Merck & Co has suffered a major setback following the news that its much-touted blood clotter vorapaxar has been pulled from one major study and discontinued from stroke patients in another.

The US giant has been studying vorapaxar in two major clinical endpoint trials: TRACER, a 13,000-patient study for acute coronary syndrome, and the 26,5000-patient TRA-2P (also known as TIMI 50), in people with prior heart attack, stroke and peripheral artery disease. Now, the combined data and safety monitoring board for the two studies has reviewed the available safety and efficacy data, and called for changes, which Merck has agreed to implement.

Specifically, with TRACER, patients will discontinue study drug and investigators "are to begin now to close out the study in a timely and orderly fashion", while in TRA-2P, vorapaxar will be continued in patients who had experienced a previous heart attack or peripheral arterial disease (approximately 75% of the patients enrolled in the study), but immediately discontinued in patients who experienced a stroke prior to entry or during the course of the study. No details as to why vorapaxar may be inappropriate for stroke patients have been disclosed yet.

Vorapaxar,  a first-in-class thrombin receptor antagonist vorapaxar, was touted as one of the most promising compounds in the pipeline of Schering-Plough, which was acquired by Merck for $41 billion in 2009.  However investors have reacted badly to the latest news and Merck's shares fell 6.7% on the news, wiping almost $8 billion from the stock.

Some analysts are claiming that vorapaxar could be dead in the water. Sanford Bernstein analyst Tim Anderson issued a less dismissive research, saying that the possible safety risk, plus the reduced potential for the drug "will partly call into question the value of Merck buying S-P". He added that "it appears that excessive bleeding may have been the culprit" in terms of treatment for stroke patients.

The company is not giving up on the compound and Peter Kim, president of Merck Research Laboratories, said "we remain committed to conducting large clinical trials such as TRACER and TRA-2P". He added that "we thank the investigators and the patients involved in these two studies for their continued efforts to understand the potential role of vorapaxar".