RS-66271, a C-terminally substituted analog of human parathyroid hormone-related protein (1–34), increases trabecular and cortical bone in ovariectomized, osteopenic rats

Abstract It was predicted from the amino acid sequence of the bone anabolic peptides, parathyroid hormone (PTH) (1–34) and PTH related protein (PTHrP) (1–34), that the C-terminal amino acids form an amphipathic α-helix. Therefore, we substituted a model amphipathic α-helical peptide (MAP) sequence in the C-terminal region of hPTHrP(1–34), obtaining RS-66271 ([MAP1–10]]22–31 hPTHrP(1–34)-NH2). The anabolic activities of RS-66271 and hPTHrP(1–34) were evaluated in 3-month-old, ovariectomized (OVX) osteopenic rats. Subcutaneous injection of hPTHrP(1–34) at 80 μg/kg/day partially reversed estrogen depletion trabecular bone loss but was ineffective in the cortex. In contrast, RS-66271 dose-relatedly reversed loss at both sites and, at 80 μg/kg/day, returned both trabecular and cortical bone calcium to the level of sham-operated controls. Histomorphometric analysis showed significantly elevated bone formation rates over vehicle-treated OVX in both trabecular and cortical tibial bone following treatment with RS-66271. Electron microscopy showed an increase in the relative surface area of vertebral trabeculae covered by osteoblasts in animals treated with RS-66271. These studies demonstrate that the C-terminal amino acids of hPTHrP(1–34) can be replaced by a model amphipathic helix and that the new chemical entity has greater anabolic activity than the parent peptide. The results suggest that RS-66271 may be a candidate molecule for the treatment of human osteoporosis.