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Drug delivery systems employing 1,6-elimination: releasable poly(ethylene glycol) conjugates of proteins.

Using lysozyme as a representative protein substrate that loses its activity when PEGylation takes place on the epsilon-amino group of lysine residues, various amounts of a novel releasable PEG linker (rPEG) were conjugated to the protein. rPEG-lysozyme conjugates were relatively stable in pH 7.4 buffer for over 24 h. However, regeneration of native protein from the rPEG conjugates occurred in a predictable manner during incubation in high pH buffer or rat plasma, as demonstrated by enzymatic activity and structural characterization. The rates of regeneration were also correlated with PEG number: native lysozyme was released more rapidly from the monosubstituted conjugate than from the disubstituted conjugate, suggesting possible steric hindrance to the approach of cleaving enzymes. Recovery of normal activity and structure for the regenerated native lysozyme was shown by a variety of assays.