Abstract 6189: Identifying efficacy of targeted HER2 antibodies in sensitization of HER2+ breast cancer to fractionated radiation

Background: Radiation therapy is one component of standard-of-care therapy for patients with breast cancer; however, treatment is often associated with patient toxicity. Fractionated radiation has emerged as an alternative to single dose radiation; although, there have been limited studies investigating it in combination with other therapies. Positron emission tomography (PET) imaging allows for detection of biological and molecular alternations. In this study, we utilized [18F]-fluoromisonidazole (FMISO)-PET imaging to monitor changes in tumor hypoxia to quantify tumor response to combination targeted plus radiation therapy. Methods: To assess for cell death, breast cancer cells (BT474, SKBR3, MDA MB 361, and MCF-7) were treated with 1 ug/mL trastuzumab (anti-HER2 targeted therapy) and radiation therapy and assessed for cell death four days later (N = 4 replicates per group). To assess for DNA double strand breaks (DSB), cells were probed for γ-H2AX with flow cytometry 30 minutes post treatment. For in vivo studies, 107 BT474 cells were subcutaneously injected into athymic nude mice (N = 3 for control and trastuzumab treated, N = 4 for radiation and combination therapy). Mice were treated with trastuzumab (4 mg/kg) on days 0 and 3 and radiation (6/3 Gy) on days 1, 2 and 3. Controls included single agent therapies and saline treated. Changes in hypoxia were assessed with FMISO PET imaging on days 0, 3 and 7. Mean and standard error of standardized uptake value (SUV) was quantified at each time point. Treatment synergy and statistical significance were assessed with Bliss test of independence and non-parametric T-test, respectively. Results:In vitro cell death assay revealed that trastuzumab or radiation treated groups exhibited 19% ± 2% or 50% ± 9% cell death respectively, while trastuzumab prior to radiation exhibited 76% ± 5.3% cell death (p = 0.01) in BT474 cells. Analysis of γ-H2AX flow cytometry showed that trastuzumab prior to radiation exhibited 40.8% ± 11% DSB, which significantly increased from trastuzumab (0.5% ± 0.4%) or radiation only groups (9% ± 7.2%) (p = 0.01). Bliss test of independence confirmed synergy of trastuzumab and fractionated radiation. In vivo data revealed that BT474 tumors treated with trastuzumab and radiation experienced a 45.8% ± 9.1% decrease in volume, while radiation only experienced a 23.2% ± 13.3% decrease from day 0 to 7 (p = 0.26). FMISO PET imaging indicated that tumors treated with trastuzumab and radiation therapy experienced a 20.5% ± 4% decrease in [18F]-FMISO tumor:muscle SUV, while radiation only tumors experience a 61.2% ± 33.3% increase in tumor:muscle SUV from day 0 to 7 (p = 0.01). Conclusions: Preliminary data reveals that treatment of cancer cells with trastuzumab synergistically increases the efficacy of fractionated radiation in vitro and in vivo. FMISO PET imaging may provide insight regarding the optimal treatment window of radiation therapy. Citation Format: Patrick Song, Yun Lu, Sharon Samuel, Anna Sorace. Identifying efficacy of targeted HER2 antibodies in sensitization of HER2+ breast cancer to fractionated radiation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6189.