An in vivo assay for anti-LH-RH and anti-FSH-RH activity of inhibitory analogues of LH-RH.

A convenient and sensitive method for testing anti-LH- and FSH-releasing activities of antagonists of LH-RH was devised. The peptides to be tested were injected subcutaneously together with synthetic LH-releasing hormone (LH-RH) in immature male rats. Serum LH and FSH concentrations were measured 30 min later by radioimmunoassay. In this system, [DesHis2]-LH-RH failed to block the pituitary response to LH-RH in agreement with previous data. The administration of [DesHis2, desGly10]-LH-RH ethylamide, [Des-His2, Leu3, des Gly10]-LH-RH ethylamide, [Des-His2, D-Ala6, des-Gly10]-LH-RH ethylamide, and [DESHis2, D-Ala6]-LH-RH in doses of 200 mug/rat, and [DesHis2, Leu3, D-Ala, desGly10]-LH-RH ethylamide in doses ranging from 100-400 mug/rat caused a significant inhibition of LH-RH-induced release of LH and FSH. On the other hand, [DesHis2, desGly10]-LH-RH propylamide, and [Leu2, Leu3, D-Ala6, desGly10]-LH-RH ethylamide failed to block the response to LH-RH. Among the peptides tested, [DesHis2, d-ala6, desGly10]-LH-RH etylamide, [Leu2, Leu3, D-Ala, desGly10]-LH-RH ethylamide in doses of 200 mug and [DesHis2, Leu3, D-Ala6, desGly10]-LH-RH ethylamide in a dose of 400 mug/rat showed some intrinsic LH and FSH-releasing activities. The method described here is simple and convenient for studying the anti-LH-RH release activity of inhibitory analogues of LH-RH, and also enabled us to demonstrate for the first time an in vivo inhibition of FSH release by these analogues.