Clinical pharmacokinetics of propafenone

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

The Effect of CYP2D6 Phenotypes on the Pharmacokinetics of Propafenone: A Systematic Review and Meta-Analysis

Propafenone (PPF) is a class 1C antiarrhythmic agent mainly metabolized by cytochrome (CYP) 2D6, CYP1A2, and CYP3A4. Previous studies have shown that CYP2D6 polymorphism influences the pharmacokinetics (PK) of PPF. However, the small sample sizes of PK studies can lead to less precise estimates of the PK parameters. Thus, this meta-analysis was performed to merge all current PK studies of PPF to determine the effects of the CYP2D6 phenotype more accurately on the PPF PK profile. We searched electronic databases for published studies to investigate the association between the PPF PK and CYP2D6 phenotype. Four PK-related outcomes were included: area under the time–concentration curve (AUC), maximum concentration (Cmax), apparent clearance (CL/F), and half-life (t1/2). A total of five studies were included in this meta-analysis (n = 56). Analyses were performed to compare PK parameters between poor metabolizers (PMs) versus extensive metabolizers (EMs). PPF has a non-linear pharmacokinetics; therefore, analyses were performed according to dose (300 mg and 400 mg). At 300 mg, the AUC mean (95% CI), Cmax, and t1/2 of PPF in PMs were 15.9 (12.5–19.2) µg·h/mL, 1.10 (0.796–1.40) µg/mL, and 12.8 (11.3–14.3) h, respectively; these values were 2.4-, 11.2-, and 4.7-fold higher than those in the EM group, respectively. At 400 mg, a comparison was performed between S- and R-enantiomers. The CL/F was approximately 1.4-fold higher for the R-form compared with the S-form, which was a significant difference. This study demonstrated that CYP2D6 metabolizer status could significantly affect the PPF PK profile. Adjusting the dose of PPF according to CYP2D6 phenotype would help to avoid adverse effects and ensure treatment efficacy.

Propafenone Poisoning of a Female Adolescent After a Suicide Attempt

Propafenone is an antiarrhythmic agent for the management of ventricular and supraventricular tachycardia and atrial fibrillation. Propafenone poisoning is rare but may be life-threatening due to drug-induced arrhythmias. Electrocardiographic changes in PR, QRS, and QT intervals have been recorded. We present a case of a 15-year-old female adolescent who developed arrhythmias and convulsions due to propafenone intoxication, in an attempt to commit suicide. The outcome of the case was a full recovery from the arrhythmias and the seizures. The aim of this article is to highlight the possibility of a lethal intoxication by a common antiarrhythmic drug. Our case aims to present our therapeutic strategy that relies mainly on close monitoring of patients and cardiac output support.

Chiral nanomaterials for tumor therapy: autophagy, apoptosis, and photothermal ablation

Chirality is a fundamental characteristic of natural molecules and a crucial factor in the biochemical reactions of living cells and organisms. Recently, researchers have successfully introduced chiral molecules to the surfaces of nanomaterials, creating chiral nanomaterials that exhibit an upscaling of chiral behavior from the molecular scale to the nanoscale. These chiral nanomaterials can selectively induce autophagy, apoptosis, and photothermal ablation in tumor cells based on their chirality, making them promising for application in anti-tumor therapy. However, these interesting and important phenomena have hitherto received little attention. Accordingly, we herein present a review of recent research progress in the field of chiral nanomaterials for tumor therapy along with brief looks at the mechanistic details of their actions. Finally, the current challenges and future perspectives of chiral nanomaterials in terms of maximizing their potential in tumor therapy are discussed. Thus, this review provides a helpful introduction to the design of chiral nanomaterials and will hopefully highlight the importance of chirality in tumor therapy.

Effect of Environmental Exposure and Pharmacogenomics on Drug Metabolism

BACKGROUND

Pesticides are major xenobiotic compounds and environmental pollutants, which are able to alter drug-metabolizing enzyme as well as pharmacokinetics of drugs. Subsequent to the release of the human genome project, genetic variations (polymorphism) become an integral part of drug development due to their influence on disease susceptibility/ progression of the disease and their impact on drug absorption, distribution, metabolism of active metabolites and finally excretion of the drug. Genetic polymorphisms crucially regulate pharmacokinetics and pharmacodynamics of drugs under the influence of physiological condition, lifestyle, as well as pathological conditions collectively.

OBJECTIVE

To review all the evidence concerning the effect of environmental exposure on drug metabolism with reference to pharmacogenomics.

METHODS

Scientific data search and review of basic, epidemiological, pharmacogenomics and pharmacokinetics studies were undertaken to evaluate the influence of environmental contaminants on drug metabolism.

RESULTS

Various environmental contaminants like pesticides effectively alter drug metabolism at various levels under the influence of pharmacogenomics, which interferes with pharmacokinetics of drug metabolism. Genetic polymorphism of phase I and phase II xenobiotic-metabolizing enzymes remarkably alters disease susceptibility as well as the progression of disease under the influence of various environmental contaminants at various levels.

CONCLUSION

Individual specific drug response may be attributed to a large variety of factors alone or in combination ranging from genetic variations (SNP, insertion, deletion, duplication etc.) to physiological setting (gender, age, body size, and ethnicity), environmental or lifestyle factors (radiation exposure, smoking, alcohol, nutrition, exposure to toxins, etc.); and pathological conditions (obesity, diabetes, liver and renal function).

Effect of CYP2D6 polymorphisms on the pharmacokinetics of propafenone and its two main metabolites

AIM OF THE STUDY

Propafenone (PPF) is an antiarrhythmic drug, metabolized mainly by CYP2D6 to 5-hydroxypropafenone (5OH-PPF) and by CYP3A4 to norpropafenone (NOR-PPF). CYP2D6 shows a high degree of genetic polymorphism which is associated with diminished antiarrhythmic efficacy or cardiac seizures/cardiotoxicity. This study aimed to investigate the effect of the CYP2D6 polymorphism on the pharmacokinetics of PPF and its two main metabolites. The usefulness of PPF/5OH-PPF ratio for CYP2D6 phenotyping in healthy adults was also evaluated.

METHODS

Twelve healthy volunteers, 3 poor metabolizers (PM), 2 intermediate metabolizers (IM) and seven extensive metabolizers (EM) received an oral dose of PPF. Concentrations of PPF and its metabolites were analyzed in serum samples over 27h.

RESULTS

The PPF/5OH-PPF ratio distinguished EMs from PMs, but not from IMs. In PMs, the mean transit time (MTT) values were almost the same for PPF and NOR-PPF and much higher than those of EMs and IMs. 5OH-PPF was not detected in EMs. Mean MTT values of 5OH-PPF and NOR-PPF in IMs were 5.27- and 1.52-fold higher than those of EMs.

CONCLUSION

A single time point serum PPF-MR approach is a useful tool to identify PMs. CYP2D6 polymorphism significantly affects the pharmacokinetics of PPF and its two metabolites.

Pharmacokinetics of propafenone hydrochloride sustained-release capsules in male beagle dogs

This paper describes the development and validation of a liquid chromatography-mass spectrometric assay for propafenone and its application to a pharmacokinetic study of propafenone administered as a new propafenone hydrochloride sustained-release capsule (SR-test), as an instant-release tablet (IR-reference) and as the market leader sustained-release capsule (Rythmol, SR-reference) in male beagle dogs (n=8). In Study A comparing SR-test with IR-reference in a crossover design T max and t 1/2 of propafenone for SR-test were significantly higher than those for IR-reference while C max and AUC were lower demonstrating the sustained release properties of the new formulation. In Study B comparing SR-test with SR-reference the observed C max and AUC of propafenone for SR-test (124.5±140.0 ng/mL and 612.0±699.2 ng·h/mL, respectively) were higher than for SR-reference (78.52±72.92 ng/mL and 423.6±431.6 ng·h/mL, respectively) although the differences were not significant. Overall, the new formulation has as good if not better sustained release characteristics to the market leader formulation.

Evaluating the safety and efficacy of dextromethorphan/quinidine in the treatment of pseudobulbar affect

Pseudobulbar affect (PBA) is a common manifestation of brain pathology associated with many neurological diseases, including amyotrophic lateral sclerosis, Alzheimer's disease, stroke, multiple sclerosis, Parkinson's disease, and traumatic brain injury. PBA is defined by involuntary and uncontrollable expressed emotion that is exaggerated and inappropriate, and also incongruent with the underlying emotional state. Dextromethorphan/quinidine (DM/Q) is a combination product indicated for the treatment of PBA. The quinidine component of DM/Q inhibits the cytochrome P450 2D6-mediated metabolic conversion of dextromethorphan to its active metabolite dextrorphan, thereby increasing dextromethorphan systemic bioavailability and driving the pharmacology toward that of the parent drug and away from adverse effects of the dextrorphan metabolite. Three published efficacy and safety studies support the use of DM/Q in the treatment of PBA; significant effects were seen on the primary end point, the Center for Neurologic Study-Lability Scale, as well as secondary efficacy end points and quality of life. While concentration-effect relationships appear relatively weak for efficacy parameters, concentrations of DM/Q may have an impact on safety. Some special safety concerns exist with DM/Q, primarily because of the drug interaction and QT prolongation potential of the quinidine component. However, because concentrations of dextrorphan (which is responsible for many of the parent drug's side effects) and quinidine are lower than those observed in clinical practice with these drugs administered alone, some of the perceived safety issues may not be as relevant with this low dose combination product. However, since patients with PBA have a variety of other medical problems and are on numerous other medications, they may not tolerate DM/Q adverse effects, or may be at risk for drug interactions. Some caution is warranted when initiating DM/Q treatment, particularly in patients with underlying risk factors for torsade de pointes and in those receiving medications that may interact with DM/Q.

Anti-Arrhythmic Agents in the Treatment of Atrial Fibrillation

Although atrial fibrillation (AF) is the most common sustained arrhythmia seen during daily cardiovascular physician practice, its management remained a challenge for cardiology physician as there was no single anti-arrhythmic agents proved to be effective in converting atrial fibrillation and kept its effectiveness in maintaining sinus rhythm over long term. Moreover all the anti-arrhythmic agents that are used in treatment of AF were potentially pro-arrhythmic especially in patients with coronary artery disease and structurally abnormal heart. Some of these drugs also have serious non cardiac side effects that limit its long term use in the management of atrial fibrillation. Several new and investigational anti-arrhythmic agents are emerging but data supporting their effectiveness and safety are still limited. In this systematic review we examine the efficacy and safety of these medications supported by the major published randomized trials, meta-analyses and review articles and conclude with a summary of guidelines recommendations.

Capillary electrophoretic investigation of the enantioselective metabolism of propafenone by human cytochrome P-450 SUPERSOMES: Evidence for atypical kinetics by CYP2D6 and CYP3A4

An enantioselective CE method was used to identify the ability of CYP450 enzymes and their stereoselectivity in catalyzing the transformation of propafenone (PPF) to 5-hydroxy-propafenone (5OH-PPF) and N-despropyl-propafenone (NOR-PPF). Using in vitro incubations with single CYP450 enzymes (SUPERSOMES), 5OH-PPF is shown to be selectively produced by CYP2D6 and N-dealkylation is demonstrated to be mediated by CYP2D6, CYP3A4, CYP1A2, and CYP1A1. For the elucidation of kinetic aspects of the metabolism with CYP2D6 and CYP3A4, incubations with individual PPF enantiomers and racemic PPF were investigated. With the exception of the dealkylation in presence of R-PPF only, which can be described by the Michaelis-Menten model, all CYP2D6-induced reactions were found to follow autoactivation kinetics. For CYP3A4, all NOR-PPF enantiomer formation rates as function of PPF enantiomer concentration were determined to follow substrate inhibition kinetics. The formation of NOR-PPF by the different enzymes is stereoselective and is reduced significantly when racemic PPF is incubated. Clearance values obtained for CYP3A4 dealkylation are stereoselective whereas those of CYP2D6 hydroxylation are not. This paper reports the first investigation of the PPF hydroxylation and dealkylation kinetics by the CYP2D6 enzyme and represents the first report in which enantioselective CE data provide the complete in vitro kinetics of metabolic steps of a drug.

Validated capillary electrophoresis assay for the simultaneous enantioselective determination of propafenone and its major metabolites in biological samples

A robust, inexpensive, and fully validated CE method for the simultaneous determination of the enantiomers of propafenone (PPF), 5-hydroxy-propafenone (5OH-PPF) and N-despropyl-propafenone (NOR-PPF) in serum and in in vitro media is described. It is based upon liquid-liquid extraction at alkaline pH followed by analysis of the reconstituted extract by CE in presence of a pH 2.0 running buffer composed of 100 mM sodium phosphate, 19% methanol, and 0.6% highly sulfated beta-CD. For each compound, the S-enantiomers are shown to migrate ahead of their antipodes, and the overall run time is about 30 min. Enantiomer levels between 25 and 1000 ng/mL provide linear calibration graphs, and the LOD for all enantiomers is between 10 and 12 ng/mL. The assay is shown to be suitable for the determination of the enantiomers of PPF and its metabolites in in vitro incubations comprising human liver microsomes or single CYP450 enzymes (SUPERSOMES). Incubations with CYP2D6 SUPERSOMES revealed, for the first time, the simultaneous formation of the enantiomers of 5OH-PPF and NOR-PPF with that enzyme. CE data can be used for the evaluation of the enzymatic N-dealkylation and hydroxylation rates.

A Rapid HPLC Assay for the Simultaneous Determination of Propafenone and Its Major Metabolites in Human Serum

A rapid and specific HPLC method has been developed and validated for the simultaneous determination of propafenone, an antiarrhythmic agent, and its major metabolites in human serum. The sample preparation was a simple deproteinization with a mixture of ZnSO4 and methanol, yielding almost 100% recoveries of three compounds. Separation was developed on a reverse-phase tracer excel C18 column (25 x 0.46 cm i.d., 5 microm), using an acetonitrile-phosphate buffer gradient at a flow rate of 1.7 ml min(-1), and UV detection of 210 nm. The calibration curves were linear (r2 > 0.999) in the concentration range of 10 - 750 ng ml(-1). The lower limit of quantification was 10 ng ml(-1) for all of the compounds studied. The within and between day precisions in the measurement of QC samples at four tested concentrations were in the range of 1.4 - 8.1% and 4.2 - 11.5% RSD, respectively. The developed procedure was applied to assess the pharmacokinetics of propafenone and its major metabolites following administration of a single 300 mg oral dose of propafenone hydrochloride to three healthy male volunteers.

An organic psychosis due to a venlafaxine-propafenone interaction

OBJECTIVE

An extraordinary case of an organic psychosis during the treatment of a patient with a bipolar affective disorder.

METHODS

Clinical observation and further investigations including monitoring of serum levels (case report).

RESULTS

A significant interaction between venlafaxine and propafenone causes markedly increased serum levels of venlafaxine correlating with unexpected psychopathological changes.

CONCLUSIONS

There is a potentially dangerous interaction between venlafaxine and propafenone. Serum levels of venlafaxine should be monitored if propafenone is added.

Plasma protein binding of amphotericin B and pharmacokinetics of bound versus unbound amphotericin B after administration of intravenous liposomal amphotericin B (AmBisome) and amphotericin B deoxycholate

Unilamellar liposomal amphotericin B (AmBisome) (liposomal AMB) reduces the toxicity of this antifungal drug. The unique composition of liposomal AMB stabilizes the liposomes, producing higher sustained drug levels in plasma and reducing renal and hepatic excretion. When liposomes release their drug payload, unbound, protein-bound, and liposomal drug pools may exist simultaneously in the body. To determine the amounts of drug in these pools, we developed a procedure to measure unbound AMB in human plasma by ultrafiltration and then used it to characterize AMB binding in vitro and to assess the pharmacokinetics of nonliposomal pools of AMB in a phase IV study of liposomal AMB and AMB deoxycholate in healthy subjects. We confirmed that AMB is highly bound (>95%) in human plasma and showed that both human serum albumin and alpha(1)-acid glycoprotein contribute to this binding. AMB binding exhibited an unusual concentration dependence in plasma: the percentage of bound drug increased as the AMB concentration increased. This was attributed to the low solubility of AMB in plasma, which limits the unbound drug concentration to <1 microg/ml. Subjects given 2 mg of liposomal AMB/kg of body weight had lower exposures (as measured by the maximum concentration of drug in serum and the area under the concentration-time curve) to both unbound and nonliposomal drug than those receiving 0.6 mg of AMB deoxycholate/kg. Most of the AMB in plasma remained liposome associated (97% at 4 h, 55% at 168 h) after liposomal AMB administration, so that unbound drug concentrations remained at <25 ng/ml in all liposomal AMB-treated subjects. Although liposomal AMB markedly reduces the total urinary and fecal recoveries of AMB, urinary and fecal clearances based on unbound AMB were similar (94 to 121 ml h(-1) kg(-1)) for both formulations. Unbound drug urinary clearances were equal to the glomerular filtration rate, and tubular transit rates were <16% of the urinary excretion rate, suggesting that net filtration of unbound drug, with little secretion or reabsorption, is the mechanism of renal clearance for both conventional and liposomal AMB in humans. Unbound drug fecal clearances were also similar for the two formulations. Thus, liposomal AMB increases total AMB concentrations while decreasing unbound AMB concentrations in plasma as a result of sequestration of the drug in long-circulating liposomes.

Single oral loading dose of propafenone for pharmacological cardioversion of recent-onset atrial fibrillation

The efficacy and safety of the single dose oral loading regimen of propafenone for pharmacological cardioversion of recent-onset atrial fibrillation (AFib) was evaluated by analyzing the trials on the subject identified through a comprehensive literature search. Most of the trials used a single dose of 600 mg for oral loading. The success rates ranged from 56% to 83%, depending on the duration of AFib and follow-up after drug administration. The conversion time ranged from 110 +/- 59 to 287 +/- 352 min, depending on the duration of observation after drug administration. The single dose oral loading regimen of propafenone was significantly more efficacious than placebo in the first 8 h after administration but not at 24 h. Compared with the intravenous regimen, the oral regimen resulted in fewer conversions in the first 2 h, but both regimens were equally efficacious afterward. The oral propafenone regimen was as efficacious as the single dose oral loading regimen of flecainide but was superior to those of quinidine and amiodarone. The adverse effects reported were transient arrhythmia, reversible QRS-complex widening, transient hypotension and mild noncardiac side effects. The transient arrhythmias were chiefly at the time of conversion and included appearance of atrial flutter, bradycardia, pauses and junctional rhythm. No life-threatening proarrhythmic adverse effects were reported. The single oral loading dose of propafenone appears to be highly effective for conversion of recent-onset AFib, with a relatively rapid effect within 2 to 3 h and freedom from serious adverse effects.

Management of Seizures in Brain Tumor Patients at the End of Life

Few data exist on the management of seizures in brain tumor patients near the end of life. This article provides information on the epidemiology and phenomenology of seizures, as well as on differential diagnostic considerations. Based largely on empirical data from the pediatric epilepsy literature, guidelines for management of seizures near the end of life are given, with emphasis on the use of rectal antiepileptic drugs.

Effect of selective serotonin reuptake inhibitors on the oxidative metabolism of propafenone: in vitro studies using human liver microsomes

Propafenone is mainly metabolized by CYP2D6 to form 5-hydroxypropafenone (5-OHP) and to a minor extent by CYP1A2 and CYP3A4 to form N-depropylpropafenone (N-DPP). The in vitro inhibitory effect of selective serotonin reuptake inhibitors (SSRIs) on the formation of both metabolites was studied, using human liver microsomes. The 5-OHP formation from racemic propafenone and from its individual enantiomers followed one-enzyme Michaelis-Menten kinetics. Incubation with the racemate yielded a mean Vmax of 64 pmol x min(-1) x mg(-1) and a mean Km of 0.12 microM (N = 3). Stereoselectivity in Vmax and Km values was observed, with (S)-propafenone displaying higher Km and Vmax values. N-DPP formation from racemic propafenone followed one-enzyme Michaelis-Menten kinetics and yielded a mean Vmax of 403 pmol x min(-1) x mg(-1) and a mean Km of 116 microM (N = 3). No stereoselectivity in propafenone N-dealkylation was observed. The influence of SSRIs and quinidine, a prototypical CYP2D6 inhbitor, on propafenone 5-hydroxylation was investigated. Quinidine was the most potent inhibitor, followed by fluoxetine, norfluoxetine, and paroxetine. Sertraline, desmethylsertraline, and fluvoxamine had only a moderate inhibitory effect, whereas citalopram displayed slight or no inhibition when racemic propafenone was used as substrate. Mean Ki values of quinidine, fluoxetine, norfluoxetine, and paroxetine were 0.13, 0.33, 0.55, and 0.54 microM, respectively (N = 3). Quinidine and paroxetine were also tested as inhibitors using the individual enantiomers, but no stereoselectivity was observed. Among the SSRIs tested, only fluvoxamine substantially inhbited propafenone N-dealkylation with a mean IC50 of 7.0 microM (N = 3). There was a more pronounced inhibitory effect of fluvoxamine on (R)-propafenone than on (S)-propafenone N-dealkylation. In conclusion, these in vitro data suggest that an in vivo interaction between propafenone and the SSRIs, fluoxetine and paroxetine, can be expected, which can lead to clinically relevant beta-blockade and an increased risk of side effects in the central nervous system. An interaction with fluvoxamine may be of importance in poor metabolizers for CYP2D6.

Propafenone disposition during continuous venovenous hemofiltration

OBJECTIVE

To determine the extent of removal of propafenone by continuous venovenous hemofiltration (CVVH) in a critically ill pediatric patient.

CASE SUMMARY

A three-year-old white-Japanese girl was admitted to the critical care unit following cardiac surgery. Her postoperative course was complicated by the development of junctional ectopic tachycardia, requiring propafenone, and acute renal failure, which necessitated the use of CVVH. The serum and ultrafiltrate concentrations of propafenone and its 5-hydroxy metabolite were measured to determine both total and CVVH clearance.

CONCLUSIONS

The data from this case report showed that propafenone was not significantly removed by CVVH. Furthermore, the total clearance of propafenone was not affected by the patient's renal or liver function impairment.

The influence of CYP2D6 activity on the kinetics of propafenone enantiomers in Chinese subjects

AIMS

To determine role of CYP2D6 activity in the pharmacokinetics of propafenone (PPF) enantiomers in native Chinese subjects.

METHODS

Sixteen extensive metabolizers (EMs) and one poor metabolizer (PM), whose phenotype had been previously assessed with dextromethorphan metabolic phenotyping, were enrolled. Blood samples (0 approximately 15 h) were taken after oral administration of a single dose (400 mg) of racemic-propafenone hydrochloride. A reverse-phase h.p.l.c. method with pre-column derivatization was employed to quantitate enantiomeric concentrations of propafenone in plasma.

RESULTS

For the EM subjects, S-PPF was less rapidly metabolized and had higher peak plasma concentrations than R-PPF (413+/-143 vs 291+/-109 ng ml-1, P<0.001). The AUC was markedly higher for S-PPF than for R-PPF (2214+/-776 vs 1639+/-630 microg h l-1, P<0.001), whereas the clearance of S-PPF was significantly lower than that of R-PPF (96.0+/-39.0 vs 138+/-78 l h-1, P<0.01). There were no differences in t1/2, and Cmax between the two isomers (P >0.05). In the one PM subject, not only did S-PPF appear to undergo less rapid metabolism than R-PPF, but the subject also showed 2 approximately 3 fold differences in Cmax, CL and AUC compared with EMs. The correlation coefficients (rs ) between dextromethorphan metabolic ratio (lg MR) and pharmacokinetic parameters (Cmax, CL and AUC) were 0.63, -0.87, 0.87 for S-PPF and 0. 57, -0.73, 0.86 for R-PPF, respectively.

CONCLUSIONS

Our results suggest that CYP2D6 activity contributes to the pharmacokinetic variability of propafenone enantiomers in Chinese subjects.

Stereoselective steady state disposition and action of propafenone in Chinese subjects

AIMS

To investigate the steady state disposition and action of racemic propafenone and its enantiomers and the potential for an enantiomer-enantiomer interaction in Chinese subjects.

METHODS

Eight healthy male Chinese individuals received in a double-blind, randomized, cross-over study racemic propafenone (150 mg every 6 h), (S)-, and (R)-propafenone (150 mg each every 6 h) and placebo orally for 4 days. During the last dosing interval the plasma concentrations of both enantiomers of propafenone were measured and ECG, blood pressure (MAP) and heart rate were monitored.

RESULTS

Whereas the apparent elimination half-life (t1/2,z), mean residence time (MRT) and time to reach peak concentrations (tmax) of (S)- and (R)-propafenone were similar and independent of the administered agent, significant differences were observed in the apparent oral clearance (CLO) of the enantiomers. During dosing with racemic propafenone CLO of (S)- and (R)-propafenone averaged (+/-s.d.) 1226+751 and 1678+625 ml min(-1), respectively (P=0.024). Following the administration of the pure enantiomers CLO of (S)-propafenone increased (P= 0.007) to 2028+/-959 ml min(-1) and that of (R)-propafenone was reduced (P= 0.042) to 1318+/-867 ml min(-1). Both enantiomers and the racemate caused about a 10% increase in the QRS duration (P<0.05) and PR-interval (P<0.01) when compared with placebo. The increase in maximum exercise heart rate was significantly (P<0.05) attenuated only at 3 h following the administration of the racemate and the S-enantiomer. MAP and QTC were not affected significantly.

CONCLUSIONS

These data indicate that the stereoselective disposition of propafenone is similar in Chinese and Caucasian subjects (previously published findings), the (R)-enantiomer being cleared more rapidly. A similar enantiomer-enantiomer interaction also occurred. Thus, when the racemate was given, the elimination of (S)-propafenone was impaired and that of the (R)-form accelerated compared with single enantiomer administration. The results indicate that dosage adjustments are probably not required in Chinese patients receiving propafenone.

Therapeutic drug monitoring: antiarrhythmic drugs

Antiarrhythmic agents are traditionally classified according to Vaughan Williams into four classes of action. Class I antiarrhythmic agents include most of the drugs traditionally thought of as antiarrhythmics, and have as a common action, blockade of the fast-inward sodium channel on myocardium. These agents have a very significant toxicity, and while they are being used less, therapeutic drug monitoring (TDM) does significantly increase the safety with which they can be administered. Class II agents are antisympathetic drugs, particularly the beta-adrenoceptor blockers. These are generally safe agents which do not normally require TDM. Class III antiarrhythmic agents include sotalol and amiodarone. TDM can be useful in the case of amiodarone to monitor compliance and toxicity but is generally of little value for sotalol. Class IV antiarrhythmic drugs are the calcium channel blockers verapamil and diltiazem. These are normally monitored by haemodynamic effects, rather than using TDM. Other agents which do not fall neatly into the Vaughan Williams classification include digoxin and perhexiline. TDM is very useful for monitoring the administration (and particularly the safety) of both of these agents.

ECG changes and plasma concentrations of propafenone and its metabolites in a case of severe poisoning

CASE REPORT

Propafenone is a class IC antiarrhythmic agent metabolized into two major metabolites, 5-hydroxypropafenone and N-depropylpropafenone. The potency of 5-hydroxypropafenone to block fast sodium channels is comparable to that of its parent. We report the positive correlation between plasma concentrations and electrocardiographic changes in a patient with severe oral self-poisoning. Serial ECG changes were measured and plasma concentrations were determined by high-performance liquid chromatography. The initial plasma concentrations of propafenone were in the toxic range and correlated with the widening of the QRS-complex. The slow decline in concentration during this first phase might relate to saturation of the isoenzyme CYP2D6. The half-life of propafenone, calculated from the second phase, was approximately 3 hours, defining the patient as a fast metabolizer. The initial concentrations of the metabolite N-depropylpropafenone were surprisingly higher than those of 5-hydroxypropafenone which may also be due to saturation of CYP2D6.

Acute cholestatic hepatitis and the sudden rise of the pacing threshold 5 weeks after VVI-pacemaker implantation

The patient we describe developed a cholestatic hepatitis and an unusual threshold elevation of about 425%, coincidently, 5 weeks after pacemaker implantation. Lead dislocation, myocardial infarction or metabolic disorder could be excluded as a reason for the pacemaker complication. Neither a biliary obstruction nor an infection of the liver could be found to explain the liver injury. Most probably, both disorders were caused by propafenone (450 mg day-1), withdrawal of the drug resulted in normalization of elevated enzyme and threshold levels.

The hepatic first-pass metabolism of problematic drugs

The first-pass hepatic metabolism of a number of important therapeutic agents is inconsistent with traditional models that assume that the hepatic extraction ratio of a drug is constant in each individual (independent of the concentration of drug in the hepatic sinusoidal blood and also independent of the history of exposure to the drug). In this review, the authors examine the first-pass metabolism of five "problematic drugs" (propranolol, lidocaine, propafenone, verapamil, and nitroglycerin). Each of these compounds has unique facets to its hepatic clearance and pharmacokinetics as well as striking similarities. Selected aspects of first-pass metabolism are reviewed, and a theory that may explain some of the unusual behavior of the four lipophilic bases (propranolol, lidocaine, propafenone, and verapamil) is presented. Finally, the unusual and variable clearance of nitroglycerin is discussed.