CLARITHROMYCIN

Clarithromycin is a semi-synthetic macrolide antibiotic. Chemically, it is 6- 0 -methylerythromycin. The molecular formula is C 38 H 69 NO 13 , and the molecular weight is 747.96. The structural formula is: Clarithromycin is a white or almost white, crystalline powder. It is soluble in acetone, slightly soluble in methanol, ethanol, and acetonitrile, and practically insoluble in water. Clarithromycin tablets, USP are available as immediate-release tablets. Each film-coated tablet contains 250 mg or 500 mg of clarithromycin and the following inactive ingredients: microcrystalline cellulose, croscarmellose sodium, colloidal silicon dioxide, magnesium stearate, povidone, hypromellose, titanium dioxide, hydroxypropyl cellulose, iron oxide yellow, propylene glycol, vanillin, and sorbic acid. Chemcial Structure

Clarithromycin tablets, USP are indicated for the treatment of mild to moderate infections caused by susceptible isolates of the designated bacteria in the conditions as listed below: Adults (Clarithromycin Tablets, USP) Pharyngitis/Tonsillitis due to Streptococcus pyogenes (The usual drug of choice in the treatment and prevention of streptococcal infections and the prophylaxis of rheumatic fever is penicillin administered by either the intramuscular or the oral route. Clarithromycin is generally effective in the eradication of S. pyogenes from the nasopharynx; however, data establishing the efficacy of clarithromycin in the subsequent prevention of rheumatic fever are not available at present). Acute maxillary sinusitis due to Haemophilus influenzae , Moraxella catarrhalis, or Streptococcus pneumoniae. Acute bacterial exacerbation of chronic bronchitis due to Haemophilus influenzae , Haemophilus parainfluenzae , Moraxella catarrhalis , or Streptococcus pneumoniae. Community-Acquired Pneumonia due to Haemophilus influenzae , Mycoplasma pneumoniae , Streptococcus pneumoniae , or Chlamydophila pneumoniae (TWAR). Uncomplicated skin and skin structure infections due to Staphylococcus aureus , or Streptococcus pyogenes (Abscesses usually require surgical drainage). Disseminated mycobacterial infections due to Mycobacterium avium , or Mycobacterium intracellulare Clarithromycin tablets, USP in combination with amoxicillin and PREVACID (lansoprazole) or PRILOSEC (omeprazole) Delayed-Release Capsules, as triple therapy, are indicated for the treatment of patients with Helicobacter pylori infection and duodenal ulcer disease (active or five-year history of duodenal ulcer) to eradicate H. pylori . Clarithromycin tablets, USP in combination with PRILOSEC (omeprazole) capsules or TRITEC (ranitidine bismuth citrate) tablets are also indicated for the treatment of patients with an active duodenal ulcer associated with H. pylori infection. However, regimens which contain clarithromycin as the single antimicrobial agent are more likely to be associated with the development of clarithromycin resistance among patients who fail therapy. Clarithromycin-containing regimens should not be used in patients with known or suspected clarithromycin resistant isolates because the efficacy of treatment is reduced in this setting. In patients who fail therapy, susceptibility testing should be done if possible. If resistance to clarithromycin is demonstrated, a non-clarithromycin-containing therapy is recommended. (For information on development of resistance see Microbiology section.) The eradication of H. pylori has been demonstrated to reduce the risk of duodenal ulcer recurrence. Children (clarithromycin tablets, USP) Pharyngitis/Tonsillitis due to Streptococcus pyogenes. Community-Acquired Pneumonia due to Mycoplasma pneumoniae , Streptococcus pneumoniae , or Chlamydophila pneumoniae (TWAR) Acute maxillary sinusitis due to Haemophilus influenzae , Moraxella catarrhalis , or Streptococcus pneumoniae Acute otitis media due to Haemophilus influenzae , Moraxella catarrhalis , or Streptococcus pneumoniae NOTE: For information on otitis media, see CLINICAL STUDIES - Otitis Media. Uncomplicated skin and skin structure infections due to Staphylococcus aureus , or Streptococcus pyogenes (Abscesses usually require surgical drainage.) Disseminated mycobacterial infections due to Mycobacterium avium , or Mycobacterium intracellulare Prophylaxis Clarithromycin tablets, USP are indicated for the prevention of disseminated Mycobacterium avium complex (MAC) disease in patients with advanced HIV infection. To reduce the development of drug-resistant bacteria and maintain the effectiveness of clarithromycin and other antibacterial drugs, clarithromycin should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.

Clarithromycin tablets may be given with or without food. Clarithromycin may be administered without dosage adjustment in the presence of hepatic impairment if there is normal renal function. In patients with severe renal impairment (CL CR < 30 mL/min), the dose of clarithromycin should be reduced by 50%. However, when patients with moderate or severe renal impairment are taking clarithromycin concomitantly with atazanavir or ritonavir, the dose of clarithromycin should be reduced by 50% or 75% for patients with CL CR of 30 to 60 mL/min or < 30 mL/min, respectively. ADULT DOSAGE GUIDELINES Clarithromycin Tablets Infection Dosage (q 12 h) Duration (Days) Pharyngitis/Tonsillitis due to S. pyogenes 250 mg 10 Acute maxillary sinusitis due to 500 mg 14 H. influenzae M. catarrhalis S. pneumoniae Acute exacerbation of chronic bronchitis due to H. influenzae 500 mg 7 to 14 H. parainfluenzae 500 mg 7 M. catarrhalis 250 mg 7 to 14 S. pneumoniae 250 mg 7 to 14 Community-Acquired Pneumonia due to H. influenzae 250 mg 7 H. parainfluenzae - - M. catarrhalis - - S. pneumoniae 250 mg 7 to 14 C. pneumoniae 250 mg 7 to 14 M. pneumoniae 250 mg 7 to 14 Uncomplicated skin and skin structure 250 mg 7 to 14 S. aureus S. pyogenes H. pylori Eradication to Reduce the Risk of Duodenal Ulcer Recurrence Triple therapy: clarithromycin/lansoprazole/amoxicillin The recommended adult dose is 500 mg clarithromycin, 30 mg lansoprazole, and 1 gram amoxicillin, all given twice daily (q 12 h) for 10 or 14 days (see INDICATIONS AND USAGE and CLINICAL STUDIES sections). Triple therapy: clarithromycin/omeprazole/amoxicillin The recommended adult dose is 500 mg clarithromycin, 20 mg omeprazole, and 1 gram amoxicillin, all given twice daily (q 12 h) for 10 days (see INDICATIONS AND USAGE and CLINICAL STUDIES sections). In patients with an ulcer present at the time of initiation of therapy, an additional 18 days of omeprazole 20 mg once daily is recommended for ulcer healing and symptom relief. Dual therapy: clarithromycin/omeprazole The recommended adult dose is 500 mg clarithromycin given three times daily (q 8 h) and 40 mg omeprazole given once daily (qAM) for 14 days (see INDICATIONS AND USAGE and CLINICAL STUDIES sections). An additional 14 days of omeprazole 20 mg once daily is recommended for ulcer healing and symptom relief. Dual therapy: clarithromycin/ranitidine bismuth citrate The recommended adult dose is 500 mg clarithromycin given twice daily (q 12 h) or three times daily (q 8 h) and 400 mg ranitidine bismuth citrate given twice daily (q 12 h) for 14 days. An additional 14 days of 400 mg twice daily is recommended for ulcer healing and symptom relief. Clarithromycin and ranitidine bismuth citrate combination therapy is not recommended in patients with creatinine clearance less than 25 mL/min (see INDICATIONS AND USAGE and CLINICAL STUDIES sections). Children The usual recommended daily dosage is 15 mg/kg/day divided q 12 h for 10 days. PEDIATRIC DOSAGE GUIDELINES Based on Body Weight Dosing Calculated on 7.5 mg/kg q 12 h Weight Dose Kg lbs (q 12 h) 9 20 62.5 mg 17 37 125 mg 25 55 187.5 mg 33 73 250 mg Mycobacterial Infections Prophylaxis The recommended dose of clarithromycin for the prevention of disseminated Mycobacterium avium disease is 500 mg b.i.d. In children, the recommended dose is 7.5 mg/kg b.i.d. up to 500 mg b.i.d. No studies of clarithromycin for MAC prophylaxis have been performed in pediatric populations and the doses recommended for prophylaxis are derived from MAC treatment studies in children. Dosing recommendations for children are in the table above. Treatment Clarithromycin is recommended as the primary agent for the treatment of disseminated infection due to Mycobacterium avium complex. Clarithromycin should be used in combination with other antimycobacterial drugs that have shown in vitro activity against MAC or clinical benefit in MAC treatment (see CLINICAL STUDIES ). The recommended dose for mycobacterial infections in adults is 500 mg b.i.d. In children, the recommended dose is 7.5 mg/kg b.i.d. up to 500 mg b.i.d. Dosing recommendations for children are in the table above. Clarithromycin therapy should continue if clinical response is observed. Clarithromycin can be discontinued when the patient is considered at low risk of disseminated infection.

Clarithromycin tablets are contraindicated in patients with a known hypersensitivity to clarithromycin or any of its excipients, erythromycin, or any of the macrolide antibiotics. Clarithromycin tablets are contraindicated in patients with a history of cholestatic jaundice/hepatic dysfunction associated with prior use of clarithromycin. Concomitant administration of clarithromycin and any of the following drugs is contraindicated: cisapride, pimozide, astemizole, terfenadine, and ergotamine or dihydroergotamine (see Drug Interactions ). There have been postmarketing reports of drug interactions when clarithromycin and/or erythromycin are coadministered with cisapride, pimozide, astemizole, or terfenadine resulting in cardiac arrhythmias (QT prolongation, ventricular tachycardia, ventricular fibrillation, and torsades de pointes) most likely due to inhibition of metabolism of these drugs by erythromycin and clarithromycin. Fatalities have been reported. Concomitant administration of clarithromycin and colchicine is contraindicated in patients with renal or hepatic impairment. Clarithromycin should not be given to patients with history of QT prolongation or ventricular cardiac arrhythmia, including torsades de pointes . Clarithromycin should not be used concomitantly with HMG-CoA reductase inhibitors (statins) that are extensively metabolized by CYP3A4 (lovastatin or simvastatin), due to the increased risk of myopathy, including rhabdomyolysis (see WARNINGS ). For information about contraindications of other drugs indicated in combination with clarithromycin tablets, refer to the CONTRAINDICATIONS section of their package inserts.

The most frequent and common adverse reactions related to clarithromycin therapy for both adult and pediatric populations are abdominal pain, diarrhea, nausea, vomiting and dysgeusia. These adverse reactions are consistent with the known safety profile of macrolide antibiotics. There was no significant difference in the incidence of these gastrointestinal adverse reactions during clinical trials between the patient population with or without preexisting mycobacterial infections. Adverse Reactions Observed During Clinical Trials of Clarithromycin The following adverse reactions were observed in clinical trials with clarithromycin at a rate greater than or equal to 1%: Gastrointestinal Disorders Diarrhea, vomiting, dyspepsia, nausea, abdominal pain Hepatobiliary Disorders Liver function test abnormal Immune System Disorders Anaphylactoid reaction Infection and Infestations Candidiasis Nervous System Disorders Dysgeusia, headache Psychiatric Disorders Insomnia Skin and Subcutaneous Tissue Disorders Rash Other Adverse Reactions Observed During Clinical Trials of Clarithromycin The following adverse reactions were observed in clinical trials with clarithromycin at a rate less than 1%: Blood and Lymphatic System Disorders Leukopenia, neutropenia, thrombocythemia, eosinophilia Cardiac Disorders Electrocardiogram QT prolonged, cardiac arrest, atrial fibrillation, extrasystoles, palpitations Ear and Labyrinth Disorders Vertigo, tinnitus, hearing impaired Gastrointestinal Disorders Stomatitis, glossitis, esophagitis, gastrooesophageal reflux disease, gastritis, proctalgia, abdominal distension, constipation, dry mouth, eructation, flatulence General Disorders and Administration Site Conditions Malaise, pyrexia, asthenia, chest pain, chills, fatigue Hepatobiliary Disorders Cholestasis, hepatitis Immune System Disorders Hypersensitivity Infections and Infestations Cellulitis, gastroenteritis, infection, vaginal infection Investigations Blood bilirubin increased, blood alkaline phosphatase increased, blood lactate dehydrogenase increased, albumin globulin ratio abnormal Metabolism and Nutrition Disorders Anorexia, decreased appetite Musculoskeletal and Connective Tissue Disorders Myalgia, muscle spasms, nuchal rigidity Nervous System Disorders Dizziness, tremor, loss of consciousness, dyskinesia, somnolence Psychiatric Disorders Anxiety, nervousness Renal and Urinary Disorders Blood creatinine increased, blood urea increased Respiratory, Thoracic and Mediastinal Disorders Asthma, epistaxis, pulmonary embolism Skin and Subcutaneous Tissue Disorders Urticaria, dermatitis bullous, pruritus, hyperhidrosis, rash maculo-papular In the acute exacerbation of chronic bronchitis and acute maxillary sinusitis studies overall gastrointestinal adverse events were reported by a similar proportion of patients taking either clarithromycin tablets or clarithromycin extended-release tablets; however, patients taking clarithromycin extended-release tablets reported significantly less severe gastrointestinal symptoms compared to patients taking clarithromycin tablets. In addition, patients taking clarithromycin extended-release tablets had significantly fewer premature discontinuations for drug-related gastrointestinal or abnormal taste adverse events compared to clarithromycin tablets. In community-acquired pneumonia studies conducted in adults comparing clarithromycin to erythromycin base or erythromycin stearate, there were fewer adverse events involving the digestive system in clarithromycin-treated patients compared to erythromycin-treated patients (13% vs. 32%; p < 0.01). Twenty percent of erythromycin-treated patients discontinued therapy due to adverse events compared to 4% of clarithromycin-treated patients. In two U.S. studies of acute otitis media comparing clarithromycin to amoxicillin/potassium clavulanate in pediatric patients, there were fewer adverse events involving the digestive system in clarithromycin-treated patients compared to amoxicillin/potassium clavulanate-treated patients (21% vs. 40%, p < 0.001). One-third as many clarithromycin-treated patients reported diarrhea as did amoxicillin/potassium clavulanate-treated patients. Postmarketing Experience The following adverse reactions have been identified during post approval use of clarithromycin. 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. Blood and Lymphatic System Disorders Thrombocytopenia, agranulocytosis Cardiac Disorders Torsades de pointes , ventricular tachycardia, ventricular arrhythmia Ear and Labyrinth Disorders Deafness was reported chiefly in elderly women and was usually reversible. Gastrointestinal Disorders Pancreatitis acute, tongue discoloration, tooth discoloration was reported and was usually reversible with professional cleaning upon discontinuation of the drug. There have been reports of clarithromycin extended-release tablets in the stool, many of which have occurred in patients with anatomic (including ileostomy or colostomy) or functional gastrointestinal disorders with shortened GI transit times. In several reports, tablet residues have occurred in the context of diarrhea. It is recommended that patients who experience tablet residue in the stool and no improvement in their condition should be switched to a different clarithromycin formulation (e.g., suspension) or another antibacterial drug. Hepatobiliary Disorders Hepatic failure, jaundice hepatocellular. Adverse reactions related to hepatic dysfunction have been reported with clarithromycin (see WARNINGS - Hepatotoxicity ). Immune System Disorders Anaphylactic reaction Infections and Infestations Pseudomembranous colitis Investigations Prothrombin time prolonged, white blood cell count decreased, international normalized ratio increased. Abnormal urine color has been reported, associated with hepatic failure. Metabolism and Nutrition Disorders Hypoglycemia has been reported in patients taking oral hypoglycemic agents or insulin. Musculoskeletal and Connective Tissue Disorders Myopathy, rhabdomyolysis was reported and in some of the reports, clarithromycin was administered concomitantly with statins, fibrates, colchicine or allopurinol (see CONTRAINDICATIONS and WARNINGS ). Nervous System Disorders Convulsion, ageusia, parosmia, anosmia, paraesthesia Psychiatric Disorders Psychotic disorder, confusional state, depersonalization, depression, disorientation, manic behavior, hallucination, abnormal behavior, abnormal dreams. These disorders usually resolve upon discontinuation of the drug. There are no data on the effect of clarithromycin on the ability to drive or use machines. The potential for dizziness, vertigo, confusion and disorientation, which may occur with the medication, should be taken into account before patients drive or use machines. Renal and Urinary Disorders Nephritis interstitial, renal failure Skin and Subcutaneous Tissue Disorders Stevens-Johnson syndrome, toxic epidermal necrolysis, drug rash with eosinophilia and systemic symptoms (DRESS), Henoch-Schonlein purpura, acne Vascular Disorders Hemorrhage There have been reports of colchicine toxicity with concomitant use of clarithromycin and colchicine, especially in the elderly, some of which occurred in patients with renal insufficiency. Deaths have been reported in some such patients (see WARNINGS and PRECAUTIONS ).

Clarithromycin use in patients who are receiving theophylline may be associated with an increase of serum theophylline concentrations. Monitoring of serum theophylline concentrations should be considered for patients receiving high doses of theophylline or with baseline concentrations in the upper therapeutic range. In two studies in which theophylline was administered with clarithromycin (a theophylline sustained-release formulation was dosed at either 6.5 mg/kg or 12 mg/kg together with 250 or 500 mg q 12 h clarithromycin), the steady-state levels of C max , C min , and the area under the serum concentration time curve (AUC) of theophylline increased about 20%. Hypotension, bradyarrhythmias, and lactic acidosis have been observed in patients receiving concurrent verapamil, belonging to the calcium channel blockers drug class. Concomitant administration of single doses of clarithromycin and carbamazepine has been shown to result in increased plasma concentrations of carbamazepine. Blood level monitoring of carbamazepine may be considered. When clarithromycin and terfenadine were coadministered, plasma concentrations of the active acid metabolite of terfenadine were threefold higher, on average, than the values observed when terfenadine was administered alone. The pharmacokinetics of clarithromycin and the 14-OH-clarithromycin were not significantly affected by coadministration of terfenadine once clarithromycin reached steady-state conditions. Concomitant administration of clarithromycin with terfenadine is contraindicated (see CONTRAINDICATIONS ). Clarithromycin 500 mg every 8 hours was given in combination with omeprazole 40 mg daily to healthy adult subjects. The steady-state plasma concentrations of omeprazole were increased (C max , AUC 0-24 , and t ½ increases of 30%, 89%, and 34%, respectively), by the concomitant administration of clarithromycin. The mean 24-hour gastric pH value was 5.2 when omeprazole was administered alone and 5.7 when coadministered with clarithromycin. Coadministration of clarithromycin with ranitidine bismuth citrate resulted in increased plasma ranitidine concentrations (57%), increased plasma bismuth trough concentrations (48%), and increased 14-hydroxy-clarithromycin plasma concentrations (31%). These effects are clinically insignificant. Simultaneous oral administration of clarithromycin tablets and zidovudine to HIV-infected adult patients may result in decreased steady-state zidovudine concentrations. Following administration of clarithromycin 500 mg tablets twice daily with zidovudine 100 mg every 4 hours, the steady-state zidovudine AUC decreased 12% compared to administration of zidovudine alone (n=4). Individual values ranged from a decrease of 34% to an increase of 14%. When clarithromycin tablets were administered two to four hours prior to zidovudine, the steady-state zidovudine C max increased 100% whereas the AUC was unaffected (n=24). Administration of clarithromycin and zidovudine should be separated by at least two hours. Simultaneous administration of clarithromycin tablets and didanosine to 12 HIV-infected adult patients resulted in no statistically significant change in didanosine pharmacokinetics. Following administration of fluconazole 200 mg daily and clarithromycin 500 mg twice daily to 21 healthy volunteers, the steady-state clarithromycin C min and AUC increased 33% and 18%, respectively. Steady-state concentrations of 14-OH clarithromycin were not significantly affected by concomitant administration of fluconazole. No dosage adjustment of clarithromycin is necessary when coadministered with fluconazole. Ritonavir Concomitant administration of clarithromycin and ritonavir (n = 22) resulted in a 77% increase in clarithromycin AUC and a 100% decrease in the AUC of 14-OH clarithromycin. Clarithromycin may be administered without dosage adjustment to patients with normal renal function taking ritonavir. Since concentrations of 14-OH clarithromycin are significantly reduced when clarithromycin is co‑administered with ritonavir, alternative antibacterial therapy should be considered for indications other than infections due to Mycobacterium avium complex (see PRECAUTIONS - Drug Interactions ). Doses of clarithromycin greater than 1000 mg per day should not be coadministered with protease inhibitors. Spontaneous reports in the postmarketing period suggest that concomitant administration of clarithromycin and oral anticoagulants may potentiate the effects of the oral anticoagulants. Prothrombin times should be carefully monitored while patients are receiving clarithromycin and oral anticoagulants simultaneously. Digoxin is a substrate for P-glycoprotein (Pgp) and clarithromycin is known to inhibit Pgp. When clarithromycin and digoxin are coadministered, inhibition of Pgp by clarithromycin may lead to increased exposure of digoxin. Elevated digoxin serum concentrations in patients receiving clarithromycin and digoxin concomitantly have been reported in postmarketing surveillance. Some patients have shown clinical signs consistent with digoxin toxicity, including potentially fatal arrhythmias. Monitoring of serum digoxin concentrations should be considered, especially for patients with digoxin concentrations in the upper therapeutic range. Coadministration of clarithromycin, known to inhibit CYP3A, and a drug primarily metabolized by CYP3A may be associated with elevations in drug concentrations that could increase or prolong both therapeutic and adverse effects of the concomitant drug. Clarithromycin should be used with caution in patients receiving treatment with other drugs known to be CYP3A enzyme substrates, especially if the CYP3A substrate has a narrow safety margin (e.g., carbamazepine) and/or the substrate is extensively metabolized by this enzyme. Dosage adjustments may be considered, and when possible, serum concentrations of drugs primarily metabolized by CYP3A should be monitored closely in patients concurrently receiving clarithromycin. The following are examples of some clinically significant CYP3A based drug interactions. Interactions with other drugs metabolized by the CYP3A isoform are also possible. Carbamazepine and Terfenadine Increased serum concentrations of carbamazepine and the active acid metabolite of terfenadine were observed in clinical trials with clarithromycin. Colchicine Colchicine is a substrate for both CYP3A and the efflux transporter, P-glycoprotein (Pgp). Clarithromycin and other macrolides are known to inhibit CYP3A and Pgp. When a single dose of colchicine 0.6 mg was administered with clarithromycin 250 mg BID for 7 days, the colchicine C max increased 197% and the AUC 0-∞ increased 239% compared to administration of colchicine alone. The dose of colchicine should be reduced when coadministered with clarithromycin in patients with normal renal and hepatic function. Concomitant use of clarithromycin and colchicine is contraindicated in patients with renal or hepatic impairment (see WARNINGS ). Efavirenz, Nevirapine, Rifampicin, Rifabutin, and Rifapentine Inducers of CYP3A enzymes, such as efavirenz, nevirapine, rifampicin, rifabutin, and rifapentine will increase the metabolism of clarithromycin, thus decreasing plasma concentrations of clarithromycin, while increasing those of 14-OH-clarithromycin. Since the microbiological activities of clarithromycin and 14-OH-clarithromycin are different for different bacteria, the intended therapeutic effect could be impaired during concomitant administration of clarithromycin and enzyme inducers. Alternative antibacterial treatment should be considered when treating patients receiving inducers of CYP3A. Concomitant administration of rifabutin and clarithromycin resulted in an increase in rifabutin, and decrease in clarithromycin serum levels together with an increased risk of uveitis. Etravirine Clarithromycin exposure was decreased by etravirine; however, concentrations of the active metabolite, 14-OH-clarithromycin, were increased. Because 14-OH-clarithromycin has reduced activity against Mycobacterium avium complex (MAC), overall activity against this pathogen may be altered; therefore alternatives to clarithromycin should be considered for the treatment of MAC. Sildenafil, Tadalafil, and Vardenafil Each of these phosphodiesterase inhibitors is primarily metabolized by CYP3A, and CYP3A will be inhibited by concomitant administration of clarithromycin. Coadministration of clarithromycin with sildenafil, tadalafil, or vardenafil will result in increased exposure of these phosphodiesterase inhibitors. Coadministration of these phosphodiesterase inhibitors with clarithromycin is not recommended. Tolterodine The primary route of metabolism for tolterodine is via CYP2D6. However, in a subset of the population devoid of CYP2D6, the identified pathway of metabolism is via CYP3A. In this population subset, inhibition of CYP3A results in significantly higher serum concentrations of tolterodine. Tolterodine 1 mg twice daily is recommended in patients deficient in CYP2D6 activity (poor metabolizers) when coadministered with clarithromycin. Triazolobenzodiazepines (e.g., alprazolam, midazolam, triazolam) When a single dose of midazolam was coadministered with clarithromycin tablets (500 mg twice daily for 7 days), midazolam AUC increased 174% after intravenous administration of midazolam and 600% after oral administration. When oral midazolam is coadministered with clarithromycin, dose adjustments may be necessary and possible prolongation and intensity of effect should be anticipated. Caution and appropriate dose adjustments should be considered when triazolam or alprazolam is coadministered with clarithromycin. For benzodiazepines which are not metabolized by CYP3A (e.g., temazepam, nitrazepam, lorazepam), a clinically important interaction with clarithromycin is unlikely. There have been postmarketing reports of drug interactions and central nervous system (CNS) effects (e.g., somnolence and confusion) with the concomitant use of clarithromycin and triazolam. Monitoring the patient for increased CNS pharmacological effects is suggested. Atazanavir Both clarithromycin and atazanavir are substrates and inhibitors of CYP3A, and there is evidence of a bi-directional drug interaction. Following administration of clarithromycin (500 mg twice daily) with atazanavir (400 mg once daily), the clarithromycin AUC increased 94%, the 14-OH clarithromycin AUC decreased 70% and the atazanavir AUC increased 28%. When clarithromycin is co‑administered with atazanavir, the dose of clarithromycin should be decreased by 50%. Since concentrations of 14-OH clarithromycin are significantly reduced when clarithromycin is coadministered with atazanavir, alternative antibacterial therapy should be considered for indications other than infections due to Mycobacterium avium complex (see PRECAUTIONS - Drug Interactions ). Doses of clarithromycin greater than 1000 mg per day should not be coadministered with protease inhibitors. Itraconazole Both clarithromycin and itraconazole are substrates and inhibitors of CYP3A, potentially leading to a bi-directional drug interaction when administered concomitantly. Clarithromycin may increase the plasma concentrations of itraconazole, while itraconazole may increase the plasma concentrations of clarithromycin. Patients taking itraconazole and clarithromycin concomitantly should be monitored closely for signs or symptoms of increased or prolonged adverse reactions. Saquinavir Both clarithromycin and saquinavir are substrates and inhibitors of CYP3A and there is evidence of a bi-directional drug interaction. Following administration of clarithromycin (500 mg b.i.d.) and saquinavir (soft gelatin capsules, 1200 mg t.i.d.) to 12 healthy volunteers, the steady-state saquinavir AUC and C max increased 177% and 187% respectively compared to administration of saquinavir alone. Clarithromycin AUC and C max increased 45% and 39% respectively, whereas the 14-OH clarithromycin AUC and C max decreased 24% and 34% respectively, compared to administration with clarithromycin alone. No dose adjustment of clarithromycin is necessary when clarithromycin is co‑administered with saquinavir in patients with normal renal function. When saquinavir is coadministered with ritonavir, consideration should be given to the potential effects of ritonavir on clarithromycin (refer to interaction between clarithromycin and ritonavir) (see PRECAUTIONS - Drug Interactions ). The following CYP3A based drug interactions have been observed with erythromycin products and/or with clarithromycin in postmarketing experience: Antiarrhythmics There have been postmarketing reports of torsades de pointes occurring with concurrent use of clarithromycin and quinidine or disopyramide. Electrocardiograms should be monitored for QTc prolongation during coadministration of clarithromycin with these drugs. Serum concentrations of these medications should also be monitored. Ergotamine/Dihydroergotamine Postmarketing reports indicate that coadministration of clarithromycin with ergotamine or dihydroergotamine has been associated with acute ergot toxicity characterized by vasospasm and ischemia of the extremities and other tissues including the central nervous system. Concomitant administration of clarithromycin with ergotamine or dihydroergotamine is contraindicated (see CONTRAINDICATIONS ). Triazolobenzodiazepines (Such as Triazolam and Alprazolam) and Related Benzodiazepines (Such as Midazolam) Erythromycin has been reported to decrease the clearance of triazolam and midazolam, and thus, may increase the pharmacologic effect of these benzodiazepines. There have been postmarketing reports of drug interactions and CNS effects (e.g., somnolence and confusion) with the concomitant use of clarithromycin and triazolam. Sildenafil (Viagra) Erythromycin has been reported to increase the systemic exposure (AUC) of sildenafil. A similar interaction may occur with clarithromycin; reduction of sildenafil dosage should be considered. (See Viagra package insert.) There have been spontaneous or published reports of CYP3A based interactions of erythromycin and/or clarithromycin with cyclosporine, carbamazepine, tacrolimus, alfentanil, disopyramide, rifabutin, quinidine, methylprednisolone, cilostazol, bromocriptine, vinblastine, phenobarbital and St. John’s Wort. Concomitant administration of clarithromycin with cisapride, pimozide, astemizole, or terfenadine is contraindicated (see CONTRAINDICATIONS ). In addition, there have been reports of interactions of erythromycin or clarithromycin with drugs not thought to be metabolized by CYP3A, including hexobarbital, phenytoin, and valproate.