Electroporation-enhanced gene delivery in mammary tumors

Electroporation was applied to enhance gene transfer into subcutaneous MC2 murine breast tumors. Cultured MC2 cells were also transfected by electroporation or by cationic liposomes in the presence of serum using pSV-luc plasmids. Electroporation parameters and liposome formulation were optimized to achieve the highest relative levels of transfection. An electric field threshold for successful electrotransfection in cultured cells appeared around 800-900 V/cm. The liposomes used contained the cationic lipid dioleoyl-3-trimethylammonium propane (DOTAP). Multilamellar vesicles (MLV) had a 10-fold advantage over small unilamellar vesicles (SUV) in cell culture transfection. For in vivo gene delivery, the plasmids were injected either alone, or in complex with MLV or SUV DOTAP liposomes. A series of six electric pulses 1 ms long were applied across tumors, using caliper electrodes on the skin surface. Electric field strengths ranged from 400-2300 V/cm. Luciferase expression was approximately two orders of magnitude higher than controls in tumors treated with pulses > or =800 V/cm. Differences between enhanced relative levels of transfection using uncomplexed plasmid and lipoplexes were not statistically significant. Distribution of DNA into tumor tissues was monitored by fluorescence in situ PCR. The highest numbers of fluorescent cells were found in tumors electroporated following the injection of plasmid. The significant transfection improvement shows that in vivo electroporation is a powerful tool for local gene delivery to tumors.