Optimization of electric pulse parameters for efficient electrically-assisted gene delivery into canine skeletal muscle Darja Pavlin 1 , Nata{a Tozon 1 , Gregor Ser{a 2 , Azra Poga~nik 1 , Maja ^ema`ar 2 1 Veterinary Faculty, University of Ljubljana, Gerbi~eva 60, SI-Ljubljana; 2 Department of Experimental Oncology, Institute of Oncology, Zalo{ka 2, SI-Ljubljana Skeletal muscle is an attractive target tissue for delivery of therapeutic genes, since it is usually large mass of well vascularized and easily accessible tissue with high capacity for synthesis of proteins, which can be secreted either locally or systemically. Electrically-assisted gene delivery into skeletal muscle of a number of experimental animals has already been achieved using two different types of electroporation (EP) protocols. The first one utilized only low voltage electric pulses with long duration (e.g. 100-200 V/cm, 20-50 ms). Lately it has been shown, that better transfection efficiency can be achieved using combination of high voltage (HV) electric pulses (600-800 V/cm, 100 µs), which cause permeabilization of cell membrane, followed by low voltage (LV) electric pulses to enable electrophoresis of DNA across destabilized cell membrane. The aim of this study was to determine optimal EP protocol for delivery of plasmid DNA into canine skeletal muscle. For this purpose we injected 150 µg/150 µl of plasmid, encoding green fluorescence protein (GFP), intramusculary into m. semitendinosus of 6 beagle dogs. Electric pulses were delivered 20 minutes after plasmid injection, with electric pulses generator Cliniporator (IGEA, Carpi, Italy), using needle electrodes. Altogether 5 different EP protocols were utilized, each applied to two muscles. Three of these protocols utilized combination of one HV pulse (600 V/cm, 100 µs), followed by different number of LV pulses. Two protocols were performed by application of LV pulses only. The control group received only injection of plasmid without application of electric pulses. Incisional biopsies of transfected muscles were performed 2 and 7 days after the procedure and transfection efficiency was determined using fluorescence microscope on frozen muscle samples. 100p28 The highest level of GFP fluorescence in the muscle was observed in EP protocol, using either 1 HV pulse, followed by 4 LV pulses (80 V/cm, ms, 1Hz) or with 8 LV pulses (200 V/cm, 20 ms, 1Hz). In both protocols significant GFP fluorescence was detectable both at 2 and 7 days after transfection. Markedly lower degree of transfection was achieved using EP protocol, utilizing 1 HV, followed by 8 LV (80 V/cm, 400 ms, 1Hz). GFP fluorescence was less pronounced compared to the first two protocols. Furthermore, it was detectable only on muscle samples, taken at day 2 after electrotransfection. No GFP fluorescence was detectable either at 2 or 7 days after electrotransfection on muscle samples, taken from control group and from groups, where 1 HV, followed by 1 LV (80 V/cm, 400 ms) or 6 LV pulses (100 V/cm, 60 ms, 1Hz) were used.
Tissue swelling at the site of electroporation, which spontaneously resolved within 2 to 3 days after the procedure, was the only observed side-effect of the procedure. In conclusion, according to our study, it is possible to achieve good transfection efficiency of canine skeletal muscle using two different EP protocols, either combination of 1 HV (600 V/cm, 100 µs) pulse, followed by 4 LV pulses (80 V/cm, 100 ms, 1Hz), or 8 LV pulses (200 V/cm, 20 ms, 1Hz), resulting in expression of transgene, lasting at least 7 days. 101