Elsevier

Bone

Volume 46, Issue 4, April 2010, Pages 1082-1088
Bone

A soluble activin receptor Type IIA fusion protein (ACE-011) increases bone mass via a dual anabolic-antiresorptive effect in Cynomolgus monkeys

https://doi.org/10.1016/j.bone.2010.01.370Get rights and content

Abstract

Activin A belongs to the TGF-β superfamily and plays an important role in bone metabolism. It was reported that a soluble form of extracellular domain of the activin receptor type IIA (ActRIIA) fused to the Fc domain of murine IgG, an activin antagonist, has an anabolic effect on bone in intact and ovariectomized mice. The present study was designed to examine the skeletal effect of human ActRIIA-IgG1-Fc (ACE-011) in non-human primates. Young adult female Cynomolgus monkeys were given a biweekly subcutaneous injection of either 10 mg/kg ACE-011 or vehicle (VEH) for 3 months. Treatment effects were evaluated by histomorphometric analysis of the distal femur, femoral midshaft, femoral neck and 12th thoracic vertebrae, by μCT analysis of femoral neck and by biomarkers of bone turnover. Compared to VEH, at the distal femur ACE-011-treated monkeys had significantly increased cancellous bone volume (+ 93%), bone formation rate per bone surface (+ 166%) and osteoblast surface (+ 196%) indicating an anabolic action. Monkeys treated with ACE-011 also had decreased osteoclast surface and number. No differences were observed in parameters of cortical bone at the midshaft of the femur. Similar to distal femur, ACE-011-treated monkeys had significantly greater cancellous bone volume, bone formation rate and osteoblast surface at the femoral neck relative to VEH. A significant increase in bone formation rate and osteoblast surface with a decrease in osteoclast surface was observed in thoracic vertebrae. μCT analysis of femoral neck indicated more plate-like structure in ACE-011-treated monkeys. Monkeys treated with ACE-011 had no effect on serum bone-specific alkaline phosphatase and CTX at the end of the study. These observations demonstrate that ACE-011 is a dual anabolic-antiresorptive compound, improving cancellous bone volume by promoting bone formation and inhibiting bone resorption in non-human primates. Thus, soluble ActRIIA fusion protein may be useful in the prevention and/or treatment of osteoporosis and other diseases involving accelerated bone loss.

Introduction

Most of the drugs approved for the prevention and treatment of osteoporosis (i.e. selective estrogen modulators, bisphosphonates and calcitonin) act by inhibiting bone resorption and thereby preventing further bone loss [1]. They do not stimulate new bone formation, and thus fail to reverse prior bone loss. In contrast, parathyroid hormone (PTH), the only FDA-approved anabolic agent, increases bone remodeling and improves bone mass and microarchitecture [2]. However, PTH has adverse effects, including nausea, dizziness, headache, hypercalcemia and hypercalciuria and can be prescribed for only two years in the US [2], [3]. Until now, there are no FDA-approved drugs that play dual role in inhibiting bone resorption and also stimulating bone formation. Thus, there is a clear clinical need to develop anabolic-antiresorptive compounds since targeting both osteoblasts and osteoclasts might be the most effective therapeutic approach to reverse osteoporosis.

Activins are multifunctional proteins that belong to the TGF-β superfamily and are composed of four different β-subunits (βA, βB, βC and βE) [4], [5]. Homo- and heterodimerization of βA and βB subunits gives rise to three biologically active glycoproteins, including activin A (βA–βA), activin B (βB–βB) and activin AB (βA–βB). Activins mediate their signal transduction cascades through serine–threonine kinase receptors, including the type I receptors, comprising the activin receptor-like kinase 2 (ALK2) and 4 (ALK4), and the type II receptors, comprising the activin type IIA (ActRIIA) and type IIB (ActRIIB) [6], [7], [8]. Activins bind with high affinity to ActRIIA or ActRIIB receptors leading to the recruitment and phosphorylation of ALK4 followed by activation of intracellular signaling molecules, Smad 2 and 3 [9].

Mice with a mutation in the gene encoding the activin βA subunit have development-related defects in their secondary palates and die within 24 h of birth while mice deficient in activin βB are viable, indicating a crucial role of activin βA in craniofacial development [10]. Activin A is abundantly localized in extracellular bone matrix but its precise role in bone homeostasis is controversial. It has been shown that activin A enhances the induction of ectopic bone formation when implanted concurrently with BMPs [11], increases osteoblast proliferation and collagen synthesis [12] and stimulates fracture healing [13]. Local injection of activin A onto the periosteum of parietal bone in newborn rats increases the thickness of periosteum and bone matrix layers [14]. However, several studies demonstrated an inhibitory effect of activin A on osteoblast differentiation in murine, rat, and human in vitro[15], [16], [17]. Additionally, activin A stimulates osteoclastogenesis in vitro whereas inhibin exhibits an opposite effect [18]. It has been recently reported that an activin antagonist, the soluble extracellular domain of ActRIIA fused to a murine IgG2a-Fc (ActRIIA-mFc), stimulates bone formation, resulting in increased bone mass and strength in intact and ovariectomized (OVX) mice [19]. In addition, ActRIIA-mFc decreases osteoclast surface after 2 weeks of treatment in 12-week-old mice.

The purpose of the present study was to extend previous work by testing a human ActRIIA-IgG1-Fc fusion protein (ACE-011, Fig. 1) in non-human primates. We addressed the following questions: (1) does ACE-011 increase cancellous bone volume in Cynomolgus monkeys, a species in which bone remodeling is similar to that in humans; (2) if so, is increased bone volume due to an increase in bone formation, a decrease in bone resorption or both; (3) does ACE-011 alter cortical bone mass; and (4) what impact does ACE-011 have on biomarkers of bone turnover? The skeletal actions of ACE-011 were determined using bone histomorphometry, μCT and serum markers of bone metabolism. Our findings indicate that biweekly treatment with 10 mg/kg ACE-011 for 3 months is well-tolerated and acts as anabolic and antiresorptive agent, increasing cancellous bone mass at various sites in Cynomolgus monkeys.

Section snippets

Study design

Young adult female Cynomolgus monkeys (Macaca fascicularis, n = 11) were quarantined and acclimated to laboratory conditions for 30 days prior to release to a stock colony (Bridge Laboratories, Maryland). Animals from the stock colony were transferred to an experimental room and housed individually in stainless steel cages for 7 days prior to experiments. They were fed fresh food twice daily (Certified Global Harlan Teklad Laboratory 2055C Primate Diet) and provided water ad libitum. The study room

Results

Based on previous findings that the pharmacological blockade of activin signaling through the ActRIIA receptor has skeletal anabolic effect in mice, it was of interest to further examine the effects of activin antagonist on bone turnover in non-human primates. Biweekly subcutaneous dosing of 10 mg/kg ACE-011 was generally well tolerated. No treatment-related changes in clinical observation were reported over 3-month period. However, there was an incidence of diarrhea in one monkey on study days

Discussion

The present study investigates the skeletal effects of soluble ActRIIA fusion protein ACE-011 in non-human primates. Treatment with 10 mg/kg ACE-011 every 14 days for 3 months was well-tolerated and not associated with any serious adverse events. ACE-011 had a positive effect on cancellous bone mass and architecture primarily at appendicular sites whereas the midshaft diaphyseal femur, an ideal site for the analysis of cortical bone in non-human primates, remained relatively unchanged. The

Acknowledgments

The study was sponsored by Acceleron Pharma, Cambridge, MA. We thank Rajaram Manoharan for assistance with the µCT imaging.

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    Current address: Department of Orthopaedics, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229, USA.

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