Sharon Gordon

Cellular and molecular mechanisms of action of bisphosphonates

Bisphosphonates currently are the most important class of antiresorptive agents used in the treatment of metabolic bone diseases, including tumor‐associated osteolysis and hypercalcemia, Pagets disease, and osteoporosis. These compounds have high affinity for calcium and therefore target to bone mineral, where they appear to be internalized selectively by bone‐resorbing osteoclasts and inhibit osteoclast function.

Molecular mechanisms of action of bisphosphonates: current status.

Purpose: Bisphosphonates are currently the most important class of antiresorptive agents used in the treatment of metabolic bone diseases, including tumor-associated osteolysis and hypercalcemia. These compounds have high affinity for calcium ions and therefore target bone mineral, where they are internalized by bone-resorbing osteoclasts and inhibit osteoclast function. Experimental Design: This article reviews the pharmacology of bisphosphonates and the relationship between chemical structure and antiresorptive potency. We also describe new insights into their intracellular molecular mechanisms of action, methods for assessing the effects of bisphosphonates on protein prenylation, and their potential as direct antitumor agents. Results: Nitrogen-containing bisphosphonates act intracellularly by inhibiting farnesyl diphosphate synthase, an enzyme of the mevalonate pathway, thereby preventing prenylation of small GTPase signaling proteins required for normal cellular function. Inhibition of farnesyl diphosphate synthase also seems to account for their antitumor effects observed in vitro and for the activation of γ,δ T cells, a feature of the acute-phase response to bisphosphonate treatment in humans. Bisphosphonates that lack a nitrogen in the chemical structure do not inhibit protein prenylation and have a different mode of action that seems to involve primarily the formation of cytotoxic metabolites in osteoclasts. Conclusions: Bisphosphonates are highly effective inhibitors of bone resorption that selectively affect osteoclasts in vivo but could also have direct effects on other cell types, such as tumor cells. After >30 years of clinical use, their molecular mechanisms of action on osteoclasts are finally becoming clear but their exact antitumor properties remain to be clarified.

The bisphosphonate zoledronic acid has antimyeloma activity in vivo by inhibition of protein prenylation

Nitrogen‐containing bisphosphonates (N‐BPs) are effective antiosteolytic agents in patients with multiple myeloma. Preclinical studies have also demonstrated that these agents have direct antitumor effects in vitro and can reduce tumor burden in a variety of animal models, although it is not clear whether such effects are caused by direct actions on tumor cells or by inhibition of bone resorption. N‐BPs prevent bone destruction in myeloma by inhibiting the enzyme farnesyl pyrophosphate synthase in osteoclasts, thereby preventing the prenylation of small GTPase signaling proteins. In this study, utilizing a plasmacytoma xenograft model without complicating skeletal lesions, treatment with zoledronic acid (ZOL) led to significant prolongation of survival in severe combined immunodeficiency mice inoculated with human INA‐6 plasma cells. Following treatment with a clinically relevant dose of ZOL, histological analysis of INA‐6 tumors from the peritoneal cavity revealed extensive areas of apoptosis associated with poly (ADP‐ribose) polymerase cleavage. Furthermore, Western blot analysis of tumor homogenates demonstrated the accumulation of unprenylated Rap1A, indicative of the uptake of ZOL by nonskeletal tumors and inhibition of farnesyl pyrophosphate synthase. These studies provide, for the first time, clear evidence that N‐BPs have direct antitumor effects in plasma cell tumors in vivo and this is executed by a molecular mechanism similar to that observed in osteoclasts.

Pamidronate causes apoptosis of plasma cells in vivo in patients with multiple myeloma

Summary. Anti‐resorptive bisphosphonates, such as pamidronate, are an effective treatment for osteolytic disease and hypercalcaemia in patients with multiple myeloma, but have also been shown to cause apoptosis of myeloma cell lines in vitro. In this study, we found that a single infusion of pamidronate, in 16 newly diagnosed patients with multiple myeloma, caused a marked increase in apoptosis of plasma cells in vivo in 10 patients and a minimal increase in four patients (P < 0·05). The nitrogen‐containing bisphosphonates pamidronate and zoledronic acid also induced apoptosis of authentic, human bone marrow‐derived plasma cells in vitro. Apoptosis of plasma cells in vitro was probably caused by inhibition of the mevalonate pathway and loss of prenylated small GTPases, as even low concentrations (≥ 1 µmol/l) of zoledronic acid caused accumulation of unprenylated Rap1A in cultures of bone marrow mononuclear cells in vitro. GGTI‐298, a specific inhibitor of geranylgeranyl transferase I, also induced apoptosis in human plasma cells in vitro, suggesting that geranylgeranylated proteins play a role in signalling pathways that prevent plasma cell death. Our results suggest that pamidronate may have direct and/or indirect anti‐tumour effects in patients with multiple myeloma, which has important implications for the further development of the more potent nitrogen‐containing bisphosphonates, such as zoledronic acid, in the treatment of myeloma.

Hip fracture incidence and mortality in chronic kidney disease : the GLOMMS-II record linkage cohort study

Background Individuals on renal replacement therapy (RRT) have increased fracture risk, but risk in less advanced chronic kidney disease (CKD) is unclear. Objective To investigate CKD associations with hip fracture incidence and mortality. Design Record linkage cohort study Grampian Laboratory Outcomes Mortality and Morbidity Study II. Setting Single health region in Scotland. Participants All individuals (≥15 years) with sustained CKD stages 3–5 and those on RRT, and a 20% random sample of those with normal renal function, in the resident population in 2003. Outcome measures Outcomes were (1) incident hip fracture measured with (A) admissions or (B) deaths, with at least 5.5 years follow-up and (2) post-hip fracture mortality. Unadjusted and adjusted, incident rate ratios (IRRs) and mortality rate ratios were calculated using Poisson regression. Results Of 39 630 individuals identified in 2003 (41% males, mean age 63.3 years), 19 537 had CKD stages 3–5, 345 were on RRT and 19 748 had normal estimated glomerular filtration rate (eGFR). Hip fracture incidence, measured by admissions, was increased in CKD stages 3–5 (compared with normal eGFR), both overall (adjusted IRR 1.49 (95% CI 1.24 to 1.79)) and for individual CKD stages 3a, 3b and 4. Hip fracture incidence, measured using deaths, was increased in those with CKD stages 3b and 4. Post-hip fracture mortality was only increased in CKD stage 4. There was only a small number of individuals and events for CKD stage 5, resulting in insufficient statistical power. Conclusion Hip fracture incidence was higher in CKD stages 3–5 compared with normal eGFR. Post-hip fracture mortality was only increased in CKD stage 4. Reducing hip fracture incidence in CKD through regular fall and fracture risk review should reduce overall deaths after hip fracture in the population.