US regulators are investigating whether two transplant rejection drugs made by Roche and Novartis, CellCept and Myfortic respectively, may be linked to a rare central nervous system disorder.

The US Food and Drug Administration said that as part of an ongoing safety review, it is looking at a potential link between CellCept (mycophenolate mofetil) and Myfortic (mycophenolate acid) and the development of progressive multifocal leukoencephalopathy. The agency said it is examining data submitted by Roche last November about cases of PML in patients who took CellCept plus other immunosuppressants.

It is also evaluating the Swiss firm's recommendation to update the drug's labelling to include information about the disease. CellCept's label in the European Union was changed earlier this year to include information about PML in patients taking the drug.

The FDA added that it has also asked Novartis to submit data about PML cases in patients receiving Myfortic and to make similar revisions to the drug's labelling. The agency concluded by saying that it may take about two months to complete its review.