Tysabri Risk Redux: Additional PML Cases Create REMS Challenge For FDA

The emergence of two new cases of progressive multifocal leukoencephalopathy linked to Biogen Idec/Elan's Tysabri (natalizumab) comes just before the deadline for the firms to submit a formal Risk Evaluation and Mitigation Strategy for the product, and while FDA is still experimenting with how to apply its new REMS authorities