Biogen Idec says that, following the withdrawal of its multiple sclerosis drug, Tysabri (natalizumab), after it was linked to two cases of the rare central nervous system disease, progressive multifocal leukoencephalopathy, in patients taking the drug together with Avonex (interferon beta-1a) [[01/03/05a]], [[04/03/05a]], the US Food and Drug Administration has analysed its post-marketing safety database and said it had found no cases of PML reported in patients receiving a beta interferon.

The firm says that the findings support its own ongoing safety reviews, which revealed no previously reported cases of PML in MS patients treated with interferon beta monotherapy.

The company says that it still remains unclear why PML developed in the two patients treated with this combination therapy, but confirmed that, together with partner Elan, it was continuing to evaluate the problem and that the firms were working to complete their review as quickly as possible.