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Synopsis: Clarithromycin is a broad spectrum macrolide antibacterial agent active in vitro and effective in vivo against the major pathogens responsible for respiratory tract infections in immunocompetent patients. It is highly active in vitro against pathogens causing atypical pneumonia (Chlamydia pneumoniae, Mycoplasma pneumoniae and Legionella spp.) and has similar activity to other macrolides against Staphylococcus aureus, Streptococcus pyogenes, Moraxella catarrhalis and Streptococcus pneumoniae. Haemophilus influenzae is susceptible or intermediately susceptible to clarithromycin alone, but activity is enhanced when the parent drug and metabolite are combined in vitro.

Absorption of clarithromycin is unaffected by food. More than half of an oral dose is systemically available as the parent drug and the active 14-hydroxy metabolite. Pharmacokinetics are nonlinear, with plasma concentrations increasing in more than proportion to the dosage. First-pass metabolism results in the rapid appearance of the active metabolite 14-hydroxy-clarithromycin in plasma. Clarithromycin and its active metabolite are found in greater concentrations in the tissues and fluids of the respiratory tract than in plasma. Dosage adjustments are required for patients with severe renal failure, but not for elderly patients or those with hepatic impairment. Drug interactions related to the cytochrome P450 system may occur with clarithromycin use.

In addition to the standard immediate-release formulation for administration twice daily, a modified-release formulation of clarithromycin is now available for use once daily.

In dosages of 500 to 1000 mg/day for 5 to 14 days, clarithromycin was as effective in the treatment of community-acquired upper and lower respiratory tract infections in hospital and community settings as [beta]-lactam agents (with or without a [beta]-lactamase inhibitor), cephalosporins and most other macrolides. Clarithromycin was similar in efficacy to azithromycin in comparative studies and is as effective as and better tolerated than erythromycin. Adverse events are primarily gastrointestinal in nature, but result in fewer withdrawals from therapy than are seen with erythromycin.

Clarithromycin provides similar clinical and bacteriological efficacy to that seen with [beta]-lactam agents, cephalosporins and other macrolides. It offers a costsaving alternative to intravenous erythromycin use in US hospitals and is available in both once-daily and twice-daily formulations. The spectrum of activity of clarithromycin against common and emerging respiratory tract pathogens may make it suitable for use in the community as empirical therapy of respiratory tract infections in both children and adults.

Antibacterial Activity: The activity of clarithromycin against most respiratory pathogens in vitro is similar to that of other macrolides. Streptococcus pyogenes isolates showed excellent susceptibility to clarithromycin. Clarithromycin is very active against pathogens causing atypical pneumonia (Mycoplasma pneumoniae, Chlamydia pneumoniae and Legionella pneumophila) and against Moraxella catarrhalis. The only characterised resistance pattern results in cross-resistance between macrolides, as seen in strains of Staphylococcus aureus and Streptococcus pyogenes. Isolates that are susceptible to erythromycin or penicillin are also susceptible to clarithromycin.

Haemophilus influenzae is susceptible or intermediately susceptible to clarithromycin alone. The combination of the parent drug and metabolite shows at least additive if not synergistic activity, such that Haemophilus influenzae is susceptible in vitro to the combination. 14-Hydroxy-clarithromycin was also active against L. pneumophila isolates.

In vitro studies in blood show that clarithromycin is active against bacteria both intra- and extracellularly. Postantibiotic effects may occur with some species and bactericidal effects are seen with H. influenzae, Streptococcus pneumoniae, and possibly M. catarrhalis. In addition to its antimicrobial effects, clarithromycin appears to improve immune function and improves the viscoelastic properties of mucus and sputum.

Pharmacokinetic Properties: Absorption of clarithromycin is unaffected by food, oral bioavailability of the parent drug is 52 to 55% and first-pass metabolism produces the active 14-hydroxy metabolite. Pharmacokinetics are nonlinear as a result of hepatic elimination, with capacity-limited formation of the active 14-hydroxy metabolite occurring at doses >600mg. Total body clearance decreases and the elimination half-life increases with increasing dose, whereas plasma concentrations and the area under the plasma concentration-time curve (AUC) increase more than proportionately with increases in the dose. However, once steady-state is reached, no accumulation occurs with additional doses administered.

Clarithromycin is not extensively protein bound but has a volume of distribution of 191 to 306L. Concentrations of the drug in respiratory tract tissues and fluids greatly exceed those in plasma. Furthermore, tissue concentrations of both clarithromycin and 14-hydroxy-clarithromycin exceed the MICs for most respiratory pathogens. About 40% of the drug is removed from the body in the faeces and [almost equal to]53% in the urine.

Reductions in clarithromycin clearance seen in elderly patients and those with hepatic disease were well tolerated and do not appear to require dosage adjustments. In patients with renal failure, however, plasma concentrations and AUC values of the parent drug and active metabolite increase markedly, necessitating dosage reductions in patients with creatinine clearance (CLCR) values of <=30 ml/min (<=1.8 L/h). Clarithromycin is found in breastmilk of nursing mothers treated with the drug.

A modified-release formulation of clarithromycin has been developed to allow administration of the drug once daily. This formulation delivers the same peak and trough concentrations of the parent drug and metabolite and reaches equivalent AUC values to those seen with the twice-daily immediate-release formulation in the 24 hours after administration. The elimination half-life of clarithromycin and its 14-hydroxy metabolite are unaltered by formulation, although peak plasma concentrations are delayed with the once-daily dosage form.

Clinical Efficacy: Results of comparative trials in adults have shown similar efficacy for clarithromycin and other antibacterial drugs in the treatment of community-acquired pneumonia, acute bronchitis, acute exacerbations of chronic bronchitis (AECB), sinusitis, pharyngitis and otitis media. Comparator agents included the [beta]-lactam agents ampicillin, amoxicillin with or without clavulanic acid, phenoxymethyl-penicillin (penicillin V), the cephalosporins cefaclor, cefuroxime axetil, cefpodoxime proxetil, ceftibuten and cefixime, and the macrolides erythromycin, azithromycin, dirithromycin, roxithromycin and josamycin.

Clarithromycin 250mg twice daily for 5 days was as effective in the treatment of acute bronchitis as a 10-day course of the same daily dosage. No statistically significant differences in efficacy were apparent between smoking and nonsmoking patients or between elderly (>=65 years) and younger patients receiving clarithromycin. In hospitalised patients with community-acquired pneumonia who were also receiving intravenous cefuroxime, oral clarithromycin was as effective as intravenous erythromycin, and intravenous clarithromycin followed by oral clarithromycin was as effective as intravenous followed by oral erythromycin.

In children aged 5 months to 16 years, comparative trials have shown generally similar efficacy for clarithromycin and other antibacterial drugs in the treatment of pneumonia, acute bronchitis, pharyngitis and otitis media. Comparator agents with similar efficacy included amoxicillin, azithromycin, erythromycin, cefaclor, cefixime, clindamycin, cefadroxil and benzathine penicillin. A recent noncomparative Japanese study has shown clarithromycin to be an effective treatment of sinusitis associated with otitis media in children. Clarithromycin tended to produce better clinical success and bacteriological eradication than amoxicillin/clavulanic acid in patients with streptococcal pharyngitis and acute bronchitis and produced similar clinical efficacy but superior bacterial eradication to phenoxymethylpenicillin in patients with streptococcal pharyngitis.

Tolerability: Clarithromycin is generally well tolerated, with most adverse events being mild to moderate. The most common events were diarrhoea, abnormal taste, nausea, dyspepsia, headache and abdominal pain or discomfort, leading to treatment withdrawal rate of <3%. A similar pattern but slightly higher incidence of events occurs in children, but leads to treatment discontinuation in <2% of patients.

Fewer digestive tract adverse events and withdrawals from treatment occur with clarithromycin than erythromycin or amoxicillin/clavulanic acid.

Drug Interactions: Clarithromycin inhibits the cytochrome P450 3A enzyme (CYP3A) and its metabolism is affected by P450 enzyme induction. Thus, interactions increasing plasma concentrations and possible adverse effects of theophylline, cyclosporin, tacrolimus, carbamazepine, digoxin and rifabutin may necessitate therapeutic drug monitoring and dosage adjustments. Both rifampicin and rifabutin may induce clarithromycin metabolism. Clarithromycin may inhibit metabolism of omeprazole and omeprazole may inhibit the metabolism and/or increase the absorption of clarithromycin. Concomitant clarithromycin may also interact with zidovudine, ritonavir, methylprednisolone and loratidine, and can reduce the tolerability of fluoxetine, nitrazepam, ergotamine and possibly terfenadine and oral anticoagulants.

Pharmacoeconomic Considerations: Variations in drug acquisition costs and antimicrobial susceptibility patterns for clarithromycin relative to other alternative therapeutic agents mean that local conditions must always be considered when assessing costs of clarithromycin use. However, in US hospitals, switching patients from intravenous erythromycin or cefuroxime to oral clarithromycin appears to reduce costs without affecting outcomes.

Dosage and Administration: Clarithromycin is usually administered orally, with or without food. Adult dosages for pharyngitis and tonsillitis (10-day courses) and for pneumonia or AECB (7- to 14-day courses) are 500 mg/day (as 1 or 2 doses, depending on the formulation used). AECB caused by H. influenzae (7- to 14-day course) and acute maxillary sinusitis (14-day course) are treated with 1000 mg/day. Paediatric dosages are usually 15 mg/kg/day for 10 days. Dosage adjustments are not usually needed in elderly patients or those with hepatic disease. Dose reductions or prolonged dosage intervals should be used in patients with renal failure (CLCR <=30 ml/min or <=1.8 L/h), in whom the use of the once-daily modified-release formulation should be avoided. The safety of clarithromycin has not been established in children <6 months old and the drug should be avoided during pregnancy and in patients with previous hypersensitivity to macrolide drugs. Breastmilk from mothers receiving clarithromycin should not be given to infants until treatment is completed.

Copyright 1997 Adis International