Structure-function relationship of the inhibition of the 3,5,3'-triiodothyronine binding to the alpha(1)- and beta(1)-thyroid hormone receptor by amiodarone analogs

Desethylamiodarone (DEA), the major metabolite of the potent antiarrhythmic drug amiodarone (A), acts as a competitive inhibitor of T-3 binding to the alpha(1)-thyroid hormone receptor (alpha(1)-T(3)R), but as a noncompetitive inhibitor with respect to the beta(1)-T(3)R. To gain insight into the structure-function relationship of the interaction between A metabolites and T(3)Rs, we investigated the effects of several A analogs on T-3 binding to the alpha(1)-T(3)R and beta(1)-T(3)R 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 T-3 binding to alpha(1)-T(3)R and beta(1)-T(3)R, 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 K-a, but no change in the maximum binding capacity (MBC) of T-3 binding to alpha(1)-T(3)R. Langmuir plots clearly indicated competitive inhibition of T-3 binding to alpha(1)-T(3)R by DDEA, DDIA, and L3373. In contrast, these three analogs acted differently with respect to the beta(1)-T(3)R. DDEA and DDIA decreased both K-a and MBC in Scatchard plots using beta(1)-T(3)R, demonstrating noncompetitive inhibition. L3373 decreased dose-dependently K-a, but not MBC, values of T-3 binding to the beta(1)-T(3)R and clearly acted as a competitive inhibitor. K-i plots indicated that DDEA, DDIA, and L3373 do not interfere significantly with occupied T(3)Rs. K-I (inhibition constant for the unoccupied receptor) plots demonstrated increasing inhibition of the T-3 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 T(3)R 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 alpha(1)-T(3)R. The behavior to the beta(1)-T(3)R 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 T-3 binding was twice as high for the beta(1)-T(3)R as for the alpha(1)-T(3)R. 3) All tested A analogs preferentially interfere with T-3 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 T-3 binding site of T(3)Rs; and 2) differences in the hormone-binding domain of alpha(1)- and beta(1)-T(3)Rs are likely to account for the competitive or noncompetitive nature of inhibition of T-3 binding by A analogs.