AstraZeneca Said on Monday it was exercising an option with Merck & Co to take back full rights to certain drugs marketed in the United States and would pay the U.S. group $647 million in April.

The payout, which had been anticipated by investors, is part of the staged unwinding of a complex relationship between the companies that entitles Merck to a royalty on U.S. sales of many AstraZeneca medicines.

Products covered by the option include Entocort for Crohn's disease, the heart drugs Atacand and Plendil, and certain products still in development, including AstraZeneca's potential blockbuester blood-thinner Brilinta.

AstraZeneca also has a second option to repurchase Merck's interest in its top-selling heartburn drug Nexium and the older product Prilosec. That option is exercisable in 2012, or 2017, or if combined annual sales of the two products fall below a minimum amount.

The agreement with Merck dates back to a selling and distribution joint venture originally set up with Sweden's Astra, which was restructured in 1998.