French pharmaceutical firm Sanofi-Aventis has reached an agreement to acquire US-based biotechnology firm Genzyme for $20.1bn.

Sanofi-Aventis had initially made a formal offer for Genzyme in August 2010, but Genzyme rejected it claiming that it did not reach the company’s true valuation or take into account pipeline medicines.

Under terms of the new deal, Genzyme shareholders will now be entitled to future payments based on sales of Genzyme’s products, including multiple-sclerosis medication Lemtrada.

Sanofi-Aventis hope that the acquisition will give it access to the rare disease treatment market, as it aims to stem flagging sales figures and ease the impact of soon-to-expire patents.

Sanofi-Aventis chief executive Christopher Viehbacher said, “This transaction will create a meaningful growth platform for Sanofi-Aventis while expanding our footprint in biotechnology.”