541. Species-Specific Characteristics in Muscle Resistance and Their Impact on In Vivo Electroporation

Numerous investigators have outlined the effectiveness of electroporation for enhancing the uptake, and consequently, the expression of plasmids encoding antigens and therapeutic proteins in skeletal muscle in various species of mammals, from mice to men. Nevertheless, constant-voltage electroporation techniques do not measure the resistance of the tissue and may cause tissue damage, inflammation and loss of plasmid expression. In the present study, we have analyzed the muscle resistance of various mammalian species (e.g., mice, dogs, rabbits, pigs, horses and non-human primates) using a software-driven constant-current electroporator that can measure the tissue impedance before and during each electroporation pulse (over 100 measurements in a typical 52msec pulse). Also, voltage and amperage during pulses were automatically recorded and analyzed. Optimal conditions of electroporation were found to be highly dependent on the individual tissue resistance, even when the same muscle is targeted in the different species. For example, 6-week-old pigs had higher muscle resistance measured in the semimembranosus muscle (Pulse 1: 196.8 ± 2.4 Ω, Pulse 2: 203.9 ± 0.7 Ω, Pulse 3: 224.6 ± 1.0 Ω with Pulse 3 versus Pulse 2: p < 0.02, Pulse 1 versus Pulse 3: p < 0.001) compared to 1-year-old pigs (Pulse 1: 165.1 ± 0.4 Ω, P2: 142.8 ± 0.5 Ω, P3: 190.5 ± 1.3 Ω with P1 versus P2: p < 0.001, P3 versus P2: p < 10 -5 , and P1 versus P3: p < 0.003), but both values were 4 to 5 times lower than the resistance of the mouse muscle (Pulse 1: 1153.9 ± 6.3 Ω, Pulse 2: 1388.1 ± 8.4 Ω, p < 0.008). Rabbits had similar resistances to younger pigs but lower than non-human primates. One-year old pigs and horses have comparable muscle resistances, but lower than younger pigs and rabbits. The muscle resistance is also variable in function of the target muscle group and even changes during the electroporation process, with changes in muscle resistance of more than 37% from Pulse 1 to Pulse 3 in non-human primates (33% in 1-year-old pigs, 32% in rabbits, 20% in mice, and 14% in younger pigs). These results indicate that the size of the animal, muscle type and age of the animal, as well as the electroporation sequence impact the electroporation parameters in each species. Thus, muscle resistance will need to be considered for the specific target animals, and accordingly, the electroporation parameters will need to be measured and optimized for each separate species in order to achieve maximum p