Site‐Specific Zwitterionic Polymer Conjugates of a Protein Have Long Plasma Circulation

Many proteins suffer from suboptimal pharmacokinetics (PK) that limit their utility as drugs. The efficient synthesis of polymer conjugates of protein drugs with tunable PK to optimize their in vivo efficacy is hence critical. We report here the first study of the in vivo behavior of a site‐specific conjugate of a zwitterionic polymer and a protein. To synthesize the conjugate, we first installed an initiator for atom‐transfer radical polymerization (ATRP) at the N terminus of myoglobin (Mb‐N‐Br). Subsequently, in situ ATRP was carried out in aqueous buffer to grow an amine‐functionalized polymer from Mb‐N‐Br. The cationic polymer was further derivatized to two zwitterionic polymers by treating the amine groups of the cationic polymer with iodoacetic acid to obtain poly(carboxybetaine methacrylate) with a one‐carbon spacer (PCBMA; C1), and sequentially with 3‐iodopropionic acid and iodoacetic acid to obtain PCBMA(mix) with a mixture of C1 and C2 spacers. The Mb‐N‐PCBMA polymer conjugates had a longer in vivo plasma half‐life than a PEG‐like comb polymer conjugate of similar molecular weights (MW). The structure of the zwitterion plays a role in controlling the in vivo behavior of the conjugate, as the PCBMA conjugate with a C1 spacer had significantly longer plasma circulation than the conjugate with a mixture of C1 and C2 spacers.