Sanofi is quitting sales of the Genzyme leukemia drug Campath as it anticipates regulatory approval of a lower dosage of the same drug to be sold as Lemtrada, a relapsing multiple sclerosis therapy.

The move is intended to prevent off-label use of Campath, which would sell for about one tenth the price of multiple sclerosis therapies at the appropriate dose were it used to treat the condition, according to a 2011 Datamonitor analysis.

By pulling Campath, Sanofi will effectively eliminate the risk that off-label use of the drug for MS would cannibalize Lemtrada sales, a scenario that would both hurt Sanofi’s bottom line and deflate the value of contingent value rights. Those rights, issued by Sanofi to Genzyme stockholders when it bought the company in February 2011, are tied to Lemtrada hitting regulatory and sales milestones.

If Campath remained on the market, Sanofi could face a problem similar to the one Roche faced with Lucentis and Avastin. Doctors often prescribe the cancer drug Avastin off-label to patients with wet age-related macular degeneration instead of the more expensive Lucentis, which is approved for that indication. Both drugs block the abnormal growth of blood vessels through the same mechanism of action. In the case of Sanofi, Campath features the active ingredient alemtuzumab, the same as Lemtrada.

While Sanofi will no longer sell Campath in the United States or Europe, it will provide Campath for free “for appropriate patients” in most markets through a company-administered patient access program.

“Our primary goal is to ensure appropriate use of the drug,” says Sanofi Senior Vice President and Head of Multiple Sclerosis Bill Sibold, during an April conference call with analysts.

In June, Genzyme submitted a supplemental Biologics License Application to the U.S. Food and Drug Administration and a marketing authorization application to the European Medicines Agency seeking approval of Lemtrada, which the company is developing in collaboration with Bayer HealthCare.

Sanofi, which lost exclusivity on Plavix and Avapro in the United States during the second quarter of 2012, is counting on Lemtrada’s success. In a November 2011 report, research firm Datamonitor said it expects the drug to generate sales of more than $600 million in the United States by 2020.

Genzyme has worldwide rights to the drug while Bayer retains an option to co-promote it in MS and, upon regulatory approval and commercialization, would receive contingent payments based on sales revenue.