Evaluation of two nucleophilic syntheses routes for the automated synthesis of 6-[18F]fluoro-l-DOPA.

Two different strategies for the nucleophilic radiosynthesis of [18F]F-DOPA were evaluated regarding their applicability for an automated routine production on an Ecker&Ziegler Modular-Lab Standard module. Initially, we evaluated a promising 5-step synthesis based on a chiral, cinchonidine-derived phase-transfer catalyst (cPTC) being described to give the product in high radiochemical yields (RCY), high specific activities (AS) and high enantiomeric excesses (ee). However, the radiosynthesis of [18F]F-DOPA based on this strategy showed to be highly complex, giving the intermediate products as well as the final product in insufficient yields for automatization. Furthermore, the automatization proved to be problematic due to incomplete radiochemical conversions and the formation of precipitates during the enantioselective reaction step. Furthermore, the required use of HI at 180°C during the last reaction step led to partial decomposition of lines and seals of the module which further counteracts an automatization. Further on, we evaluated a 3-step synthesis using the commercially available, enantiomerically pure precursor AB1336 for automatization. This synthesis approach gave much better results and [18F]F-DOPA could be produced fully automated within 114min in RCYs of 20±1%, ee of >96%, a radiochemical purity (RCP) of >98% and specific activities of up to 2.2GBq/µmol.

Investigations into the synthesis, radiofluorination and conjugation of a new [¹8F]fluorocyclobutyl prosthetic group and its in vitro stability using a tyrosine model system.

The [(18)F]fluorocyclobutyl group has the potential to be a metabolically stable prosthetic group for PET tracers. The synthesis of the radiolabeling precursor cis-cyclobutane-1,3-diyl bis(toluene-4-sulfonate) 8 was obtained from epibromohydrin in 7 steps (2% overall yield). The radiolabeling of this precursor 8 and its conjugation to L-tyrosine as a model system was successfully achieved to give the new non-natural amino acid 3-[(18)F]fluorocyclobutyl-L-tyrosine (L-3-[(18)F]FCBT) [(18)F]17 in 8% decay-corrected yield from the non-carrier-added [(18)F]fluoride. L-3-[(18)F]FCBT was investigated in vitro in different cancer cell lines to determine the uptake and stability. The tracer [(18)F]17 showed a time dependent uptake into different tumor cell lines (A549, NCI-H460, DU145) with the best uptake of 5.8% injected dose per 5×10(5) cells after 30min in human lung carcinoma cells A549. The stability of L-3-[(18)F]FCBT in human and rat plasma and the stability of the non-radioactive L-3-FCBT in rat hepatocytes were both found to be excellent. These results show that the non-natural amino acid L-3-[(18)F]FCBT is a promising metabolically stable radiotracer for positron emission tomography.

5-(2-18F-fluoroethoxy)-L-tryptophan as a substrate of system L transport for tumor imaging by PET.

UNLABELLED: Large neutral l-amino acids are substrates of system L amino acid transporters. The level of one of these, LAT1, is increased in many tumors. Aromatic l-amino acids may also be substrates of aromatic l-amino acid decarboxylase (AADC), the level of which is enhanced in endocrine tumors. Increased amino acid uptake and subsequent decarboxylation result in the intracellular accumulation of the amino acid and its decarboxylation product. (18)F- and (11)C-labeled neutral aromatic amino acids, such as l-3,4-dihydroxy-6-(18)F-fluorophenylalanine ((18)F-FDOPA) and 5-hydroxy-l-[ß-(11)C]tryptophan, are thus successfully used in PET to image endocrine tumors. However, 5-hydroxy-l-[ß-(11)C]tryptophan has a relatively short physical half-life (20 min). In this work, we evaluated the in vitro and in vivo characteristics of the (18)F-labeled tryptophan analog 5-(2-(18)F-fluoroethoxy)-l-tryptophan ((18)F-l-FEHTP) as a PET probe for tumor imaging. METHODS: (18)F-l-FEHTP was synthesized by no-carrier-added (18)F fluorination of 5-hydroxy-l-tryptophan. In vitro cell uptake and efflux of (18)F-l-FEHTP and (18)F-FDOPA were studied with NCI-H69 endocrine small cell lung cancer cells, PC-3 pseudoendocrine prostate cancer cells, and MDA-MB-231 exocrine breast cancer cells. Small-animal PET was performed with the respective xenograft-bearing mice. Tissues were analyzed for potential metabolites. RESULTS: (18)F-l-FEHTP specific activity and radiochemical purity were 50-150 GBq/µmol and greater than 95%, respectively. In vitro cell uptake of (18)F-l-FEHTP was between 48% and 113% of added radioactivity per milligram of protein within 60 min at 37°C and was blocked by greater than 95% in all tested cell lines by the LAT1/2 inhibitor 2-amino-2-norboranecarboxylic acid. (18)F-FDOPA uptake ranged from 26% to 53%/mg. PET studies revealed similar xenograft-to-reference tissue ratios for (18)F-l-FEHTP and (18)F-FDOPA at 30-45 min after injection. In contrast to the (18)F-FDOPA PET results, pretreatment with the AADC inhibitor S-carbidopa did not affect the (18)F-l-FEHTP PET results. No decarboxylation products of (18)F-l-FEHTP were detected in the xenograft homogenates. CONCLUSION: (18)F-l-FEHTP accumulates in endocrine and nonendocrine tumor models via LAT1 transport but is not decarboxylated by AADC. (18)F-l-FEHTP may thus serve as a PET probe for tumor imaging and quantification of tumor LAT1 activity. These findings are of interest in view of the ongoing evaluation of LAT1 substrates and inhibitors for cancer therapy.

Faster analysis of radiopharmaceuticals using ultra performance liquid chromatography (UPLC) in combination with low volume radio flow cell.

With the aim of reducing analysis time of radiopharmaceuticals, especially for carbon-11 and fluorine-18, radio-high performance liquid chromatography (radio-HPLC) is the analysis method of choice. Faster and more sensitive analytic methods are needed. Recently, ultra performance liquid chromatography (UPLC) has become an accepted analysis method but has not yet been established in radiopharmacy. This study demonstrates with the established positron emission tomography (PET) tracer [(18)F]fluoromisonidazole (FMISO) the applicability of using UPLC) in combination with low volume radio flow cell for a fast, sensitive and efficient analytic method. The developed UPLC) method showed a sharp and high radio signal. The total analysis time was thus reduced from 15 to 2 min.

Strategy for improved [(11)C]DAA1106 radiosynthesis and in vivo peripheral benzodiazepine receptor imaging using microPET, evaluation of [(11)C]DAA1106.

INTRODUCTION: The peripheral benzodiazepine receptor (PBR) has shown considerable potential as a clinical marker of neuroinflammation and tumour progression. [(11)C]DAA1106 ([(11)C]N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)-acetamide) is a promising positron emission tomography (PET) radioligand for imaging PBRs. METHODS: A four-step synthetic route was devised to prepare DAA1123, the precursor for [(11)C]DAA1106. Two robust, high yielding methods for radiosynthesis based on [(11)C]-O-methylation of DAA1123 were developed and implemented on a nuclear interface methylation module, producing [(11)C]DAA1106 with up to 25% radiochemical yields at end-of-synthesis based on [(11)C]CH(3)I trapped. Evaluation of [(11)C]DAA1106 for in vivo imaging was performed in a rabbit model with microPET, and the presence of PBR receptor in the target organ was further corroborated by immunohistochemistry. RESULTS: The standard solution method produced 2.6-5.2 GBq (n=19) of [(11)C]DAA1106, whilst the captive solvent method produced 1.6-6.3 GBq (n=10) of [(11)C]DAA1106. Radiochemical purities obtained were 99% and specific radioactivity at end-of-synthesis was up to 200 GBq/micromol for both methods. Based on radiochemical product, shorter preparation times and simplicity of synthesis, the captive solvent method was chosen for routine productions of [(11)C]DAA1106. In vivo microPET [(11)C]DAA1106 scans of rabbit kidney demonstrated high levels of binding in the cortex. The subsequent introduction of nonradioactive DAA1106 (0.2 micromol) produced considerable displacement of the radioactive signal in this region. The presence of PBR in kidney cortex was further corroborated by immunohistochemistry. CONCLUSIONS: A robust, high yielding captive solvent method of [(11)C]DAA1106 production was developed which enabled efficacious in vivo imaging of PBR expressing tissues in an animal model.