LONDON — Clinical trials of GlaxoSmithKline PLC's experimental multiple sclerosis drug have been halted by the U.S. Food and Drug Administration, the British pharmaceutical company said Wednesday.

The U.S. regulator's decision is part of a move to place a clinical hold on new drugs in the same class as Tysabri, the multiple sclerosis treatment marketed by Ireland's Elan Corp. PLC and Cambridge, Mass.-based Biogen Idec Inc. that was withdrawn in February after one patient died.

Multiple sclerosis is a chronic disease of the central nervous system.

Elan and Biogen voluntarily suspended sales of their much-awaited multiple sclerosis drug Tysabri in February, after two patients contracted a rare disease of the central nervous system.

Glaxo said that while its drug has the same molecular target as Tysabri, it is chemically unrelated to that drug.

There are 11 drugs in active development that are in the same class as Tysabri, which work by targeting an immune system protein called VLA-4.

Preclinical compounds include Schering-Plough Corp.'s TBC-4746, which is being tested for asthma and multiple sclerosis.

Glaxo's American Depositary Receipts fell 29 cents, or 0.6 per cent, to $48.41 in early afternoon trading Wednesday on the New York Stock Exchange.

Multiple sclerosis drugs that are already on the market stand to benefit from the FDA's stance, said Karl Heinz Koch, an analyst with Zurich-based private bank Lombard Odier Darier Hentsch.

"The FDA almost certainly will no longer allow an MS drug to the market that has less than two years of data," he said.

Swiss drug companies Serono SA and Novartis AG are also working on oral multiple sclerosis treatments. But representatives for both companies said their drugs aren't affected because they have different modes of action from Tysabri.

Novartis said it plans to start Phase 2 trials on its compound, FTY720, in the first half of 2005.

Serono's Mylinax, which is being jointly developed with Miami-based Ivax Corp., is to enter Phase 3 clinical testing in the first quarter.

In Germany, Schering AG sells a multiple sclerosis drug called Betaseron, which has been on the market for 12 years and has a strong safety record.