Comparative toxicity of arginine deiminase formulated with poly(ethylene glycol) 5000 or 20,000 and the effects of arginine

J.S. Bomalaski, J.L. Ivett, M. Vegarra, F.W. Holtsberg, C.M. Ensor and M.A. Clark
Preclinica (2003), Vol. 1 pages 284-293

Arginine deiminase (ADI), an enzyme found in microorganisms but not in humans, converts arginine into citrulline and ammonia. ADI coupled to either monomethoxy poly(ethylene glycol) (PEG) of 5000 or 20,000 molecular weight (ADI-SS PEG_{5000 MW} or ADI-SS PEG_{20,000 MW}, respectively) using a succinimidyl succinate linker inhibits melanomas and hepatocellular carcinomas in vitro and in vivo. We have performed animal preclinical safety testing of these compounds in mice and rats. Repeated injections of ADI-SS PEG_{5000 MW} or ADI-SS PEG_{20,000 MW} at 800 U/m² in mice resulted in undetectable levels of serum arginine. These drugs were well tolerated in mice with some toxicity but no drug-induced deaths. Furthermore, ADI-SS PEG_{5000 MW} resulted in significantly more anaphylaxis in guinea pigs than ADI-SS PEG_{20,000 MW}. In contrast to mice and humans, rats have an absolute requirement for arginine. Single doses in rats at 800 U/m² also resulted in undetectable levels of serum arginine but caused diarrhea and a 100x decrease in white blood cell count by day 3, prior to death on days 4 and 5. Lower doses that resulted in milder lowering of serum arginine levels were better tolerated in this argi¬nine-requiring species. In mice and guinea pigs, ADI-SS PEG_{20,000 MW} was less toxic than ADI-SS PEG_{5000 MW}. These data illustrate the significance of species selection for toxicology testing of potential human therapeutics.