PREVIEW-Investors to eye MS drug safety at neurology meeting

* Annual Meeting of American Academy of Neurology

* Seattle, April 25 to May 2

* Investors focus on MS oral drug safety data

By Toni Clarke and Ben Hirschler

BOSTON/LONDON, April 24 (Reuters) - Data to be presented next week could give clues about how the next generation of multiple sclerosis drugs will stack up against existing treatments and against each other.

Swiss drugmaker Novartis AG (NOVN.VX) and Germany's Merck KGaA (MRCG.DE) are leading the charge to develop multiple sclerosis drugs that can be taken orally, rather than by infusion or injection.

But companies must persuade physicians and investors that the drugs are safe, and experts will be looking closely at the side-effect profile of each drug when data is released at the annual meeting of the American Academy of Neurology, which begins April 25th in Seattle.

Novartis's drug, FTY720, has been associated with potentially fatal infections, skin cancer and, just recently, a case of hemorrhaging focal encephalitis, an inflammation of the brain with bleeding. It is unclear whether the drug was responsible for the events.

Merck's drug, cladribine, has been linked to a reduction in white blood cells, which can lead to infection, and in January the company said four patients in a late-stage trial were diagnosed with cancer.

"Clearly, none of these drugs are aspirin or orange juice," said Bruno Musch, head of global clinical development for neurodegenerative diseases at Merck. "You have to counterbalance your safety risk with a high level of efficacy."

TYSABRI MOST EFFECTIVE

Biogen's drug Tysabri, which is given by infusion, is the most effective drug on the market, reducing the relapse rate in patients with multiple sclerosis by 68 percent. But sales of the drug have been crimped because of its link with progressive multifocal leukoencephalopathy, or PML, a potentially deadly brain infection.

Earlier this year Merck said results of a late-stage trial showed cladribine reduced the number of relapses per year by as much as 58 percent, compared with those taking a placebo. But safety will likely dominate the discussion.

"Expect a focus on the four known cases of cancer, infection rates and any unknown side effects," said Kevin Wilson, an analyst at Citi Investment Research, in a recent report.

Novartis said a late-stage trial of FTY720 known as TRANSFORMS reduced the annual relapse rate by up to 52 percent compared with patients taking Biogen's injectable multiple sclerosis drug Avonex.

"Concerns over the drug remain on skin cancer, macular odema and fatal viral infections," said Wilson.

Biogen is developing an oral MS drug, known as BG-12, but it is not as far advanced as the Merck or Novartis drugs, which are expected to be submitted for approval this year.

Data on FTY720 and cladribine will be presented on Wednesday, April 29. (Editing by Brian Moss)