The threat of emerging bacterial resistance has resulted in adjustments to standard treatment regimens and the exploration of methods to improve patient compliance using established therapy. Azithromycin has the flexibility of once-daily dosing and effectiveness against many pathogens when administered as a large, single-dose treatment.
Azithromycin is a macrolide antibiotic and interferes with microbial protein synthesis. At its lower dosage range, the antibiotic effect is bacteriostatic; bactericidal activity is possible against Streptococcus pyogenes, S pneumoniae, and Haemophilus influenzae. The antibiotic concentrates on phagocytes, a capacity necessary for its action against intracellular pathogens.
The bioavailability of azithromycin following oral administration is about 40%, with the coadministration of food increasing the maximum plasma concentration, but not the overall oral absorption.
Streptococci and staphylococci exhibit a cross-resistance to azithromycin when resistance to erythromycin has been established. Whereas amoxicillin remains the treatment of choice for acute otitis media, azithromycin is recommended as an alternative for patients with true penicillin allergy.
Administration and Dosing
Specific indications for the oral tablet forms of azithromycin include acute bacterial infections associated with chronic obstructive pulmonary disease, acute bacterial sinusitis, community- acquired pneumonia (CAP), pharyngitis, tonsillitis, uncomplicated infections of the skin, urethritis, cervicitis, and genital ulcer disease in men. The 600-mg dosage form is additionally indicated for the prophylaxis or treatment of Mycobacterium avium complex infections. For pediatric patients, the tablets have been approved to treat otitis media, CAP, tonsillitis, and pharyngitis.
Dosages for azithromycin vary with the infection being treated. For pharyngitis, chronic bronchitis, skin infections, and CAP, 500 mg is given on the first day, followed by daily doses of 250 mg for a total of 5 days. Acute sinusitis responds to 500 mg daily for 3 days. Uncomplicated chlamydial infections are treated with a single 1-g dose, and gonorrhea therapy involves a single 2-g treatment.
Side Effects and Drug Interactions
Like most macrolide antibiotics, the use of azithromycin is associated with diarrhea, nausea, vomiting, headache, and pruritus. These effects are observed in up to 6% of patients.
More severe, but comparatively rare, are the potential for cholestatic jaundice, angioedema, Stevens-Johnson syndrome, and nephritis. Although hepatic or renal impairment could interfere with the metabolism and elimination of azithromycin, there are no established protocols for dosing adjustments in these situations. As with many antibiotics, the possibility of superinfections and consequential pseudomembranous colitis should be considered.
Many drug interactions commonly associated with macrolide antibiotics (especially clarithromycin or erythromycin) do not appear to be a problem with azithromycin. Apparently, many of the macrolide-based interactions do not emerge with azithromycin since this antibiotic does not induce or activate cytochrome P- 450 enzymes. Nevertheless, the manufacturers suggest using extra care with concurrent dosing of digoxin, ergotamine, and phenytoin.
A specific drug-drug contraindication remains with concomitant use of pimozide (Orap), due to potential prolongation of cardiac QT intervals.
Oral azithromycin should not be administered with aluminum-or magnesium-containing antacids, since this will reduce peak antibiotic concentrations by up to 24%.
Of the 3-hydroxy-3-methyl-glutaryl-CoA agents used to treat dyslipidemia, lovastatin (Mevacor) appears the most problematic. This problem is based on a single report of a patient receiving long-term therapy with lovastatin, who then received azithromycin daily for 5 days. The rhabdomyolysis that emerged in this instance appeared to be related to this combination of therapies.
Since its introduction in 1991, azithromycin has become among the most commonly prescribed antibiotics in the United States. It continues to be an effective antibiotic despite its frequent use. Reexamination of its standard 5-day course of therapy has created new dosage regimens and an expansion of antimicrobial indications. Its availability in generic form will no doubt increase its widespread utilization, with treatment cost becoming less of a consideration.
The FDA has approved generic 250-, 500-, and 600-mg tablet versions of azithromycin manufactured by Teva Pharmaceutical Industries Ltd and Sandoz Inc.
Mr. Middleton is an instructor of pharmacology for Kellogg Community College in Battle Creek, Mich.