Merck (NYSE:MRK) will discontinue some clinical trials of its investigational blood clot preventer following recommendations from a safety board that deemed the drug inappropriate for stroke patients.

The move led to a massive sell off as hopes of the drug’s multibillion-dollar sales potential began to dwindle, sending its shares on Thursday down more than 6%.

The drug company said it made changes in two trials for the experimental cardiovascular medicine, vorapaxar, as per a data and safety monitoring board’s suggestion. 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.

One of the trials, called the Tracer study, which target patients with acute coronary syndrome, will be discontinued in what Merck called a “timely and orderly fashion.”

The TRA-2P trial, which targets patients with prior heart attack, stroke and peripheral artery disease, will be immediately discontinued in patients with a history of strokes, however experiments will continue on patients with previous heart attacks or peripheral arterial disease, which make up roughly 75% of participants.

Merck said it plans on updating its projections for regulatory filings for vorapaxar once it receives the efficacy and safety data from Tracer, slated for later this year, and can determine an updated completion date for TRA-2P.

The researchers remain committed to conducting large clinical trials such as Tracer and TRA-2P to study new cardiovascular medicines, according to Dr. Peter S. Kim, president of Merck Research Laboratories, who noted the ultimate goal is to advance the standard of care for patients with heart disease.

“Cardiovascular disease remains the world's leading killer,” he said, “and despite all of the advances that have been made in the field, millions of patients remain at risk for major cardiovascular events.”