PURPOSE:To obtain a new calcitonin derivative as a new and useful peptide, having a specific amino acid sequence, exhibiting excellent serum calcium level- lowering activity, thus useful for e.g. treating osteoporosis, hypercalcemia, Paget' s disease, etc. CONSTITUTION:A new calcitonin derivative having excellent serum calcium level-lowering activity, thus useful for e.g. treating osteoporosis, hypercalcemia or Paget's disease, etc., and consisting of a peptide expressed by formula I ((a), (b), (c), (d) and (e) each is 1 or 2, their total is 6). This new peptide is obtained by the following processes: using a peptide synthesizer, protected amino acids are bound successively to a carrier such as a p-methylbenzhydrylamine(MBHA)resin by solid phase synthesis to synthesize a peptide resin of formula II (Cl-Z is o-chlorobenzyloxycarbonyl; Bzl is benzyl; Cl2Bzl is 2,6-dichlobenzyl) followed by binding a peptide of formula III to this resin, and then deprotection and release from the resin.
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