* Says 6 new PML cases as of June 7th

* Total PML cases stand at 55, up from 49 as of May 6th

BOSTON (Reuters) - Six more patients taking Biogen
Idec Inc's multiple sclerosis drug Tysabri have
developed a potentially deadly brain infection known as PML,
the company said.

The Cambridge, Massachusetts-based biotechnology company
said in its latest monthly update that as of June 7th, there
were 55 confirmed cases of progressive multifocal
leukoencephalopathy, or PML, up from 49 as of May 6th.

Biogen, which markets the drug with partner Elan Corp Plc
, said the number of deaths from PML remained at 11.

Sales of Tysabri, the most important growth driver for both
Elan and Biogen, have been crimped by concerns over PML as
investors and physicians try to gauge how great the risk the
risk of contracting the disease might be over time.

Last week, a panel of advisers to the U.S. Food and Drug
Administration recommended the agency approve a rival product
from Novartis AG called Gilenia, potentially hurting
Tysabri even further.

Tysabri was temporarily withdrawn from the market in 2005
after being linked with PML, but reintroduced in 2006 with
stricter safety warnings.

The rate of PML increases with the length of time a patient
remains on the drug. The incidence of PML in patients taking
Tysabri for roughly a year is 0.1 per 1,000 patients. That rate
rises to 0.35 cases per 1,000 in patients taking the drug for
between one and two years, and to 1.47 cases for patients
taking it between two and three years.

The company says the rate of PML is still within the 1 in
1,000 level seen in its clinical trials and noted on the drug's
label.

The May 6th figure of 49 represented an increase of three
new cases compared with the 46 cases reported as of April 6th.
That in turn represented an increase of four from March 10th,
when there were 42 cases.

(Reporting by Toni Clarke; Editing by Bernard Orr)