Stable Human Calcitonin Analogues with High Potency on Bone Together with Reduced Anorectic and Renal Actions

Abstract

Various derivatives of human calcitonin have been synthesized and their biological characteristics compared with those of existing calcitonins. The acute effects of these analogues in reducing serum calcium levels and suppressing appetite were assessed in rats. A calcitonin analogue, PO-1 (CGNLSTCMLGKLSQELHKLQTYPQTAIGVGAP-NH2), having both the N- and C-terminal ten amino acid sequences those of human calcitonin, and the 12 amino acid central region that of salmon calcitonin, was found to have equal effectiveness with salmon calcitonin and elcatonin for reducing serum calcium lavels. Strong hypocalcemic activity was also exhibited by PO23 ([cyclo-Asp¹, Lys⁷]-[des-Gly²]-[Leu⁸]-PO-1) and PO-29 ([Asp¹⁵, Asn¹⁷, Phe¹⁹, His²⁰]-PO-23). PO-23 was prepared by replacing the N-terminal Cys-Cys S-S bond of PO-1 with a ring structure composed of an Asp-Lys peptide bond to enhance physicochemical stability. PO-29 was prepared by modifying the central area of the PO-23 molecule to more closely mimic human calcitonin. When tested in vitro, human calcitonin analogues with a [cyclo-Asp¹, Lys⁷] structure showed biological activities on osteoclast-like cells comparable to those of existing calcitonins (salmon calcitonin and elcatonin) in keeping with their relative potencies for in vivo hypocalcemic action. Acute anorectic activity in rats was strong with salmon calcitonin and elecatonin but relatively reduced with human calcitonin analogues having a [cyclo-Asp¹, Lys⁷] structure. The activities of these analogues on kidney cells were also weaker than that of salmon calcitonin or elecatonin. These results suggest that stable human calcitonin analogues with a [cyclo-Asp¹, Lys⁷] structure suppress bone resorption to a degree similar to that of salmon calcitonin or elcatonin with weaker activities on non-osseous tissues which might be related to adverse reaction.