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 alpha-helix. Therefore, we substituted a model amphipathic alpha-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 micrograms/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 micrograms/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.