Osteoprotegerin: A Potential, Novel Therapeutic Agent for Treatment of Humoral Hypercalcemia of Malignancy
Osteoprotegerin: A Potential, Novel Therapeutic Agent for Treatment of Humoral Hypercalcemia of Malignancy
By Casey Capparelli and Paul J. Kostenuik, PhD
Recently, a novel member of the tnf receptor family was discovered, termed osteoprotegerin (OPG) or osteoclastogenesis-inhibitory factor (OCIF), that inhibits osteoclast formation and activity in vitro and in vivo.1,2 OPG acts as a decoy receptor, binding to and neutralizing osteoprotegerin ligand (OPGL).3 OPGL is expressed by osteoblasts and bone marrow stromal cells where it acts locally in the bone microenvironment, binding to its receptor, RANK (receptor-activator of NF-kB), on the osteoclast.3,4 The interaction between OPGL and RANK is pivotal, if not essential for the differentiation and activation of osteoclasts both in vivo and in vitro.3,5,6 The specific and potent action of OPG accordingly is because of the prevention of the OPGL/RANK interaction. Several genetically altered mouse lines have been instrumental in establishing the specificity and importance of OPG, OPGL, and RANK in the regulation of bone homeostasis. Transgenic mice overexpressing OPG are osteopetrotic and have significant decreases in osteoclast number, whereas OPG-deficient mice are osteoporotic, with bone phenotype characterized by excessive bone remodeling and increases in osteoclast number.1,7 OPGL and RANK are also essential in the homeostasis of the skeleton, as mice deficient in either OPGL or RANK are severely osteopetrotic.5,6
OPGL in Hypercalcemia
The importance of OPGL in bone resorption is further supported by evidence demonstrating that OPGL expression is upregulated by factors known to increase bone resorption and potentially cause hypercalcemia, including parathyroid hormone (PTH).4,8 Recombinant OPGL delivered in pharmacological doses also increases osteoclast activity, resulting in severe hypercalcemia and loss of bone density.3,9 Other studies have shown that recombinant OPG can inhibit bone resorption because of PTHrP, IL-1, 1,25(OH)2D3, PTH, TNF, or OPGL, as well as to inhibit the bone loss due to ovariectomy.1,3,10
Hypercalcemia is a frequent complication of cancer. In some cases, hypercalcemia occurs as a result of metastasis of the cancer to bone, producing localized bone destruction, and, thus, excessive calcium release into the circulation. Humoral hypercalcemia of malignancy (HHM) occurs with little or no evidence of metastasis to bone, and results from the systemic actions of tumor-generated factors that alter calcium metabolism.11 The primary cytokine involved in HHM is parathyroid hormone-related protein (PTHrP), which when secreted by a solid tumor, acts systemically to induce hypercalcemia.12-14
Systemic Actions of PTHrP
The systemic actions of PTHrP are most likely mediated through the PTH/PTHrP receptor, which binds both PTH and PTHrP with a similar affinity. PTHrP is identical to PTH in eight out of its first 13 amino acids, which is the domain of the molecule that is essential for activating the PTH/PTHrP receptor.15 Similar to PTH, PTHrP administered in vivo increases osteoclastic bone resorption and renal calcium reabsorption.16 HHM is also characterized by increases in osteoclastic bone resorption and in renal calcium reabsorption, with bone resorption contributing the majority of the calcium load.17 The calcemic effects of PTHrP on renal calcium metabolism contribute to hypercalcemia that is independent of osteoclast function. Current therapies to treat HHM in humans include treatment with anti-resorptive agents that target osteoclastic bone resorption, such as pamidronate (a bisphosphonate) or calcitonin.17,18 The need to maximally inhibit bone resorption in managing HHM is extremely important because of the currently untreatable effects of PTHrP on renal calcium reabsorption.
OPG in Prevention and Treatment of HHM
Because of the specific and potent actions of recombinant OPG in bone, we tested OPG in the prevention and treatment of HHM in immune competent mice inoculated with syngeneic colon-26 (C-26) adenocarcinoma cells. The C-26 cells grow as a solid tumor when injected into the flank of mice. These tumor cells express PTHrP mRNA, and tumor-bearing mice have significantly elevated plasma PTHrP levels.19 The production of PTHrP, as well as other cytokines by the C-26 cells, results in the development of significant hypercalcemia and excessive bone resorption.
In a prevention model, OPG was administered daily for seven days, with treatment beginning prior to the development of hypercalcemia.19 C-26 tumor burden was associated with a 47% increase in blood ionized calcium, and OPG treatment limited the tumor-associated increase to only 10%. Histomorphometry of the tibial metaphysis demonstrated that osteoclast number was increased by 120% in the vehicle-treated tumor-bearing mice. OPG treatment prevented this tumor-associated increase and reduced osteoclast number (to sub-physiologic levels) by 95% when compared to normal, non-tumor bearing mice. Despite the almost complete eradication of osteoclasts, OPG did not completely normalize calcium levels. This might be attributed to the effects of PTHrP on renal calcium reabsorption, which also contributes to the overall calcium burden. Tumor size, plasma PTHrP levels, and body weight loss were not significantly effected by OPG treatment, further confirming the specific action of OPG on bone.19
In a treatment model, C-26 tumor-bearing mice were allowed to become hypercalcemic prior to OPG treatment.19 Hypercalcemia was defined as a 60% increase in blood ionized calcium over vehicle-treated, non-tumor-bearing mice. OPG treatment resulted in a rapid reduction in ionized calcium and by 24 hours, ionized calcium in the OPG-treated mice was only increased 20% above normal. Within 48 hours, statistically normal calcium levels were established in the OPG-treated mice. Ionized calcium remained decreased in the OPG-treated group for the final two days of the study. Histologically, OPG had similar effects in the treatment model as in the prevention model. Following a four-day OPG treatment, osteoclast number was reduced to a sub-physiological level.19
Summary
The activity of OPG in this model indicates that blocking OPG ligand is an effective strategy to prevent bone resorption induced by PTHrP and/or other tumor-derived factors. The results of this study suggest that OPG might have clinical utility in treating hypercalcemia caused by malignant tumors and other disorders of excessive bone resorption. OPG appears to have low toxicity, as transgenic mice over-expressing OPG develop normally and have a normal life expectancy (unpublished data).1 OPG was also well tolerated in a phase 1 clinical trial.20 If clinical trials demonstrate safety and efficacy in human HHM, OPG could provide an alternative to bisphosphonate therapy for patients with HHM. (Mr. Capparelli is research associate and Dr. Kostenuik is research scientist at Amgen, Inc, Thousand Oaks, CA.)
References
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Osteoprotegerin:
a. promotes osteoclast formation in vivo.
b. promotes osteoclast activity in vitro.
c. binds to and neutralizes osteoprotegerin ligand.
d. raises serum calcium.
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