AMIODARONE Drug Interactions: What You Need to Know

INTERACTIONS Amiodarone is metabolized to the active metabolite desethylamiodarone by the cytochrome P450 (CYP450) enzyme group, specifically cytochromes P4503A4 (CYP3A) and CYP2C8. The CYP3A isoenzyme is present in both the liver and intestines. Amiodarone is an inhibitor of CYP3A. Therefore, amiodarone has the potential for interactions with drugs or substances that may be substrates, inhibitors or inducers of CYP3A. While only a limited number of in vivo drug-drug interactions with amiodarone have been reported, chiefly with the oral formulation, the potential for other interactions should be anticipated. This is especially important for drugs associated with serious toxicity, such as other antiarrhythmics. If such drugs are needed, reassess their dose and, where appropriate, measure plasma concentrations. In view of the long and variable half-life of amiodarone, potential for drug interactions exists not only with concomitant medication but also with drugs administered after discontinuation of amiodarone. Since amiodarone is a substrate for CYP3A and CYP2C8, drugs/substances that inhibit these isoenzymes may decrease the metabolism and increase serum concentration of amiodarone. Reported examples include the following: Protease inhibitors: Protease inhibitors are known to inhibit CYP3A to varying degrees. A case report of one patient taking amiodarone 200 mg and indinavir 800 mg three times a day resulted in increases in amiodarone concentrations from 0.9 mg/L to 1.3 mg/L. DEA concentrations were not affected. There was no evidence of toxicity. Consider monitoring for amiodarone toxicity and serial measurement of amiodarone serum concentration during concomitant protease inhibitor therapy. Histamine H 1 antagonists: Loratadine , a non-sedating antihistaminic, is metabolized primarily by CYP3A. QT interval prolongation and TdP have been reported with the coadministration of loratadine and amiodarone. Histamine H 2 antagonists: Cimetidine inhibits CYP3A and can increase serum amiodarone levels. Antidepressants: Trazodone , an antidepressant, is metabolized primarily by CYP3A. QT interval prolongation and TdP have been reported with the coadministration of trazodone and amiodarone. Other substances: Grapefruit juice given to healthy volunteers increased amiodarone AUC by 50% and C max by 84%, resulting in increased plasma levels of amiodarone. Do not take grapefruit juice during treatment with amiodarone. Amiodarone inhibits p-glycoprotein and certain CYP450 enzymes, including CYP1A2, CYP2C9, CYP2D6, and CYP3A. This inhibition can result in unexpectedly high plasma levels of other drugs which are metabolized by those CYP450 enzymes or are substrates for p-glycoprotein. Reported examples of this interaction include the following: Immunosuppressives: Cyclosporine (CYP3A substrate) administered in combination with oral amiodarone has been reported to produce persistently elevated plasma concentrations of cyclosporine resulting in elevated creatinine, despite reduction in dose of cyclosporine. HMG-CoA Reductase Inhibitors: Simvastatin (CYP3A substrate) in combination with amiodarone has been associated with reports of myopathy/rhabdomyolysis. Cardiovasculars: Cardiac glycosides : In patients receiving digoxin therapy, administration of oral amiodarone regularly results in an increase in serum digoxin concentration that may reach toxic levels with resultant clinical toxicity. Amiodarone taken concomitantly with digoxin increases the serum digoxin concentration by 70% after one day. On administration of oral amiodarone, review the need for digitalis therapy and reduce the dose of digitalis by approximately 50% or discontinue digitalis. If digitalis treatment is continued, monitor serum levels closely and observe patients for clinical evidence of toxicity. Antiarrhythmics: Other antiarrhythmic drugs, such as quinidine, procainamide, disopyramide, and phenytoin, have been used concurrently with amiodarone. There have been case reports of increased steady-state levels of quinidine, procainamide, and phenytoin during concomitant therapy with amiodarone. Phenytoin decreases serum amiodarone levels. Amiodarone taken concomitantly with quinidine increases quinidine serum concentration by 33% after two days. Amiodarone taken concomitantly with procainamide for less than seven days increases plasma concentrations of procainamide and n-acetyl procainamide by 55% and 33%, respectively. Reduce quinidine and procainamide doses by one-third when either is administered with amiodarone. Plasma levels of flecainide have been reported to increase in the presence of oral amiodarone; adjust the dose of flecainide when these drugs are administered concomitantly. In general, initiate any added antiarrhythmic drug at a lower than usual dose and monitor the patient carefully. Reserve the combination of amiodarone with other antiarrhythmic therapy to patients with life-threatening ventricular arrhythmias who are incompletely responsive to a single agent or incompletely responsive to amiodarone. During transfer to oral amiodarone, reduce the dose levels of previously administered agents by 30 to 50% several days after the addition of oral amiodarone. Review the continued need for the other antiarrhythmic agent after the effects of amiodarone have been established, and attempt discontinuation. If the treatment is continued, carefully monitor these patients for adverse effects, especially for conduction disturbances and exacerbation of tachyarrhythmias. In amiodarone-treated patients who require additional antiarrhythmic therapy, the initial dose of such agents should be approximately half of the usual recommended dose. Antihypertensives: Use amiodarone with caution in patients receiving ß-receptor blocking agents (e.g., propranolol , a CYP3A inhibitor) or calcium channel antagonists (e.g., verapamil , a CYP3A substrate, and diltiazem , a CYP3A inhibitor) because of the possible potentiation of bradycardia, sinus arrest, and AV block; if necessary, amiodarone can continue to be used after insertion of a pacemaker in patients with severe bradycardia or sinus arrest. Anticoagulants: Potentiation of warfarin -type (CYP2C9 and CYP3A substrate) anticoagulant response is almost always seen in patients receiving amiodarone and can result in serious or fatal bleeding. Since the concomitant administration of warfarin with amiodarone increases the prothrombin time by 100% after 3 to 4 days, reduce the dose of the anticoagulant by one-third to one-half, and monitor prothrombin times closely. Clopidogrel , an inactive thienopyridine prodrug, is metabolized in the liver by CYP3A to an active metabolite. A potential interaction between clopidogrel and amiodarone resulting in ineffective inhibition of platelet aggregation has been reported. Some drugs/substances are known to accelerate the metabolism of amiodarone by stimulating the synthesis of CYP3A (enzyme induction). This may lead to low amiodarone serum levels and potential decrease in efficacy. Reported examples of this interaction include the following: Antibiotics: Rifampin is a potent inducer of CYP3A. Administration of rifampin concomitantly with oral amiodarone has been shown to result in decreases in serum concentrations of amiodarone and desethylamiodarone. Other substances, including herbal preparations: St. John's Wort ( Hypericum perforatum ) induces CYP3A. Since amiodarone is a substrate for CYP3A, St. John's Wort likely reduces amiodarone levels. Other reported interactions with amiodarone: Fentanyl (CYP3A substrate) in combination with amiodarone may cause hypotension, bradycardia, and decreased cardiac output. Sinus bradycardia has been reported with oral amiodarone in combination with lidocaine (CYP3A substrate) given for local anesthesia. Seizure, associated with increased lidocaine concentrations, has been reported with concomitant administration of intravenous amiodarone. Dextromethorphan is a substrate for both CYP2D6 and CYP3A. Amiodarone inhibits CYP2D6. Cholestyramine increases enterohepatic elimination of amiodarone and may reduce its serum levels and t ½ . Disopyramide causes QT prolongation which could induce arrhythmia. Fluoroquinolones, macrolide antibiotics, and azoles are known to cause QTc prolongation. There have been reports of QTc prolongation, with or without TdP, in patients taking amiodarone when fluoroquinolones, macrolide antibiotics, or azoles were administered concomitantly [see Warnings and Precautions ( 5.4 )] . Hemodynamic and electrophysiologic interactions have also been observed after concomitant administration with propranolol, diltiazem, and verapamil .

Volatile Anesthetic

Agents : Patients who are on amiodarone therapy may be more sensitive to the myocardial depressant and conduction defects of halogenated inhalational anesthetics [see Warnings and Precautions ( 5.9 )] . In addition to the interactions noted above, chronic (> 2 weeks) oral amiodarone administration impairs metabolism of phenytoin, dextromethorphan, and methotrexate. Since amiodarone is a substrate for CYP3A and CYP2C8, drugs/substances that inhibit these isoenzymes may decrease the metabolism and increase serum concentration of amiodarone. Amiodarone inhibits p-glycoprotein and certain CYP450 enzymes, including CYP1A2, CYP2C9, CYP2D6, and CYP3A. This inhibition can result in unexpectedly high plasma levels of other drugs that are metabolized by those CYP450 enzymes or are substrates for p-glycoprotein. Fluoroquinolones, macrolide antibiotics, and azoles are known to cause QTc prolongation. There have been reports of QTc prolongation, with or without TdP, in patients taking amiodarone when fluoroquinolones, macrolide antibiotics, or azoles were administered concomitantly. ( 7 )

Amiodarone is contraindicated in patients with:

• Known hypersensitivity to any of the components of Amiodarone Hydrochloride Injection, USP, including iodine. Hypersensitivity reactions may involve rash, angioedema, cutaneous/mucosal hemorrhage (bleeding), fever, arthralgias (joint pains), eosinophilia (abnormal blood counts), urticaria (hives), thrombotic thrombocytopenic purpura, or severe periateritis (inflammation around blood vessels)
• Cardiogenic shock
• Marked sinus bradycardia
• Second- or third-degree atrio-ventricular (AV) block unless of a functioning pacemaker is available. Amiodarone is contraindicated in patients with (4):
• Known hypersensitivity to any of the components of amiodarone, including iodine
• Cardiogenic shock
• Marked sinus bradycardia
• Second- or third-degree atrio-ventricular (AV) block unless of a functioning pacemaker is available.

AND PRECAUTIONS Amiodarone should be administered only by physicians who are experienced in the treatment of life-threatening arrhythmias, who are thoroughly familiar with the risks and benefits of amiodarone therapy, and who have access to facilities adequate for monitoring the effectiveness and side effects of treatment. Because of the long half-life of amiodarone and its metabolite desethylamiodarone, the potential for adverse reactions or interactions, as well as observed adverse effects, can persist following amiodarone withdrawal.

• Hypotension: Slow the infusion; as needed, add vasopressor drugs, positive inotropic agents, and volume expansion. ( 5.1 )
• Bradycardia and AV block: Slow the infusion or discontinue. ( 5.2 )

5.1 Hypotension Hypotension is the most common adverse reaction seen with intravenous amiodarone. In clinical trials, treatment-emergent, drug-related hypotension was reported as an adverse effect in 288 (16%) of 1836 patients treated with intravenous amiodarone. Clinically significant hypotension during infusions was seen most often in the first several hours of treatment and was not dose related, but appeared to be related to the rate of infusion. Hypotension necessitating alterations in intravenous amiodarone therapy was reported in 3% of patients, with permanent discontinuation required in less than 2% of patients. Treat hypotension initially by slowing the infusion; additional standard therapy may be needed, including the following: vasopressor drugs, positive inotropic agents, and volume expansion. Monitor the initial rate of infusion closely and do not exceed the recommended rate <span class="opacity-50 text-xs">[see Dosage and Administration (2) ]</span> . In some cases, hypotension may be refractory and result in a fatal outcome <span class="opacity-50 text-xs">[see Adverse Reactions (6.2) ]</span> .

5.2 Bradycardia and Atrio-ventricular Block In 90 (4.9%) of 1836 patients in clinical trials, drug-related bradycardia that was not dose-related occurred while they were receiving intravenous amiodarone for life-threatening VT/VF. Treat bradycardia by slowing the infusion rate or discontinuing amiodarone. In some patients, a pacemaker is required. Despite such measures, bradycardia was progressive and terminal in 1 patient during the controlled trials. Treat patients with a known predisposition to bradycardia or AV block with amiodarone in a setting where a temporary pacemaker is available.

5.3 Hepatic Injury Elevations of blood hepatic enzyme values [alanine aminotransferase (ALT), aspartate aminotransferase (AST), and gamma-glutamyl transferase (GGT)] are commonly seen in patients with immediately life-threatening VT/VF. Interpreting elevated AST activity can be difficult because the values may be elevated in patients who have had recent myocardial infarction, congestive heart failure, or multiple electrical defibrillations.

Approximately

54% of patients receiving intravenous amiodarone in clinical studies had baseline liver enzyme elevations, and 13% had clinically significant elevations.

In

81% of patients with both baseline and on-therapy data available, the liver enzyme elevations either improved during therapy or remained at baseline levels. Baseline abnormalities in hepatic enzymes are not a contraindication to treatment. Elevated bilirubin levels have been reported in patients administered intravenous amiodarone. Acute, centrolobular confluent hepatocellular necrosis leading to hepatic coma, acute renal failure, and death has been associated with the administration of intravenous amiodarone [see Dosage and Administration (2) ] . In patients with life-threatening arrhythmias, the potential risk of hepatic injury should be weighed against the potential benefit of amiodarone therapy. Carefully monitor patients receiving amiodarone for evidence of progressive hepatic injury. In such cases, consider reducing the rate of administration or withdrawing amiodarone.

5.4 Proarrhythmia Like all antiarrhythmic agents, amiodarone may cause a worsening of existing arrhythmias or precipitate a new arrhythmia sometimes leading to fatal outcomes <span class="opacity-50 text-xs">[see Adverse Reactions (6.2) ]</span> . Proarrhythmia, primarily torsade de pointes (TdP), has been associated with prolongation, by intravenous amiodarone, of the QTc interval to 500 ms or greater. Although QTc prolongation occurred frequently in patients receiving intravenous amiodarone, TdP or new-onset VF occurred infrequently (less than 2%). Monitor patients for QTc prolongation during infusion with amiodarone. Reserve the combination of amiodarone with other antiarrhythmic therapies that prolong the QTc to patients with life-threatening ventricular arrhythmias who are incompletely responsive to a single agent. Correct hypokalemia, hypomagnesemia or hypocalcemia whenever possible before initiating treatment with amiodarone, as these disorders can exaggerate the degree of QTc prolongation and increase the potential for TdP. Give special attention to electrolyte and acid-base balance in patients experiencing severe or prolonged diarrhea or in patients receiving concomitant diuretics and laxatives. Amiodarone causes thyroid dysfunction in some patients, which may lead to potentially fatal breakthrough or exacerbated arrhythmias.

5.5 Pulmonary Injury Early-onset Pulmonary Toxicity There have been postmarketing reports of acute-onset (days to weeks) pulmonary injury in patients treated with intravenous amiodarone. Findings have included pulmonary infiltrates and masses on X-ray, bronchospasm, wheezing, fever, dyspnea, cough, hemoptysis, and hypoxia. Some cases have progressed to respiratory failure or death.

Ards

Two percent (2%) of patients were reported to have adult respiratory distress syndrome (ARDS) during clinical studies involving 48 hours of therapy.

Pulmonary Fibrosis

There have been reports of early development of pulmonary fibrosis (within 1 to 3 months) following initiation of amiodarone treatment.

Only

1 of more than 1000 patients treated with intravenous amiodarone in clinical studies developed pulmonary fibrosis. In that patient, the condition was diagnosed 3 months after treatment with intravenous amiodarone, during which time the patient received oral amiodarone. Pulmonary toxicity is a well-recognized complication of long-term amiodarone use (see package insert for oral amiodarone).

5.6 Loss of Vision Cases of optic neuropathy and optic neuritis, usually resulting in visual impairment, have been reported in patients treated with oral amiodarone or intravenous amiodarone. In some cases, visual impairment has progressed to permanent blindness. Optic neuropathy and neuritis may occur at any time following initiation of therapy. A causal relationship to the drug has not been clearly established. Perform an ophthalmic examination if symptoms of visual impairment appear, such as changes in visual acuity and decreases in peripheral vision. Re-evaluate the necessity of amiodarone therapy if optic neuropathy or neuritis is suspected. Perform regular ophthalmic examination, including fundoscopy and slit-lamp examination, during administration of amiodarone.

5.7 Thyroid Abnormalities Amiodarone inhibits peripheral conversion of thyroxine (T4) to triiodothyronine (T3) and may cause increased T4 levels, decreased T3 levels, and increased levels of inactive reverse T3 (rT3) in clinically euthyroid patients. Amiodarone is also a potential source of large amounts of inorganic iodine and can cause either hypothyroidism or hyperthyroidism. Evaluate thyroid function prior to treatment and periodically thereafter, particularly in elderly patients, and in any patient with a history of thyroid nodules, goiter, or other thyroid dysfunction. Because of the slow elimination of amiodarone and its metabolites, high plasma iodide levels, altered thyroid function, and abnormal thyroid-function tests may persist for months following amiodarone withdrawal. There have been postmarketing reports of thyroid nodules/thyroid cancer in patients treated with amiodarone. In some instances hyperthyroidism was also present. Hyperthyroidism and Thyrotoxicosis Amiodarone causes hyperthyroidism in about 2% of patients. Thyrotoxicosis and arrhythmia with fatal outcome has been reported in the presence of pre-existing hyperthyroidism even following a single intravenous amiodarone dose. Consider the possibility of hyperthyroidism if any new signs of arrhythmia appear. Hyperthyroidism may result from iodine load (type 1 amiodarone-induced thyrotoxicosis [type 1 AIT]; in particular in patients with underlying autonomous thyroid nodules or latent Grave’s disease). Hyperthyroidism may also result from direct amiodarone-induced destructive thyroiditis that occurs in individuals with no underlying thyroid disease (type 2 AIT), resulting in the release of preformed thyroid hormone into the bloodstream from damaged thyroid follicular epithelium. Mixed forms of hyperthyroidism as a result of both pathogenic mechanisms (excessive thyroid hormone production and thyroid destruction) can also occur. The risk of hyperthyroidism may be higher among patients with prior inadequate dietary iodine intake. Identify hyperthyroidism by relevant clinical signs and symptoms, subnormal serum levels of thyroid stimulating hormone (TSH), abnormally elevated serum free T4, and elevated or normal serum T3. Since arrhythmia breakthroughs may accompany amiodarone-induced hyperthyroidism, aggressive medical treatment is indicated, including, if possible, dose reduction or withdrawal of amiodarone. Amiodarone hyperthyroidism may be followed by a transient period of hypothyroidism. The institution of antithyroid drugs, β-adrenergic blockers or temporary corticosteroid therapy may be necessary. The action of antithyroid drugs may be especially delayed in amiodarone-induced thyrotoxicosis because of substantial quantities of preformed thyroid hormones stored in the gland. Radioactive iodine therapy is not recommended because of the low radioiodine uptake associated with amiodarone-induced hyperthyroidism. When aggressive treatment of amiodarone-induced thyrotoxicosis has failed or amiodarone cannot be discontinued because it is the only drug effective against the resistant arrhythmia, surgical management may be an option. Experience with thyroidectomy as a treatment for amiodarone-induced thyrotoxicosis is limited, and this form of therapy could induce thyroid storm. Therefore, surgical and anesthetic management require careful planning.

Hypothyroidism

Hypothyroidism has been reported in 2% to 10% of patients receiving amiodarone and may be primary or subsequent to resolution of preceding amiodarone-induced hyperthyroidism. This condition may be identified by clinical symptoms and elevated serum TSH levels. Cases of severe hypothyroidism and myxedema coma, sometimes fatal, have been reported in association with amiodarone therapy. In some clinically hypothyroid amiodarone-treated patients, free thyroxine index values may be normal. Manage hypothyroidism by reducing the dose of or discontinuing amiodarone and considering the need for thyroid hormone supplement.

5.8 Neonatal Injury Amiodarone can cause fetal harm when administered to a pregnant woman. Fetal exposure may increase the potential for adverse experiences including cardiac, thyroid, neurodevelopmental, neurological and growth effects in neonates. Inform the patient of the potential hazard to the fetus if amiodarone is administered during pregnancy or if the patient becomes pregnant while taking amiodarone <span class="opacity-50 text-xs">[see Pregnancy (8.1) ]</span> .

5.9 Exaggerated Effects of Perisurgical Therapy Perform close perioperative monitoring in patients undergoing general anesthesia who are on amiodarone therapy as they may be more sensitive to the myocardial depressant and conduction defects of halogenated inhalational anesthetics.

5.10 Interference with Corneal Refractive Laser Surgery Advise patients that most manufacturers of corneal refractive laser surgery devices contraindicate corneal refractive laser surgery in patients taking amiodarone.

5.11 Hypersensitivity Reactions Anaphylactic/anaphylactoid reactions have been reported with intravenous amiodarone including shock (sometimes fatal), cardiac arrest, and the following manifestations: hypotension, tachycardia, hypoxia, cyanosis, rash, flushing, hyperhidrosis and cold sweat.