New effective gonadotropin releasing hormone antagonists with minimal potency for histamine release in vitro

In order to minimize the adverse effect of histamine release in the rat of some gonadotropin releasing hormone (GnRH) antagonists, such as [Ac-D2Nal1,D4FPhe2,DTrp3,DArg6]-GnRH, new structures with modifications at positions 1, 2, 3, 5, 6, 7, and 10 were synthesized and tested in several biological systems. In vitro: the affinity for the pituitary GnRH receptor was measured as was the ability of the analogues to inhibit GnRH-stimulated release of luteinizing hormone (LH) by dispersed anterior pituitary cells in culture and to release histamine from rat mast cells. In vivo: inhibition of ovulation in the cycling rat was determined after subcutaneous (sc) injection of the peptides at noon on the day of proestrus; the duration of action of the peptides was evaluated by measuring LH levels in the castrated male rat after sc injection of some selected analogues. [Ac-D2Nal1,D4ClPhe2,D3Pal3,Arg5,D-4-p-methoxy benzoyl-2-aminobutyric acid6,DAla10]-GnRH was found to be one of the most potent analogues of this series, causing a 100% inhibition of ovulation at 5 micrograms/kg or less. Release of histamine was observed at doses 10-25 times that required for [Ac-D2Nal1,D4FPhe2,DTrp3,DArg6]-GnRH. Thus, introduction of arginine in position 5 with a hydrophobic amino acid in position 6 is compatible with high potency in several biological systems and results in compounds with lowered potency to release histamine compared to homologous peptides with tyrosine in position 5 and D-arginine in position 6.