Anaphylactoid and anti-ovulatory activities of LHRH antagonists in rats

Potent pituitary-gonadal axis suppression and extremely low anaphylactoid activity of a new gonadotropin releasing hormone (GnRH) receptor antagonist "azaline B"

We report here the biological characterization of azaline B, a new gonadotropin releasing hormone (GnRH) receptor antagonist, with the following amino acid sequence: [Ac-D-Nal1, D-Cpa2, D-Pal3, Aph5(atz), D-Aph6(atz), Ilys8, D-Ala10]-GnRH. Azaline B was shown to suppress several reproductive processes in rats including ovulation, and had very low anaphylactoid activity compared with other GnRH antagonists. Azaline B inhibited histrelin (a GnRH agonist)-mediated follicle stimulating hormone (FSH) and luteinizing hormone (LH) release from cultured rat pituitary cells. Three antagonists ([Nal-Glu]-GnRH, [Nal-Lys]-GnRH ("antide"), and azaline B) inhibited 0.1 nM histrelin-mediated gonadotropin release to baseline levels with EC50 values of approximately 0.6 nM. Azaline B, when injected s.c. into rats on the afternoon of proestrus, was more potent at inhibiting ovulation than either [Nal-Glu]-GnRH or [Nal-Lys]-GnRH. The relative order of antiovulatory potencies of the three antagonists was azaline B > [Nal-Glu]-GnRH > [Nal-Lys]-GnRH. Similar azaline B potency was shown by its ability to suppress gonadotropin levels in castrated rats. The improved selectivity of azaline B was demonstrated when it was compared with other GnRH antagonists in the cutaneous anaphylactoid assay (local wheal response) in rats. Results with azaline B were not significantly different from results with vehicle in this assay. [Nal-Glu]-GnRH was more than twice as potent as [Nal-Lys]-GnRH in stimulating a wheal response. Furthermore, the maximal wheal response produced by azaline B was only 0.6 times that of [Nal-Lys]-GnRH, currently one of the most selective antagonists identified. Finally, both azaline B and [Nal-Lys]-GnRH were much less potent than [Nal-Glu]-GnRH in the guinea pig cardiopulmonary anaphylactoid assay after i.v. administration. These data show that azaline B is a potent and selective GnRH receptor antagonist with little or no anaphylactoid activity in animal models, and therefore has potential for use in the treatment of many reproductive endocrine disorders, as well as for use as a contraceptive.

Evaluation of the anaphylactoid activity of a new LHRH antagonist

ORF 23541 [N-Ac-D-Nal(2)1,D-pCl-Phe2,D-Pal(3)3,Ser4,Nic-Lys5,D-Nic-Lys 6, Leu7, I-Lys8, Pro9,D-Ala10NH2; "Nal-Lys antagonist"] was identified as a potent LHRH antagonist without significant anaphylactoid activity. It blocked ovulation in proestrus rats when administered subcutaneously with an ED50 of 5.8 micrograms/kg. Much higher doses of ORF 23541 than of other antagonists were required to induce a cutaneous anaphylactoid-like reaction. Intradermal administration of ORF 23541 caused an 8.75 x 8.75 mm wheal response with estimated doses of 10.9 and 13.7 micrograms in rats and guinea pigs, respectively. These doses were at least 10 times greater than that required of other LHRH antagonists for the same response. ORF 23541 also did not alter pulmonary function in guinea pigs or dogs when administered intravenously at doses up to 10 mg. These results indicate that ORF 23541 represents a new generation of LHRH antagonists with an improved safety margin.