Effects of dronedarone and amiodarone on plasma thyroid hormones and on the basal and postischemic performance of the isolated rat heart

The present study investigated the effects of dronedarone and amiodarone on plasma thyroid hormones and the possible consequences on the response of the heart to ischemia. Amiodarone (30 mg/kg/day per os) or dronedarone (30 mg/kg/day per os) were administered for 2 weeks in normal and thyroxine-treated animals (25 microg/100 g body weight od sc, for 2 weeks), while animals without amiodarone and dronedarone served as controls. Isolated rat hearts were perfused in a Langendorff mode and subjected to 20 and 30 min of zero-flow global ischemia followed by 45 min of reperfusion. Functional changes were assessed by measuring left ventricular developed pressure (LVDP) under resting conditions and in response to ischemia-reperfusion, LVDP%, as well as the severity of ischemic contracture. Amiodarone resulted in increased T4, T4/T3 and rT3, whereas dronedarone did not alter the thyroid hormone profile in normal animals. In thyroxine-treated animals, amiodarone increased T4/T3 ratio but T4, T3 and rT3 levels were not altered. Basal functional parameters and ischemic contracture did not change by amiodarone and/or dronedarone neither in normal nor in thyroxine-treated hearts. In normal hearts, postischemic functional recovery, LVDP%, was not altered by amiodarone or dronedarone administration. LVDP% was statistically higher in thyroxine-treated hearts than in normal and this beneficial effect was not abolished by amiodarone or dronedarone treatment.

Chronic and acute effects of dronedarone on the action potential of rabbit atrial muscle preparations: comparison with amiodarone

Dronedarone, a noniodinated derivative of amiodarone, is under evaluation as a potentially less toxic anti-arrhythmic alternative to amiodarone. The acute and chronic electrophysiologic effects of dronedarone and amiodarone were compared in isolated rabbit atrial muscle by microelectrode techniques. Four-week PO treatment with dronedarone or amiodarone increased action potential duration (APD90) (58 +/- 4 ms control versus 69 +/- 2 ms dronedarone, p < 0.01; 68 +/- 3 ms amiodarone, p < 0.01 for a 100-mg/kg/d dose) and effective refractory period (49 +/- 6 ms control versus 68 +/- 4 ms dronedarone, p < 0.01; 63 +/- 3 ms amiodarone, p < 0.01). The APD90 prolonged reverse rate-dependency. In contrast, acute superfusion with 10 microM dronedarone or amiodarone decreased APD90 (61 +/- 6 ms control versus 53 +/- 4 ms dronedarone, p < 0.05; 52 +/- 6 ms amiodarone, p < 0.05), effective refractory period (50 +/- 5 ms control versus 44 +/- 4 ms dronedarone, p < 0.05; 43 +/- 6 ms amiodarone, p < 0.05), and the maximum upstroke slope of the action potential (Vmax) (188 +/- 9 V/s control versus 182 +/- 11 V/s dronedarone p < 0.05; 182 +/- 11 V/s amiodarone, p < 0.05). Thus, chronic and acute electrophysiologic effects of dronedarone on rabbit atrial muscle are similar to those of amiodarone, suggesting a similar potential against atrial arrhythmias.

Determination of amiodarone and desethylamiodarone in human plasma by high-performance liquid chromatography-electrospray ionization tandem mass spectrometry with an ion trap detector

A sensitive and specific high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS-MS) method has been developed for the simultaneous determination of amiodarone and desethylamiodarone in human plasma. After the addition of the internal standard tamoxifen, plasma samples were extracted using Oasis MCX solid-phase extraction cartridges. The compounds were separated on a 5 microm Symmetry C18 (Waters) column (150 x 3.0 mm, internal diameter) with a mobile phase of acetonitrile-0.1% forrmic acid (46:54, v/v) at a flow-rate of 0.5 ml/min. The overall extraction efficiency was more than 89% for both compounds. The assay was sensitive down to 1 microg/l for amiodarone and down to 0.5 microg/l for desethylamiodarone. Within-run accuracies for quality-control samples were between 95 and 108% of the target concentration, with coefficients of variation <8%. The proposed method enables the unambiguous identification and quantitation of amiodarone and desethylamiodarone in both clinical and forensic specimens.

First clinical experience with the rapid-, short-acting amiodarone derivative E 047/1 after cardiac surgery

BACKGROUND AND OBJECTIVE

Amiodarone is very effective against a variety of dysrhythmias but has poor pharmacodynamic properties and many undesired side-effects. Its short- and rapid-acting derivative E 047/1 may circumvent some of these drawbacks. It is easier to titrate while retaining the high efficacy of amiodarone and may have acceptable influences on haemodynamics and cardiac conduction in patients who develop serious, destabilizing ventricular tachydysrhythmias after cardiac surgery.

METHODS

Testing E 047/1 was performed prospectively in two consecutive phase II open, clinical studies. Out of 504 patients scheduled for surgery using cardiopulmonary bypass for coronary artery grafting and/or valve repair, 35 developed serious, haemodynamically destabilizing ventricular dysrhythmias (Lown 2-Lown 4b) after surgery and were treated with a 1 mg kg(-1) (pilot study, n = 15) or randomized to a 2 or 3 mg kg(-1) bolus of E 047/1, followed by a 1 mg kg(-1) h(-1) continuous infusion for 2 h (n = 10 in each group). Dysrhythmias, PQ, QTc intervals and haemodynamics using the thermodilution technique were evaluated for up to 24 h after drug initiation.

RESULTS

At the time of final inclusion the patients had between 6 and 12 (or more) ventricular ectopics per minute. Within the first 2-3 min of application in the pilot trial E 047/1 induced a decrease of ventricular dysrhythmias to between 0 and 4 per min, a decrease that held for the duration of treatment. The area under the curve decreased from 434 (322, 855; median, quartiles) to 114 (9, 477, P < 0.01) events per hour. In the randomized trial, E 047/1 administered in either dose rapidly reduced ventricular dysrhythmias at least as effectively as in the pilot trial 565 (478, 701) to 33 (8, 238, P < 0.05) after a 2 mg bolus; 482 (339, 482) to 95 (13, 540, P < 0.01) events per hour after a 3 mg bolus. Approximately 4-6 h after drug termination, dysrhythmias reappeared in the majority of patients. In only three patients did the incidence of dysrhythmias return to inclusion criteria levels. In contrast to the pilot trial, in the randomized trial there was a slight increase of mean pulmonary artery pressure, central venous pressure and pulmonary arterial wedge pressure and a slight decrease of LCWI in both groups. E 047/1 did not cause QTc prolongation.

CONCLUSIONS

E 047/1 appears to be a safe alternative to amiodarone in the perioperative setting of cardiac surgery when serious, destabilizing dysrhythmias occur.

Chronic amiodarone evokes no torsade de pointes arrhythmias despite QT lengthening in an animal model of acquired long-QT syndrome

BACKGROUND

Amiodarone is an effective antiarrhythmic drug rarely associated with torsade de pointes arrhythmias (TdP). The noniodinated compound dronedarone could resemble amiodarone and be devoid of the adverse effects. In the dog with chronic complete atrioventricular (AV) block (CAVB) and acquired long-QT syndrome, the electrophysiological and proarrhythmic properties of the drugs were compared after 4 weeks of oral treatment.

METHODS AND RESULTS

Amiodarone (n=7, 40 mg. kg(-1). d(-1)) and dronedarone (n=8, 20 mg/kg BID) were started at 6 weeks of CAVB (baseline). Six dogs served as controls. Surface ECGs and endocardially placed monophasic action potential catheters in the left (LV) and right (RV) ventricles were recorded to assess QTc time, action potential duration (APD), interventricular dispersion (DeltaAPD=LV APD minus RV APD), early afterdepolarizations (EADs), ectopic beats, and TdP. Both amiodarone (+21%) and dronedarone (+31%) increased QTc time. Amiodarone showed no increase in DeltaAPD in 4 of 7 dogs, whereas dronedarone augmented DeltaAPD in 7 of 8 animals. After dronedarone, TdP occurred in 4 of 8 dogs with the highest DeltaAPD (105+/-20 ms). TdP was never seen with amiodarone, not even in the dogs that had DeltaAPD values comparable to those with dronedarone. Furthermore, a difference existed in EADs and ectopic activity incidence (dronedarone 3 of 8; amiodarone 0 of 7), which was also seen during an epinephrine challenge.

CONCLUSIONS

In the CAVB dog model, both amiodarone and dronedarone prolong QT time (class III effect). The absence of TdP with amiodarone seems to be related to homogeneous APD lengthening in the majority of dogs and the lack of EADs and/or ventricular ectopic beats in all.

Amiodarone-Induced thyroid dysfunction and ventricular tachyarrhythmias during long-term therapy in Japan

In 232 Japanese patients receiving long-term amiodarone therapy for life-threatening ventricular tachyarrhythmias, hyperthyroidism and hypothyroidism developed in 29 patients (12.5%) and 25 patients (10.8%), respectively. In patients with hyperthyroidism, the recurrence of sustained ventricular tachycardia was significantly higher with thyrotoxicosis than in the euthyroid period (31% vs 3%, p<0.01). Holter monitoring showed that the average heart rate and ventricular premature complexes significantly increased with hyperthyroidism. On the other hand, there was no increase in the recurrence of ventricular tachyarrhythmia with hypothyroidism. There was no change in the dose or the plasma concentration of amiodarone or desethylamiodarone in the euthyroid period or when hyperthyroidism or hypothyroidism manifested. It is important to monitor for arrhythmia when hyperthyroidism develops during amiodarone therapy.

Altered metabolite concentrations with amiodarone generic substitution cannot be observed without monitoring

The use of amiodarone has grown rapidly, resulting in the marketing of several generic formulations. The adequacy of the testing used to approve these formulations as bioequivalent has been questioned, and mounting clinical evidence suggests that in some patients, substitution with generic amiodarone can cause serious problems. The effects of switching amiodarone formulations may take weeks to develop, leaving the relationship between the events unrecognized. In animal models, the toxicity of desethylamiodarone, an active metabolite partly formed during amiodarone absorption, is greater than that of its parent compound. High metabolite to amiodarone ratios have been associated with clinical toxicity. Because measuring serum amiodarone and metabolite is not standard clinical practice, aberrations after switching formulations will be missed. Major changes in metabolite concentrations were documented in four patients switched to a generic formulation, suggesting that the tests used for regulatory approval failed to identify the cumulative effects of differing excipients on amiodarone metabolism during absorption. Physicians should monitor patients for several months after a switch in amiodarone formulation is made. Regulatory criteria for bioequivalence of amiodarone need to be reconsidered.

Toxicity of amiodarone and amiodarone analogues on isolated rat liver mitochondria

BACKGROUND

Amiodarone is a well-known mitochondrial toxin consisting of a benzofuran ring (ring A) coupled to a p-OH-benzene structure substituted with 2 iodines and a diethyl-ethanolamine side chain (ring B).

AIM

To find out which part of amiodarone is responsible for mitochondrial toxicity.

METHODS

Amiodarone, ring A and B without the ethanolamine side-chain and iodines (B0), ring A and B with iodines but no ethanolamine (B2), ring B with 1 iodine and no ethanolamine (C1) and ring B with ethanolamine and 2 iodines (D2) were studied.

RESULTS

In freshly isolated rat liver mitochondria, amiodarone inhibited state 3 glutamate and palmitoyl-CoA oxidation and decreased the respiratory control ratios. B0 and B2 were more potent inhibitors than amiodarone and B2 more potent than B0. C1 and D2 showed no significant mitochondrial toxicity. After disruption, mitochondrial oxidases and complexes of the electron transport chain were inhibited by amiodarone, B0 and B2, whereas C1 and D2 revealed no inhibition. Beta-oxidation showed a strong inhibition by amiodarone, B0 and B2 but not by C1 or D2. Ketogenesis was almost unaffected.

CONCLUSIONS

Amiodarone, B0 and B2 are uncouplers of oxidative phosphorylation, and inhibit complexes I, II and III, and beta-oxidation. The benzofuran structure is responsible for mitochondrial toxicity of amiodarone and the presence of iodine is not essential.

Beneficial effects of amiodarone and dronedarone (SR 33589b), when applied during low-flow ischemia, on arrhythmia and functional parameters asssessed during reperfusion in isolated rat hearts

The effects of short-term amiodarone and dronedarone treatments on action potential characteristics and arrhythmia (ventricular tachycardia ) induced by reperfusion after global low-flow ischemia were studied in rat hearts. The actions of amiodarone and SR on recovery of coronary flow and contractile function were also determined. Isolated hearts were stabilized for 40 min and were then submitted to 25-min global low-flow ischemia (constant coronary flow, 0.3 ml/min) followed by 30 min of reperfusion at constant pressure. Drugs were applied only during ischemia: consequently, action potential duration (APD) tended to widen. During reperfusion, APD tended to recover or shorten, and the more complete the recovery, the less the arrhythmia. Despite its ability to widen APD during ischemia, amiodarone facilitated APD recovery during reperfusion. Moreover, APD shortening and ventricular tachycardia suppression exhibit a bell-shaped concentration-response relation, implying that the drugs affect ventricular tachycardia by a class III-independent action. These results point to an anti-ischemic action supported by improvement in function and inhibition of reactive hyperemia.

Role of desethylamiodarone in the anticoagulant effect of concurrent amiodarone and warfarin therapy

BACKGROUND

The concurrent use of amiodarone and warfarin inhibits metabolism of S-warfarinby cytochrome P450 (CYP) 2C9, thereby increasing the anticoagulant effect of warfarin. Amiodarone primarily inhibits CYP1A2 and CYP3A4, and desethylamiodarone primarily inhibits CYP2C9. We investigate whether a relationship exists between the plasma concentration of desethylamiodarone and anticoagulation when amiodarone is administered to patients receiving warfarin therapy.

METHODS AND RESULTS

The correlation between the plasma concentration of either amiodarone or desethylamiodarone, and prolongation of prothrombin time-international normalized ratio/dose of warfarin (Delta INR/Dose) on day 7 of amiodarone administration was studied in 25 patients (22-74 years old) with structural heart disease and refractory arrhythmias receiving stable warfarin therapy.

RESULTS

No correlation was found between the plasma concentration of amiodarone and Delta INR/Dose, but a correlation was found between the plasma concentration of desethylamiodarone and Delta INR/Dose.

CONCLUSIONS

It was suggested that inhibition of CYP2C9 by desethylamiodarone, the active metabolite of amiodarone, plays an important role in the interaction of warfarin and amiodarone.

Effects of vitamin E on cytotoxicity of amiodarone and N-desethylamiodarone in isolated hamster lung cells

Amiodarone (AM) is a potent and efficacious antidysrhythmic agent that can cause potentially life-threatening pulmonary fibrosis. Vitamin E has been demonstrated to decrease AM-induced pulmonary fibrosis in vivo in hamsters. In the present in vitro study, we investigated the effects of vitamin E on cell death induced by AM and its primary metabolite, N-desethylamiodarone (DEA), in freshly isolated hamster lung cells. Following incubation for 24 or 36 h, 300 microM vitamin E decreased (P<0.05) 100 microM AM-induced cytotoxicity (0.5% trypan blue uptake) in alveolar macrophages by 11.7+/-3% or 21.4+/-12%, respectively, but did not decrease cytotoxicity in fractions enriched with alveolar type II cells or non-ciliated bronchiolar epithelial (Clara cells) or in isolated unseparated cells (cell digest). Vitamin E had no effect on 50 microM DEA-induced cytotoxicity. Vitamin E did not alter cellular levels of AM or DEA in any cell fraction. Lipid peroxidation (assessed by isoprostane formation) was increased (P<0.05) in cell digest, alveolar type II cell and Clara cell enriched fractions incubated with 500 microM carbon tetrachloride (CCl(4)) for 4 h but not in enriched fractions of cells exposed to 100 microM AM or 50 microM DEA. No AM-induced loss of viability was observed at this time point, but DEA decreased (P<0.05) Clara cell viability by approximately 25%. These results demonstrate cell type selective protection against AM-induced cytotoxicity by vitamin E, and suggest that lipid peroxidation does not initiate AM- or DEA-induced cytotoxicity in isolated hamster lung cells.

Disruption of mitochondrial function and cellular ATP levels by amiodarone and N-desethylamiodarone in initiation of amiodarone-induced pulmonary cytotoxicity

Amiodarone (AM), a potent antidysrhythmic agent, can cause potentially life-threatening pulmonary fibrosis. In the present investigation of mechanisms of initiation of AM lung toxicity, we found that 100 microM AM decreased mitochondrial membrane potential in intact hamster lung alveolar macrophages and preparations enriched in isolated alveolar type II cells and nonciliated bronchiolar epithelial (Clara) cells, following 2 h of incubation. This was followed by a drop in cellular ATP content (by 32--77%) at 4 to 6 h, and 30 to 55% loss of viability at 24 h. Supplementation of incubation media with 5.0 mM glucose or 2.0 mM niacin did not reduce AM-induced ATP depletion or cell death in macrophages, and the mitochondrial permeability transition inhibitor cyclosporin A (1.0 microM) did not affect AM cytotoxicity. At 50 microM, the AM metabolite N-desethylamiodarone (DEA) produced effects similar to those of AM, but more rapidly and extensively, with the Clara cell-enriched preparation being particularly susceptible. In isolated whole lung mitochondria, DEA was accumulated to a greater extent than AM. Both AM and DEA inhibited complex I- and complex II-supported respiration, but DEA inhibited complex II to a greater degree than AM. These results demonstrate that AM and DEA disrupt mitochondrial membrane potential prior to ATP depletion and subsequent lung cell death, that DEA is more potent than AM, and that the mitochondrial permeability transition is not involved in mitochondrial perturbation by AM. This suggests that AM- and DEA-induced perturbations of mitochondrial function may initiate AM-induced pulmonary toxicity.

Desethylamiodarone antagonizes the effect of thyroid hormone at the molecular level

OBJECTIVE

To evaluate the molecular mechanisms of the inhibitory effects of amiodarone and its active metabolite, desethylamiodarone (DEA) on thyroid hormone action.

MATERIALS AND METHODS

The reporter construct ME-TRE-TK-CAT or TSHbeta-TRE-TK-CAT, containing the nucleotide sequence of the thyroid hormone response element (TRE) of either malic enzyme (ME) or TSHbeta genes, thymidine kinase (TK) and chloramphenicol acetyltransferase (CAT) was transiently transfected with RSV-TRbeta into NIH3T3 cells. Gel mobility shift assay (EMSA) was performed using labelled synthetic oligonucleotides containing the ME-TRE and in vitro translated thyroid hormone receptor (TR)beta.

RESULTS

Addition of 1 micromol/l T4 or T3 to the culture medium increased the basal level of ME-TRE-TK-CAT by 4.5- and 12.5-fold respectively. Amiodarone or DEA (1 micromol/l) increased CAT activity by 1.4- and 3.4-fold respectively. Combination of DEA with T4 or T3 increased CAT activity by 9.4- and 18.9-fold respectively. These data suggested that DEA, but not amiodarone, had a synergistic effect with thyroid hormone on ME-TRE, rather than the postulated inhibitory action; we supposed that this was due to overexpression of the transfected TR into the cells. When the amount of RSV-TRbeta was reduced until it was present in a limited amount, allowing competition between thyroid hormone and the drug, addition of 1 micromol/l DEA decreased the T3-dependent expression of the reporter gene by 50%. The inhibitory effect of DEA was partially due to a reduced binding of TR to ME-TRE, as assessed by EMSA. DEA activated the TR-dependent down-regulation by the negative TSH-TRE, although at low level (35% of the down-regulation produced by T3), whereas amiodarone was ineffective. Addition of 1 micromol/l DEA to T3-containing medium reduced the T3-TR-mediated down-regulation of TSH-TRE to 55%.

CONCLUSIONS

Our results demonstrate that DEA, but not amiodarone, exerts a direct, although weak, effect on genes that are regulated by thyroid hormone. High concentrations of DEA antagonize the action of T3 at the molecular level, interacting with TR and reducing its binding to TREs. This effect may contribute to the hypothyroid-like effect observed in peripheral tissues of patients receiving amiodarone treatment.

Electrophysiological effects of dronedarone (SR 33589), a noniodinated amiodarone derivative in the canine heart: comparison with amiodarone

The electrophysiological effects of dronedarone, a new nonionidated analogue of amiodarone were studied after chronic and acute administration in dog Purkinje fibres, papillary muscle and isolated ventricular myocytes, and compared with those of amiodarone by applying conventional microelectrode and patch-clamp techniques. Chronic treatment with dronedarone (2x25 mg(-1) kg(-1) day p.o. for 4 weeks), unlike chronic administration of amiodarone (50 mg(-1) kg(-1) day p.o. for 4 weeks), did not lengthen significantly the QTc interval of the electrocardiogram or the action potential duration (APD) in papillary muscle. After chronic oral treatment with dronedarone a small, but significant use-dependent V(max) block was noticed, while after chronic amiodarone administration a strong use-dependent V(max) depression was observed. Acute superfusion of dronedarone (10 microM), similar to that of amiodarone (10 microM), moderately lengthened APD in papillary muscle (at 1 Hz from 239.6+/-5.3 to 248.6+/-5.3 ms, n=13, P<0.05), but shortened it in Purkinje fibres (at 1 Hz from 309.6+/-11.8 to 287.1+/-10.8 ms, n=7, P<0.05). Both dronedarone (10 microM) and amiodarone (10 microM) superfusion reduced the incidence of early and delayed afterdepolarizations evoked by 1 microM dofetilide and 0.2 microM strophantidine in Purkinje fibres. In patch-clamp experiments 10 microM dronedarone markedly reduced the L-type calcium current (76.5+/-0.7 %, n=6, P<0.05) and the rapid component of the delayed rectifier potassium current (97+/-1.2 %, n=5, P<0.05) in ventricular myocytes. It is concluded that after acute administration dronedarone exhibits effects on cardiac electrical activity similar to those of amiodarone, but it lacks the 'amiodarone like' chronic electrophysiological characteristics.

Metabolism of amiodarone. II. High-performance liquid chromatographic assay for mono-N-desethylamiodarone hydroxylation in liver microsomes

Amiodarone (AMI) is a potent antiarrhythmic drug. In vivo and in vitro, AMI is biotransformed to mono-N-desethylamiodarone (MDEA). Recently, it was observed that MDEA was further hydroxylated to n-3'-hydroxybutyl-MDEA (3'OH-MDEA). The performance of a HPLC-UV assay being developed for the quantification of the new compound was investigated. Liver microsomes isolated from rabbit, rat and human biotransformed MDEA to 3'OH-MDEA. Their estimates of Michaelis-Menten parameters were Km=6.39, 25.2, 19.4 microM; Vmax=560, 54, 17.3 pmol/mg protein/min), respectively. Thus, hydroxylase activity in mammals may be the origin of the species dependence observed in the AMI metabolism.

Metabolism of amiodarone (Part III): identification of rabbit cytochrome P450 isoforms involved in the hydroxylation of mono-N-desethylamiodarone

1. Amiodarone (AMI) is a potent anti-arrhythmic drug and mono-N-desethylamiodarone (MDEA) is its only known metabolite. It was found recently that in rabbit liver microsomes MDEA was biotransformed to n-3-hydroxybutyl-MDEA (3OH-MDEA). 2. In liver microsomes isolated from the untreated rabbit, the formation of 3OH-MDEA obeyed Michaelis-Menten enzyme kinetics with Km = 6.39 +/- 1.07 microM and Vmax = 0.56 +/- 0.21 nmolmin(-1) mg(-1) protein. 3. Furthermore, (1) among chemicals usually used as inhibitors of cytochrome P450, only midazolam (MDZ), cyclosporin A and ketoconazole inhibited the MDEA hydroxylase activity significantly (>60% inhibition), (2) MDZ, a substrate of CYP3A, inhibited the 30OH-MDEA formation competitively (Ki = 10 +/- 5 microM), (3) the formation rates of 3OH-MDEA correlated positively with those of 1'OH-MDZ (r = 0.81; n = 6), and (4) MDEA hydroxylase activity of microsomes isolated from rabbit rifampicin-induced cultured hepatocytes was 4-fold more active than the control. 4. Since CYP3A6 is mainly induced by rifampicin in rabbit-cultured hepatocytes, the data suggest that this isoform is involved in the biotransformation of MDEA to 3OH-MDEA. 5. Since alpha-naphthoflavone, cimetidine and quinidine also partially inhibited the MDEA hydroxylase activity, it is possible that other CYPs, such as 1A, 2C and 2D, may also be active in the metabolism of amiodarone.

Structure-activity relationships and electrophysiological effects of short-acting amiodarone homologs in guinea pig isolated heart

Antiarrhythmic agents with amiodarone-like electrophysiological actions, but with a more favorable pharmacokinetic profile than amiodarone would be extremely useful for the treatment of many tachyarrhythmias. We designed a series of amiodarone homologs with an alkyl ester group at position 2 of the benzofurane moiety. It was hypothesized that the electrophysiological and pharmacokinetic properties of these compounds are closely related to the size and branching of the ester group. The magnitude and time course of electrophysiological effects caused by methyl (ATI-2001), ethyl (ATI-2010), isopropyl (ATI-2064), sec-butyl (ATI-2042), and neopentyl (ATI-2054) homologs, and their common metabolite (ATI-2000) were investigated in guinea pig isolated heart. In paced hearts (atrial cycle length = 300 ms), each homolog (1 microM) was infused for 90 min followed by a 90-min washout. The stimulus-to-atrium (St-A), atrium-to-His bundle (AH), His bundle-to-ventricle (HV), QRS, and QT intervals, and ventricular monophasic action potential duration at 90% repolarization (MAPD(90)) were measured every 10 min. ATI-2001 and ATI-2064 significantly lengthened the St-A, HV, and QRS intervals, whereas ATI-2042 and ATI-2054 prolonged only the St-A interval. All compounds except the metabolite prolonged the AH interval. The relative rank order for the homologs to lengthen ventricular repolarization (MAPD(90)) was ATI-2042 > or = 2001 = 2010 = 2064 > 2054 > or = 2000. The metabolite was electrophysiologically inactive. Thus, modification of the benzofurane moiety ester group size and branching markedly altered the magnitude and time course of the electrophysiological effects caused by the ATI compounds. The different structure-activity relationships among the amiodarone homologs may have important consequences for further development of amiodarone-like antiarrhythmic agents.

Influence of CYP3A metabolizer status on the pharmacokinetics and pharmacodynamics of amiodarone

The present work was designed to determine whether the individual differences in pharmacokinetics and pharmacodynamics of amiodarone and its N-desethyl metabolite are related to cytochrome CYP3A metabolizer status.

METHODS

12 cardiac patients with inducible ventricular tachyarrhythmias during the baseline electrophysiologic study were enrolled in this study. Urinary 24-hour excretion of 6 beta-hydroxycortisol (6 beta-OHC and the ratio of 6 beta-hydroxycortisol to urinary free cortisol (6 beta-OHC/UFC) were measured before the first amiodarone administration. Trough plasma concentrations of amiodarone and N-desethylamiodarone (N-DEA) were measured after 79 +/- 11 days (2nd period) and after 182 +/- 25 days (3rd period). Electrophysiologic effects of amiodarone therapy were established with serial electrophysiologic studies in 9 of these patients at the baseline and after 79 +/- 11 days (during the second period).

RESULTS

Both the 6 beta-OHC excretion and 6 beta-OHC/UFC ratio varied approximately 6-fold between the patients. We found significant inverse correlation between the 6 beta-OHC excretion and the trough plasma concentrations of amiodarone at the time of the 3rd period (rs = -0.58, p < 0.05). Similarly, there was correlation between the 24-hour urinary 6 beta-OHC excretion and trough plasma concentrations of amiodarone during the 3rd period (rs = -0.64, p < 0.025). We were unable to detect any association between CYP3A activity and amiodarone pharmacodynamics.

CONCLUSION

This study points toward important information value of CYP3A metabolizer status in the context of therapeutic drug monitoring of amiodarone.

Metabolism of amiodarone (part I): identification of a new hydroxylated metabolite of amiodarone

Amiodarone (AMI) is a potent antiarrhythmic drug, but its metabolism has not yet been fully documented. Mono-N-desethylamiodarone (MDEA) is its only known metabolite. Our preliminary investigations using rabbit liver microsomes had shown that in vitro AMI was biotransformed to MDEA, and the latter was rapidly further biodegraded to other unknown products. The aim of the present study was to investigate the chemical structure of the biotransformed compound of MDEA. Upon incubation of MDEA with rabbit liver microsomes and NADPH as cofactor, MDEA was biotransformed into three unknown products: X1, X2, and X3. The products were purified using chromatography. The chemical structure of the major product, X1, was investigated in detail. HPLC-ESI-MS revealed that MDEA had been oxygenated. Hydrogen-deuterium exchange experiments showed that the X1 molecule contained one exchangeable hydrogen atom more than its precursor MDEA, indicating that MDEA had been hydroxylated. Further results from ESI-MS/MS analysis indicated that the site of hydroxylation was the n-butyl side chain. NMR analysis (1H NMR, one-dimensional-total correlation spectroscopy, and heteronuclear multiple-bond correlation spectroscopy) established the 3-position (omega-1) of the butyl moiety as the specific carbon atom that is hydroxylated. Rat liver microsomes were also able to catalyze MDEA hydroxylation. Compound X1, as analyzed by HPLC-ESI-MS and ESI-MS/MS, was detected in the liver, heart, lung, and kidney tissue of four rats receiving AMI, suggesting that the hydroxylated MDEA was a secondary metabolite of AMI.

CONCLUSION

in mammals, MDEA is hydroxylated to the secondary metabolite of AMI [2-(3-hydroxybutyl)-3-[4-(3-ethylamino-1-oxapropyl)-3,5-diiodobenzoyl]-benzofuran].

N-desethylamiodarone modulates intracellular calcium concentration in endothelial cells

The main in-vivo metabolite of amiodarone, N-desethylamiodarone (DEAM), possesses clinically relevant class-II antiarrhythmic and vasodilator activities. Vasodilation by DEAM is endothelium dependent and involves a sustained and biphasic increase in cytosolic free Ca2+ concentration ([Ca2+]i). The aims of this study were to explore the mechanisms mediating the DEAM-induced increase in [Ca2+]i in endothelial cells and to determine whether this increase in [Ca2+]i was associated with altered cell proliferation. Cultured bovine aortic endothelial cells were loaded with the Ca2+-sensitive fluorescent dye Fura-2/AM, and [Ca2+]i measured spectrofluorimetrically. DEAM increased [Ca2+]i concentration dependently (EC50 approximately 6 microM) both in the presence and absence of extracellular Ca2+. In the presence of extracellular Ca2+, the response of [Ca2+]i to DEAM (10 microM) consisted of an initial rise to a plateau followed by a second increase to micromolar levels. The initial plateau was reduced by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (200 nM) and by the antioxidant ascorbic acid (100 microM). The initial rate of rise in [Ca2+]i was decreased by blocking mitochondrial Ca2+ release with cyclosporine A (1 microM). Under Ca2+-free conditions, the response of [Ca2+]i to DEAM (10 microM) was also biphasic, consisting of an initial transient peak and a second slow increase. When extracellular Ca2+ was restored, [Ca2+]i rose to micromolar concentrations. The initial peak was abolished by thapsigargin, but not altered by ascorbic acid or cyclosporine A. Both the second [Ca2+]i increase and that due to restoring extracellular Ca2+ were reduced by ascorbic acid but not affected by thapsigargin or cyclosporine A. The DEAM-induced generation of free radicals and sustained increase in [Ca2+]i might alter cell proliferation and endothelial cell proliferation was indeed concentration-dependently inhibited by DEAM (IC50 approximately 2.5 microM). In conclusion, the DEAM-induced [Ca2+]i increase in endothelial cells is due to Ca2+ influx from the extracellular space and to Ca2+ release from endoplasmic reticulum and mitochondria and involves enhanced generation of free radicals.

Inhibitory effects of dronedarone on muscarinic K+ current in guinea pig atrial cells

Dronedarone (SR33589), an amiodarone-like noniodinated antiarrhythmic agent, is undergoing clinical trials in atrial fibrillation. Because vagal activation plays a role in the pathophysiology of supraventricular arrhythmias, we have assessed the ability of dronedarone (0.01, 0.1, and 1 microM), compared with amiodarone (0.1, 1, and 10 microM) to inhibit the muscarinic acetylcholine receptor-operated K+ current (I(K(ACh))) in single cells isolated from guinea pig atria (patch-clamp technique). I(K(ACh)) was activated by extracellular application of carbachol (10 microM) or by intracellular loading with GTP-gamma-S (100 microM). Dronedarone and amiodarone reduced the carbachol-induced I(K(ACh)) with an IC50 (concentration required for 50% inhibition) slightly above 10 nM and 1 microM, respectively. Dronedarone also inhibited the GTP-gamma-S induced K+ current by 28% and 58% at 0.01 and 0.1 microM, respectively. These data suggest that dronedarone inhibits I(K(ACh)) by depressing the function of K(ACh) channel itself or associated GTP-binding proteins. Compared with amiodarone, dronedarone is approximately 100 times more potent on I(K(ACh)) and seems more selective in inhibiting I(K(ACh)) with respect to its antagonism of other inward and outward currents reported in the literature. This relative high potency of dronedarone to reduce I(K(ACh)) may be involved, at least in part, in the antiarrhythmic action of dronedarone against atrial fibrillation.