Progressive Multifocal Leukoencephalopathy (PML) is a life-threatening, progressive demyelinating disease affecting the white matter tracts of the brain. It is caused by reactivation of the John Cunningham (JC) virus. Although JC virus infects 80 – 90% of the world’s population, PML is exceptionally rare and almost exclusively occurs in people who are immunosuppressed, either due to a disease such as HIV/AIDS or due to medication.
In the MS world, cases of PML have been linked to the use of natalizumab, a highly-effective disease-modifying drug which prevents entry of immune cells into the central nervous system. PML was thought to occur in around 0.5% of people treated with natalizumab. Given that there is no effective treatment for PML, it is crucial that we have a method for working out who is at risk of developing PML and who can be safely treated with natalizumab.
We knew already that the three risk factors for PML in people getting natalizumab are:
- The presence of antibodies against the JC virus (implying prior infection)
- Previous immunosuppressive treatment
- Duration of natalizumab therapy
Here, we will keep you updated on the latest PML figures and recommended action.