Genzyme Corp. rejected an $18.5 billion takeover offer from French drugmaker Sanofi-Aventis, saying it dramatically undervalues the company. The Cambridge, Mass.-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.”