Genzyme rejected an $18.5 billion takeover offer from French drugmaker Sanofi-Aventis, saying it dramatically undervalues the company. The Cambridge, Massachusetts-based biotechnology company said its board of directors met Sunday night and unanimously affirmed a previous rejection of Sanofi's proposal.

In a letter to Sanofi's chief executive, Chris Viehbacher, Genzyme said Sanofi's proposal to acquire Genzyme for $69 a share is the same as an offer it  made last month. "It provides no new information and no improvement in price, and therefore fails to establish a basis for engagement by the Genzyme board," Genzyme said.

Genzyme said that in its first response letter to Sanofi, dated July 29, the board said it was not the right time to sell the company "because your  opportunistic takeover proposal does not begin to recognize the significant progress underway to rectify our manufacturing challenges or the potential for our new-product pipeline."

Genzyme said in its latest letter to Sanofi, "Our board met last evening in response to your second letter and unanimously confirmed those views." Genzyme said Sanofi and its advisers claim Sanofi is willing to pay more but that the company is unwilling to "bid against" itself.

"The Genzyme board is not prepared to engage in merger negotiations with Sanofi based upon an opportunistic proposal with an unrealistic starting price that dramatically undervalues the company."

Sources previously told press representatives that Genzyme wants an offer of at least $75 per share before Sanofi could review its books. Some shareholders want as much as $80 a share to clinch a deal.

The French group stopped short of making a direct approach to Genzyme shareholders on Sunday, and Viehbacher sounded a conciliatory tone in an interview with Les Echos newspaper that nonetheless suggested there was a limit to his patience. "We want to show that we are determined and serious, without being threatening straightaway. Quite some time can go by yet," Viehbacher -- dubbed the "Smiling Killer" by some staff for his cost-cutting zeal -- said in the interview.