Relationship between the extent of lipid substitution on poly(L-lysine) and the DNA delivery efficiency.

Poly(L-lysine) (PLL) is a commonly used polymer for nonviral gene delivery. However, the polymer exhibits significant toxicity and is not very effective for transgene expression. To enhance the gene delivery efficiency of the polymer, we imparted an amphiphilic property to PLL by substituting approximately 10% of epsilon-NH2 with several endogenous lipids of variable chain lengths (lipid carbon chain ranging from 8 to 18). Lipid-modified PLL with high molecular weight (approximately 25 vs 4 kDa) was found to be more effective in delivering plasmid DNA intracellularly in clinically relevant bone marrow stromal cells (BMSC). For lipid-substituted 25 kDa PLL, a correlation between the extent of lipid substitution and the plasmid DNA delivery efficiency was obtained. Additionally, transgene expression by BMSC significantly increased (20-25%) when amphiphilic PLLs were used for plasmid delivery as compared to native PLL and the commercial transfection agent Lipofectamine-2000. The transfection efficiency of the polymers was positively correlated with the extent of lipid substitution. The amphiphilic polymers were able to modify the cells up to 7 days after transfection, after which the expression was decreased to background levels within 1 week. We conclude that lipid-substituted PLL can be used effectively as a nonviral carrier for DNA, and the extent of lipid substitution was an important determinant of gene delivery.