Sanofi Aventis will keep its $18.5bn offer to acquire the US-based biotechnology company Genzyme open for a further six weeks.

The French pharmaceutical major had previously attempted to acquire a majority percentage in the company, but the announced that it could only attract less than 1% of Genzyme shares from stockholders during the previous offer.

In response, the firm has extended its offer, priced at $69 per share, open until January 21 with the aim of acquiring a majority to take control over board objections.

Genzyme chief executive Henri A Termeer dismissed the approach, suggesting that Sanofi Aventis is undervaluing the company, with its true value closer to $89 per share.

“The results of the tender offer reported today demonstrate that our shareholders strongly support the view of the board that the Sanofi offer substantially undervalues Genzyme,” he added.