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.

Avonex (CSL)
vials containing 30 microgram as lyophilised powder
Approved indication: relapsing multiple sclerosis
Australian Medicines Handbook Section 14.2.2

Autoimmune mechanisms have an important role in multiple sclerosis. This led to the approval in 1995 of interferon beta-1b for the treatment of the relapsing-remitting type of multiple sclerosis. The structurally similar interferon beta-1a has now been approved for the same indication. These interferons are cytokines which act by altering the immune response, but their exact mechanism of action in multiple sclerosis is unknown.

The main trial of interferon beta-1a involved 158 patients who were compared with 143 patients who were given a placebo. The patients were given weekly injections for two years. At the end of the study, there had been fewer exacerbations in the treated group. There was evidence that the progression of disability was slower in the treated group. Disability increased in 22% of the treated group and 35% of the placebo group. Magnetic resonance imaging showed fewer lesions in the brains of patients given interferon beta-1a.1

The half-life of interferon beta-1a is 10 hours. As its biological effects last much longer, it only has to be injected weekly. After instruction, able patients can give their own intramuscular injections.

The manufacturer suggests that paracetamol be given before the injection and for the next day. This is to counteract the flu-like symptoms which occur in most patients. In addition to fever, chills and muscle aches, the risk of developing anaemia is also significantly greater for patients using the drug than for those given a placebo. Only 4% of the patients taking interferonbeta-1a in clinical trials had to withdraw because of adverse effects. Safety and effectiveness have not been established beyond two years of therapy.

One of the problems with interferon beta-1b is the development of antibodies which may reduce its effectiveness. After two years, 22% of the patients given interferon beta-1a had developed antibodies, but the effect on efficacy is unclear.

When interferon beta-1b was marketed, there was no evidence of its effect on disability, whereas interferon beta-1a has been shown to have some effect. The clinical importance of reducing exacerbations from 0.9 per year to 0.6 per year will also have to be weighed against treatment costs. Overseas information suggests one month's treatment with interferon beta-1ais likely to be cheaper than with interferon beta-1b. When deciding which interferon to recommend, Australian prescribers will also have to consider that the recombinant interferon beta-1a available here differs from that used in the clinical trials.