LH-RH RECEPTOR ANTAGONIST

PROBLEM TO BE SOLVED: To obtain an antagonist, a new cyclic pentapeptide (salt), having a specific amino acid sequence, presenting LH-RH receptor antagonistic action, thus useful for preventing and treating e.g. prostate cancer, uterine cancer, endometriosis, and as e.g. a contraceptive. SOLUTION: This antagonist, a new cyclic pentapeptide (salt), is expressed by the formula (A1 is Phe, Gly, Ala, etc.; A2 is Arg, Lys, Trp, etc.; A3 is Phe, Nal, Tyr, etc.; A4 is Ala, Leu, Phe, etc.; A5 is Trp, Phe, Leu, etc.; namely, A1 -A5 are each an &alpha -amino acid residue), has LH-RH receptor antagonistic action, and then is useful, as a LH-RH receptor antagonist, for preventing and treating e.g. sex hormone-dependent cancer, sex hormone-dependent diseases, and as e.g. a regulator for gestation or menstrual cycle.This new compound is obtained by the following process: using a solid phase synthetic method for peptide, a chain pentapeptide is synthesized according to the amino acid sequence of a peptide followed by eliminating the protecting groups on both ends of the pentapeptide and then conducting an intramolecular cyclization.