Electrochemotherapy (ECT) involves the use of locally applied electric pulses to increase delivery of chemotherapeutic drugs into cells in tissues. ECT with bleomycin (BLM) is a very effective local treatment, but different tumors have different response rates to ECT. The aim of our study was to compare the responsiveness of SA-1 and EAT tumors to BLM and ECT in vitro and in vivo, in order to find possible reasons for the observed difference in response rate. The difference in sensitivity to ECT in vitro between the SA-1 and EAT cells was 10-fold and was the same as the difference in sensitivity to chronic BLM exposure, as measured by tetrazolium-based colorimetric (MTT) assay. This difference in sensitivity between SA-1 and EAT to ECT was also reflected in tumor cure rate. A six-times lower dose of BLM was needed to obtain local tumor control in SA-1 than in EAT tumors. Therefore, we suggest that the difference in sensitivity to BLM and ECT predominantly reflects the difference in intrinsic sensitivity of the cells to BLM.Key words: Electrochemotherapy -Bleomycin -Electroporation -Experimental tumor -Mice Bleomycin (BLM) is a water-soluble glycopeptidic antibiotic with limited antitumor effectiveness, and is currently used only in combined chemotherapeutic schedules in the treatment of cancer.1) The major reason for its limited antitumor effectiveness is the hampered transport of BLM through the plasma membrane. However, once inside the cell, BLM is highly cytotoxic, inducing single and double strand DNA breaks.2) Different approaches have been tested to increase the antitumor effectiveness of BLM, [3][4][5] mainly with the aim of facilitating entry of the drug into the cells. Electropermeabilization (electroporation), i.e., a technique for introduction of molecules into cells by exposure of the cells to intense electrical pulses, proved to be very effective. Under specific conditions, electropermeabilization is a reversible process which does not impair cell viability.
6)Use of electropermeabilization to increase BLM uptake into the cells and consequently to increase the antitumor effectiveness was demonstrated in vitro, in vivo and also in clinical trials.7-21) This treatment was termed electrochemotherapy (ECT) and was also extended to other chemotherapeutic drugs. [22][23][24] Its application to cisplatin has proven to be very effective. 23,24) Preclinical studies of ECT with BLM were performed on a variety of transplantable and spontaneous tumors in immunocompetent and immunodeficient mice. [9][10][11][12][13][14][15][16][17][18] In all of these studies ECT proved to be an effective antitumor treatment, inducing a prolonged tumor growth delay compared to BLM treatment only. In addition, ECT results in tumor cures. In these preclinical studies a certain degree of variation in responsiveness to ECT was observed among the tumor types treated, as well as in the first clinical trials on basal cell carcinoma, malignant melanoma, head and neck squamous cell carcinoma and breast adenocarcinoma.
7-24)The aim of our...