Sanofi-Aventis has upped the pressure on Genzyme Corp, urging the management of its takeover target not to use "defensive tactics" and let shareholders decide on its $18.50 billion hostile offer.

The French drugmaker has published a letter from chief executive Chris Viehbacher to his Genzyme counterpart  Henri Termeer who has repeatedly rejected the $69.00 per share on the table. The letter notes that the biotech major has decided to 'probe and evaluate alternatives', including contacting third parties, and "we were encouraged to hear this". However, "to date, we have not been contacted or included in this process".

Mr Viehbacher went on to note that while Genzyme has said "you are committed to maximising shareholder returns and that you value shareholders’ voices," he is concerned about "certain comments...that appear to be inconsistent with that objective". He was referring to possible moves to extend the terms of some of its current directors, the adoption of a poison pill or the use of Massachusetts anti-takeover statutes.

These moves would prevent Sanofi from acquiring Genzyme without the cooperation of the board, "notwithstanding your shareholders’ support of a transaction", Mr Viehbacher stated, adding that "we believe it would be inappropriate for the board to take these defensive actions". He concludes by saying that "if we are unable to have a direct dialogue with you, in all fairness you should allow your shareholders the opportunity to decide for themselves whether or not to accept our proposal."

Mr Termeer has been touting $89.00 per share as a fair price through what he says are "detailed reports on our significant progress and detailed information about our near-term plans and future prospects" including the results of an independent third party study of alemtuzumab's revenue potential as a treatment for multiple sclerosis. Responding to Mr Viehbacher's letter, he says Sanofi "has chosen to dismiss or ignore this information and stick to its opportunistic and inadequate $69.00 per share offer, even as analysts have adjusted their targets to reflect the new information we have shared and we continue to have the support of our shareholders".

He added that 'we are open to a transaction that appropriately recognises Genzyme's intrinsic value and prospects. We will meet with Sanofi if it makes an offer that gives our board of directors reason to believe it will lead to that result".