plummeted Thursday after the company halted enrollment in a clinical trial of an experimental cancer drug due to heart risks.
Shares were down $1.31, or 14%, to $8.03.
The decision came after more than 10% of patients enrolled through the end of September experienced serious heart-related side effects after being treated with the drug, dubbed XL999. Four out of 14 enrolled in October also experienced similar problems.
However, the company says it will continue to give the drug to the 115 of the 131 patients already enrolled who reported no side effects. All but one of the adverse events occurred after the first administration of the drug, Exelixis says.
"The apparent increase in the frequency of cardiovascular events during October concerns us," said Dr. George Scangos, president and chief executive of Exelixis. "These are recent observations, and we are in the process of collecting and analyzing all of the relevant primary data."