France's Sanofi-Aventis plans to press ahead with a formal offer for Genzyme after its informal overture failed to strike interest, a source familiar with the situation said on Wednesday.

The board of Sanofi met in Paris on Wednesday and voted to make a formal offer for Genzyme, though details of the proposal were not immediately available. Sanofi, which is scheduled to report second-quarter earnings on Thursday, declined to comment.

Genzyme could not be immediately reached for comment.

Reports that Sanofi was making a run at Genzyme surfaced on Friday, sending the U.S. company's shares up more than 15% as investors figured the company would garner a hefty premium for its portfolio of expensive treatments for rare genetic disorders and a pipeline of drugs in development.

Some company watchers say Genzyme shareholders may be willing to accept a price of $70 to $80 per share, particularly newer investors who were drawn to the company when it was targeted by activist investor Carl Icahn.