Increasing the time between Tysabri doses reduces chance of PML

February 9, 2018


Natalizumab (Tysabri) is a disease-modifying drug currently given by intravenous infusion every 4 weeks and is commonly prescribed for the treatment of very active MS. It is quite effective at reducing relapse rates by around two-thirds and there is some evidence that it slows progression to disability. However, its use must be balanced against the risk of a rare but potentially deadly brain infection, progressive multifocal leukoencephalopathy (PML).

PML is thought to occur as a result of a mutation of the John Cunningham Virus (JCV), which is carried harmlessly in around half to three-quarters of the population. However, when the immune system is suppressed, as in the case of Tysabri, the virus can become uncontrolled and lead to PML. If diagnosed, there is around a 1 in 4 chance of death or serious disability.

As of January 2018, there have been 756 reported cases of PML, with a global incidence of 4.19 per 1,000 people treated with Tysabri. So this is a real problem.

If a patient with MS tests positive for JCV, they are usually either advised not to use the drug at all, or have treatment stopped after 2 years, when the risk of PML becomes too high. For those who test negative for JCV, the risk is much lower, at around 1 in 10,000.

When a person with MS stops Tysabri however, there is a risk of reactivation of their disease (immune reconstitution inflammatory syndrome – IRIS) to the same level as before starting the therapy. There is also a 1 in 10 chance of rebound in disease activity to a level worse than when they started.

This makes for very difficult discussions and decisions for neurologists and people with MS. They have to weigh up the positive effects of the drug in terms of relapse reduction and preventing disability, against the possibility of potentially deadly brain infection or worsening of their disease.

Positive news

There is some recent positive news though. Researchers from NYU Langone Health recently presented a study at the Americas Committee for Treatment and Research in Multiple Sclerosis (ACTRIMS) Forum 2018 in San Diego. They showed that, if the interval between doses was extended to 5 – 12 weeks, over 6 years the risk of PML was reduced by nearly 100%.

Whilst their study didn’t assess whether the drug worked as well when the dosing was extended, previous research has shown that there was no negative impact on effectiveness when doses were 8 weeks apart, looking back at the records of 2,000 people.

The team are now planning to prospectively test extended dosing, with the aim that many more people with MS can remain on it in the longer term. This research is very reassuring for those taking or considering starting Tysabri, and certainly warrants detailed discussion with one’s neurologist.

Dr Jonathan White

Read more on Tysabri & PML:


  2. Jelinek, G; Overcoming Multiple Sclerosis; 2nd Edition; A&U; pp283-283)

11 thoughts on ‘Increasing the time between Tysabri doses reduces chance of PML

  1. I was on Copaxone, for 2 1/2 years, my Neuro wanted me to go onto Tysabri, so with a lot of thought, decided to try it, and for a year at least, but at 9 months with no change at all in my physical state, plus diagnosed with JC virus at a small scale, decided to come off it. I have gone back to Copaxone, which my Neuro is happy for me to do, but have to see what is happening on my next MRI when i have that done in June, to see if theres any change in lesions, which I haven’t had any change in 10 years!!!!!!

  2. Hi every body, I am diagnosed with MS in 2005 and have been ok since my last relapse in 2005. Untill two weeks ago:( I am going to see my neurologist next week and potentially going on a medication but I am not sure.
    The three medicidines I might go on are Tysabri, Gilenya CMI and Tecfidera ! I appreciate your comments

  3. I have a friend who has been on it for years. She does not have PML yet. I wouldn’t try anything that could give me yet another problem in life, it scares me. If you are doing fine great.

  4. Hello, I was diagnosed 7 weeks ago, Neuro talked to me about trating me with this medicine, Im super scared to start, im JCV negative, and I have 2 legions on my brain none on my spine. She doesnt think its aggressive. I am very scared to start!!!

  5. Hey Debra, completely understand your fear. It’s a horrible disease that requires a lot of tough choices. I was diagnosed four years ago. Started with tecfidera but was still progressing and losing my hair. Then I tried Copaxone. As a RN the injections didn’t bother me, but I hated the way I felt and again.. I still progressed. I’ve been doing Tysabri for 19 months and I LOVE it. I feel so great after my infusion and can feel when I’m due for my next round. For me, it’s a no-brainer. My neuro watches my JCV status closely and I pray I never have to stop this medication.
    Bottom line, find what works best for you! Be your own advocate and insist on answers.
    Lots of love, Kris

  6. been on tysabri for almost 2 years, i am jc positive and am really worried about pml, anyone shed any light on the statistics in plain english?

  7. I am jc positive and have been getting tysabri infusions for 18 months. My jc count is very high and my neuro has put me on a six week cycle instead of a r week cycle. I have noticed more fatigue and a little more stiffness.

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