High-performance liquid chromatographic isolation and fast atom bombardment mass spectrometric identification of Di-N-desethylamiodarone, a new metabolite of amiodarone in the dog

Examining the safety of amiodarone

INTRODUCTION

Amiodarone is the most widely used antiarrhythmic agent, with demonstrated effectiveness against all the spectrum of cardiac tachyarrhythmias. The risk of adverse effects acts as a limiting factor to its utilization especially in the long term. This article systematically reviews the published evidence on amiodarone versus placebo to examine its safety as an antiarrhythmic drug.

AREAS COVERED

Authors collected data on adverse effects reported in 49 randomized placebo-controlled trials with amiodarone. Adverse effects were classified according to the organ/system involved. Pooled estimates of the number needed to treat (NNT) and to harm (NNH) versus placebo were calculated.

EXPERT OPINION

Amiodarone is effective for both the acute conversion of atrial fibrillation (AF) (11 trials, NNT = 4 at 24 h; p = 0.003) and the prevention of postoperative AF (18 trials, NNT = 8; p < 0.001), although with an increased risk of bradycardia, hypotension, nausea or phlebitis (pooled NNH = 4; p < 0.001). Amiodarone administration for the maintenance of sinus rhythm has a favorable net clinical benefit (pooled NNT = 3; p < 0.001 versus pooled NNH for either thyroid toxicity, gastrointestinal discomfort, skin toxicity or eye toxicity = 11; p < 0.001). Treatment with amiodarone for the prophylaxis of sudden cardiac death has less favorable net clinical benefit (15 trials, NNT = 38; p < 0.001 versus NNH for either thyroid toxicity, hepatic toxicity, pulmonary toxicity or bradycardia = 14; p < 0.001). Amiodarone treatment in this setting should be used in only selected cases.

Identification and quantitation of novel metabolites of amiodarone in plasma of treated patients

In mammals, mono-N-desethylamiodarone (MDEA) is the only known metabolite of amiodarone. Our previous experiments demonstrated that in vitro MDEA may be hydroxylated, N-dealkylated, and deaminated. In this report, we investigated the concentration of these microsomal metabolites in the plasma of patients receiving amiodarone. The presence of the hydroxy-amiodarone and deiodinated amiodarone was also additionally investigated. A high-performance liquid chromatography-atmospheric pressure chemical ionization tandem mass spectrometry (HPLC-APCI-MS/MS) quantitative assay using morpholine-amiodarone as internal standard was developed for measuring these metabolites in the range of 3-250 ng ml(-1). In the concentration ranges 5-50 and 50-250 ng ml(-1), the coefficients of variation of the measurements were less than 14 and 7%, respectively. The concentrations of investigated compounds in plasma of patients (n=14) receiving amiodarone (0.2 g day(-1), orally for >2 months) varied inter-individually and were 140.0+/-85.2, 39.1+/-20.8, and 26.2+/-15.2 ng ml(-1) for 3'OH-mono-N-desethylamiodarone, di-N-desethylamiodarone, and deaminated amiodarone, respectively. The concentrations of MDEA and amiodarone in these samples were 970+/-347 and 11163+/-435 ng ml(-1), respectively. In contrast, the studied compounds were not detectable in plasma samples from eight patients receiving amiodarone intravenously. Qualitatively, in the plasma of patients receiving amiodarone orally, hydroxylated amiodarone was also positively detected by assaying the [M+H](+) ions at m/z 662, but the deiodo-metabolites of amiodarone were not detected using mass spectrometry. Thus, in humans, amiodarone and MDEA were biotransformed by dealkylation, hydroxylation, and deamination.

Disruption of hepatic lipid homeostasis in mice after amiodarone treatment is associated with peroxisome proliferator-activated receptor-alpha target gene activation

Amiodarone, an efficacious and widely used antiarrhythmic agent, has been reported to cause hepatotoxicity in some patients. To gain insight into the mechanism of this unwanted effect, mice were administered various doses of amiodarone and examined for changes in hepatic histology and gene regulation. Amiodarone induced hepatomegaly, hepatocyte microvesicular lipid accumulation, and a significant decrease in serum triglycerides and glucose. Northern blot analysis of hepatic RNA revealed a dose-dependent increase in the expression of a number of genes critical for fatty acid oxidation, lipoprotein assembly, and lipid transport. Many of these genes are regulated by the peroxisome proliferator-activated receptor-alpha (PPARalpha), a ligand-activated nuclear hormone receptor transcription factor. The absence of induction of these genes as well as hepatomegaly in PPARalpha knockout [PPARalpha-/-] mice indicated that the effects of amiodarone were dependent upon the presence of a functional PPARalpha gene. Compared to wild-type mice, treatment of PPARalpha-/- mice with amiodarone resulted in an increased rate and extent of total body weight loss. The inability of amiodarone to directly activate either human or mouse PPARalpha transiently expressed in human HepG2 hepatoma cells indicates that the effects of amiodarone on the function of this receptor were indirect. Based upon these results, we conclude that amiodarone disrupts hepatic lipid homeostasis and that the increased expression of PPARalpha target genes is secondary to this toxic effect. These results provide important new mechanistic information regarding the hepatotoxic effects of amiodarone and indicate that PPARalpha protects against amiodarone-induced hepatotoxicity.

Biomedical and biological mass spectrometry

This review focuses on biological and biomedical mass spectrometry, and covers a selection of publications in this area included in the MEDLINE database for the period 1987-2001. Over the last 15 years, biological and biomedical mass spectrometry has progressed out of all recognition. The development of soft ionization methods, such as electrospray ionization and matrix-assisted laser desorption ionization, has mainly contributed to the remarkable progress, because they can easily produce gas-phase ions of large, polar, and thermally labile biomolecules, such as proteins, peptides, nucleic acids and others. The innovations of ionization methods have led to remarkable progress in mass spectrometric technology and in biochemistry, biotechnology and molecular biology research. In addition, mass spectrometry is one of the powerful and effective technologies for drug discovery and development. It is applicable to studies on structural determination, drug metabolism, including pharmacokinetics and toxicokinetics, and de novo drug discovery by applying post-genomic approarches. In the present review, the innovative soft ionization methods are first discussed along with their features. Also, the characteristics of the mass spectrometers which are active in the biological and biomedical research fields are also described. In addition, examples of the applications of biological and biomedical mass spectrometry are provided.

Interaction between amiodarone and lidocaine

We investigated the in vitro and in vivo interaction between amiodarone and lidocaine. The interaction on a molecular level was first studied in microsomes from 11 human livers. Close correlations between amiodarone N-monodesethylase activities and (a) the amounts of cytochrome P-4503A4 (CYP3A4), and (b) the rates of lidocaine N-monodesethylation were observed. Lidocaine inhibited amiodarone N-monodesethylation (Ki = 120 microM) competitively; inversely, amiodarone suppressed lidocaine N-monodesethylase activity in the same manner (Ki = 47 microM). Moreover, the metabolite N-monodesethylamiodarone (DEA) was stable and inhibited lidocaine metabolism in a concentration-dependent manner. The in vivo interaction was investigated in 6 cardiac patients. Each of them received a dose of 1 mg/kg lidocaine hydrochloride intravenously (i.v.) on three different occasions: before amiodarone treatment (control), and after cumulative doses of 3 g (phase I) and 13 g (phase II), respectively, amiodarone hydrochloride. The analysis of lidocaine pharmacokinetics showed an increase in lidocaine area under the curve (AUC) when amiodarone was administered, whereas that of N-monodesethylated lidocaine decreased. Moreover, the systemic clearance of lidocaine decreased, while the elimination half-life (t1/2) and the distribution volume at steady state of lidocaine remained unchanged. The pharmacokinetic parameters during phase II were the same as those during phase 1, indicating that the interaction had already occurred early in the loading phase of amiodarone administration. The interaction between amiodarone and lidocaine may be explained by the inhibition of CYP3A4 by amiodarone and/or by its main metabolite DEA.

A systematic review and critical comparison of internal standards for the routine liquid chromatographic assay of amiodarone and desethylamiodarone

Despite potential adverse effects, clinical use of amiodarone is increasing because of its efficacy in treating arrhythmias. Thus there is a continued need for a rapid, practical amiodarone assay to better study the relationship between serum concentrations and clinical effects and to guide safer dosing. Because the most widely used internal standard, L8040, is no longer available, a systematic comparison of potential alternatives was undertaken based on physicochemical and chromatographic characteristics. All amiodarone assays indexed on Medline were reviewed to produce a list of alternatives and five other potential substances considered based on previous experiences. An isocratic high-performance liquid chromatographic method was modified to allow simultaneous resolution of multiple compounds. The internal standard was expected to perform well in the solid-phase extraction of small sample volumes. No commercially available substances were able to duplicate all the advantages of L8040. Tamoxifen, the most acceptable alternative, was used to develop an assay to measure amiodarone and desethylamiodarone at concentrations as low as 0.25 mg/L in 100 microliters of serum (5 ng detected in a 20 microliters injection). Standard curves were linear over the range of concentrations found in our patients (0.25 to 8 mg/L), within-run coefficients of variation (CVs) averaged 5.3% for amiodarone and 2.9% for desethylamiodarone, and between-run CVs were 4.5% for amiodarone and 1.6% for desethylamiodarone.

Amiodarone- and desethylamiodarone-induced myelinoid inclusion bodies and toxicity in cultured rat hepatocytes

Hepatocytes isolated from Sprague-Dawley rats were incubated with various concentrations of either amiodarone or desethylamiodarone for 0 to 96 hr. Both drugs produced a concentration-dependent increase of lactate dehydrogenase release in the culture medium, which correlated well with cell death as measured by trypan blue exclusion test. Desethylamiodarone was more toxic than amiodarone in the cultured hepatocytes. Incubation with subtoxic concentrations of either amiodarone (7.6 microM) or desethylamiodarone (8 microM) for 24 hr resulted in the development of myelinoid inclusion bodies in the hepatocytes without any excess release of lactate dehydrogenase. In experimental protocols where the hepatocytes were exposed to either amiodarone or desethylamiodarone for up to 96 hr, there was an increase in lactate dehydrogenase and the percent volume-density of multilamellar inclusion bodies with cumulative drug exposure with time. A linear correlation between hepatocyte drug concentration and multilamellar inclusion bodies was found for both amiodarone and desethylamiodarone. These results demonstrate that both amiodarone and its major metabolite, desethylamiodarone, induce lysosomal inclusions, which, under appropriate conditions, can be dissociated from cell death. Withdrawal of the drug after 24 hr exposure did not result in disappearance of the inclusion bodies from the hepatocytes for up to 96 hr of tissue culture. The concentrations at which amiodarone- or desethylamiodarone-induced electron microscopic changes and hepatotoxicity were only two to five times as high as the usual serum drug levels in patients given antiarrhythmic therapy with amiodarone.

Basic and clinical pharmacology of amiodarone: relationship of antiarrhythmic effects, dose and drug concentrations to intracellular inclusion bodies

Amiodarone is a unique class III antiarrhythmic drug with several unusual pharmacokinetic, pharmacodynamic, and toxicological actions which are quite distinct from those of the standard antiarrhythmic drugs. Extensive animal and clinical studies have demonstrated that amiodarone and its major metabolite, desethylamiodarone, both produce a marked increase in the duration of transmembrane action potential, which may be related to their antiarrhythmic as well as clinical electrophysiological activity. Unlike most other cardiovascular drugs, it has been recognized for more than 20 years that optimal antiarrhythmic effects may take several days to weeks after onset of oral therapy. Amiodarone is highly lipid soluble and exhibits at least three separate compartments of drug distribution, with a long elimination half-life of 14-120 days after chronic therapy. The pharmacokinetic profile of desethylamiodarone is qualitatively similar to that of amiodarone, but its elimination half-life is even longer and its tissue distribution may be slightly different. Although there may not be any correlation between serum drug levels and clinical toxicity of amiodarone during long-term therapy, recent animal as well as clinical data suggest that multilamellar intracellular inclusions can be dissociated from cell death or clinical toxicity. Thus, it is possible that amiodarone toxicity can be minimized with low doses or low serum drug concentrations. The metabolite(s) of amiodarone may play a major role in its pharmacological and toxicological actions.

Amiodarone pharmacokinetics. II. Disposition kinetics following subchronic administration in rats

A 30 mg kg-1 intravenous bolus of 14C-amiodarone (19 microCi kg-1) was given to male Sprague-Dawley rats pretreated with 0 (vehicle), 25 or 100 mg kg-1 day-1 of amiodarone HCl orally for 37-42 days to determine the effects of dose and duration of administration on the disposition kinetics of amiodarone. Serial blood samples and total urine were collected over 48 hours and assayed for 14C-amiodarone by liquid scintillation counting following separation by HPLC. In all three groups, the blood 14C-amiodarone concentration-time curves declined bioexponentially with terminal half-lives (t1/2 beta) ranging from 14-22 hours. No differences in beta, t1/2 beta, or central compartment volume (Vc) were observed between the three groups of rats. In the rats pretreated with 100 mg kg-1 day-1 of amiodarone HCl for 5-6 weeks, amiodarone clearance (CL) and steady state volume of distribution (Vss) were reduced 52 per cent (12.2 to 5.9 ml min-1 kg-1) and 41 per cent (11.73 to 6.97 l kg-1), respectively. At the lower amiodarone daily dose, no changes in CL or Vss were observed. Negligible levels of radioactivity were detected in the urine. Amiodarone accounted for approximately 30-40 per cent of the total radioactivity in each blood specimen. This study demonstrated that CL and Vss were dose-dependent, and that beta, t1/2 beta and Vc were dose-independent. The results further suggested that the disposition kinetics of amiodarone were independent of the duration of drug administration.

Effects of acute intravenous and chronic oral amiodarone on defibrillation energy requirements

Amiodarone is commonly used with the automatic implantable defibrillator to treat recurrent ventricular tachyarrhythmias. The effects of acute intravenous and chronic oral amiodarone on the energy requirements for successful defibrillation were evaluated in 12 dogs chronically instrumented with right atrial spring and left ventricular patch defibrillation electrodes. Multiple shocks of varying energy were applied in balanced random order to construct curves of percent successful defibrillation vs energy (DF curves) on each test day. Dogs were studied on days 1, 11, 18, 25, and 32. On day 11, DF curves were determined before and after infusing saline (n = 6) or amiodarone (n = 6), 10 mg/kg loading and 0.33 mg/kg/min maintenance doses. Dogs administered intravenous amiodarone were continued on oral drug (300 mg twice daily) for the remainder of the study. Data were analyzed by logistic regression and the energy required for 50% (E50) and 80% (E80) successful defibrillation were compared. Differences between controls and animals receiving chronic oral amiodarone were not significant on any day. After acute intravenous infusion, dogs given amiodarone had a 21.7 +/- 12.8% decrease in E50 (p less than 0.01) and a 19.7 +/- 17.8% decrease in E80 (p less than 0.05), while controls had an 11.4 +/- 30.5% increase (p = NS) in E50 and 6.30 +/- 30.5 increase in E80 (p = NS). It is concluded that the energy required for successful defibrillation is decreased by acute intravenous amiodarone, while chronic oral administration has no significant effect.

Sensitive method for the measurement of amiodarone and desethylamiodarone in serum and tissue and its application to disposition studies

A high-performance liquid chromatographic (HPLC) method for the measurement of amiodarone (AM) and its metabolite(s) in serum and tissues was developed. The method uses a 5-micron silica column, methanol containing 0.02% perchloric acid at pH 4 as the mobile phase, and ultraviolet detection at 240 nm. The standard curves for AM and desethylamiodarone (DAM) were linear for serum (range 0.025-6.0 microgram/ml) and tissues (range 0.1-0.5 micrograms for 10-25 mg wet weight). There was a significant decrease as a function of time in AM and DAM concentrations in patients' sera left at ambient temperature in the presence of light. This HPLC method was applied to studies on serum AM elimination kinetics in patients and on tissue uptake during chronic AM administration to rabbits. The elimination half-life (5.8 h) of AM after a 5 mg/kg intravenous dose to a patient was similar to that after acute oral doses. AM, being lipophilic, accumulated maximally in the fat tissue (56 micrograms/g wet weight), followed by lung and liver in rabbits injected with AM for six weeks. The latter two tissues also contained nearly equal quantities of DAM. The high concentrations of AM and DAM in the liver and lungs may be related to the hepatic toxicity and pulmonary fibrosis associated with chronic AM therapy. Two new metabolites were found in the lung and bile of AM-treated rabbits, but these have not yet been identified.

Clinical pharmacokinetics of the newer antiarrhythmic agents

This article reviews clinical pharmacokinetic data on 8 new antiarrhythmic agents. Some of these drugs have been studied extensively while others are relatively new, with incomplete data due to limited evaluation. Amiodarone is a class III antiarrhythmic drug which is effective in treating many atrial and ventricular arrhythmias that are refractory to other drugs. Amiodarone accumulates extensively in tissues and its disposition characteristics are best described by models with 3 and 4 compartments. Its apparent volume of distribution is very large (1300 to 11,000L) and its elimination half-life very long (53 days). A delay of up to 28 days from of treatment to onset of antiarrhythmic effect may be observed, and the antiarrhythmic effect may persist for weeks to months following cessation of therapy. Clinically significant drug interactions have been observed with amiodarone and warfarin, digoxin, quinidine and procainamide. Encainide is a class Ic antiarrhythmic drug. Although it has a short elimination half-life (1 to 3h), 2 major metabolites with antiarrhythmic effects accumulate in the plasma of patients during long term therapy. Plasma concentrations of O-demethyl encainide appear to correlate with the antiarrhythmic effect. Flecainide, another class Ic antiarrhythmic agent, has an elimination half-life of 14 hours which makes it suitable for twice daily dosing. Flecainide elimination is prolonged in patients with low output heart failure. Significant drug interactions with digoxin and cimetidine have been reported. Lorcainide is also a class Ic antiarrhythmic drug, the bioavailability of which is nonlinear. Clearance of the drug is reduced during long term therapy. A major active metabolite, norlorcainide, accumulates in the plasma of patients during long term therapy and its concentration exceeds that of lorcainide by a factor of 2. The elimination half-lives of lorcainide (9h) and norlorcainide (28h) allow for once or twice daily dosing. Mexiletine, a class Ib antiarrhythmic drug, is structurally similar to lignocaine (lidocaine). A sustained release formulation provides effective plasma concentrations when administered twice daily. The apparent volume of distribution of mexiletine is 5.0 to 6.6 L/kg, and the elimination half-life varies from 6 to 12 hours in normal subjects and from 11 to 17 hours in cardiac patients. Mexilitine is extensively metabolised but the metabolites are not pharmacologically active. Renal elimination of mexiletine is pH dependent. Drugs which induce hepatic metabolism significantly alter the pharmacokinetics of mexiletine.(ABSTRACT TRUNCATED AT 400 WORDS)