A drug used to treat cutaneous T-cell lymphoma that can bind the cytokine IL-2 and that has not responded to other treatment. It is also being studied in the treatment of other types of cancer. Denileukin diftitox is made by combining a part of IL-2 with a bacterial toxin. The IL-2 part of the drug attaches to the cancer cells and then the toxin kills the cells. Denileukin diftitox is a type of immunotoxin and a type of fusion toxin. Also called Ontak.