Decreased bone remodeling and porosity are associated with improved bone strength in ovariectomized cynomolgus monkeys treated with denosumab, a fully human RANKL antibody☆
Research highlights
► Denosumab markedly reduced cortical and trabecular bone remodeling in OVX cynos. ► Denosumab reduced trabecular eroded surfaces and cortical porosity. ► Lowest levels of remodeling at each site were associated with greatest structural strength. ► Cortical remodeling parameters had little or no relationship with cortical material properties. ► Denosumab effects on remodeling were fully reversible upon clearance of drug.
Introduction
Bone remodeling is a continuous lifelong process by which packets of bone matrix are resorbed by osteoclasts and then replaced by matrix-secreting osteoblasts [1]. Bone remodeling rate increases after the menopause in humans, and with ovariectomy in non-human primates, leading to reductions in bone mass that can reduce bone strength and increase fracture risk [2], [3]. Bone remodeling is reduced by antiresorptive agents, and clinical trials have shown that 3–5 years of reduced bone remodeling with antiresorptives decreases fracture risk in subjects with low bone mass [4], [5], [6]. Greater decreases in biochemical markers of bone turnover with various antiresorptive agents have been associated with greater fracture risk reduction [7], [8], [9], [10].
Most data examining the effects of remodeling inhibition on bone strength and fracture risk are from bisphosphonate studies. The recent introduction of a new class of antiresorptive agents, the RANKL inhibitors, provides new tools to re-examine relationships between bone remodeling and bone strength. RANKL inhibitors, including the fully human monoclonal antibody denosumab (Prolia®), can reduce bone remodeling to a greater extent than other antiresorptive agents, including bisphosphonates [11], [12], [13], [14], [15]. Denosumab was recently shown to reduce hip, spine and non-vertebral fractures in postmenopausal women [16]. Iliac crest biopsies from a subgroup of this trial population exhibited markedly reduced bone resorption and formation parameters after 2–3 years of denosumab treatment, but this sub-study was not powered to examine relationships between histomorphometric remodeling parameters and fracture risk [17]. And while the iliac crest is a common site for histomorphometry assessment, it is subjected to a different loading environment compared to sites where fragility fractures commonly occur. Non-clinical studies allow bone histomorphometry analyses at multiple sites including diaphyseal cortices, and relationships between tissue-level remodeling and bone strength can be examined to further explore potential safety signals related to remodeling inhibition.
This paper describes histology and histomorphometry analyses of cortical and trabecular bone from a number of skeletal sites after 6 to 16 months of denosumab administration to mature ovariectomized (OVX) cynomolgus monkeys. This animal model is widely used for bone quality assessments of osteoporosis agents due to their skeletal maturity, increased trabecular and intracortical remodeling, and systemic osteopenia that is typical of postmenopausal osteoporosis [18]. The denosumab dosing regimens (25 or 50 mg/kg/month, subcutaneous) resulted in systemic exposure levels that greatly exceeded those achieved with clinical dosing for osteoporosis, providing robust safety margins. The primary goal of the study was to determine whether denosumab's anti-remodeling effects were associated with any adverse effects on bone histology or on extrinsic (structural) or intrinsic (material) properties of bone strength.
Section snippets
Materials and methods
All animal activities were approved by the Charles River Montreal Animal Care Committee and performed in an AAALAC-accredited facility. The study was performed under Good Laboratory Practice (GLP) conditions according to the protocol and Standard Operating Procedures established at Charles River Laboratories, Quebec, Canada, and Amgen Inc., Thousand Oaks, CA, USA.
Bone histology and histopathology assessments
Qualitative microscopic examination across all skeletal sites revealed no detrimental denosumab-related histological findings at either dose; the regular lamellar collagen arrangement was maintained and no osteoid accumulation was found. In many denosumab-treated monkeys, fluorochrome double labels were rare (Table 2) and had to be sought outside of the measured regions in order to confirm that the absence of labels in individual bones reflected very low bone formation rate and not an
Discussion
This paper provides the first description of cortical and trabecular bone histomorphometry data for adult OVX cynos treated with denosumab, a fully human anti-RANKL antibody that inhibits bone resorption and thereby reduces bone remodeling. The study's primary purpose was to determine whether the low-turnover state induced by high doses of denosumab led to detrimental effects on bone strength, in accordance with regulatory guidelines for new osteoporosis therapies [22], [23]. Other data from
Conflict of Interest
P.J.K., J.S., I.P., and M.S.O. are employees and own stock in Amgen Inc.
S.Y. Smith and J. Jolette are employees and own stock in Charles River Laboratories.
Charles River Laboratories received funding from Amgen Inc. for the study.
Acknowledgments
This study was supported by Amgen Inc. Rogely Boyce, D.V.M., Ph.D., provided helpful comments, and Michelle N. Bradley, Ph.D., provided editorial support on behalf of Amgen Inc. We are also grateful to the excellent technical expertise of the histomorphometry teams at Charles River Laboratories.
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Parts of the manuscript were presented at the 28th Annual Meeting of the American Society for Bone and Mineral Research in Philadelphia, PA in 2006, and at the 29th Annual Meeting of the American Society for Bone and Mineral Research in Honolulu, Hawaii in 2007.