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Nonviral nucleic acid delivery to cells and tissues is considered a standard tool in life science research. However, although an ideal delivery system should have high efficacy and minimal toxicity, existing materials fall short, most of them being either too toxic or little effective. We hypothesized that disulfide cross-linked lowmolecular-weight (MW) linear poly(ethylene imine) (MW <4.6 kDa) would overcome this limitation. Investigations with these materials revealed that the extracellular high MW provided outstandingly high transfection efficacies (up to 69.62 ؎ 4.18% in HEK cells). We confirmed that the intracellular reductive degradation produced mainly nontoxic fragments (cell survival 98.69 ؎ 4.79%). When we compared the polymers in >1,400 individual experiments to seven commercial transfection reagents in seven different cell lines, we found highly superior transfection efficacies and substantially lower toxicities. This renders reductive degradation a highly promising tool for the design of new transfection materials.biodegradable ͉ polyethylenimine ͉ nucleic acid ͉ disulfide bond ͉ transfection G ene transfer into cells has become a standard procedure in life science research. Nonviral carriers have gained in importance in recent years because of their safety in handling and ease of application compared with viral vectors. Browsing databases such as PubMed or Chemical Abstracts reveals that almost every publication related to genetics, biochemistry, molecular biology, developmental biology, or neurology incorporated lipid-or polymer-based transfection as a pivotal tool for the investigation of various cellular processes. Therefore, it appears that the delivery of nucleic acids is a well established method for the transfection of mammalian cells, and, given the amount of research data that has been produced, one would assume that the procedures for nonviral transfection would be well optimized. A closer look at the contemporary literature, however, reveals that doubts seem justified. Contemporary transfection reagents are almost universally toxic because of their cationic or amphiphilic character (1-4).The dilemma that we face is exemplified by considering the polymeric transfection agent poly(ethylene imine) (PEI). Since its introduction in 1995, PEI has been considered the gold standard for polymer-based gene carriers because of the relatively high transfection efficacy of its polyplexes (complex of nucleic acid and polymer) (5, 6). However, it has been shown that both the efficacy and toxicity of PEI are strongly correlated with its molecular weight (MW) as well as its structure (branched or linear: bPEI or lPEI, respectively) (7-17). Efficacy and adverse reactions seem thereby to be strongly associated. PEI is not the only material that suffers from this ''malignant'' correlation. Nonviral transfection seems like choosing between Scylla and Charybdis: either a high transfection efficacy, which is associated with a devastating toxicity, or a low efficacy, which does not affect the cell viability at ...

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Our study suggests that LPEIs with low MWs are promising candidates for non-viral gene delivery, because they are more efficient and substantially less toxic than their higher MW counterparts.