Synthesis and radiolabeling of a nitrofuran derivate with 18F for identification of areas of hypoxia in the tumor microenvironment

Abstract

Abstract Background Positron Emission Tomography (PET) is a non-invasive molecular imaging technique widely known for studying hypoxia mostly employing 2-nitroimidazole-based radiotracers. These probes are based on the oxygen-mimetic chemical sensitizers of hypoxic cells developed for cancer therapy during the 1970s. 5-nitrofuran derivates are more electron affinic than nitroimidazoles, therefore, higher specificity for hypoxic regions is expected for the formers, and new radiotracer probes bearing a 5-nitrofuran ring could be used for imaging hypoxia. Results A nitrofuran-based radiotracer for detection of hypoxic areas in the tumor microenvironment, (E)-1-(4-[18F]-fluorophenyl)-3-(5-nitrofuran-2-yl)prop-2-en-1-one, baptized as [18F]FNFP, was obtained. Two copper-mediated nucleophilic radiofluorination procedures were tested and compared using the same pinacol-derived aryl boronic ester precursor: method 1, using K222/K2CO3 and [Cu(OTf)2(py)4] afforded the product in 56 ± 8% (n = 5) RCY after HPLC analysis of the crude reaction mixture; method 2: an azeotropic drying-free [18F]-labelling procedure, using Cu(OTf)2 as [18F]-elution agent and copper source, yielded [18F]FNFP in 88 ± 4% (n = 5) RCY. Method 2 was chosen as the standard for the synthesis of the radiotracer, obtaining the product with an overall radiochemical yield of 38,4 ± 3% (n = 5), high radiochemical purity (> 99%), total synthesis time of 85 minutes and a molar activity of 41.56 GBq/µmol. [18F]FNFP was found to be stable in serum and Phosphate-buffered saline for up to 6h, and lipophilicity measurements concluded that it is more hydrophilic than [18F]FMISO (log10𝑃=2.6), with log10𝑃=1.05. Conclusion: The first nitrofuran-based radiotracer to be used as a PET hypoxia imaging agent was efficiently radiolabeled with 18F. In vitro and in vivo studies are being lined up to compare [18F]FNFP with [18F]FMISO and [18F]FAZA.