Denileukin diftitox, sold under the brand name Ontak among others, is an anti-cancer medication used for the treatment of adults with T-cell lymphoma.

It is an engineered protein combining interleukin-2 and diphtheria toxin.

[3] Denileukin diftitox is indicated for the treatment of adults with relapsed or refractory stage I-III cutaneous T-cell lymphoma after at least one prior systemic therapy.

[1][2] The prescription label contains a boxed warning that capillary leak syndrome (CLS), including life-threatening or fatal reactions, can occur in people receiving denileukin diftitox.

[1][2] In 1999, denileukin diftitox was approved by the US Food and Drug Administration (FDA) for the treatment of cutaneous T-cell lymphoma.