The PEG modification of protein drugs is mainly to convert the hydroxyl group of PEG into various functional groups, and then control the reaction conditions to positionally modify the naked amino group, carboxyl group, sulfhydryl group, hydroxyl group, and even positional modification of arginine. 

But there is a problem that it cannot be quantitatively modified. Protein-based drugs all have the disadvantage of reduced activity or even disappearance after being modified by PEG. In order to avoid this problem, the PEG modification of synthetic peptide drugs mostly adopts the method of fixed-point quantification. The main modification sites of peptide synthetic drugs: the C-terminal and N-terminal of the peptide chain, the side chain amino group, carboxyl group and sulfhydryl group in the peptide chain.