Cysteine-specific PEGylation of rhG-CSF via selenylsulfide bond

A new PEGylation reagent enabling selective modification of free thiol groups is described in this article. The reagent was synthesized by attaching linear polyethylene glycol (PEG) N-hydroxysuccinimide to selenocystamine. The reaction was very fast, resulting in over 95% conversion yield. The active group of this new PEG-Se reagent is a diselenide, reacting with thiols via thiol/diselenide exchange reaction. Recombinant human granulocyte colony-stimulating factor (rhG-CSF) with an unpaired cysteine at the position 18 (Cys18) was used as a model protein. It was comparatively PEGylated with the new PEG-Se reagent, as well as with commercially available maleimide (PEG-Mal) and ortho-pyridyl disulfide (PEG-OPSS) PEG reagents. The highest PEGylation yield was obtained with PEG-Mal, followed by PEG-OPSS and PEG-Se. The reaction rates of PEG-Mal and PEG-Se were comparable, while the reaction rate of PEG-OPSS was lower. Purified monoPEGylated rhG-CSF conjugates were characterized and compared. Differences in activity, stability, and in vivo performance were observed, although all conjugates contained a 20 kDa PEG attached to the Cys18. Minor conformational changes were observed in the conjugate prepared with PEG-Mal. These changes were also reflected in low in vitro biological activity and aggregate formation of the maleimide conjugate. The conjugate prepared with PEG-Se had the highest in vitro biological activity, while the conjugate prepared with PEG-OPSS had the best in vivo performance.