CONFORMATIONAL STUDIES OF RS-66271, AN ANALOG OF PARATHYROID HORMONE-RELATED PROTEIN WITH PRONOUNCED BONE ANABOLIC ACTIVITY

Both the parathyroid hormone (PTH) and the functionally similar parath yroid hormone-related protein (PTHrP) have served as templates for the development of novel bone anabolic agents for the treatment of osteop orosis. The PTHrP analog RS-66271 (Vickery, B. H.; Avnur, Z.; Cheng, Y .; Chiou, S.-S.; Leaffer, D.; Caulfield, J. P.; Kimmel, D. B.; Ho, T.; Krstenansky, J. L. J. Bone Miner. Res. 1996, 11, 1943-1951), in which the amino acids 22-31 have been substituted by the sequence E-22-L-L- E-K-L-L-E-K-L-31 (a model amphiphilic peptide), is a potent bone anabo lic agent in vivo. Therefore, RS-66271 is a good candidate for structu ral analysis with the aim of developing a structure-activity relations hip. The structural characterization described here was carried out in aqueous solution employing circular dichroism and nuclear magnetic re sonance spectroscopy. We find that the incorporated amphiphilic decape ptide is indeed helical. In addition, it induces the adjacent residues , up to residue 16, to adopt the helical conformation. The helical dom ain, including residues 16-32, incorporates most of the previously ide ntified principal receptor binding domain PTHrP(25-34). We discuss the relevance of the distinct and extensive helicity in light of the redu ced in vitro receptor affinity/activity and the enhanced in vivo bone anabolic efficacy of RS-66271.