Ontak (denileukin diftitox), is a recombinant DNA-derived cytotoxic protein composed of the amino acid sequences for diphtheria toxin fragments A and B (Met1-Thr387)-His and the sequences for human interleukin-2 (IL-2; Ala1-Thr133). It is produced in an E. coli expression system and has a molecular weight of 58 kD. Neomycin is used in the fermentation process but is undetectable in the final product. Ontak (denileukin diftitox) is supplied in single use vials as a sterile, frozen solution intended for intravenous (IV) administration. Each 2 mL vial of Ontak contains 300 mcg of recombinant denileukin diftitox in a sterile solution of citric acid (20 mM), EDTA (0.05 mM) and polysorbate 20 ( < 1%) in Water for Injection, USP. The solution has a pH range of 6.9 to 7.2.