Significant impact of different oxygen breathing conditions on noninvasive in vivo tumor-hypoxia imaging using [18F]-fluoro-azomycinarabino-furanoside ([18F]FAZA)

Background: [F-18]FAZA is a PET biomarker with great potential for imaging tumor hypoxia. Aim of our study was to compare [F-18]FAZA uptake in mice with subcutaneous exogenous CT26 colon carcinomas and endogenous polyoma middle-T (PyV-mT) mammary carcinomas and to analyze the influence of different breathing protocols in CT26 colon carcinomas as well as the reversibility or irreversibility of [F-18]FAZA uptake. Methods: We injected subcutaneous CT26 colon carcinoma or polyomavirus middle-T (PyV-mT) mammary carcinoma-bearing mice intravenously with F-18-FAZA and performed PET scans 1-3 h post injection (p.i.). To analyze the impact of oxygen supply in CT26 carcinomas we used three different breathing protocols: (P0) air; (P1) 100% oxygen 1 h prior injection until 3 h p.i.; (P2) 100% oxygen breathing starting 2 min prior tracer injection until 1 h p.i. and during the PET scans; mice were breathing air between the 2 h and 3 h 10 min static scans. Normalized PET images were analyzed by using defined regions of interest. Finally, some mice were dissected for pimonidazole immunohistochemistry. Results: There was no difference in F-18-FAZA uptake 1-3 h p.i. between the two carcinoma types (CT26: 1.58 +/- 0.45% ID/cc; PyV-mT: 1.47 +/- 0.89% ID/cc, 1 h p.i., tumor size < 0.5 cm(3)). We measured a significant tracer clearance, which was more pronounced in muscle tissue (P0). The [F-18]FAZA tumor-to-muscle-ratios in CT26 colon carcinoma-bearing mice 2 h and 3 h, but not 1 h p.i. were significantly higher when the mice breathed air (P0: 3.56 +/- 0.55, 3 h) compared to the oxygen breathing protocols (P1: 2.45 +/- 0.58; P2: 2.77 +/- 0.42, 3 h). Surprisingly, the breathing protocols P1 and P2 showed no significant differences in T/M ratios, thus indicating that the crucial [F-18]FAZA uptake phase is during the first hour after [F-18]FAZA injection. Importantly, the muscle clearance was not affected by the different oxygen breathing conditions while the tumor clearance was lower when mice were breathing air. Conclusion: Exogenous CT26 colon carcinomas and endogenous polyoma middle-T (PyV-mT) mammary carcinomas showed no differences in [F-18]FAZA uptake 1-3 h p.i. Our analysis using various breathing protocols with air (P0) and with pure oxygen (P1, P2) clearly indicate that [F-18]FAZA is an appropriate PET biomarker for in vivo analysis of hypoxia revealing an enhanced tracer uptake in tumors with reduced oxygen supply. [F-18]FAZA uptake was independent of tumor-type.