PARIS (Reuters) - Sanofi's rare disease unit Genzyme is pulling leukaemia drug Campath to prepare for its launch under a different dosage and as a multiple sclerosis treatment that will be branded as Lemtrada.

The withdrawal, meant to prevent the off-label use of Campath as a multiple sclerosis drug, is already under way in some European countries and will be effective in the United States on September 4, a Genzyme spokesman said.

Lemtrada, which Sanofi submitted for approval with health regulators in Europe and the United States in June, could be launched in 2013 if it wins approval.

If approved, it will be given far less frequently and in lower doses than Campath, and is one of the new products the French drugmaker is betting on to restore growth after losing several aging blockbusters to generic rivals.

"We think that this stoppage shows Sanofi's confidence in the approval of Lemtrada in multiple sclerosis," said Bryan Garnier analyst Eric Le Berrigaud, who estimates the drug could generate sales of $400 million in 2018, if approved.

The withdrawal will also enable Sanofi to adjust Lemtrada's price closer to that of rival multiple sclerosis drugs, Le Berrigaud said. Rivals include Tysabri, an injectable drug sold by Biogen Idec and Elan, while Novartis markets MS pill Gilenya.

Campath, which last year had sales of $76 million, will continue to be available through patient access programs in the 50 countries where it has been available since its launch in 2001.

"In most countries we will provide the drug for free, where this is permitted," the Genzyme spokesman said.

The news was first reported by trade publication BioCentury.

Sanofi shares, which have risen around 18 percent since the start of the year, closed 0.8 percent higher at 67.1 euros, outperforming France's main CAC-40 index, which closed 0.2 percent lower.

(Reporting by Elena Berton; Editing by David Cowell)