Comparison of three 18F-labeled 2-nitroimidazoles for imaging hypoxia in breast cancer xenografts: [18F]FBNA, [18F]FAZA and [18F]FMISO

Background: Tumour hypoxia is associated with increased metastasis, invasion, poor therapy response and prognosis. Most PET radiotracers developed and used for clinical hypoxia imaging belong to the 2-nitroimidazole family. Recently we have developed novel 2-nitroimidazole-derived PET radiotracer [F-18]FBNA (N-(4-[18F]fluorobenzyl)-2-(2-nitro-1H-imidazol-1-yl)-acet-amide), an F-18-labeled analogue of antiparasitic drug benznidazole. The present study aimed to analyze its radio-pharmacological properties and systematically compare its PET imaging profiles with [F-18]FMISO and [F-18]FAZA in preclinical triple-negative (MDA-MB231) and estrogen receptor-positive (MCF-7) breast cancer models. Methods: In vitro cellular uptake experiments were carried out in MDA-MB321 and MCF-7 cells under normoxic and hypoxic conditions. Metabolic stability in vivo was determined in BALB/c mice using radio-TLC analysis. Dynamic PET experiments over 3 h post-injection were performed in MDA-MB231 and MCF-7 tumour-bearing mice. Those PET data were used for kinetic modelling analysis utilizing the reversible two-tissue-compartment model. Autoradiography was carried out in tumour tissue slices and compared to HIF-1a immunohistochemistry. Detailed ex vivo biodistribution was accomplished in BALB/c mice, and this biodistribution data were used for dosimetry calculation. Results: Under hypoxic conditions in vitro cellular uptake was elevated in both cell lines, MCF-7 and MDA-MB231, for all three radiotracers. After intravenous injection, [18F]FBNA formed two radiometabolites, resulting in a final fraction of 65 +/- 9 % intact [18F]FBNA after 60 min p.i. After 3 h p.i., [18F]FBNA tumour uptake reached SUV values of 0.78 +/- 0.01 in MCF-7 and 0.61 +/- 0.04 in MDA-MB231 tumours (both n = 3), representing tumourto-muscle ratios of 2.19 +/- 0.04 and 1.98 +/- 0.15, respectively. [F-18]FMISO resulted in higher tumour uptakes (SUV 1.36 +/- 0.04 in MCF-7 and 1.23 +/- 0.08 in MDA-MB231 (both n = 4; p < 0.05) than [F-18]FAZA (0.66 +/- 0.11 in MCF-7 and 0.63 +/- 0.14 in MDA-MB231 (both n = 4; n.s.)), representing tumour-to-muscle ratios of 3.24 +/- 0.30 and 3.32 +/- 0.50 for [F-18]FMISO, and 2.92 +/- 0.74 and 3.00 +/- 0.42 for [18F]FAZA, respectively. While the fraction per time of radiotracer entering the second compartment (k3) was similar within uncertainties for all three radiotracers in MDA-MB231 tumours, it was different in MCF-7 tumours. The ratios k3/(k3 + k2) and K1*k3/(k3 + k2) in MCF-7 tumours were also significantly different, indicating dissimilar fractions of radiotracer bound and trapped intracellularly: K1*k3/(k2 + k3) [F-18]FMISO (0.0088 +/- 0.001)/min, n = 4; p < 0.001) > [F-18]FAZA (0.0052 +/- 0.002)/min, n = 4; p < 0.01) > [F-18]FBNA (0.003 +/- 0.001)/min, n = 3). In contrast, in MDA-MB231 tumours, only K1 was significantly elevated for [F-18]FMISO. However, this did not result in significant differences for K1*k3/(k2 + k3) for all three 2-nitroimidazoles in MDA-MB231 tumours. Conclusion: Novel 2-nitroimidazole PET radiotracer [18F]FBNA showed uptake into hypoxic breast cancer cells and tumour tissue presumably associated with elevated HIF1- a expression. Systematic comparison of PET imaging performance with [F-18]FMISO and [F-18]FAZA in different types of preclinical breast cancer models revealed a similar tumour uptake profile for [F-18]FBNA with [18F]FAZA and, despite its higher lipophilicity, still a slightly higher muscle tissue clearance compared to [F-18]FMISO.

Automated summary

The novel 2-nitroimidazole PET radiotracer [F-18]FBNA showed uptake in hypoxic breast cancer cells and tumor tissue potentially linked to high HIF1-a expression. Comparison with [F-18]FMISO and [F-18]FAZA in preclinical breast cancer models revealed similar tumor uptake profiles for [F-18]FBNA with [18F]FAZA, but slightly higher muscle tissue clearance compared to [F-18]FMISO.