* AstraZeneca exercises first option under drug rights deal
* AstraZeneca to pay Merck $647 million in April
* Deal was expected after option terms set out in 2008
* AstraZeneca shares up 1.6 pct (Adds shares, background)
LONDON, March 1 (Reuters) - AstraZeneca (AZN.L) said on Monday it was exercising an option with Merck & Co (MRK.N) 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 blockbuster blood-thinner Brilinta.
The appraised value of $647 million was first set out in 2008.
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 1982 selling and distribution joint venture originally set up with Sweden’s Astra, which was restructured in 1998.
The British-based group has long argued that ending the alliance with Merck makes good business sense since it will relieve it from costly contingent payments and increase its freedom to operate.
AstraZeneca shares were 1.6 percent higher at 1320 GMT, outperforming a 0.6 percent gain in the STOXX Europe Healthcare index .SXDP. (Reporting by Ben Hirschler; Editing by Dan Lalor and Jon Loades-Carter)