Effects of long-acting thyroid stimulator (LATS) and LATS protector on human thyroid adenyl cyclase activity

Teprotumumab: Interpreting the Clinical Trials in the Context of Thyroid Eye Disease Pathogenesis and Current Therapies

Teprotumumab, a monoclonal antibody targeted against the insulin-like growth factor 1 (IGF-1) receptor, was recently approved by the United States Food and Drug Administration for the treatment of thyroid eye disease (TED). Phase 1 studies of teprotumumab for the treatment of malignancies demonstrated an acceptable safety profile but limited effectiveness. Basic research implicating the IGF-1 receptor on the CD-34+ orbital fibrocyte in the pathogenesis of TED renewed interest in the drug. Two multicenter, randomized, double-masked, clinical trials (phase 2 and 3) evaluated the efficacy of 8 infusions of teprotumumab every 3 weeks versus placebo in 170 patients with recent-onset active TED, as defined by a clinical activity score (CAS) of at least 4. Teprotumumab was superior to placebo for the primary efficacy end points in both studies: overall responder rate as defined by a reduction of 2 or more CAS points and a reduction of 2 mm or more in proptosis (69% vs. 20%; P < 0.001; phase 2 study) and proptosis responder rate as defined by a reduction of 2 mm or more in proptosis (83% vs. 10%; P < 0.001; phase 3 study). In both studies, treatment with teprotumumab compared with placebo achieved a significant mean reduction of proptosis (-3.0 mm vs. -0.3 mm, phase 2 study; -3.32 mm vs. -0.53 mm, phase 3 study) and CAS (-4.0 vs. -2.5, phase 2 study; -3.7 vs. -2.0, phase 3 study). Teprotumumab also resulted in a greater proportion of patients with a final CAS of 0 or 1, higher diplopia responder rate, and a larger improvement in the Graves' Ophthalmopathy Quality of Life overall score. More than half of patients (62%, phase 2 trial; 56%, phase 3 trial) who were primary end point responders maintained this response at 51 weeks after the last dose of therapy. The most common adverse events reported with teprotumumab included muscle spasms (25%), nausea (17%), alopecia (13%), diarrhea (13%), fatigue (10%), hearing impairment (10%), and hyperglycemia (8%). Teprotumumab is contraindicated for those with inflammatory bowel disease and who are pregnant. Although the current dosing regimen has proven effective for TED, dose-ranging studies including variable concentrations, infusion frequencies, and durations of teprotumumab therapy in the setting of TED have not been performed.

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.

Structure-function relations of the thyrotropin receptor

The thyrotropin (thyroid-stimulating hormone or TSH) receptor is an amphiphilic membrane component with a relative molecular mass of about 200,000 as judged by gel filtration and an isoelectric point close to pH 5. Analyses with chemical, enzymic and affinity probes indicate that the receptor is a glycoprotein containing a disulphide bridge and that the integrity of the disulphide bond is essential for maintaining the structure of the TSH-binding site. Serum from patients with Graves' disease contains antibodies which inhibit the binding of TSH to its receptor and there is considerable evidence that this effect is due to a direct interaction between the antibodies and the receptor. The antibody-receptor interaction is probably responsible for the TSH agonist properties of Graves' serum and, similarly, the TSH antagonist properties of the sera from a small number of patients can be explained on the basis of antibody-receptor binding. Although TSH and IgG from Graves' disease patients appear to bind to the same receptor, the relationship between the sites for the two substances is not clearly understood. However, Fab fragments of Graves' IgG are as effective as intact IgG in competing with TSH for the receptor and gel filtration and immunoprecipitation studies indicate that the binding of hormone and antibody to the receptor is mutually exclusive. Current evidence suggests therefore that the binding sites for TSH and TSH receptor antibodies are very closely related and may well be identical.

Short- and long-term evaluation of normal and abnormal human thyroid cells in monolayer culture

We have investigated the TSH responsiveness of normal and abnormal human thyroid cells cultured in the short term with high serum concentrations and for up to 6 months in a low serum, chemically-defined, medium. Cells from normal human thyroid tissue (n = 9), multinodular goitre (n = 6), benign follicular adenomata (n = 6), and differentiated thyroid carcinoma (n = 3) formed confluent monolayers which were sensitive to bovine TSH (bTSH) in concentrations greater than 25 microU/ml when assessed by the intracellular response of cyclic AMP at 7 d of culture. Such sensitivity was less than that observed with a continuously proliferating thyroid cell line (FRTL-5) derived from Fisher rat thyroid and which responded to concentrations of bTSH as low as 5-10 microU/ml. Human cells derived from iodine/antithyroid drug treated Graves' thyroid tissue (n = 6) were less sensitive than normal cells requiring up to 500 microU/ml bTSH to increase intracellular cyclic AMP and poorly differentiated thyroid cancer cells (n = 3) failed to respond to bTSH. Long-term human thyroid cultures of normal and follicular adenoma cells in the chemically-defined medium used for the FRTL-5 cells had absent fibroblast growth and continued in monolayer form without significant follicle formation. These cells remained highly sensitive to bTSH stimulation when tested after 4, 13, and 26 weeks of continuous culture. All such cell preparations failed to proliferate under conditions which favoured the rapid growth of the rat thyroid cells. These data demonstrated that while thyroid cell culture conditions described in the literature do not permit proliferation of human thyroid cells, they do allow an assessment of their functional state in vitro which may lead to a further understanding of thyroid cell pathophysiology.

Stimulation of iodine organification in porcine thyroid cells by thyroid stimulators

Several Graves' sera were simultaneously assessed in a bioassay based on the ability of porcine thyroid cells to organify 125I and in a radioreceptor assay for TSH receptor binding activity. Both assay systems were sensitive to 1 mcU/ml (final concentration) of unlabelled bovine TSH. Six Graves' sera were studied in detail over a wide (0-1.0 mcl sera) dose response range in repeat determinations. Two sera exhibited parallel binding and stimulating. However, two sera revealed significant inhibition of 125I-TSH binding prior to the demonstration of stimulation and the other two sera showed stimulatory capabilities before significant binding was evident. IgG was prepared from one serum by ammonium sulphate precipitation and chromatography on Sepharose 6B and then subjected to preparative isoelectric focusing. The isoelectric distribution of the two activities were found to be identical with major peaks of activity at pl=9.5 and pl=8.5. In summary: 1) each Graves' sera exhibits different dose-response curves with respect to binding and stimulation, 2) at certain concentrations of sera, only binding or stimulation were evident, 3) neither assay was consistently more sensitive for the presence of Graves' immunoglobulins, 4) for one Graves' sera, binding and stimulation could not be separated by isoelectric focusing. These studies would suggest each Graves' immunoglobulin has inherently different characteristics in its interaction with the TSH receptor.

The nude mouse. A possible experimental model for investigation of human thyroid tissue

The effect of transplanting hyperfunctioning human thyroid tissue to athymic, nude mice was explored. The thyroid tissue was obtained from a patient with diffuse, thyrotoxic goiter preoperatively treated with a beta-adrenoreceptor blocker. One transplant was placed in each groin of 4 nude mice. Light microscopy after 4 weeks showed that transplants consisted mainly of typical follicles; capillaries were common at the periphery of the transplant but not in between the follicles. The ultrastructure of the follicle cells in the transplants was similar to that of follicle cells in general. All 8 transplants accumulated 125I as revealed by external counting; the uptake in each transplant 24 h after administration of radioiodine was 4-25% of that in the thyroid of the transplanted mouse. The release of radioiodine, measured during a period of 14 days, from the thyroid of a transplanted mouse was delayed as compared to a nontransplanted control; this indicates that the mouse thyroid activity was suppressed due to hormone production in the transplants. Electron microscopic autoradiography 24 h after injection of 125I showed that protein-bound label in transplants was located mainly in follicle lumens. The radioiodine was incorporated in thyroglobulin as demonstrated by separation of soluble proteins from transplants by sucrose density gradient centrifugation. The present study shows that human thyroid tissue can be transplanted to nude mice with maintained structural and functional properties. Transplantation of human thyroid tissue might be a useful model for studies of thyroid diseases.

Thyroid adenylate cyclase stimulating immunoglobulins in thyroid diseases

The occurrence of serum immunoglobulins with capacity to stimulate thyroid adenylate cyclase (TSAb) was studied in seventy-two healthy volunteers and 120 unselected patients with various thyroid diseases. A high frequency of TSAb (82.5%, P less than 0.00006) was found in Graves' disease, while TSAb was present only in 13--20% of serum from patients with nontoxic nodular goitre, nontoxic diffuse goitre, toxic adenoma, toxic nodular goitre and myxoedema. These patients had low level of TSAb compared to patients with Graves' disease. In patients with Graves' disease there was no correlation between the level of TSAb and hormonal status except serum triiodothyronine (rs = 0.29, P less than 0.05), and no relation with eye involvement or presence of microsomal thyroid antibodies was found. The results indicate that the human thyroid adenylate cyclase assay system with 1 hour incubation periods is a sensitive method for detection of immunoglobulins with TSH-like capacity to stimulate the thyroid gland.

TSH-responsive adenylate cyclase activity in thyroid tissue from patients with Graves' disease

Basal adenylate cyclase activity of thyroid plasma membranes obtained from six patients with Graves' disease was slightly but not significantly lower than normal (83.3 +/- 13.9 pmol cAMP/10 min/mg of protein versus 120.9 +/- 19.5 pmol cAMP/10 min/mg of protein). In five of these patients the adenylate cyclase activity was stimulated by bovine TSH with an apparent Km value similar to that of normal thyroid (3.1 +/- 0.5 X 10-9 M versus 3.4 +/- 0.6 X 10-9 M). The response to prostaglandin E2 was also normal. In the sixth patient adenylate cyclase activity was stimulated by prostaglandin E2 but not by bovine TSH. The distribution of basal adenylate cyclase activity in various gradient layers was studied in two TSH-responsive patients. A relative increase of this activity was found in the denser layer when compared to normal thyroid tissue. This could be the expression of an altered ratio between the protein and lipid components of the plasma membranes in patients with Graves' disease.

Characterization of antibodies to the insulin receptor: a cause of insulin-resistant diabetes in man

We have characterized the circulating inhibitor of insulin receptor binding found in several patients with a new syndrome of extreme insulin resistance. The inhibitor is an immunoglobulin by multiple criteria, including precipitation by 33% ammonium sulfate, migration on G-200 Sephadex gel filtration and DEAE chromatography, and immuno-precipitation with specific anti-human immuno-globulins. Although predominantly IgG, some activity is found in the IgM fraction of the immunoglobulins in one patient. The inhibitory immunoglobulins reacted with antisera to both kappa and lambda light chain determinants and are therefore polyclonal. In addition, activity is retained in the F(ab')2 fraction of pepsin-digested IgG. Evidence suggests that these antibodies are directed at determinants on or near the insulin receptor, and that they are responsible for the observed clinical insulin resistance.

Relation of thyroid-stimulating immunoglobulins to thyroid function and effects of surgery, radioiodine, and antithyroid drugs

Thyroid-stimulating immunoglobulins (T.S.I.) have been detected in the serum of all patients with untreated Graves' disease, and in these patients the levels of T.S.I. correlated significantly with the early uptake of 131I by the thyroid. The frequency of T.S.I. in patients treated solely by antithyroid drugs, by radioiodine, or by partial thyroidectomy was 53 per cent, 50 per cent, and 17 per cent, respectively. The reduced frequency of T.S.I. in the serum of patients treated by drugs or radioiodine was probably due to spontaneous remission, but in the case of partial thyroidectomy the operation itself clearly had a dramatic effect on the serum-T.S;I. These results accorded well with the reported frequency of thyroid autonomy in similar groups of patients and suggested that thyroid-stimulating immunoglobulins were responsible for hyperthyroidism in Graves' disease.

Association of the long-acting thyroid stimulator protector with the immunoglobulin G fraction of serum from patients with thyrotoxicosis

Both the long-acting thyroid stimulator and the long-acting thyroid stimulator protector are associated with the immunoglobulin G fraction of human serum proteins. Fractionation of globulins by (NH4)2SO4 precipitation and DEAE-cellulose chromatography showed that the protector activity was associated with subfractions of differing electrophoretic mobility. Attempts to concentrate the protector in immunochemically pure immunoglobulin G prepared in this way may fail because of loss of protector activity with other immunoglobulin G fractions. Gel filtration on Sephadex G-150 indicated that protector activity was associated with 7-S proteins.

A radio-ligand receptor assay for the long-acting thyroid stimulator. Inhibition by the long-acting thyroid stimulator of the binding of radioiodinated thyroid-stimulating hormone to human thyroid membranes

Highly purified bovine TSH (thyroid-stimulating hormone) was labelled with 125I by using very low concentrations of chloramine-T. Human thyroid membranes prepared by discontinuous sucrose-density-gradient centrifugation were homogeneous on examination by electron microscopy. Incubation of radioiodinated TSH with the membranes showed that radioactivity could be bound to the membranes. Under the experimental conditions described here, binding was dependent on time and temperature and was a saturable phenomenon. Preincubation of the membranes with unlabelled hormone inhibited the subsequent binding of 125I-labelled TSH. Similarly, inhibition by the long-acting thyroid stimulator also showed a saturation behaviour. A rapid and sensitive method for the detection of the long-acting thyroid stimulator is described.