Extended presence of antide (Nal-Lys GnRH antagonist) in circulation: prolonged duration of gonadotropin inhibition may derive from antide binding to serum proteins.

Previous data from this laboratory revealed a rapid (-12h) and unexpectedly long (-30 days) inhibition of pituitary gonadotropin secretion after a single injection of Antide (Nal-Lys GnRH antagonist) in ovariectomized (OVX) monkeys. Although the apparent mechanism of action of Antide is competitive occupancy of GnRH receptors, the etiology of the prolonged action is unknown. Here, we report development of a radioreceptor assay to measure circulating Antide levels to determine the mechanism(s) of its long duration of action. Five long-term OVX monkeys were injected with Antide (3.0 mg/kg). Blood samples were collected daily for 30 days, and thereafter on alternate days until day 60. Following sc or iv Antide injection, peripheral luteinizing hormone (LH) levels declined from 281 +/- 19 ng/ml to 29 +/- 3 ng/ml within one day (P less than 0.05). LH levels slowly recovered to pretreatment levels within 35 +/- 7 days. Peripheral Antide levels were 16,531 +/- 4,432 ng/ml within 15 minutes following iv injection, and 52 +/- 21 ng/ml at 1 day after sc Antide injection. Interestingly, thereafter clearance of Antide-from the peripheral circulation was very slow, with an apparent t1/2 (second phase) of 6.5 days following iv administration. Detectable Antide levels were present in the peripheral circulation for more than one month in all five monkeys. In a second experiment, incubation of 125I-Tyro Antide with OVX monkey serum resulted in binding of the labelled peptide to serum proteins and reduction of 125I-Tyro Antide binding to pituitary receptors. Following gel permeation chromatography, greater than 70% of the radioactivity was associated with a 66 kDa protein(s). In conclusion, the prolonged duration of gonadotropin inhibition by Antide seems to derive from the long circulatory half-life of this molecule. In turn, this extended action of Antide may be manifest, at least in part, by binding to serum protein(s) that serves as a built-in peripheral depot release mechanism.