Rythmol: Package Insert / Prescribing Info

5.1 Proarrhythmic Effects

Propafenone has caused new or worsened arrhythmias. Such proarrhythmic effects include sudden death and life-threatening ventricular arrhythmias such as ventricular fibrillation, ventricular tachycardia, asystole, and torsade de pointes. It may also worsen premature ventricular contractions or supraventricular arrhythmias, and it may prolong the QT interval. It is therefore essential that each patient given RYTHMOL be evaluated electrocardiographically prior to and during therapy to determine whether the response to RYTHMOL supports continued treatment. Because propafenone prolongs the QRS interval in the electrocardiogram, changes in the QT interval are difficult to interpret [see Clinical Pharmacology (12.2)].

In a US uncontrolled, open-label, multicenter trial in subjects with symptomatic supraventricular tachycardia (SVT), 1.9% (9/474) of these subjects experienced ventricular tachycardia (VT) or ventricular fibrillation (VF) during the trial. However, in 4 of the 9 subjects, the ventricular tachycardia was of atrial origin. Six of the 9 subjects that developed ventricular arrhythmias did so within 14 days of onset of therapy. About 2.3% (11/474) of all subjects had a recurrence of SVT during the trial which could have been a change in the subjects’ arrhythmia behavior or could represent a proarrhythmic event. Case reports in patients treated with propafenone for atrial fibrillation/flutter have included increased premature ventricular contractions (PVCs), VT, VF, torsade de pointes, asystole, and death.

Overall in clinical trials with RYTHMOL (which included subjects treated for ventricular arrhythmias, atrial fibrillation/flutter, and PSVT), 4.7% of all subjects had new or worsened ventricular arrhythmia possibly representing a proarrhythmic event (0.7% was an increase in PVCs; 4.0% a worsening or new appearance of VT or VF). Of the subjects who had worsening of VT (4%), 92% had a history of VT and/or VT/VF, 71% had coronary artery disease, and 68% had a prior myocardial infarction. The incidence of proarrhythmia in subjects with less serious or benign arrhythmias, which include subjects with an increase in frequency of PVCs, was 1.6%. Although most proarrhythmic events occurred during the first week of therapy, late events also were seen and the CAST trial [see Boxed Warning: Mortality] suggests that an increased risk of proarrythmia is present throughout treatment.

In a trial of sustained-release propafenone (RYTHMOL SR®), there were too few deaths to assess the long-term risk to patients. There were 5 deaths, 3 in the pooled group for RYTHMOL SR (0.8%), and 2 in the placebo group (1.6%). In the overall database of 8 trials of RYTHMOL SR and immediate-release RYTHMOL, the mortality rate was 2.5% per year on propafenone and 4.0% per year on placebo. Concurrent use of propafenone with other antiarrhythmic agents has not been well studied.

5.2 Unmasking Brugada Syndrome

Brugada Syndrome may be unmasked after exposure to RYTHMOL. Perform an ECG after initiation of RYTHMOL, and discontinue the drug if changes are suggestive of Brugada Syndrome [see Contraindications (4)].

5.3 Use with Drugs that Prolong the QT Interval and Antiarrhythmic Agents

The use of RYTHMOL in conjunction with other drugs that prolong the QT interval has not been extensively studied. Such drugs may include many antiarrhythmics, some phenothiazines, tricyclic antidepressants, and oral macrolides. Withhold Class IA and III antiarrhythmic agents for at least 5 half-lives prior to dosing with RYTHMOL. Avoid the use of propafenone with Class IA and III antiarrhythmic agents (including quinidine and amiodarone). There is only limited experience with the concomitant use of Class IB or IC antiarrhythmics.

5.4 Drug Interactions: Simultaneous Use with Inhibitors of Cytochrome P450 Isoenzymes 2D6 and 3A4

Propafenone is metabolized by CYP2D6, CYP3A4, and CYP1A2 isoenzymes. Approximately 6% of Caucasians in the US population are naturally deficient in CYP2D6 activity and other demographic groups are deficient to a somewhat lesser extent. Drugs that inhibit these CYP pathways (such as desipramine, paroxetine, ritonavir, sertraline for CYP2D6; ketoconazole, erythromycin, saquinavir, and grapefruit juice for CYP3A4; and amiodarone and tobacco smoke for CYP1A2) can be expected to cause increased plasma levels of propafenone.

Increased exposure to propafenone may lead to cardiac arrhythmias and exaggerated beta-adrenergic blocking activity. Because of its metabolism, the combination of CYP3A4 inhibition and either CYP2D6 deficiency or CYP2D6 inhibition in users of propafenone is potentially hazardous. Therefore, avoid simultaneous use of RYTHMOL with both a CYP2D6 inhibitor and a CYP3A4 inhibitor.

5.5 Use in Patients with a History of Heart Failure

Propafenone exerts a negative inotropic activity on the myocardium as well as beta-blockade effects and may provoke overt heart failure.

In clinical trial experience with RYTHMOL, new or worsened congestive heart failure (CHF) has been reported in 3.7% of subjects with ventricular arrhythmia; of those 0.9% were considered probably or definitely related to propafenone HCl. Of the subjects with CHF probably related to propafenone, 80% had pre-existing heart failure and 85% had coronary artery disease. CHF attributable to propafenone HCl developed rarely (less than 0.2%) in subjects with ventricular arrhythmia who had no previous history of CHF. CHF occurred in 1.9% of subjects studied with PAF or PSVT.

In a US trial of RYTHMOL SR in subjects with symptomatic AF, heart failure was reported in 4 (1.0%) subjects receiving RYTHMOL SR (all doses) compared with 1 (0.8%) subject receiving placebo.

5.6 Conduction Disturbances

Propafenone slows atrioventricular conduction and may also cause dose-related first-degree AV block. Average PR interval prolongation and increases in QRS duration are also dose-related. Do not give propafenone to patients with atrioventricular and intraventricular conduction defects in the absence of a pacemaker [see Contraindications (4), Clinical Pharmacology (12.2)].

The incidence of first-degree, second-degree, and third-degree AV block observed in 2,127 subjects with ventricular arrhythmia was 2.5%, 0.6%, and 0.2%, respectively. Development of second- or third-degree AV block requires a reduction in dosage or discontinuation of propafenone HCl. Bundle branch block (1.2%) and intraventricular conduction delay (1.1%) have been reported in subjects receiving propafenone. Bradycardia has also been reported (1.5%). Experience in patients with sick sinus node syndrome is limited and these patients should not be treated with propafenone.

In a US trial in 523 subjects with a history of symptomatic AF treated with RYTHMOL SR, sinus bradycardia (rate less than 50 beats per min) was reported with the same frequency with RYTHMOL SR and placebo.

5.7 Effects on Pacemaker Threshold

Propafenone may alter both pacing and sensing thresholds of implanted pacemakers and defibrillators. During and after therapy, monitor and re-program these devices accordingly.

5.8 Agranulocytosis

Agranulocytosis has been reported in patients receiving propafenone. Generally, the agranulocytosis occurred within the first 2 months of propafenone therapy, and upon discontinuation of therapy the white count usually normalized by 14 days. Unexplained fever or decrease in white cell count, particularly during the initial 3 months of therapy, warrant consideration of possible agranulocytosis or granulocytopenia. Instruct patients to report promptly any signs of infection such as fever, sore throat, or chills.

5.9 Use in Patients with Hepatic Dysfunction

Propafenone is highly metabolized by the liver. Severe liver dysfunction increases the bioavailability of propafenone to approximately 70% compared with 3% to 40% in patients with normal liver function. In 8 subjects with moderate to severe liver disease, the mean half-life was approximately 9 hours. Increased bioavailability of propafenone in these patients may result in excessive accumulation. Carefully monitor patients with impaired hepatic function for excessive pharmacological effects [see Overdosage (10)].

5.10 Use in Patients with Renal Dysfunction

Approximately 50% of propafenone metabolites are excreted in the urine following administration of RYTHMOL.

In patients with impaired renal function, monitor for signs of overdosage [see Overdosage (10)].

5.11 Use in Patients with Myasthenia Gravis

Exacerbation of myasthenia gravis has been reported during propafenone therapy.

5.12 Elevated ANA Titers

Positive ANA titers have been reported in patients receiving propafenone. They have been reversible upon cessation of treatment and may disappear even in the face of continued propafenone therapy. These laboratory findings were usually not associated with clinical symptoms, but there is one published case of drug-induced lupus erythematosis (positive rechallenge); it resolved completely upon discontinuation of therapy. Carefully evaluate patients who develop an abnormal ANA test and, if persistent or worsening elevation of ANA titers is detected, consider discontinuing therapy.

5.13 Impaired Spermatogenesis

Reversible disorders of spermatogenesis have been demonstrated in monkeys, dogs, and rabbits after high-dose intravenous administration of propafenone. Evaluation of the effects of short-term administration of RYTHMOL on spermatogenesis in 11 normal subjects suggested that propafenone produced a reversible, short-term drop (within normal range) in sperm count.

6.1 Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared with rates in the clinical trials of another drug and may not reflect the rates observed in practice.

Adverse reactions associated with RYTHMOL occur most frequently in the gastrointestinal, cardiovascular, and central nervous systems. About 20% of subjects treated with RYTHMOL have discontinued treatment because of adverse reactions.

Adverse reactions reported for greater than 1.5% of 474 subjects with SVT who received RYTHMOL in US clinical trials are presented in Table 1 by incidence and percent discontinuation, reported to the nearest percent.

In controlled trials in subjects with ventricular arrhythmia, the most common reactions reported for RYTHMOL and more frequent than on placebo were unusual taste, dizziness, first-degree AV block, intraventricular conduction delay, nausea and/or vomiting, and constipation. Headache was relatively common also, but was not increased compared with placebo. Other reactions reported more frequently than on placebo or comparator and not already reported elsewhere included anxiety, angina, second-degree AV block, bundle branch block, loss of balance, congestive heart failure, and dyspepsia.

Adverse reactions reported for greater than or equal to 1% of 2,127 subjects with ventricular arrhythmia who received propafenone in US clinical trials were evaluated by daily dose. The most common adverse reactions appeared dose-related (but note that most subjects spent more time at the larger doses), especially dizziness, nausea and/or vomiting, unusual taste, constipation, and blurred vision. Some less common reactions may also have been dose-related such as first-degree AV block, congestive heart failure, dyspepsia, and weakness. Other adverse reactions included rash, syncope, chest pain, abdominal pain, ataxia, and hypotension.

In addition, the following adverse reactions were reported less frequently than 1% either in clinical trials or in marketing experience. Causality and relationship to propafenone therapy cannot necessarily be judged from these events.

Cardiovascular System

Atrial flutter, AV dissociation, cardiac arrest, flushing, hot flashes, sick sinus syndrome, sinus pause or arrest, supraventricular tachycardia.

Nervous System

Abnormal dreams, abnormal speech, abnormal vision, confusion, depression, memory loss, numbness, paresthesias, psychosis/mania, seizures (0.3%), tinnitus, unusual smell sensation, vertigo.

Gastrointestinal

Cholestasis, elevated liver enzymes (alkaline phosphatase, serum transaminases), gastroenteritis, hepatitis .

Hematologic

Agranulocytosis, anemia, bruising, granulocytopenia, leukopenia, purpura, thrombocytopenia.

Other

Alopecia, eye irritation, impotence, increased glucose, positive ANA (0.7%), muscle cramps, muscle weakness, nephrotic syndrome, pain, pruritus.

6.2 Postmarketing Experience

The following adverse reactions have been identified during post-approval use of RYTHMOL. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure.

Gastrointestinal

A number of patients with liver abnormalities associated with propafenone therapy have been reported in postmarketing experience. Some appeared due to hepatocellular injury, some were cholestatic, and some showed a mixed picture. Some of these reports were simply discovered through clinical chemistries, others because of clinical symptoms including fulminant hepatitis and death. One case was rechallenged with a positive outcome.

Blood and Lymphatic System:

Increased bleeding time.

Immune System

Lupus erythematosis.

Nervous System

Apnea, coma.

Renal and Urinary

Hyponatremia/inappropriate ADH secretion, kidney failure.

7.1 CYP2D6 and CYP3A4 Inhibitors

Drugs that inhibit CYP2D6 (such as desipramine, paroxetine, ritonavir, sertraline) and CYP3A4 (such as ketoconazole, ritonavir, saquinavir, erythromycin, grapefruit juice) can be expected to cause increased plasma levels of propafenone. The combination of CYP3A4 inhibition and either CYP2D6 deficiency or CYP2D6 inhibition with administration of propafenone may increase the risk of adverse reactions, including proarrhythmia. Therefore, simultaneous use of RYTHMOL with both a CYP2D6 inhibitor and a CYP3A4 inhibitor should be avoided [see Warnings and Precautions (5.4), Dosage and Administration (2)].

Amiodarone

Concomitant administration of propafenone and amiodarone can affect conduction and repolarization and is not recommended.

Cimetidine

Concomitant administration of propafenone immediate-release tablets and cimetidine in 12 healthy subjects resulted in a 20% increase in steady-state plasma concentrations of propafenone.

Fluoxetine

Concomitant administration of propafenone and fluoxetine in extensive metabolizers increased the S-propafenone Cmax and AUC by 39% and 50%, respectively, and the R propafenone Cmax and AUC by 71% and 50%, respectively.

Quinidine

Small doses of quinidine completely inhibit the CYP2D6 hydroxylation metabolic pathway, making all patients, in effect, slow metabolizers [see Clinical Pharmacology (12.3)]. Concomitant administration of quinidine (50 mg 3 times daily) with 150 mg immediate-release propafenone 3 times daily decreased the clearance of propafenone by 60% in extensive metabolizers, making them slow metabolizers. Steady-state plasma concentrations more than doubled for propafenone, and decreased 50% for 5-OH-propafenone. A 100-mg dose of quinidine tripled steady-state concentrations of propafenone. Avoid concomitant use of propafenone and quinidine.

Rifampin:

Concomitant administration of rifampin and propafenone in extensive metabolizers decreased the plasma concentrations of propafenone by 67% with a corresponding decrease of 5-OH-propafenone by 65%. The concentrations of norpropafenone increased by 30%. In slow metabolizers, there was a 50% decrease in propafenone plasma concentrations and an increase in the AUC and Cmax of norpropafenone by 74% and 20%, respectively. Urinary excretion of propafenone and its metabolites decreased significantly. Similar results were noted in elderly patients: Both the AUC and Cmax of propafenone decreased by 84%, with a corresponding decrease in AUC and Cmax of 5-OH‑propafenone by 69% and 57%, respectively.

7.2 Digoxin

Concomitant use of propafenone and digoxin increased steady-state serum digoxin exposure (AUC) in patients by 60% to 270%, and decreased the clearance of digoxin by 31% to 67%. Monitor plasma digoxin levels of patients receiving propafenone and adjust digoxin dosage as needed.

7.3 Warfarin

The concomitant administration of propafenone and warfarin increased warfarin plasma concentrations at steady state by 39% in healthy volunteers and prolonged the prothrombin time (PT) in patients taking warfarin. Adjust the warfarin dose as needed by monitoring INR (international normalized ratio).

7.4 Orlistat

Orlistat may limit the fraction of propafenone available for absorption. In postmarketing reports, abrupt cessation of orlistat in patients stabilized on propafenone has resulted in severe adverse events including convulsions, atrioventricular block, and acute circulatory failure.

7.5 Beta-Antagonists

Concomitant use of propafenone and propranolol in healthy subjects increased propranolol plasma concentrations at steady state by 113%. In 4 patients, administration of metoprolol with propafenone increased the metoprolol plasma concentrations at steady state by 100% to 400%. The pharmacokinetics of propafenone was not affected by the coadministration of either propranolol or metoprolol. In clinical trials using propafenone immediate-release tablets, subjects who were receiving beta-blockers concurrently did not experience an increased incidence of side effects.

7.6 Lidocaine

No significant effects on the pharmacokinetics of propafenone or lidocaine have been seen following their concomitant use in patients. However, concomitant use of propafenone and lidocaine has been reported to increase the risks of central nervous system side effects of lidocaine.

8.1 Pregnancy

Pregnancy Category C. There are no adequate and well-controlled studies in pregnant women. RYTHMOL should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus.

Animal Data

Teratogenic Effects: Propafenone has been shown to be embryotoxic (decreased survival) in rabbits and rats when given in oral maternally toxic doses of 150 mg per kg per day (about 3 times the maximum recommended human dose [MRHD] on a mg per m2 basis) and 600 mg per kg per day (about 6 times the MRHD on a mg per m2 basis), respectively. Although maternally tolerated doses (up to 270 mg per kg per day, about 3 times the MRHD on a mg per m2 basis) produced no evidence of embryotoxicity in rats; post-implantation loss was elevated in all rabbit treatment groups (doses as low as 15 mg per kg per day, about 1/3 the MRHD on a mg per m2 basis).

Non-teratogenic Effects: In a study in which female rats received daily oral doses of propafenone from mid-gestation through weaning of their offspring, doses as low as 90 mg per kg per day (equivalent to the MRHD on a mg per m2 basis) produced increases in maternal deaths. Doses of 360 or more mg per kg per day (4 or more times the MRHD on a mg per m2 basis) resulted in reductions in neonatal survival, body weight gain, and physiological development.

8.2 Labor and Delivery

It is not known whether the use of propafenone during labor or delivery has immediate or delayed adverse effects on the fetus, or whether it prolongs the duration of labor or increases the need for forceps delivery or other obstetrical intervention.

8.3 Nursing Mothers

Propafenone is excreted in human milk. Because of the potential for serious adverse reactions in nursing infants from propafenone, decide whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

8.4 Pediatric Use

The safety and effectiveness of propafenone in pediatric patients have not been established.

8.5 Geriatric Use

Clinical trials of RYTHMOL did not include sufficient numbers of subjects aged 65 and older to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger subjects. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

12.1 Mechanism of Action

Propafenone is a Class 1C antiarrhythmic drug with local anesthetic effects, and a direct stabilizing action on myocardial membranes. The electrophysiological effect of propafenone manifests itself in a reduction of upstroke velocity (Phase 0) of the monophasic action potential. In Purkinje fibers, and to a lesser extent myocardial fibers, propafenone reduces the fast inward current carried by sodium ions. Diastolic excitability threshold is increased and effective refractory period prolonged. Propafenone reduces spontaneous automaticity and depresses triggered activity.

Studies in anesthetized dogs and isolated organ preparations show that propafenone has beta-sympatholytic activity at about 1/50 the potency of propranolol. Clinical studies employing isoproterenol challenge and exercise testing after single doses of propafenone indicate a beta-adrenergic blocking potency (per mg) about 1/40 that of propranolol in man. In clinical trials, resting heart rate decreases of about 8% were noted at the higher end of the therapeutic plasma concentration range. At very high concentrations in vitro, propafenone can inhibit the slow inward current carried by calcium, but this calcium antagonist effect probably does not contribute to antiarrhythmic efficacy. Moreover, propafenone inhibits a variety of cardiac potassium currents in in vitro studies (i.e., the transient outward, the delayed rectifier, and the inward rectifier current). Propafenone has local anesthetic activity approximately equal to procaine. Compared with propafenone, the main metabolite, 5-hydroxypropafenone, has similar sodium and calcium channel activity, but about 10 times less beta-blocking activity. (N-depropylpropafenone has weaker sodium channel activity but equivalent affinity for beta-receptors.)

12.2 Pharmacodynamics

Electrophysiology

Electrophysiology trials in subjects with ventricular tachycardia have shown that propafenone prolongs atrioventricular conduction while having little or no effect on sinus node function. Both atrioventricular nodal conduction time (AH interval) and His-Purkinje conduction time (HV interval) are prolonged. Propafenone has little or no effect on the atrial functional refractory period, but AV nodal functional and effective refractory periods are prolonged. In patients with Wolff-Parkinson-White syndrome, RYTHMOL reduces conduction and increases the effective refractory period of the accessory pathway in both directions.

Electrocardiograms

Propafenone slows prolongs the PR and QRS intervals. Prolongation of the QRS interval makes it difficult to interpret the effect of propafenone on the QT interval.

a Change and percent change based on mean baseline values for each treatment group.

In any individual patient, the above ECG changes cannot be readily used to predict either efficacy or plasma concentration.

RYTHMOL causes a dose-related and concentration-related decrease in the rate of single and multiple PVCs and can suppress recurrence of ventricular tachycardia. Based on the percent of patients attaining substantial (80% to 90%) suppression of ventricular ectopic activity, it appears that trough plasma levels of 0.2 to 1.5 mcg per mL can provide good suppression, with higher concentrations giving a greater rate of good response.

When 600 mg per day propafenone was administered to subjects with paroxysmal atrial tachyarrhythmias, mean heart rate during arrhythmia decreased 14 beats per min and 37 beats per min for subjects with PAF and subjects with PSVT, respectively.

Hemodynamics

Trials in humans have shown that propafenone HCl exerts a negative inotropic effect on the myocardium. Cardiac catheterization trials in subjects with moderately impaired ventricular function (mean C.I. = 2.61 L per min per m2) utilizing intravenous propafenone infusions (loading dose of 2 mg per kg over 10 min followed by 2 mg per min for 30 min) that gave mean plasma concentrations of 3.0 mcg per mL (a dose that produces plasma levels of propafenone greater than recommended oral dosing) showed significant increases in pulmonary capillary wedge pressure, systemic and pulmonary vascular resistances, and depression of cardiac output and cardiac index.

12.3 Pharmacokinetics

Absorption/Bioavailability

Propafenone HCl is nearly completely absorbed after oral administration with peak plasma levels occurring approximately 3.5 hours after administration in most individuals. Propafenone exhibits extensive saturable presystemic biotransformation (first-pass effect) resulting in a dose dependent and dosage form dependent absolute bioavailability; e.g., a 150-mg tablet had absolute bioavailability of 3.4%, while a 300-mg tablet had absolute bioavailability of 10.6%. A 300-mg solution which was rapidly absorbed had absolute bioavailability of 21.4%. At still larger doses, above those recommended, bioavailability increases still further.

Propafenone HCl follows a nonlinear pharmacokinetic disposition presumably because of saturation of first-pass hepatic metabolism as the liver is exposed to higher concentrations of propafenone and shows a very high degree of inter-individual variability. For example, for an increase in daily dose from 300 to 900 mg per day there is a 10-fold increase in steady-state plasma concentration. The top 25% of subjects given 337.5 mg per day, however, had a mean concentration of propafenone larger than the bottom 25%, and about equal to the second 25%, of subjects given a dose of 900 mg. Although food increased peak blood level and bioavailability in a single-dose trial, during multiple-dose administration of propafenone to healthy volunteers, food did not change bioavailability significantly.

Distribution

Following intravenous administration of propafenone, plasma levels decline in a bi-phasic manner consistent with a 2-compartment pharmacokinetic model. The average distribution half-life corresponding to the first phase was about 5 minutes. The volume of the central compartment was about 88 liters (1.1 L per kg) and the total volume of distribution about 252 liters.

In serum, propafenone is greater than 95% bound to proteins within the concentration range of 0.5 to 2 mcg per mL.

Metabolism

There are 2 genetically determined patterns of propafenone metabolism. In over 90% of patients, the drug is rapidly and extensively metabolized with an elimination half-life from 2 to 10 hours. These patients metabolize propafenone into 2 active metabolites: 5-hydroxypropafenone which is formed by CYP2D6 and N-depropylpropafenone (norpropafenone) which is formed by both CYP3A4 and CYP1A2.

In less than 10% of patients, metabolism of propafenone is slower because the 5-hydroxy metabolite is not formed or is minimally formed. In these patients, the estimated propafenone elimination half-life ranges from 10 to 32 hours. Decreased ability to form the 5-hydroxy metabolite of propafenone is associated with a diminished ability to metabolize debrisoquine and a variety of other drugs (such as encainide, metoprolol, and dextromethorphan) whose metabolism is mediated by the CYP2D6 isozyme. In these patients, the N-depropylpropafenone metabolite occurs in quantities comparable to the levels occurring in extensive metabolizers.

There are significant differences in plasma concentrations of propafenone in slow and extensive metabolizers, the former achieving concentrations 1.5 to 2.0 times those of the extensive metabolizers at daily doses of 675 to 900 mg per day. At low doses the differences are greater, with slow metabolizers attaining concentrations more than 5 times that of extensive metabolizers. Because the difference decreases at high doses and is mitigated by the lack of the active 5-hydroxy metabolite in the slow metabolizers, and because steady-state conditions are achieved after 4 to 5 days of dosing in all patients, the recommended dosing regimen is the same for all patients. The greater variability in blood levels requires that the drug be titrated carefully in patients with close attention paid to clinical and ECG evidence of toxicity [see Dosage and Administration (2)].

Stereochemistry: RYTHMOL is a racemic mixture. The R- and S-enantiomers of propafenone display stereoselective disposition characteristics. In vitro and in vivo studies have shown that the R-isomer of propafenone is cleared faster than the S-isomer via the 5-hydroxylation pathway (CYP2D6). This results in a higher ratio of S-propafenone to R-propafenone at steady state. Both enantiomers have equivalent potency to block sodium channels; however, the S-enantiomer is a more potent beta-antagonist than the R-enantiomer. Following administration of RYTHMOL immediate-release tablets, the S/R ratio for the area under the plasma concentration-time curve was about 1.7. In addition, no difference in the average values of the S/R ratios is evident between genotypes or over time.

Special Populations

Hepatic Impairment: Decreased liver function increases the bioavailability of propafenone. Absolute bioavailability of RYTHMOL immediate-release tablets is inversely related to indocyanine green clearance, reaching 60% to 70% at clearances of 7 mL per min and below. Protein binding decreases to about 88% in patients with severe hepatic dysfunction. The clearance of propafenone is reduced and the elimination half-life increased in patients with significant hepatic dysfunction [see Warnings and Precautions (5.9)].

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Lifetime maximally tolerated oral dose studies in mice (up to 360 mg per kg per day, about twice the maximum recommended human oral daily dose [MRHD] on a mg per m2 basis) and rats (up to 270 mg per kg per day, about 3 times the MRHD on a mg per m2 basis) provided no evidence of a carcinogenic potential for propafenone HCl.

Propafenone HCl tested negative for mutagenicity in the Ames (salmonella) test and in the in vivo mouse dominant lethal test. It tested negative for clastogenicity in the human lymphocyte chromosome aberration assay in vitro and in rat and Chinese hamster micronucleus tests, and other in vivo tests for chromosomal aberrations in rat bone marrow and Chinese hamster bone marrow and spermatogonia.

Propafenone HCl, administered intravenously to rabbits, dogs, and monkeys, has been shown to decrease spermatogenesis. These effects were reversible, were not found following oral dosing of propafenone HCl, were seen at lethal or near lethal dose levels, and were not seen in rats treated either orally or intravenously [see Warnings and Precautions (5.13)]. Treatment of male rabbits for 10 weeks prior to mating at an oral dose of 120 mg per kg per day (about 2.4 times the MRHD on a mg per m2 basis) or an intravenous dose of 3.5 mg per kg per day (a spermatogenesis-impairing dose) did not result in evidence of impaired fertility. Nor was there evidence of impaired fertility when propafenone HCl was administered orally to male and female rats at dose levels up to 270 mg per kg per day (about 3 times the MRHD on a mg per m2 basis).

13.2 Animal Toxicology and/or Pharmacology

Renal changes have been observed in the rat following 6 months of oral administration of propafenone HCl at doses of 180 and 360 mg per kg per day (about 2 and 4 times, respectively, the MRHD on a mg per m2 basis). Both inflammatory and non-inflammatory changes in the renal tubules, with accompanying interstitial nephritis, were observed. These changes were reversible, as they were not found in rats allowed to recover for 6 weeks. Fatty degenerative changes of the liver were found in rats following longer durations of administration of propafenone HCl at a dose of 270 mg per kg per day (about 3 times the MRHD on a mg per m2 basis). There were no renal or hepatic changes at 90 mg per kg per day (equivalent to the MRHD on a mg per m2 basis).