A drug used to treat cutaneous T-cell lymphoma that has recurred (come back) or has not gotten better with other treatment. It is used in patients whose cancer has a receptor for the protein interleukin-2 (IL-2). It is also being studied in the treatment of other types of cancer. Denileukin diftitox is made by combining IL-2 with a bacterial toxin. The IL-2 part of the drug attaches to the cancer cells and the toxin kills them. Denileukin diftitox is a type of immunotoxin and a type of fusion protein. Also called Ontak.