Merck (NYSE:MRK) finally came clean on what the FDA wants before the drug formally known as Cordaptive (MK-0524A) can be approved. From the looks of it, it's going to be a long wait for Merck.

The Food and Drug Administration rejected MK-0524A back in April, but Merck didn't give much other information -- except that the FDA had rejected the brand name, too. On Friday, Merck elaborated that the FDA wants to see the results of a trial (HPS2-THRIVE) that won't be available until January 2013. That's a serious wait to turn around Merck's cholesterol franchise, which was hurt earlier this year when it and Schering-Plough (NYSE:SGP) presented results suggesting that Vytorin doesn't lower plaque in arteries any more than a generic statin.

Ironically, it was likely the Vytorin trial is what spurred the FDA to wait for the results of HPS2-THRIVE. Because Vytorin was approved based on a surrogate endpoint -- lowering bad cholesterol -- but hasn't been proven to save lives, the FDA now appears reluctant to approve MK-0524A based on its ability to raise good cholesterol and instead would like to see the results of HPS2-THRIVE, which will measure MK-0524A's ability to delay a major vascular event, such as a heart attack or a stroke. The shift away from using surrogate endpoints for cholesterol drugs is also delaying Isis Pharmaceuticals' (NASDAQ:ISIS) and Genzyme's (NASDAQ:GENZ) mipomersen drug from being approved for use in the general population.

It's a little surprising that the FDA would require Merck to wait for an outcomes study, because MK-0524A is a combination of extended-release niacin and laropiprant, and niacin is already approved as a treatment for cholesterol issues. The laropiprant part is a flushing inhibitor that reduces a hot-flash-like side effect of niacin. It's possible that the FDA is worried about the safety of the relatively new laropiprant, but it shouldn't take a trial of 20,000 subjects to detect any safety issues.

Merck has already received a positive recommendation for MK-0524A in Europe, so the drug should hit the market there by the end of the year, I would guess. That's some solace, but Merck's comeback in the cholesterol drug space in the U.S. is going to have to wait a few years.