The development of biodegradable cationic polymers for use in somatic gene therapy is desirable because degradable polymers have the potential to overcome cellular toxicities that are related to the high charge densities of the polycationic delivery system. Therefore, to produce a biocompatible gene delivery vehicle, we have designed a novel biodegradable, high molecular weight multiblock copolymer (MBC) of the type (AB)(n) which consists of repeating units of low molecular weight poly(ethylene glycol) (PEG) conjugated to low molecular weight cationic poly(L-lysine) (PLL). PEG was used not only to impart steric stabilization properties onto the polymer/pDNA complexes but also to introduce biodegradable ester bond linkages into the backbone of the MBCs. Also, to improve the endosome-disrupting capabilities of the polymer, N,N-dimethylhistidine (His) was coupled at various mole ratios (5 mol % His, 9 mol % His, 16 mol % His, 22 mol % His) to the e-amines of PLL to produce PEG-PLL-grafted-His (PEG-PLL-g-His) MBCs. Polymer screening revealed that MBCs with 16% His grafted (PEG-PLL-g-16% His) (31 kDa) produced the highest transfection efficiency with minimal cytotoxicity in murine smooth muscle cells (A7r5). The MBCs condensed plasmid DNA (pDNA) into nanostructures with an average particle size between 150 and 200 nm with no aggregation and surface charge of similar to4-45 mV. These MBCs also protected pDNA from endonuclease digestion for at least 2 h. The polymers showed exponential decay with a half-life t(1/2) of similar to5 h in PBS, pH 7.4 at 37degreesC. However, complexes incubated in PBS buffer showed complete stability up to 6 days despite the short polymer t(1/2). The pK of the conjugated imidazoles was found to be 4.75 which would facilitate buffering at low pH environments of the late endosome/lysosome. Finally, the ability of the imidazoles to protonate and destabilize membrane vesicles was investigated by the use of bafilomycin A(1) which showed that the MBCs produced about five times higher transfection efficiency in vitro in A7r5 cells compared to the treated cells. This supports the function of histidine as an endosomal disrupting moiety. Therefore, these results suggest that biodegradable multiblock copolymers are promising candidates for long-term gene delivery.