An ongoing study into natalizumab (Tysabri) dosage in MS patients has revealed that the dosage interval could be doubled from every four weeks to eight weeks, without any negative effect on the efficacy of the treatment. And critically, reducing the overall exposure to the drug potentially reduces the risk of developing PML, even in those patients with a higher-than-average PML risk.
Progressive multifocal leukoencephalopathy (PML) is a rare and untreatable viral infection of the brain which is associated with the use of Tysabri. The risk of PML increases with years of exposure to Tysabri, currently most noticeable in patients receiving the treatment for over two years.
Among patients who test positive for the JC virus, which is the actual causative agent, between 0.5 to 1% will develop PML, and of this group 20% of cases will be fatal. PML is the biggest deterrent to using Tysabri, which is an otherwise highly effective and safe drug in the treatment of relapsing-remitting multiple sclerosis. Of roughly 2000 patients participating in the study, half received Tysabri according to the label recommendation of every 28 days, and the other half were given the drug at intervals ranging from 31 to 61 days. The initial results showed a similar, or better, efficacy of the drug in those on the extended dosing interval with no cases of PML recorded at all, compared to four cases amongst those on the 28-day schedule.
Lana Zhovtis-Ryerson, MD, of NYU Langone Medical Center in New York City, who presented these findings at ECTRIMS in Barcelona last week, highlighted however that the data is currently insufficient to fully support the findings on PML risk at this stage, and that it could take 18 months (in order to accrue the correct number of patient-years of exposure in both groups) before the differences can be definitely defined. But this is potentially great news for people currently taking Tysabri, and may make Tysabri a much safer treatment option for people with very active MS.