Desethylamiodarone prolongation of cardiac repolarization is dependent on gene expression: a novel antiarrhythmic mechanism

Points to consider emerging from a mini-workshop on cardiac safety: assessing torsades de pointes liability

A mini-workshop on cardiac safety focusing on assessing drug-induced Torsades de Pointes (TdP) liability was convened as part of the 6th Annual Meeting of the Safety Pharmacology Society. The purpose of this brief publication is to disseminate the salient points emanating from this workshop as a means of engaging the scientific community in the appropriate discussions needed to advance this important field of human safety. The recommendations in this publication extend those of the workshop on "Moving Towards Better Predictors of Drug-Induced Torsades de Pointes" held in November 2005 under the auspice of the International Life Sciences Institute, Health and Environmental Sciences Institute; they fall into four key areas: molecular and cellular biology underlying TdP, dynamics of periodicity, models of TdP proarrhythmia and key considerations for demonstrating utility of non-clinical models. The reader is encouraged to consider the recommendations emanating from the two workshops and align these with ongoing studies in their laboratories. The authors intend to convene a workshop in 2009/2010 to judge advancements in the field of study of drug-induced TdP and make recommendations for a focused validation of those methods holding the greatest promise of improving the predictivity of this unwanted human cardiac risk.

Are hERG channel inhibition and QT interval prolongation all there is in drug-induced torsadogenesis? A review of emerging trends

Contemporary preclinical in vitro and in vivo methods have been imperfect in predicting drug-induced Torsades de Pointes (TdP) in humans. A better understanding of additional relevant factors in the genesis of drug-induced TdP is necessary. New sophisticated in vitro techniques, such as arterially perfused ventricular wedge preparations or isolated perfused hearts, potentially offer a better understanding of torsadogenic mechanisms and a refinement of drug testing. Of particular interest are the dispersion of repolarization and the refractoriness of different cell types across the ventricular wall, triangulation of the action potential, reverse use dependence and instability of the action potential duration. In vivo models are currently refined by establishing parameters such as beat-to-beat variability and T-wave morphology as derived from the in vitro proarrhythmia indices. Animal models of proarrhythmia are to date not recommended for routine evaluation. A pharmacodynamic interaction with combinations of torsadogenic compounds is another area to be considered. Little is known about channel/receptor cross talk, although considerable evidence exists that cardiac G protein-coupled receptors can modulate hERG channel function. More investigations are necessary to further evaluate the role of altered gene expression, mutations, and polymorphisms in drug-induced TdP. A novel mechanism of drug-induced torsadogenesis is the reduced expression of hERG channel protein on the plasma membrane due to a trafficking defect. Pharmacokinetic and metabolism data are crucial for calculating the risk of a torsadogenic potential in man. Consideration of intracardiac accumulation can help in delineating pharmacokinetic-pharmacodyamic relationships. In silico virtual screening procedures with new chemical entities to predict hERG block may develop as a promising tool. The role of in silico modeling of TdP arrhythmia is likely to become increasingly important for organizing and integrating the vast amount of generated data. At present, however, in silico methods cannot replace existing preclinical in vitro and in vivo models.

Long-Term Amiodarone Administration Remodels Expression of Ion Channel Transcripts in the Mouse Heart

Background— The basis for the unique effectiveness of long-term amiodarone treatment on cardiac arrhythmias is incompletely understood. The present study investigated the pharmacogenomic profile of amiodarone on genes encoding ion-channel subunits.

Methods and Results— Adult male mice were treated for 6 weeks with vehicle or oral amiodarone at 30, 90, or 180 mg · kg −1 · d −1 . Plasma and myocardial levels of amiodarone and N -desethylamiodarone increased dose-dependently, reaching therapeutic ranges observed in human. Plasma triiodothyronine levels decreased, whereas reverse triiodothyronine levels increased in amiodarone-treated animals. In ECG recordings, amiodarone dose-dependently prolonged the RR, PR, QRS, and corrected QT intervals. Specific microarrays containing probes for the complete ion-channel repertoire (IonChips) and real-time reverse transcription–polymerase chain reaction experiments demonstrated that amiodarone induced a dose-dependent remodeling in multiple ion-channel subunits. Genes encoding Na + (SCN4A, SCN5A, SCN1B), connexin (GJA1), Ca 2+ (CaCNA1C), and K + channels (KCNA5, KCNB1, KCND2) were downregulated. In patch-clamp experiments, lower expression of K + and Na + channel genes was associated with decreased I to,f , I K,slow , and I Na currents. Inversely, other K + channel α- and β-subunits, such as KCNA4, KCNK1, KCNAB1, and KCNE3, were upregulated.

Conclusions— Long-term amiodarone treatment induces a dose-dependent remodeling of ion-channel expression that is correlated with the cardiac electrophysiologic effects of the drug. This profile cannot be attributed solely to the amiodarone-induced cardiac hypothyroidism syndrome. Thus, in addition to the direct effect of the drug on membrane proteins, part of the therapeutic action of long-term amiodarone treatment is likely related to its effect on ion-channel transcripts.

Protective effect of amiodarone but not N-desethylamiodarone on postischemic hearts through the inhibition of mitochondrial permeability transition

Amiodarone is a widely used and potent antiarrhythmic agent that is metabolized to desethylamiodarone. Both amiodarone and its metabolite possess antiarrhythmic effect, and both compounds can contribute to toxic side effects. Here, we compare the effect of amiodarone and desethylamiodarone on mitochondrial energy metabolism, membrane potential, and permeability transition and on mitochondria-related apoptotic events. Amiodarone but not desethylamiodarone protects the mitochondrial energy metabolism of the perfused heart during ischemia in perfused hearts. At low concentrations, amiodarone stimulated state 4 respiration due to an uncoupling effect, inhibited the Ca2+-induced mitochondrial swelling, whereas it dissipated the mitochondrial membrane potential (Deltapsi), and prevented the ischemia-reperfusion-induced release of apoptosis-inducing factor (AIF). At higher concentrations, amiodarone inhibited the mitochondrial respiration and simulated a cyclosporin A (CsA)-independent mitochondrial swelling. In contrast to these, desethylamiodarone did not stimulate state 4 respiration, did not inhibit the Ca2+-induced mitochondrial permeability transition, did not induce the collapse of Deltapsi in low concentrations, and did not prevent the nuclear translocation of AIF in perfused rat hearts, but it induced a CsA-independent mitochondrial swelling at higher concentration, like amiodarone. That is, desethylamiodarone lacks the protective effect of amiodarone seen at low concentrations, such as the inhibition of calcium-induced mitochondrial permeability transition and inhibition of the nuclear translocation of the proapoptotic AIF. On the other hand, both amiodarone and desethylamiodarone at higher concentration induced a CsA-independent mitochondrial swelling, resulting in apoptotic death that explains their extracardiac toxic effect.

Class I or class III agents for atrial fibrillation: are asking the right question?

Atrial fibrilliation (AF) is often combined with advanced age and structural heart disease, conditions known to invite serious proarrhythmic complications of antiarrhythmic drug therapy. Recent controlled trials comparing two AF treatment strategies-rhythm control requiring atrial defibrilliation and antiarrhythmic drugs to prevent AF and ventiricular rate control obviating sinus rhythm maintenance with such drugs-showed equal or superior results with rate control. AF is associated with derepressions of "fetospecific" expression patterns that may profoundly alter the responsiveness of atrial muscle to antiarrhythmic drugs. Therefore, effects of drugs predicted according to pharmacological classifications evaluating drug actions in intact myocardium only should be interpreted cautiously. The classification proposed by Vaughan Williams fails to distinguish between acute and chronic drug efficacy and toxicity as recommended in classical pharmacology. There is, however, overwhelming evidence that acute and chronic drug effects often differ fundamentally. For instance, amiodarone acts acutely as a sodium channel blocker, whereas chronic effects may be mediated by a downregulation of thyroid hormone receptors. Meaningful direct effects of amiodarone on atrial potassium channels is questionable, since the main candidate target-current (IKr) may not be expressed in human atrial muscle. Multiple biophysical factors contribute to the lack of ion channel-selective actions of antiarrhythmic agents. Nonselectivity becomes particularly important in the context of mechanisms of action of Vaughan Williams Class I and III agents on human atrial muscle. Preclinical studies indicate that Class I agents such as flecainide and propafenone may act in AF predominantly as Class III agents. Meta-analyses of antiarrhythmic agents for the prevention of AF have failed to reveal superior drugs or drug classes. Superiority of amiodarone over other agents may depend on arbitrary amiodarone-favoring loading protocols producing significant differential effects exclusively during the acute phase of treatment. In conclusion, the classification of current antiarrhythmic agents into Class I and III may not be a useful simplification when applied to the pharmacotherapy of AF.

Desethylamiodarone interferes with the binding of co-activator GRIP-1 to the beta 1-thyroid hormone receptor

Ligand binding to the thyroid hormone nuclear receptor beta1 (TRbeta(1)) is inhibited by desethylamiodarone (DEA), the major metabolite of the widely used anti-arrhythmic drug amiodarone. Gene expression of thyroid hormone (triiodothyronine, T(3))-regulated genes can therefore be affected by amiodarone due to less ligand binding to the receptor. Previous studies have indicated the possibility of still other explanations for the inhibitory effects of amiodarone on T(3)-dependent gene expression, probably via interference with receptor/co-activator and co-repressor complex. The binding site of DEA is postulated to be on the outside surface of the receptor protein overlapping the regions where co-activator and co-repressor bind. Here we show the effect of a drug metabolite on the interaction of TRbeta(1) with the co-activator GRIP-1 (glucocorticoid receptor interacting protein-1). The T(3)-dependent binding of GRIP-1 to the TRbeta(1) is disrupted by DEA. A DEA dose experiment showed that the drug metabolite acts like an antagonist under 'normal' conditions (at 10(-7) M T(3) and 5x10(-6)-->10(-3) M DEA), but as an agonist under extreme conditions (at 0 and 10(-9) M T(3) and >10(-4) M DEA). To our knowledge, these results show for the first time that a metabolite of a drug which was not devised for this purpose can interfere with nuclear receptor/co-activator interaction.

Amiodarone-induced thyroid disorders: a clinical review

Although amiodarone is regarded as a highly effective anti-arrhythmic agent, its use may lead to alterations in thyroid gland function and/or thyroid hormone metabolism, partly because of its rich iodine content. Patients treated with amiodarone may manifest altered thyroid hormone profile without thyroid dysfunction, or they may present with clinically significant amiodarone-induced hypothyroidism or amiodarone-induced thyrotoxicosis. The former results from the inability of the thyroid to escape from the Wolff-Chaikoff effect. It prevails in areas with high dietary iodine intake, and it is readily managed by discontinuation of amiodarone or thyroid hormone replacement. Amiodarone-induced thyrotoxicosis occurs more frequently in areas with low iodine intake; it may arise from iodine-induced excessive thyroid hormone synthesis (type I) or destructive thyroiditis with release of preformed hormones (type II). Type I should be treated with thionamides alone or in combination with potassium perchlorate, whereas type II benefits from treatment with glucocorticoids. Surgery may be a feasible option for patients who require long-term amiodarone treatment.