Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration.

Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and metabolism. Recent studies have implicated TH signaling in cone photoreceptor viability. Using mouse models of retinal degeneration, we demonstrated that antithyroid drug treatment and targeting iodothyronine deiodinases (DIOs) to suppress cellular tri-iodothyronine (T3) production or increase T3 degradation preserves cones. In this work, we investigated the effectiveness of inhibition of the TH receptor (TR). Two genes, THRA and THRB, encode TRs; THRB2 has been associated with cone viability. Using TR antagonists and Thrb2 deletion, we examined the effects of TR inhibition. Systemic and ocular treatment with the TR antagonists NH-3 and 1-850 increased cone density by 30-40% in the Rpe65-/- mouse model of Leber congenital amaurosis and reduced the number of TUNEL+ cells. Cone survival was significantly improved in Rpe65-/- and Cpfl1 (a model of achromatopsia with Pde6c defect) mice with Thrb2 deletion. Ventral cone density in Cpfl1/Thrb2-/- and Rpe65-/- /Thrb2-/- mice was increased by 1- to 4-fold, compared with age-matched controls. Moreover, the expression levels of TR were significantly higher in the cone-degeneration retinas, suggesting locally elevated TR signaling. This work shows that the effects of antithyroid treatment or targeting DIOs were likely mediated by TRs and that suppressing TR protects cones. Our findings support the view that inhibition of TR locally in the retina is a therapeutic strategy for retinal degeneration management.-Ma, H., Yang, F., Butler, M. R., Belcher, J., Redmond, T. M., Placzek, A. T., Scanlan, T. S., Ding, X.-Q. Inhibition of thyroid hormone receptor locally in the retina is a therapeutic strategy for retinal degeneration.

Ester-to-amide rearrangement of ethanolamine-derived prodrugs of sobetirome with increased blood-brain barrier penetration.

Current therapeutic options for treating demyelinating disorders such as multiple sclerosis (MS) do not stimulate myelin repair, thus creating a clinical need for therapeutic agents that address axonal remyelination. Thyroid hormone is known to play an important role in promoting developmental myelination and repair, and CNS permeable thyromimetic agents could offer an increased therapeutic index compared to endogenous thyroid hormone. Sobetirome is a clinical stage thyromimetic that has been shown to have promising activity in preclinical models related to MS and X-linked adrenoleukodystrophy (X-ALD), a genetic disease that involves demyelination. Here we report a new series of sobetirome prodrugs containing ethanolamine-based promoieties that were found to undergo an intramolecular O,N acyl migration to form the pharmacologically relevant amide species. Several of these systemically administered prodrugs deliver more sobetirome to the brain compared to unmodified sobetirome. Pharmacokinetic properties of the parent drug sobetirome and amidoalcohol prodrug 3 are described and prodrug 3 was found to be more potent than sobetirome in target engagement in the brain from systemic dosing.

Elevated Serum Tetrac in Graves Disease: Potential Pathogenic Role in Thyroid-Associated Ophthalmopathy.

Context: The sources and biological impact of 3,3',5,5' tetraiodothyroacetic acid (TA4) are uncertain. CD34+ fibrocytes express several proteins involved in the production of thyroid hormones. They infiltrate the orbit in Graves disease (GD), an autoimmune process known as thyroid-associated ophthalmopathy. It appears that the thyrotropin receptor plays an important role in the pathogenesis of thyroid-associated ophthalmopathy. Objective: To quantify levels of TA4 in healthy participants and those with GD, determine whether fibrocytes generate this thyroid hormone analogue, and determine whether TA4 influences the actions of thyroid-stimulating hormone and thyroid-stimulating immunoglobulins in orbital fibroblasts. Design/Setting/Participants: Patients with GD and healthy donors in an academic medical center clinical practice were recruited. Main Outcome Measures: Liquid chromatography-tandem mass spectrometry, autoradiography, real-time polymerase chain reaction, hyaluronan immunoassay. Results: Serum levels of TA4 are elevated in GD. TA4 levels are positively correlated with those of thyroxine and negatively correlated with serum levels of triiodothyronine. Several cell types in culture generate TA4 from ambient thyroxine, including fibrocytes, HELA cells, human Müller stem cells, and retinal pigmented epithelial cells. Propylthiouracil inhibits TA4 generation. TA4 enhances the induction by thyrotropin and thyroid-stimulating immunoglobulins of several participants in the pathogenesis of thyroid-associated ophthalmopathy, including interleukin 6, hyaluronan synthase 1, prostaglandin endoperoxide H synthase 2, and haluronan production. Conclusion: TA4 may be ubiquitously generated in many tissues and enhances the biological impact of thyrotropin and thyroid-stimulating immunoglobulins in orbital connective tissue. These findings may identify a physiologically important determinant of extrathyroidal thyroid-stimulating hormone action.

Sobetirome prodrug esters with enhanced blood-brain barrier permeability.

There is currently great interest in developing drugs that stimulate myelin repair for use in demyelinating diseases such as multiple sclerosis. Thyroid hormone plays a key role in stimulating myelination during development and also controls the expression of important genes involved in myelin repair in adults. Because endogenous thyroid hormone in excess lacks a generally useful therapeutic index, it is not used clinically for indications other than hormone replacement; however, selective thyromimetics such as sobetirome offer a therapeutic alternative. Sobetirome is the only clinical-stage thyromimetic that is known to cross the blood-brain-barrier (BBB) and we endeavored to increase the BBB permeability of sobetirome using a prodrug strategy. Ester prodrugs of sobetirome were prepared based on literature reports of improved BBB permeability with other carboxylic acid containing drugs and BBB permeability was assessed in vivo. One sobetirome prodrug, ethanolamine ester 11, was found to distribute more sobetirome to the brain compared to an equimolar peripheral dose of unmodified sobetirome. In addition to enhanced brain levels, prodrug 11 displayed lower sobetirome blood levels and a brain/serum ratio that was larger than that of unmodified sobetirome. Thus, these data indicate that an ester prodrug strategy applied to sobetirome can deliver increased concentrations of the active drug to the central nervous system (CNS), which may prove useful in the treatment of CNS disorders.

Increasing Thyromimetic Potency through Halogen Substitution.

Sobetirome is one of the most studied thyroid hormone receptorâ€…ß (TRß)-selective thyromimetics in the field due to its excellent selectivity and potency. A small structural change-replacing the 3,5-dimethyl groups of sobetirome with either chlorine or bromine-produces significantly more potent compounds, both in vitro and in vivo. These halogenated compounds induce transactivation of a TRß-mediated cell-based reporter with an EC50 value comparable to that of T3, access the central nervous system (CNS) at levels similar to their parent, and activate an endogenous TR-regulated gene in the brain with an EC50 value roughly five-fold lower than that of sobetirome. Previous studies suggest that this apparent increase in affinity can be explained by halogen bonding between the ligand and a backbone carbonyl group in the receptor. This makes the new analogues potential candidates for treating CNS disorders that may respond favorably to thyroid-hormone-stimulated pathways.

New synthetic routes to thyroid hormone analogs: d6-sobetirome, 3H-sobetirome, and the antagonist NH-3.

New synthetic routes for the preparation of isotopically labeled versions of thyroid hormone agonist sobetirome were developed using Knochel's iodine-magnesium exchange. A more efficient synthesis of the thyroid hormone antagonist NH-3 was developed from a common intermediate in the sobetirome route. Using the new synthetic routes, d6- and 3H-sobetirome were prepared for their use in studying biodistribution and the cellular uptake of sobetirome. The new route to NH-3 allows for a more rapid and efficient synthesis and provides access to an advanced intermediate to facilitate antagonist analog production in the final bond-forming synthetic step.

Synthesis of frame-shifted farnesyl diphosphate analogs.

A set of synthetic approaches were developed and applied to the synthesis of eight frame-shifted farnesyl diphosphate (FPP) analogs. These analogs bear increased or decreased methylene units between the double bonds and/or diphosphate moieties of the isoprenoid structure. Evaluation versus mammalian FTase revealed that small structural changes can lead to dramatic changes in substrate ability.

New synthetic methodology for the construction of 7-substituted farnesyl diphosphate analogs.

Through the use of a 1,2-metalate rearrangement, six 7-substituted farnesol analogs were generated in a concise manner. This new synthetic route allowed us to quickly prepare several diverse farnesyl diphosphate analogs with interesting biological activities against mammalian protein-farnesyl transferase.

Lipidomic metabolism analysis of the endogenous cannabinoid anandamide (N-arachidonylethanolamide).

Elucidation of pathways involved with lipid metabolism has been limited by analytical challenges associated with detection and structure identification. A discovery-based mass spectrometry lipidomic approach has been applied to identify metabolites of the endogenous cannabinoid anandamide (N-arachidonylethanolamide). Previously, a model system was established to show that anandamide can be recycled by cells to form new endocannabinoids suggesting recycling of the arachidonate carbon chain. We hypothesized that distinct cellular pathways exist to direct the anandamide-derived arachidonate chain into a specific set of metabolites, different from the metabolite pool that is comprised of non-anandamide-derived arachidonic acid. Using stable isotope encoding and liquid chromatography-mass spectrometry, we identified a distinct pool of lipid metabolites derived from exogenous anandamide or arachidonic acid in RBL-2H3 cells. We discovered that arachidonic acid-derived metabolites were primarily comprised of the eicosanoid lipid class, whereas anandamide-derived arachidonic acid, in addition to eicosanoids, was metabolized into diradylglycerols, fatty acid amides, sterols, and glycerophospholipids. From the list of anandamide metabolites of particular interest was 1-O-arachidonyl-sn-glycero-3-phosphocholine. Furthermore, we determined that while 1-O-arachidonyl-sn-glycero-3-phosphocholine may be a metabolite of anandamide, the sn-2 compound was more abundant in mouse brain tissue. Overall, our results provide a novel approach to study the metabolic fate of endocannabinoids and fatty acid-derived signaling molecules.

Detection of nonsterol isoprenoids by HPLC-MS/MS.

Isoprenoids constitute an important class of biomolecules that participate in many different cellular processes. Most available detection methods allow the identification of only one or two specific nonsterol isoprenoid intermediates following radioactive or fluorescent labeling. We here report a rapid, nonradioactive, and sensitive procedure for the simultaneous detection and quantification of the eight main nonsterol intermediates of the isoprenoid biosynthesis pathway by means of tandem mass spectrometry. Intermediates were analyzed by HPLC-MS/MS in the multiple reaction monitoring mode using a silica-based C(18) HPLC column. For quantification, their stable isotope-labeled analogs were used as internal standards. HepG2 cells were used to validate the method. Mevalonate, phosphomevalonate, and the six subsequent isoprenoid pyrophosphates were readily determined with detection limits ranging from 0.03 to 1.0mumol/L. The intra- and interassay variations for HepG2 cell homogenates supplemented with isoprenoid intermediates were 3.6-10.9 and 4.4-11.9%, respectively. Under normal culturing conditions, isoprenoid intermediates in HepG2 cells were below detection limits. However, incubation of the cells with pamidronate, an inhibitor of farnesyl pyrophosphate synthase, resulted in increased levels of mevalonate, isopentenyl pyrophosphate/dimethylallyl pyrophosphate, and geranyl pyrophosphate. This method will be suitable for measuring profiles of isoprenoid intermediates in cells with compromised isoprenoid biosynthesis and for determining the specificity of potential inhibitors of the pathway.