Electrochemotherapy (ECT) is an effective local tumor ablative technique, with an objective response over 80%. Nevertheless, it has different effectiveness in different histological types of tumors. ECT treatment of basal cell carcinoma is the most effective, with approximately 90% of complete responses (CR), followed by melanoma (60-70% CR) while in breast cancer there are only around 50% of CR (2, 3). Therefore combining ECT with cisplatin or bleomycin with other therapies to increase the complete response rate is required. One of the possibilities is addition of DNA damage repair mechanism inhibitors, in our case poly (adenosin diphosphate-ribose(ADP)) polymerase or PARP. PARP1 and 2 are required to repair single strand breaks (SSBs), while PARP1 is also involved in repair of double strand breaks (DSBs) and in stalling of replication forks. Therefore, the aim of our study was to combine ECT with cisplatin or bleomycin with PARP inhibitor olaparib in murine breast cancer cell line MCF7 in vitro. We used standard ECT pulses (8 pulses, 100 µs, 1300 v/cm, 1Hz). The cytotoxic effect of both drugs was first determined. Concentration escalation of olaparib was also tested in MCF7 cells, but without pulses since olaparib is highly permeable drug. Combination of IC50 dose of cisplation or bleomycin and olaparib was then tested in cells and later on 3D cell cultures, spheroids, in vitro. Growth of cells after the combined therapy was measured by PrestoBlue assay and growth of spheroids after the combined therapy by measurement of the diameter and area. We determined IC50 doses for cisplatin and for bleomycin. For combination study, we selected the dose of olaparib, which alone did not cause any statistically significant cell death (5 µM). After combination IC50 dose of cytotoxic drug for ECT and olaparib, growth of MCF7 cells was significantly reduced for 80% at day 5 after the treatment. Similar effect was observed in spheroids, where diameter and their area decreased after the combined treatment with bleomycin. Immunofluorescent staining showed increased number of ?-H2AX foci after both ECT and ECT + olaparib compared to pertinent controls To conclude, we showed that combining ECT with PARP inhibitor olaparib in MCF7 human breast cancer cell line increase effectiveness of ECT. ECT causes DNA breaks, whose repair is abrogated because of the addition of PARP inhibitor olaparib.
COBISS.SI-ID: 3333755