The Tarrytown, N.Y., company said it received a notice about the planned transaction from the Federal Trade Commission. The notice said Sanofi intends to own more than $500 million in voting shares. Sanofi is Regeneron's largest shareholder. It owns about 15.8 million shares of Regeneron, which is worth about $2.6 billion and is a one-sixth stake in the company.
Under the terms of the agreements between the two companies, Sanofi is not allowed to own more than 30 percent of Regeneron's Class A stock and common stock. However Sanofi said it does have the right to buy more shares and may do so. It described the filing as a technical move, and added that it is "very happy" with its relationship with Regeneron and has no plans to take a controlling stake in the company.
Shares of Regeneron rose $6.50, or 3.9 percent, to $172.38 after being halted early in the day. Earlier the shares gained as much as 10.1 percent. Regeneron stock is up more than 60 percent over the last year and reached an all-time high of $188.95 in November.
Sanofi and Regeneron collaborate on the cancer drug Zaltrap. The Food and Drug Administration approved Zaltrap as a treatment for colorectal cancer, and the companies are marketing it together in the U.S. European regulators approved the drug Tuesday. The two companies are also studying treatments for rheumatoid arthritis, "bad" cholesterol and asthma. They are also working on other cancer drugs.
Regeneron also makes the eye drug Eylea, a treatment for "wet" age-related macular degeneration and for macular edema with central retinal vein occlusion. It expects to report $790 million to $815 million in sales of the drug in 2012, including about $230 million to $250 million in the fourth quarter.
Regeneron will report its quarterly results Thursday.