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 gon adotropin releasing hormone (GnRH) receptor antagonist, with the follo wing amino acid sequence: [Ac-D-Nal(1), D-Cpa(2), D-Pal(3), Aph(5)(atz ), D-Aph(6)(atz), Ilys(8), D-Ala(10)]-GnRH. Azaline B was shown to sup press several reproductive processes in rats including ovulation, and had very low anaphylactoid activity compared with other GnRH antagonis ts. Azaline B inhibited histrelin (a GnRH agonist)-mediated follicle s timulating hormone (FSH) and luteinizing hormone (LH) release from cul tured rat pituitary cells. Three antagonists ([Nal-Glu]-GnRH, [Nal-Lys ]-GnRH (''antide''), and azaline B) inhibited 0.1 nM histrelin-mediate d gonadotropin release to baseline levels with EC(50) values of approx imately 0.6 nM. Azaline B, when injected s.c. into rats on the afterno on of proestrus, was more potent at inhibiting ovulation than either [ Nal-Glu]-GnRH or [Nal-Lys]-GnRH. The relative order of antiovulatory p otencies of the three antagonists was azaline B > [Nal-Glu]-GnRH > [Na l-Lys]-GnRH. Similar azaline B potency was shown by its ability to sup press gonadotropin levels in castrated rats. The improved selectivity of azaline B was demonstrated when it was compared with other GnRH ant agonists in the cutaneous anaphylactoid assay (local wheal response) i n rats. Results with azaline B were not significantly different from r esults with vehicle in this assay. [Nal-Glu]-GnRH was more than twice as potent as [Nal-Lys]-GnRH in stimulating a wheal response. Furthermo re, the maximal wheal response produced by azaline B was only 0.6 time s that of [Nal-Lys]-GnRH, currently one of the most selective antagoni sts identified. Finally, both azaline B and [Nal-Lys]-GnRH were much l ess potent than [Nal-Glu]-GnRH in the guinea pig cardiopulmonary anaph ylactoid assay after i.v. administration. These data show that azaline B is a potent and selective GnRH receptor antagonist with little or n o 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.