Comparison of biological effects of a sustained delivery system and nonencapsulated LH-RH antagonist SB-75 in rats

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.

Systemic delivery of the GnRH antagonist cetrorelix by intratracheal instillation in anesthetized rats

Pulmonary absorption of the decapeptide cetrorelix acetate was studied in rats by a non-surgical intratracheal instillation method. The pharmacological effect (decrease of testosterone plasma concentration) following intratracheal (i.t.) instillation was determined in four groups of seven rats each at three different concentrations (0.5, 1.0 and 2.5 mg/kg body weight). The applied doses reduced testosterone plasma concentration to subnormal level (</=1 ng/ml), for 24, 34 and 72 h, respectively. Pharmacokinetic data of cetrorelix were determined in two additional groups of ten and nine rats, respectively, at doses of 0.5 and 1 mg/kg body. After i.t. administration the mean terminal t(1/2) was 12.94+/-1.74 (0.5 mg/kg) and 13.03+/-3.15 h (1 mg/kg); mean residence time (MRT) was 6. 85+/-3.01 and 8.72+/-2.33 h; the C(max) (277.72+/-252.11 and 274. 23+/-113.49 ng/ml) were observed in the first or the second plasma sample, suggesting that the drug was rapidly absorbed (t=1 and 2 h). Comparing the plasma concentration after i.t. administration with data after i.v. administration from a previous study undertaken in the same laboratory, the mean i.t. bioavailability was calculated as 75.80+/-45.42 and 58.97+/-18.25%. The data from the group of 0.5 mg/kg were confirmed in a subsequent experiment. Our studies show that intratracheal instillation via the adopted method of non-surgical cannulation provides reproducible results. In addition, we demonstrated that pharmacologically active amounts of cetrorelix were absorbed from the lungs.

Luteal phase and clinical outcome after human menopausal gonadotrophin/gonadotrophin releasing hormone antagonist treatment for ovarian stimulation in in-vitro fertilization/intracytoplasmic sperm injection cycles

The luteal phase hormonal profile and the clinical outcome of 69 patients undergoing in-vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) after ovarian stimulation with human menopausal gonadotrophin (HMG) and the gonadotrophin-releasing hormone (GnRH) antagonist Cetrorelix were analysed. Twenty-four patients received Cetrorelix 0.5 mg (group I) while in 45 patients Cetrorelix 0.25 mg was administered (group II). Human chorionic gonadotrophin (HCG) was used as luteal support. Nine clinical pregnancies were obtained in group I (37.5%) and 12 in group II (26. 6%). These results were not significantly different. Serum progesterone and oestradiol concentrations did not differ between the two groups either in pregnant or non-pregnant patients. An expected decrease of the same hormones was observed 8 days after the pre-ovulatory HCG injection in non-pregnant women. With regard to serum luteinizing hormone concentrations, a decrease was observed 2 days after the pre-ovulatory HCG injection and was maintained at almost undetectable levels throughout the entire luteal phase in both conception and non-conception cycles of group I and group II. This study demonstrates that different doses of GnRH antagonist do not have any impact on the luteal phase of IVF/ICSI cycles when hormonal support is given.

Autocrine regulation of gonadotropin-releasing hormone secretion in cultured hypothalamic neurons

Episodic hormone secretion is a characteristic feature of the hypothalamo-pituitary-gonadal system, in which the profile of gonadotropin release from pituitary gonadotrophs reflects the pulsatile secretory activity of GnRH-producing neurons in the hypothalamus. Pulsatile release of GnRH is also evident in vitro during perifusion of immortalized GnRH neurons (GT1-7 cells) and cultured fetal hypothalamic cells, which continue to produce bioactive GnRH for up to 2 months. Such cultures, as well as hypothalamic tissue from adult rats, express GnRH receptors as evidenced by the presence of high-affinity GnRH binding sites and GnRH receptor transcripts. Furthermore, individual GnRH neurons coexpress GnRH and GnRH receptors as revealed by double immunostaining of hypothalamic cultures. In static cultures of hypothalamic neurons and GT1-7 cells, treatment with the GnRH receptor antagonist, [D-pGlu1, D-Phe2, D-Trp(3,6)]GnRH caused a prominent increase in GnRH release. In perifused hypothalamic cells and GT1-7 cells, treatment with the GnRH receptor agonist, des-Gly10-[D-Ala6]GnRH N-ethylamide, reduced the frequency and increased the amplitude of pulsatile GnRH release, as previously observed in GT1-7 cells. In contrast, exposure to the GnRH antagonist analogs abolished pulsatile secretion and caused a sustained and progressive increase in GnRH release. These findings have demonstrated that GnRH receptors are expressed in hypothalamic GnRH neurons, and that receptor activation is required for pulsatile GnRH release in vitro. The effects of GnRH agonist and antagonist analogs on neuropeptide release are consistent with the operation of an ultrashort-loop autocrine feedback mechanism that exerts both positive and negative actions that are necessary for the integrated control of GnRH secretion from the hypothalamus.

Rational use of agonists and antagonists of luteinizing hormone-releasing hormone (LH-RH) in the treatment of hormone-sensitive neoplasms and gynaecologic conditions

Analogues of luteinizing hormone-releasing hormone (LH-RH) have made possible new approaches to the treatment of some hormone-dependent cancers and diseases and conditions which result from inappropriate sex hormone levels. In the fields of both gynaecology and oncology, the development of sustained delivery depot systems has played a key role in the clinical use of LH-RH agonists and will be also essential for the LH-RH antagonists. Clinical results show that therapy with agonists of LH-RH is the preferred method of treatment for men with advanced prostate cancer. For prostate cancer and other indications, the new LH-RH antagonists such as Cetrorelix may offer an advantage based on the fact that they inhibit LH, FSH and sex-steroid secretion from the start of the administration and thus reduce the time of the onset of therapeutic effects. The use of antagonists would avoid the temporary clinical "flare-up" of the disease which can occur with the agonists in men with prostate cancer. The rapid shrinkage of the prostate and improvement in urinary symptoms obtained with Cetrorelix in men with benign prostatic hyperplasia (BHP) suggests that LH-RH antagonists offer a therapeutic alternative in patients who are considered poor surgical risks. Various experimental and clinical studies suggest that analogues of LH-RH might be useful for treatment of premenopausal women with oestrogen-dependent breast cancer. LH-RH antagonists such as Cetrorelix could be also considered for hormonal therapy of epithelial ovarian cancer which responds only marginally to the agonists, and for treatment of endometrial cancer. Many investigators have reported beneficial effects of LH-RH agonists in the treatment of patients with leiomyomas. LH-RH antagonists also appear to be promising for therapy of uterine leiomyomas, and in addition might be useful for treatment of endometriosis and polycystic ovarian disease (PCOD). LH-RH agonists have been employed in in vitro fertilization and embryo transfer (IVF-ET) programs to prevent a premature rise in LH and various results suggest that the use of antagonist Cetrorelix in assisted reproduction procedures, could be even more advantageous. For most of these indications, the use of sustained release depot preparations will be required.

Down-regulation of pituitary receptors for luteinizing hormone-releasing hormone (LH-RH) in rats by LH-RH antagonist Cetrorelix

Antagonists of luteinizing hormone-releasing hormone (LH-RH), unlike the LH-RH agonists, suppress gonadotropins and sex steroid secretion immediately after administration, without initial stimulatory effects. [Ac-D-Nal(2)1,D-Ph(4Cl)2,D-Pal(3)3,D-Cit6,D-Ala10]LH-R H (SB-75; Cetrorelix) is a modern, potent antagonistic analog of LH-RH. In this study, the binding characteristics of receptors for LH-RH in membrane fractions from rat anterior pituitaries were investigated after a single injection of Cetrorelix at a dose of 100 microg per rat. To determine whether the treatment with Cetrorelix can affect the concentration of measurable LH-RH binding sites, we applied an in vitro method to desaturate LH-RH receptors by chaotropic agents such as manganous chloride (MnCl2) and ammonium thiocyanate (NH4SCN). Our results show that the percentages of occupied LH-RH receptors at 1, 3, and 6 h after administration of Cetrorelix were approximately 28%, 14%, and 10%, respectively, of total receptors. At later time intervals, we could not detect occupied LH-RH binding sites. Ligand competition assays, following in vitro desaturation, demonstrated that rat pituitary LH-RH receptors were significantly (P < 0.01) down-regulated for at least 72 h after administration of Cetrorelix. The lowest receptor concentration was found 3-6 h after Cetrorelix treatment and a recovery in receptor number began within approximately 24 h. The down-regulation of LH-RH binding sites induced by Cetrorelix was accompanied by serum LH and testosterone suppression. Higher LH-RH receptor concentrations coincided with elevated serum hormone levels at later time intervals. Our results indicate that administration of LH-RH antagonist Cetrorelix produces a marked down-regulation of pituitary receptors for LH-RH and not merely an occupancy of binding sites.

Evaluation of the in vitro and in vivo activity of the L-, D,L- and D-Cit6 forms of the LH-RH antagonist Cetrorelix (SB-75)

The objective of this study was to examine the in vivo and in vitro gonadotropin-inhibiting potencies, edematogenic activities and the receptor binding affinities of the D-Cit6, D,L-Cit6 and L-Cit6 forms of the LH-RH antagonist Cetrorelix (SB-75) [Ac-D-Nal(2)1,D-Phe(4Cl)2,D-Pal(3)3,D-Cit6,D-Ala10]LH- RH. In order to demonstrate the suppressive effects of two different diastereomers of SB-75 and their racemic mixture on LH and FSH release, [D-Cit6] SB-75 was injected subcutaneously in doses of 2.5 and 10 micrograms/rat, [D,L-Cit6]-SB-75 in doses of 5 and 20 micrograms/rat and [L-Cit6] SB-75 in doses of 12.5 and 50 micrograms/rat to castrated male rats. Two hours after administration, there was no difference in LH levels between rats injected with the L-form and control animals, indicating a low activity and/or a rapid enzymatic degradation of this peptide. The (1:1) diastereomeric mixture was only about half as potent in suppression of LH release compared to [D-Cit6] SB-75. Serum FSH levels were suppressed significantly (p < 0.01) for more than 48 h after the administration of 10 micrograms [D-Cit6] SB-75 and 20 micrograms of [D,L-Cit6] SB-75, respectively. [D-Cit6] SB-75 administered at a dose of 2 micrograms/rat induced 100% inhibition of ovulation, while 4 micrograms/rat of the D,L-Cit6 peptide were necessary to produce the same effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of bombesin/gastrin-releasing peptide antagonist RC-3095 and luteinizing hormone-releasing hormone antagonist SB-75 on the growth of MCF-7 MIII human breast cancer xenografts in athymic nude mice

BACKGROUND

The results of several clinical trials using various luteinizing hormone-releasing hormone agonists for treatment of advanced breast cancer are encouraging. However, only about 30% of breast cancers are estrogen-dependent and can be treated by hormonal manipulation. New therapeutic approaches combining estrogen ablation therapy with other compounds must be explored. Various studies suggest that bombesin or gastrin-releasing peptide acts as an autocrine growth factor and may play a role in the initiation and progression of some cancers, including that of the breast.

METHODS

Female athymic nude mice bearing xenografts of the MCF-7 MIII human breast cancer cell line were treated for 7 weeks with bombesin/gastrin-releasing peptide antagonist (D-Tpi6, Leu13 psi[CH2NH]-Leu14) bombesin(6-14) (RC-3095) injected subcutaneously daily at a dose of 20 micrograms and luteinizing hormone-releasing hormone antagonist SB-75 (Cetrorelix) administered biweekly in the form of microgranules releasing 45 micrograms/day.

RESULTS

After 2 weeks of treatment, a significant inhibition of tumor volume was observed in the groups treated with RC-3095 alone or in combination with SB-75 but not in those treated with SB-75 as a single agent. After 7 weeks, tumor growth as measured by tumor volume and percentage changes in tumor volume and tumor weight was greatly inhibited in all of the treated groups. Uterine and ovarian weights were reduced and serum luteinizing hormone levels decreased by administration of SB-75 alone or in combination with RC-3095. Histologically, a significant decrease in argyrophilic nucleolar organizer region count in tumor cell nuclei was observed in all of the treated groups, indicating a lower proliferation of these cells. High-affinity binding sites for bombesin were detected in cultured MCF-7 MIII cells. Chronic treatment with RC-3095 caused a significant down-regulation of epidermal growth factor receptors in tumor cell membranes, which might be related to tumor inhibition. In studies in vitro, SB-75 inhibited proliferation of MCF-7 cells in culture but not proliferation of MCF-7 MIII cells.

CONCLUSIONS

Because previously we demonstrated that RC-3095 inhibits the proliferation of MCF-7 MIII cells in vitro, it appears that the major antitumoral effect of RC-3095 on the MCF-7 MIII cancer line is direct, whereas that of SB-75 is indirect, and that it is mediated by suppression of the pituitary-gonadal axis. In view of its immediate and powerful inhibitory effect on MCF-7 MIII tumors, bombesin/gastrin-releasing peptide antagonist RC-3095 might be considered as a possible new agent for the treatment of breast cancer.