French drugmaker Sanofi-Aventis extended its snubbed $18.5 billion tender offer for U.S. biotech Genzyme for a second time as the companies continue behind-the-scenes talks to reach an agreement.

Only 0.4% of shares were tendered at $69 each by a Friday deadline, Sanofi said on Monday. This time the offer will be extended until Feb. 15. At the previous tender offer deadline in mid-December, 0.9% of the shares were handed in.

Genzyme, specialized in rare diseases, has rejected Sanofi's $69-a-share-bid as too low given that it is recovering from a manufacturing crisis and that it has high hopes for experimental multiple sclerosis treatment Lemtrada, which is sole under the name Campath.

Following months of deadlock, the companies this month have come together at the negotiating table by addressing the possible use of contingent value rights, an extra fee Genzyme shareholders will receive if Lemtrada hits certain milestones.

"Although discussions have continued and have included commercial teams and executives of both Sanofi-Aventis and Genzyme, there remain significant differences on the potential CVR and value of Sanofi-Aventis's offer and there is no guarantee that the parties will come to an agreement," Sanofi said in a statement on Monday.

Genzyme has forecast peak annual sales of $3.5 billion for the drug, while Sanofi, using the average outcome of several analyst estimates, expects sales of about $700 million per year.

A CVR would come on top of Sanofi's cash bid, which analysts widely expect will be raised. Genzyme shares have been trading at around $72 each. Still, a final agreement may not include such a payout.

Genzyme Chief Executive Henri Termeer said on Wednesday that at the very least, talks on a CVR were proving useful in establishing the right value for Genzyme.

Some investors expect a deal in the high $70s or possibly $80-per-share level, depending on how it is structured.