A cytotoxic recombinant fusion protein consisting of the human cytokine interleukin-2 (IL-2) fused to diphtheria toxin fragments A and B, containing both the catalytic and translocation domains, with potential antineoplastic activity. Upon administration, the IL-2 moiety of denileukin difitox targets and binds to IL-2 receptors. After internalization by IL-2 receptor-expressing cells via endocytosis, denileukin difitox is proteolytically cleaved. This releases the catalytic domain of the toxin moiety, which catalyzes the transfer of the ADP-ribose moiety of NAD to a diphthamide residue of elongation factor 2 (EF-2). This covalent modification inactivates EF-2 and disrupts polypeptide chain elongation, resulting in an inhibition of translation and cell death.

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. Ontak 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. Ontak is a type of immunotoxin and a type of fusion toxin.