Native protein-initiated ATRP: A viable and potentially superior alternative to PEGylation for stabilizing biologics

Comparison of in vitro serum stability and enzyme activity retention for PEGylated chymotrypsin and structurally different, biocompatible vinyl polymer grafts of chymotrypsin was performed. These polymer grafts were synthesized by atom transfer radical polymerization (ATRP) initiated by chymotrypsin covalently modified with 2-bromoisobutyric acid, the ATRP initiator. The maximum number of ATRP initiators attached to chymotrypsin was adjusted to be as close as possible to the maximum number of polyethylene glycol chains attached to chymotrypsin for better comparison and then polymerizations were conducted. In mouse serum, native and PEGylated chymotrypsin deactivated within 24 h, whereas chymotrypsin-graft-poly(N-2-hydroxypropylmethacrylamide) retained >50% of its catalytic activity even after 5 days of incubation. In human serum, PEGylated chymotrypsin deactivated within 4 days of incubation, whereas native chymotrypsin and chymotrypsin-graft-poly(N-2-hydroxypropylmethacrylamide) and chymotrypsin-graft-poly(2-methacryloyloxyethyl phosphorylcholine) retained >25% catalytic activity after 5 days of incubation. Biocompatible vinyl polymer grafts of chymotrypsin synthesized by protein-initiated ATRP had higher catalytic activity retention and molecular weights and lower polydispersity than PEGylated chymotrypsin. In summary, studying the effects of structures of conjugated polymers on the stability and activity retention of modified proteins can lead to identification of a polymer–protein conjugate having superior pharmacological properties than conventionally PEGylated protein. Also, since vinyl monomers that form biocompatible polymers are easily polymerizable by ATRP, protein-initiated ATRP can become a viable and potentially superior alternative to PEGylation for stabilizing biologics.