Abaloparatide, a Novel Osteoanabolic PTHrP Analog, Increases Cortical and Trabecular Bone Mass and Architecture in Orchiectomized Rats by Increasing Bone Formation Without Increasing Bone Resorption

Male osteoporosis can occur with advanced age and with hypogonadism, with increased bone resorption and/or inadequate bone formation contributing to reduced bone mass and increased fracture risk. Abaloparatide is a selective PTH receptor agonist that increases bone formation and bone mass in postmenopausal women with osteoporosis and in estrogen-deficient animals. The current study evaluated the effects of abaloparatide in orchiectomized (ORX) rats, a model of male osteoporosis. Four-month-old Sprague-Dawley rats underwent ORX or sham surgery; 8 weeks later the ORX groups exhibited relative osteopenia vs sham controls, based on dual X-ray absorptiometry (DXA) and/or peripheral quantitative computed tomography (pQCT) assessments at the total body, lumbar spine, femur, and tibia. ORX rats (n = 10/group) were then injected daily (s.c.) for 8 weeks with vehicle or abaloparatide at 5 (ABL5) or 25 μg/kg/d (ABL25). Sham controls (n = 10) received s.c. vehicle. DXA and pQCT showed that one or both abaloparatide groups gained more areal and volumetric BMD at all sites analyzed compared with vehicle controls, leading to substantial or complete reversal of ORX-induced BMD deficits. pQCT also indicated greater gains in tibial cortical thickness in both abaloparatide groups versus vehicle controls. Tibial bone histomorphometry showed greater trabecular bone formation and bone volume and improved micro-architecture with abaloparatide, with no increase in osteoclasts. Abaloparatide also led to significant improvements in the balance of biochemical bone formation markers versus bone resorption markers, which correlated with BMD changes. These findings suggest that abaloparatide may have therapeutic benefits in men with osteoporosis.