GnRH Antagonists: A New Generation of Long Acting Analogues Incorporating p-Ureido-phenylalanines at Positions 5 and 6†

A series of antagonists of gonadotropin-releasing hormone (GnRH) of the general formula Ac-d2Nal-d4Cpa-d3Pal-Ser-4Aph/4Amf(P)-d4Aph/d4Amf(Q)-Leu-ILys-Pro-dAla-NH2 was synthesized, characterized, and screened for duration of inhibition of luteinizing hormone release in a castrated male rat assay. Selected analogues were tested in a reporter gene assay (IC50 and pA2) and an in vitro histamine release assay. P and Q contain urea/carbamoyl functionalities designed to increase potential intra- and intermolecular hydrogen bonding opportunities for structural stabilization and peptide/receptor interactions, respectively. These substitutions resulted in analogues with increased hydrophilicity and a lesser propensity to form gels in aqueous solution than azaline B [Ac-d2Nal-d4Cpa-d3Pal-Ser-4Aph(Atz)-d4Aph(Atz)-Leu-ILys-Pro-dAla-NH2 with Atz = 3‘-amino-1H-1‘,2‘,4‘-triazol-5‘-yl, 5], and in some cases they resulted in a significant increase in duration of action after subcutaneous (sc) administration. Ac-d2Nal-d4Cpa...