NEW YORK ( TheStreet) -- Shares of pharmaceutical giant Merck ( MRK) are down by more than 6% on Thursday morning after it announced that an experimental blood clot prevention drug that held the promise of billions in sales had suffered serious setbacks in two trials.

Trading in Merck shares was heavy on Thursday morning, with more than twice its average daily volume of 22 million shares reached an hour after the market open.

The race is on for big pharma companies like Merck and Pfizer ( PFE) to find the next blockbuster drug. While Pfizer has the most notable patent expiration of 2011 with Lipitor set to go generic in November, the pipeline of both Merck and Pfizer are keys for these stocks breaking out of the narrow trading range in which their stable cash flow and dividend-friendly profile keep them hemmed in.

Merck said that one trial for vorapaxar would be discontinued immediately, while a second trial would be discontinued for patients who had previously suffered a stroke.

Les Funtleyder, health care analyst at Miller Tabak, noted in an email to TheStreet, "There will always be days like this in health care. The 'but' is when a big drug does go down, it puts more pressure on the rest of the pipeline to be successful."

The blood clot drug has been among the most promising in the Merck pipeline as a revenue replacement for the company. Two key blood clot pills with sales in the billions went generic in 2010.

Merck shares fell from above $37 to below $35 on Thursday morning. The stock's 52-week high above $41 dates back to January 2010.

Merck is much more richly valued by the market than Pfizer in terms of price to earnings, though all the drug stocks trade at multiples that are considered relatively low compared to other sectors in the equities market.

Vorapaxar was acquired by Merck in its acquisition of Schering-Plough.

Both Merck and Pfizer start 2011 with new CEOs. Merck's new CEO Kenneth Frazier took over on January 1, with a vow of not throwing drug exploration money around loosely but rather, focusing on innovative drugs.

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