PEG–protein conjugates: nonclinical and clinical toxicity considerations

Abstract Polyethylene glycol (PEG) is an inert amphiphilic polymer in contrast to ethylene glycol oxide, which is commonly known as a toxic compound used for synthesis of PEG. A consequence of its low reactivity is that PEG does not exhibit toxicity in humans also because mammals do not have enzymes which are able to metabolize PEG. Therefore, once ingested or parenterally administered PEG remains intact until it is eliminated via urine or feces. Upon high-dose exposure in animal toxicology studies not occurring under therapeutic treatment regimens with PEGylated proteins PEG has shown intracellular accumulation in form of vacuoles in certain cells. This is likely the only undesired effect PEG has. With the current understanding of pharmacology, it is unclear whether or not vacuolation should be considered a toxic effect as it does not result in cellular and organ dysfunction. Toxicity seen with PEGylated proteins is always driven by the drug protein but not by PEG. This chapter describes the toxicity of PEG, its precursors and impurities, clearance mechanisms, and immunogenicity of PEG. It then focuses on preclinical safety evaluation of PEGylated biologicals by showing examples of chemistry and toxicology of approved PEGylated proteins. A special paragraph in this respect compares three PEGylated recombinant human coagulation factor VIII products based on three different PEG reagents allowing direct assessment of the toxicological contribution of PEG. Finally, this chapter discusses the relevance of cellular vacuolation observed in nonclinical studies and potential risk for human therapies. It concludes by summarizing toxicological recommendations and regulatory guidance to support clinical development and therapeutic use of PEG–protein conjugates.