Towards an optimal dose-response relationship in gene electrotransfer protocols

In search of an optimal gene electrotransfer (GET) protocol, an electroporation-based (EP) tumor treatment with great potential as a non-viral gene-delivery system, the concept of the dose-response relationship is introduced. It is shown that a reliable dose parameter is the pulse dosage and reliable response parameters are the reversibly electroporated tissue area as well as the unwanted damaged tissue area and plasmid damage due to pH. The standard stationary EP model consists in computing the reversibly electroporated tissue area in the first pulse as the region of tissue subjected to an electric field distribution higher than an electric field threshold for EP, where the electric field threshold comes from an experimental measurement and the electric field distribution from the solution of the nonlinear stationary Laplace equation for the electrostatic potential. The extended standard EP model introduced here consists in replicating for n consecutive pulses the standard EP model, via the experimental measurement in time of the successive thresholds. Because experimental data of this threshold variation is lacking, an exponential time decay function is assumed based on experimental measurements. The damage induced by pH fronts is defined as the tissue area subjected to pH abrupt changes above a basic threshold or below an acid threshold, where these changes come from numerical solutions via the electrolytic ablation (EA) model for EP-based protocols and the basic and acid thresholds from experiments. An optimal dose-response relationship in a GET protocol, for the range of pulse intensities with fixed pulse length and frequency, tested here, is predicted as the critical pulse dosage yielding maximum reversibly electroporated tissue area with minimal tissue area damage induced by pH fronts. Moreover, since damage induced by pH changes is proportional to the Coulomb dosage, damage induced by pH fronts is negligible in typical EP-based tumor protocols such as in electrochemotherapy (ECT) and irreversible electroporation (IRE) but not in GET, due to the most often longer pulses applied/used, i.e. higher dosage applied.