A simplified synthesis of the hypoxia imaging agent 2-(2-Nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-[(18)F]pentafluoropropyl)-acetamide ([18F]EF5).

INTRODUCTION: [(18)F]EF5 is a validated marker for PET imaging of tumor hypoxia. It is prepared by reacting a trifluoroallyl precursor with carrier-added [(18)F]F(2) gas in trifluoroacetic acid (TFA) solvent. We report here an improved radiosynthesis and purification of [(18)F]EF5 by utilizing an electroformed nickel (Ni) target for [(18)F]F(2) production, and Oasis® HLB cartridges for on-line solid phase extraction of [(18)F]EF5 prior to HPLC purification. METHODS: [(18)F]F(2) was produced by deuteron bombardment of neon plus F(2) in an Ni target, and bubbled through the radiolabelling precursor solution. Purification was achieved by extracting the contents of the crude reaction mixture onto Oasis HLB cartridges, and subsequently eluted onto a semi-preparative HPLC column for further separation. Purified [(18)F]EF5 was evaluated in small animal PET studies using HCT116 tumor xenografts in nude mice. RESULTS: The electroformed Ni target enabled recovery of >75% of the radioactivity from the cyclotron target, resulting in 16.2 ± 2.2 GBq (438 ± 58 mCi) of [(18)F]F(2) available for the synthesis. Use of Oasis cartridges yielded a less complex mixture for purification. On average, 1140 ± 200 MBq (30.8 ± 5.4 mCi) of [(18)F]EF5 were collected at EOS. Small animal PET imaging studies showed specific retention of [(18)F]EF5 in tumors, with tumor-to-muscle ratios of 2.7 ± 0.3 at about 160 min after injection. CONCLUSION: A simple procedure has been developed for the routine synthesis of [(18)F]EF5 in amounts and purity required for clinical studies. This new method avoids the need for TFA evaporation and also enables facile automation of the synthesis using commercially available radiosynthesis modules.

Tracer level electrophilic synthesis and pharmacokinetics of the hypoxia tracer [(18)F]EF5.

PURPOSE: 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide labeled with [(18)F]-fluorine ([(18)F]EF5), a promising tracer for tumor hypoxia, has previously been synthesized in low yields and low specific radioactivity. In pharmacokinetic evaluations, in the presence of non-radioactive EF5, a uniform and low background uptake and high in vivo stability of [(18)F]EF5 have been demonstrated. Our purpose was to increase the specific radioactivity of [(18)F]EF5 to enable to study the pharmacokinetics at trace level. PROCEDURES: [(18)F]EF5 was synthesized using high specific radioactivity electrophilic [(18)F]F(2) as labelling reagent. Biodistribution of [(18)F]EF5 was determined in a prostate tumor mouse model, and formation of radiolabelled metabolites was studied in mouse, rat and human plasma. RESULTS: On average, 595 ± 153 MBq of [(18)F]EF5 was produced. Specific radioactivity was 6.6 ± 1.9 GBq/µmol and the radiochemical purity exceeded 99.0%. [(18)F]EF5 was distributed uniformly in tissues, with highest uptake in liver, kidney, and intestine. Several radiolabelled metabolites were detected in mouse plasma and tissues, whereas low amounts of metabolites were detected in human and rat plasma. CONCLUSIONS: [(18)F]EF5 was synthesized by electrophilic labelling with high quality and high yields. Pharmacokinetics of [(18)F]EF5 was determined at trace level in several species. Our results suggest that the trace-level approach does not affect the biodistribution of [(18)F]EF5. Extensive metabolism was seen in mouse.

[18F]-EF5, a marker for PET detection of hypoxia: synthesis of precursor and a new fluorination procedure.

There is a great deal of clinical and experimental interest in determining tissue hypoxia using non-invasive imaging methods. We have developed EF5, 2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide, with both invasive and non-invasive hypoxia detection in mind. EF5 and other 2-nitroimidazoles are used to detect hypoxia, because the rate of their bioreductive metabolism is inversely dependent on oxygen partial pressure. Such metabolism leads to the formation of covalent adducts within the metabolizing cells. Previously, we have described the invasive detection of these adducts by highly specific monoclonal antibodies after tissue biopsy. In this report, we demonstrate the synthesis of 18F-labeled EF5, [18F]-2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide, in greater than 10% yield by direct fluorination of the newly synthesized precursor 2-(2-nitro-1[H]-imidazol-1-yl)-N-(2,3,3-trifluoroallyl)-acetamide by [18F]-F2 in trifluoroacetic acid. Our objective was to optimize the electrophilic fluorination of the fluorinated alkene bond with fluorine gas, a new method of 18F-labeling of polyfluorinated molecules. Previous biodistribution studies in mice have demonstrated uniform access of EF5 to all tissues with bioelimination dominated by renal excretion. When [18F]-EF5 was injected into a rat followed by urine collection and analysis, we found no detectable metabolism to other radioactive compounds. Thus, [18F]-EF5 should be well suited for use as a non-invasive hypoxia marker with detection using positron emission tomography (PET).

Detection of hypoxic cells by monoclonal antibody recognizing 2-nitroimidazole adducts.

Hypoxic cells in tissue pose many medical problems, and there is a need for more accurate measurements of tissue hypoxia. However, measurement of the pO2 and the extent of hypoxia within normal and tumor tissue have proven difficult. One of the most sensitive of the currently available methodologies involves the oxygen-dependent metabolic activation of nitroheterocyclic drugs, leading to adducts between the drugs and cellular macromolecules. Limitations of the present drugs and adduct-detection methods prompted the present studies. A pentafluorinated derivative [EF5; 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetam ide] of etanidazole was synthesized with the expectation of lessening some of the non-oxygen-dependent variability in adduct formation observed previously with other nitroaromatic compounds. EF5-protein conjugates, prepared by radiochemical reduction, were found to be immunogenic and allowed the development of monoclonal antibodies. One of these antibodies, ELK2-4, has been characterized and found to be highly specific for the EF5 adducts whether produced radiochemically or by cellular bioreductive metabolism. 9L rat glioma cells pretreated with EF5 under hypoxic, compared with aerobic, conditions were readily discriminated immunochemically using fluorochrome-conjugated secondary antibodies which recognize the ELK2-4 antibody subtype (IgG1). Similarly, the central region of multicenter spheroids, composed of EMT6 mouse mammary sarcoma cells, was selectively visualized by immunohistochemistry after the spheroids were incubated for 4 h in 0.5 mM EF5. Tumor biopsy, preparation, and immunohistochemical staining 24 h after treatment of tumor-bearing animals with drug also demonstrated high contrast regions within EMT6 mouse or Morris 7777 hepatoma rat tumors. The use of this new compound and its highly specific monoclonal antibody may allow elucidation of bioreductive metabolism of the nitroheterocyclics and significantly improve technologies for the quantitation of tissue pO2.