Development of Potent, Protease-Resistant Agonists of the Parathyroid Hormone Receptor with Broad β Residue Distribution

The
parathyroid hormone receptor 1 (PTHR1) is a member of the B-family
of GPCRs; these receptors are activated by long polypeptide hormones
and constitute targets of drug development efforts. Parathyroid hormone
(PTH, 84 residues) and PTH-related protein (PTHrP, 141 residues) are
natural agonists of PTHR1, and an N-terminal fragment of PTH, PTH(1–34),
is used clinically to treat osteoporosis. Conventional peptides in
the 20–40-mer length range are rapidly degraded by proteases,
which may limit their biomedical utility. We have used the PTHR1–ligand
system to explore the impact of broadly distributed replacement of
α-amino acid residues with β-amino acid residues on susceptibility
to proteolysis and agonist activity. This effort led us to identify
new PTHR1 agonists that contain α → β replacements
throughout their sequences, manifest potent agonist activity in cellular
assays, and display remarkable resistance to proteolysis, in cases
remaining active after extended exposure to simulated gastric fluid.
The strategy we have employed suggests a path toward identifying protease-resistant
agonists of other B-family GPCRs.