Synthesis and biological properties of (D-Ala-6, des-Gly-NH2-10)-LH-RH ethylamide, a peptide with greatly enhanced LH- and FSH- releasing activity

A nonapeptide analog of luteinizing hormone-releasing hormone (LH-RH), D-Ala⁶, des-Gly-NH₂¹⁰]-LH-RH ethylamide, was prepared by solid-phase methodology. The peptide was assayed against LH-RH in two $\text{in vivo}$ systems and was found to be many times more potent than the naturally occurring hormone. In one of the tests, based on elevation of LH and FSH levels after infusion into immature male rats, the analog showed LH-releasing activity of 1600% and FSH-releasing activity of 1200% compared to LH-RH.