Background: Poor endosomal release is one major barrier of gene delivery. Endosomolytic polyethylenimine-melittin conjugates have shown to enhance gene transfer efficiency; however, cytotoxicity due to their general membrane-destabilizing properties limits their application. To overcome this drawback we grafted a polycation with a masked pH-responsive melittin derivate and investigated lytic activity, gene transfer efficiency and cytotoxicity of the resulting conjugate.
Methods: Melittin (Mel) was modified with dimethylmaleic anhydride (DMMAn) and covalently coupled to poly-L-lysine (PLL). The membrane lytic activity was analyzed after incubation at neutral or endosomal pH. PLL-DMMAn-Mel polyplexes were generated in HEPES-buffered glucose and tested in transfection experiments using luciferase as reporter gene. Cellular cytotoxicity was analyzed by measurement of membrane integrity and metabolic activity.
Results: Covalent attachment of DMMAn-modified melittin to PLL resulted in a pH-responsive conjugate. No lytic activity was observed at neutral pH; after acidic cleavage of the protecting groups at pH 5 lytic activity was regained. Acute toxicity was greatly reduced (as compared to PLL-Mel or even unmodified PLL) and high gene expression levels (up to 1800-fold higher than unmodified PLL) were obtained.
Conclusions: Modification of the polycationic carrier PLL with DMMAn-masked melittin not only enhances gene transfer efficiency, but also strongly reduces the acute toxicity of melittin and PLL. Hence this modification might be useful for optimizing polycationic gene carriers.