Merck said it will retain its interest in certain AstraZeneca non-heartburn drugs, which means in 2012 AstraZeneca can buy out an interest Merck holds in its heartburn drugs, including blockbuster medication Nexium. The two companies have been partnering in a long-running joint venture. Last month, Merck said it expected its full-year 2008 earnings to include a gain of about $2.2 billion, as AstraZeneca begins to buy out Merck's share in the joint venture. Merck said it will receive payments from AstraZeneca in the first quarter of 2008 in the amount of $2.6 billion -- a payment of about $4.0 billion offset by a loan repayment of $1.4 billion. As a result of this decision, AstraZeneca will have its own option to buy Merck's interest in the non-heartburn, products in the first half of 2010, meaning the ownership would change hands. Sales of Nexium declined 9% to $1.3 billion in the fourth quarter and pressure is expected to increase with the arrival in the US of generic copies of rival heartburn drug Protonix, sold by US drug maker Wyeth.