Merck & Co’s bid to revitalise its cholesterol portfolio has been dealt a blow as US regulators rejected the firm’s new combination treatment MK-0524A.

The US Food and Drug Administration has issued a not approvable letter for MK-0524A (extended-release niacin/laropiprant) for the treatment of primary hypercholesterolemia or mixed dyslipidemia. The move has taken the firm and analysts by surprise, seeing as how European regulators recommended approval just days before for the drug, which lowers ‘bad’ low-density lipoprotein cholesterol and triglyceride levels, and also increases cardioprotective ‘good’ high-density lipoprotein cholesterol levels .

The New Jersey-based giant did not give any specific details as to what was wrong with the New Drug Application but Peter Kim, president of Merck Research Laboratories, said that “we plan to meet with the FDA and to submit additional information to enable the agency to further evaluate the benefit/risk profile of MK-0524A". The FDA also rejected the proposed trade name Cordaptive for the drug and Merck said it will use Tredaptive for use in the USA if it finally gets on the market.

Dr Kim said “we firmly believe that MK-0524A provides physicians with an important option to manage their patients' cholesterol”. He added that the firm was encouraged that on April 24, the Committee for Medicinal Products for Human Use recommended marketing approval in Europe “and we will continue to pursue approval within individual markets in the EU and around the world".

Merck’s chief executive Richard Clark was upbeat about the not-approvable letter, saying that the firm’s broad portfolio of medicines and vaccines, “including eight products in launch phase, enables us to weather challenges that come our way". The company said its guidance for 2008, issued last week, of earnings of $3.28-$3.38 per share remains unchanged.

There had been a fair degree of excitement about MK-0524A, especially as studies suggested that laropiprant would prevent the flushing side effect common to niacin. It was also supposed to be a blockbuster for the firm.

The FDA’s decision comes at a time when Merck’s cholesterol franchise is looking less than healthy. It lost billions of dollars when Zocor (simvastatin) went off-patent in 2006 and then of course there is the controversial drug Vytorin, a combination of Zetia (ezetimibe) and Zocor, sold with Schering-Plough which is no more effective than cheaper statins, notably generic Zocor, in preventing heart disease. Panellists at the American College of Cardiology meeting in Chicago recommended that widespread use of Vytorin and Zetia should be curtailed.

Bad news then for Merck but the rejection of MK-0524A is a huge boost for Abbott Laboratories, especially its recently-approved Simcor,which combines Niaspan (extended-release niacin) and Zocor. Sales of the latter can only benefit from the news that Merck’s drug will not be on the US market any time soon.