complication by 30 to 40% in both breast cancer and multiple myeloma. Oral clondronate and intravenous pamidronate have similar efficacy in both diseases, but the duration of efficacy may differ between drugs. Both agents have shown intriguing survival benefits in subgroups of patients. The numbers needed to treat (NNT) to prevent a skeletal complication during one year are lowest in metastatic skeletal disease in breast cancer (NNT < 8) but also compare very favourably with other disease for patients with recurrent nonskeletal breast cancer or multiple myeloma (NNTs 7 to 31 depending on the complication to be prevented). Treatment costs of both breast cancer and multiple myloma are driven by inpatient and outpatient hospital visits so that bisphosphonate regimens should be developed that reduce both. Further research is required to determine if subgroups of patients can be better identified that will derive particular benefit, or perhaps no benefit at all, from bisphosphonate therapy. It is not known whether more potent bisphosphonates will deliver greater clinical efficacy in the future.