ESTRO 2024 - Abstract Book

S5314

Radiobiology - Tumour biology

ESTRO 2024

3 Gunma University, Radiation Oncology, Mebashi, Japan. 4 Dokkyo Medical University, Radiology, Shimotsugagun, Japan. 5 Fukushima Medical University, Radiation Oncology, Fukushima, Japan

Purpose/Objective:

Breast cancer, which have been incidence one million women annually worldwide, is one of the most common malignancies in women. 1) There are five subtypes of breast cancer: Luminal A, Luminal B HER2 negative, Luminal B HER2 positive, HER2 positive non-luminal, and Triple negative, of which Triple negative has the worst prognosis. 2) However, no effective treatment has yet been established. So, focusing on the fact that triple negative type of breast cancer expresses a lot of epidermal growth factor receptor:EGFR, 3) we hypothesized that combination of EGFR inhibitors and radiotherapy on triple negative type of breast cancer would be an effective therapy.

Material/Methods:

We used human breast cancer cell line which is triple negative type. Specifically, we used three cell lines: MDA231, MDA436, and MDA468. And two types of EGFR inhibitors were used: cetuximab and geftinib. Cytotoxicity was assessed by alamerblue or clonogenic assays in cell lines treated with EGFR inhibitor alone or in combinations. There were two protocols for combination EGFR inhibitors and 60 Co γ ray irradiation. One was a pre-treat in which the drug was administrated before 60 Co γ ray irradiation. The other is post-treat, in which the drug is administrated after irradiation. Furthermore, we investigated protein expression to investigate the reason of the radiosensitizing effect caused by EGFR inhibitors. Western blotting was used for protein expression.

Results:

To determine an adequate concentrations of EGFR inhibitor for combination with 60 Co γ ray irradiation, the cytotoxicity of EGFR inhibitor on triple negative breast cancer cell was investigated with the alamerblue assay. In terms of drug dose, an ideal radiosensitizer has little or no cytotoxic effect by itself to minimize the damage to normal tissue and increase the therapeutic ratio. Therefore, the EGFR inhibitor concentration that results in an approximately 20% inhibition of cell viability was used for experiments involving EGFR inhibitor administration for 24 h combined with 60 Co γ ray. The protocol-specific effect of EGFR inhibitor treatment on radiation was confirmed. When TNBC cells were first subjected to radiation before by EGFR inhibitor, this treatment resulted in an enhancement of the radiation effect. However, administering EGFR inhibitor after radiation did not enhance the effect of radiation. In the case of MDA231 cells, comparing the cell survival at 8 Gy, 60 Co γ ray alone was 0.006 and the pre-treat was 0.0003, respectively. (P> 0.01) In addition, the results of examining EGFR protein expression by Western blotting indicated that EGFR activation was suppressed by the combination of radiation and an EGFR inhibitor compared to radiation alone.

Conclusion:

EGFR inhibitor has potential to enhance radiation effect in triple negative breast cancer cell lines. The radiosensitizing effect of the EGFR inhibitor was thought to be caused by the EGFR inhibitor suppressing EGFR signaling pathways in triple negative breast cancer cells, as our data showed that only sensitizing effects showed pre-treat. We will have continued further studies and are going to investigate the mechanism why EGFR inhibitor has enhanced the radiation