Carbohydrate-specifically polyethylene glycol-modified ricin A-chain with improved therapeutic potential

Ricin A-chain, which exhibits excellent cytotoxicity to tumor cells, has been widely used as an immunotoxin source. However, it has the fatal shortcoming of poor pharmacokinetics due to the tremendous liver uptake via carbohydrate-mediated recognition. Modification of proteins with polyethylene glycol, PEGylation, has the advantages of shielding the specific sites and prolonging the biological half-life. In this study, the carbohydrate-specific PEGylation of ricin A-chain was considered to be a novel approach to overcome this limitation. The carbohydrate group of ricin A-chain was oxidized by sodium m-periodate and further specifically conjugated with hydrazide-derivatized PEG. For a comparative study, the PEGylated ricin A-chain at amino groups was prepared using the hydroxysuccinimide ester-derivatized PEG. The carbohydrate-specifically PEGylated ricin A-chain showed a markedly lower liver uptake and systemic clearance compared with the amine-directly PEGylated ricin A-chain as well as the unmodified ricin A-chain. Furthermore, carbohydrate-specifically PEGylated ricin A-chain showed a significantly higher in vitro ribosome-inactivating activity than the amine-directly PEGylated ricin A-chain. These findings clearly demonstrate that the carbohydrate-specificity as well as PEGylation plays an important role in improving the in vivo pharmacokinetic properties and in vitro bioactivity. Therefore, these results suggest that the therapeutic use of immunotoxins constructed using this carbohydrate-specifically PEGylated ricin A-chain has potential as a cancer therapy.