Metal ion-induced conformational changes of phosphorylated fragments of human neurofilament (NF-M) protein.

The NF-M subunit of human neurofilaments has a C-terminal repeating 13-mer sequence. The 13-mer (Lys-Ser-Pro-Val-Pro-Lys-Ser-Pro-Val-Glu-Glu-Lys-Gly) (NF-M13) and 17-mer (Glu-Glu-Lys-Gly)-(NF-M13) sequences were synthesized, as were both the mono- and diphosphorylated Ser species. Circular dichroism (c.d.) studies and c.d. titrations with Al3+ and Ca2+ were performed. The conformation of the phosphorylated and unphosphorylated material was random in water. Deconvolution of the c.d. spectra, in trifluoroethanol, of the untitrated samples yielded a high content of unordered structure, similar to the poly-L-proline II structure. Titration of the phosphorylated species with Al3+ or Ca2+ caused a surprising conformational change to occur, yielding a high content of beta-pleated sheet structure. A mechanism of metal binding to the phosphofragments is proposed which may be relevant to the formation of neurofibrillary tangles in Alzheimer's disease.

Epidermal growth factor increases 32P incorporation into phosphatidylcholine and protein kinase C activity in colon carcinoma cell line (HT29).

There are conflicting data about the effect of the epidermal growth factor (EGF) on protein kinase C (PKC) enzyme activity. The aim of our study was to find out which type of phospholipids [phosphatidylinositol 4,5-bisphosphate PI4,5P2 or the other phospholipids-phosphatidylcholine (PC) or phosphatidic acid (PA)] could be the source of 1,2-diacylglycerol (1,2-DAG) in PKC activation. In colon carcinoma cells (HT29) we observed a more than 2-fold increase in the PC pool and at the same time decreased tyrosine kinase activity (50%). With increasing incubation time EGF affects the pools of both phosphatidylinositols and other phospholipids parallel with the activation of the tyrosine kinase activity. EGF increases the activity of PKC in the HT29 cell line and PC could be the source of 1,2-DAG which may stimulate PKC activity.

Phosphatidylcholine could be the source of 1,2-DAG which activates protein kinase C in EGF-stimulated colon carcinoma cells (HT29).

In our previous study (A. Balogh et al, Cell. Signalling 5 (6), 795-802, 1993.), we have shown that epidermal growth factor (EGF) increased protein kinase C (PKC) activities in colon carcinoma cell line (HT29), possibly through the increased 1,2-diacylglycerol (1,2-DAG) production via phosphatidylcholine (PC). Here we investigate the effect of well-known PKC activator 12-O-tetradecanoyl-2 phorbol-13-acetate (TPA), on the levels of 32P incorporation into EGF induced phosphatidylinositols (PI, PI4P, PI4, 5P2) and different phospholipids (PC, PA, PS) as well as on induced tyrosine kinase activity. TPA significantly decreased the effects of EGF and it had the biggest inhibitory effect on EGF induced PC level. These data support our contention that PC plays an important role in the activation of PKC via 1,2-DAG production in the EGF stimulated pathway.

Synthesis of gonadotropin-releasing hormone III analogs. Structure-antitumor activity relationships.

Following the observation that the activity of gonadotropin-releasing hormone III (GnRH-III) in the suppression of growth of MDA-MB-231 and MCF-7 breast cancer cells surpasses that of GnRH and other analogs thereof, analogs of GnRH-III were synthesized to investigate the structural basis for the improved antitumor activity. Compounds synthesized include analogs with changes in the central sequence in which GnRH-III differs from GnRH and in the C- and N-terminal regions. The results indicate that a salt bridge between Asp6 and Lys8 stabilizes the active conformation of GnRH-III and show the importance of the Trp7. Replacement of the C-terminal Gly-NH2 with D-Ala-NH2 was not well tolerated, but replacement with ethylamide was. Replacement of pGlu1 with Ac-D-Trp appears to have a significantly deleterious effect on a unique conformation of GnRH-III which is responsible for its binding to the receptors on cancer cell lines and the resultant antitumor activity.

Desensitization of luteinizing hormone release in cultured pituitary cells by gonadotropin-releasing hormone.

Preincubation of cultured pituitary cells with GnRH caused a marked decrease in subsequent LH release. The rate of desensitization increased when the preincubating concentration of GnRH and the preincubation time were increased. Pituitary cells obtained from male rats were not as sensitive to GnRH as cells obtained from female rats and the extent of desensitization was also smaller in cells from male rats. Densensitization was found to be a long-lasting effects, without any change in the viability of the cells. A superactive analogue of GnRH (D-Phe6-GnRH) caused almost complete desensitization of LH secretion, while a competitive inhibitory analogue of GnRH caused a much smaller decrease in LH response which could be overcome by increasing the concentration of GnRH used for reincubation. These data suggest that the desensitization is closely related to the biological activity of GnRH and does not correlate with receptor binding. High concentrations of potassium also induced desensitization, although to a lower extent than GnRH. Since K+ induces LH release by a different mechanism than GnRH, our data suggest that the desensitization phenomenon cannot be explained only at the receptor level. The time curve of desensitization supports the idea that GnRH action has two-phases: an acute effect which cannot be desensitized, and a secondary phase which can be densensitized.

Antiovulatory doses of antagonists of LH-RH inhibit LH and progesterone but not FSH and estradiol release.

The differential regulation of immunoactive FSH and LH secretion by endogenous LH-RH was studied using LH-RH antagonists (Ac-D-Trp1,2, D-Cpa2, D-Lys6, D-Ala10LH-RH (MI-1544) and (Ac-D-Nal1, D-Phe(pCl2), D- Trp3, D-Cit6, D-Ala10LH-RH (SB-030) in ovariectomized (OVX) and regularly cycling rats. Single injections of 10 micrograms and 100 micrograms doses and long-term treatment with 10 micrograms doses of MI-1544 were used in OVX animals. Serum and pituitary LH and FSH, as well as serum estradiol and progesterone was determined by RIA during and/or after the treatment. Single injections of MI-1544 in OVX animals caused prompt (in 2 h) and long-lasting (for more than 24 h) suppression of the serum LH, while no or late decrease (after more than 6 h) of the serum FSH. Long-term treatment with the same analog decreased the serum LH (by 50%) and moderately increased the pituitary LH (by 21%) but did not change the serum and the pituitary FSH concentrations. In normal rats, long-term treatment with both of our analogs also resulted in divergent alterations in the LH and FSH concentrations. Serum LH dropped to undetectable levels,while serum FSH did not change significantly. Pituitary LH increased (by 31 to 41%), while FSH decreased (by 27 to 38%). Marked depression was found in the serum progesterone (by 64%) but no significant change in the serum estradiol levels, after the long-term treatment for 21 days. The ovarian cycles were interrupted, and no ovulation appeared during the treatment. Significant decrease was detectable in the weight of the ovaries (by 46%), whereas the weight of the uteri did not change or slightly elevated (by 22%), after the treatment with SB-030 or MI-1544, respectively.

Desensitization of pituitary cells by gonadotrophin-releasing hormone or its analogues in the superfusion system: different pattern for males and females.

The effect of a 6-h infusion of gonadotrophin-releasing hormone (GnRH) or its analogues on dispersed anterior pituitary cells from male or female rats was investigated. The cells were stimulated with 3-min pulses of K(+) and GnRH. Thereafter GnRH (1 nM) or GnRH analogues ([D-Trp(6) ]GnRH-ethylamide ([D-Trp(6) ]GnRH, 50 pM), [D-Phe(6) , Gln(8) ]GnRH-ethylamide (Folligen, 100 pM) and [Asu(6) ]GnRH-ethylamide ([Asu(6) ]GnRH, 33 pM)) were applied for 6 h. In cells from female rats this treatment resulted in a 20-fold increase in luteinizing hormone (LH) secretion during the first 90-min period of the 6-h incubation. Following this a gradual decrease in LH release occurred, and during the fourth 90-min period the amount of LH secreted was only one-third or less of the initial value. The pituitary cells of male rats responded to the same treatment with only a 7-fold rise of LH secretion during the first period. In the second 90-min of the 6-h incubation a 20% to 30% increase was observed. Even in the fourth 90-min period the amount of LH secreted was two-thirds or more greater than that of the first 90-min period. When using 10-fold greater concentrations of the same peptides in males, the increase in hormone secretion in the second 90-min was not seen and the hormone release decreased to around 50%. We found definite differences in the responses of male and female rat pituitary cells to the 6-h infusion of GnRH or its analogues: the initial amplitude of the response in females was higher but desensitization was stronger. In males, the initial response was weaker; however, even using doses one magnitude greater, the level of desensitization did not reach the values obtained in females. The results were similar both with GnRH and the analogues. The responses to 3-min K(+) and GnRH stimuli given after the 6-h incubation were strongly reduced in cells from female rats compared to the initial responses; however, in cells from male rats the reaction was higher or unchanged. The ratio of LH released by the final K(+) stimulus relative to the actual LH content of the cells decreased in females but increased in males. Our data show that the differences between the pattern of desensitization in cells from male and female rats may be caused by the differences in the amount and ratio of immediately releasable hormone and the hormone replenishment into these pools.

Lamprey gonadotropin hormone-releasing hormone-III has no selective follicle-stimulating hormone-releasing effect in rats.

Lamprey gonadotropin releasing-hormone (LGnRH)-III, a hypothalamic neurohormone recently isolated from sea lamprey, was reported to have a selective stimulatory effect on follicle-stimulating hormone (FSH) release in rats and suggested to be the mammalian FSH-releasing factor. In this study, we determined the relative luteinizing hormone (LH)- and FSH-releasing potency of LGnRH-III compared to mammalian gonadotropin-releasing hormone (LHRH) in normal female rats, ovariectomized (OVX) and oestrogen/progesterone substituted rats and the superfused rat-pituitary cell system. The specificity of LGnRH-III for the mammalian LHRH receptor was investigated by blocking the receptor with an LHRH antagonist, MI-1544. In vitro, LGnRH-III dose-dependently stimulated both LH and FSH secretion from rat pituitary cells at 10(-7) to 10(-5) M concentrations, while LHRH stimulated gonadotropin secretion at a 1000-fold lower doses (10(-10) to 10(-8) M). The difference between its LH- and FSH-releasing potency was similar to that of LHRH. LGnRH-III bound to high affinity binding sites on rat pituitary cells with a Kd of 6.7 nM, B(max)=113 +/- 27 fmol/mg protein. In vivo, LGnRH-III also stimulated both LH and FSH secretion in a dose-dependent manner and, similar to LHRH, induced a greater rise in the serum LH than the FSH level. In normal cycling rats, it showed 180-650-fold weaker potency than LHRH in stimulating LH secretion and 70-80-fold weaker effect in stimulating FSH secretion. In OVX rats, LGnRH-III demonstrated a similarly weak effect on both gonadotropins. It was found to be 40-210-fold less potent than LHRH regarding LH release and 50-160-fold weaker regarding FSH release. LHRH-receptor antagonist MI-1544 prevented both the LH- and the FSH-releasing effect of LGnRH-III both in vitro and in vivo. These results do not support the hypothesis that LGnRH-III might be the mammalian FSH-releasing factor but demonstrate that it is a weak agonist for the pituitary LHRH receptor and stimulates both gonadotropins in a dose-dependent fashion.

Effects of continuous and repetitive administration of a potent analog of GH-RH(1-30)-NH2 on the GH release in rats treated with monosodium glutamate.

To assess the efficacy of a potent GH-releasing hormone (GH-RH) analog (D-Ala2,Nle27,Gaba30-GH-RH-(1-30)-amide) in the treatment of GH deficiency, we investigated the effects of chronic administration of this analog (A-495) on growth responses in monosodium glutamate (MSG)-lesioned rats. Basal serum GH concentrations, GH responses to bolus injections of GH-RH, as well as acceleration of body gain and linear growth were compared after long-term continuous and repetitive administration of A-495. The effects of continuous and repetitive administration of the analog on GH responses in vitro were also compared using the superfused pituitary cell system method. Treatment with MSG reduced the body weight and linear growth of the animals (-22% and -11%, respectively), the basal serum GH concentration (-66%), and the GH-RH-induced absolute GH responses (-61%) but did not alter the relative GH responses (to basal GH concentrations). Repetitive administration of 10 micrograms daily doses of A-495 at 24 h intervals for 2 weeks highly increased the GH responsiveness to GH-RH and induced catch-up growth, by which MSG-treated animals achieved the growth rate of normal controls. However, basal serum GH concentrations were only modestly enhanced. Continuous infusion of A-495 at the same daily dose resulted in slight increases in the GH-RH-induced GH rises, moderate acceleration of body gain, and no change in linear growth. Basal serum GH concentrations were not significantly influenced by this treatment. These results demonstrate that exogenous GH-RH pulses administered at lower frequency than the frequency of the physiological GH secretion are able to fully restore the normal growth rate of the GH deficient rats. The effectivity of the treatment is rather dependent on the magnitude of GH rises than the basal GH level. Although continuous administrations of the GH-RH is also have some effect on the body gain, repetitive administration is more effective at the same daily dose. Our results from in vitro experiments show that, in addition to the low magnitude of the GH-RH-stimulated GH rises, desensitization of the GH secretory response might also be accounted for the low effectivity of the continuously administered GH-RH. Present results demonstrate the therapeutic usefulness of our new GH-RH analog and are the first to evidence that GH-RH need not be administered as frequently as the appearance of the endogenous GH pulses to restore the normal growth of the GH deficient rats.

Antitumour effect of a gonadotropin-releasing-hormone antagonist (MI-1544) and its conjugate on human breast cancer cells and their xenografts.

Our gonadotropin-releasing hormone (GnRH) antagonist analogue MI-1544 ([Ac-D-Trp1,3,D-Cpa2,D-Lys6,D-Ala10]GnRH) was developed as a potential contraceptive material, because it decreased the luteinizing hormone level without unfavourable side-effects. The antagonist was covalently bound to poly[Lys-(Ac-Glu0.96-DL-Ala3.1)] (AcEAK)-a branched polypeptide having a polylysine backbone--resulting in a MI-1544-AcEAK conjugate. According to our in vitro experiments the MI-1544 induced a 33%-35% decrease in cell numbers of MCF-7 and MDA-MB-231 human breast cancer cell lines at a dose of 30 microM. The biodegradable polymeric carrier, AcEAK, alone inhibited cell proliferation by only 13%-15%, while the MI-1544-AcEAK conjugate, applied at the same dose, was capable of producing 45%-50% inhibition of cell proliferation. Our in vivo experiments using immunosuppressed mice showed that MI-1544, applied twice daily s.c., inhibited the growth of oestrogensensitive and -insensitive xenografts by 65% and 30% respectively. This effect was potentiated (70%) in both types of xenografts by the presence of the polymeric carrier in the conjugate; however, the carrier by itself did not cause tumour growth inhibition. The polymeric polypeptide carrier is supposed to increase the stability of the GnRH antagonist and to prevent the rapid excretion of the covalently bound peptide molecule. The antagonist and its conjugate may have various direct and indirect effects on breast cancer cells and, as a consequence, the new GnRH antagonist conjugates are suitable for treating an extended range of breast cancers.

Influence of luteinizing hormone-releasing hormone agonists on human mammary carcinoma cell lines and their xenografts.

The specific binding of luteinizing hormone-releasing hormone (LH-RH) agonist in estradiol-dependent MCF-7 and estradiol-independent MDA-MB-231 human breast cancer cells has been studied using [3H]Ovurelin [(D-3H-Phe6),des-Gly10-LH-RH- ethylamide]. The results of Scatchard analyses suggest the presence of a single class of receptor sites, both in cell suspensions and membrane fractions. Evaluation of these peptide receptors appears to reflect additional characteristics of biological behaviour of these human breast cancer cells. The synthetic LH-RH agonist Ovurelin [(D-Phe6),des-Gly10-LH-RH-ethylamide] can directly interfere (25-30%) with the proliferation of MDA-MB-231 human breast cancer cells in culture. The inhibitory effect of Ovurelin in vitro was negligible in the MCF-7 cell line. In the in vivo experiments the treated immunosuppressed mice bearing either MCF-7 or MDA-MB-231 xenografts responded to the high-dose LH-RH analogue Zoladex depot and Decapeptyl depot therapy. Since the MDA-MB-231 tumour was found to be ER-negative it seems possible that the regression of this xenograft results from the direct antitumor action of the LH-RH agonist.

Novel antitumor peptide hormones and their effect on signal transduction.

A series of novel gonadotropin releasing hormone (GnRH) and Somatostatin analogs have been developed in our laboratory and were screened for antiproliferative and signal transduction inhibitory effect. Our GnRH analog Folligen, had significant antitumor activity on DMBA induced mammary carcinomas in rats without blocking ovarian functions. The direct effect of Folligen and Buserelin has been compared on the human breast cancer cell line MDA-MB-231. Folligen was found to be more effective in inhibiting cell proliferation and significant differences were found in the signal transduction pathways activated by these analogs. Our novel Somatostatin analogs were screened for tyrosine kinase inhibition and for antiproliferative effect on human colon tumor cells and for growth hormone (GH) release inhibition in vitro and in vivo. The analog TT-2-50 was significantly more active inhibiting GH release in superfused rat pituitary cells and in vivo than native Somatostatin and it strongly inhibited tyrosine kinase and proliferation while it stimulated protein kinase C activity.

Inhibition of LH-release by synthetic analogues of LH-RH in rat pituitary cell culture.

Conditions for testing the inhibitory activity of synthetic analogues of LH-RH in monolayer cultures of rat pituitary cells were optimized. Nine synthetic derivatives of LH-RH were tested to evaluate their inhibitory dose50(ID50) values. The peptide antagonists tested included linear, branched-chain, dimeric and tetrameric derivatives of LH-RH, Ac-D-Phe1, D-Cpa2, D-Trp3,6-LH-RH (analogue V) was found to be the most effective competitive inhibitor of the series with an ID50 value of 6 x 10(-9)M in the presence of 10(-9) M LH-RH. The kinetics of the LH-RH-blocking action of two inhibitors were also studied. Both the linear (analogue I) and the dimeric (analogue VI) peptide showed immediate inhibition of HL-release. Prolonged inhibitory activity of the same two peptides was observed when cells were incubated with LH-Rh subsequent to preincubation with the inhibitors.

Effects of long-term administration of a superactive agonistic and an antagonistic GnRH analog on the pituitary-gonad system.

A powerful GnRH antagonist: [Ac-D-Trp1,3,D-Cpa2,D-Lys6,D-Ala10]-GnRH (MI-1544) and a superactive GnRH agonist: [D-Phe6,desGly10]-GnRH(1-9)EA (OVURELIN) were used in long-term administration to compare their effects on the inhibition of ovulation, LH and progesterone (P) release, LH content of pituitaries as well as on the recovery period. Both analogs showed 100% inhibitory effects on ovulation in very low doses during the daily treatment for 21 days. The antagonist prevented LH release already after the first injection, decreased the serum P level to 40%, and increased the LH content of the pituitary up to 180%, inhibiting only the release but not the synthesis of LH. The agonist showed marked LH-releasing effects on the first day of the treatment, which were reduced to 12% on the 7th day. Serum P concentration was dropped to 68% by the end of the treatment. No change was found in the LH content of pituitaries in the group treated with the agonist. Ovaries showed polifollicular pictures in the antagonist-treated group, and persistent corpora lutea were seen in the ovaries from the agonist-treated group. Regular estrous cycles returned 13-15 days after ceasing the treatment with the antagonist and 3-5 days after ceasing the treatment with the agonist. No edema-inducing effect was observed after the injections of the antagonist in doses of 100 times higher than the single antiovulatory dose.

Synthesis and biological activity of LH-RH antagonists modified in position 1.

As part of our studies on the design of more potent antagonists of the LH-RH (luteinizing hormone-releasing hormone) decapeptide, twelve new highly soluble D-Arg6-analogs have been synthesized. These peptides contain modifications in position 1 and are typified by the general formula (N-acetyl-X1, D-p-Cl-Phe2, D-Trp3, D-Arg6, D-Ala10) LH-RH. We have found that a lyophilic, aromatic substituent is required in position 1 in order to elicit antiovulatory activity at a dose as low as 3 micrograms. The larger the hydrophobic amino acid (X: p-Br-Phe, beta-Nal-2) in position 1, the higher is the antiovulatory activity that can be attained. Analogs with non-aromatic or hydrophilic amino acids (X: Gly, Leu, Arg, His, Glu) in position 1 generally have much lower activities in this series of LH-RH antagonists.

The role of N-acyl groups in the inhibitory activity of LH-RH analogues.

Inhibitory analogues of luteinizing hormone-releasing hormone (LH-RH) were prepared with formyl-D-Trp1, acetyl-D-Trp1, valeryl-D-Trp1, tartaryl-D-Trp1, diacetyl-tartaryl-D-Trp1, acetyl-Gly1, and acetyl-Sar1 successively replacing the position one in the analogue [D-Trp1, D-p-Cl-Phe2, D-Trp3, D-Phe6, D-Ala10]-LH-RH. The formyl-D-Trp1 and acetyl-D-Trp1 analogues yielded 100% blockade of ovulation at the 10 micrograms dose; the others were less potent and inhibited ovulation at the 50 micrograms dose. The inhibitory potency seems to correlate with the polarity of the acyl group.

Distribution of 3H-alpha-MSH in rat brain.

Radioactive synthetic alpha-MSH, prepared by selective tritiation of its dibromo-L-tyrosine-derivative, was injected into the carotid artery of intact, hypophysectomized or pinealectomized rats. More radioactivity was found in the occipital cortex, cerebellum, and pons-medulla than in most other brain parts of rats decapitated 15 sec and, to a less significant extent, 30 min after injection. Part of this radioactivity behaved like alpha-MSH in several identification procedures. After administration of 3H-tyrosine, the least radioactivity was consistently found in the pons-medulla. Large amounts of radioactivity after 3H-alpha-MSH were found in the pituitary and, particularly in the hypophysectomized rats, in the pineal. The half-time disappearance of the first component of radioactivity from the blood of each group of rats injected with tritiated-alpha-MSH was no more than 2.5 minutes, a time considerably shorter than the behavioral and EEG effects of this pituitary hormone on visual attention.

Improvement of the reproductive performance of sows by treatment with a GnRH superactive analogue.

The effect of a Hungarian-made superactive analogue of GnRH (Ovurelin, D-Phe6-GnRH-EA, Reanal, Hungary) on the postpartal sexual function of sows was monitored. GnRH treatment was carried out on day 19 before weaning. The sows were inseminated at the first oestrus after weaning. GnRH treatment markedly increased litter size at weaning, substantially reduced (to 25 and 50%, respectively) the number of sows failing to come into oestrus after weaning, and increased the number of sows coming into oestrus within one week after weaning by 42.5% and 9%, respectively. These beneficial effects were particularly apparent on the far using closed management technology.

Effect of treatment with Prolan or with a GnRH superactive analog on the sexual function of sows after weaning.

On a large, closed pig farm using artificial insemination (AI), 29 sows were treated with Prolan S-öl injection (Bayer, FRG) and 31 sows with a GnRH superactive analog (Ovurelin, D-Phe6-GnRH-EA, Reanal, Hungary) 48 h after weaning. The effect of treatment on the sows' sexual function was monitored by serum progesterone radioimmunoassay (RIA). The conception rate in the control group (36 sows) was 69.4%. In the groups treated with Prolan and Ovurelin it was 79.3 and 71%, respectively. The use of Prolan S-öl injection markedly reduced the number of acyclic sows and of those having an irregular oestrous cycle. Treatment with the GnRH analog inhibited the manifestation of weaning-induced heat; subsequently, however, it induced a regular cycle in 30 out of the 31 sows treated.

Role, mechanism of action and application of gonadoliberins in reproductive processes.

Gonadoliberin (gonadotropin releasing hormone, GnRH) plays a central role in the regulation of reproductive functions as it regulates the release of both luteinizing hormone (LH) and follicle stimulating hormone (FSH). The isolation and structure determination of GnRH opened the possibility of its use for influencing reproductive processes. This possibility initiated a rapid development in the design of potent and long-acting GnRH agonists and antagonists. The most important structural modifications of GnRH leading to superagonists are the D-amino acid substitutions in position 6 combined with Pro9-ethylamide or azaGly10 at the C-terminus. We have synthesized several superagonists of GnRH according to these substitution principles. Furthermore, our L-isoaspartyl modification in position 6, as a new approach to GnRH agonist design, also resulted in superactive analogs. The recently discovered sequences of non-mammalian GnRH-s opened new routes for us to synthesize species specific GnRH agonists. All three groups of the above mentioned GnRH analogs have been successfully used for the treatment of sexual disorders of different animals (cattle, pigs, rabbits, etc.). Ovulation synchronization and a 30% increase in the fertility rate could be achieved by using GnRH agonists in cattle breeding. Analogs derived from species specific sequences could be applied for the induced artificial propagation of fish even out of the spawning season. It is known that superactive GnRH analogs can suppress the growth of certain hormone-dependent tumours. In vitro and in vivo tests of our analogs showed promising antitumour activity in breast cancer which might be explained by the mechanism of desensitization. Almost a hundred antagonist analogs of GnRH have been developed in our laboratory. The most effective ones contain 4 or 5 D-amino acids, and one of them is even orally active. The inhibition of ovulation can also be achieved by the administration of GnRH superagonists. This phenomenon might also be explained by the desensitization of LH-release. Radioactive analogs specifically labeled with tritium in different amino acid residues have been synthesized and used for studying tissue distribution and biodegradation of gonadoliberins. Analogs containing a photoreactive group have been prepared and applied for the trials of GnRH receptor isolation.