Merck & Co stock plunges as firm halts one vorapaxar study, changes another on safety concerns

US pharma giant Merck & Co (NYSE: MRK) suffered a setback yesterday, seeing its shares slump 7% to $35.00 in early trading, after the company said it was halting one late-stage study of its experimental blood-thinner vorapaxar and modifying a second trial due to safety concerns.

Industry analysts have previously touted vorapaxar - acquired along with its $41 billion buy of Schering-Plough in 2009 - as Merck's most promising experimental drug, and Cowen & Co predicted it will generate annual sales of $1 billion by 2015. Other analysts have been much more upbeat, suggesting that the anticoagulant could generate turnover of $5 billion or more a year. However, any negatives - such as excluding patients who had previously had a stroke - could limit its performance, assuming of course that it gains regulatory approval at all.

Merck is studying vorapaxar in two major clinical endpoint trials to evaluate the investigational medicine for the prevention of cardiac events: TRACER, a study in patients with acute coronary syndrome, and TRA-2P (also known as TIMI 50), a trial in patients with prior heart attack, stroke and peripheral artery disease. Both studies are event-driven trials in which patients were planned to be followed for a minimum of one year, and both had completed enrollment.