Progressive multifocal leukoencephalopathy (PML) is the most feared serious adverse effect of use of Tysabri for relapsing MS. The medication is given by infusion every 4 weeks, with that interval determined by studies showing that 80% of the receptors continued to be blocked at that point. At 12 weeks, the therapeutic effect is reduced to 50% and at 6 months almost to zero. When Tysabri is stopped, there may be clinical and MRI worsening (rebound), but this usually starts at 10 to 12 weeks.

Several reports indicate that women who weigh less than 160 pounds may safely be dosed every 6 weeks with no loss of efficacy of the medication. A new study did not limit patients by weight. About 1000 patients who received standard dose Tysabri were compared with about another 1000 who received extended interval dose therapy, usually 6 to 8 weeks and as long as 8-1/2 weeks. No patient in the extended dose arm had a relapse and none developed PML. On the other hand, the standard dosing group had 4 PML cases during the study.

This strategy may allow continued dosing of a very effective medication for relapsing MS.

This study was presented at the 2015 ECTRIMS Meeting. Lead author was Zhovtis Ryerson, MD.

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