Hammett Analysis of Selective Thyroid Hormone Receptor Modulators Reveals Structural and Electronic Requirements for Hormone Antagonists

Abstract

Selective thyroid hormone modulators that function as isoform-selective agonists or antagonists of the thyroid hormone receptors (TRs) might be therapeutically useful in diseases associated with aberrant hormone signaling. The most potent thyroid hormone antagonist reported to date is NH-3. To explore the significance of the 5'-p-nitroaryl moiety of NH-3 and understand what chemical features are important to confer antagonism, we sought to expand the structure-activity relationship data for the class of 5'-phenylethynyl GC-1 derivatives. Herein, we describe an improved synthetic route utilizing palladium-catalyzed chemistry for efficient access to a series of 5'-phenylethynyl compounds with varying size and electronic properties. We prepared and tested sixteen analogues for TR binding and transactivation activity. Substitution at the 5'-position decreased binding affinity, but retained TRbeta-selectivity. In transactivation assays, the analogues displayed a spectrum of agonist, antagonist, and mixed agonist/antagonist activity that correlated with electronic character in a Hammett analysis between sigma substituent value and TR modulation. Analogues NH-5, NH-7, NH-9, NH-11, and NH-23 displayed full antagonist activity with reduced potency compared to NH-3, indicating the nitro group is not required for antagonism. However, para-substitution with strong electron withdrawing properties on the 5'-aryl extension is important for antagonist activity, and antagonist potency-but not ligand receptor binding-was found to correlate linearly with the sigma values for the electron withdrawing substituents.

Differential effects of TR ligands on hormone dissociation rates: Evidence for multiple ligand entry/exit pathways

“…We previously described GC-1 derivatives with bulky 5′ extensions that exhibit diverse activities, from full agonist to full antagonist, and bind TRs with a range of affinities [25, 26]. Of this series, eight of ten compounds displaced T 3 more rapidly than native hormone (Fig.…”

Pharmacological Profile of the Thyroid Hormone Receptor Antagonist NH3 in Rats

“…1) that selectively and competitively blocks TR and shows in vivo activity in a tadpole tail resorption assay (Lim et al, 2002;Malm, 2004a). This antagonist was designed on the hypothesis that effects of modification of a nuclear hormone ligand can be predicted by the placement of molecular extensions that disrupt folding of the carboxylterminal helix 12, preventing coactivator recruitment (Arnold et al, 2005;Nguyen et al, 2005). Therefore, NH3 competitively binds to TR, but it also inhibits cofactor recruitment, explaining its antagonist activity.…”

Complex actions of thyroid hormone receptor antagonist NH-3 on gene promoters in different cell lines

“…In this study, we examined actions of NH-3, which blocks T 3 action in cell free systems (Nguyen, Apriletti et al 2002; Moore, Galicia et al 2004; Nguyen, Apriletti et al 2005), transfected cells (Nguyen, Apriletti et al 2002; Nguyen, Apriletti et al 2005), tadpoles (Lim, Nguyen et al 2002) and rats (Grover, Dunn et al 2007). Since unliganded TRs bind DNA and are transcriptionally active we suspected that NH-3 may influence TR conformation and activity in a manner that is independent of effects on T 3 binding.…”

“…Since hormone is buried in the core of the domain (Wagner, Apriletti et al 1995), we proposed that derivatives of agonists with appropriately placed extensions should compete with T 3 for binding and inhibit NR activity by displacing H12 (Scanlan, Baxter et al 1996; Webb, Nguyen et al 2002). We used this strategy to identify lead compounds that inhibit TRs (Yoshihara, Apriletti et al 2001; Baxter, Goede et al 2002; Nguyen, Apriletti et al 2002; Nguyen, Apriletti et al 2005). The best, NH-3, is derived from the TRβ selective agonist GC-1 and contains a bulky 5′ nitrophenylethynyl extension (Chiellini, Apriletti et al 1998; Nguyen, Apriletti et al 2002; Webb, Nguyen et al 2002).…”