Cyclic Parathyroid Hormone Related Protein Antagonists: Lysine 13 to Aspartic Acid 17 [i to (ì + 4)] Side Chain to Side Chain Lactamization

Cyclizatiön of parathyroid hormone related protein (7-34)amide [PTHrP(7-34)NH₂] via covalent bond formation between the e-amino of Lys¹³ and the β-carboxyl of Asp¹⁷ yielded a 20-membered ring lactam. This analogue, [formula omitted]PTHrP(7-34)NH₂, was 5-10-fold more potent than the linear parent peptide (K_b = 15 and 18 nM in PTH receptor binding assays, and K_i = 130 and 17 nM in PTH-stimulated adenylate cyclase assays in bovine renal cortical membrane and in human bone derived B10 cells, respectively). In contrast, a linear analogue in which charges in positions 13 and 17 were eliminated and other stereoisomers of the above-mentioned lactam in which either Lys¹³ and/or Asp¹⁷ were replaced by the corresponding D-amino acids were much less potent with regard to antagonist bioactivity than the parent peptide. The rationale for the design of the lactam as well as the conformational implications for the PTHrP sequence in light of reported models suggested for the 1-34 peptide are described. The potential use of conformationally constrained analogues for elucidating the “bioactive conformation” of antagonists and for the design of substantially simplified molecular structures for antagonists is discussed.