(Des-Gly10,Leu6,Pro-NHEt9)-LHRH

To assess the effects of a gonadotropin-releasing hormone agonist, [D-Leu6,des-Gly10] luteinizing hormone-releasing hormone ethylamide, in ameliorating the damage caused by ionizing radiation, gonadotropin-releasing hormone agonist was administered to rats from day 22 to 37 of age in doses of 0.1, 0.4, and 1.0 microgram/day or vehicle and the rats were sacrificed on day 44 of age. There were no effects on estradiol, progesterone, luteinizing, or follicle-stimulating hormone, nor an effect on ovarian follicle numbers or development. In separate experiments, rats treated with gonadotropin-releasing hormone agonist in doses of 0.04, 0.1, 0.4, or 1.0 microgram/day were either irradiated or sham irradiated on day 30 and all groups sacrificed on day 44 of age. Irradiation produced a reduction in ovarian weight and an increase in ovarian follicular atresia. Pretreatment with the agonist prevented the reduction in ovarian weight and numbers of primordial and preantral follicles but not healthy or atretic antral follicles. Such putative radioprotection should be tested on actual reproductive performance.

Gonadotropin-releasing hormone (GnRH) receptor agonists have wide clinical applications including the treatment of prostate cancer and endocrine disorders. However, such agonists are characterized by poor pharmacokinetic properties, often requiring repeated administration or special formulations. Therefore, the development of novel peptide analogs with enhanced in vivo stability could potentially provide therapeutic alternatives. The pharmacological evaluation of a bioactive peptide [Des-Gly¹⁰,Tyr⁵(OMe),D-Leu⁶,Aze-NHEt⁹]GnRH, analog 1, is presented herein and compared with leuprolide. Peptide stability was evaluated using mouse kidney membrane preparations, followed by a liquid chromatography-tandem mass spectrometry-based approach that afforded identification and quantification of its major metabolites. The analog was significantly more stable in vitro in comparison with leuprolide. In vitro and in vivo stability results correlated well, encouraging us to develop a clinically relevant pharmacokinetic mouse model, which facilitated efficacy measurements using testosterone as a biomarker. Analog 1, an agonist of the GnRH receptor with a binding affinity in the nanomolar range, caused testosterone release in mice that was acutely dose-dependent, an effect blocked by the GnRH receptor antagonist cetrorelix. Repeated dosing studies in mice demonstrated that analog 1 was well tolerated and had potency similar to that of leuprolide, based on plasma and testis testosterone reduction and histopathological findings. Analog 1 also shared with leuprolide similar significant antiproliferative activity on androgen-dependent prostate cancer (LNCaP) cells. On the basis of pharmacokinetic advantages, we expect that analog 1 or analogs based on this new design will be therapeutically advantageous for the treatment of cancer and endocrine disorders.

The chronic administration of superactive agonists of gonadotropin releasing hormone (GnRH-A) have been reported to have a direct inhibitory effect on the sex tissues of the male rat. In an attempt to confirm or refute this statement, adult male rats were either left intact or were castrated and then treated daily for 14 days with either testosterone (T), dihydrotestosterone (DHT) or sesame oil (vehicle). Half of the intact and castrate animals also received daily injections of 200 ng of the GnRH agonist, D-Leu6, des-Gly10-GnRH ethylamide for 14 days. Twenty-four hours after completing treatment, blood levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH) and T were measured by radioimmunoassay and the ventral prostate gland (VP), seminal vesicle (SV) and penis were weighed. After 2 weeks of GnRH-A treatment, the plasma T level was reduced from 2506 +/- 170 (pg/ml +/- SEM) in the intact, nontreated animals to 907 +/- 69 in the intact, GnRH-A-treated group, indicating that the dosage of GnRH-A used in this study had an inhibiting effect on T secretion. No differences were observed in the VP, SV and penile weights between the castrate, GnRH-A and the castrate, nontreated groups. When exogenous T or DHT was given for 14 days to these castrated animals, the concomitant administration of GnRH-A did not appear to have any effect on the plasma T levels or the sex accessory tissue weights. These data suggest that GnRH-A itself does not appear to have a direct inhibitory or stimulatory effect on the sex tissues of the adult male rat.