RBC Capital maintained an Outperform rating on Biogen (NASDAQ: BIIB) with a price target of $375. Analyst Michael J. Yee commented on today's decline, linking it to light Tecfidera results. He also tied the fall in part to the disclosure of a PML case for Tecfidera. After 4.5 years of use, he said the case wasn't surprising, and his thesis hasn't changed.

"Today, Biogen disclosed the first case of PML for a patient on Tecfidera who had been on the drug for 4.5 years, since clinical trials. While obviously not a positive –it’s also not that surprising: 1) BIIB has said there could be risk of PML (albeit more because of patients who were coming from Tysabri and this case wasn’t from prior Tysabri), 2) there were a few cases of PML already known in the history and have been discussed many times from use of the old version of the drug (Fumaderm) in Europe and this was widely known/discussed during the regulatory process where FDA fully approved the drug without an FDA panel," said Yee.

"So again not too surprised there is a PML case after 4-5 years of therapy, severe lymphopenia, and continuing to stay on the drug……and the fact that 100k patients have been on the drug (mostly less than 2 years since the drug was approved <2 years ago). We think “RISK/BENEFIT” is clearly still in favor of using Tecfidera for nearly all patients. If Tysabri is on $2B run-rate with hundreds of PML cases and only goes after half the market – this seems obvious that the risk/benefit for MS patients is this won’t change thinking much," he continued.

"More facts/history of what we know so far: Patient did not have prior Tysabri. JCV status was not disclosed. BIIB plans to discuss with the FDA but we don’t think any major changes are likely and possible label change to note it and just to be more vigilant. In clinical trials, rate of lymphopenia was 2% for Tecfidera and <1% for placebo," added the analyst.

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Shares of Biogen closed at $326.77 yesterday.