Billionaire investor Carl Icahn has raised his stake in

ImClone Systems


to 11.68% from just under 10%, according to regulatory documents filed Monday.

Last week, ImClone said it was inviting Icahn to stand for election to its board of directors. That announcement came as ImClone, the maker of the cancer drug Erbitux, said it would no longer pursue a sale of the company and would instead remain independent.

Shares of ImClone rose 25 cents, or 0.9%, to $28.36 in premarket trading. The disclosure that Icahn had boosted his holdings was made through a filing with the

Securities and Exchange Commission