Furosemide, fenclofenac, diclofenac, mefenamic acid and meclofenamic acid inhibit specific T₃ binding in isolated rat hepatic nuclei

Abstract

Previous studies with Phenytoin (DPH) show that this inhibitor of thyroid hormone binding to plasma proteins also interacts with specific nuclear T₃ binding sites. In order to further define the nuclear effects of drugs that inhibit plasma protein binding of thyroid hormones, we assessed furosemide and a number of non-steroidal antiinflammatory drugs using isolated rat liver nuclei. The effects were compared with those of DPH, ipodate and amiodarone. The T₃ binding site in isolated nuclei (K_a 1.2×10⁹M⁻¹) showed relative affinity triac ≈ T₃>T₄. Drugs were studied over the concentration range 10⁻³-10⁻⁷M, approximating the known therapeutic total plasma concentrations, in competition with ¹²⁵|-T₃ 0.1 nM, expressing inhibition as the percent decrement from maximum specific binding of ¹²⁵|-T₃ in drug vehicle (assay buffer or ethanol 1–10%). Specific T₃ binding was inhibited by furosemide to 78.8 ± 3.5% at 2 mM, by fenclofenac to 37.6 ± 2.8% at 1 mM, by meclofenamic acid to 70.2 ± 2.4% at 0.1 mM, by mefenamic acid to 60.6 ± 4.6% at 0.05 mM (each p< 0.02) and by diclofenac to 87.4 ± 5.6% at 0.2 mM (p < 0.05). In comparison, DPH inhibited T₃ binding to only 88.1 ± 0.6% at 0.3 mM, as did calcium ipodate (68 ± 3.5% at 1 mM, p<0.02). Amiodarone (0.3 mM), sodium salicylate (1 mM) and phenylbutazone (0.1 mM) were inactive. In order to achieve a level of nuclear receptor occupancy that approaches in vivo occupancy, the concentration ¹²⁵|-T₃ was increased over the range 0.1–0.5 nM. At constant drug concentrations, the decrease in the concentration of bound ¹²⁵|-T₃ induced by furosemide, fenclofenac, diclofenac, mefenamic acid, and meclofenamic acid and ipodate was accentuated by increased receptor occupancy, as previously demonstrated with DPH. These studies demonstrate that a wide range of bicyclic drugs may interact with specific nuclear T₃ binding sites, over a range of total concentrations similar to those previously reported as inhibitory for DPH and ipodate. The molecular basis for the relationship between inhibition of specific nuclear receptor and plasma protein binding remains unknown, and the potential for these drugs to alter expression of thyroid hormone action is not yet defined. Effects on TSH secretion, previously attributed to changes in circulating free hormone concentration, could also result from interaction with nuclear receptors. It will be necessary to consider the relationship between free intranuclear drug and T₃ concentrations to further define these potentially-important pharmacologic interactions.