Update: cardiac antiarrhythmic drugs

The last ten years have been a period of extensive research and development of new agents for the treatment of cardiac rhythm abnormalities. Several new subclass Ic agents have been developed, and more recently the class III agents have become the focus of attention. These new agents are all remarkable for their potency and potential for producing side effects. While none of these agents offers the perfect cure for the treatment or prevention of cardiac arrhythmias, they all offer advantages and options that are valuable for clinical management of patients.

Rate-related electrophysiologic effects of long-term administration of amiodarone on canine ventricular myocardium in vivo

The electrophysiologic effects of amiodarone were examined in 13 dogs that received 30 g amiodarone orally during 3 weeks and compared with 13 control dogs that did not receive amiodarone. Longitudinal and transverse epicardial conduction velocities were estimated with a square array of 64 closely spaced electrodes and a computer-assisted acquisition and analysis system. Amiodarone caused a rate-dependent decrease in conduction velocity with a slightly greater effect in the longitudinal direction of propagation. Rate-related depression of conduction velocity developed rapidly after abrupt shortening of the pacing cycle length; 67% of the change occurred between the first two beats of the rapid train, and little change occurred after the 10th beat. Recovery from use-dependent depression of conduction velocity was exponential with a mean time constant of 447 +/- 172 msec in the longitudinal direction and 452 +/- 265 msec in the transverse direction. Repolarization intervals, defined as the interval between the activation time and the repolarization time in the unipolar electrograms, correlated highly with refractory period determinations in the absence and presence of amiodarone at each cycle length tested. The increase in repolarization intervals and refractory periods resulting from amiodarone treatment did not vary with cycle length. Amiodarone treatment also resulted in a significant rate-related reduction in systolic blood pressure. The systolic blood pressure in the group that received amiodarone decreased by a mean of 50 +/- 23% between steady-state pacing cycle lengths of 1,000 and 200 msec, whereas the corresponding decrease in the control group was 21 +/- 32% (p less than 0.05). Plasma and myocardial amiodarone and desethylamiodarone levels were comparable to those observed clinically. We conclude that long-term amiodarone administration causes rate-dependent reductions in conduction velocity and blood pressure and causes rate-independent increases in repolarization intervals.

In vivo evaluation of the effects of altered cyclosporine metabolism on its immunosuppressive potency

The dose-response and plasma concentration-response relationships of cyclosporine after both inducing and inhibiting its metabolism were studied in a mouse heart transplant model. The metabolism of cyclosporine was altered by coadministering phenobarbital and cimetidine as metabolism inducing and inhibiting agents, respectively. We found that phenobarbital depressed the immunosuppressive potency of cyclosporine by enhancing its metabolism resulting in lower cyclosporine blood levels. On the other hand, when cimetidine was administered concurrently with cyclosporine, the immunosuppressive effect was enhanced due to inhibition of the metabolism of cyclosporine which produced higher cyclosporine blood levels. When graft survival was evaluated relative to blood cyclosporine concentrations, however, it appeared that cimetidine had a direct negative effect on the survival of transplanted organs independent and contrary to its effect on the accumulation of cyclosporine. The immunosuppression produced by cyclosporine at these elevated blood levels was less than expected. The accrued data support the conclusion that cyclosporine and not its metabolites are primarily responsible for its immunosuppressive activity in the mouse.

Effect of albumin on the electrophysiologic stability of isolated perfused rabbit hearts

The electrophysiologic stability of isolated perfused rabbit hearts was evaluated over a period of 5 h. Hearts perfused with protein-free buffer deteriorated over time, with significant shortening of the ventricular effective refractory period (ERP) and development of ventricular fibrillation. When serum albumin (6.01 x 10(-4) mM) was added to the perfusate, hearts were more stable, the ventricular ERPs remained relatively constant throughout the 5-h perfusion period, and hearts did not fibrillate. When a fatty acid-free protein source was used, the hearts demonstrated similar stability to those perfused with protein containing fatty acids. Despite marked changes in the refractory periods of hearts perfused with protein-free buffer, the QRS interval did not change over time, indicating that this is a very insensitive parameter for monitoring the electrophysiologic stability of isolated perfused hearts. In studies utilizing isolated hearts to evaluate antiarrhythmic drug effects, it is preferable to use a protein-containing buffer.

Pharmacodynamics of procainamide in patients with ventricular tachyarrhythmias

The onset and offset of the electropharmacologic effect of procainamide was studied in nine patients with ventricular arrhythmias. Procainamide was given at a constant infusion rate of 0.27 +/- 0.05 mg/kg/min for 50 to 60 minutes to an average total dose of 15.5 +/- 4.4 mg/kg. The QRS interval (used as an index of electropharmacologic effect) at a paced cycle length of 500 ms, and the plasma procainamide concentration were measured simultaneously every 5 minutes during infusion and at frequent intervals for up to 4 hours during a washout period. The average peak plasma concentration was 15.8 +/- 9.6 micrograms/ml and the average maximum QRS interval prolongation was 23.9 +/- 6.8% from baseline. The temporal and static plasma concentration-effect relationships were evaluated by pharmacodynamic modeling and linear regression. For six patients, there was a minimal (less than 2 minutes) delay in the plasma concentration-effect relationship, and the data fit a linear relationship with an average slope of 3.2 +/- 1.1 msec/microgram/ml. For the other three patients, there was a significant delay (3, 10, and 18 minutes respectively) in the plasma concentration-effect relationship. In most patients, the electropharmacologic effect of procainamide is rapid and proportional to plasma concentration; but in a minority of patients, significant delay occurs and could influence the results and interpretation of electropharmacologic studies.

Effect of pH on the myocardial uptake and pharmacodynamics of propafenone in the isolated rabbit heart

The influence of pH on the myocardial disposition of propafenone was studied in isolated perfused rabbit hearts. Five pH groups were evaluated: pH 7.0, 7.2, 7.4, 7.6, and 7.8. Hearts were perfused with a modified Krebs-Henseleit buffer containing approximately 100 ng/ml propafenone. Myocardial propafenone accumulation was determined from differences in the aortic perfusate and coronary sinus effluent propafenone concentrations. The myocardial accumulation of propafenone was significantly pH dependent. The steady-state propafenone concentration increased from 5.9 +/- 1.3 micrograms/g at pH 7.0 to 13.2 +/- 3.8 micrograms/g at pH 7.4 and 24.2 +/- 3.5 micrograms/g at pH 7.6. The time to reach steady-state myocardial propafenone levels increased proportionately with the increased propafenone levels. Steady-state was not reached by 150 min at pH of 7.6 or 7.8. Percent change in QRS duration was measured to monitor the electrophysiologic effect of propafenone. The relationship between the myocardial drug concentration and the measured changes in QRS also was evaluated. The myocardial concentration-effect relationships were linear over the observed myocardial concentration range. The slopes of these concentration-effect relationships were similar for three groups (pH 7.0, 7.2, and 7.4). Over the pH range 7.0-7.4, the steady-state effect increased as a function of pH and correlated with the differences in propafenone steady-state myocardial concentrations. However, at alkalotic pH, the concentration-effect relationship was shifted to the right; less effect was observed than might be predicted for the myocardial propafenone concentration. Thus, small changes in pH may significantly alter the myocardial distribution and pharmacodynamics of propafenone.

Evaluation of the in vivo dose-response relationship of immunosuppressive drugs using a mouse heart transplant model: application to cyclosporine

We have developed a 2-week bioassay that quantitates the effect of immunosuppressive drugs on organ allograft rejection. This assay is not only rapid and reliable, but also simple and relatively inexpensive and sparing of test substances. We refined the method by which neonatal mouse hearts are transplanted into pouches in the pinnae of ears of adult recipient mice and used cyclosporine treatment as an example of how this method might be generally applied to study the dose-response relationship of immunosuppressive drugs. Five- to 10-week-old C3H/km mice were used as cardiac recipients, and unsexed newborn BALB/c (allograft) or C3H/km (isograft) mice (24-48 hr old) were used as cardiac graft donors. The heart grafts were examined by two independent observers every other day at 10- to 20-fold magnification for up to 14 days. The immunosuppressive effect of cyclosporine was studied at doses of 3, 7.5, 15, 22.5 and 30 mg/kg per day administered i.p. The accrued data were analyzed by two methods: dose-response curves at days 12 and 14 and mean survival scores from day 8 to day 14. The dose-response curves on days 12 and 14 were similar, and the calculated ED50 values were 9.83 and 15 mg/kg/day, respectively. The results of this study demonstrate the potential usefulness and the sensitivity of the ear-heart transplantation bioassay for relative potency evaluations of immunosuppressive drugs.

Pharmacodynamics and pharmacokinetics of oral pirmenol

The efficacy, pharmacokinetics, and pharmacodynamics of pirmenol, a class Ia antiarrhythmic agent, were studied in patients with frequent symptomatic premature ventricular complexes (PVCs). Pirmenol was given every 12 hours to eight patients in a dose-ranging protocol, and median PVC suppression of 94% (range 72% to 100%) was achieved. The median effective pirmenol dose was 300 mg/day (range 200 to 500 mg/day), and mean (+/- SD) trough plasma pirmenol concentration at the effective dose was 0.98 +/- 0.29 micrograms/ml. The mean half-life of elimination was 10.5 +/- 2 hours. There was considerable overlap among patients with respect to plasma pirmenol concentration and times at which PVC frequency returned to 25%, 50%, and 75% of baseline during drug washout trials. Altering pirmenol's dose interval (while maintaining a constant daily dose) from 12 to 6 hours did not improve drug efficacy. Pirmenol was given to seven patients for long-term therapy (24 to 44 months). Median PVC suppression at 24 months was 70%. Pirmenol is safe and well tolerated, and it can be administered twice daily for PVC suppression.

Interaction between warfarin and propafenone in healthy volunteer subjects

The effect of propafenone on the pharmacokinetics and pharmacologic effects of warfarin was studied in healthy normal male volunteer subjects. Each drug was administered alone for 1 week followed by a combined administration for 1 additional week. Blood samples were analyzed for propafenone and warfarin concentrations and the effect of each treatment on the prothrombin time was assessed. The concurrent administration of warfarin did not produce any changes in the absorption or disposition kinetics of propafenone. Concurrent propafenone administration did lead to a reduction in the clearance of warfarin, resulting in an average increase of 38% in the mean steady-state plasma warfarin concentration. During the combined therapy phase, the prothrombin time increased significantly (P less than 0.01) from the "warfarin alone" phase. We conclude from this study that the concomitant administration of propafenone and warfarin may lead to an enhanced anticoagulant effect that may require a reduction in the warfarin dose.

Influence of hepatic dysfunction on the pharmacokinetics of propafenone

Hepatic metabolism is the primary process of elimination of propafenone. It therefore is important to understand the effect of altered liver function on the disposition and elimination kinetics of this drug. Patients with abnormal liver function probably will require treatment with propafenone for cardiac arrhythmias; an understanding of the relationship between liver function and the pharmacokinetics of propafenone will provide a rational basis for optimal dosage adjustments in these individuals. Our results demonstrate that both systemic clearance and bioavailability of propafenone are sensitive to variability in liver function. The bioavailability of propafenone is inversely related to the clearance of indocyanine green (ICG), whereas a direct relationship exists between systemic clearance of propafenone and ICG clearance. Comparisons of clinical parameters with the propafenone data yielded interesting results. An overall clinical grading of severity of liver disease based on the presence or absence of portal hypertension (i.e., varices and/or splenomegaly), prior encephalopathy, and ascites did not correlate well with propafenone results. However, albumin, total bilirubin, serum glutamic oxaloacetic transaminase (SGOT) concentrations and prothrombin time values correlated strongly with the overall results. No definite relationships with subjects' age; weight; and hemoglobin, alkaline phosphatase, lactic acid dehydrogenose, cholesterol, blood urea nitrogen, or creatinine levels were detected. These results demonstrate that moderate to severe liver disease significantly affects the absorption and disposition of propafenone. In patients with cirrhosis, and presumably other forms of hepatic dysfunction, careful adjustments of propafenone doses are needed to optimize therapy.

Pharmacodynamic modeling of antiarrhythmic drug effects--application to 3-methoxy-O-demethyl encainide

The pharmacodynamic characteristics of 3-methoxy-O-demethyl encainide (MODE) were studied in instrumented, chloralose-anesthetized dogs. The HIS Purkinje conduction times (HV) were utilized to assess drug effect. Two protocols were conducted; the first protocol involved multiple pairs of loading and maintenance infusions to achieve several steady-state plasma drug concentrations. The second protocol involved a single short infusion. The data from both protocols supported a linear concentration-effect relationship for the concentration range studied. The slopes and intercepts were similar for both data sets. The data from the second protocol were also analyzed to assess the temporal aspects of the pharmacodynamics of MODE. Data analysis indicated that there is significant hysteresis in the plasma concentration-effect relationship that is characterized by a first-order rate constant corresponding to a half-life (t1/2) of approximately 10 min. These study results also demonstrated the advantages of single-infusion protocols over multiple-infusion studies for evaluating the concentration-effect relationship of antiarrhythmic drugs.

Cyclosporine serum concentrations soon after heart or heart-lung transplantation

The accumulation of cyclosporine was evaluated in 11 patients following either heart or heart-lung transplantation. Blood samples were drawn frequently for the first few postoperative days, and the plasma was analyzed by both radioimmunoassay (RIA) and high-performance liquid chromatography (HPLC). The levels determined by RIA were higher than those assessed by HPLC, and it is hypothesized that the difference is due to antibody reactive metabolites of cyclosporine, which are measured by the RIA procedure. We did not find any consistent trends in the accumulation of these metabolites. Analysis of early clinical data did not suggest any relationship between levels of cyclosporine or its metabolites and clinical outcome.

Influence of protein binding on the myocardial uptake and pharmacodynamics of propafenone

The relationship between plasma protein binding, myocardial uptake, and cardiac pharmacodynamics of propafenone was studied in an isolated perfused rabbit heart model. Hearts were perfused with buffer which contained varied concentrations of alpha 1-acid glycoprotein (alpha 1AGP). The rate and extent of myocardial uptake was assessed, as was the time course and magnitude of the change in QRS duration. The addition of alpha 1AGP has significant influence on both the rate and extent of myocardial accumulation as well as the resultant changes in QRS duration. The steady-state changes in QRS duration were linearly related to the free perfusate propafenone concentration. At times of disequilibrium, the relationship between free concentration and effect was not evident. However, at all times, a linear relationship between the myocardial concentration of propafenone and the resultant effect was observed. This relationship was not influenced by addition of alpha 1AGP protein to the perfusate; however, the amount of drug which was able to partition into the myocardium was greatly affected. These studies demonstrate that protein binding does influence the partitioning of drug into the myocardium and subsequently modulates the cardiac effect.

Relation of blood level and metabolites to the antiarrhythmic effectiveness of encainide

Encainide is a potent new antiarrhythmic agent with 2 major active metabolites and 2 distinct phenotypes for metabolism, extensive (approximately 92%) and nonextensive (8%). Encainide is an active compound with close correlation of plasma levels with antiarrhythmic effectiveness and electrocardiographic changes in nonextensive metabolizers. Its metabolites, O-demethyl-encainide and 3-methoxy-O-demethyl-encainide, are active against experimental and clinical arrhythmias. They have longer half-lives than and equal or greater potency than the parent compound. All 3 compounds contribute to the antiarrhythmic profile in extensive metabolizers. There is no readily apparent relation between encainide and its metabolites, blood levels and efficacy because of the complexity of the 3 active compounds and individual variation in pharmacokinetic and arrhythmia responsiveness. Encainide has been given for up to 2 years in 140 patients with sustained ventricular tachycardia or ventricular fibrillation. The survival curves are similar to historical control data from patients reported by Graboys and Swerdlow. The survival curves for long-term administration in patients with frequent ventricular premature complexes (greater than 30/min) are comparable to data from Califf. While these data must be viewed cautiously, it seems fair to conclude that encainide is as effective as any combination of drugs for preventing sudden death in patients with life-threatening ventricular arrhythmias.

The importance of pharmacokinetics and pharmacodynamics in the clinical evaluation of antiarrhythmic drugs

Many pharmacokinetic and pharmacodynamic factors affect clinical evaluation of antiarrhythmic agents. The route of administration and the duration of treatment are variables that influence the study outcome. Due to metabolic differences and pharmacodynamic factors, different results may be obtained depending on whether drugs are administered orally or intravenously. Moreover the achievement of steady state blood level does not insure the achievement of adequate myocardial drug level nor does it indicate that a steady state effect has been achieved. There may be significant delay in the accumulation of antiarrhythmic drugs in the myocardium. Only when the metabolic profile and pharmacodynamic characteristics of a drug are understood can the most efficient testing protocol be designed for a particular antiarrhythmic agent.

Qualitative and quantitative comparison of the cardiac effects of encainide and its three major metabolites in the dog

We have evaluated the electrophysiologic effect of encainide and its three major metabolites, O-demethyl encainide, 3-methoxy-O-demethyl encainide and N-demethyl encainide in an anesthetized dog model. Our results support previous reports that O-demethyl encainide and 3-methoxy-O-demethyl encainide are both more potent than encainide in the depression of conduction. We also have shown that N-demethyl encainide is of about equal potency to encainide. Whereas the major differences between these compounds is primarily one of potency, there are some qualitative differences. Although O-demethyl encainide did not change the ventricular or atrial effective refractory periods significantly, 3-methoxy-O-demethyl encainide and N-demethyl encainide prolonged both. Encainide increased the atrial effective refractory period but did not produce significant changes in the ventricular refractory period. These data support previous suggestions of an important role for these metabolites as modulators of the clinical efficacy of encainide.

Myocardial uptake kinetics and pharmacodynamics of propafenone in the isolated perfused rabbit heart

The myocardial disposition of propafenone was studied in an isolated perfused rabbit heart. Six hearts were perfused with a modified Krebs-Henseleit buffer containing propafenone 110 +/- 5 ng/ml. Pharmacokinetic parameters were determined by fitting the coronary sinus effluent propafenone concentration-time data to a one-compartment pharmacokinetic model. The mean half-life of myocardial uptake (T1/2d) was 22.3 +/- 5.9 min and the average time to approach steady-state tissue levels was 112 +/- 29 min. Propafenone accumulated extensively in myocardium and at equilibrium the average (+/- S.D.) myocardial concentration was 114 +/- 21 times that of the perfusate. The electrophysiological effect was measured as percentage of change of the baseline QRS duration. The half-life of onset of effect (T1/2e) and the effect at steady state were determined by fitting the effect-time data to a monoexponential function. The T 1/2e averaged 26.0 +/- 9.4 min and did not differ significantly from T 1/2d. The relationship between myocardial propafenone concentration and effect was linear but there was interexperimental variability in the slopes of the lines of this concentration-effect relationship. The interexperimental differences in steady-state effect could not be accounted for by differences in myocardial concentration.

Effects of encainide and its metabolites on energy requirements for defibrillation

Encainide, a class IC antiarrhythmic agent, has been associated with proarrhythmic responses of ventricular tachycardia and fibrillation requiring defibrillation in patients. We examined the short-term effects of intravenous encainide and its two major metabolites, O-demethyl-encainide (ODE) and 3-methoxy-ODE (MODE), on the energy requirements for successful defibrillation in 25 pentobarbital-anesthetized, open-chest dogs. Truncated exponential (60% tilt) defibrillation shocks were administered through right atrial spring and left ventricular epicardial patch electrodes identical to those used in man with the automatic implantable defibrillator. At baseline multiple shocks of varying energy were applied to construct curves of percent successful defibrillation as a function of energy (DF curves) for each animal. Encainide, ODE, or MODE was then infused in loading and maintenance doses to achieve QRS widening of 20% to 50%. Saline was administered to animals serving as controls. Determination of the DF curve was repeated, after which the infusion was discontinued. After 1 hr washout period, an additional DF curve was constructed. The data were analyzed by logistic regression, and the energies required for 50% successful defibrillation (E50) were compared. No significant differences existed between the four groups in body or heart weight, extent of QRS widening, or baseline E50 values. After administration of encainide and ODE, the E50 increased by 129 +/- 43% (p less than .001) and 76 +/- 34% (p less than .005), respectively. Return of E50 toward baseline was observed after the washout periods in both groups (p less than .025), demonstrating the reversibility of the drugs' effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of antiarrhythmic drugs to purified human alpha 1-acid glycoprotein

The binding of lidocaine, verapamil, propafenone and propranolol to isolated, purified human alpha 1-acid glycoprotein was studied using equilibrium dialysis. Lidocaine and verapamil bound to a single class of binding sites which was characterized by high affinity (kd1 for lidocaine was 5.79 x 10(-6)M-1 and for verapamil 3.43 X 10(-6)M-1) and low capacity (n = 0.40 for lidocaine and 0.62 for verapamil). The binding of propafenone revealed two classes of binding sites, both with high affinity (kd1 was 7.62 X 10(-6)M-1 and kd2 was 6.00 X 10(-8)M-1) and low capacity (n1 = 0.79 and n2 = 0.20). Propranolol bound to at least two classes of binding sites (kd1 was 2.56 X 10(-6)M-1; n1 = 0.58). Complete characterization of the binding parameters of the second site was not possible due to failure to achieve saturation.

Metabolite cumulation during chronic propafenone dosing in arrhythmia

The cumulation of propafenone and two of its metabolites, 5-hydroxypropafenone (5-OHP) and N-depropylpropafenone (NDPP), was examined in patients with frequent ventricular ectopy. After 2 weeks of propafenone therapy (300 mg twice a day), propafenone was discontinued and blood samples were drawn for 24 hours. The mean (+/- SD) steady-state concentrations of propafenone, 5-OHP, and NDPP were 1010 +/- 411, 174 +/- 113, and 179 +/- 93 ng/ml. The concentration ratios of 5-OHP/propafenone and NDPP/propafenone were 0.177 +/- 0.049 and 0.227 +/- 0.203. Plasma concentrations of 5-OHP and NDPP did not decay in a log-linear manner during the sampling period and thus estimates of their disappearance t1/2s were not possible. At 24 hours after propafenone dosing, concentrations of 5-OHP and NDPP were 63% +/- 37% and 50% +/- 21% of their mean steady-state levels. Our data indicate that these propafenone metabolites cumulate in the plasma during chronic oral propafenone therapy, and that their clinical role needs to be elucidated.

Chronic lorcainide therapy for symptomatic premature ventricular complexes: efficacy, pharmacokinetics and evidence for norlorcainide antiarrhythmic effect

Chronic premature ventricular complexes (PVCs) have been effectively suppressed by oral lorcainide as reported in previous short-term studies. The plasma level-effect relation of lorcainide may be affected by the possible cardioactivity of norlorcainide, a metabolite that accumulates after repeated oral doses. This study evaluated the long-term efficacy of lorcainide in suppressing chronic symptomatic PVCs, and examined the relation of arrhythmia suppression to plasma concentrations of lorcainide and norlorcainide. Fourteen patients were treated with lorcainide, 200 to 400 mg/day, 12 of whom achieved nearly complete suppression of arrhythmias after treatment for 1 year. Chronic lorcainide treatment was well tolerated; no patient discontinued treatment because of adverse effects. Lorcainide and norlorcainide plasma concentrations remained stable after the first week of therapy. Antiarrhythmic activity persisted throughout the year. Upon drug withdrawal, the mean lorcainide washout half-life was 14.3 +/- 3.7 hours and the mean norlorcainide washout half-life was 31.9 +/- 8.9 hours. The return of arrhythmias occurred well after the lorcainide plasma concentration had decreased to subtherapeutic levels, suggesting an antiarrhythmic effect of norlorcainide. Thus, long-term lorcainide therapy is effective in treating chronic symptomatic PVCs and is well tolerated by most patients. The metabolite norlorcainide appears to have antiarrhythmic activity independent of lorcainide.

Metabolites of cardiac antiarrhythmic drugs: their clinical role

Most antiarrhythmic drugs are extensively metabolized, and the accumulation of the metabolites of several of these drugs has been documented. In some cases, the steady-state plasma concentrations of metabolites are considerably greater than is the concentration of the parent drug. Several of these metabolites have been evaluated in animal models for antiarrhythmic activity and their potencies have been defined relative to the activity of their parent compound. Evaluations of activity are generally conducted in animal arrhythmia models, and very few metabolites of antiarrhythmic drugs have been evaluated directly in patients. However, from knowledge of antiarrhythmic activity in animals and the degree to which a metabolite accumulates in the plasma of patients, one can make qualitative judgments about its therapeutic role. Such judgments, however, need to be recognized as tenuous. Quantitative judgments require further information regarding the relationship between the parent drug and metabolite when present simultaneously in the myocardium. One must consider whether the effects of the parent drug and metabolite are additive, synergistic, or even antagonistic. The latter case is most possible with drug-metabolite pairs where the metabolite accumulates substantially, but does not have significant antiarrhythmic potency. Other considerations include noncardiac effects of the metabolites. As in the case of the mono-desethyl metabolite of lidocaine, the significance of its accumulation relates more to central nervous system side effects than to direct cardiac actions. The role of active metabolites also much be considered in regard to differences in the disposition kinetics between the parent drug and metabolite. The most obvious situation where this is important is in designing clinical drug evaluation protocols. As illustrated by the metabolites of encainide and lorcainide, the time course of accumulation and disappearance of the metabolites may be much longer than that of the parent drug. Clinical evaluations at steady state must take into account the time required to achieve steady-state concentrations of the metabolites as well. Similarly, after discontinuation of drug administration, the time required before washout is complete may be totally dependent on the kinetics of the metabolite, and not the parent drug. Variability in metabolic activity also needs to be considered. It has been shown with procainamide and encainide that genetic factors can influence the rate of production of active metabolites and consequently influence the clinical efficacy of these drugs. Another consideration that deserves attention is the question of drug interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

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

Amiodarone is an antiarrhythmic drug which has received considerable attention in recent years. It has been suggested that the unusual pharmacodynamic characteristics of this drug may be due in part to the influence of active metabolites. Using fast atom bombardment (FAB) mass spectrometry we have identified a new metabolite of amiodarone, the di-N-desethyl analog (DDEA). This metabolite was present in the blood of dogs treated with the parent drug, and showed a greater affinity for myocardium than did the parent drug. The unique features of FAB mass spectrometry over electron impact mass spectrometry was an essential element in facilitating the identification of this new metabolite. Whether or not this metabolite has pharmacologic activity or is responsible for some of the side effects occurring during amiodarone administration is not known.