Covalent conjugation of a biologically stable polymer to a therapeutic protein, e.g., an antibody, holds many benefits such as prolonged plasma exposure of the protein and improved tumor uptake. Generation of defined conjugates is advantageous in many applications, and a range of site-selective conjugation methods have been reported. Many current coupling methods lead to dispersity in coupling efficiencies with subsequent conjugates of less-well-defined structure, which impacts reproducibility of manufacture and ultimately may impact successful translation to treat or image diseases. We explored designing stable, reactive groups for polymer conjugation reactions that would lead to conjugates through the simplest and most abundant residue on most proteins, the lysine residue, yielding conjugates in high purity and demonstrating retention of mAb efficacy through surface plasmon resonance (SPR), cell targeting, and in vivo tumor targeting. We utilized squaric acid diesters as coupling agents for selective amidation of lysine residues and were able to selectively conjugate one, or two, high-molecular-weight polymers to a therapeutically relevant antibody, 528mAb, that subsequently retained full binding specificity. Water-soluble copolymers of N-(2-hydroxypropyl) methacrylamide (HPMA) and N-isopropylacrylamide (NIPAM) were prepared by Reversible Addition-Fragmentation chain-Transfer (RAFT) polymerization and we demonstrated that a dual-dye-labeled antibody-RAFT conjugate (528mAb-RAFT) exhibited effective tumor targeting in model breast cancer xenografts in mice. The combination of the precise and selective squaric acid ester conjugation method, with the use of RAFT polymers, leads to a promising strategic partnership for improved therapeutic protein-polymer conjugates having a very-well-defined structure.
Keywords: RAFT polymerization; antibody conjugation; bioconjugation chemistry; polymer−protein conjugation; squaramide.