Biogen Idec have revealed yet another new case of progressive multifocal leukoencephalopathy in a multiple sclerosis patient being treated with Tysabri.

That means that six patients have developed the potentially deadly brain infection since Tysabri (natalizumab), which is partnered with Ireland’s Elan Corp, was reintroduced to the market in July 2006. The case was confirmed late on Friday, just after Biogen had presented first-quarter results which showed that Tysabri brought in $165 million, up 44%.

In the latest case, the patient had been on the drug for 31 months, the longest duration so far. The previous high had been 26 months and the average now is 19 months.

The reaction to the case from analysts was somewhat mixed. Christopher Raymond, an analyst at Robert W Baird, issued a research note saying that “while we applaud management's decision to update PML cases weekly, we think it's fair to ponder why this wasn't fully discussed on the earnings call”.

Ian Hunter at Irish broker Goodbody however, said “we do not believe that this sixth case will have much impact on physician/ patient sentiment on the drug, where efficacy greatly outweighs risk”. He noted that the US Food and Drug Administration set a risk limit of one PML case per 1,000 patients and currently, there have been six cases among the 39,300 commercial patients on the drug as of March.

Biogen added that 6,800 patients have been on Tysabri over 24 months, 14,400 over 18 months and 24,900 at least one year.