The U.S. Food and Drug Administration announced it is reviewing data involving a link between two drugs used to prevent organ rejection and the developmental of progressive multifocal leukoencephalopathy (PML).

PML is rare but life-threatening disorder that affects the central nervous system. The disease usually occurs when a patient’s immune system is suppressed to disease or medicines. PML is caused by activation resulting from unknown factors of a polyomavirus, also known as the JC virus, which is latent in 80 percent of healthy adults.

Symptoms of PML include localized neurological signs, vision changes, loss of coordination, clumsiness, memory loss, difficulty speaking or understanding what others say and weakness in the legs.

The reviewed drugs are CellCept (mycophenolate mofetil), manufactured by Hoffman-LaRoche Inc. and Myfortic (mycophenolic acid) manufactured by Novartis Pharmaceuticals Inc. CellCept is approved to prevent heart, liver and kidney transplant rejection, while Myfortic is approved to prevent kidney transplant.

According to FDA representative Christopher Kelly, the agency has received 16 reports of PML in CellCept patients since 1995 including some deaths. The agency is currently checking its database to see if there were additional cases, Kelly added.

A representative for Novartis said that the company was not aware of any cases of the disease associated with the drug.

On the other hand, Roche representative Christopher Vancheri said the company had 10 confirmed PML cases and 6 cases of possible PML.

The FDA said it may take about 2 months to complete its review and as soon as it is finished, the agency will communicate the conclusions and recommendations to the public.

“Until further information is available, patients and healthcare professionals should be aware of the possibility of PML, such as localized neurologic signs and symptoms in the setting of a suppressed immune system, including during therapy with CellCept and Myfortic,” the FDA said on its Web site.

However, patients shouldn’t change the way they use either medication, during the ongoing review, the FDA added.

Both drugs received a black box warning last fall, warning about possible first-trimester miscarriage and congenital malformations. The ongoing review could also bring new warning information on the packages.