In order to be used as fertility regulators in humans, gonadotropin releasing hormone (GnRH) antagonists must be extremely potent and long acting and exhibit negligible side effects such as stimulating histamine release. To this aim, we have recently synthesized a series of analogues with the standard Ac-DNal1-DCpa2-DPal3 substitutions, where the N omega-amino function of ornithine, lysine, or p-aminophenylalanine (Aph) was converted to the aminotriazolyl (atz) derivatives at positions 5 and 6 with further modifications at positions 7 and 10. The analogues were tested for their ability to bind to pituitary cell membranes, to release histamine in a mast cell assay, to inhibit luteinizing hormone (LH) secretion by castrated male rats or cultured pituitary cells, and to interfere with the ovulation in intact female rats. While the subcutaneous (sc) injection of 50 micrograms of Azaline A (7, [Ac-DNal1,DCpa2,DPal3,Lys5(atz),DLys6++ +(atz),ILys8,DAla10]GnRH) dissolved in 0.2 mL of an aqueous media significantly inhibited LH release in the castrated male rat for 24 h, the same dose of Azaline B (11), [Ac-DNal1,DCpa2,DPal3,Aph5(atz),DAph6++ +(atz),ILys8,DAla10]GnRH, inhibited LH release for 72 h. A similar long duration of action was observed for Antide ([Ac-DNal1,DCpa2,DPal3,Lys5(Nic),DLys6(Nic ),ILys8,DAla10]GnRH) but not for Nal-Glu ([Ac-DNal1,DCpa2,DPal3,Arg5,4-(pmethoxybenzoy l)-D-2-Abu6,DAla10]GnRH). In the same paradigm, a 5-fold dilution of the peptide (50 micrograms in 1 mL) and the use of three injection sites rather than one resulted in significantly shorter duration of action for most of the peptides tested. This suggested that long duration of action might be the result of slow release from the injection site(s). In order to investigate this possibility, Nal-Glu and Azaline B were injected intravenously (i.v.) at three doses (10, 50, 250 micrograms) to castrated male rats. At all doses, both peptides significantly lowered LH levels for 8 h. By 24 h, Nal-Glu (250 micrograms) and Azaline B (50 and 250 micrograms) still measurably inhibited LH secretion. Finally, only Azaline B (250 micrograms) was still active at 48 h. These findings demonstrate that subtle structural modifications will yield peptides with different half-lives after iv administration. These findings led us to investigate the effects of other structural modifications on duration of action. We observed that systematic substitutions at positions 7 (NMeLeu) and 10 (Pro9-NHEt, and Gly-NH2) were found to be deleterious. Of interest was the observation that only the DAla10-NH2 substitution led to long duration of action and enzymatic stability under the conditions tested.(ABSTRACT TRUNCATED AT 400 WORDS)