Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.

Zithromax (Pfizer)
250 mg capsules
Indication: specified infections

Azithromycin is structurally related to erythromycin. Although its antibiotic properties are similar to erythromycin, the structural differences give azithromycin different pharmacokinetics.

The oral bioavailability of azithromycin is 37%. Absorption is reduced if the drug is taken with food. There is a large volume of distribution with azithromycin being concentrated in the tissues. This tissue concentration results in a half life of 68 hours and excretion over several days. Most indications require only a 3-day course given as a single daily dose. Genital infections with Chlamydia trachomatis can be treated with one dose. Azithromycin inhibits bacterial protein synthesis. In vitro, it is less effective than erythromycin against Gram positive organisms, e.g. streptococci and staphylococci, but it has greater activity against some Gram negative organisms e.g. Moraxella catarrhalis, Haemophilus influenzae and parainfluenzae.

In addition to chlamydial infections, azithromycin has also been studied in respiratory tract infections. It has been approved for use in sinusitis and community acquired pneumonia due to Streptococcus pneumoniae or Haemophilus influenzae and acute bronchitis due to these organisms or Moraxella catarrhalis. Azithromycin can also be used to treat acute streptococcal pharyngitis.

Uncomplicated skin infections due to Staphylococcus aureus, Streptococcus pyogenes or Streptococcus agalactiae are also approved indications for azithromycin.

In clinical trials, adverse effects occurred in approximately 13% of patients, but less than 1% had to discontinue treatment. The most frequent adverse reactions affected the gastrointestinal system. Rare adverse effects include angioedema and cholestatic jaundice.

Azithromycin offers a more convenient treatment regimen than erythromycin and may have fewer adverse effects, but it is likely to cost much more. It is not approved for use in children.