WEDNESDAY, Jan. 27, 2016 (HealthDay News) -- People with multiple sclerosis who are treated with the drug Tysabri (natalizumab) may have up to a 10 times greater risk for a rare and potentially deadly viral infection, a new study finds.
The germ in question is the John Cunningham virus (JCV), a pathogen thought to cause a deadly brain condition known as progressive multifocal leukoencephalopathy (PML).
The link between Tysabri and PML isn't new: Numerous studies published over the past few years have shown an increase in risk for the disease in patients taking the drug.
However, even though the new study showed a link between Tysabri and JCV infection, experts stressed that the drug can be of great help to patients, who should weigh its benefits against its risks.
The new research was led by Dr. Heinz Wiendl of the University of Muenster in Germany. The findings are published in the Jan. 27 online edition of the journal Neurology: Neuroimmunology & Neuroinflammation.
The study authors explained that the virus is usually kept in check by the human immune system. However, people taking drugs that compromise the immune system, such as Tysabri, appear to be more vulnerable to JCV. The researchers believe that Tysabri may prevent immune cells from reaching the brain and fighting the virus.
The new study involved 525 German and 711 French multiple sclerosis patients. All were taking Tysabri. The German patients had their blood levels of JCV antibodies monitored for 15 months, while the French patients were monitored for more than two years.
Those taking Tysabri had higher blood levels of JCV antibodies -- an indicator of exposure to the virus, the study found. As a result, the researchers concluded these patients were at greater risk for PML.
The patients from Germany transitioned from being negative for JCV antibodies to positive at an annual rate of about one in every 10 patients (10 percent). The patients in France made this transition at an annual rate of 9 percent, the findings showed.
The researchers said that these rates are significantly higher than the annual rate of 1 percent among the general population and those with MS who are not treated with Tysabri.
Meanwhile, patients who initially tested positive for JCV antibodies in their blood at the start of the study showed a rise in those levels over the course of the study. Overall, treatment with Tysabri was linked to a 13 percent yearly rise in the level of JCV antibodies, the study revealed.
"An increase in the levels of anti-JCV antibodies could signify an increased risk of PML," study author Weindl theorized in a news release from the American Academy of Neurology.
He said that it's important for people with multiple sclerosis who are taking Tysabri to "speak with their doctor before making any changes to their treatment."
In addition, he said that Tysabri "did appear to increase the levels of anti-JCV antibodies and this higher level may be associated with a higher risk of PML. The results of this study underscore the need for frequent monitoring of anti-JCV antibodies in people who are being treated with [Tysabri] for multiple sclerosis."
However, his team stressed that while the study showed a link between use of Tysabri and levels of JCV antibodies in the blood, it could not go the extra step and prove that the drug causes the virus to replicate at higher rates.
According to the author of a corresponding journal editorial, Dr. Adil Javed, from the University of Chicago, "Even though anti-JCV antibodies were present at a higher level, it does not necessarily mean that an individual will get PML."
He agreed that patients need to weigh the risks of taking the drug against the benefits.
"The risk of PML in JCV-positive people being treated for multiple sclerosis with [Tysabri] without prior immunosuppressant therapy is one in 1,000 people," Javed noted. On the other hand, "the risk of a multiple sclerosis attack in untreated patients is one in every two people."
Dr. Paul Wright is chair of neurology at North Shore University Hospital in Manhasset, N.Y. He agreed with the study authors that even though there was a 10-fold rate of conversion to JCV-positive status, "it did not necessarily imply that patients were 10-fold likely to get PML."
Tysabri "has been shown to be very effective in treating relapses of MS," added Wright, who is also head of neurology at Long Island Jewish Medical Center in New Hyde Park, N.Y. In his opinion, "patients should contact their physicians if they have any concerns or questions regarding their treatment."
Efforts by HealthDay to reach Biogen, the maker of Tysabri, for comment were not successful.
The U.S. National Library of Medicine has more about Tysabri.