ZURICH: A person in Germany treated with Roche's new multiple sclerosis drug Ocrevus has been diagnosed with an often-deadly brain infection after switching from another medication earlier this year, the Swiss drugmaker said on Wednesday.

Roche said it was investigating a case of Progressive Multifocal Leukoencephalopathy (PML) in a patient previously being treated for three years with Biogen Inc's Tysabri and who had received a single dose of Ocrevus in February.

Ocrevus was approved in the United States in March.

Roche said the case of the rare brain disease that is usually fatal or disabling was reported as a carry-over from Tysabri, also known as natalizumab, by the physician who had been treating the patient.

Biogen declined to make any immediate comment.

Ocrevus is an integral part of Roche's strategy to replace revenue from older blockbuster cancer medicines Rituxan, Avastin and Herceptin whose patents are expiring, leaving them increasingly exposed to cheaper copies, called biosimilars.

"Patient safety is Roche's highest priority and we are gathering more details about the case and the patient's history," Roche said. "We will continue to share information with healthcare providers and global health authorities as we know more."

The patient who developed PML had previously tested positive for John Cunningham virus, a common virus that is normally harmless but which can lead to the brain disease in MS patients who have weakened immune systems.

Dozens of patients undergoing treatment with Tysabri have been diagnosed with PML. Use of the drug was temporarily halted a decade ago, but resumed after patients for whom the medication was effective demanded its return and regulators added restrictions to its use.

Roche has said no PML cases emerged during its trials of Ocrevus, but the company included warnings to patients taking the medication that there was a risk they could get the disease. (Reporting by John Miller; Editing by Edmund Blair, Greg Mahlich)