The drug, if approved, could help advance the treatment of relapse and remitting MS significantly by allowing patients to stop taking betainterfon injections, but still offering similar protection against relapses offered by betainterferon.

But the drug was issued with a ‘refuse to file’ (RTF) letter from the FDA at the end of November, a move which usually means there are serious ‘omissions or inadequacies’ in the document submitted.

Despite this setback, Merck Serono say they are talking to the FDA about re-submitting a New Drug Application (NDA) to the regulator in the immediate future.

Pharma sector research specialists Decision Resources say the ‘refuse to file’ (RTF) letter has heightened the competition between cladribine and its main competitor, Novartis/Mitsubishi Tanabe’s FTY-720 (fingolimod), to be the first oral disease-modifying multiple sclerosis therapy to reach the US market, but believes Merck Serono’s drug will retain its lead.

The Decision Resources report says although cladribine's first-to-market advantage over FTY-720 will be cut back because of the delay, cladribine is still expected to be launched in the US during 2010, whereas FTY-720 remains likely to reach the market sometime in early 2011. Merck Serono have not released details about the contents of the letter, but Decision Resources say this procedural delay will not be directly related to the data itself.

“Typically, the FDA will issue an RTF letter if some aspect of a new drug application is incomplete but this action is not a reflection of the FDA’s opinion regarding a drug's efficiency or safety profiles,” said Decision Resources analyst Bethany Kiernan, who also added: “At this time, we assume the RTF letter will not significantly delay the 2010 launch of oral cladribine in the United States.” The analysts predict that by 2018, oral cladribine and FTY-720 will together capture more than $2 billion in sales in the US, France, Germany, Italy, Spain, the UK and Japan.

Cladribine is already a licenced drug (brand name Leustat) in a different therapy area, the treatment of leukaemia as an intravenous infusion. Merck Serono have reformulated the drug into an oral version with a different dosing pattern for relapsing remitting MS.

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