Inhibition of Osteoblast function by bisphosphonates

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ABSTRACTS / Bone 42 (2008) S17–S110

Conclusions: HA-TCP demonstrated greater osteogenic capability than ACS, with the ACS groups showing a slight healing advantage over control. Placement of BMP in these carriers did not contribute to significant increases in bone growth, which may be due to lack of a sustained release carrier system. However, inherent osteoconductive properties of a scaffold may play a greater role in bone growth than the addition of osteoinductive growth factors. doi:10.1016/j.bone.2007.12.152

143 Inhibition of osteoblast function by bisphosphonates Isabel R. Orriss a , Michelle L. Key a, Kay W. Colston b, Timothy R. Arnett a a Department of Anatomy and Developmental Biology, University College London, London, United Kingdom b Division of Cellular and Molecular Medicine, St George’s, University of London, London, United Kingdom Bisphosphonates are analogues of pyrophosphate, a key inhibitor of mineralisation. They are widely recognised as potent inhibitors of osteoclastic resorption (as well as for their potential cytotoxicity). Surprisingly, the direct actions of bisphosphonates on osteoblast function are not well documented. We studied this question using cultures of primary, bone-forming osteoblasts derived from neonatal rat calvariae. We found that exposure to zoledronate at concentrations of 1 and 10 μM reduced osteoblast numbers by 61% and 90%, respectively after 14 days; pamidronate exerted a small inhibitory action and clodronate was without effect. Exposure to 10 μM zoledronate was rapidly toxic, resulting in a 3-fold decrease in osteoblast viability after 2 days. Treatment with 1 μM zoledronate for 7 or 14 days inhibited soluble collagen production by up to 82%, whereas pamidronate and clodronate had no effect at concentrations ≤ 10 μM. In control osteoblast cultures, abundant formation of ‘trabecular’ mineralised bone matrix nodules began from about day 10. Zoledronate selectively inhibited the mineralisation of bone nodules at low concentrations (IC50 ≈ 10 nM); pamidronate and clodronate exerted similar effects but at higher concentrations (IC50 values ≈ 100 nm –1 μM and 1– 10 μM, respectively). Complete abolition of organic matrix deposition occurred in the presence of 1 μM zoledronate and 10 μM pamidronate. Consistent with its low-dose effects on mineralisation, zoledronate caused dose-dependent inhibition of osteoblast alkaline phosphatase activity (IC50 ≈ 100 nM). In contrast, pamidronate and clodronate stimulated alkaline phosphatase activity at high concentrations (100 nM–10 μM and 10 μM, respectively). Our results indicate important differences in the mode of action of bisphosphonates on osteoblasts. Chronic exposure to zoledronate, the most potent bisphosphonate studied, inhibited bone formation in two distinct ways: first, a relatively non-toxic, selective inhibition of mineralisation at concentrations in the low nanomolar range and second, a cytotoxic inhibition of osteoblast growth and function at concentrations ≥ 100 nM. Although no data are available on the

bisphosphonate concentrations that osteoblasts could be exposed to in vivo, our results are consistent with previous reports that bisphosphonates may inhibit bone formation, including the anabolic response to parathyroid hormone. doi:10.1016/j.bone.2007.12.153

144 Treatment with RAP-011 (A soluble activin receptor type IIA) increases bone mineral density in mice with prior bisphosphonate exposure R. Scott Pearsall, Michelle Mangini, Milton Cornwall-Brady, Travis Monnell, Tod Marvell, Brendan Haigis, Ravindra Kumar, Kathryn W. Underwood, Jasbir Seehra Acceleron Pharma, Inc., Cambridge, USA RAP-011 is a soluble activin receptor type IIa (ActRIIA) fused to a murine IgG1-Fc region that is a surrogate compound for our human therapeutic, ACE-011. Previously, we demonstrated that treatment with RAP-011, an activin antagonist, reversed bone loss in ovariectomized mice. Here we present data on the use of RAP-011 in ovariectomized mice receiving previous bisphosphonate treatment. In this study 8 week old female C57BL/6 mice (n = 40) were ovariectomized and allowed to develop bone loss for 8 weeks. At 16 weeks of age mice were divided into 5 groups, with 3 groups receiving a single dose of zoledronic acid (ZOL, IP, 20 ug/kg) and 2 groups received vehicle (VEH). Three days later, mice were injected (IP) with RAP-011 (1 or 10 mg/kg) or VEH twice per week for 8 weeks. Whole body DXA and pQCT scanning was performed at baseline, 3 weeks and at the conclusion of the study. In the ovariectomized mice, treatment with RAP-011 showed a significant (p b 0.01) increase in BMD (+11%) than either ZOL (+ 5%) or VEH treated mice (+ 1%). In addition, pre-treatment with ZOL followed by RAP-011 increased BMD to a greater extent than either treatment by itself (+ 15%). The data demonstrate that RAP-011 can increase bone density in the context of a previously administered bisphosphonate. The data provide support for the use of soluble ActRIIA fusion proteins for the treatment of skeletal fragility despite prior antiresorptive therapy. Scott Pearsall; Acceleron Pharma, Inc; Full time employee. doi:10.1016/j.bone.2007.12.154

145 Effect of intravenous pamidronate for treatment of severe osteoporosis in postmenopausal women Vladyslav V. Povoroznyuk, Marina A. Bystrytska, Tatyana V. Orlyk, Natalija I. Dzerovych, Vladymyr M. Vayda, Natalija A. German Department of Clinical Physiology and Pathology of Locomotor Apparatus, Institute of Gerontology AMS Ukraine, Ukrainian Scientific-Medical Centre for the Problems of Osteoporosis, Kiev, Ukraine