Evaluation of the impact of concurrent gonadotropin-releasing hormone (GnRH) antagonist administration on GnRH agonist-induced gonadotrope desensitization

The mechanism responsible for the supraphysiologic gonadotropin surge in females treated with gonadotropin-releasing hormone (GnRH) agonist and primed with GnRH antagonist

OBJECTIVE

To elucidate the physiologic mechanism responsible for the supraphysiologic gonadotropin release from the pituitary induced by gonadotropin-releasing hormone (GnRH) agonist in female rats primed with GnRH antagonist.

DESIGN

Controlled experimental intervention.

SETTING

Government research facility.

ANIMAL(S)

Forty 8-week-old Sprague-Dawley rats.

INTERVENTION(S)

Forty oophorectomized rats were randomized into four treatment groups of 10: group A, control vehicles; group B, GnRH agonist (leuprolide acetate; 1.7 microg/kg twice a day) on day 4; group C, GnRH antagonist (Nal-Lys; 3 mg/kg each day) days 1 to 4; or group D, GnRH antagonist (Nal-Lys; 3 mg/kg each day) days 1 to 4 plus GnRH agonist (1.7 microg/kg twice a day) on day 4.

MAIN OUTCOME MEASURE(S)

Immunohistochemical methods, Northern and in situ hybridization to quantitate pituitary follicle-stimulating hormone beta (FSH-beta), luteinizing hormone beta (LH-beta), and GnRH receptor (GnRH-R) messenger RNA (mRNA), and receptor protein levels in all treatment groups.

RESULT(S)

Treatment with GnRH antagonist was associated with increased storage of gonadotropin in the pituitary for FSH-beta and LH-beta, but mRNA levels were unchanged. The GnRH-R mRNA decreased after GnRH-agonist treatment but remained stable in the GnRH-antagonist treatment groups. Levels of GnRH-R were decreased after GnRH-antagonist treatment.

CONCLUSION(S)

These data indicate that the in vivo mechanism responsible for the exaggerated release of gonadotropins in rats primed with GnRH antagonist and treated with GnRH agonist was an increase in releasable gonadotropin pools coupled with a reduction in GnRH-R, but receptor function was preserved.

GnRH antagonist-induced down-regulation of the mRNA expression of pituitary receptors: comparisons with GnRH agonist effects

In order to compare the mechanism for the down regulation of the mRNA expression of pituitary receptors induced by GnRH antagonist (GnRHant) to that by GnRH agonist (GnRHa), we examined the effects of GnRHant (Cetrorelix, 333 mug/kg/day), GnRHa (leuprolide depot, 333 microg/kg), and GnRHant combined with GnRHa on LH response to exogenous GnRH, pituitary LH content, LH beta subunit mRNA, and GnRH receptor (GnRH-R) mRNA levels at 2, 5, 24, 72 hours, and 7 days after the treatment in ovariectomized rats. GnRHant significantly decreased serum LH, the LH response of the pituitary to exogenous GnRH, and the pituitary LH content compared to the control treatment, though GnRHa significantly increased serum LH. GnRHant with GnRHa significantly diminished the GnRHa-induced flare-up phenomenon. GnRHant significantly decreased LH beta mRNA and GnRH-R mRNA levels, but the magnitude of the decrease in these mRNA levels by GnRHant was significantly less than those by GnRHa until 72 hours following treatment. Prolonged treatment of GnRHant caused a marked inhibition of LH beta mRNA and GnRH-R mRNA expression, similar to that caused by GnRHa. Combination treatment with GnRHa and GnRHant was demonstrated to decrease LH beta mRNA and GnRH-R mRNA levels as much as GnRHa alone and GnRHant alone over 7 days of the treatment. The present study showed differences between GnRHant and GnRHa treatment in the reduction of GnRH-R mRNA levels up to 72 hours after the treatment, and indicated that the suppression of GnRH-R mRNA by GnRHant was the maximal by GnRHa 7 days after the treatment because more profound suppression was not observed upon additional treatment with GnRHa. The findings in the present study support the hypothesis that the mechanism by which GnRHant leads to down-regulation of the mRNA expression of pituitary receptors is similar to that of GnRHa.

Evaluation of Orntide microspheres in a rat animal model and correlation to in vitro release profiles

Orntide acetate, a novel luteinizing hormone-releasing hormone (LHRH) antagonist, was prepared and evaluated in vivo in 30-day and 120-day sustained delivery formulations using a rat animal model. Orntide poly(d,l-lactide-co-glycolide) (PLGA) and poly(d,l- lactide) (PLA) microspheres were prepared by a dispersion method and administered subcutaneously in a liquid vehicle to rats at 2.2 mg Orntide/kg of body weight (30-day forms) or 8.8 mg Orntide/kg (120-day forms). Serum levels of Orntide and testosterone were monitored by radioimmunoassays, and a dose-response study at 4 doses (3, 2.25, 1.5, and 1.75 mg Orntide/kg) was conducted to determine the effective dose of Orntide. Microspheres with diameters between 3.9 and 14 micron were prepared. The onset and duration of testosterone suppression varied for different microsphere formulations and were influenced both by polymer properties and by microsphere characteristics. Microspheres prepared with 50:50 and 75:25 copolymers effectively sustained peptide release for 14 to 28 days, whereas an 85:15 copolymer and the PLA microspheres extended the pharmacological response for more than 120 days. Increase in drug load generally accelerated peptide release from the microspheres, resulting in higher initial serum levels of Orntide and shorter duration of the release. In general, apparent release was faster in vivo than under in vitro conditions. Orntide microspheres effectively suppressed testosterone in rats, providing rapid onset of release and extended periods of chemical castration. Testosterone suppression occurred immediately after microsphere administration without the initial elevation seen with LHRH superagonists.

Effect of the concurrent LHRH antagonist administration with a LHRH superagonist in rats

PURPOSE

The purpose of this study was to investigate the effect of a novel LHRH antagonist, Orntide acetate, on the initial testosterone elevation in rats during treatment with a LHRH superagonist, Leuprolide acetate.

METHODS

Thirteen groups of a rat animal model were administered either liquid Orntide or Orntide PLGA microspheres before or simultaneously with Leuprolide injections. Serum levels of testosterone were monitored during the time course of the study using a radioimmunoassay method.

RESULTS

Administration of a single daily dose of liquid Orntide resulted in testosterone suppression within 6 h to levels below 0.5 ng/ml (castration level). However, combined administration of liquid Orntide and liquid Leuprolide did not have a significant effect on the initial testosterone elevation in studied rats. Similarly, there was no effect when liquid Orntide was co-administered with Leuprolide microspheres. Administration of Orntide microspheres 48 h before Leuprolide microspheres suppressed testosterone levels below the castration level within 24 h, however, did not prevent a rise in testosterone serum concentration upon administration of Leuprolide microspheres. Also, a second testosterone peak was observed between days 3 and 15 in the animals which were simultaneously treated with Orntide microspheres and Leuprolide microspheres.

CONCLUSIONS

Orntide acetate was found to be an effective LHRH antagonist with a rapid onset of pharmacological action and a short biological half-life. Administration of a single dose of liquid Orntide or Orntide microspheres, resulted in rapid testosterone suppression without an initial elevation, as seen with LHRH superagonists. However, combined administration of Orntide and Leuprolide did not have an effect on the initial testosterone elevation in rats.