Eli Lilly said on Wednesday it stopped giving its most important experimental medicine -- blood-clot preventer prasugrel -- to patients in two small trials because the dosage may need to be changed for certain patients.
Lilly , whose shares fell 4 percent in after-hours trading, said developments in the two trials "should not be interpreted to represent the outcome of the
overall prasugrel clinical development program."
The company did not identify what types of patients had aroused concern in the trials.
Prasugrel is considered by Wall Street to be a linchpin of the Indianapolis drugmaker's future earnings growth and a serious potential rival to Bristol-Myers Squibb's multibillion-dollar Plavix anti-clot drug.
The company said enrollment of new patients had been stopped in the two trials, which involved people with coronary artery disease taking either prasugrel or Plavix.
Lilly said enrollment in the trials would not resume until new analyses of clinical data were completed and more was known about prasugrel's pharmacokinetics, meaning how the body absorbs, breaks down or excretes a drug.
Lilly noted that any changes to the design of the trials must first be approved by institutional review boards, panels of experts that assess clinical trials and ensure that patient safety is protected while they are being conducted.
Shares of Lilly fell to $53.95 in electronic trading from their closing share price on Wednesday of $56.15 on the New York Stock Exchange. Bristol-Myers' stock rose 2 percent to $29 a share.
Investors are eagerly awaiting data, expected on November 4, from a very large late-stage trial that compares the safety and effectiveness of prasugrel and Plavix.
Cowen and Co. analyst Steve Scala on Wednesday said prasugrel could generate sales of $2 billion in 2012 if data from the Phase III trial show prasugrel has superior effectiveness without raising risk of serious bleeding.
Although Scala said he believes prasugrel will indeed show superior effectiveness, he predicted it will be shown to significantly raise the incidence of serious bleeding. Under that scenario, he said prasugrel could have "zero" sales in 2012.