Drugmakers Merck & Co. and AstraZeneca PLC have amended their U.S. partnership to give AstraZeneca a 2014 buyout option that could cost more than $400 million.

The companies said Tuesday that London-based AstraZeneca will have an option to purchase Merck’s remaining interest in AstraZeneca’s heartburn drug Nexium and its older product Prilosec for $327 million. It also will pay an additional amount based on Merck’s profit from the partnership in the three years before the option is exercised.