Localization of c-ERB A proteins in rat liver using monoclonal antibodies

Dronerarone acts as a selective inhibitor of 3,5,3'-triiodothyronine binding to thyroid hormone receptor-alpha1: in vitro and in vivo evidence

Dronedarone (Dron), without iodine, was developed as an alternative to the iodine-containing antiarrhythmic drug amiodarone (AM). AM acts, via its major metabolite desethylamiodarone, in vitro and in vivo as a thyroid hormone receptor alpha(1) (TRalpha(1)) and TRbeta(1) antagonist. Here we investigate whether Dron and/or its metabolite debutyldronedarone inhibit T(3) binding to TRalpha(1) and TRbeta(1) in vitro and whether dronedarone behaves similarly to amiodarone in vivo. In vitro, Dron had a inhibitory effect of 14% on the binding of T(3) to TRalpha(1), but not on TRbeta(1). Desethylamiodarone inhibited T(3) binding to TRalpha(1) and TRbeta(1) equally. Debutyldronedarone inhibited T(3) binding to TRalpha(1) by 77%, but to TRbeta(1) by only 25%. In vivo, AM increased plasma TSH and rT(3), and decreased T(3). Dron decreased T(4) and T(3), rT(3) did not change, and TSH fell slightly. Plasma total cholesterol was increased by AM, but remained unchanged in Dron-treated animals. TRbeta(1)-dependent liver low density lipoprotein receptor protein and type 1 deiodinase activities decreased in AM-treated, but not in Dron-treated, animals. TRalpha(1)-mediated lengthening of the QTc interval was present in both AM- and Dron-treated animals. The in vitro and in vivo findings suggest that dronedarone via its metabolite debutyldronedarone acts as a TRalpha(1)-selective inhibitor.

Desethylamiodarone is a competitive inhibitor of the binding of thyroid hormone to the thyroid hormone alpha 1-receptor protein

Desethylamiodarone (DEA), the major metabolite of the potent antiarrythmic drug amiodarone, is a non-competitive inhibitor of the binding of thyroid hormone (T3) to the beta 1-thyroid hormone receptor (T3R). In the present study, we investigated whether DEA acts in a similar way with respect to the alpha 1-T3R. The chicken alpha 1-T3R, expressed in an E. coli system, was incubated in the presence or absence of DEA with [125I]T3 in buffer containing 0.05% Triton X-100, 0.05% BSA and 1% ethanol (v/v) in order to solubilise DEA. DEA, but not amiodarone, inhibited T3 binding in a dose-dependent manner; the IC50 value was 3.5 x 10(-5) M. Scatchard analyses in the presence of DEA demonstrated a dose-dependent decrease in Ka values, but no change in MBC. Lineweaver-Burk plots clearly indicated competitive inhibition by DEA. Pre-incubation of the alpha 1-receptor with DEA decreased maximal [125I]T3 binding, which was independent of the duration of pre-incubation. In conclusion, in contrast to the beta 1-T3R, where DEA acts as a non-competitive inhibitor, we now report as a new finding the competitive action of DEA to the alpha 1-T3R.