Potent nonpeptide GnRH receptor antagonists derived from substituted indole-5-carboxamides and -acetamides bearing a pyridine side-chain terminus

A phenotypic high throughput screening assay for the identification of pharmacoperones for the gonadotropin releasing hormone receptor

We describe a phenotypic high throughput screening (HTS) calcium flux assay designed to identify pharmacoperones for the gonadotropin releasing hormone receptor (GnRHR). Pharmacoperones are target-specific, small molecules that diffuse into cells, rescue misfolded protein mutants, and restore them to function. Rescue is based on correcting the trafficking of mutants that would otherwise be retained in the endoplasmic reticulum and unable to function correctly. This approach identifies drugs with a significant degree of novelty, relying on cellular mechanisms that are not currently exploited. Development of such assays is important, since the extensive use of agonist/antagonist screens alone means that useful chemical structures may be present in existing libraries but have not been previously identified using existing methods. Our assay utilizes cell lines stably expressing a GnRHR mutant under the control of a tetracycline (OFF) transactivator. This allows us to quantitate the level of functional and properly trafficked G protein coupled receptors present in each test well. Furthermore, since we are able to turn receptor expression on and off, we can rapidly eliminate the majority of false positives from our screening results. Our data show that this approach is likely to be successful in identifying hits from large chemical libraries.

Pharmacoperones: a new therapeutic approach for diseases caused by misfolded G protein-coupled receptors

G Protein-coupled receptors (GPCRs) are cell membrane proteins that recognize specific chemical signals such as drugs and hormones and transduce these signals into cellular responses by activating G-proteins. As is the case for all newly synthesized proteins, GPCRs are subjected to conformational scrutiny at the endoplasmic reticulum prior to processing and trafficking to the cell surface membrane. Because of this stringent quality control screening mechanism, mutations that result in protein misfolding frequently lead to retention in the endoplasmic reticulum, aggregation or other misrouting and, eventually, to disease. This article reviews some patents and new therapeutic opportunities based on the misfolding and retention of otherwise functional GPCRs that represent promising approaches to correct conformational abnormalities leading to distinct disease states.

Pharmacological chaperones correct misfolded GPCRs and rescue function: protein trafficking as a therapeutic target

G-protein-coupled receptors (GPCRs) are a large superfamily of plasma membrane proteins that play central roles in transducing endocrine, neural and -sensory signals. In humans, more than 30 disorders are associated with mutations in GPCRs and these proteins are common drug development targets, with 30-50% of drugs targeting them. GPCR mutants are frequently misfolded, recognized as defective by the cellular quality control system, retained in the endoplasmic reticulum and do not traffic to the plasma membrane. The use of small molecules chaperones (pharmacological chaperones or "pharmacoperones") to rescue misfolded GPCRs has provided a new approach for treatment of human diseases caused by misfolding and misrouting. This chapter provides an overview of the molecular basis of this approach using the human gonadotropin-releasing hormone receptor (hGnRHR) as model for treatment of conformational diseases provoked by -misfolded GPCRs.

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.

Pharmacological chaperones for misfolded gonadotropin-releasing hormone receptors

Structural alterations provoked by mutations or genetic variations in the gene sequence of G protein-coupled receptors (GPCRs) may lead to abnormal function of the receptor molecule. Frequently, this leads to disease. While some mutations lead to changes in domains involved in agonist binding, receptor activation, or coupling to effectors, others may cause misfolding and lead to retention/degradation of the protein molecule by the quality control system of the cell. Several strategies, including genetic, chemical, and pharmacological approaches, have been shown to rescue function of trafficking-defective misfolded GPCRs. Among these, pharmacological strategies offer the most promising therapeutic tool to promote proper trafficking of misfolded proteins to the plasma membrane (PM). Pharmacological chaperones or "pharmacoperones" are small compounds that permeate the PM, enter cells, and bind selectively to misfolded proteins and correct folding allowing routing of the target protein to the PM, where the receptor may bind and respond to agonist stimulation. In this review, we describe new therapeutic opportunities based on mislocalization of otherwise functional human gonadotropin-releasing hormone receptors. This particular receptor is highly sensitive to single changes in chemical charge, and its intracellular traffic is delicately balanced between expression at the PM or retention/degradation in the endoplasmic reticulum; it is, therefore, a particularly instructive model to understand both the protein routing and the molecular mechanisms, whereby pharmacoperones rescue misfolded intermediates or conformationally defective receptors.

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.

Refolding of misfolded mutant GPCR: post-translational pharmacoperone action in vitro

All reported GnRH receptor mutants (causing human hypogonadotropic hypogonadism) are misfolded proteins that cannot traffic to the plasma membrane. Pharmacoperones correct misfolding and rescue mutants, routing them to the plasma membrane where they regain function. Because pharmacoperones are often peptidomimetic antagonists, these must be removed for receptor function after rescue; in vivo this necessitates pulsatile pharmacoperone administration. As an antecedent to in vivo studies, we determined whether pharmacoperones need to be present at the time of synthesis or whether previously misfolded proteins could be refolded and rescued. Accordingly, we blocked either protein synthesis or intra-cellular transport. Biochemical and morphological studies using 12 mutants and 10 pharmacoperones representing three different chemical classes show that previously synthesized mutant proteins, retained by the quality control system (QCS), are rescued by pharmacoperones, showing that pharmacoperone administration in vivo likely need not consider whether the target protein is being synthesized at the time of drug administration.

Regulation of G Protein-coupled Receptor Trafficking by Inefficient Plasma Membrane Expression

Despite the prevalence of G protein-coupled receptors as transducers of signals from hormones, neurotransmitters, odorants, and light, little is known about mechanisms that regulate their plasma membrane expression (PME), although misfolded receptors are recognized and retained by a cellular quality control system (QCS). Convergent evolution of the gonadotropin-releasing hormone (GnRH) receptor (GnRHR) progressively decreases inositol phosphate production in response to agonist, validated as a measure of PME of receptor. A pharmacological chaperone that optimizes folding also increases PME of human, but not of rat or mouse, GnRHR because a higher percentage of human GnRHRs are misfolded structures due to their failure to form an apparent sulfhydryl bridge, and they are retained by the QCS. Bridge formation is increased by deleting (primate-specific) Lys191. In rat or mouse GnRHR that lacks Lys191, the bridge is non-essential and receptor is efficiently routed to the plasma membrane. Addition of Lys191 alone to the rat sequence did not diminish PME, indicating that other changes are required for its effects. A strategy, based on identification of amino acids that both 1) co-evolved with the Lys191 and 2) were thermodynamically unfavorable substitutions, identified motifs in multiple domains of the human receptor that control the destabilizing influence of Lys191 on a particular Cys bridge, resulting in diminished PME. The data show a novel and underappreciated means of posttranslational control of a G protein-coupled receptor by altering its interaction with the QCS and provide a biochemical explanation of the basis of disease-causing mutations of this receptor.

In vitro coexpression and pharmacological rescue of mutant gonadotropin-releasing hormone receptors causing hypogonadotropic hypogonadism in humans expressing compound heterozygous alleles

We analyzed the function of mutant GnRH receptor (GnRHR) pairs associated with compound heterozygous patients showing complete or partial forms of hypogonadotropic hypogonadism. We did this to examine potential interactions between misfolded mutants that may influence net receptor function and response to pharmacological rescue. Nine pairs of GnRHR mutants and an unreported combination (L314X((stop))/R262Q) were studied. Coexpression of each pair of mutants in COS-7 cells resulted in an active predominant effect (Q106R/L266R, A171T/Q106R, T32I/C200Y, and R262Q/A129D mutant GnRHR pairs), an additive effect (R262Q/Q106R, N10K/Q106R, and R262Q/Y284C human GnRHR pairs), or a dominant-negative effect (L314X((stop))/Q106R, Q106R+S217R/R262Q, and L314X((stop))/R262Q GnRHRs). For all combinations, addition of the pharmacoperone IN3 increased both agonist binding and effector coupling. The IN3 response was unpredictable because responses could be either similar, higher, or lower, compared with that exhibited by the less affected mutant. The clinical phenotype in patients expressing complex heterozygous alleles appears to be dictated by both the contribution from each mutant and a dominant-negative effect similar to that reported for mutants and wild-type receptor. Depending on the genotype, partial or full restoration of receptor function in response to pharmacological chaperones may be achievable goals in patients bearing inactivating mutations in the GnRHR gene.

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Evolved regulation of gonadotropin-releasing hormone receptor cell surface expression

Dominant negative effects of mutant gonadotropin-releasing hormone (GnRH) receptors (GnRHR; isolated from patients with idiopathic hypogonadotropic hypogonadism) on plasma membrane expression (PME) and function of the wt GnRHR were examined. In addition, we assessed the effect of mutants on wt GnRHR with receptor modifications that, by themselves, diminished PME. Among such mechanisms that restrict PME of GnRHR in primates are: (a) addition of the primate-specific K191 and (b) deletion of the carboxyl tail ("Ctail") found in pre-mammalian species (fish, birds) of GnRHR. We prepared rat (r) and human (h) GnRHR plasmids (88% homologous), each with or without the K191; chimeras were then made with C-tail or each of four truncated fragments (selected to isolate consensus sites for palmitoylation or phosphorylation) of the 51-amino-acid Ser-rich piscine GnRHR C-tail and then expressed in COS-7 cells. The data suggest that the dominant negative effect of the mutants on the hGnRHR requires intrinsic low PME that co-evolved with the dominant-negative effect. The data further reveal that additional modifications must have occurred in primates that are important for both the diminution of the PME and the development of the dominant negative effect of the mutants.

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.

Lead compounds discovered from libraries: part 2

Many lead compounds with the potential to progress to viable drug candidates have been identified from libraries during the past two years. There are two key strategies most often employed to find leads from libraries: first, high-throughput biological screening of corporate compound collections; and second, synthesis and screening of project-directed libraries (i.e. target-based libraries). Numerous success stories, including the discovery of several clinical candidates, testify to the utility of chemical library collections as proven sources of new leads for drug development.

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.

Structure-activity relations of successful pharmacologic chaperones for rescue of naturally occurring and manufactured mutants of the gonadotropin-releasing hormone receptor

We expressed a test system of wild-type (WT) rat (r) and human (h) gonadotropin-releasing hormone (GnRH) receptors (GnRHRs), including naturally occurring (13) and manufactured (five) "loss-of-function" mutants of the GnRHR. These were used to assess the ability of different GnRH peptidomimetics to rescue defective GnRHR mutants and determine their effect on the level of membrane expression of the WT receptors. Among the manufactured mutants were the shortest rGnRHR C-terminal truncation mutant that resulted in receptor loss-of-function (des(325-327)-rGnRHR), two nonfunctional deletion mutants (des(237-241)-rGnRHR and des(260-265)-rGnRHR), two nonfunctional Cys mutants (C(229)A-rGnRHR and C(278)A-rGnRHR); the naturally occurring mutants included all 13 full-length GnRHR point mutations reported to date that result in full or partial human hypogonadotropic hypogonadism. The 10 peptidomimetics assessed as potential rescue molecules ("pharmacoperones") are from three differing chemical pedigrees (indoles, quinolones, and erythromycin-derived macrolides) and were originally developed as GnRH peptidomimetic antagonists. These structures were selected for this study because of their predicted ability to permeate the cell membrane and interact with a defined affinity with the GnRH receptor. All peptidomimetics studied with an IC(50) value (for hGnRHR) <or=2.3 nM had measurable efficacy in rescuing GnRHR mutants, and within a single chemical class, this ability correlated to these IC(50) values. Erythromycin-derived macrolides with IC(50) values as high as 669.5 nM showed efficacy as rescue compounds. The ability to rescue a particular receptor was a reasonable predictor of the ability to rescue others, even across species lines, although particular mutants could not be rescued by any of the drugs tested.

Misrouted cell surface receptors as a novel disease aetiology and potential therapeutic target: the case of hypogonadotropic hypogonadism due to gonadotropin-releasing hormone resistance

Molecules that are incorrectly folded or defectively assembled are recognised by cellular quality control mechanisms. This leads such conformationally abnormal molecules to intracellular retention and eventual degradation. A number of diseases caused by mutations that interfere with proper processing and intracellular trafficking of key cell surface proteins have been described. These include a particular variant of hypogonadotropic hypogonadism, which results from mislocalisation of the gonadotropin-releasing hormone (GnRH) receptor. It has been shown recently that membrane expression and function of misfolded GnRH receptor mutants can be rescued by a peptidomimetic antagonist of GnRH (IN3) that permeates into the cell and reaches the abnormally manufactured nascent receptor, stabilising a conformation compatible with cell-surface transport and reversing intracellular retention. This approach seems applicable for the development of defined therapeutic strategies for an array of diseases caused by incorrectly routed cell surface or secreted proteins.

Characterization of mono- and diaminopyrimidine derivatives as novel, nonpeptide gonadotropin releasing hormone (GnRH) receptor antagonists

A novel series of derivatives of mono- and diaminopyrimidines 1 potently displaced binding of a radiolabeled GnRH analogue to human and rat GnRH receptors. Analogues from these series competitively antagonized GnRH-stimulated increases in extracellular acidification in vitro and suppressed GnRH-mediated increases in circulating luteinizing hormone (LH) in castrated rats and testosterone in intact rats. These compounds or their analogues may be useful in treating sex hormone-dependent disease.

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.

2-Arylindoles as gonadotropin releasing hormone (GnRH) antagonists: optimization of the tryptamine side chain

A series of 2-arylindoles containing novel heteroaromatic substituents on the tryptamine tether, based on compound 1, was prepared and evaluated for their ability to act as gonadotropin releasing hormone (GnRH) antagonists. Successful modifications of 1 included chain length variation (reduction) and replacement of the pyridine with heteroaromatic groups. These alterations culminated in the discovery of compound 27kk which had excellent in vitro potency and oral efficacy in rodents.

Orally bioavailable, indole-based nonpeptide GnRH receptor antagonists with high potency and functional activity

Stereospecific introduction of a methyl group to the indole-3-side chain enhanced activity in our tryptamine-derived series of GnRH receptor antagonists. Further improvements were achieved by variation of the bicyclic amino moiety of the tertiary amide and by adjustment of the tether length to a pyridine or pyridone terminus. These modifications culminated in analogue 24, which had oral activity in a rat model and acceptable oral bioavailability and half-life in dogs and monkeys.