New multivalent cationic lipids reveal bell curve for transfection efficiency versus membrane charge density: lipid-DNA complexes for gene delivery

Background Gene carriers based on lipids or polymers—rather than on engineered viruses—constitute the latest technique for delivering genes into cells for gene therapy. Cationic liposome–DNA (CL‐DNA) complexes have emerged as leading nonviral vectors in worldwide gene therapy clinical trials. To arrive at therapeutic dosages, however, their efficiency requires substantial further improvement. Methods Newly synthesized multivalent lipids (MVLs) enable control of headgroup charge and size. Complexes comprised of MVLs and DNA have been characterized by X‐ray diffraction and ethidium bromide displacement assays. Their transfection efficiency (TE) in L‐cells was measured with a luciferase assay. Results Plots of TE versus the membrane charge density (σM, average charge/unit area of membrane) for the MVLs and monovalent 2,3‐dioleyloxypropyltrimethylammonium chloride (DOTAP) merge onto a universal, bell‐shaped curve. This bell curve leads to the identification of three distinct regimes, related to interactions between complexes and cells: at low σM, TE increases with increasing σM; at intermediate σM, TE exhibits saturated behavior; and unexpectedly, at high σM, TE decreases with increasing σM. Conclusions Complexes with low σM remain trapped in the endosome. In the high σM regime, accessible for the first time with the new MVLs, complexes escape by overcoming a kinetic barrier to fusion with the endosomal membrane (activated fusion), yet they exhibit a reduced level of efficiency, presumably due to the inability of the DNA to dissociate from the highly charged membranes in the cytosol. The intermediate, optimal regime reflects a compromise between the opposing demands on σM for endosomal escape and dissociation in the cytosol. Copyright © 2005 John Wiley & Sons, Ltd.