Shares of Eli Lilly fell more than 6 percent on Thursday, after the drugmaker halted two small trials of blood clot preventer prasugrel, the company's most important experimental drug.
The pharmaceutical firm's shares slipped $3.97, or 7.07 percent, trading at $52.18 on the New York Stock Exchange.
Lilly and its partner Daiichi Sankyo late on Wednesday said they had stopped giving prasugrel to patients in the trials because the dosage may need to be changed for certain patients. The company did not provide detailed reasons.
Morgan Stanley analyst Jami Rubin cut her rating on Lilly to "underweight," due in part to Lilly's decision to stop enrollment in the trials.
She cited "new concerns surrounding safety and efficacy" of prasugrel, and cut her 2011 sales forecast for the drug to $550 million, from her earlier view of $900 million.