NCI: A substance being studied in the treatment of some types of cancer. It blocks the action of certain proteins that are part of the epidermal growth factor receptor (EGFR) family of proteins. These proteins may be found in increased amounts on the surface of some types of cancer cells. Blocking the action of these proteins may stop cancer cells from growing and may kill cancer cells. Pelitinib is a type of EGFR inhibitor. Also called EKB-569.