Merck KGaA has suffered a major blow this morning with the news that regulators in the USA want more information on its multiple sclerosis pill cladribine.

The German drugmaker has received a complete response letter from the US Food and Drug Administration which states that "substantial evidence" of cladribine’s effectiveness was provided by the CLARITY study it submitted. However, the agency has asked the company to provide "an improved understanding of safety risks and the overall benefit-risk profile either through additional analyses or by additional studies".

Merck will now request an end-of-review meeting with the FDA to clarify its next steps and to see whether data from completed and ongoing clinical studies will be enough to address the agency's questions. Top-line results from the CLARITY extension trial and ORACLE MS2 study are expected by the end of 2011, while data from the ONWARD study is due in the first half of 2012.

The company is not giving up and  Fereydoun Firouz, head of Merck’s US subsidiary EMD Serono, said its commitment to MS "remains steadfast”. The FDA's decision comes a fortnight after it was revealed Merck had pulled a submission for cladribine in Europe, after the regulator required more data.

Cladribine is already approved (and sold as Movectro) in Australia and Russia but the last couple of weeks has seen the drug fall yet further behind Novartis' oral MS drug Gilenya (fingolimod), which was recommended for approval in Europe in January and was given the green light by the FDA last September.