Synthesis of [18F]FMISO, a hypoxia-specific imaging probe for PET, an overview from a radiochemist's perspective.

BACKGROUND: [18F]fluoromisonidazole ([18F]FMISO, 1H-1-(3-[18F]fluoro-2-hydroxypropyl)-2-nitroimidazole) is a commonly used radiotracer for imaging hypoxic conditions in cells. Since hypoxia is prevalent in solid tumors, [18F]FMISO is in clinical application for decades to explore oxygen demand in cancer cells and the resulting impact on radiotherapy and chemotherapy. RESULTS: Since the introduction of [18F]FMISO as positron emission tomography imaging agent in 1986, a variety of radiosynthesis procedures for the production of this hypoxia tracer has been developed. This paper gives a brief overview on [18F]FMISO radiosyntheses published so far from its introduction until now. From a radiopharmaceutical chemist's perspective, different precursors, radiolabeling approaches and purification methods are discussed as well as used automated radiosynthesizers, including cassette-based and microfluidic systems. CONCLUSION: In a GMP compliant radiosynthesis using original cassettes for FASTlab we produced [18F]FMISO in 49% radiochemical yield within 48 min with radiochemical purities > 99% and molar activities > 500 GBq/µmol. In addition, we report an easy and efficient radiosynthesis of [18F]FMISO, based on in-house prepared FASTlab cassettes, providing the radiotracer for research and preclinical purposes in good radiochemical yields (39%), high radiochemical purities (> 99%) and high molar activity (> 500 GBq/µmol) in a well-priced option.

Imaging of the brain serotonin transporters (SERT) with 18F-labelled fluoromethyl-McN5652 and PET in humans.

PURPOSE: [(11)C]DASB is currently the most frequently used highly selective radiotracer for visualization and quantification of central SERT. Its use, however, is hampered by the short half-life of (11)C, the moderate cortical test-retest reliability, and the lack of quantifying endogenous serotonin. Labelling with (18)F allows in principle longer acquisition times for kinetic analysis in brain tissue and may provide higher sensitivity. The aim of our study was to firstly use the new highly SERT-selective (18)F-labelled fluoromethyl analogue of (+)-McN5652 ((+)-[(18)F]FMe-McN5652) in humans and to evaluate its potential for SERT quantification. METHODS: The PET data from five healthy volunteers (three men, two women, age 39 ± 10 years) coregistered with individual MRI scans were semiquantitatively assessed by volume-of-interest analysis using the software package PMOD. Rate constants and total distribution volumes (V (T)) were calculated using a two-tissue compartment model and arterial input function measurements were corrected for metabolite/plasma data. Standardized uptake region-to-cerebellum ratios as a measure of specific radiotracer accumulation were compared with those of a [(11)C]DASB PET dataset from 21 healthy subjects (10 men, 11 women, age 38 ± 8 years). RESULTS: The two-tissue compartment model provided adequate fits to the data. Estimates of total distribution volume (V (T)) demonstrated good identifiability based on the coefficients of variation (COV) for the volumes of interest in SERT-rich and cortical areas (COV V (T) <10%). Compared with [(11)C]DASB PET, there was a tendency to lower mean uptake values in (+)-[(18)F]FMe-McN5652 PET; however, the standard deviation was also somewhat lower. Altogether, cerebral (+)-[(18)F]FMe-McN5652 uptake corresponded well with the known SERT distribution in humans. CONCLUSION: The results showed that (+)-[(18)F]FMe-McN5652 is also suitable for in vivo quantification of SERT with PET. Because of the long half-life of (18)F, the widespread use within a satellite concept seems feasible.

Experimental hypoxia is a potent stimulus for radiotracer uptake in vitro: comparison of different tumor cells and primary endothelial cells.

Hypoxia causes upregulation of vascular endothelial growth factor (VEGF) which is a key regulator in tumor angiogenesis and essential for the proliferation of endothelial cells. Endothelial cells have been described to accumulate radiotracers like (18)F-FDG. However, the contribution of radiotracer uptake by endothelial cells to uptake measured in tumors by positron emission tomography (PET) is still unclear. In this study (18)F-FDG and (18)F-FMISO radiotracer uptake in various tumor and primary endothelial cells cultured at hypoxic conditions was investigated. Experimental hypoxia was confirmed by significant upregulation of VEGF mRNA. In comparison to normoxic conditions, cellular uptake of (18)F-FDG was significantly increased at hypoxic conditions in two of the tumor and all endothelial cells, whereas (18)F-FMISO uptake was only enhanced in tumor cell lines HT-29 and MCF-7. Our data showed a marked influence of experimental hypoxia on the metabolism and gene expression of tumor and endothelial cells in vitro. This indicates an important contribution of endothelial cells to (18)F-FDG radiotracer uptake in tumors and for the visualization of tumors by means of PET.

Biodistribution and catabolism of (18)F-labeled neurotensin(8-13) analogs.

4-([(18)F]fluoro)benzoyl-neurotensin(8-13) ((18)FB-Arg(8)-Arg(9)-Pro(10)-Tyr(11)- Ile(12)-Leu(13)-OH, 1) and two analogs stabilized in one and two positions ((18)FB-Arg(8)psi(CH(2)NH)Arg(9)-Pro(10)-Tyr(11)- Ile(12)-Leu(13)-OH, 2, (18)FB-Arg(8)psi(CH(2)NH)Arg(9)-Pro(10)-Tyr(11)-Tle(12)-Leu(13)-OH, 3) were synthesized in a radiochemical yield of 25-36% and a specific activity of 5-15 GBq/mmol. The peptides were evaluated in vitro and in vivo for their potential to image tumors overexpressing neurotensin receptor 1 (NTR1) by positron emission tomography (PET). All analogs exhibited in vitro binding affinity in the low nanomolar range to NTR1-expressing human tumors, measured by quantitative receptor autoradiography, HT-29 and WiDr cells, and to sections of tumors derived from these cell lines in mice. The radiotracers were internalized in the cells in vitro, and the fluorinated peptides were able to mobilize intracellular Ca(2+) of WiDr cells. In in vivo studies in rats and in mice bearing HT-29 cell tumors, only a moderate uptake of the radioligands into the studied tumors was observed, presumed to be due to degradation in vivo and fast elimination by the kidneys. In comparison with the other analogs, the specific tumor uptake expressed as tumor-to-muscle relation was highest for the radioligand 3. The blood clearance of 3 was reduced by co-injection of peptidase inhibitors. The catabolic pathways of the radiofluorinated peptides were elucidated. The results suggest that the high binding affinity to NTR1 and the stabilization against proteolytic degradation are not yet sufficient for tumor imaging by PET.