Efficient in vivo gene delivery by the negatively charged complexes of cationic liposomes and plasmid DNA

We examined changes in zeta potential (the surface charge density, zeta) of the complexes of liposome (nmol)/DNA (microg) (L/D) formed in water at three different ratios (L/D=1, 10 and 20) by changing the ionic strength or pH to find an optimum formulation for in vivo gene delivery. At high DNA concentrations, zeta of the complexes formed in water at L/D=10 was significantly lowered by adding NaCl (zeta=+8.44+/-3.1 to -27.6+/-3.5 mV) or increasing pH from 5 (zeta=+15.3+/-1.0) to 9 (zeta=-22.5+/-2.5 mV). However, the positively charged complexes formed at L/D=20 (zeta=+6.2+/-3.5 mV) became negative as NaCl was added at alkaline pH as observed in medium (zeta=-19.7+/-9.9 mV). Thus, the complexes formed in water under the optimum condition were stable and largely negatively charged at L/D=1 (zeta=-58.1+/-3.9 mV), unstable and slightly positively charged at L/D=10 (zeta=+8.44+/-3.7 mV), and unstable and largely positively charged at L/D=20 (zeta=+24.3+/-3.6 mV). The negatively charged complexes efficiently delivered DNA into both solid and ascitic tumor cells. However, the positively charged complexes were very poor in delivering DNA into solid tumors, yet were efficient in delivering DNA into ascitic tumors grown in the peritoneum regardless of complex size. This slightly lower gene transfer efficiency of the negatively charged complexes can be as efficient as the positively charged ones when an injection is repeated (at least two injections), which is the most common case for therapy regimes. The results indicate that optimum in vivo lipofection may depend on the site of tumor growth.