Intrinsic sensitivity of tumor cells to bleomycin as an indicator of tumor response to electrochemotherapy.

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.