MONOCYCLIC AND BICYCLIC ANALOGS OF PARATHYROID HORMONE-RELATED PROTEIN .1. SYNTHESIS AND BIOLOGICAL STUDIES

The bioactive conformation of parathyroid hormone-related protein (PTH rP), a single-chain linear peptide structurally similar to parathyroid hormone (PTH), is of considerable interest because PTH and PTHrP both recognize and bind to a shared G-protein-coupled receptor. Both hormo nes are thought to present a bioactive conformation to the receptor wh ich is substantially a-helical in nature. To better characterize this putative biologically relevant conformation, we prepared a series of c onformationally constrained analogs of PTHrP with enhanced or-helical stability. A combination of structural constraint and helix stabilizat ion was achieved through side chain-to-side chain lactam ring formatio n between Lys' and Asp(i+4) residues (13-to-17 and 26-to-30) along the PTHrP sequence. Mono- and bicyclic analogs derived from the agonist P THrP-(1-34)NH2 and the antagonist PTHrP-(7-34)NH2 were prepared and ch aracterized in terms of receptor binding and stimulation (or antagonis m) of PTH-stimulated adenylyl cyclase activity in osteoblast-like cell s. The binding affinity of monocyclic [Lys(13),Asp(17)]-(I) and bicycl ic [Lys(13,)Asp(17),Lys(26),Asp(30)]PTHrP-(1-34)NH2 (III) agonists was in the low nanomolar range and similar to that of the parent linear p eptide. Furthermore, their efficacy was in the sub-nanomolar range and about 10-fold higher than that of the corresponding linear parent pep tide. Analogs I and III are the first cyclic PTH/PTHrP receptor agonis ts and amongst the most potent PTHrP analogs yet designed. The rank-or der of potency in the cyclic antagonist series does not correlate with the binding affinities. In light of the positional dependence and the differential effects of lactam bridge formation on the biological act ivities of agonist vs antagonists, these analogs may provide insight r egarding the biologically relevant conformations of PTHrP-derived liga nds [Maretto et al. (1997) Biochemistry 36, 3300-3307].