Efficient gene transfer using reversibly cross-linked low molecular weightpolyethylenimine

Polyethylenimine (PEI) is a polycation with potential application as a nonv iral vector for gene delivery. Here we show that after conjugation with hom obifunctional amine reactive reducible cross-linking reagents, low molecula r weight polyethylenimine efficiently mediates in vitro gene delivery to Ch inese hamster ovary (CHO) cells. Two cross-linking reagents, dithiobis(succ inimidylpropionate) (DSP) and dimethyl-3,3'-dithiobispropionimidate-2HCI (D TBP), were utilized based on their reactivity and chemical properties. Both reagents react with primary amines to form reducible cross-links; however, unlike DSP, the DTBP cross-linker maintains net polymer charge through ami dine bond formation. PEI with a reported weight-average molecular weight (M -w) of 800 Da was reacted with either DSP or DTBP at PEI primary amine:cros s-link reactive group ratios of 1:1 and 2:1. The transfection efficiencies of the resulting cross-linked products were evaluated in CHO cells using a luciferase reporter gene under a cytomegalovirus (CMV) promoter. Our result s show that cross-linked polymers mediate variable levels of transfection d epending on the cross-linking reagent, the extent of conjugation, and the N IP ratio. In general, we found conjugate size to be proportional to gene tr ansfer efficiency. Using gel retardation analysis, we also evaluate the cap acity of the cross-linked polymers to condense plasmid DNA before and after reduction with 45 mM dithiothreitol (DTT). DTT mediated reduction of intra cross-link disulfide bonds and inhibited condensation of DNA by conjugates cross-linked with DSP at a ratio of 1:1, but had little effect on the remai ning polymers. Analogous intracellular reduction of transfection complexes by reduced glutathione could facilitate uncoupling of PEI from DNA to enhan ce gene expression.