Sanofi-Aventis (NYSE:SNY) and Genzyme (NASDAQ:GENZ) are reportedly discussing potential values for Genzyme's key multiple sclerosis drug, according to a report on Sunday by Reuters.

Genzyme is looking to market Campath as a treatment for multiple sclerosis under the name Lemtrada. While discussions are undergoing, French drug maker Sanofi warned that there was no guarantee of an agreement.

The move comes on the heels of Sanofi’s $18.5 billion, or $69 a share, hostile bid to acquire Genzyme. The U.S. drug maker turned down the offer late last year, saying it was too low and undervalued the company.

The two parties are reportedly discussing a way to bridge the valuation gap by focusing on Campath’s future success and prospects, according to the report. Genzyme has argued that the drug could generate as much as $3.5 billion in annual sales, while Sanofi has only estimated $700 million.

The companies are moving toward a deal structure known as a contingent value right, which would offer Genzyme shareholders an additional payout based on the drug’s approval and reach of certain revenue targets, according to Reuters.