PEGylation

PEGylation attaches one or more polyethylene glycol polymers, usually through a reactive group aimed at a lysine side chain or an engineered cysteine, to a peptide or protein. The polymer is heavily hydrated, so it drags a shell of water with it and makes the conjugate behave as though it were far larger than its true mass. That inflated hydrodynamic radius is the whole mechanism: it pushes the molecule above the glomerular filtration threshold so the kidney stops clearing it efficiently, and it sterically shields the surface from proteases and from antibody recognition.

The effect on exposure can be dramatic. Peginterferon alfa-2a, which carries a branched 40 kDa PEG, has a half-life measured in tens of hours against a few hours for the unmodified interferon, which is what turned a three-times-weekly injection into a weekly one. Pegfilgrastim replaced daily filgrastim dosing after chemotherapy on the same principle. Larger and branched PEGs extend exposure further than small linear ones, and the relationship is driven by size rather than by the chemistry of the attachment point.

The tradeoff is potency. The same shield that hides the molecule from an enzyme also gets in the way of its receptor, so PEGylated conjugates commonly lose intrinsic activity and win only because the loss is outweighed by the longer exposure. Site of attachment matters enormously, and a conjugate PEGylated at a residue near the binding face can be nearly inactive.

Two things are routinely overstated. PEG is not immunologically silent: anti-PEG antibodies are measurable in treatment-naive populations and have been linked to accelerated clearance and to infusion reactions with pegloticase. And for peptides, PEGylation now competes with fatty acid acylation, which achieves comparable half-life extension through albumin binding without adding a large non-degradable polymer.