A new small-molecule antagonist inhibits Graves' disease antibody activation of the TSH receptor

Graves disease: latest understanding of pathogenesis and treatment options

Graves disease is the most common cause of hyperthyroidism in iodine-sufficient areas. The main responsible mechanism is related to autoantibodies that bind and activate the thyrotropin receptor (TSHR). Although Graves hyperthyroidism is relatively common, no causal treatment options are available. Established treatment modalities are antithyroid drugs, which reduce thyroid hormone synthesis, radioactive iodine and surgery. However, emerging drugs that target the main autoantigen (monoclonal antibodies, small molecules, peptides) or block the immune pathway have been recently tested in clinical trials. Graves disease can involve the thyroid exclusively or it can be associated with extrathyroidal manifestations, among which Graves orbitopathy is the most common. The presence of Graves orbitopathy can change the management of the disease. An established treatment for moderate-to-severe Graves orbitopathy is intravenous glucocorticoids. However, recent advances in understanding the pathogenesis of Graves orbitopathy have allowed the development of new target-based therapies by blocking pro-inflammatory cytokine receptors, lymphocytic infiltration or the insulin-like growth factor 1 receptor (IGF1R), with several clinical trials providing promising results. This article reviews the new discoveries in the pathogenesis of Graves hyperthyroidism and Graves orbitopathy that offer several important tools in disease management.

Targeting Thyroid-Stimulating Hormone Receptor: A Perspective on Small-Molecule Modulators and Their Therapeutic Potential

TSHR is a member of the glycoprotein hormone receptors, a subfamily of class A G-protein-coupled receptors and plays pivotal roles in various physiological and pathological processes, particularly in thyroid growth and hormone production. The aberrant TSHR function has been implicated in several human diseases including Graves' disease and orbitopathy, nonautoimmune hyperthyroidism, hypothyroidism, cancer, neurological disorders, and osteoporosis. Consequently, TSHR is recognized as an attractive therapeutic target, and targeting TSHR with small-molecule modulators including agonists, antagonists, and inverse agonists offers great potential for drug discovery. In this perspective, we summarize the structures and biological functions of TSHR as well as the recent advances in the development of small-molecule TSHR modulators, highlighting their chemotypes, mode of actions, structure-activity relationships, characterizations, in vitro/in vivo activities, and therapeutic potential. The challenges, new opportunities, and future directions in this area are also discussed.

Receptor modulators associated with the hypothalamus -pituitary-thyroid axis

The hypothalamus-pituitary-thyroid (HPT) axis maintains normal metabolic balance and homeostasis in the human body through positive and negative feedback regulation. Its main regulatory mode is the secretion of thyrotropin (TSH), thyroid hormones (TH), and thyrotropin-releasing hormone (TRH). By binding to their corresponding receptors, they are involved in the development and progression of several systemic diseases, including digestive, cardiovascular, and central nervous system diseases. The HPT axis-related receptors include thyrotropin receptor (TSHR), thyroid hormone receptor (TR), and thyrotropin-releasing hormone receptor (TRHR). Recently, research on regulators has become popular in the field of biology. Several HPT axis-related receptor modulators have been used for clinical treatment. This study reviews the developments and recent findings on HPT axis-related receptor modulators. This will provide a theoretical basis for the development and utilisation of new modulators of the HPT axis receptors.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Non-thionamide antithyroid drug options in Graves' hyperthyroidism

INTRODUCTION

The thionamide anti-thyroid drugs namely carbimazole, methimazole, and propylthiouracil, have been the predominant therapy modality for Graves' hyperthyroidism for over 60 years. Although these agents have proven efficacy and favorable side-effect profiles, non-thionamide alternatives are occasionally indicated in patients who are intolerant or unresponsive to thionamides alone. This review examines the available non-thionamide drug options for the control of Graves' hyperthyroidism and summarizes their clinical utility, efficacy, and limitations.

AREAS COVERED

We reviewed existing literature on mechanisms, therapeutic utility, and side-effect profiles of non-thionamide anti-thyroid drugs. Established non-thionamide agents act on various phases of the synthesis, release, and metabolism of thyroid hormones and comprise historical agents such as iodine compounds and potassium perchlorate as well as drug repurposing candidates like lithium, glucocorticoids, beta-blockers, and cholestyramine. Novel experimental agents in development target key players in Graves' disease pathogenesis including B-cell depletors (Rituximab), CD40 blockers (Iscalimab), TSH-receptor antagonists, blocking antibodies, and immune-modifying peptides.

EXPERT OPINION

Non-thionamide anti-thyroid drugs are useful alternatives in Graves' hyperthyroidism and more clinical trials are needed to establish their safety and long-term efficacy in hyperthyroidism control. Ultimately, the promise for a cure will lie in novel approaches that target the well-established immunopathogenesis of Graves' disease.

Thyrotropin receptor antagonists and inverse agonists, and their potential application to thyroid diseases

The thyrotropin receptor (TSHR) plays critical roles in thyroid growth and function and in the pathogenesis of several thyroid diseases including Graves' hyperthyroidism and ophthalmopathy, non-autoimmune hyperthyroidism and thyroid cancer. Several low-molecular weight compounds (LMWCs) and anti-TSHR monoclonal antibodies (mAbs) with receptor antagonistic and inverse agonistic activities have been reported. The former binds to the pocket formed by the receptor transmembrane bundle, and the latter to the extracellular TSH binding site. Both are effective inhibitors of TSH/thyroid stimulating antibody-stimulated cAMP and/or hyaluronic acid production in TSHR-expressing cells. Anti-insulin-like growth factor 1 inhibitors are also found to inhibit TSHR signaling. Each agent has advantages and disadvantages; for example, mAbs have a higher affinity and longer half-life but are more costly than LMWCs. At present, mAbs appear most promising, yet the development of more efficacious LMWCs is desirable. These agents are anticipated to be efficacious not only for the above-mentioned diseases but also for resistance to thyroid hormone and have utility for thyroid cancer radionuclide scintigraphy/therapy as a new theranostic.

2021 update on thyroid-associated ophthalmopathy

PURPOSE

Our understanding of thyroid-associated ophthalmopathy (TAO, A.K.A Graves' orbitopathy, thyroid eye disease) has advanced substantially, since one of us (TJS) wrote the 2010 update on TAO, appearing in this journal.

METHODS

PubMed was searched for relevant articles.

RESULTS

Recent insights have resulted from important studies conducted by many different laboratory groups around the World. A clearer understanding of autoimmune diseases in general and TAO specifically emerged from the use of improved research methodologies. Several key concepts have matured over the past decade. Among them, those arising from the refinement of mouse models of TAO, early stage investigation into restoring immune tolerance in Graves' disease, and a hard-won acknowledgement that the insulin-like growth factor-I receptor (IGF-IR) might play a critical role in the development of TAO, stand out as important. The therapeutic inhibition of IGF-IR has blossomed into an effective and safe medical treatment. Teprotumumab, a β-arrestin biased agonist monoclonal antibody inhibitor of IGF-IR has been studied in two multicenter, double-masked, placebo-controlled clinical trials demonstrated both effectiveness and a promising safety profile in moderate-to-severe, active TAO. Those studies led to the approval by the US FDA of teprotumumab, currently marketed as Tepezza for TAO. We have also learned far more about the putative role that CD34+ fibrocytes and their derivatives, CD34+ orbital fibroblasts, play in TAO.

CONCLUSION

The past decade has been filled with substantial scientific advances that should provide the necessary springboard for continually accelerating discovery over the next 10 years and beyond.

Thyroid eye disease: From pathogenesis to targeted therapies

Thyroid eye disease (TED) is the most common extrathyroidal manifestation of autoimmune Graves’ hyperthyroidism. TED is a debilitating and potentially blinding disease with unclear pathogenesis. Autoreactive inflammatory reactions targeting orbital fibroblasts (OFs) lead to the expansion of orbital adipose tissues and extraocular muscle swelling within the fixed bony orbit. There are many recent advances in the understating of molecular pathogenesis of TED. The production of autoantibodies to cross-linked thyroid-stimulating hormone receptor and insulin-like growth factor-1 receptor (IGF-1R) activates OFs to produce significant cytokines and chemokines and hyaluronan production and to induce adipocyte differentiation. In moderately severe active TED patients, multicenter clinical trials showed that inhibition of IGF-1R with teprotumumab was unprecedentedly effective with minimal side effects. The emergence of novel biologics resulted in a paradigm shift in the treatment of TED. We here review the literature on advances of pathogenesis of TED and promising therapeutic targets and drugs.

Teprotumumab in Thyroid-Associated Ophthalmopathy: Rationale for Therapeutic Insulin-Like Growth Factor-I Receptor Inhibition

Thyroid-associated ophthalmopathy (TAO) is an autoimmune component of Graves' disease for which no currently available medical therapy provides reliable and safe benefit. Based on insights generated experimentally over the past several decades, the insulin-like growth factor-I receptor (IGF-IR) has been implicated in the pathogenesis of TAO. Furthermore, an IGF-IR inhibitor, teprotumumab, has emerged from 2 clinical trials as a promising treatment for active, moderate to severe TAO. This brief review intends to provide an overview of the rationale underlying the development of teprotumumab for this disease. It is possible that teprotumumab will soon take its place in our therapeutic armamentarium for active TAO.

Novel Approaches for Immunosuppression in Graves' Hyperthyroidism and Associated Orbitopathy

BACKGROUND

Both Graves' hyperthyroidism (GH) and Graves' orbitopathy (GO) are associated with significant adverse health consequences. All conventional treatment options have limitations regarding efficacy and safety. Most importantly, they do not specifically address the underlying immunological mechanisms. We aim to review the latest development of treatment approaches in these two closely related disorders.

SUMMARY

Immunotherapies of GH have recently demonstrated clinical efficacy in preliminary studies. They include ATX-GD-59, an antigen-specific immunotherapy which restores immune tolerance to the thyrotropin receptor; iscalimab, an anti-CD40 monoclonal antibody which blocks the CD40-CD154 costimulatory pathway in B-T cell interaction; and K1-70, a thyrotropin receptor-blocking monoclonal antibody. Novel treatment strategies have also become available in GO. Mycophenolate significantly increased the overall response rate combined with standard glucocorticoid (GC) treatment compared to GC monotherapy. Tocilizumab, an anti-interleukin 6 receptor monoclonal antibody, displayed strong anti-inflammatory action in GC-resistant cases. Teprotumumab, an anti-insulin-like growth factor 1 receptor monoclonal antibody, resulted in remarkable improvement in terms of disease activity, proptosis, and diplopia. Further, rituximab appears to be useful in active disease of recent onset without impending dysthyroid optic neuropathy.

KEY MESSAGES

Therapeutic advances will continue to optimize our management of GH and associated orbitopathy in an effective and safe manner.

The Molecular Function and Clinical Role of Thyroid Stimulating Hormone Receptor in Cancer Cells

The thyroid stimulating hormone (TSH) and its cognate receptor (TSHR) are of crucial importance for thyrocytes to proliferate and exert their functions. Although TSHR is predominantly expressed in thyrocytes, several studies have revealed that functional TSHR can also be detected in many extra-thyroid tissues, such as primary ovarian and hepatic tissues as well as their corresponding malignancies. Recent advances in cancer biology further raise the possibility of utilizing TSH and/or TSHR as a therapeutic target or as an informative index to predict treatment responses in cancer patients. The TSH/TSHR cascade has been considered a pivotal modulator for carcinogenesis and/or tumor progression in these cancers. TSHR belongs to a sub-group of family A G-protein-coupled receptors (GPCRs), which activate a bundle of well-defined signaling transduction pathways to enhance cell renewal in response to external stimuli. In this review, recent findings regarding the molecular basis of TSH/TSHR functions in either thyroid or extra-thyroid tissues and the potential of directly targeting TSHR as an anticancer strategy are summarized and discussed.

A Gq Biased Small Molecule Active at the TSH Receptor

G protein coupled receptors (GPCRs) can lead to G protein and non-G protein initiated signals. By virtue of its structural property, the TSH receptor (TSHR) has a unique ability to engage different G proteins making it highly amenable to selective signaling. In this study, we describe the identification and characterization of a novel small molecule agonist to the TSHR which induces primary engagement with G_αq/11. To identify allosteric modulators inducing selective signaling of the TSHR we used a transcriptional-based luciferase assay system with CHO-TSHR cells stably expressing response elements (CRE, NFAT, SRF, or SRE) that were capable of measuring signals emanating from the coupling of G_αs , G_αq/11, G_βγ, and G_α12/13, respectively. Using this system, TSH activated G_αs , G_αq/11, and G_α12/13 but not G_βγ. On screening a library of 50K molecules at 0.1,1.0 and 10 μM, we identified a novel G_q/11 agonist (named MSq1) which activated G_q/11 mediated NFAT-luciferase >4 fold above baseline and had an EC₅₀= 8.3 × 10^-9 M with only minor induction of G_αs and cAMP. Furthermore, MSq1 is chemically and structurally distinct from any of the previously reported TSHR agonist molecules. Docking studies using a TSHR transmembrane domain (TMD) model indicated that MSq1 had contact points on helices H1, H2, H3, and H7 in the hydrophobic pocket of the TMD and also with the extracellular loops. On co-treatment with TSH, MSq1 suppressed TSH-induced proliferation of thyrocytes in a dose-dependent manner but lacked the intrinsic ability to influence basal thyrocyte proliferation. This unexpected inhibitory property of MSq1 could be blocked in the presence of a PKC inhibitor resulting in derepressing TSH induced protein kinase A (PKA) signals and resulting in the induction of proliferation. Thus, the inhibitory effect of MSq1 on proliferation resided in its capacity to overtly activate protein kinase C (PKC) which in turn suppressed the proliferative signal induced by activation of the predomiant cAMP-PKA pathway of the TSHR. Treatment of rat thyroid cells (FRTL5) with MSq1 did not show any upregulation of gene expression of the key thyroid specific markers such as thyroglobulin(Tg), thyroid peroxidase (Tpo), sodium iodide symporter (Nis), and the TSH receptor (Tshr) further suggesting lack of involvement of MSq1 and G_αq/11 activation with cellular differentation. In summary, we identified and characterized a novel G_αq/11 agonist molecule acting at the TSHR and which showed a marked anti-proliferative ability. Hence, Gq biased activation of the TSHR is capable of ameliorating the proliferative signals from its orthosteric ligand and may offer a therapeutic option for thyroid growth modulation.

Thyroid-associated ophthalmopathy: Emergence of teprotumumab as a promising medical therapy

Thyroid-associated ophthalmopathy (TAO) remains a vexing autoimmune component of Graves' disease that can diminish the quality of life as a consequence of its impact on visual function, physical appearance and emotional well-being. Because of its relative rarity and variable presentation, the development of highly effective and well-tolerated medical therapies for TAO has been slow relative to other autoimmune diseases. Contributing to the barriers of greater insight into TAO has been the historical absence of high-fidelity preclinical animal models. Despite these challenges, several agents, most developed for treatment of other diseases, have found their way into consideration for use in active TAO through repurposing. Among these, teprotumumab is a fully human inhibitory monoclonal antibody against the insulin-like growth factor I receptor. It has shown remarkable effectiveness in moderate to severe, active TAO in two completed multicenter, double masked, and placebo controlled clinical trials. The drug exhibits a favorable safety profile. Teprotumumab has recently been approved by the U.S. F.D.A, and may rapidly become the first line therapy for this disfiguring and potentially blinding condition.

Immunological Aspects of Graves' Ophthalmopathy

The body's autoimmune process is involved in the development of Graves' disease (GD), which is manifested by an overactive thyroid gland. In some patients, autoreactive inflammatory reactions contribute to the development of symptoms such as thyroid ophthalmopathy, and the subsequent signs and symptoms are derived from the expansion of orbital adipose tissue and edema of extraocular muscles within the orbit. The autoimmune process, production of antibodies against self-antigens such as TSH receptor (TSHR) and IGF-1 receptor (IGF-1R), inflammatory infiltration, and accumulation of glycosaminoglycans (GAG) lead to edematous-infiltrative changes in periocular tissues. As a consequence, edema exophthalmos develops. Orbital fibroblasts seem to play a crucial role in orbital inflammation, tissue expansion, remodeling, and fibrosis because of their proliferative activity as well as their capacity to differentiate into adipocytes and myofibroblasts and production of GAG. In this paper, based on the available medical literature, the immunological mechanism of GO pathogenesis has been summarized. Particular attention was paid to the role of orbital fibroblasts and putative autoantigens. A deeper understanding of the pathomechanism of the disease and the involvement of immunological processes may give rise to the introduction of new, effective, and safe methods of treatment or monitoring of the disease activity.

Will biological agents supplant systemic glucocorticoids as the first-line treatment for thyroid-associated ophthalmopathy?

In this article, the two authors present their opposing points of view concerning the likelihood that glucocorticoids will be replaced by newly developed biological agents in the treatment of active, moderate-to-severe thyroid-associated ophthalmopathy (TAO). TAO is a vexing, disfiguring and potentially blinding autoimmune manifestation of thyroid autoimmunity. One author expresses the opinion that steroids are nonspecific, frequently fail to improve the disease and can cause sometimes serious side effects. He suggests that glucocorticoids should be replaced as soon as possible by more specific and safer drugs, once they become available. The most promising of these are biological agents. The other author argues that glucocorticoids are proven effective and are unlikely to be replaced by biologicals. He reasons that while they may not uniformly result in optimal benefit, they have been proven effective in many reports. He remains open minded about alternative therapies such as biologicals but remains skeptical that they will replace steroids as the first-line therapy for active, moderate-to-severe TAO without head-to-head comparative clinical trials demonstrating superiority. Despite these very different points of view, both authors are optimistic about the availability of improved medical therapies for TAO, either as single agents or in combination. Further, both agree that better treatment options are needed to improve the care of our patients with active moderate-to-severe TAO.

Comprehensive research on thyroid diseases associated with autoimmunity: autoimmune thyroid diseases, thyroid diseases during immune-checkpoint inhibitors therapy, and immunoglobulin-G4-associated thyroid diseases

Various thyroid diseases are associated with autoimmunity. Major autoimmune thyroid diseases are Graves' disease (GD) and Hashimoto's thyroiditis (HT). Thyrotropin receptor is an autoantigen in GD, and its immunogenicity has been examined. Immune-checkpoint inhibitor (ICI) is recently widely used for treatment of malignant tumors, but cases of thyroid diseases during ICI treatment have been increasing. Thyroid diseases during ICI therapy have been investigated in immunological and clinical aspects, and their Japanese official diagnostic guidelines were established. In addition, serum and tissue immunoglobulin-G4 levels have been examined in association with clinicopathological characteristics in GD, HT, and Riedel's thyroiditis. We review these diseases associated with thyroid autoimmunity and comprehensively discuss their potential application in future research and therapeutic options.

Potential Roles of CD34+ Fibrocytes Masquerading as Orbital Fibroblasts in Thyroid-Associated Ophthalmopathy

CONTEXT

Orbital tissues in thyroid-associated ophthalmopathy exhibit particular reactivity and undergo characteristic remodeling. Mechanisms underlying these changes have remained largely unexplained. Studies have characterized orbital connective tissues and derivative fibroblasts to gain insights into local manifestations of a systemic autoimmune syndrome.

EVIDENCE ACQUISITION

A systematic search of PubMed was undertaken for studies related to thyroid-associated ophthalmopathy (TAO), orbital fibroblasts, and fibrocytes involved in pathogenesis.

EVIDENCE SYNTHESIS

Orbital tissues display marked cellular heterogeneity. Fibroblast subsets, putatively derived from multiple precursors, inhabit the orbit in TAO. Among them are cells displaying the CD34+CXC chemokine receptor 4+collagen I+ phenotype, identifying them as fibrocytes, derived from the monocyte lineage. Their unique presence in the TAO orbit helps explain the tissue reactivity and characteristic remodeling that occurs in the disease. Their unanticipated expression of several proteins traditionally thought to be thyroid gland specific, including the TSH receptor and thyroglobulin, may underlie orbital involvement in Graves disease. Although no currently available information unambiguously establishes that CD34+ orbital fibroblasts originate from circulating fibrocytes, inferences from animal models of lung disease suggest that they derive from bone marrow. Further studies are necessary to determine whether fibrocyte abundance and activity in the orbit determine the clinical behavior of TAO.

CONCLUSION

Evidence supports a role for fibrocytes in the pathogenesis of TAO. Recognition of their presence in the orbit now allows development of therapies specifically targeting these cells that ultimately could allow the restoration of immune tolerance within the orbit and perhaps systemically.

Cambridge Ophthalmological Symposium 2018: introduction and reflections on the day

I was privileged to be one of the co-chairs, along with Professor Tim Sullivan (Brisbane, Australia), for the Cambridge Ophthalmological Society (COS) annual international symposium, which, this year, was dedicated to thyroid eye disease (TED). Together with the organisers, Miss Rachna Murthy and Professor Keith Martin from COS, we compiled an impressive programme covering all aspects of the condition from events happening in a single orbital cell to improved surgical approaches.

A New Highly Thyrotropin Receptor-Selective Small-Molecule Antagonist with Potential for the Treatment of Graves' Orbitopathy

BACKGROUND

The thyrotropin receptor (TSHR) is the target for autoimmune thyroid stimulating antibodies (TSAb) triggering hyperthyroidism. Whereas elevated thyroid hormone synthesis by the thyroid in Graves' disease can be treated by antithyroid agents, for the pathogenic activation of TSHR in retro-orbital fibroblasts of the eye, leading to Graves' orbitopathy (GO), no causal TSHR directed therapy is available.

METHODS

Due to the therapeutic gap for severe GO, TSHR inhibitors were identified by high-throughput screening in Chinese hamster ovary cells expressing the TSHR. Stereo-selective synthesis of the screening hits led to the molecule S37, which contains seven chiral centers. Enantiomeric separation of the molecule S37 resulted in the enantiopure molecule S37a-a micro-molar antagonist of thyrotropin-induced cyclic adenosine monophosphate accumulation in HEK 293 cells expressing the TSHR.

RESULTS

The unique rigid bent shape of molecule S37a may mediate the observed high TSHR selectivity. Most importantly, the closely related follitropin and lutropin receptors were not affected by this compound. S37a not only inhibits the TSHR activation by thyrotropin itself but also activation by monoclonal TSAb M22 (human), KSAb1 (murine), and the allosteric small-molecule agonist C2. Disease-related ex vivo studies in HEK 293 cells expressing the TSHR showed that S37a also inhibits cyclic adenosine monophosphate formation by oligoclonal TSAb, which are highly enriched in GO patients' sera. Initial in vivo pharmacokinetic studies revealed no toxicity of S37a and a remarkable 53% oral bioavailability in mice.

CONCLUSION

In summary, a novel highly selective inhibitor for the TSHR is presented, which has promising potential for further development for the treatment of GO.

ANNIVERSARY REVIEW: Antithyroid drug therapy: 70 years later

The thionamide antithyroid drugs were discovered in large part following serendipitous observations by a number of investigators in the 1940s who found that sulfhydryl-containing compounds were goitrogenic in animals. This prompted Prof. Edwin B Astwood to pioneer the use of these compounds to treat hyperthyroidism in the early 1940s and to develop the more potent and less toxic drugs that are used today. Despite their simple molecular structure and ease of use, many uncertainties remain, including their mechanism(s) of action, clinical role, optimal use in pregnancy and the prediction and prevention of rare but potentially life-threatening adverse reactions. In this review, we summarize the history of the development of these drugs and outline their current role in the clinical management of patients with hyperthyroidism.

Challenges in Orphan Drug Development: Identification of Effective Therapy for Thyroid-Associated Ophthalmopathy

Thyroid-associated ophthalmopathy (TAO), the ocular manifestation of Graves' disease, is a process in which orbital connective tissues and extraocular muscles undergo inflammation and remodeling. The condition seems to result from autoimmune responses to antigens shared by the thyroid and orbit. The thyrotropin receptor (TSHR), expressed at low levels in orbital tissues, is a leading candidate antigen. Recent evidence suggests that another protein, the insulin-like growth factor-I receptor (IGF-IR), is overexpressed in TAO, and antibodies against IGF-IR have been detected in patients with the disease. Furthermore, TSHR and IGF-IR form a physical and functional complex, and signaling initiated at TSHR requires IGF-IR activity. Identification of therapy for this rare disease has proven challenging and currently relies on nonspecific and inadequate agents, thus representing an important unmet need. A recently completed therapeutic trial suggests that inhibiting IGF-IR activity with a monoclonal antibody may be an effective and safe treatment for active TAO.

New advances in understanding thyroid-associated ophthalmopathy and the potential role for insulin-like growth factor-I receptor

Thyroid-associated ophthalmopathy (TAO), a localized periocular manifestation of the autoimmune syndrome known as Graves’ disease, remains incompletely understood. Discussions of its pathogenesis are generally focused on the thyrotropin receptor, the proposed role for which is supported by substantial evidence. Considerations of any involvement of the insulin-like growth factor-I receptor (IGF-IR) in the disease are frequently contentious. In this brief, topically focused review, I have attempted to provide a balanced perspective based entirely on experimental results that either favor or refute involvement of IGF-IR in TAO. Discussion in this matter seems particularly timely since the currently available treatments of this disfiguring and potentially sight-threatening disease remain inadequate. Importantly, no medical therapy has thus far received approval from the US Food and Drug Administration. Results from a very recently published clinical trial assessing the safety and efficacy of teprotumumab, an inhibitory human anti–IGF-IR monoclonal antibody, in active, moderate to severe TAO are extremely encouraging. That double-masked, placebo-controlled study involved 88 patients and revealed unprecedented clinical responses in the improvement of proptosis and clinical activity as well as a favorable safety profile. Should those results prove reproducible in an ongoing phase III trial, therapeutic inhibition of IGF-IR could become the basis for paradigm-shifting treatment of this vexing disease.

Nonthionamide Drugs for the Treatment of Hyperthyroidism: From Present to Future

Hyperthyroidism is a common endocrine disease. Although thionamide antithyroid drugs are the cornerstone of hyperthyroidism treatment, some patients cannot tolerate this drug class because of its serious side effects including agranulocytosis, hepatotoxicity, and vasculitis. Therefore, nonthionamide antithyroid drugs (NTADs) still have an important role in controlling hyperthyroidism in clinical practice. Furthermore, some situations such as thyroid storm or preoperative preparation require a rapid decrease in thyroid hormone by combination treatment with multiple classes of antithyroid drugs. NTADs include iodine-containing compounds, lithium carbonate, perchlorate, glucocorticoid, and cholestyramine. In this narrative review, we summarize the mechanisms of action, indications, dosages, and side effects of currently used NTADs for the treatment of hyperthyroidism. In addition, we also describe the state-of-the-art in future drugs under development including rituximab, small-molecule ligands (SMLs), and monoclonal antibodies with a thyroid-stimulating hormone receptor (TSHR) antagonist effect.

Discovery of a Positive Allosteric Modulator of the Thyrotropin Receptor: Potentiation of Thyrotropin-Mediated Preosteoblast Differentiation In Vitro

Recently, we showed that TSH-enhanced differentiation of a human preosteoblast-like cell model involved a β-arrestin 1 (β-Arr 1)-mediated pathway. To study this pathway in more detail, we sought to discover a small molecule ligand that was functionally selective toward human TSH receptor (TSHR) activation of β-Arr 1. High-throughput screening using a cell line stably expressing mutated TSHRs and mutated β-Arr 1 (DiscoverX1 cells) led to the discovery of agonists that stimulated translocation of β-Arr 1 to the TSHR, but did not activate G_s-mediated signaling pathways, i.e., cAMP production. D3-βArr (NCGC00379308) was selected. In DiscoverX1 cells, D3-βArr stimulated β-Arr 1 translocation with a 5.1-fold greater efficacy than TSH and therefore potentiated the effect of TSH in stimulating β-Arr 1 translocation. In human U2OS-TSHR cells expressing wild-type TSHRs, which is a model of human preosteoblast-like cells, TSH upregulated the osteoblast-specific genes osteopontin (OPN) and alkaline phosphatase (ALPL). D3-βArr alone had only a weak effect to upregulate these bone markers, but D3-βArr potentiated TSH-induced upregulation of ALPL and OPN mRNA levels 1.6-fold and 5.5-fold, respectively, at the maximum dose of ligands. Furthermore, the positive allosteric modulator effect of D3-βArr resulted in an increase of TSH-induced secretion of OPN protein. In summary, we have discovered the first small molecule positive allosteric modulator of TSHR. As D3-βArr potentiates the effect of TSH to enhance differentiation of a human preosteoblast in an in vitro model, it will allow a novel experimental approach for probing the role of TSH-induced β-Arr 1 signaling in osteoblast differentiation.

[Pharmacological approaches for correction of thyroid dysfunctions in diabetes mellitus]

Thyroid diseases are closely associated with the development of types 1 and 2 diabetes mellitus (DM), and as a consequence, the development of effective approaches for their treatment is one of the urgent problems of endocrinology. Traditionally, thyroid hormones (TH) are used to correct functions of the thyroid system. However, they are characterized by many side effects, such as their negative effect on the cardiovascular system as well as the ability of TH to enhance insulin resistance and to disturb insulin-producing function of pancreas, exacerbating thereby diabetic pathology. Therefore, the analogues of TH, selective for certain types of TH receptors, that do not have these side effects, are being developed. The peptide and low-molecular weight regulators of thyroid-stimulating hormone receptor, which regulate the activity of the thyroid axis at the stage of TH synthesis and secretion in thyrocytes, are being created. Systemic and intranasal administration of insulin, metformin therapy and drugs with antioxidant activity are effective for the treatment of thyroid pathology in types 1 and 2 DM. In the review, the literature data and the results of own investigations on pharmacological approaches for the treatment and prevention of thyroid diseases in patients with types 1 and 2 DM are summarized and analyzed.

Cyclic Peptides for Effective Treatment in a Long-Term Model of Graves Disease and Orbitopathy in Female Mice

A model for human Graves disease in mice was used to compare several treatment approaches. The mice received regular adenovirus (Ad) thyroid-stimulating hormone receptor (TSHR) A subunit immunizations (injections every 4 weeks). The generation of anti-TSHR antibodies, enlarged thyroid sizes (goiter), elevated serum thyroxine levels, retro-orbital fibrosis, and cardiac involvement (tachycardia and hypertrophy) were consistently observed over 9 months. Treatment of established disease in these mice using cyclic peptides that mimic one of the cylindrical loops of the TSHR leucine-rich repeat domain improved or cured all investigated parameters after six consecutive monthly injections. The first significant beneficial effects were observed 3 to 4 months after starting these therapies. In immunologically naïve mice, administration of any of the cyclic peptides did not induce any immune response. In contrast, monthly injections of the full antigenic TSHR A domain as fusion protein with immunoglobulin G crystallizable fragment induced clinical signs of allergy in Ad-TSHR-immunized mice and anti-TSHR antibodies in naïve control mice. In conclusion, cyclic peptides resolved many clinical findings in a mouse model of established Graves disease and orbitopathy. In contrast to blocking TSHR by allosteric modulation, the approach does not incur a direct receptor antagonism, which might offer a favorable side effect profile.

Review of Mouse Models of Graves' Disease and Orbitopathy-Novel Treatment by Induction of Tolerance

Various approaches have been used to model human Graves' disease in mice, including transfected fibroblasts, and plasmid or adenoviral immunisations with the extracellular A subunit of the human thyrotropin receptor (TSHR). Some of these models were only observed for a short time period or were self-limiting. A long-term model for human Graves' disease was established in mice using continuing immunisations (4-weekly injections) with recombinant adenovirus expressing TSHR. Generation of TSHR binding cAMP-stimulatory antibodies, thyroid enlargement and alterations, elevated serum thyroxin levels, tachycardia and cardiac hypertrophy were maintained for at least 9 months in all Ad-TSHR-immunised mice. Here, we show that these mice suffer from orbitopathy, which was detected by serial orbital sectioning and histomorphometry. Attempts to treat established Graves' disease in preclinical mouse model studies have included small molecule allosteric antagonists and specific antagonist antibodies which were isolated from hypothyroid patients. In addition, novel peptides have been conceived which mimic the cylindrical loops of the TSHR leucine-rich repeat domain, in order to re-establish tolerance toward the antigen. Here, we show preliminary results that one set of these peptides improves or even cures all signs and symptoms of Graves' disease in mice after six consecutive monthly injections. First beneficial effects were observed 3-4 months after starting these therapies. In immunologically naïve mice, administration of the peptides did not induce any immune response.

TSHR as a therapeutic target in Graves' disease

INTRODUCTION

Graves' disease (GD) and thyroid-associated ophthalmopathy (TAO) are thought to result from actions of pathogenic antibodies mediated through the thyrotropin receptor (TSHR). This leads to the unregulated consequences of the antibody-mediated receptor activity in the thyroid and connective tissues of the orbit. Recent studies reveal antibodies that appear to be directed against the insulin-like growth factor-I receptor (IGF-IR). Areas covered: In this brief article, I attempt to review the fundamental characteristics of the TSHR, its role in GD and TAO, and its relationship to IGF-IR. Strong evidence supports the concept that the two receptors form a physical and functional complex and that IGF-IR activity is required for some of the down-stream signaling initiated through TSHR. Recently developed small molecules and monoclonal antibodies that block TSHR and IGF-IR signaling are also reviewed in the narrow context of their potential utility as therapeutics in GD and TAO. The Pubmed database was searched from its inception for relevant publications. Expert opinion: Those agents that can interrupt the TSHR and IGF-IR pathways possess the potential for offering more specific and better tolerated treatments of both hyperthyroidism and TAO. This would spare patients exposure to toxic drugs, ionizing radiation and potentially hazardous surgeries.

Structural-Functional Features of the Thyrotropin Receptor: A Class A G-Protein-Coupled Receptor at Work

The thyroid-stimulating hormone receptor (TSHR) is a member of the glycoprotein hormone receptors, a sub-group of class A G-protein-coupled receptors (GPCRs). TSHR and its endogenous ligand thyrotropin (TSH) are of essential importance for growth and function of the thyroid gland and proper function of the TSH/TSHR system is pivotal for production and release of thyroid hormones. This receptor is also important with respect to pathophysiology, such as autoimmune (including ophthalmopathy) or non-autoimmune thyroid dysfunctions and cancer development. Pharmacological interventions directly targeting the TSHR should provide benefits to disease treatment compared to currently available therapies of dysfunctions associated with the TSHR or the thyroid gland. Upon TSHR activation, the molecular events conveying conformational changes from the extra- to the intracellular side of the cell across the membrane comprise reception, conversion, and amplification of the signal. These steps are highly dependent on structural features of this receptor and its intermolecular interaction partners, e.g., TSH, antibodies, small molecules, G-proteins, or arrestin. For better understanding of signal transduction, pathogenic mechanisms such as autoantibody action and mutational modifications or for developing new pharmacological strategies, it is essential to combine available structural data with functional information to generate homology models of the entire receptor. Although so far these insights are fragmental, in the past few decades essential contributions have been made to investigate in-depth the involved determinants, such as by structure determination via X-ray crystallography. This review summarizes available knowledge (as of December 2016) concerning the TSHR protein structure, associated functional aspects, and based on these insights we suggest several receptor complex models. Moreover, distinct TSHR properties will be highlighted in comparison to other class A GPCRs to understand the molecular activation mechanisms of this receptor comprehensively. Finally, limitations of current knowledge and lack of information are discussed highlighting the need for intensified efforts toward TSHR structure elucidation.

DYNAMIC CHANGES OF TRAb AND TPOAb AFTER RADIOIODINE THERAPY IN GRAVES' DISEASE

Context

To analyze the dynamic changes of serum thyrotrophin receptor antibody (TRAb) and thyroid peroxidase antibody (TPOAb) in Graves' disease (GD) patients before and after radioactive iodine (RAI) treatment and to investigate if TRAb and TPOAb play a role in the occurrence of early hypothyroidism after ¹³¹I therapy for Graves' hyperthyroidism.

Subjects and Methods

A total of 240 patients newly diagnosed with GD were selected to study. A clinical and laboratory assessment was performed before and at 3, 6, and 12 months after ¹³¹I therapy. Chemiluminescent immunoassays were used to detect serum free triiodothyronine (FT3), free thyroxine (FT4), sensitive thyroid-stimulating hormone (TSH) and TPOAb concentration. Radio-receptor assay was used to measure serum TRAb concentration. According to the early onset of hypothyroidism in a year after RAI therapy, patients were divided into early hypothyroidism group (group A) and non-early hypothyroidism group (group B).

Results

In both groups, serum TRAb and TPOAb increased at 3 months, reached the highest level at 6 months and returned to the baseline at 12 months after RAI therapy. TRAb showed a significant difference between the two groups at 6 months (P<0.01). Serum TPOAb in group A was higher than that in group B before and at 3, 6, 12 months after RAI therapy (P<0.05).

Conclusions

Serum TRAb and TPOAb are closely related to the occurrence of the early hypothyroidism, and play an important role in judging prognosis after ¹³¹I treatment in Graves' disease.

Current trends in antithyroid drug treatment of Graves' disease

INTRODUCTION

Graves' hyperthyroidism is associated with significant morbidity and mortality risk. The thionamides, methimazole, its pro-drug derivative carbimazole, and propylthiouracil, remain a cornerstone of management. Yet despite decades of use, optimal strategies for maximising treatment response and curtailing adverse effect risk remains uncertain.

AREAS COVERED

We reviewed the current literature on the evidence based medical management of Graves' disease. Specifically, we evaluated current approaches to the use of thionamides, adjunctive therapies, and potential novel agents for controlling Graves' hyperthyroidism.

EXPERT OPINION

Primary medical therapy is successful in less than 50% of cases and so careful selection of patients for medical treatment based on a combination of pathological and pragmatic considerations is essential. Carbimazole or methimazole is the treatment of choice in the non-pregnant population driven by its more favourable pharmacokinetic and adverse effect profile over propylthiouracil. In pregnancy the choice of treatment is less straightforward and an approach that minimises undue fetal exposure to all thionamides should be adopted. Additional data is needed on the value of adjunctive therapies including potassium perchlorate, iodides, glucocorticoids, lithium, and cholestyramine. Novel agents directed against pathogenetic targets including TSH receptor blocking monoclonal antibodies and small molecule antagonists may hold promise for the future.

Thyrotropin Receptor Epitope and Human Leukocyte Antigen in Graves' Disease

Graves' disease (GD) is an organ-specific autoimmune disease, and thyrotropin (TSH) receptor (TSHR) is a major autoantigen in this condition. Since the extracellular domain of human TSHR (TSHR-ECD) is shed into the circulation, TSHR-ECD is a preferentially immunogenic portion of TSHR. Both genetic factors and environmental factors contribute to development of GD. Inheritance of human leukocyte antigen (HLA) genes, especially HLA-DR3, is associated with GD. TSHR-ECD protein is endocytosed into antigen-presenting cells (APCs), and processed to TSHR-ECD peptides. These peptide epitopes bind to HLA-class II molecules, and subsequently the complex of HLA-class II and TSHR-ECD epitope is presented to CD4+ T cells. The activated CD4+ T cells secrete cytokines/chemokines that stimulate B-cells to produce TSAb, and in turn hyperthyroidism occurs. Numerous studies have been done to identify T- and B-cell epitopes in TSHR-ECD, including (1) in silico, (2) in vitro, (3) in vivo, and (4) clinical experiments. Murine models of GD and HLA-transgenic mice have played a pivotal role in elucidating the immunological mechanisms. To date, linear or conformational epitopes of TSHR-ECD, as well as the molecular structure of the epitope-binding groove in HLA-DR, were reported to be related to the pathogenesis in GD. Dysfunction of central tolerance in the thymus, or in peripheral tolerance, such as regulatory T cells, could allow development of GD. Novel treatments using TSHR antagonists or mutated TSHR peptides have been reported to be effective. We review and update the role of immunogenic TSHR epitopes and HLA in GD, and offer perspectives on TSHR epitope specific treatments.

Pathogenesis of thyroid eye disease: review and update on molecular mechanisms

Orbital changes in thyroid orbitopathy (TO) result from de novo adipogenesis, hyaluronan synthesis, interstitial oedema and enlargement of extraocular muscles. Cellular immunity, with predominantly CD4+ T cells expressing Th1 cytokines, and overexpression of macrophage-derived cytokines, perpetuate orbital inflammation. Orbital fibroblasts appear to be the major effector cells. Orbital fibroblasts express both thyrotropin receptor (TSHR) and insulin-like growth factor-1 receptor (IGF-1R) at higher levels than normal fibroblasts. TSHR expression increases in adipogenesis; TSHR agonism enhances hyaluronan production. IGF-1R stimulation leads to adipogenesis, hyaluronan synthesis and production of the chemokines, interleukin (IL)-16 and Regulated on Activation, Normal T Cell Expression and Secreted, which facilitate lymphocyte trafficking into the orbit. Immune activation uses a specific CD40:CD154 molecular bridge to activate orbital fibroblasts, which secrete pro-inflammatory cytokines including IL-1β, IL-1α, IL-6, IL-8, macrophage chemoattractant protein-1 and transforming growth factor-β, to perpetuate orbital inflammation. Molecular pathways including adenylyl cyclase/cyclic adenosine monophosphate, phophoinositide 3 kinase/AKT/mammalian target of rapamycin, mitogen-activated protein kinase are involved in TO. The emergence of a TO animal model and a new generation of TSHR antibody assays increasingly point towards TSHR as the primary autoantigen for extrathyroidal orbital involvement. Oxidative stress in TO resulting from imbalances of the oxidation-reduction state provides a framework of understanding for smoking prevention, achieving euthyroidism and the use of antioxidants such as selenium. Progress has been made in the understanding of the pathogenesis of TO, which should advance development of novel therapies targeting cellular immunity, specifically the CD40:CD40 ligand interaction, antibody-producing B cells, cytokines, TSHR and IGF-1R and its signalling pathways. Further studies in signalling networks and molecular triggers leading to burnout of TO will further our understanding of TO.

Forkhead Transcription Factor FOXO1 Is Regulated by Both a Stimulatory Thyrotropin Receptor Antibody and Insulin-Like Growth Factor-1 in Orbital Fibroblasts from Patients with Graves' Ophthalmopathy

BACKGROUND

Activation of thyrotropin receptor (TSHR) and/or insulin-like growth factor (IGF-1) receptor (IGF-1R) enhances HA production and adipogenesis in orbital fibroblasts from patients with Graves' ophthalmopathy (GO) and recapitulates the tissue remodeling characteristic of the orbit in GO. A functional relationship between TSHR and IGF-1R has long been postulated, and recently bidirectional crosstalk between the receptors in GO fibroblasts was demonstrated. Because the transcription factor Forkhead box O-1 (FOXO1) was recently shown to be a critical downstream mediator of TSH and IGF-1 effects on thyrocyte proliferation, studies were designed to determine whether FOXO1 might similarly act as a common mediator of M22, a stimulatory TSHR antibody (TSAb), and IGF-1 in GO orbital fibroblasts.

METHODS

FOXO1 mRNA and protein were measured in orbital tissue specimens derived from normal individuals and patients with GO. In addition, the control of FOXO1 cellular localization was investigated using quantitative Western blotting of fractionated cell lysates from orbital fibroblasts treated with M22 and/or IGF-1 with or without specific TSHR, IGF-1R, or PI3K/AKT1/2 inhibitors.

RESULTS

Significantly lower levels of both FOXO1 mRNA and protein were found in GO orbital tissue specimens compared with normal orbital tissues (M = 39%, p = 0.043; M = 46.4%; p = 0.028, respectively). In addition, treatment of GO orbital cultures with M22, IGF-1, or M22 plus IGF-1 increased cytoplasmic FOXO1 compared with control (1.63-fold, p = 0.008; 1.68-fold, p = 0.001; 1.61-fold, p ≤ 0.001, respectively) and decreased nuclear FOXO1 (M = 28%, p = 0.002; M = 38%, p ≤ 0.001; M = 35%, p = 0.007, respectively). These effects were inhibited by co-treatment with the respective, but not the opposite, receptor antagonist. AKT inhibition of M22 or IGF-1-treated cultures was found to increase nuclear (1.4-fold, p = 0.026; 1.3-fold, p = 0.001, respectively) and decrease cytoplasmic (24.2%, p = 0.001; 36%, p = 0.004, respectively) FOXO1 localization.

CONCLUSIONS

These data point to FOXO1 as an important mediator of TSAb and IGF-1 action via their cognate receptors in GO orbital fibroblasts. These findings provide a link between the low FOXO1 protein levels demonstrated in GO orbital tissue and the tissue remodeling characteristic of GO, and suggest novel therapy for GO aimed at increasing nuclear expression of FOXO1 in GO target cells.

Targeting the thyroid-stimulating hormone receptor with small molecule ligands and antibodies

INTRODUCTION

The thyroid-stimulating hormone receptor (TSHR) is the essential molecule for thyroid growth and thyroid hormone production. Since it is also a key autoantigen in Graves' disease and is involved in thyroid cancer pathophysiology, the targeting of the TSHR offers a logical model for disease control.

AREAS COVERED

We review the structure and function of the TSHR and the progress in both small molecule ligands and TSHR antibodies for their therapeutic potential.

EXPERT OPINION

Stabilization of a preferential conformation for the TSHR by allosteric ligands and TSHR antibodies with selective modulation of the signaling pathways is now possible. These tools may be the next generation of therapeutics for controlling the pathophysiological consequences mediated by the effects of the TSHR in the thyroid and other extrathyroidal tissues.

Bidirectional TSH and IGF-1 receptor cross talk mediates stimulation of hyaluronan secretion by Graves' disease immunoglobins

CONTEXT

There is no pathogenetically linked medical therapy for Graves' ophthalmopathy (GO). Lack of animal models and conflicting in vitro studies have hindered the development of such therapy. Recent reports propose that Graves' Igs bind to and activate thyrotropin receptors (TSHRs) and IGF-1 receptors (IGF-1Rs) on cells in orbital fat, stimulating hyaluronan (HA) secretion, a component of GO.

OBJECTIVE

The objective of the study was to investigate potential cross talk between TSHRs and IGF-1Rs in the pathogenesis of GO using a sensitive HA assay.

DESIGN/SETTING/PARTICIPANTS

Orbital fibroblasts from GO patients were collected in an academic clinical practice and cultured in a research laboratory. Cells were treated with TSH, IGF-1, and a monoclonal Graves' Ig M22.

MAIN OUTCOME MEASURES

HA was measured by a modified ELISA.

RESULTS

Simultaneous activation by TSH and IGF-1 synergistically increased HA secretion from 320 ± 52 for TSH and 430 ± 65 μg/mL for IGF-1 alone, to 1300 ± 95 μg/mL. IGF-1 shifted the TSH EC50 19-fold to higher potency. The dose response to M22 was biphasic. An IGF-1R antagonist inhibited the higher potency phase but had no effect on the lower potency phase. M22 did not cause IGF-1R autophosphorylation. A TSHR antagonist abolished both phases of M22-stimulated HA secretion.

CONCLUSIONS

M22 stimulation of HA secretion by GO fibroblasts/preadipocytes involves cross talk between TSHR and IGF-1R. This cross talk relies on TSHR activation rather than direct activation of IGF-1R and leads to synergistic stimulation of HA secretion. These data propose a model for GO pathogenesis that explains previous contradictory results and argues for TSHR as the primary therapeutic target for GO.

Discovery and Development of Small Molecule Allosteric Modulators of Glycoprotein Hormone Receptors

Glycoprotein hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are heterodimeric proteins with a common α-subunit and hormone-specific β-subunit. These hormones are dominant regulators of reproduction and metabolic processes. Receptors for the glycoprotein hormones belong to the family of G protein-coupled receptors. FSH receptor (FSHR) and LH receptor are primarily expressed in somatic cells in ovary and testis to promote egg and sperm production in women and men, respectively. TSH receptor is expressed in thyroid cells and regulates the secretion of T3 and T4. Glycoprotein hormones bind to the large extracellular domain of the receptor and cause a conformational change in the receptor that leads to activation of more than one intracellular signaling pathway. Several small molecules have been described to activate/inhibit glycoprotein hormone receptors through allosteric sites of the receptor. Small molecule allosteric modulators have the potential to be administered orally to patients, thus improving the convenience of treatment. It has been a challenge to develop a small molecule allosteric agonist for glycoprotein hormones that can mimic the agonistic effects of the large natural ligand to activate similar signaling pathways. However, in the past few years, there have been several promising reports describing distinct chemical series with improved potency in preclinical models. In parallel, proposal of new structural model for FSHR and in silico docking studies of small molecule ligands to glycoprotein hormone receptors provide a giant leap on the understanding of the mechanism of action of the natural ligands and new chemical entities on the receptors. This review will focus on the current status of small molecule allosteric modulators of glycoprotein hormone receptors, their effects on common signaling pathways in cells, their utility for clinical application as demonstrated in preclinical models, and use of these molecules as novel tools to dissect the molecular signaling pathways of these receptors.

An update on the medical treatment of Graves' hyperthyroidism

Medical treatment of Graves' hyperthyroidism is based on the use of thionamides; namely, methimazole and propylthiouracil. In the past, methimazole was preferred by European endocrinologists, whereas propylthiouracil was the first choice for the majority of their North American colleagues. However, because of the recent definition of a better side-effect profile, methimazole is nowadays the first choice world while. Although thionamides are quite effective for the short-term control of Graves' hyperthyroidism, a relatively high proportion of patients relapses after thionamide withdrawal. Other possible medical treatments, include iodine and compounds containing iodine, perchlorate, lithium (as an adjuvant in patients undergoing radioiodine therapy), β-adrenergic antagonists, glucocorticoids, and some new molecules still under investigation. Management of Graves' hyperthyroidism using thionamides as well as the other available medical treatments is here reviewed in detail, with a special mention of situations such as pregnancy and lactation, as well as neonatal and fetal thyrotoxicosis.

Teprotumumab, an IGF-1R blocking monoclonal antibody inhibits TSH and IGF-1 action in fibrocytes

CONTEXT

Thyroid-associated ophthalmopathy (TAO) is the component of Graves' disease characterized by orbital inflammation and connective tissue remodeling. The IGF-1 receptor (IGF-1R) and TSH receptor (TSHR) form a physical and functional complex in orbital fibroblasts. A subset of these fibroblasts is derived from infiltrating CD34(+) fibrocytes. Teprotumumab (RV 001, R1507) is a human monoclonal anti-IGF-1R blocking antibody currently undergoing a phase 2 clinical trial in patients with active TAO.

OBJECTIVE

To determine whether teprotumumab inhibits the induction by TSH of IL-6 and IL-8 in fibrocytes.

DESIGN

Fibrocytes were treated without or with teprotumumab in combination with IGF-1 or TSH.

MAIN OUTCOME MEASURES

IL-6 and IL-8 mRNA expression and protein production were analyzed by real-time PCR and Luminex, respectively. Phosphorylated Akt (S473) levels were analyzed by Western blot. TSHR and IGF-1R display was assessed by flow cytometry.

RESULTS

Fibrocyte display of IGF-1R and TSHR was reduced with teprotumumab, as were IGF-1- and TSH-dependent phosphorylated Akt levels. TSH induction of IL-6 and IL-8 mRNA and protein was also reduced by the monoclonal antibody.

CONCLUSIONS

Teprotumumab attenuates the actions of both IGF-1 and TSH in fibrocytes. Specifically, it blocks the induction of proinflammatory cytokines by TSH. These results provide, at least in part, the molecular rationale for interrogating the therapeutic efficacy of this antibody in TAO.

Regulation of IL-1 receptor antagonist by TSH in fibrocytes and orbital fibroblasts

CONTEXT

The IL-1 family plays important roles in normal physiology and mediates inflammation. The actions of IL-1 are modulated by multiple IL-1 receptor antagonists (IL-1RA), including intracellular and secreted forms. IL-1 has been implicated in autoimmunity, such as that occurring in Graves' disease (GD) and its inflammatory orbital manifestation, thyroid-associated ophthalmopathy (TAO). We have previously reported that CD34(+) fibrocytes, monocyte-lineage bone marrow-derived cells, express functional TSH receptor, the central antigen in GD. When activated by TSH, they produce IL-6, IL-8, and TNF-α. Moreover, they infiltrate the orbit in TAO in which they transition into CD34(+) fibroblasts and comprise a population of orbital fibroblasts (OFs). Little is known currently about any relationship between TSH, TSH receptor, and the IL-1 pathway.

OBJECTIVE

The objective of the study was to determine whether TSH regulates IL-1RA in fibrocytes and OFs.

DESIGN/SETTING/PARTICIPANTS

Fibrocytes and OFs were collected and analyzed from healthy individuals and those with GD in an academic clinical practice.

MAIN OUTCOME MEASURES

Real-time PCR, Western blot analysis, reporter gene assays, and cell transfections were performed.

RESULTS

TSH induces the expression of IL-1RA in fibrocytes and GD-OFs. The patterns of induction diverge quantitatively and qualitatively in the two cell types. This results from relatively small effects on gene transcription-related events but a greater influence on secreted IL-1RA and intracellular IL-1RA mRNA stabilities. These actions of TSH are dependent on the intermediate induction of IL-1α and IL-1β.

CONCLUSIONS

Our findings for the first time directly link activities of the TSH and IL-1 pathways. Furthermore, they identify novel molecular interactions that could be targeted as therapy for TAO.

A modified ELISA accurately measures secretion of high molecular weight hyaluronan (HA) by Graves' disease orbital cells

Excess production of hyaluronan (hyaluronic acid [HA]) in the retro-orbital space is a major component of Graves' ophthalmopathy, and regulation of HA production by orbital cells is a major research area. In most previous studies, HA was measured by ELISAs that used HA-binding proteins for detection and rooster comb HA as standards. We show that the binding efficiency of HA-binding protein in the ELISA is a function of HA polymer size. Using gel electrophoresis, we show that HA secreted from orbital cells is primarily comprised of polymers more than 500 000. We modified a commercially available ELISA by using 1 million molecular weight HA as standard to accurately measure HA of this size. We demonstrated that IL-1β-stimulated HA secretion is at least 2-fold greater than previously reported, and activation of the TSH receptor by an activating antibody M22 from a patient with Graves' disease led to more than 3-fold increase in HA production in both fibroblasts/preadipocytes and adipocytes. These effects were not consistently detected with the commercial ELISA using rooster comb HA as standard and suggest that fibroblasts/preadipocytes may play a more prominent role in HA remodeling in Graves' ophthalmopathy than previously appreciated.

A selective TSH receptor antagonist inhibits stimulation of thyroid function in female mice

Because the TSH receptor (TSHR) plays an important role in the pathogenesis of thyroid disease, a TSHR antagonist could be a novel treatment. We attempted to develop a small molecule, drug-like antagonist of TSHR signaling that is selective and active in vivo. We synthesized NCGC00242364 (ANTAG3) by chemical modification of a previously reported TSHR antagonist. We tested its potency, efficacy, and selectivity in a model cell system in vitro by measuring its activity to inhibit stimulation of cAMP production stimulated by TSH, LH, or FSH. We tested the in vivo activity of ANTAG3 by measuring its effects to lower serum free T4 and thyroid gene expression in female BALB/c mice continuously treated with ANTAG3 for 3 days and given low doses of TRH continuously or stimulated by a single administration of a monoclonal thyroid-stimulating antibody M22. ANTAG3 was selective for TSHR inhibition; half-maximal inhibitory doses were 2.1 μM for TSHR and greater than 30 μM for LH and FSH receptors. In mice treated with TRH, ANTAG3 lowered serum free T4 by 44% and lowered mRNAs for sodium-iodide cotransporter and thyroperoxidase by 75% and 83%, respectively. In mice given M22, ANTAG3 lowered serum free T4 by 38% and lowered mRNAs for sodium-iodide cotransporter and thyroperoxidase by 73% and 40%, respectively. In conclusion, we developed a selective TSHR antagonist that is effective in vivo in mice. This is the first report of a small-molecule TSHR antagonist active in vivo and may lead to a drug to treat Graves' disease.

Constitutive activities in the thyrotropin receptor: regulation and significance

The thyroid-stimulating hormone receptor (TSHR, or thyrotropin receptor) is a family A G protein-coupled receptor. It not only binds thyroid-stimulating hormone (TSH, or thyrotropin) but also interacts with autoantibodies under pathological conditions. The TSHR and TSH are essential for thyroid growth and function and thus for all thyroid hormone-associated physiological superordinated processes, including metabolism and development of the central nervous system. In vitro studies have found that the TSHR permanently stimulates ligand-independent (constitutive) activation of Gs, which ultimately leads to intracellular cAMP accumulation. Furthermore, a vast variety of constitutively activating mutations of TSHR-at more than 50 different amino acid positions-have been reported to enhance basal signaling. These lead in vivo to a "gain-of-function" phenotype of nonautoimmune hyperthyroidism or toxic adenomas. Moreover, many naturally occurring inactivating mutations are known to cause a "loss-of-function" phenotype, resulting in resistance to thyroid hormone or hyperthyrotropinemia. Several of these mutations are also characterized by impaired basal signaling, and these are designated here as "constitutively inactivating mutations" (CIMs). More than 30 amino acid positions with CIMs have been identified so far. Moreover, the permanent TSHR signaling capacity can also be blocked by inverse agonistic antibodies or small drug-like molecules, which both have a potential for clinical usage. In this chapter, information on constitutive activity in the TSHR is described, including up- and downregulation, linked protein conformations, physiological and pathophysiological conditions, and related intracellular signaling.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.

A comprehensive metabolic modeling of thyroid pathway in relation to thyroid pathophysiology and therapeutics

The thyroid pathway represents a complex interaction of different glands for thyroid hormone synthesis. Thyrotropin releasing hormone is synthesized in the hypothalamus and regulates thyrotropin stimulating hormone gene expression in the pituitary gland. In order to understand the complexity of the thyroid pathways, and using experimental data retrieved from the biomedical literature (e.g., NCBI, HuGE Navigator, Protein Data Bank, and KEGG), we constructed a metabolic map of the thyroid hormone pathway at a molecular level and analyzed it topologically. A total of five hub nodes were predicted in regards to the transcription thyroid receptor (TR), cAMP response element-binding protein (CREB), signal transducer and activator of transcription 3 (STAT 3), nuclear factor kappa-light-chain-enhancer of activated B cells (NFkB), and activator protein 1 (AP-1) as being potentially important in study of thyroid disorders and as novel putative therapeutic drug targets. Notably, the thyroid receptor is a highly connected hub node and currently used as a therapeutic target in hypothyroidism. Our analysis represents the first comprehensive description of the thyroid pathway, which pertains to understanding the function of the protein and gene interaction networks. The findings from this study are therefore informative for pathophysiology and rational therapeutics of thyroid disorders.

A small molecule antagonist inhibits thyrotropin receptor antibody-induced orbital fibroblast functions involved in the pathogenesis of Graves ophthalmopathy

CONTEXT

Graves ophthalmopathy (GO) is an autoimmune disorder characterized by increased adipogenesis and hyaluronan (HA) production by orbital fibroblasts. Circulating autoantibodies (thyroid-stimulating antibodies [TSAbs]) directed at the thyrotropin receptor (TSHR) on these cells stimulate or augment these cellular processes. A recently developed drug-like small molecule inverse agonist of TSHR, NCGC00229600, termed 1, binds to TSHR and blocks basal and stimulated signal transduction.

OBJECTIVE

The purpose of this article was to determine whether 1 might inhibit HA production and relevant signaling pathways in orbital fibroblasts cultured in the presence of monoclonal TSAbs or bovine TSH (bTSH).

DESIGN

Primary cultures of undifferentiated GO orbital fibroblasts (n = 13) were untreated or treated with a TSAb (M22 or MS-1) or bTSH in serum-free medium, with or without 1 or a TSHR neutral antagonist, NCGC00242595, termed 2, which does not inhibit basal signaling but does inhibit stimulated signaling.

MAIN OUTCOME MEASURES

cAMP production, Akt phosphorylation (Ser473pAkt in media and immunoblotting for pAkt/total Akt), and HA production were analyzed.

RESULTS

Compound 1 inhibited basal cAMP, pAkt, and HA production and that stimulated by M22 in undifferentiated orbital fibroblasts. Inhibition of HA production was dose-dependent, with a half-maximal inhibitory dose of 830 nM. This compound also inhibited MS-1- and bTSH-stimulated cAMP, pAkt, and HA production. Compound 2 did not inhibit basal HA production but did inhibit M22-stimulated HA production.

CONCLUSIONS

Because cAMP, pAkt, and HA production are fibroblast functions that are activated via TSHR signaling and are important in the pathogenesis of GO, small molecule TSHR antagonists may prove to be effective in the treatment or prevention of the disease in the future.