Tysabri (natalizumab) is a medication primarily used to treat multiple sclerosis (MS) and Crohn’s disease by modulating the immune system. While it is effective in reducing MS relapses and slowing disease progression, it carries a serious risk of causing brain infections, most notably a rare but often fatal condition called progressive multifocal leukoencephalopathy (PML).
PML is a severe brain infection caused by the reactivation of the John Cunningham virus (JC virus), which is normally kept in check by a healthy immune system. Tysabri works by blocking immune cells from crossing the blood-brain barrier, which helps reduce inflammation in MS but also impairs the immune system’s ability to surveil and control latent infections in the brain. This immune suppression can allow the JC virus to reactivate and damage the white matter of the brain, leading to PML.
The risk of developing PML while on Tysabri depends on several factors:
– **Presence of JC virus antibodies:** Patients who have been exposed to the JC virus (which is common in the general population) and test positive for antibodies are at higher risk.
– **Duration of Tysabri treatment:** The risk increases significantly after about two years of continuous therapy.
– **Prior immunosuppressant use:** Patients who have used other immunosuppressive drugs before starting Tysabri have an elevated risk.
– **Immune system status:** The degree of immune suppression influences susceptibility.
Symptoms of PML can include progressive weakness, vision problems, speech difficulties, cognitive decline, and seizures. Because these symptoms can overlap with MS progression, diagnosis requires careful neurological evaluation and brain imaging, often confirmed by detecting JC virus DNA in cerebrospinal fluid.
Besides PML, Tysabri may increase the risk of other infections, including meningitis and encephalitis caused by herpes viruses or other opportunistic pathogens, due to its immune-modulating effects. These infections, while less common than PML, can also be serious and require prompt medical attention.
Doctors carefully weigh the benefits and risks before prescribing Tysabri and monitor patients closely during treatment. Regular testing for JC virus antibodies and neurological assessments are standard to detect early signs of brain infections. If PML or other serious infections are suspected, Tysabri is discontinued immediately to allow immune recovery.
In summary, Tysabri can cause brain infections, with PML being the most significant and feared complication. This risk arises from the drug’s mechanism of suppressing immune surveillance in the brain, allowing latent viruses to reactivate. Vigilant monitoring and risk assessment are essential parts of managing patients on Tysabri to minimize the chance of these devastating infections.
