An acetal-based PEGylation reagent for pH-sensitive shielding of DNA polyplexes

p-Piperazinobenzaldehyde methoxy poly(ethylene glycol) (mPEG, 5 kDa) acetal was synthesized by the Buchwald-Hartwig coupling reaction from piperazine and p-bromobenzaldehyde mPEG acetal. Introduction of a maleimide moiety yielded a novel acetal-based PEGylation reagent (PEG-acetal-MAL) for pH-sensitive conjugation of PEG to thiol-functionalized biomolecules. For reversible shielding of polyplexes, PEG-acetal-MAL was conjugated to polyethylenimine (PEI). At 37 degrees C, the PEG-acetal-PEI conjugate had a half-life of 3 min at endosomal pH 5.5 and 2 h at physiological pH 7.4, respectively. PEI polyplexes containing PEG-acetal-PEI had a zeta potential of +3 mV and were stable to salt-induced aggregation for 2 h at pH 7.4. In contrast, at endosomal pH, the particles were deshielded and aggregated within 0.5 h. Epidermal growth factor or transferrin receptor-targeted polyplexes shielded with the pH-sensitive PEG-acetal mediated enhanced luciferase gene expression in receptor-expressing target cells (Renca-EGFR or K562) as compared to stably shielded control polyplexes. Thus, the novel PEG-acetal-MAL reagent may present a versatile tool for drug and gene delivery formulations when pH-sensitive PEGylation is preferred.