* FDA panel to meet on Nov. 13, FDA verdict due by year-end
* FDA staff cite risk of blood disorders, infections, cancer
* Shares down around 1 percent after the news
By Ben Hirschler and Natalie Huet
Nov 8 (Reuters) - U.S. regulatory officials have raised concerns about "multiple serious and potentially fatal safety issues" in patients given Sanofi's new multiple sclerosis drug Lemtrada, fuelling uncertainty about whether it will be approved.
Food and Drug Administration (FDA) staff said in a report prepared ahead of a Nov. 13 advisory panel that the risks might be too great to justify approval in the world's biggest pharmaceutical market, unless the drug showed "substantial clinical benefit."
Lemtrada, an injectable treatment also known as alemtuzumab, is one of two new Sanofi drugs for MS, an autoimmune disease that attacks the central nervous system and affects more than 2 million people worldwide.
The FDA is expected to rule on the drug by the end of the year. Lemtrada won marketing authorisation from the European Commission in September, shortly after obtaining approval for its MS pill Aubagio.
The current consensus for annual Lemtrada sales worldwide is $752 million by 2018, according to forecasts compiled by Thomson Reuters Pharma.
Shares in Sanofi fell as much as 2.5 percent after the documents were posted on the FDA's website on Friday. By 1555 GMT, they had narrowed their loss and were down 0.8 percent at 78.01 euros.
"Briefing documents usually raise many questions that are open for discussion at the upcoming meeting. But when we read the document it is clearly more negative than usual," a Paris-based trader said.
IMMUNE SYSTEM RISK
Multiple sclerosis is a chronic, often disabling disease that attacks the central nervous system and can lead to numbness, paralysis and loss of vision.
Lemtrada is given via an intravenous drip for five days and for three days one year later. It is designed to re-programme the immune system, but in doing so can make the body more vulnerable to other diseases.
The FDA staff review cited risks of autoimmune diseases including blood disorders, infections and cancer, and, noting that these issues were "serious and potentially fatal", did not recommend approval of Lemtrada "unless substantial clinical benefit exists".
Natixis analyst Beatrice Muzard said the FDA's note signalled a greater risk that the drug might not reach the U.S. market.
But another Paris-based analyst, who declined to be named, was less alarmed, pointing out that the side effects cited in the note were already known.
"Our company is confident that Lemtrada offers an important step forward in the way physicians and patients will think about treating multiple sclerosis," a spokesman for Sanofi said in an emailed statement.
Sanofi acquired Lemtrada when it bought U.S. biotech firm Genzyme for $20.1 billion in 2011. Lemtrada's prospects were at centre-stage in that drawn-out takeover battle, leading to an eventual deal that included listed contingent value rights linked to Lemtrada's future success.