Understanding PML, JCV, and the potential risks

What is PML?

  • Progressive multifocal leukoencephalopathy (PML) is a rare brain infection that usually leads to death or severe disability
  • There is no known treatment, prevention, or cure for PML

JCV is something you may or may not have in your body

  • Typically, people with JCV are exposed to it during childhood
  • JCV is common and is harmless in most people, but can cause PML in people who have weakened immune systems, such as people taking TYSABRI
  • Your doctor may do a blood test to check if you have antibodies to JCV before you start receiving TYSABRI or during your treatment

A standard blood test can determine if you have antibodies to JCV

  • The JCV antibody test shows if you have developed antibodies to JCV, which is a natural response from your immune system
    • If the test is positive for JCV antibodies, your risk of developing PML is increased

People who are JCV negative should be retested periodically

  • It's possible to come into contact with JCV and have it enter your body after being tested
  • There's a possibility of a false-negative result. Even if you test negative for JCV antibodies, you could still be at risk of developing PML*

*There's a possibility (3%) of a false-negative test result, which occurs when the blood test wrongly indicates JCV antibodies have not been detected.

A closer look at the estimated risk of PML

People who test negative for JCV antibodies are at a lower risk for developing PML. No one can predict who will get PML, but we can estimate the risk based on 3 known factors.

  • JCV antibody positive test result
  • Prior use of certain medicines that can weaken your immune system
  • Length of time taking TYSABRI
    • Especially for more than 2 years

Your risk of getting PML is greatest if you have all 3 risk factors listed above. There may be other risk factors for getting PML during TYSABRI treatment that we do not know about yet.

Even if you test positive for JCV antibodies, you and your doctor can decide if TYSABRI is right for you.

Your chance of getting PML increases if you have antibodies to the JCV (John Cunningham Virus)

  • The presence of JCV in people taking TYSABRI® (natalizumab) can potentially cause PML

Estimated United States Incidence of PML Stratified by Risk Factor

 antibody dosage table

*Based on US postmarketing PML data as of September 3, 2013, and TYSABRI use data as of August 31, 2013.

†Calculation based on 2 cases of anti-JCV antibody negative PML in patients exposed for at least 1 month of therapy as of September 3, 2013. Data for anti-JCV antibody negative patients reflect worldwide exposure.

‡ Data beyond 6 years of treatment are limited.


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TYSABRI® (natalizumab) is a prescription medicine used to treat adults with relapsing forms of Multiple Sclerosis (MS). TYSABRI can slow the worsening of symptoms common in people with MS and can decrease the number of flare-ups (relapses). TYSABRI increases the risk of PML. When starting and continuing treatment with TYSABRI, it is important that you discuss with your doctor whether the expected benefit of TYSABRI is enough to outweigh this risk.

Important Safety Information

Please see Full Prescribing Information (PDF) including Boxed Warning and Patient Medication Guide (PDF).

This information is not intended to replace discussions with your healthcare provider.