LH-RH antagonists: state of the art and future perspectives

Population pharmacokinetic/pharmacodynamic (PK/PD) modelling of the hypothalamic-pituitary-gonadal axis following treatment with GnRH analogues

AIMS

To develop a population pharmacokinetic/pharmacodynamic (PK/PD) model of the hypothalamic-pituitary-gonadal (HPG) axis describing the changes in luteinizing hormone (LH) and testosterone concentrations following treatment with the gonadotropin-releasing hormone (GnRH) agonist triptorelin and the GnRH receptor blocker degarelix.

METHODS

Fifty-eight healthy subjects received single subcutaneous or intramuscular injections of 3.75 mg of triptorelin and 170 prostate cancer patients received multiple subcutaneous doses of degarelix of between 120 and 320 mg. All subjects were pooled for the population PK/PD data analysis. A systematic population PK/PD model-building framework using stochastic differential equations was applied to the data to identify nonlinear dynamic dependencies and to deconvolve the functional feedback interactions of the HPG axis.

RESULTS

In our final PK/PD model of the HPG axis, the half-life of LH was estimated to be 1.3 h and that of testosterone 7.69 h, which corresponds well with literature values. The estimated potency of LH with respect to testosterone secretion was 5.18 IU l(-1), with a maximal stimulation of 77.5 times basal testosterone production. The estimated maximal triptorelin stimulation of the basal LH pool release was 1330 times above basal concentrations, with a potency of 0.047 ng ml(-1). The LH pool release was decreased by a maximum of 94.2% by degarelix with an estimated potency of 1.49 ng ml(-1).

CONCLUSIONS

Our model of the HPG axis was able to account for the different dynamic responses observed after administration of both GnRH agonists and GnRH receptor blockers, suggesting that the model adequately characterizes the underlying physiology of the endocrine system.

Influence on antiproliferative activity of structural modification and conjugation of gonadotropin-releasing hormone (GnRH) analogues

The effect of various GnRH analogues, and their conjugates on proliferation, clonogenicity and cell cycle phase distribution of MCF-7 and Ishikawa human cancer cell lines was studied. GnRH-III, a sea lamprey GnRH analogue reduced cell proliferation by 35% and clonogenicity by 55%. Structural modifications either decreased, or did not alter biological activity. Conjugation of GnRH analogues including MI-1544, MI-1892, and GnRH-III with poly(N-vinylpyrrolidone-co-maleic acid) (P) through a tetrapeptide spacer GFLG(X) substantially increased the inhibitory effect of the GnRH analogues. The conjugate P-X-GnRH-III induced significant accumulation of cells in the G2/M phase; from 8% to 15.6% at 24 h and 9.8% to 15% at 48 h. It was concluded that conjugation of various GnRH analogues substantially enhanced their antiproliferative activity, strongly reduced cell clonogenicity and retarded cell progression through the cell division cycle at the G2/M phase.

Return to fertility after extended chemical castration with a GnRH antagonist

BACKGROUND

Antagonistic analogues of GnRH for the treatment of prostate cancer may be used clinically in persons for whom return to fertility after such treatment is important or desirable. The purpose of this study was, therefore, to evaluate the effects of a long term treatment with orntide, a GnRH antagonist, on testosterone levels and fertility in male rats.

METHODS

Two groups of male rats received either 120-day orntide microspheres (8.8 mg orntide/kg/120 days) or vehicle alone (control group). Serum orntide and testosterone levels in both groups were monitored at certain intervals for 9 months from the initiation of treatment. After recovery of normal serum testosterone levels in the treated animals, each rat was housed with two proven breeder, but drug-naive, females.

RESULTS

All mates of treated rats achieved pregnancy as rapidly as the mates of control rats although two of the control rats did not sire a litter with either female and one sired only one litter. The mean size of the litters of treated (12.3 offspring per litter) and control (10.6 offspring per litter) were similar. All offspring were grossly normal morphologically and behaviorally during the time to weaning.

CONCLUSIONS

These results suggest that lack of fertility due to testosterone suppression is reversible after cessation of treatment with this GnRH antagonist.

Preparation, characterization, and in vitro evaluation of 1- and 4-month controlled release orntide PLA and PLGA microspheres

PURPOSE

To prepare, characterize and evaluate in vitro sustained delivery formulations for a novel LHRH antagonist, Orntide acetate, using biodegradable microspheres (ms).

METHODS

Poly(d,l-lactide) (PLA) and poly(d,l-lactide-co-glycolide) (PLGA) were characterized for molecular weight (Mw, Mn) using gel permeation chromatography (GPC) and content of free end carboxyl groups (acid number, AN) by a titration method. 1- and 4-month Orntide ms were prepared by a dispersion/solvent extraction/evaporation process and characterized for drug content (HPLC), bulk density (tapping method), particle size (laser diffraction method), surface morphology (scanning electron microscopy, SEM), and structural integrity of encapsulated peptide by Fourier Transform Matrix Assisted Laser Desorption mass spectrometry (FT-MALDI). Peptide binding to PLA and PLGA and non-specific adsorption to blank ms was studied in 0.1 M phosphate buffer pH 7.4 (PB) and 0.1 M acetate buffer pH 4.0 (AB). In vitro release of peptide was assessed in PB and AB.

RESULTS

Mw for the PLGA copolymers varied from 10,777 to 31,281 Da and was 9,489 Da for PLA. AN was between 4.60 and 15.1 for the hydrophilic resomers and 0.72 for the hydrophobic 50:50 PLGA copolymer. Spherical ms (3.9 mu to 14 mu in diameter) with mostly nonporous surface and varying degree of internal porosity were prepared. FT-MALDI mass spectra of the extracted peptide showed that the encapsulation process did not alter its chemical structure. Peptide binding to PLGA and PLA and non-specific adsorption to blank PLGA ms were dependent upon pH and were markedly higher in PB than in AB. The initial in vitro release in PB varied from 0.5 to 26%/24 h but due to substantial binding of the peptide to the polymeric matrix the long-term release in PB could not be determined. Application of a dialysis method allowed for a more accurate determination of in vitro release and a good total drug recovery.

CONCLUSIONS

Orntide acetate was successfully incorporated into PLA and PLGA ms and the 1- and 4-month in vitro release profiles were achieved by polymer selection and optimization of the manufacturing parameters.

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.

Germ cell-specific cyclic adenosine 3',5'-monophosphate response element modulator expression in rodent and primate testis is maintained despite gonadotropin deficiency

cAMP response element modulator (CREM) is an important component of the cAMP-mediated signaling pathway and is essential for differentiation of haploid male germ cells. In the rodent, testicular expression of CREM is believed to be controlled by FSH. We studied the expression pattern of CREM and gonadotropic control in the nonhuman primate and rodent testis. Adult cynomolgus monkeys (Macaca fascicularis) received daily either vehicle or the potent GnRH antagonist (ANT) cetrorelix for periods of 25 and 56 days. Rats were also exposed to vehicle or ANT for periods of 14 and 42 days. ANT treatment suppressed pituitary gonadotropin secretion, reduced testis size, and altered spermatogenesis. A rabbit polyclonal antibody raised against recombinant CREM tau and reacting with CREM alpha, -beta, -gamma, -tau1, and -tau2 at similar affinities was used for immunocytochemistry and Western blotting. CREM expression was seen in round spermatids, with highest levels during spermatogenic stages V-VII, but declined with progression of spermatid development in the primate. Similar observations were made for the rat testis. Thus, CREM expression was maximal at the onset of acrosome formation and was low or undetectable upon initiation of spermatid elongation in both species. A weak, but specific, CREM signal was seen in mid- to late pachytene spermatocytes and during meiotic division in both species. After ANT exposure, the germ cell- and stage-specific pattern of CREM expression was quantitatively retained at all time points and in both species. Northern and Western blot analysis confirmed the maintenance of testicular CREM expression despite 25 days of ANT treatment. A retrospective immunocytochemical analysis of rat testes 14 days posthypophysectomy revealed CREM signals in round spermatids. These findings demonstrate that the testicular expression of CREM is not entirely dependent on gonadotropic hormones but, rather, on the maturational stage of haploid round germ cells.

Gonadotropin-releasing hormone analogue conjugates with strong selective antitumor activity

Conjugation of gonadotropin-releasing hormone (GnRH) analogues GnRH-III, MI-1544, and MI-1892 through lysyl side chains and a tetrapeptide spacer, Gly-Phe-Leu-Gly (X) to a copolymer, poly(N-vinylpyrrolidone-co-maleic acid) (P) caused increased antiproliferative activity toward MCF-7 and MDA-MB-231 breast, PC3 and LNCaP prostate, and Ishikawa endometrial cancer cell lines in culture and against tumor development by xenografts of the breast cancer cells in immunodeficient mice. MCF-7 cells treated with P-X-1544 and P-X-1892 displayed characteristic signs of apoptosis, including vacuoles in the cytoplasm, rounding up, apoptotic bodies, bleb formation, and DNA fragmentation. Conjugates, but not free peptides, inhibited cdc25 phosphatase and caused accumulation of Ishikawa and PC3 cells in the G2/M phase of the cell cycle after 24 h at lower doses and in the G1 and G2 phases after 48 h. Since P-X-peptides appear to be internalized, the increased cytotoxicity of the conjugates is attributed to protection of peptides from proteolysis, enhanced interaction of the peptides with the GnRH receptors, and/or internalization of P-X-peptide receptor complexes so that P can exert toxic effects inside, possibly by inhibiting enzymes involved in the cell cycle. The additional specificity of P-X-peptides compared with free peptides for direct antiproliferative effects on the cancer cells but not for interactions in the pituitary indicates the therapeutic potential of the conjugates.

Pharmacological studies with cetrorelix (SB-75), a potent antagonist of luteinising hormone-releasing hormone

The antitumour and hormone-suppressive effects of the luteinising hormone-releasing hormone LH-RH antagonist Cetrorelix (D-20761) and its pamoate salt (D-20762) were investigated in the model of the DMBA-induced mammary carcinoma of female rats and by testosterone determinations in normal male rats. Treatment with single high doses of D-20761 induced a rapid decrease of tumour weights with a dose-dependent duration of action. Strong antitumour effects were also observed by applying different multiple dose schedules, including a initial high dose (3.16 mg/kg, s.c.) followed by a low maintenance dose (31.6 micrograms/kg, s.c.). The stability of the molecule against degrading enzymes led to the idea of using the poorly soluble pamoate salt for facilitating a sustained release of active compound. This salt indeed induced a prolonged suppression of tumour growth and of testosterone levels. In conclusion, we found that Cetrorelix is a highly effective LH-RH antagonist which should be further developed for the treatment of hormone-dependent diseases.

The future of antihormone therapy: innovations based on an established principle

Endocrine therapy of mammary and prostate cancer has been established for decades. The therapies available to block sex-hormone-receptor-mediated tumor growth are based on two principles: (i) ligand depletion, which can be achieved surgically, by use of luteinizing-hormone-releasing hormone analogues or inhibitors of enzymes involved in steroid biosynthesis or by interfering with the feedback mechanisms of sex hormone synthesis at the pituitary/hypothalamic level; (ii) blockade of sex hormone receptor function by use of antihormones. The antiestrogen tamoxifen, which is the compound of choice for the treatment of mammary carcinoma, has the drawback of being a partial agonist. A complete blockade of estrogen receptor (ER) function can be achieved by a new class of compounds, pure antiestrogens. In contrast to aromatase inhibitors, pure antiestrogens are able to block ER activation by ligands other than estradiol and can also interfere with ligand-independent ER activation. In addition to estradiol, progesterone has a strong proliferative effect in mammary carcinomas. Antiprogestins are promising new tools for clinical breast cancer therapy. These compounds clearly need a functionally expressed progesterone receptor to block tumor growth, but there is strong experimental evidence that their tumor inhibition is based on more than just progesterone antagonism. The ability of these compounds to induce tumor cell differentiation that leads to apoptosis is unique among all other endocrine therapeutics. In prostate tumors that have relapsed from current androgen-ablation therapies the androgen receptor (AR) is still expressed and, compared to the primary tumors, its level is often even enhanced. Mutated AR that can be activated by other compounds such as adrenal steroids, estrogens, progestins and even antiandrogens have been detected in recurrent tumors. Thus, relapse of tumors under the selective pressure of common androgen-ablation therapies can be caused by acquired androgen hypersensitivity and AR activation by ligands other than (dihydro-)testosterone. There is a clinical need for future compounds that produce a complete blockade of AR activity even in recurrent tumors. Preclinical experiments indicate that combination therapy as well as the extension of endocrine treatments to several other tumor entities are promising approaches for further developments. Examples are the combination of antiestrogens and antiprogestins for breast cancer treatment, or the treatment of prostate carcinomas with antiprogestins.

Gonadotrophin control of testicular germ cell development

Successful and complete male germ cell development is dependent on the balanced, endocrine interplay of the hypothalamus, the pituitary and the testis. The hypothalamus secretes gonadotrophin-releasing hormone in a pulsatile manner which, in turn, elicits the pulsatile release of the gonadotrophins LH and FSH from the pituitary. Luteinizing hormone stimulates spermatogenesis indirectly via testosterone, whereas FSH acts directly on the seminiferous tubules. The synthesis and release of gonadotrophic hormones is under the feedback control of testosterone. Whether other testicular peptides such as inhibin and activin are also involved is not yet clear. Luteinizing hormone/testosterone and FSH are the prime regulators of germ cell development. On their own, these hormones are capable of exerting clear-cut stimulatory effects on the spermatogenic process. However, the quantitative production of spermatozoa generally requires the presence of both LH/testosterone and FSH. Since receptors for androgens and FSH are confined to the somatic cells of the testis, the trophic effects of these hormones on germ cells must be indirect. However, it is not known as yet precisely which genes/factors mediate the beneficial effects of androgens and FSH on spermatogenesis. The gonadotrophic hormones have been found in a number of isoforms and multiple transcripts of the LH and FSH receptor have been detected. Therefore, the possibility must be considered that certain forms of male infertility could be due to dysfunctional hormones and/or mutated receptors.

Comparison of the antigonadotropic activity of three GnRH antagonists (Nal-Glu, Antide and Cetrorelix) in a non-human primate model (Macaca fascicularis)

We compared the antigonadotropic activity of the GnRH antagonists Nal-Glu, Antide and Cetrorelix in a non-human primate model (Macaca fascicularis). Orchidectomized animals received a single subcutaneous injection at doses of 250 micrograms kg-1 (n = 4), 625 micrograms kg-1 (n = 4) and 1250 micrograms kg-1 (n = 3) of the compounds Nal-Glu ([Ac-D-Nal(2)1, D-4-Cl-Phe2, D-Pal3, Arg5, D-Glu(AA)6, D-Ala10]-GnRH), Antide (Nal-Lys, [Ac-D-Nal1, D-4-Cl-Phe2, D-Pal3, Nic-Lys5, D-Nic-Lys6, Ip-Lys8, D-Ala10]-GnRH) or Cetrorelix ([Ac-D-Nal1, D-4-Cl-Phe2, D-Pal3, D-Cit6, D-Ala10]-GnRH). Blood samples were collected before and 3, 6, 12, 24, 48, 72, and 96 h after GnRH antagonist administration. Serum was analysed for concentrations of bioactive LH and immunoactive LH and FSH. All three compounds decreased LH secretion within 3-12 h (P < 0.05) and FSH secretion within 12-48 h (P < 0.05) after injection. Major differences between the GnRH antagonists were observed with regard to the effective dose and duration of action. At a dose of 250 micrograms kg-1 Nal-Glu and Antide only transiently suppressed LH and FSH release, whereas Cetrorelix induced complete inhibition (P < 0.05) which lasted for the entire observation period. At a dose of 625 micrograms kg-1 Cetrorelix exhibited the longest duration of action and Nal-Glu the shortest. At the highest dose of 1250 micrograms kg-1 Nal-Glu, Antide and Cetrorelix markedly inhibited LH and FSH secretion throughout the entire study period.(ABSTRACT TRUNCATED AT 250 WORDS)