Biotechnology Exelixis Inc. said Thursday it is suspending new enrollment in a mid-stage study of its anticancer compound XL999 because of possible serious side effects.

The company said 12 patients, or nearly 10 percent of those enrolled, experienced "serious adverse cardiovascular events."

Through the end of September, 117 patients had been dosed with the compound. Also, the company said 4 of 14 patients enrolled in October experienced similar adverse events, "which raised a concern with the company's internal safety monitoring committee."

Shares tumbled $1.04, or 11.1 percent, to $8.30 in premarket trading on the Nasdaq, having closed Wednesday at $9.34 on the Nasdaq.

But, Exelixis said, because 115 of the 131 people enrolled in the study have received repeated doses with no reported events, those already enrolled patients may continue on with the Phase II clinical trial.

"Since all but one of the events occurred on first administration of XL999, we are continuing to treat those patients presently enrolled in the trial," said President and Chief Executive George A. Scangos, in a statement.

The enrollment delay is expected to last between two weeks and three months.