As expected, Sanofi-Aventis has extended its $18.50 billion offer for Genzyme Corp without changing the terms, after only 0.9% of shares had been tendered by last Friday’s deadline.

The French drugmaker issued a statement saying that its $69.00 per share offer will now expire at midnight on January 21. Just 2.2 million shares have been offered up and seeing that Genzyme stock has been trading over $70 per share, it seems unlikely that many of the latter’s investors will go over to Sanofi’s side at the moment.

The battle for ownership of the US biotech could therefore go on for some time. Genzyme has repeatedly rejected Sanofi's hostile offer but it has been reported that it is exploring the possibility of a contingent value right being added to any deal. A CVR means that the purchase price would be supplemented by extra payments in the future, in this case reflecting the potential of Campath (alemtuzumab), Genzyme's leukaemia drug which is in late-stage trials for multiple sclerosis.

It has also been rumoured that Genzyme has been integrating the CVR in its discussions with third parties who may be interested, including Pfizer, Johnson & Johnson and Merck & Co. However, none of these firms have made any interest public and it seems that the stalemate with Sanofi will run well into 2011.