Bioinorganic chemistry in thyroid gland: effect of antithyroid drugs on peroxidase-catalyzed oxidation and iodination reactions

Selenization of Small Molecule Drugs: A New Player on the Board

There is an urgent need to develop safer and more effective modalities for the treatment of a wide range of pathologies due to the increasing rates of drug resistance, undesired side effects, poor clinical outcomes, etc. Throughout the years, selenium (Se) has attracted a great deal of attention due to its important role in human health. Besides, a growing body of work has unveiled that the inclusion of Se motifs into a great number of molecules is a promising strategy for obtaining novel therapeutic agents. In the current Perspective, we have gathered the most recent literature related to the incorporation of different Se moieties into the scaffolds of a wide range of known drugs and their feasible pharmaceutical applications. In addition, we highlight different representative examples as well as provide our perspective on Se drugs and the possible future directions, promises, opportunities, and challenges of this ground-breaking area of research.

Novel potent natural peroxidases inhibitors with in vitro assays, inhibition mechanism and molecular docking of phenolic compounds and alkaloids

Peroxidase inhibition produced by phenolic compounds as hispidin and gallic acid, alkaloids as harmine and natural extracts of Inonotus hispidus, and Marrubium vulgare were investigated in this study. No further studies have been found in this context. Thus, the results show that the phenolic and the alkaloidal extracts with the three molecules are potent inhibitors of horseradish peroxidase. Uric acid is used as a substrate reaction to finding the enzymatic inhibition for the first time. The results show that the best inhibitor is hispidin with a value of IC50 = 23 µg/ml. Moreover, Molecular docking has been carried out using the AutoDock Vina program to discuss the nature of interactions and the mechanism of inhibition between both peroxidases (horseradish and thyroid) which is performed with and without heme group for the first time. The three studied compounds were further subjected to ADEMT and Lipinski filtering analyses for drug-likeness prediction analysis. However, the results show that all the docked molecules are competitive inhibitors confirming that no further studies have been published before. Thus, hispidin is a more potent irreversible TPO inhibitor then propylthiouracil anti-thyroid drug. Its inhibition mechanism is well described through this work for the first time; which suggests is used as an anti-thyroid drug to treat hyperthyroidism. Furthermore, the studied phenolic compounds (Hispidin and Gallic acid) and one alkaloid (Harmine) are non-toxic, that bind to the receptor-binding site and catalytic dyad of peroxidases were identified from the predictive ADMET and Lipinski filter analysis.Communicated by Ramaswamy H. Sarma.

Effect of Thyroperoxidase and Deiodinase Inhibition on Anterior Swim Bladder Inflation in the Zebrafish

A set of adverse outcome pathways (AOPs) linking inhibition of thyroperoxidase and deiodinase to impaired swim bladder inflation in fish has recently been developed. These AOPs help to establish links between these thyroid hormone (TH) disrupting molecular events and adverse outcomes relevant to aquatic ecological risk assessment. Until now, very little data on the effects of TH disruption on inflation of the anterior chamber (AC) of the swim bladder were available. The present study used zebrafish exposure experiments with three model compounds with distinct thyroperoxidase and deiodinase inhibition potencies (methimazole, iopanoic acid, and propylthiouracil) to evaluate this linkage. Exposure to all three chemicals decreased whole body triiodothyronine (T3) concentrations, either through inhibition of thyroxine (T4) synthesis or through inhibition of Dio mediated conversion of T4 to T3. A quantitative relationship between reduced T3 and reduced AC inflation was established, a critical key event relationship linking impaired swim bladder inflation to TH disruption. Reduced inflation of the AC was directly linked to reductions in swimming distance compared to controls as well as to chemical-exposed fish whose ACs inflated. Together the data provide compelling support for AOPs linking TH disruption to impaired AC inflation in fish.

Coordination of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone to zinc and cadmium: Monotonic and non-monotonic bond length variations for [H(sebenzimMe)]2MCl2 complexes (M = Zn, Cd, Hg)

The reactions of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), towards the zinc and cadmium halides, MX2 (M = Zn, Cd; X = Cl, Br, I), afford the adducts, [H(sebenzimMe)]2MX2, which have been structurally characterized by X-ray diffraction. The halide ligands of each of these complexes participate in hydrogen bonding interactions with the imidazole N-H moieties, although the nature of the interactions depends on the halide. Specifically, the chloride and bromide derivatives, [H(sebenzimMe)]2ZnX2 and [H(sebenzimMe)]2CdX2 (X = Cl, Br), exhibit two intramolecular N-H•••X interactions, whereas the iodide derivatives, [H(sebenzimMe)]2ZnI2 and [H(sebenzimMe)]2CdI2, exhibit only one intramolecular N-H•••I interaction. Comparison of the M-Se and M-Cl bond lengths of the chloride series, [H(sebenzimMe)]2MCl2 (M = Zn, Cd, Hg), indicates that while the average M-Cl bond lengths progressively increase as the metal becomes heavier, the variation in M-Se bond length exhibits a non-monotonic trend, with the Cd-Se bond being the longest. These different trends provide an interesting subtlety concerned with use of covalent radii in predicting bond length differences. In addition to tetrahedral [H(sebenzimMe)]2CdCl2, [H(sebenzimMe)]3,CdCl2•[H(sebenzim)Me]4CdCl2, which features both five-coordinate and six-coordinate coordinate centers, has also been structurally characterized. Finally, the reaction between CdI2 and H(sebenzimMe) at elevated temperatures affords the 1-methylbenzimidazole complex, [H(sebenzimMe)]-[H(benzimMe)]CdI2, a transformation that is associated with cleavage of the C-Se bond.

Effect of antithyroid drugs on the occurrence of antibodies against type 2 deiodinase (DIO2), which are involved in hyperthyroid Graves' disease influencing the therapeutic efficacy

Graves' disease is an organ-specific autoimmune disease with hyperthyroidism, diffuse goiter and autoantibodies against TSH receptor, thyroid peroxidase (TPO) and/or thyroglobulin (Tg). Graves' hyperthyroidism is characterized by T3 dominance due to the conversion of T4 into T3 through type 1 and 2 deiodinase enzymes (DIO1, DIO2). Methimazole (MMI) and propylthiouracil (PTU) therapies inhibit thyroid hormone synthesis blocking the activity of deiodinase and TPO enzymes. The study investigated the occurrence of autoantibodies against DIO2 peptides (cys- and hom-peptides) with the effect of antithyroid drugs on their frequencies in 78 patients with Graves' disease and 30 controls. In hyperthyroidism, the presence of DIO2 peptide antibodies was as follows: 20 and 11 cases out of 51 for cys- and hom-peptide antibodies, respectively, of whom 8 cases possessed antibodies against both peptides. Antithyroid drugs differently influenced their frequencies, which were greater in PTU than in MMI (3/6 vs 13/45 cases, P < 0.016 for cys- and 0/6 vs 2/45 cases for hom-peptide antibodies). Antibodies against both peptides demonstrated more reduced levels of anti-TPO (P < 0.003) and anti-Tg antibodies (P < 0.002) compared with those without peptide antibodies. PTU compared with MMI increased the levels of TSH receptor antibodies (32.5 UI/l vs 2.68 IU/l, P < 0.009). MMI treatment led to more reduced FT3 levels and FT3/FT4 ratios in hyperthyroid Graves' ophthalmopathy (P < 0.028 for FT3, P < 0.007 for FT3/FT4 ratio). In conclusion, the presence of DIO2 peptide antibodies is connected to Graves' hyperthyroidism influencing the levels of antibodies against TPO, Tg and TSH receptor, as well as the therapeutic efficacy of antithyroid drugs.

Dual binding mode of antithyroid drug methimazole to mammalian heme peroxidases - structural determination of the lactoperoxidase-methimazole complex at 1.97 Å resolution

Lactoperoxidase (LPO, EC 1.11.1.7) is a member of the mammalian heme peroxidase family which also includes thyroid peroxidase (TPO). These two enzymes have a sequence homology of 76%. The structure of LPO is known but not that of TPO. In order to determine the mode of binding of antithyroid drugs to thyroid peroxidase, we have determined the crystal structure of LPO complexed with an antithyroid drug, methimazole (MMZ) at 1.97 Å resolution. LPO was isolated from caprine colostrum, purified to homogeneity and crystallized with 20% poly(ethylene glycol)‐3350. Crystals of LPO were soaked in a reservoir solution containing MMZ. The structure determination showed the presence of two crystallographically independent molecules in the asymmetric unit. Both molecules contained one molecule of MMZ, but with different orientations. MMZ was held tightly between the heme moiety on one side and the hydrophobic parts of the side chains of Arg255, Glu258, and Leu262 on the opposite side. The back of the cleft contained the side chains of Gln105 and His109 which also interacted with MMZ. In both orientations, MMZ had identical buried areas and formed a similar number of interactions. It appears that the molecules of MMZ can enter the substrate‐binding channel of LPO in two opposite orientations. But once they reach the distal heme pocket, their orientations are frozen due to equally tight packing of MMZ in both orientations. This is a novel example of an inhibitor binding to an enzyme with two orientations at the same site with nearly equal occupancies.

Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow

In the present study, a hypothesized adverse outcome pathway linking inhibition of thyroid peroxidase (TPO) activity to impaired swim bladder inflation was investigated in two experiments in which fathead minnows (Pimephales promelas) were exposed to 2-mercaptobenzothiazole (MBT). Continuous exposure to 1mg MBT/L for up to 22 days had no effect on inflation of the posterior chamber of the swim bladder, which typically inflates around 6 days post fertilization (dpf), a period during which maternally-derived thyroid hormone is presumed to be present. In contrast, inflation of the anterior swim bladder, which occurs around 14dpf, was impacted. Specifically, at 14dpf, approximately 50% of fish exposed to 1mg MBT/L did not have an inflated anterior swim bladder. In fish exposed to MBT through 21 or 22dpf, the anterior swim bladder was able to inflate, but the ratio of the anterior/posterior chamber length was significantly reduced compared to controls. Both abundance of thyroid peroxidase mRNA and thyroid follicle histology suggest that fathead minnows mounted a compensatory response to the presumed inhibition of TPO activity by MBT. Time-course characterization showed that fish exposed to MBT for at least 4 days prior to normal anterior swim bladder inflation had significant reductions in anterior swim bladder size, relative to the posterior chamber, compared to controls. These results, along with similar results observed in zebrafish (see part II, this issue) are consistent with the hypothesis that thyroid hormone signaling plays a significant role in mediating anterior swim bladder inflation and development in cyprinids, and that role can be disrupted by exposure to thyroid hormone synthesis inhibitors. Nonetheless, possible thyroid-independent actions of MBT on anterior swim bladder inflation cannot be ruled out based on the present results. Overall, although anterior swim bladder inflation has not been directly linked to survival as posterior swim bladder inflation has, potential links to adverse ecological outcomes are plausible given involvement of the anterior chamber in sound production and detection.

Protolytic cleavage of Hg-C bonds induced by 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone: synthesis and structural characterization of mercury complexes

Multinuclear ((1)H, (77)Se, and (199)Hg) NMR spectroscopy demonstrates that 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzim(Me)), a structural analogue of the selenoamino acid, selenoneine, binds rapidly and reversibly to the mercury centers of HgX2 (X = Cl, Br, I), while X-ray diffraction studies provide evidence for the existence of adducts of composition [H(sebenzim(Me))]xHgX2 (X = Cl, x = 2, 3, 4; X = I, x = 2) in the solid state. H(sebenzim(Me)) also reacts with methylmercury halides, but the reaction is accompanied by elimination of methane resulting from protolytic cleavage of the Hg-C bond, an observation that is of relevance to the report that selenoneine demethylates CysHgMe, thereby providing a mechanism for mercury detoxification. Interestingly, the structures of [H(sebenzim(Me))]xHgX2 exhibit a variety of different hydrogen bonding patterns resulting from the ability of the N-H groups to form hydrogen bonds with chlorine, iodine, and selenium.

Halogen vs hydrogen bonding in thiazoline-2-thione stabilization with σ- and π-electron acceptors adducts: theoretical and experimental study

Molecular charge-transfer complexes (CT) between thiazoline-2-thione (THZ) and different σ- (I2) and π-acceptors (Tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL)) were investigated. UV-Vis absorption spectroscopy and theoretical calculations using both MP2/aug-cc-pVDZ-PP and B3LYP/6-311++G(d,p) level of theory were corroborated to study the nature of the stabilizing forces for THZ-I2, THZ-DDQ, THZ-TCNE, and THZ-CHL. Halogen bonding (XB) was the stabilizing attractive force in THZ-I2 and THZ-CHL whereas; hydrogen bonding (HB) was dominated in both THZ-TCNE, and THZ-DDQ complexes. Formation constant (K), extinction coefficient (ɛ), thermodynamic parameters such as enthalpy change (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) were measured in different solvents.

Mode of binding of the antithyroid drug propylthiouracil to mammalian haem peroxidases

The mammalian haem peroxidase superfamily consists of myeloperoxidase (MPO), lactoperoxidase (LPO), eosinophil peroxidase (EPO) and thyroid peroxidase (TPO). These enzymes catalyze a number of oxidative reactions of inorganic substrates such as Cl(-), Br(-), I(-) and SCN(-) as well as of various organic aromatic compounds. To date, only structures of MPO and LPO are known. The substrate-binding sites in these enzymes are located on the distal haem side. Propylthiouracil (PTU) is a potent antithyroid drug that acts by inhibiting the function of TPO. It has also been shown to inhibit the action of LPO. However, its mode of binding to mammalian haem peroxidases is not yet known. In order to determine the mode of its binding to peroxidases, the structure of the complex of LPO with PTU has been determined. It showed that PTU binds to LPO in the substrate-binding site on the distal haem side. The IC50 values for the inhibition of LPO and TPO by PTU are 47 and 30 µM, respectively. A comparision of the residues surrounding the substrate-binding site on the distal haem side in LPO with those in TPO showed that all of the residues were identical except for Ala114 (LPO numbering scheme), which is replaced by Thr205 (TPO numbering scheme) in TPO. A threonine residue in place of alanine in the substrate-binding site may affect the affinity of PTU for peroxidases.

Se- and s-based thiouracil and methimazole analogues exert different inhibitory mechanisms on type 1 and type 2 deiodinases

The thioamide anti-thyroid drugs methimazole (MMI) and propylthiouracil (PTU) play a pivotal role in the treatment of hyperthyroidism. MMI exerts its effect via inhibiting one of the key enzymes involved in synthesis of thyroid hormones (TH), thyroid peroxidase (TPO). PTU is both an inhibitor of TPO and type 1 deiodinase (D1), which catalyzes TH deiodination at both aromatic rings. In contrast, no selective inhibitors are known for type 2 deiodinase (D2) or type 3 deiodinase, which deiodinate TH at the phenolic or tyrosyl ring, respectively. We aimed to identify specific inhibitors for D1 or D2. New Se- and S-based PTU and MMI-like compounds have been generated. The D1 and D2 inhibiting capacity of several compounds was tested in vitro. Our data show that compounds based on a PTU and MMI backbone can differentially influence the reaction kinetics of deiodinases. For inhibition of D1, the addition of a phenyl group to the PTU backbone increases potency by at least 10-fold over PTU. For inhibition of D2, the addition of an aromatic ring structure to MMI and its Se isomer increases inhibitory potency by an order of magnitude. Furthermore, S-methylation of the MMI changes its reaction kinetics from non-competitive to uncompetitive with respect to the cofactor dithiothreitol. These results open perspectives for further investigations on identifying specific inhibitors of the deiodinase isoenzymes, potentially based on the addition of aromatic ring structures or alkyl groups to PTU and MMI.

Evaluation of drug-induced injury and human response in precision-cut tissue slices

1.Drug induced organ injury is multifaceted, encompassing a spectrum of cell types and numerous networks reflecting cell-cell and cell-matrix interactions. Characterization of drug induced side effects and human response can be addressed in organ slice models. 2.The application of human tissue to various organ slice models including liver, intestine, kidney, liver-blood co-cultures and thyroid enhances our ability to focus on the clinical relevance of side effects identified in animal studies for human, and to evaluate potential biomarkers of the side effects. Dose-response relationships can help discern drug concentrations which alter organ function or affect morphology, to identify drug concentrationswhich could pose a risk for humans. 3.Insight into pathways of organ injury, by incorporating gene and protein expression profiling, with functional measurements and morphology, aid to define species differences and sensitivity. 4.Human organ slice studies are valuable for bridging the extrapolation of animal derived data and for identifying mechanisms relevant for humans, thereby expanding the scope of translational research for drug safety assessment.

Feasibility study of the zebrafish assay as an alternative method to screen for developmental toxicity and embryotoxicity using a training set of 27 compounds

To anticipate to increased testing needs for reproductive toxicity and 3R approaches, we studied zebrafish embryo/larva as an alternative for animal testing for developmental toxicity and embryotoxicity and evaluated a training set of 27 compounds with a standardized protocol. The classification of compounds in the zebrafish embryo/larva assay, based on a prediction model using a TI (teratogenic index) cut-off value of 2, was compared to available animal and human data. When comparing the classification of compounds in the zebrafish embryo/larva assay to available animal classification, a sensitivity of 72% and specificity of 100% were obtained. The predictive values obtained in comparison to a limited set of human data were 50, 60% respectively for teratogens, non-teratogens. Overall, we demonstrated that the zebrafish embryo/larva assay, may be used as screening tool for prioritization of compounds and could contribute to reduction of animal experiments in the field of developmental toxicology.

Cellular distributions of molecules with altered expression specific to thyroid proliferative lesions developing in a rat thyroid carcinogenesis model

To identify differentially regulated molecules related to early and late stages of tumor promotion in a rat two-stage thyroid carcinogenesis model by an antithyroid agent, sulfadimethoxine, microarray-based microdissected lesion-specific gene expression profiling was carried out. Proliferative lesions for profiling were divided into two categories: (i) focal follicular cell hyperplasias (FFCH) and adenomas (Ad) as early lesions; and (ii) carcinomas (Ca) as more advanced. In both cases, gene expression was compared with that in surrounding non-tumor follicular cells. Characteristically, upregulation of cell cycle-related genes in FFCH + Ad, downregulation of genes related to tumor suppression and transcription inhibitors of inhibitor of DNA binding (Id) family proteins in Ca, and upregulation of genes related to cell proliferation and tumor progression in common in FFCH + Ad and Ca, were detected. The immunohistochemical distributions of molecules included in the altered expression profiles were further examined. In parallel with microarray data, increased localization of ceruloplasmin, cyclin B1, and cell division cycle 2 homolog A, and decreased localization of poliovirus receptor-related 3 and Id3 were observed in all types of lesion. Although inconsistent with the microarray data, thyroglobulin immunoreactivity appeared to reduce in Ca. The results thus suggest cell cycling facilitation by induction of M-phase-promoting factor consisting of cyclin B1 and cell division cycle 2 homolog A and generation of oxidative responses as evidenced by ceruloplasmin accumulation from an early stage, as well as suppression of cell adhesion involving poliovirus receptor-related 3 and inhibition of cellular differentiation regulated by Id3. Decrease of thyroglobulin in Ca may reflect dedifferentiation with progression.