Increasing Thyromimetic Potency through Halogen Substitution.

Sobetirome is one of the most studied thyroid hormone receptor†β (TRβ)-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 TRβ-mediated cell-based reporter with an EC(50) 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 EC(50) 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.