Initial structure-activity relationship studies of a novel series of pyrrolo[1,2-a]pyrimid-7-ones as GnRH receptor antagonists

Recent Development of Non-Peptide GnRH Antagonists

The decapeptide gonadotropin-releasing hormone, also referred to as luteinizing hormone-releasing hormone with the sequence (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH₂) plays an important role in regulating the reproductive system. It stimulates differential release of the gonadotropins FSH and LH from pituitary tissue. To date, treatment of hormone-dependent diseases targeting the GnRH receptor, including peptide GnRH agonist and antagonists are now available on the market. The inherited issues associate with peptide agonists and antagonists have however, led to significant interest in developing orally active, small molecule, non-peptide antagonists. In this review, we will summarize all developed small molecule GnRH antagonists along with the most recent clinical data and therapeutic applications.

Discovery of 1-{4-[1-(2,6-difluorobenzyl)-5-[(dimethylamino)methyl]-3-(6-methoxypyridazin-3-yl)-2,4-dioxo-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidin-6-yl]phenyl}-3-methoxyurea (TAK-385) as a potent, orally active, non-peptide antagonist of the human gonadotropin-releasing hormone receptor

We previously discovered an orally active human gonadotropin-releasing hormone (GnRH) receptor antagonist, thieno[2,3-d]pyrimidine-2,4-dione derivative 1 (sufugolix). To reduce the cytochrome P450 (CYP) inhibitory activity and improve in vivo GnRH antagonistic activity, further optimization of this scaffold was carried out. We focused our synthetic efforts on chemical modification at the 5 and 3 positions of the thieno[2,3-d]pyrimidine-2,4-dione ring based on computational modeling, which resulted in the discovery of 1-{4-[1-(2,6-difluorobenzyl)-5-[(dimethylamino)methyl]-3-(6-methoxypyridazin-3-yl)-2,4-dioxo-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidin-6-yl]phenyl}-3-methoxyurea (16b) as a highly potent and orally active GnRH antagonist. Compound 16b showed potent in vitro GnRH antagonistic activity in the presence of fetal bovine serum (FBS) without CYP inhibition. Oral administration of 16b maintained the suppressive effect of the plasma luteinizing hormone levels in castrated cynomolgus monkeys at a 3 mg/kg dose for more than 24 h. Compound 16b is currently under clinical development with the code name of TAK-385.

Microwave-assisted synthesis of substituted 2-(benzylthio)imidazo[1,2a]pyrimidin-5-ones

A microwave-assisted solid-phase synthesis of 2-(benzylthio)imidazo[1,2a]pyrimidin-5-ones has been developed. Using microwave irradiation, the reaction time was significantly reduced from a few days to 80 min. A representative set of 10 2-(benzylthio)-6,7-substituted-imidazo[1,2a]pyrimidin-5-ones was prepared. These compounds were subsequently N-alkylated and formylated in good yields.

G protein-coupled receptors of the hypothalamic-pituitary-gonadal axis: a case for Gnrh, LH, FSH, and GPR54 receptor ligands

The hypothalamic-pituitary-gonadal (HPG) axis, important in reproduction and sex hormone-dependent diseases, is regulated by a number of G protein-coupled receptors. The recently "deorphanized" GPR54 receptor activated by the peptide metastin is thought to be the key regulator of the axis, mainly by releasing gonadotropin-releasing hormone (GnRH) from the hypothalamus. The latter decapeptide, through the activation of the GnRH receptor in the anterior pituitary, causes the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which subsequently activate their respective receptors on the gonadotrope cells. In this review we will discuss the small molecule agonists and antagonists that are currently being developed to intervene with the action of these four receptors. For GnRH receptors, 14 different chemical classes of non-peptidic antagonists have been reported, while for the LH receptor three classes of agonists have been described. Both agonists and antagonists have been introduced for the FSH receptor. Recently, the first non-peptidic agonist for GPR54 was reported.

QSAR models for predicting the activity of non-peptide luteinizing hormone-releasing hormone (LHRH) antagonists derived from erythromycin A using quantum chemical properties

Multiple linear regression (MLR) combined with genetic algorithm (GA) and Bayesian-regularized Genetic Neural Networks (BRGNNs) were used to model the binding affinity (pK(I)) of 38 11,12-cyclic carbamate derivatives of 6-O-methylerythromycin A for the Human Luteinizing Hormone-Releasing Hormone (LHRH) receptor using quantum chemical descriptors. A multiparametric MLR equation with good statistical quality was obtained that describes the features relevant for antagonistic activity when the substituent at the position 3 of the erythronolide core was varied. In addition, four-descriptor linear and nonlinear models were established for the whole dataset. Such models showed high statistical quality. However, the BRGNN model was better than the linear model according to the external validation process. In general, our linear and nonlinear models reveal that the binding affinity of the compounds studied for the LHRH receptor is modulated by electron-related terms.

Bayesian-regularized genetic neural networks applied to the modeling of non-peptide antagonists for the human luteinizing hormone-releasing hormone receptor

Bayesian-regularized genetic neural networks (BRGNNs) were used to model the binding affinity (IC(50)) for 128 non-peptide antagonists for the human luteinizing hormone-releasing hormone (LHRH) receptor using 2D spatial autocorrelation vectors. As a preliminary step, a linear dependence was established by multiple linear regression (MLR) approach, selecting the relevant descriptors by genetic algorithm (GA) feature selection. The linear model showed to fit the training set (N=102) with R(2)=0.746, meanwhile BRGNN exhibited a higher value of R(2)=0.871. Beyond the improvement of training set fitting, the BRGNN model overcame the linear one by being able to describe 85% of test set (N=26) variance in comparison with 73% the MLR model. Our non-linear QSAR model illustrates the importance of an adequate distribution of atomic properties represented in topological frames and reveals the electronegativities, masses and polarizabilities as the most influencing atomic properties in the structures of the heterocycles under analysis for having an appropriate LHRH antagonistic activity. Furthermore, the ability of the non-linear selected variables for differentiating the data was evidenced when total data set was well distributed in a Kohonen self-organizing map (SOM).

Fluorosubstitution and 7-alkylation as prospective modifications of biologically active 6-aryl derivatives of tricyclic acyclovir and ganciclovir analogues

A series of fluorine containing tricyclic analogues of acyclovir (ACV, 1) and ganciclovir (GCV, 2) were synthesized and evaluated for their activity against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) and cytostatic activity against HSV-1 thymidine kinase (TK) gene-transduced human osteosarcoma tumour cells. It was found that fluorine substitution reduced the antiviral activity, but most of the new compounds were pronounced cytostatic agents with potency and selectivity similar to those of parental ACV and GCV. Compounds 12, 13 and 16 seem to be promising as labeled substrates for (19)F NMR studies of the HSV TK-ligand interaction and/or monitoring of their metabolites in cells expressing HSV TK.

Species selectivity of nonpeptide antagonists of the gonadotropin-releasing hormone receptor is determined by residues in extracellular loops II and III and the amino terminus

Efforts to develop orally available gonadotropin-releasing hormone (GnRH) receptor antagonists have led to the discovery of several classes of potent nonpeptide antagonists. Here we investigated molecular interactions of three classes of nonpeptide antagonists with human, rat, and macaque GnRH receptors. Although all are high affinity ligands of the human receptor (K(i) <5 nm), these compounds show reduced affinity for the macaque receptor and bind only weakly (K(i) >1 microm) to the rat receptor. To identify residues responsible for this selectivity, a series of chimeric receptors and mutant receptors was constructed and evaluated for nonpeptide binding. Surprisingly, 4 key residues located in the amino terminus (Met-24) and extracellular loops II (Ser-203, Gln-208) and III (Leu-300) of the GnRH receptor appear to be primarily responsible for species-selective binding. Comparisons of reciprocal mutations suggest that these may not be direct contacts but rather may be involved in organizing extracellular portions of the receptor. These data are novel because most previous reports of residues involved in binding of nonpeptide ligands to peptide-activated G protein-coupled receptors, including the GnRH receptor as well as mono-amine receptors, have identified binding sites in the transmembrane regions.

Synthesis and structure–activity relationships of ( R )-1-alkyl-3-[2-(2-amino)phenethyl]-5-(2-fluorophenyl)-6-methyluracils as human GnRH receptor antagonists

The synthesis of a series of (R)-1-alkyl-3-[2-(2-amino)phenethyl]-5-(2-fluorophenyl)-6-methyluracils is discussed. SAR around N-1 of the uracil was explored, which led to the discovery that an electron-deficient 2,6-disubstituted benzyl group is required for optimal receptor binding. The best compound from the series had binding affinity of 0.7 nM (K(i) for the human GnRH receptor, which was 8-fold better than the 2,6-difluorobenzyl analog.

Synthesis and structure-activity relationships of 1-arylmethyl-3-(1-methyl-2-amino)ethyl-5-aryl-6-methyluracils as antagonists of the human GnRH Receptor

A new class of small molecule GnRH antagonists, the 1-arylmethyl-3-(1-methyl-2-amino)ethyl-5-aryl-6-methyluracils, was designed and a novel stereoselective synthesis for these compounds was developed. The stereochemical integrities of key intermediates (S)-6 and (R)-6 were confirmed by a combination of X-ray crystallography and chiral HPLC determinations. SAR studies were performed, which allowed the identification of derivatives (R)-9f, (R)-9h and (R)-12 as potent hGnRH antagonists (K(i)=20 nM).

Synthesis and structure-activity relationships of 1-arylmethyl-3-(2-aminopropyl)-5-aryl-6-methyluracils as potent GnRH receptor antagonists

The novel synthesis and SAR studies of 6-methyluracils as human GnRH receptor antagonists are discussed. Introduction of a small methyl substituent at the beta-position from N3 of the uracil improved the GnRH binding potency by 5- to 10-fold. The best compound from the series had binding affinity of 5 nM (K(i)) to the human GnRH receptor.

Synthesis and structure–Activity relationships of thieno[2,3- d ]pyrimidine-2,4-dione derivatives as potent GnRH receptor antagonists

The synthesis and SAR studies of thieno[2,3-d]pyrimidine-2,4-diones as human GnRH receptor antagonists to treat reproductive diseases are discussed. It was found that the 2-(2-pyridyl)ethyl group on the 5-aminomethyl functionality of the core structure was a key feature for good receptor binding activity. SAR study of the 6-(4-aminophenyl) group suggests that hydrophobic substituents were preferred. The best compound from this series had binding affinity (K(i)) of 0.4 nM to the human GnRH receptor.

Biological characterization of a novel, orally active small molecule gonadotropin-releasing hormone (GnRH) antagonist using castrated and intact rats

Gonadotropin-releasing hormone (GnRH) receptor antagonists have potential in treating numerous hormone-dependent pathologies including cancers of the prostate, breast, and ovary, endometriosis, and fertility disorders. An unmet clinical need exists for an orally available GnRH receptor antagonist. Guided by structure-activity relationships, ligand-based targeted library designs, and biomarker measurements, our discovery efforts have yielded a novel, small molecule GnRH receptor antagonist, 5-[(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-naphthalenyl)methyl]-N-(2,4,6-trimethoxyphenyl)-2-furamide (CMPD1). CMPD1 bound with low nanomolar affinities to human, rat, and mouse GnRH receptors (6.0, 3.8, and 2.2 nM, respectively). CMPD1 was more than 100-fold selective for GnRH receptors versus various G-protein-coupled receptors and other enzymes and ion channels. In cells expressing recombinant rat GnRH receptors, CMPD1 was a competitive antagonist of GnRH-stimulated increases in extracellular acidification rates in Cytosensor microphysiometer assays. In cells expressing recombinant human GnRH receptors, CMPD1 was a potent inhibitor of GnRH-stimulated total inositol phosphate accumulation. The effects of CMPD1 on circulating levels of luteinizing hormone (LH) and testosterone were studied in castrated and intact male rats, respectively. Intravenous and oral administration of CMPD1 dose dependently suppressed GnRH-mediated elevations of LH in castrated male rats and testosterone in gonad-intact male rats. Moreover, CMPD1, when given at 20 mg/kg i.v. to intact male rats, inhibited the elevations of LH and testosterone stimulated by the superagonist of GnRH, [d-Ala(6), des-Gly(10)]GnRH (GnRH-A). These data suggest that CMPD1 is a potent, selective, orally active GnRH receptor antagonist that may have potential application as a therapeutic agent for treating hormone-dependent cancers and diseases.

A novel synthesis of 7-aryl-8-fluoro-pyrrolo[1,2-a]pyrimid-4-ones as potent, stable GnRH receptor antagonists

A new class of small molecule GnRH antagonists, the 7-aryl-8-fluoro-pyrrolo[1,2-a]pyrimid-4-ones, was designed and a novel synthesis for these compounds was developed. The synthesis utilizes a base-catalyzed intramolecular cyclization of fluoromethyl pyrimidone 5 to generate the bicyclic core. Amongst the compounds synthesized, we discovered some highly potent GnRH receptor antagonists (e.g., 12, K(i)=9 nM), which showed enhanced stability towards acidic physiological conditions compared to the des-fluoro analogues.

The discovery of novel small molecule non-peptide gonadotropin releasing hormone (GnRH) receptor antagonists

A novel series of non-peptide derivatives 1, 14, and 15 that bind with high affinity to the human GnRH receptors is discussed. The discovery was made from screening our in-house libraries that contained the active structure 2 along with a trace amount of a second active structure 1 that was derived from an acid-induced rearrangement. From this structure type 1, a series of guanidine and non-guanidine containing analogues were prepared and tested as GnRH receptor antagonists. Compounds derived from this series bind to both human and rat GnRH receptors and antagonize GnRH-mediated increases in inositol phosphate production in cells containing recombinant human receptors. These compounds or their analogues may be useful as therapeutic agents for the treatment of hormone-dependent pathologies including prostate, breast and ovarian cancers.

Synthesis and initial structure-activity relationships of a novel series of imidazolo[1,2-a]pyrimid-5-ones as potent GnRH receptor antagonists

SAR studies of 2-arylimidazolo[1,2-a]pyrimid-5-ones 10a-m, which were derived from initial lead 3a, resulted in the discovery of a series of potent nonpeptide human GnRH receptor antagonists. Compounds with good potency (e.g., 10e, K(i)=7.5 nM) were prepared by introduction of a 2-(2-pyridyl)ethyl at the basic nitrogen and a 3-pentyl ester at the 6-position of the bicyclic core.

Design, synthesis and structure-activity relationships of novel imidazolo[1,2-a]pyrimid-5-ones as potent GnRH receptor antagonists

SAR studies of lead GnRH receptor antagonists 2a and 2b reported earlier resulted in the discovery of compound 10b which showed much higher potency (K(i)=4.6 nM, compared with 2b, K(i)=230 nM) in which the 7-position of the imidazolo[1,2-a]pyrimidone core was substituted with a methyl group, and the ester at the 6-position was replaced by the 3-methoxyphenyl group.

A novel synthesis of 2-arylpyrrolo[1,2-a]pyrimid-7-ones and their structure-activity relationships as potent GnRH receptor antagonists

In the process of developing GnRH receptor antagonists, a novel base-catalyzed cyclization of compounds 5a-b was discovered, which led to the formation of the 2-aryl pyrrolo[1,2-a]pyrimid-7-one core structures 6a-b. These intermediates were further modified at positions 1, 2, 4 and 6 to afford a series of potent GnRH antagonists with low nanomolar K(i) values.