No abstract

There is much interest in attaching polyethylene glycol (PEG) and other hydrophilic, neutral polymers to surfaces to reduce the extent of protein and cell adsorption. Interestingly, these same surface-bound polymers are effective in masking surface charge and reducing electrokinetic effects such as particle electrophoretic mobility, streaming potential, and electroosmosis. It is apparent that similar molecular properties are responsible for both protein and cell rejection and reduction of electrokinetic effects. In this work we compared the fibrinogen-rejecting ability and the effect on electrophoretic mobility of three polymer coatings bound to polystyrene. The three polymers were side-bound dextran, end-bound dextran, and end-bound PEG. The results of these measurements were used to elucidate the importance of polymer packing density and polymer layer thickness on protein adsorption and reduction of electrokinetic effects. Protein adsorption appears not to be sensitive to polymer layer thickness or the presence of dilute polymer tails in a surface coating, while electrokinetic effects are. Protein adsorption is, however, very sensitive to the availability of exposed surface. Finally, the unique effectiveness of PEG is apparent in this research as in previous studies.

SynopsisFive general routes for the preparation of polyoxyethylene [generally referred to as poly(ethy1ene glycol) or PEG] derivatives are described. These routes are (1) nucleophilic displacements with the alkoxide of PEG, (2) nucleophilic displacement on PEG-tosylate, -mesylate, or -bromide, (3) reductive amination of PEG-aldehyde, (4) reductive amination of PEG-amine, and (5) nucleophilic displacements on the s -triazine derivative prepared from s-triazine trichloride (cyanuric chloride) and PEG. Eighteen derivatives are prepared and potential applications to catalysis, cell purifications, and other areas are discussed briefly.

Poly(ethylene glycol)-conjugated (or PEGylated) proteins are an increasingly important class of therapeutic proteins that offer improved in vivo circulation half lives over their corresponding native forms. Their production involves covalent attachment of one or more poly(ethylene glycol) molecules to a native protein, followed by purification. Because of the extremely high costs involved in producing native therapeutic proteins it is important that subsequent PEGylation processes are as efficient as possible. In this paper, reaction engineering and purification issues for PEGylated proteins are reviewed. Paramount considerations for PEGylation reactions are specificity with respect to the conjugation site and overall yield. Batch PEGylation reaction methods are discussed, along with innovative methods using packed bed or "on-column" approaches to improve specificity and yield. Purification methods are currently dominated by ion exchange and size exclusion chromatography. Other methods in common use for protein separations, including hydrophobic interaction chromatography, affinity chromatography and membrane separations, are rarely used in PEGylated protein purification schemes. A better understanding of the effects of PEGylation on the physicochemical properties of proteins (isoelectric point, surface charge density and distribution, molecular size and relative hydrophobicity) and interactions between PEGylated proteins and surfaces is needed for the future development of optimal purification processes and media.