Molecular Requirements for Recombinant Cytotoxins Efficacy

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A restricted receptor (R) for interleukin 13 (IL13) was found in a vast majority of high-grade gliomas (HGG) patients. The HGG-associated receptor for IL13 was identified molecularly to be the IL13Ra2, and it belongs to a family of tumor antigens, termed cancer/testis tumor antigens (CTA). CTA provide high specificity for molecular targeting/recognition of cancer. Furthermore, IL13-based bacterial toxin, Pseudomonas exotoxin A (PE)-containing recombinant cytotoxins were documented to be the potent anti-glioma agents in pre- clinical evaluation, and the first generation of these cytotoxins is in Phase III clinical trials. The structure- function relationship analysis of IL13 and its recently revealed solution structure documented that alpha-helix D appears to play pivotal role in the binding of IL13to its HGG-associated receptor. It is proposed to generate novel highly specific and highly efficacious IL13-based cytotoxins that will incorporate (i) new knowledge on IL13 structure-function relationship, (ii) new information on glioma cell biology, and (iii) previous experience with the use of cytotoxins in a clinical setting. Thus, these new cytotoxins will be composed of genetically engineered forms of IL13and a derivative of another bacterial toxin, Diphtheria toxin, DT390. The principal idea behind the design of these cytotoxins is to produce novel cytotoxins that allow the binding region of IL13, the D-helix, to be freely available to the HGG-associated receptor forIL13 and to eliminate the site on the cytokine that interacts with its normal physiological receptor that is expressed in many vital organs, including the central nervous system. IL13will be engineered to have the DT toxin moiety remote form its binding site to the IL13R<x2, which cannot be fully achieved with PE. Moreover, IL13 alpha-helix D mutants, super agonists of the IL13Ra2, have been identified and they will be used in the design of optimized cytotoxins. The number of the restricted binding sites for the cytotoxins and an intracellular protease activating bacterial toxins (furin) play major roles in the cytotoxins' tumor cell killing. Thus, the regulation of their expression levels will be examined and the expression levels correlated with the novel IL13-DT cytotoxins efficacy. The novel DT390-IL13 mutant-based cytotoxins will be tested in vitro and in vivo in order to demonstrate how the changes in the molecular design in the context of biological expression of the target and toxin's activating moiety are reflected in their anti-tumor efficacy. It is expected that the new information gathered will be invaluable in further molecular design of recombinant anti-cancer cytotoxins.