US label change could boost Tysabri growth

Biogen Idec and Elan's multiple sclerosis drug Tysabri (natalizumab) has had its labelling updated to allow the use of a new companion diagnostic, which could lead to a dramatic increase in prescriptions of the drug.

The diagnostic can screen patients for an increased risk of developing a condition called progressive multifocal leukoencephalopathy (PML), which first emerged as a serious side effect with the drug in 2005.

Tysabri was taken off the market briefly after the risk of PML was identified, but reintroduced in 2006 on the grounds that its benefits outweighed the risks in some MS patients, for example those who have not responded to or cannot tolerate other therapies.

The Stratify JCV kit from Quest Diagnostics tests patients for JC virus, an infection which is invariably found in patients who go on to develop PML on Tysabri therapy. The assay was launched in the US in August and in the EU in May 2011.

In Europe, the EMA approved a change to Tysabri's labelling to allow use of the diagnostic the following June.

However, the FDA extended its review period by three months in order to give it more time to evaluate the Risk Evaluation and Mitigation Strategy (REMS) programme linked to the new labelling.

Identifying which patients are JCV-negative will dramatically increase the potential patient population eligible for treatment with the drug, and some analysts have suggested this could lead to a doubling in usage. Elan reported Tysabri revenues approached $400m in the third quarter of 2011.

On the other hand the test could also restrict Tysabri's usage in JC virus-positive patients who might otherwise have received treatment with it, but given the low rate of PML it is expected that any negative pressure on sales would be slight.