Backbone Modification of a Parathyroid Hormone Receptor-1 Antagonist/Inverse Agonist.

A backbone-modified peptide derived from parathyroid hormone (PTH) is shown to function as an inhibitor and inverse agonist of parathyroid hormone receptor-1 (PTHR1) signaling. This receptor acts to regulate calcium and phosphate homeostasis, as well as bone turnover and development. PTH is a natural agonist of PTHR1, and PTH(1-34) displays full activity relative to the natural 84-residue hormone. PTH(1-34) is used clinically to treat osteoporosis. N-terminally truncated derivatives of PTH(1-34), such as PTH(7-34), are known to bind to PTHR1 without initiating intracellular signaling and can thus act as competitive antagonists of PTH-induced signaling at PTHR1. In some cases, N-terminally truncated PTH derivatives also act as inverse agonists of PTHR1 variants that display pathologically high levels of signaling in the absence of PTH. Many analogues of PTH, however, are rapidly degraded by proteases, which may limit biomedical application. We show that backbone modification via periodic replacement of α-amino acid residues with homologous β-amino acid residues leads to an α/β-PTH(7-34) peptide that retains the antagonist and inverse agonist activities of the prototype α-peptide while exhibiting enhanced stability in the presence of aggressive proteases. These findings highlight the value of backbone-modified peptides derived from PTH as tools for investigating determinants of PTH metabolism and provide guidance for designing therapeutic agents for diseases arising from excessive ligand-dependent or ligand-independent PTHR1 activity.