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.
Lilly said suspension of the two trials was unlikely to delay U.S. approval of prasugrel. But industry analysts said the news had magnified their worries of whether the drug will beat Bristol-Myers Squibb's Plavix in a large Lilly study, called Triton, whose results will be reported on Nov. 4.
Prasugrel is deemed vital for Lilly's future earnings growth and a serious potential rival to Plavix, one of the world's biggest-selling medicines.
Lilly said it still aims to seek U.S. approval for prasugrel by the end of 2007 if it succeeds in the 13,600-patient Triton study.
Sanford Bernstein analyst Tim Anderson said he was concerned patients taking prasugrel in the two halted trials may have experienced more bleeding than those taking Plavix -- a finding he said would harm its prospects of being approved or prescribed by doctors.
"If it were something besides bleeding, we believe Lilly would seek to clarify this," Anderson said in a research note.
He said Lilly's sketchy explanation for halting the trials left many questions unanswered.
Lilly and Daiichi Sankyo are co-developing prasugrel. It was invented by Daiichi Sankyo and research partner Ube Industries.
Daiichi Sankyo shares closed down 10.3 percent, while Ube fell 5.4 percent.