Seattle-based Dendreon, the developer of a drugs used to boost the immune systems of prostate cancer patients, said Thursday afternoon that it is laying off 500 of its employees, in the face of sales issues for its drug. The firm said that it will take a $21M charge related to the layoffs. The firm said it made the cuts, in order to "align with" its launch trajectory and manufacturing requirements. The firm had 1,915 employees as of June.