Abstract 3058: Molecular imaging of hypoxia to evaluate synergy between HER2 targeted therapy and fractionated radiation in murine models of HER2+ breast cancer (2021)

Abstract Introduction: Tumor hypoxia contributes to intra-tumoral heterogeneity and decreases efficacy of cytotoxic therapy. Fractionated radiation is a component of standard-of-care therapy for patients with unresectable human epidermal growth factor receptor 2 (HER2+) breast cancer. This study investigates if [18F]-fluoromisonidazole (FMISO)-PET imaging can quantify anti-HER2 therapy-induced changes in tumor oxygenation and utilize imaging metrics to monitor the synergy of combination targeted and fractionated radiation. Improving treatment synergy in HER2+ breast cancer has potential to increase therapeutic efficacy without increasing cytotoxic therapy. Methods: For in vivo studies, HER2+ cell line (BT474 and MDA-MB-361) and HER2+ patient derived xenograft (BCM 3472) tumor models were engrafted in mice and treated with single agent or combination trastuzumab (4 mg/kg) and fractionated radiation (6/3 Gy). Mice were imaged with FMISO-PET imaging on days 0, 3, and 7 to monitor changes in tumor hypoxia. Hypoxic fraction within the tumor was quantified by a voxel-wise basis with a hypoxic threshold of mean standardized uptake value (SUV) of the muscle + 1 STD. Tumors were either extracted for biological validation of imaging or continued to measure tumor size changes for longitudinal assessment of response. Immunohistochemical analysis included staining for pimonidazole (hypoxia), CD-31 (microvessel), cleaved caspase-3 (apoptosis) and Ki-67 (proliferation). Bliss test of independence and a non-parametric T-test was used to assess for treatment synergy and significance, respectively. Results: In vivo, MDA-MB-361 tumors treated with trastuzumab and fractionated radiation revealed a SUVmean of 0.20 ± 0.09 on day 7, while tumors treated with fractionated radiation had a FMISO SUVmean of 0.31 ± 0.06 (p=0.05). MDA-MB-361 tumors treated with a control or fractionated radiation were 93.7% and 87.7% hypoxic, respectively; whereas, tumors treated with trastuzumab + fractionated radiation were significantly decreased (76.3% hypoxic, p=0.05 and p=0.26, respectively). 30 days after start of therapy, BCM3472 tumors treated with single agent fractionated radiation had a tumor volume of 285.3 ± 59 mm3, whereas tumors treated with trastuzumab and fractionated radiation had a tumor volume of 104 ± 24 mm3 (p&lt;0.05). Treatment synergy started 14 days after start of therapy. Conclusion: HER2+ breast cancer treated with trastuzumab prior to fractionated radiation decreases hypoxia and synergistically increases efficacy of radiotherapy. FMISO-PET imaging has potential to identify in vivo response to combination therapies and better understand changes in the tumor microenvironment to guide combination therapy. Citation Format: Patrick Song, Yun Lu, Sharon Samuel, Katherine Heinzman, Suzanne Lapi, Anna Sorace. Molecular imaging of hypoxia to evaluate synergy between HER2 targeted therapy and fractionated radiation in murine models of HER2+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3058.