Structure-function relationship of the inhibition of the 3,5,3'-triiodothyronine binding to the alpha1- and beta1-thyroid hormone receptor by amiodarone analogs

Desethylamiodarone (DEA), the major metabolite of the potent antiarrhythmic drug amiodarone (A), acts as a competitive inhibitor of T3, binding to the alpha1-thyroid hormone receptor (alpha1-T3R), but as a noncompetitive inhibitor with respect to the beta1-T3R. To gain insight into the structure- function relationship of the interaction between A metabolites and T3Rs, we investigated the effects of several A analogs on T3 binding to the alpha1-T3R and beta1-T3R in vitro. The analogs tested were: 1) compounds obtained by deethylation of A, DEA, and desdiethylamiodarone (DDEA); 2) compounds obtained by deiodination of A, monoiodoamiodarone and desdiiodoamiodarone (DDIA); and 3) benzofuran derivatives with various iodination grades, 2-butyl-3-(4-hydroxy-3,5-diiodo-benzoyl)benzofuran (L3373, two iodine atoms), L6424 (L3373 with one iodine atom), and L3372 (L3373, no iodine atoms). IC50, values of inhibition of T3 binding to alpha1-T3R and beta1-T3R, respectively, were as follows (mean +/- SD, expressed x 10(-5) M): DEA, 4.7 +/- 0.9 and 2.7 +/- 1.4 (P < 0.001); DDEA, 3.7 +/- 0.9 and 1.9 +/- 0.3 (P < 0.001); monoiodoamiodarone, more than 20 and more than 20; DDIA, 16.2 +/- 5.6 and 9.1 +/- 2.1 (P < 0.01); L3373, 3.8 +/- 1.0 and 3.6 +/- 0.5 (P = NS); L6424, 11.3 +/- 5.7 and 10 +/- 2.0 (P = NS); and L3372, no inhibition. Scatchard analyses in the presence of DDEA, DDIA, and L3373 demonstrated a dose-dependent decrease in Ka, but no change in the maximum binding capacity (MBC) of T3 binding to alpha1-T3R. Langmuir plots clearly indicated competitive inhibition of T3 binding to alpha1-T3R by DDEA, DDIA, and L3373. In contrast, these three analogs acted differently with respect to the beta1-T3R. DDEA and DDIA decreased both Ka and MBC in Scatchard plots using beta1-T3R, demonstrating noncompetitive inhibition. L3373 decreased dose-dependently Ka, but not MBC, values of T3 binding to the beta1-T3R and clearly acted as a competitive inhibitor. Ki plots indicated that DDEA, DDIA, and L3373 do not interfere significantly with occupied T3Rs. KI (inhibition constant for the unoccupied receptor) plots demonstrated increasing inhibition of the T3 binding to unoccupied receptors with increasing analog concentrations. In summary, 1) removal of one or two ethyl groups of A results in compounds with strong but almost equal potency of inhibiting T3R binding, whereas removal of one or two iodine atoms of A has a lower potency in this respect. The strong inhibitory potency of the benzofuran derivative L3373 (equalling that of the deethylated compounds) is lost upon deiodination. 2) All tested A analogs acted as competitive inhibitors to the alpha1-T3R. The behavior to the beta1-T3R was different; deethylation or deiodination of A resulted in noncompetitive inhibition, whereas L3373 was a competitive inhibitor. The potency of deethylated and deiodinated compounds (but not of the benzofuran derivatives) for inhibiting T3 binding was twice as high for the beta1-T3R as for the alpha1-T3R. 3) All tested A analogs preferentially interfere with T3 binding to unoccupied receptors. The implications of these findings for the structure-activity relationship are the following: 1) the size of the diethyl-substituted nitrogen group and of the two bulky iodine atoms in the A molecule hamper the binding of A at the T3 binding site of T3Rs; and 2) differences in the hormone-binding domain of alpha1- and beta1-T3Rs are likely to account for the competitive or noncompetitive nature of inhibition of T3 binding by A analogs.