DAB389IL-2 (Denileukin Diftitox, ONTAK): A New Fusion Protein Technology

Abstract

Genetic engineering has led to the development of fusion protein toxins, which are targeted effector molecules that combine a targeting ligand such as a growth factor with a cytocidal moiety such as a plant or bacterial toxin. The first genetically constructed family of fusion proteins used diphtheria toxin as the toxophore for receptor-binding domain substitution. Diphtheria toxin consists of three domains: an enzymatically active adenosine diphosphate (ADP) ribosyltransferase domain, a hydrophobic transmembrane domain, and a C-terminal receptor-binding domain. Introduction of the enzymatically active domain into the cytosol via receptor-mediated endocytosis results in inhibition of protein synthesis by ADP ribosylation of elongation-factor 2. DAB486IL-2, in which the native receptor-binding domain of diphtheria toxin was replaced with the full-length interleukin-2 (IL-2) gene, was capable of intoxicating high-affinity IL-2 receptor-bearing cells in vitro with an IC50 of 10−10 M; whereas, cells lacking the full component of the high-affinity IL-2 receptor (p55, p75, p64) were relatively resistant (IC50 of 10−8 M). Because of a short in vivo half-life of DAB486IL-2, efforts to reengineer the molecule were undertaken, leading to the DAB389IL-2 construct, which had a twofold to threefold higher Kd than DAB486IL-2 and a longer half-life, resulting in a tenfold increase in potency. Thus far, the clinical activity of both IL-2 chimeric fusion toxins has been similar, with the DAB389IL-2 molecule displaying more favorable pharmacokinetics.