(Corrects 7th paragraph to say 0.01 per 1,000, not 0.1)
* Says 6 new PML cases as of June 7th
* Total PML cases stand at 55, up from 49 as of May 6th
BOSTON, June 17 (Reuters) - Six more patients taking Biogen Idec Inc's (BIIB.O) 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 ELN.I, 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 (NOVN.VX) 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Tysabri was temporarily withdrawn from the market in 2005 after being linked with PML, but reintroduced in 2006 with stricter safety warnings.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 0.01 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.