Abacavir

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.

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Ziagen (Glaxo Wellcome)
300 mg tablets
20 mg/mL oral solution
Approved indication: HIV infection
Australian Medicines Handbook Section 5.3.3

Reverse transcriptase inhibitors can be used in combination with other drugs for the treatment of HIV infection. Abacavir is a nucleoside reverse transcriptase inhibitor which can be used in combination with drugs such as zidovudine and lamivudine.

The drug is taken twice a day. It is well absorbed with a bioavailability of 83%. Abacavir penetrates the cerebrospinal fluid. It is almost completely metabolised by the liver, with most of the metabolites being excreted in the urine.

The major trial of abacavir in untreated adults compared the combination ofzidovudine, lamivudine and abacavir with zidovudine and lamivudine. As the double-blind stage of the trial lasted for only 16 weeks surrogate end-points were used. After one month of treatment, the proportion of patients with low plasma concentrations of HIV increased in the patients given triple therapy. At week 16, significantly more of the patients given abacavir had undetectable concentrations of HIV RNA. The CD₄ cell count increased, but there was no significant difference between the rises in each group.

A similar study in previously treated children found that the advantage of abacavir was less clear. There are few data on the use of abacavir by previously treated adults.

Approximately 3% of patients will develop a potentially fatal hypersensitivity reaction. This usually occurs in the first six weeks of therapy and commonly presents with fever and rash. Patients may also complain of vomiting, diarrhea and itching. The drug should be stopped immediately and never restarted as this may kill the patient.

Most patients with HIV infection taking a combination of drugs will develop adverse effects. Common complaints are nausea, headache and fatigue.

While the Australian Drug Evaluation Committee has approved abacavir for use in Australia, the role of the drug in therapy, particularly for previously treated patients, will require more study.