Medical journal The Lancet warned that Sanofi's experimental multiple sclerosis drug Lemtrada may be too costly for patients and health insurers once it gets approved by regulators.

The journal, which published the encouraging results of two late-stage Lemtrada tests on Thursday, also criticised the drugmaker's decision to withdraw leukemia therapy Campath, the same drug given at a different dosage, depriving MS patients who had been using it off-label.

In an editorial accompanying the test results, The Lancet voiced concerns that Lemtrada would be priced higher than current MS drugs on the market and said the discontinuation of Campath may mean patients who had used it for MS would not be able to continue their treatment.

The injectable drug, chemically known as alemtuzumab, was sold until September 2012 under the name Campath as treatment for leukaemia and given more frequently at a higher dosage.

"There is concern that with a license for multiple sclerosis, the cost of alemtuzumab could rise and might become too expensive for many patients and health systems," the editorial said.

Although Campath remains available free of charge to leukaemia patients, Sanofi's rare disease unit Genzyme pulled it off the market in September to prevent its unauthorised use as an MS drug.

Analysts said the move would allow the company to adjust the price to match that of rival MS drugs on the market.

A full course of Campath, which in 2011 had sales of $76 million, cost around $60,000 when given three times a week for up to 12 weeks, according to Genzyme.

Lemtrada, instead, is given at less than half the dose of Campath for 5 consecutive days and then again for 3 days a year later. Since the drug has yet to be approved, it remains unclear how much Sanofi will charge for it.

The drug, which works by resetting a person's immune system, has shown in late-stage trials to be an effective treatment for MS patients who have failed to respond to other therapies.

It has also shown to benefit people not previously treated for the disease, suggesting it could be used as a first-line MS therapy.

But patients need regular monitoring for serious side effects that can include infections and autoimmune diseases.

"It's important that the appropriate safety monitoring is in place for patients who are prescribed Lemtrada," Genzyme's head of MS, Bill Sibold, told Reuters, responding to questions about the Lancet editorial. "Until an approved risk-management program is established, we believe the use of Lemtrada should only occur in clinical trials."

Lemtrada remains available to patients who are taking part in clinical tests.

Sibold declined to discuss pricing plans for Lemtrada, but said Genzyme has set up programmes to make its approved drugs available to patients who cannot afford them. "With Lemtrada it would be no different," he said.

Drug funding

But there are concerns that cash-strapped European governments may balk at funding the drug through their public healthcare systems.

Doug Brown, Head of Biomedical Research at U.K. charity MS Society said that while Lemtrada's results are great news for patients, the drug would only be useful to them if it were available through the country's publicly funded National Health Service.

"We urge Genzyme to price the treatment responsibly so that if it's licensed, it's deemed cost effective on the NHS," he said.

The U.K.'s cost-effectiveness body National Institute for Health and Clinical Excellence (NICE), whose opinions are also watched closely in other countries, initially rejected Novartis' MS pill Gilenya, only to make a U-turn after the company agreed to a discounted price.

Sanofi launched its MS pill Aubagio in the U.S. at a price of $45,000 for a year's treatment, making it cheaper than rivals.

Gilenya - the only other MS pill currently on the market - costs 28% more, while injectable treatments such as Biogen's Avonex and Teva Pharmaceutical Industries's Copaxone are 8 and 6.5% higher respectively.