Abaloparatide, a PTH receptor agonist with homology to PTHrP, enhances callus bridging and biomechanical properties in rats with femoral fracture.

Fractures typically heal via endochondral and intramembranous bone formation, which together form a callus that achieves union and biomechanical recovery. PTHrP, a PTH receptor agonist, plays an important physiological role in fracture healing as an endogenous stimulator of endochondral and intramembranous bone formation. Abaloparatide, a novel systemically-administered osteoanabolic PTH receptor agonist that reduces fracture risk in women with postmenopausal osteoporosis, has 76% homology to PTHrP, suggesting it may have potential to improve fracture healing. To test this hypothesis, ninety-six 12-week-old male rats underwent unilateral internally-stabilized closed mid-diaphyseal femoral fractures and were treated starting the next day with daily s.c. saline (Vehicle) or abaloparatide at 5 or 20 µg/kg/d for 4 or 6 weeks (16 rats/group/time point). Histomorphometry and histology analyses indicated that fracture calluses from the abaloparatide groups exhibited significantly greater total area, higher fluorescence scores indicating more newly-formed bone, and higher fracture bridging scores versus Vehicle controls. Callus bridging score best correlated with callus cartilage score (r = 0.64) and fluorescence score (r = 0.67) at week 4, and callus area correlated with cartilage score (r = 0.60) and fluorescence score (r = 0.89) at Week 6. By micro-CT, calluses from one or both abaloparatide groups had greater bone volume, bone volume fraction, bone mineral content, bone mineral density, and cross-sectional area at both time points versus Vehicle controls. Destructive bending tests indicated greater callus maximum load and stiffness in one or both abaloparatide groups at both time points versus Vehicle controls. These results provide preliminary preclinical evidence for improved fracture healing with systemically-administered abaloparatide.