Optimization of Irreversible Electroporation Protocols for In-vivo Myocardial Decellularization

Abstract

Background: Irreversible electroporation (IRE) is a non-thermal cell ablation approach that induces selective damage to cell membranes only. The purpose of the current study was to evaluate and optimize its use for in-vivo myocardial decellularization.

Methods: Forty-two Sprague-Dawley rats were used to compare myocardial damage of seven different IRE protocols with anterior myocardial infarction damage. An in-vivo open thoracotomy model was used, with two-needle electrodes in the anterior ventricular wall. IRE protocols included different combinations of pulse lengths (70 vs. 100 μseconds), frequency (1, 2, 4 Hz), and number (10 vs. 20 pulses), as well as voltage intensity (50, 250 and 500 Volts). All animals underwent baseline echocardiographic evaluation. Degree of myocardial ablation was determined using repeated echocardiography measurements (days 7 and 28) as well as histologic and morphometric analysis at 28 days.

Results: All animals survived 28 days of follow-up. Compared with 50V and 250V, electroporation with 500V was associated with significantly increased myocardial scar and reduction in ejection fraction (67.4%±4% at baseline vs. 34.6%±20% at 28 days; p <0.01). Also, compared with pulse duration of 70 μsec, pulses of 100 μsec were associated with markedly reduced left ventricular function and markedly increased relative scar area ratio (28%±9% vs. 16%±3%, p = 0.02). Decreasing electroporation pulse frequency (1Hz vs. 2Hz, 2Hz vs. 4Hz) was associated with a significant increase in myocardial damage. Electroporation protocols with a greater number of pulses (20 vs. 10) correlated with more profound tissue damage (p<0.05). When compared with myocardial infarction damage, electroporation demonstrated a considerable likeness regarding the extent of the inflammatory process, but with relatively higher levels of extra-cellular preservation.

Conclusions: IRE has a graded effect on the myocardium. The extent of ablation can be controlled by changing pulse length, frequency and number, as well as by changing electric field intensity.

Conflict of interest statement

We have the following interests: Dr. Maor and the Sheba Medical Center have filed a patent application entitled "Myocardial Ablation by Irreversible Electroporation" (Application #: US14/894,349). This patent relates in part to the results presented in this study. In addition, Dr. Maor has a granted patent entitled "Extracellular matrix material created using non-thermal irreversible electroporation" (US8835166 B2). There are no additional patents, products in development or marketed products to declare. This does not alter our adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.

Figures

Fig 1. Animal model and electric field distribution After sterile thoracotomy, IRE was applied to the anterior myocardium located to the left of the lower part of the left anterior descending artery (Fig 1.1 and Fig 1.2).

Electric field distribution induced by two-needle electrodes and a 500 volt pulse are illustrated in Fig 1.3. Note that electric field intensity is concentrated in the plane between the two electrodes.

Fig 2. Average ejection fraction of various study groups during the study period.

The vertical axis stands for ejection fraction (EF) in %. The horizontal axis stands for time point during the study. Each group was treated with a different IRE protocol. Note that the reduction in EF was demonstrated in all the protocols. Some of the groups demonstrated some recovery in EF between day 7 and day 28. Some of the IRE protocols caused more severe damage than the anterior MI group (protocol 8).

Fig 3. Comparison of morphometric measurements between pairs of protocols.

Morphometric measurements of pairs of protocols with only one changed parameter were compared (using unpaired Student t-test), in order to learn about the effect of different parameters on the potency of each protocol. Only pairs with a statistically significant difference in morphometric measurements are shown. From top to bottom: (3a) presents comparisons of scar areas with significant differences. (3b) presents comparisons of scar thicknesses with significant differences. (3c) presents comparisons of scar perimeters with significant differences.

Fig 4. Microscopic view of collagen staining of rat hearts.

The collagen staining demonstrates the presence of extended scar tissue in the myocardium that has been treated by IRE. The control slide (left figure) lacks bluish coloring because of lack of scar. In the right figure, the bluish coloring demonstrates well the typical scaring caused by IRE as was observed in our study.

Fig 5. Comparison of myocardial axial section of different protocols.

Every row in the figure compares the axial section of the left ventricle of rats that were treated with two different IRE protocols. Moreover, all the pairs of protocols selected for this figure are different in only one setting of the IRE protocols (i.e. voltage, frequency, etc.) and demonstrate significant differences between their morphometric measurements. Protocol numbers are in the right upper corner of each slide. The pairs of protocols are (from top to bottom): 5 and 3(70μsec and 100μsec), 8 and 3 (MI vs. IRE), 6 and 4 (10 pulses vs. 20 pulses), 5 and 4 (2Hz vs. 1HZ).

Fig 6. Microscopic view of treated myocardium.

In each row the images from right to left represent: low potency IRE protocol (protocol 2), high potency electroporation protocol (protocol 6), MI group (protocol 8). ED1 staining is presented in the upper row, while H&E staining is presented in the lower row. Note: 1) the similar extent of damage between high potency IRE protocol and MI–images 1B and 1C, 2B and 2C. 2). The greater degree of extracellular damage caused by MI compared with low and high potency IRE protocols–images 2C, 2A, 2B respectively. Arrows point to the areas of extensive inflammation.