No carrier added synthesis of O-(2′-[18F]fluoroethyl)-l-tyrosine via a novel type of chiral enantiomerically pure precursor, NiII complex of a (S)-tyrosine Schiff base

Radiosynthesis and Preclinical Evaluation of m-[18F]FET and [18F]FET-OMe as Novel [18F]FET Analogs for Brain Tumor Imaging

O-([18F]Fluoroethyl)-l-tyrosine ([18F]FET) is actively transported into the brain and cancer cells by LAT1 and possibly other amino acid transporters, which enables brain tumor imaging by positron emission tomography (PET). However, tumor delivery of this probe in the presence of competing amino acids may be limited by a relatively low affinity for LAT1. The aim of the present work was to evaluate the meta-substituted [18F]FET analog m-[18F]FET and the methyl ester [18F]FET-OMe, which were designed to improve tumor delivery by altering the physicochemical, pharmacokinetic, and/or transport properties. Both tracers could be prepared with good radiochemical yields of 41-56% within 66-90 min. Preclinical evaluation with [18F]FET as a reference tracer demonstrated reduced in vitro uptake of [18F]FET-OMe by U87 glioblastoma cells and no advantage for in vivo tumor imaging. In contrast, m-[18F]FET showed significantly improved in vitro uptake and accelerated in vivo tumor accumulation in an orthotopic glioblastoma model. As such, our work identifies m-[18F]FET as a promising alternative to [18F]FET for brain tumor imaging that deserves further evaluation with regard to its transport properties and in vivo biodistribution.

Enantiomeric purity deviations of radiolabelled amino acids obtained from chiral columns

Enantiomeric purity (EP) is an important value which denotes the relative percentage of the L-isomer with respect to the D-isomer. For 11C and 18F-labelled amino acid (AA) radiopharmaceutical (RP) production, EP represents a quality control parameter specified in European and national monographs for particular RPs. In most instances, EP value of greater then 90 or 95% (depending on AA type) is required as part of the quality control (QC) value of a RP following radiosynthesis. In common practice, two chromatographic columns are used for the EP determination of RPs: Crownpak CR(+) (Daicel), which contains a crown ether stationary phase or Chirobiotic T (Astec), which contains silica-bound glycoproteins as the stationary phase. The application of column Crownpak CR(+) requires that only perchloric acid solution (with pH 1–2) may be used, as the retention capability of the stationary phase is greatly reduced using organic solvents. This work intends to identify which chromatographic system is more accurate and reliable for EP determination as part of QC. We performed a series of parallel injections of the same batch of the widely used AA RPs [11C]MET and [18F]FET on the two aforementioned columns. The EP determination using column Crownpak CR(+) consistently provided a lower EP value compared to the Chirobiotic T column; the EP deviation between the respective columns was found to range from 2.4–4.0% for the same RP sample. Furthermore, the EP value was influenced by a sample’s dilution factor, e.g. the EP was observed to increase up to 1.5% when the radioactive sample had a fivefold dilution factor. This phenomenon was consistent for both Crownpak CR(+) and Chirobiotic T columns. Finally, a series of standard solutions of non-radioactive methionine with various ratios of L-and D-isomers was analyzed. The data obtained for non-radioactive methionine confirmed that column Crownpak CR(+) incorrectly provided a higher D-enantiomer concentration, whereas Chirobiotic T was found to provide a lower D-enantiomer concentration of the same sample. The deviation from the theoretical EP value was between 0.67 and 1.92%.

Preparation of labeled aromatic amino acids via late-stage 18F-fluorination of chiral nickel and copper complexes

A general protocol for the preparation of 18F-labeled AAAs and α-methyl-AAAs applying alcohol-enhanced Cu-mediated radiofluorination of Bpin-substituted chiral complexes using Ni/Cu-BPX templates as double protecting groups is reported. The chiral auxiliaries are easily accessible from commercially available starting materials in a few synthetic steps. The versatility of the method was demonstrated by the high-yielding preparation of a series of [18F]F-AAAs and the successful implementation of the protocol into automated radiosynthesis modules.

An Overview of PET Radiochemistry, Part 1: The Covalent Labels 18F, 11C, and 13N

This continuing educational article introduces the radiochemistry of PET tracers that exhibit a covalently bound radiolabel with the nuclides ¹¹C, ¹³N, and ¹⁸F. The overall process of PET tracer production is explained, starting from the production of the radionuclide in a cyclotron; followed by the automatization process of the radiosynthesis, including the necessary steps for the respective synthesis; and finalized with the requirements for quality control.

A new Schiff base compound N,N'-(2,2-dimetylpropane)-bis(dihydroxylacetophenone): synthesis, experimental and theoretical studies on its crystal structure, FTIR, UV-visible, 1H NMR and 13C NMR spectra

The Schiff base compound, N,N'-(2,2-dimetylpropane)-bis(dihydroxylacetophenone) (NDHA) is synthesized through the condensation of 2-hydroxylacetophenone and 2,2-dimethyl 1,3-amino propane in methanol at ambient temperature. The yellow crystalline precipitate is used for X-ray single-crystal determination and measuring Fourier transform infrared (FTIR), UV-visible, (1)H NMR and (13)C NMR spectra. Electronic structure calculations at the B3LYP, PBEPBE and PW91PW91 levels of theory are performed to optimize the molecular geometry and to calculate the FTIR, (1)H NMR and (13)C NMR spectra of the compound. Time-dependent density functional theory (TDDFT) method is used to calculate the UV-visible spectrum of NDHA. Vibrational frequencies are determined experimentally and compared with those obtained theoretically. Vibrational assignments and analysis of the fundamental modes of the compound are also performed. All theoretical methods can well reproduce the structure of the compound. The (1)H NMR and (13)C NMR chemical shifts calculated by all DFT methods are consistent with the experimental data. However, the NMR shielding tensors computed at the B3LYP/6-31+G(d,p) level of theory are in better agreement with experimental (1)H NMR and (13)C NMR spectra. The electronic absorption spectrum calculated at the B3LYP/6-31+G(d,p) level by using TD-DFT method is in accordance with the observed UV-visible spectrum of NDHA. In addition, some quantum descriptors of the molecule are calculated and conformational analysis is performed and the results were compared with the crystallographic data.

4-[18F]fluoroglutamic acid (BAY 85-8050), a new amino acid radiotracer for PET imaging of tumors: synthesis and in vitro characterization

There is a high demand for tumor specific PET tracers in oncology imaging. Besides glucose, certain amino acids also serve as energy sources and anabolic precursors for tumors. Therefore, (18)F-labeled amino acids are interesting probes for tumor specific PET imaging. As glutamine and glutamate play a key role in the adapted intermediary metabolism of tumors, the radiosynthesis of 4-[(18)F]fluoro l-glutamic acid (BAY 85-8050) as a new specific PET tracer was established. Cell-uptake studies revealed specific tumor cell accumulation.

A new salen base 5-(phenylazo)-N-(2-amino pyridine) salicyliden Schiff base ligand: synthesis, experimental and density functional studies on its crystal structure, FTIR, 1H NMR and 13C NMR spectra

A novel Schiff base ligand 5-(phenylazo)-N-(2-amino pyridine) salicyliden is prepared through the condensation of 5-(phenylazo) salicylaldehyde and 2-amino pyridine in methanol at room temperature. The orange crystalline precipitate is used for X-ray crystallography and measuring Fourier transform (FTIR), 1H NMR and 13C NMR spectra. Density functional theory (DFT) calculations at the B3LYP, MPWB1K and B3PW91 levels of theory is used to optimize the geometry and calculate the FTIR, 1H NMR and 13C NMR spectra of the compound. The vibrational frequencies determined experimentally are compared with those obtained theoretically and a vibrational assignment and analysis of the fundamental modes of the compound is performed. We found that the MPWB1K method predicts low vibrational frequencies better than the commonly used B3LYP method. Although the B3PW91 method overestimates the 1H NMR chemical shifts, the values computed at the B3LYP level of theory are in accordance with experimental 1H NMR spectrum. However, both B3LYP and B3PW91 methods tend to overestimate 13C NMR chemical shifts. In addition, a few quantum descriptors of the molecule are calculated and conformational analysis is performed and the result was compared with crystallographic data.

Why is monoalkylation versus bis-alkylation of the Ni(II) complex of the Schiff base of (S)-N-(2-benzoylphenyl)-1-benzylpyrrolidine-2-carboxamide and glycine so selective? MP2 modelling and topological QTAIM analysis of chiral metallocomplex synthons of α-amino acids used for the preparation of radiopharmaceuticals for positron emission tomography

Chiral Ni(II) complexes are used for the preparation of carbon-11 or fluorine-18 enantiomerically pure α-amino acids for positron emission tomography (PET). They enable the selective monoalkylation of a glycine synthon with high stereoselectivity and the preparation of enantiomerically pure α-amino acids with quarternary α-carbon. Molecular modelling of non-, mono- and di-substituted complexes using quantum theory of atoms-in-molecule (QTAIM) topological analysis of electron density allowed us to formulate a new theory explaining the reasons for highly selective monomethylation of the complexes. In the non-substituted complex (GK), the α-carbon atom exhibits a higher atomic volume and a more positive charge in comparison with mono- and di-substituted complexes. This unusual behaviour is accompanied by increasing the bond critical point (BCP) ellipticity of the iminic bond in GK explained by the higher mechanical strain. Both phenomena indicate the increased reactivity and probably originate in more compact core of GK where shorter distances in the internal coordination sphere result in the higher strain of its bonds.

New approach for the synthesis of [18F]fluoroethyltyrosine for cancer imaging: simple, fast, and high yielding automated synthesis

O-(2-[(18)F]fluoroethyl)-L-tyrosine ([(18)F]FET) is one of the first (18)F-labeled amino acids for imaging amino acid metabolism in tumors. This tracer overcomes the disadvantages of [(18)F]fluorodeoxyglucose, [(18)F]FDG, and [(11)C]methionine, [(11)C]MET. Nevertheless, the various synthetic methods providing (18)F[FET] exhibit a big disadvantage concerning the necessity of two purification steps during the synthesis including HPLC purification, which causes difficulties in the automation, moderate yields, and long synthesis times >60 min. A new approach for the synthesis of [(18)F]FET is developed starting from 2-bromoethyl triflate as precursor. After optimization of the synthesis parameters including the distillation step of [(18)F]-FCH(2)CH(2)Br combined with the final purification of [(18)F]FET using a simple solid phase extraction instead of an HPLC run the synthesis [(18)F]FET could be significantly simplified, shortened, and improved. The radiochemical yield (RCY) was about 45% (not decay corrected and calculated relative to [(18)F]F(-) activity that was delivered from the cyclotron). Synthesis time was only 35 min from the end of bombardment (EOB) and the radiochemical purity was >99% at the end of synthesis (EOS). Thus, this simplified synthesis for [(18)F]FET offers a very good option for routine clinical use.

[(18)F-labeled tyrosine derivatives: synthesis and experimental studies on accumulation in tumors and abscesses]

Tyrosine derivatives labeled with a short-living fluorine 18 isotope (T(1/2) 110 min), namely 2-[(18)F]fluoro-L-tyrosine (FTYR) and O-(2'-[(18)F]fluoroethyl)-L-tyrosine (FET), promising radiopharmaceutical products (RPP) for positron emission tomography (PET), were obtained by asymmetric synthesis. Accumulation of FTYR and FET in the rat tumor "35 rat glioma" and in abscesses induced in Vistar mouse muscles was studied and compared with that of a well-known glycolysis radiotracer 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG). It was shown that the relative accumulation indices of amino acid RPP were considerably lower than those of FDG. At the same time, tumor/muscle ratios were high enough (2.9 for FET and 3.9 for FTYR 120 min after injection) for reliable tumor visualization. The data obtained indicated a possibility in principle to use FTYR and FET for differentiated PET diagnostics of brain tumors and inflammation lesions. Of the tyrosine derivatives studied, FET seems to be the most promising agent due to a simple and easily automated method of preparation based on direct nucleophilic substitution of the leaving tosyloxy group of an enantiomerically pure Ni-(S)-BPB-(S)-Tyr(CH2CH2OTs) precursor by an activated [(18)F]fluoride.