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.
Mycobutin (Pharmacia)
150 mg capsules
Indication: specified infections
Rifabutin is related to rifampicin and therefore has activity against mycobacteria. It has been approved for use in cases of pulmonary tuberculosis where the Mycobacterium tuberculosis strains are resistant to rifampicin. Rifabutin can also be used in the treatment of Mycobacterium avium-intracellulare complex (MAC)and other atypical mycobacteria. MAC is found in up to 50% of AIDS patients at autopsy, so rifabutin has also been approved for prophylaxis in patients with advanced HIV infection who have less than 200 CD4 cells/ micro litre.
Rifabutin is rapidly absorbed from the gut, but the absorption is incomplete and the bioavailability is less than 20%. The daily dose can be given at anytime, independently of meals. After absorption, the drug is bound to plasma proteins and distributed widely with concentrations in the lung exceeding plasma concentration. Although rifabutin is metabolised with only 4% of the dose appearing as unchanged drug in the urine, it is recommended that the dose should be halved in patients with severe renal impairment. Rifabutin has a half-life of 35-40hours.
Most of the patients studied in trials of rifabutin have also been taking other drugs, so attributing adverse reactions can be difficult. Rifabutin maybe associated with arthralgia, myalgia, uveitis, changes in liver function, nausea and vomiting. Like rifampicin, rifabutin can cause an orange-red discolouration of the urine and body secretions. Contact lenses may become permanently stained. The frequency of adverse haematological reactions, such as leucopenia, maybe increased by concurrent treatment with isoniazid. Full blood counts and liver function tests should be monitored during long-term multi drug treatments. Rifabutin induces the cytochrome P450 enzymes, so there is a potential for interactions with drugs metabolised by that system.
