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

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 derivatives are more electron-affinic than nitroimidazoles; therefore, higher specificity for hypoxic regions is expected for the former, 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 6 h, 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.