UPDATE 3-Niaspan tops Zetia in new setback for Merck drug

* Niaspan leads to reduction in artery wall thickness

* No meaningful change seen with Zetia

* 5 times as many serious adverse events seen with Zetia

* Journal editorials cite several limitations of study (Adds analyst comment, AHA comment on Zetia safety)

By Bill Berkrot and Ransdell Pierson

ORLANDO, Nov 15 (Reuters) - Abbott Laboratories' (ABT.N) Niaspan appeared to be more effective and safer than Merck & Co's (MRK.N) Zetia as a supplementary cholesterol treatment to statins, according to a small study that is likely to further tarnish Merck's damaged cholesterol franchise.

The damage, however, may be limited by the limitations of the study itself, which included data from just 208 patients and was criticized in a major medical journal.

"Niacin is the clear winner and led to very clear reductions in the amount of atherosclerosis that patients had," Alan Taylor, the study's lead investigator who will present the data on Monday at the American Heart Association scientific meeting in Orlando, Florida, said in an interview on Sunday.

Atherosclerosis, which occurs when there is a build-up of dangerous plaque in the arteries, can lead to heart attacks.

Two earlier studies had led to questions about the value and effectiveness of Zetia and a related Merck drug, Vytorin, after which sales of the medicines plunged, although it remains a $4 billion a year franchise.

"This trial doesn't quite put the nail in the coffin for ezetimibe, but it pushes it way down on the list of medications for cholesterol-lowering therapy," Anthony DeMaria, editor-in-chief of the Journal of the American College of Cardiology, said in a statement, using the chemical name for Zetia.

Jon LeCroy, an analyst with Hapoalim Securities, said the study could have a sharp negative effect on Merck's cholesterol franchise.

"This is a huge negative because the excess heart attacks raise significant concern, and front-page headlines will cause worried patients to call their doctors," LeCroy said. He speculated U.S. sales of Zetia and Vytorin could fall another 10-15 percent on the news.

The study tested the effect on carotid artery wall thickness of adding either Niaspan, a long-acting niacin that raises good HDL cholesterol, or Zetia, which lowers bad LDL levels. It involved patients who had heart disease or high risk of heart disease whose LDL was already at target levels from taking statins -- the first-line treatment for high cholesterol -- but who had low HDL levels.

Increases in thickness of the artery wall could be an indicator of build-up of dangerous plaque that causes atherosclerosis, while a decrease could indicate regression of the disease.

The 14-month, 363-subject study sought to determine whether a patient on statins in need of additional therapy would benefit more from further driving LDL down or raising HDL. "This is a question that clinicians are faced with every single day," Taylor explained.