18F-Flortanidazole Hypoxia PET Holds Promise as a Prognostic and Predictive Imaging Biomarker in a Lung Cancer Xenograft Model Treated with Metformin and Radiotherapy.

ABSTRACT

Metformin may improve tumor oxygenation and thus radiotherapy response, but imaging biomarkers for selection of suitable patients are still under investigation. First, we assessed the effect of acute metformin administration on non-small cell lung cancer xenograft tumor hypoxia using PET imaging with the hypoxia tracer 18F-flortanidazole. Second, we verified the effect of a single dose of metformin before radiotherapy on long-term treatment outcome. Third, we examined the potential of baseline 18F-flortanidazole as a prognostic or predictive biomarker for treatment response. Methods: A549 tumor-bearing mice underwent a 18F-flortanidazole PET/CT scan to determine baseline tumor hypoxia. The next day, mice received a 100 mg/kg intravenous injection of metformin. 18F-flortanidazole was administered intravenously 30 min later, and a second PET/CT scan was performed to assess changes in tumor hypoxia. Two days later, the mice were divided into 3 therapy groups controls (group 1), radiotherapy (group 2), and metformin + radiotherapy (group 3). Animals received saline (groups 1-2) or metformin (100 mg/kg; group 3) intravenously, followed by a single radiotherapy dose of 10 Gy (groups 2-3) or sham irradiation (group 1) 30 min later. Tumor growth was monitored triweekly by caliper measurement, and tumor volume relative to baseline was calculated. The tumor doubling time (TDT), that is, the time to reach twice the preirradiation tumor volume, was defined as the endpoint. Results: Thirty minutes after metformin treatment, 18F-flortanidazole demonstrated a significant change in tumor hypoxia, with a mean intratumoral reduction in 18F-flortanidazole tumor-to-background ratio (TBR) from 3.21 ± 0.13 to 2.87 ± 0.13 (P = 0.0001). Overall, relative tumor volume over time differed across treatment groups (P < 0.0001). Similarly, the median TDT was 19, 34, and 52 d in controls, the radiotherapy group, and the metformin + radiotherapy group, respectively (log-rank P < 0.0001). Both baseline 18F-flortanidazole TBR (hazard ratio, 2.0; P = 0.0004) and change from baseline TBR (hazard ratio, 0.39; P = 0.04) were prognostic biomarkers for TDT irrespective of treatment, and baseline TBR predicted metformin-specific treatment effects that were dependent on baseline tumor hypoxia. Conclusion: Using 18F-flortanidazole PET imaging in a non-small cell lung cancer xenograft model, we showed that metformin may act as a radiosensitizer by increasing tumor oxygenation and that baseline 18F-flortanidazole shows promise as an imaging biomarker.