The two drugmakers are expanding their joint venture with plans to market a new combo pill that will feature a mix of their existing Zetia cholesterol drug and a generic version of Pfizer’s Lipitor.

The debut would be timed to coincide with the patent expiration on Lipitor, which is expected occur in either 2010 or 2011, and tap the market for the biggest-selling cholesterol pill. Lipitor dominates the $35.2 billion global market for cholesterol drugs by generated $13.6 billion last year, according to IMS Health.

“Given the size of the worldwide market in which this product would compete, we believe the combination of ezetimibe with atorvastatin could be an important contributor to Schering-Plough’s future,” says Fred Hassan, Schering-Plough’s ceo, in a statement this morning.

Although the joint venture - which sells Zetia and also a combination of Zetia and Vytorin - has sometimes been tense for both companies, Merck’s Dick Clark not surprisingly took the high road in his own comments, calling this a “logical next step for our very strong and successful partnership with Schering-Plough.”[tags]Lipitor, Merck, Schering-Plough, Vytorin, Zetia[/tags]