"Grey's Anatomy" stars Katherine Heigl and T.R. Knight are leaving the hit television drama, a co-star has confirmed. The couple have played doctors Izzie Stevens and George O'Malley in the medical drama since it first aired in 2005, but they have chosen to end their runs, according to co-star James Pickens Jr. He tells Usmagazine.com, "She (Heigl) is (leaving). Wherever Katherine goes, I wish her nothing but the best."He adds, "(Knight is) going too. He just wanted to pursue other career paths." But there is hope Knight will return in the future to reprise his role. A source says, "Writers are scrambling to find a believable exit for George, one that would leave the door open for him to come back someday."