Targeting thyroid hormone receptor-beta agonists to the liver reduces cholesterol and triglycerides and improves the therapeutic index

Abstract

Despite efforts spanning four decades, the therapeutic potential of thyroid hormone receptor (TR) agonists as lipid-lowering and anti-obesity agents remains largely unexplored in humans because of dose-limiting cardiac effects and effects on the thyroid hormone axis (THA), muscle metabolism, and bone turnover. TR agonists selective for the TRbeta isoform exhibit modest cardiac sparing in rodents and primates but are unable to lower lipids without inducing TRbeta-mediated suppression of the THA. Herein, we describe a cytochrome P450-activated prodrug of a phosphonate-containing TR agonist that exhibits increased TR activation in the liver relative to extrahepatic tissues and an improved therapeutic index. Pharmacokinetic studies in rats demonstrated that the prodrug (2R,4S)-4-(3-chlorophenyl)-2-[(3,5-dimethyl-4-(4'-hydroxy-3'-isopropylbenzyl)phenoxy)methyl]-2-oxido-[1,3,2]-dioxaphosphonane (MB07811) undergoes first-pass hepatic extraction and that cleavage of the prodrug generates the negatively charged TR agonist (3,5-dimethyl-4-(4'-hydroxy-3'-isopropylbenzyl)phenoxy)methylphosphonic acid (MB07344), which distributes poorly into most tissues and is rapidly eliminated in the bile. Enhanced liver targeting was further demonstrated by comparing the effects of MB07811 with 3,5,3'-triiodo-l-thyronine (T(3)) and a non-liver-targeted TR agonist, 3,5-dichloro-4-(4-hydroxy-3-isopropylphenoxy)phenylacetic acid (KB-141) on the expression of TR agonist-responsive genes in the liver and six extrahepatic tissues. The pharmacologic effects of liver targeting were evident in the normal rat, where MB07811 exhibited increased cardiac sparing, and in the diet-induced obese mouse, where, unlike KB-141, MB07811 reduced cholesterol and both serum and hepatic triglycerides at doses devoid of effects on body weight, glycemia, and the THA. These results indicate that targeting TR agonists to the liver has the potential to lower both cholesterol and triglyceride levels with an acceptable safety profile.

Fig. 1.

Chemical structures and mechanism of MB07811 conversion to MB07344. (A) Chemical structure of KB-141. (B) Chemical structures of MB07811 and the products generated from the CYP3A-catalyzed cleavage reaction, namely MB07344 and the glutathione conjugate (2) produced from the addition of glutathione (GSH) to the aryl vinyl ketone byproduct. Ar, 3-chlorophenyl.

Fig. 2.

MB07811 metabolism, excretion, and tissue distribution. (A) Mean (±½ range) intracellular levels (circles) and levels in culture medium (triangles) of MB07344 from isolated, cultured rat hepatocytes incubated with 10 μM MB07811 (open symbols, n = 2) or 10 μM MB07344 (filled symbols, n = 2). (B) Mean (± SEM) systemic (filled circles, n = 5) and portal vein (open circles, n = 4) plasma concentrations of MB07811 after an oral dose of MB07811 (3 mg/kg) to male SD rats; (C) Mean (± SEM) MB07344 levels in bile samples collected over the indicated periods from bile duct-cannulated male SD rats (n = 3) treated with MB07344 (10 mg/kg, i.v.). (D) Approximate mean tissue concentration (total radioactivity) for the tissues with the highest concentration 3 and 24 h after an oral dose of [¹⁴C]-MB07811 (5 mg/kg) to male SD rats (n = 4). Tissues evaluated (liver, spleen, lymph, thyroid, testes, fat, bladder, prostate, pancreas, stomach, small and large intestine, adrenals, kidneys, thymus, heart, bone marrow, muscle, eye, brain, pituitary, skin, lung, and bone) but not listed in the figure (other than stomach and intestine) have <5% of the liver concentration at 3 h and <3% at 24 h.

Fig. 3.

Relative expression of mRNA in tissues of male SD rats (n = 6) treated with vehicle (black bar), T₃ (white bar), KB-141 (white hatched bar), or MB07811 (gray bar) at 10× the CF rat ED₅₀ (0.12, 0.5, and 4.0 mg/kg, respectively). Bar graphs are grouped by mRNA and represent the fold-change relative to vehicle at the time point (3, 8, or 24 h) associated with the largest fold-change for T₃. Data from the other two time points and doses (1× and 3× ED₅₀) are in SI Appendix, Figs. 8–19. (A) Liver mRNA: CYP7A, malic enzyme (ME), SREBP-1c, and LDLR. (B) Heart and skeletal muscle mRNA: MHCβ, deiodinase 1 (D1), and uncoupling protein 3 (UCP3). (C) Pituitary and thyroid mRNA: TSHβ and D1. (D) Other tissue mRNAs: D1 in spleen, kidney, and liver.

Fig. 4.

Effects on SD rat cardiac function and THA. (A and B) Dose–response of T₃ (▴, 6.5–650 μg·kg⁻¹·day⁻¹), KB-141 (○, 0.01–5 mg·kg⁻¹·day⁻¹), and MB07811 (□, 0.1–50 mg·kg⁻¹·day⁻¹) on heart rate (A) and LV dP/dt (B) in SD rats (n = 3–6 per dose) treated for 7 days. (C–E) Effects of KB-141 and MB07811 on total and free T₄ (C), total and free T₃ (D), and plasma TSH and pituitary TSHβ mRNA (E) (relative units) in SD rats (n = 5–6 per group) treated once daily for 28 days with vehicle (black bars), KB-141 at 0.1 mg/kg (white bars) and 1 mg/kg (white hatched bars), and MB07811 at 3 mg/kg (gray bars) and 30 mg/kg (gray hatched bars). The low and high doses for KB-141 are equivalent to 2× and 20×, respectively, the CF rat ED₅₀. The doses for MB07811 are equivalent to 7.5× and 75× the CF rat ED₅₀. *, P < 0.05.

Fig. 5.

Dose–response curves in DIO mice (n = 8 per group) treated for 14 days with KB-141 (open symbols, blue) and MB07811 (filled symbols, red). Vehicle is denoted by gray-filled symbols ± SEM. (A, C, and D) Percent change from baseline for total plasma cholesterol (circles) and TGs (triangles) (A), BW (C), and blood glucose (D). (B, E, and F) Effects of KB-141 and MB07811 on liver TGs (B), heart weight (E), and serum total T₄ (circles, left axis) and total T₃ (triangles, right axis) (F). *, P < 0.05.