France's Sanofi-Aventis says it has extended its $18.5 billion takeover offer for U.S.-based biotech drugmaker Genzyme Corp.

Sanofi says it has now set an expiration date for the $69-per-share offer of one minute before midnight on Feb. 15 in New York — from Jan. 21 previously — and that all the details of the offer remain the same.

Sanofi said Monday "significant differences" remain between the companies in their talks over a contingent value right (CVR) in any deal linked to Genzyme's experimental multiple sclerosis drug — the brand name Lemtrada.

CVRs resemble an option for shareholders of an acquired company.

Sanofi said shareholders have already tendered just under 1.1 million Genzyme common shares, amounting to about 0.4 percent of its outstanding shares on a fully diluted basis.