Cell-Specific Transport and Thyroid Hormone Receptor Isoform Selectivity Account for Hepatocyte-Targeted Thyromimetic Action of MGL-3196

Abstract

Thyroid hormones (THs) and TH receptor-beta (TRβ) reduce hepatic triglycerides, indicating a therapeutic potential for TH analogs in liver steatosis. To avoid adverse extrahepatic, especially TRα-mediated effects such as tachycardia and bone loss, TH analogs with combined TRβ and hepatocyte specificity are desired. MGL-3196 is a new TH analog that supposedly meets these criteria. Here, we characterize the thyromimetic potential of MGL-3196 in cell-based assays and address its cellular uptake requirements. We studied the contribution of liver-specific organic anion transporters (OATP)1B1 and 1B3 to MGL-3196 action. The TR isoform-specific efficacy of MGL-3196 compared with 3,5,3′-triiodothyronine (T3) was determined with luciferase assays and gene expression analysis in OATP1B1 and OATP1B3 and TRα- or TRβ-expressing cells and in primary murine hepatocytes (PMHs) from wild-type and TRβ knockout mice. We measured the oxygen consumption rate to compare the effects of MGL-3196 and T3 on mitochondrial respiration. We identified OATP1B1 as the primary transporter for MGL-3196. MGL-3196 had a high efficacy (90% that of T3) in activating TRβ, while the activation of TRα was only 25%. The treatment of PMHs with T3 and MGL-3196 at EC50 resulted in a similar induction of Dio1 and repression of Serpina7. In HEK293 cells stably expressing OATP1B1, MGL-3196 had comparable effects on mitochondrial respiration as T3. These data indicate that MGL-3196’s hepatic thyromimetic action, the basis for its therapeutic use, results from a combination of hepatocyte-specific transport by OATP1B1 and the selective activation of TRβ over TRα.

Thyroid hormone receptor alpha modulates fibrogenesis in hepatic stellate cells

“…45 The very potent hepatic thyromimetic effect of MGL-3196 is a result of its´ TRβ selectivity and hepatocyte-specific transport mediated by OAPT1B1, although MGL-3196 is about 60-100 times less potent than the endogenous T3. 14 Therefore, it is unknown whether MGL-3196 has a similar effect in HSC as GC-1.…”

“…Of note, there are currently two ongoing phase 2/3 (NCT03900429 and NCT04173065) clinical trials with TRβ‐agonists in patients with NASH and fibrosis 12,13 . Those studies' rationale is the metabolic impact of TRβ agonism as a dominant receptor in hepatocytes while having little systemic side effects due to TRβ isoform specificity and, for Resmetirom (MGL‐3196), hepatocyte specificity 14 . However, there is only rare information about the relevant TR isoform in HSCs and its role in HSC activation.…”

Resmetirom: An Orally Administered, Small-molecule, Liver-directed, β-selective THR Agonist for the Treatment of Non-alcoholic Fatty Liver Disease and Non-alcoholic Steatohepatitis

“… 52 In vitro studies have demonstrated resmetirom’s preferential use of the organic anion transporting polypeptides 1B1 receptor for hepatocyte uptake and stronger activation of THR-β. 53 , 54 Moreover, oxygen consumption rate studies found that both T3 and resmetirom increased basal and maximal respiration, accompanied by an increase in the production of adenosine triphosphate, reflecting higher oxidation of substrates. 53 While it is known that increased hepatic THR-β agonism increases the level of hepatic DIO1, 55 , 56 additional in vitro and in vivo data suggest that resmetirom upregulates the expression of DIO1, supporting its role not only as a thyromimetic but also in potentially increasing the conversion of T4 to T3.…”

“… 53 , 54 Moreover, oxygen consumption rate studies found that both T3 and resmetirom increased basal and maximal respiration, accompanied by an increase in the production of adenosine triphosphate, reflecting higher oxidation of substrates. 53 While it is known that increased hepatic THR-β agonism increases the level of hepatic DIO1, 55 , 56 additional in vitro and in vivo data suggest that resmetirom upregulates the expression of DIO1, supporting its role not only as a thyromimetic but also in potentially increasing the conversion of T4 to T3. 54 Although not definitive, the low serum T4 detected in patients treated with resmetirom indirectly suggests this mechanism may be relevant.…”

“… 54 Although not definitive, the low serum T4 detected in patients treated with resmetirom indirectly suggests this mechanism may be relevant. 53 …”