(c) Application of Biomimetic Dehalogenation in Drug Design


Chemistry of the Thyroid Hormone Deiodination


Thyroid hormones, produced by the thyroid gland, are iodine-containing compounds that regulate gene expression in every vertebrate tissue and control the metabolism in the body. The iodide ions are transported into the follicular cells of the thyroid gland by the sodium/iodide symporter (NIS) and for each iodide anion (I–), NIS transports two sodium cations (Na+) into the cell. The synthesis of the prohormone thyroxine (T4) on thyroglobulin is catalyzed by thyroid peroxidase (TPO), a heme enzyme that utilizes iodide and hydrogen peroxide to perform two distinct reactions: iodination of tyrosyl residues and phenolic coupling of the resulting iodinated tyrosyl residues of thyroglobulin. Subsequently, the thyroxyl residues are cleaved by proteolysis to produce free T4 (Figure 7).

Figure 7. Biosynthesis of thyroid hormones from thyroglobulin by thyroid peroxidase and regioselective deiodination of thyroxine (T4) (inset). The figure is adapted from Manna, D.; Roy, G.; Mugesh, G. Acc. Chem. Res. 2013, 11, 2706.

Although a small amount of T3 (3,3',5-triiodothyronine), the active thyroid hormone, is also produced during synthesis, most of the circulating T3 is generated via 5'-deiodination of T4, catalyzed by the selenoenzymes, type 1 and 2 iodothyronine deiodinases (ID-1 and ID-2). The deactivation of thyroid hormone is catalyzed by the type 3 deiodinase (ID-3), which removes iodine exclusively from the tyrosyl (inner) ring of T4 to produce 3,3',5'-triiodothyronine (rT3). The triiodo derivatives T3 and rT3 are further metabolized by the three selenoenzymes to produce 3,3'-diiodothyronine (3,3'-T2) (Figure 7, inset). It is known that the activation of thyroid stimulating hormone (TSH) receptor by auto-antibodies leads to an overproduction of thyroid hormones. In addition, these auto-antibodies also stimulate ID-1 and probably ID-2, which then together produce relatively more T3, leading to hyperthyroidism. The overproduction of T3 is controlled by specific inhibitors which either block the thyroid hormone biosynthesis (Figure 8) or reduce the conversion of T4 to T3. The most commonly used drug for hyperthyroidism are the thiourea drugs, 6-n-propyl-2-thiouracil (PTU), 6-methyl-2-thiouracil (MTU), methimazole (MMI) and carbimazole (CBZ).

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Figure 8. (A) Mechanism of the antithyroid action of methimazole. (B) A simple and facile one-pot synthesis of carbimazole (12) and its methyl analogue (13) by using heterocyclic carbenes generated in situ from the corresponding imidazolium salts.

During last ten years, we have contributed significantly to the understanding of the mechanism of the inhibition of thyroid hormone synthesis by antithyroid drugs. The isosteric substitutions in the existing drugs led to compounds that can effectively and reversibly inhibit the iodination. In contrast to MMI, the chemically modified analogues inhibit the enzyme by reducing the H2O2 that is required for the oxidation of the iron center. These studies reveal that the degradation of the intracellular H2O2 by some of the anti-thyroid drugs may be beneficial to the thyroid gland as these compounds may act as antioxidants and protect thyroid cells from oxidative damage. As the inhibition is reversible, the drugs with a more controlled action could be of great importance in the treatment of hyperthyroidism. (Roy, G. and Mugesh, G. J. Am. Chem. Soc. 2004, 126, 2712; J. Am. Chem. Soc. 2005, 127, 15207, Das, D.; Roy, G.; Mugesh, G. J. Med. Chem. 2008, 51, 7313).

Recently, we have carried out extensive work on the regioselective deiodination of thyroid hormones as the regioselectivity of the reaction controls the thyroid hormone homeostasis in human body. When the work was started, nothing was known about the reaction mechanism. They demonstrated the first chemical model for the inner-ring deiodination of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) by iodothyronine deiodinase (Figure 9). This study suggests that the nature of substituents around the selenol functionality may modulate its reactivity towards outer- or inner-ring iodines. The presence of an in-built thiol group in the close proximity to selenium is important as this group not only acts as thiol cofactor in the deiodination reactions, but may also assist the selenol in polarizing the C-I bond. The effective removal of iodine from the

Figure 9. The first example of a selective removal of 5-iodine by a synthetic compound.

inner-ring of T4 indicates that an enol-keto tautomerism is not required for the deiodination. Although the activation of selenocysteine by histidine is believed to be important for the ID-1-mediated deiodination, such activation is probably not essential for the ID-3-catalyzed reaction. (Manna, D.; Mugesh, G. Angew. Chem. Int. Ed. 2010, 49, 9246). The replacement of the thiol group in the deiodinase mimetic (Figure 9) by a selenol group lead to a dramatic increase in the activity, without affecting the regioselectivity of deiodination (Manna, D.; Mugesh, G. J. Am. Chem. Soc. 2011, 133, 9980).

Figure 10. Proposed mechanism for the deiodination of thyroxine (T4) involving chalcogen and halogen bonding.

Mechanistic investigations reveal that the formation of a halogen bond between the iodine and chalcogen (S or Se) and the peri-interaction between two chalcogen atoms (chalcogen bond) are important for the deiodination reactions. Although the formation of a halogen bond leads to elongation of the C-I bond, the chalcogen bond facilitates the transfer of more electron density to the C-I σ* orbitals, leading to a complete cleavage of the C-I bond (Figure 10). The higher activity of amino-substituted selenium compounds can be ascribed to the deprotonation of thiol/selenol moiety by the amino group, which not only increases the strength of halogen bond, but also facilitates the chalcogen-chalcogen interactions (Manna, D.; Mugesh, G. J. Am. Chem. Soc. 2012, 134, 4269). This work has been highlighted in Nature Chemistry (Metrangolo, P.; Resnati, G. Nature Chem. 2012, 4, 437).

Development of synthetic deiodinase mimetics is important not only to understand the mechanism of enzyme catalysis, but also to develop novel therapeutic agents. It is known that abnormal thyroid hormone levels have implications in different diseases, such as hypoxia, myocardial infarction, critical illness, neuronal ischemia, tissue injury and cancer. In the above work, we showed that the napthyl-based compounds shown in Figure 9 (thiol-selenol pair) and Figure 10 (selenol pair) always mediate the 5-deiodination of thyroxine (T4) to produce rT3 as the only product. However, very recently, we showed that the introduction of tellurium atoms in place of sulfur/selenium in deiodinase mimetics alters not only the reactivity but also the regioselectivity of deiodination. We showed for the first time that compounds having two tellurol moieties or a thiol/tellurol pair can mediate sequential deiodination of T4 to produce all the possible thyroid hormone derivatives under physiologically relevant conditions (Figure 11).


















As shown in Figure 11, T4 is converted to the fully deiodinated derivative T0 through rT3-3,3'-T2-3-T1 pathway, involving two IRD and two ORD reactions. When compounds 16 and 17 were used in large excess (50 equiv.), a complete conversion of T4 to T0 was observed, indication that 3'-T1 also undergoes deiodination to give T0. The diiodo derivative 3,3′-T2 serves as a common intermediate for both 3-T1 and 3'-T1. These observations reveal that the reactivity of C-I bonds in T4 is remarkably altered upon deiodination. This study provides the first experimental evidence that the regioselectivity of thyroid hormone deiodination is controlled by the nucleophilicity and the strength of halogen bond between the iodine and chalcogen atoms. While the larger size and greater polarizability of Te supports more efficient deiodination than observed with Se compounds, the change in regiospecificity discovered here was not anticipated. This change will be crucial in the future for distinguishing between the various mechanisms proposed for the model reactions as well as those catalyzed by the natural deiodinases including the three isozymes of iodothyronine deiodinases (Raja, K.; Mugesh, G. Angew. Chem. Int. Ed. 2015, doi/10.1002/ anie.201502762).

Figure 11. (A) Chemical structures of compounds 14-19. (B) HPLC chromatograms for the reactions of T4 with 16 and 17. (C) Initial rates for the deiodination of T4 and its derivatives. (D) Sequential deiodination of T4 by 16 and 17. IRD: inner-ring deiodination, ORD: outer-ring deiodination, T4: L-thyroxine, T3: 3,5,3'-triiodothyronine, rT3: 3,3',5'-triiodothyronine, 3,3'-T2: 3,3'-diiodothyronine. Raja, K.; Mugesh, G. Angew. Chem. Int. Ed. 2015, doi/10.1002/ anie.201502762.

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