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Disuse osteoporosis (bone loss resulted from a reduction in mechanical loading) occurs in patients due to prolonged bed rest, paralysis and application of braces. Abaloparatide (ABL) is a synthetic peptide analog of PTHrP that has been shown to promote bone formation with limited bone resorption. ABL was approved by the FDA in 2017 to treat osteoporosis in postmenopausal women at high fracture risk. Yet, the ability of ABL to prevent bone loss in disuse is unknown. We hypothesized that ABL would prevent bone loss in the hindlimb unloading (HLU) rat model of disuse osteoporosis. Adult male Wistar rats, 13–14 weeks of age, were assigned to 1 of 4 groups (10 rats/group): ambulatory + vehicle (CON-VEH), ambulatory + ABL (CON-ABL), HLU + vehicle (HLU-VEH) or HLU + ABL (HLU-ABL). The rats received a daily subcutaneous injection of ABL (25µg/kg/day) or vehicle for 28 days. Blood serum was collected on day 0, 7, 14 and 28 to examine the expression of bone markers such as osteocalcin (OCN) and TRAcP5b. pQCT scans were acquired at the proximal tibia at day 0 and 28 to measure changes in the total and trabecular vBMD. Following euthanasia, trabecular (Tb) and cortical (Ct) bone microarchitecture from femurs, tibias and L4 vertebrae were assessed using µCT. Femurs were mechanically tested to failure in 3-point bending to determine ultimate load (N) and stiffness (N/mm). Treatment effects were evaluated using 2-way ANOVA. Effects were considered significant at p < 0.05. Data reported as mean±SD. HLU led to loss of bone density and structure that were prevented by ABL. Longitudinal pQCT revealed significant increases in total vBMD in ABL-CON (52±17%) vs. VEH-CON (20±5%); and in HLU-ABL (24±6%) vs. HLU-VEH (-2±3%) (p We demonstrated positive effects of ABL on BMD, trabecular bone mass and structure in both ambulating and unloaded rats. These results are consistent with prior studies showing positive effects of ABL on bone mass, structure and strength in OVX and ORX rats. Limits include only male rats and 1 dose of ABL. However, the results observed in this study provide a strong rationale for investigating the ability of ABL to prevent disuse bone loss in humans.

Osteoporosis is characterized by low bone mass associated with bone fragility and high fracture risk resulting in reduced quality of life and a higher mortality rate. Osteoporosis is a result of unbalanced bone remodeling with increased catabolic actions of osteoclasts. To date, mechanisms of coupling bone resorption to bone formation are not well understood limiting treatment for osteoporotic patients. Abaloaratide (ABL), a synthetic analog of PTHrP, has been recently approved by the FDA for the treatment of postmenopausal women with osteoporosis at high risk for fracture. ABL exhibits anabolic functions on bone with only limited effects on bone resorption when compared to hPTH(1-34). Noteworthy, in vitro studies suggest that certain PTH or PTHrP ligand analogs can distinguish between two high-affinity PTHR1 conformations resulting in different signaling responses, one possibility to explain the distinct effects on bone resorption. To compare the effects of ABL and PTH(1-34) on calvarial bone resorption in vitro and in vivo mouse models were used. Mice were subcutaneously injected once daily with 10µg/kg of either peptide for 5 and 12 consecutive days. Our data demonstrate that in peptide-treated mice numbers of TRAP+ cells and bone marrow filled cavities within the calvariae are significantly increased compared to controls. Intriguingly, ABL-treated mice exhibit ~50% less TRAP+ cells compared to PTH(1-34). The markedly reduced ratio of marrow cavity/region of interest in response to ABL correlates with lower serum Rankl levels. Besides, in vitro formation of pits and trails, a general hallmark of osteoclastic activity, was increased when bone-marrow derived osteoclasts were treated with conditioned media from peptide-treated osteoblasts. Similarly to in vivo data, the total resorbing area was 3-fold less in ABL- than in PTH(1-34) treated osteoclasts, and the numbers of long trenches representing continuous resorption were reduced when using conditioned medium from ABL treated cells. Moreover, osteoclastogenesis assays with Ocy454 cells co-cultured for seven days with primary bone marrow-derived macrophages and treated intermittently for 6hr/day with ABL, or PTH(1-34) resulted in more osteoclast numbers compared to vehicle controls. Consistent with our in vivo results, in PTH-treated cells, the increase in osteoclast number and size was 1.6-fold and 2-fold greater, respectively, than in ABL- treated cells. These data imply differences in the extent of induction of osteoclastogenesis by ABL and PTH(1-34)-dependent signaling, which might manifest in different amounts of bone resorption. Our studies comparing the effects of ABL and PTH (1-34) on bone resorption provide mechanistic explanation for the net bone gain observed in the ACTIVE trial indicating that increased bone mass is, at least partly, the consequence of reduced osteoclastogenesis in ABL-treated patients.