Abstract: Peptides which have substantial bioactivity to inhibit the secretion of gonadotropins by the pituitary gland and to inhibit the release of steroids by the gonads. Administration of an effective amount of such GnRH antagonists prevents ovulation and/or the release of steroids by the gonads. They may also be used to treat steroid-dependent tumors, such as prostatic and mammary tumors. The peptides are bicyclic analogs of the decapeptide GnRH having two covalent bonds, between the residues in the 4- and 10-positions and the residues in the 5- and 8-positions. The latter linkage includes peptide bonds between a residue of an .alpha.-amino acid outside of the main chain and a side-chain carboxyl group of the 5-position residue and a side-chain amino group of the 8-position residue.

Abstract: Peptides which inhibit the secretion of gonadotropins by the pituitary gland and inhibit the release of steroids by the gonads. Administration of an effective amount of such GnRH antagonists prevents ovulation of female mammalian eggs and/or the release of steroids by the gonads. These peptides may be used to treat steroid-dependent tumors, such as prostatic and mammary tumors. The peptides include cyclic, bicyclic and tricyclic analogs of the decapeptide GnRH, and preferably there are at least two covalent bonds between the residues in the 4- and 10-positions, the residues in the 5- and 8-positions and the residues in the 1- and 3-positions, respectively. Examples of such bonds include a disulfide linkage between Cys residues, an amide linkage between a side chain amino group and a side chain carboyxl group, a dicarba linkage between side-chain alkyl groups, and a carba linkage between a side-chain alkyl group and a side-chain sulfhydryl group.