[(18)F]-(fluoromethoxy)ethoxy)methyl)-1H-1,2,3-triazol-1-yl)propan-2-ol ([(18)F FPTC) a novel PET-ligand for cerebral beta-adrenoceptors.

UNLABELLED: Cerebral ß-adrenergic receptors (ß-ARs) play important roles in normal brain and changes of ß-AR expression are associated with several neuropsychiatric illnesses. Given the high density of ß-AR in several brain regions, quantification of ß-AR levels using PET is feasible. However, there is a lack of radiotracers with suitable biological properties and meeting safety requirements for use in humans. We developed a PET tracer for ß-AR by (18)F-fluorination of 1-((9H-carbazol-4-yl)oxy)-3-4(4-((2-(2-(fluoromethoxy)-ethoxy)methyl)-1H-1,2,3-triazol-1-yl)propan-2-ol ((18)F-FPTC). METHODS: [(18)F] FPTC was synthesized by Cu(I)-catalyzed alkyne-azide cycloaddition. First, (18)F-PEGylated alkyne was prepared by (18)F-fluorination of the corresponding tosylate. Next (18)F-PEGylated alkyne was reacted with an azidoalcohol derivative of 4-hydroxycarbazol in the presence of the phosphoramidite Monophos as a ligand and Cu(I) as a catalyst. After purification with radio-HPLC, the binding properties of [(18)F FPTC were tested in ß-AR-expressing C6-glioma cells in vitro and in Wistar rats in vivo using microPET. RESULTS: The radiochemical yield of (18)F-PEGylated alkyne was 74%-89%. The click reaction to prepare [(18)F]FPTC proceeded in 10min with a conversion efficiency of 96%. The total synthesis time was 55min from the end of bombardment. Specific activities were >120GBq/µmol. Propranolol strongly and dose-dependently inhibited the binding of both [(125)I]-ICYP and [(18)F]FPTC to C6 glioma cells, with IC50 values in the 50-60 nM range. However, although both FPTC and propranolol inhibited cellular [(125)I]ICYP binding, FPTC decreased [(125)I]ICYP uptake by only 25%, whereas propranolol reduced it by 83%. [(18)F]FPTC has the appropriate lipophilicity to penetrate the blood brain barrier (logP +2.48). The brain uptake reached a maximum within 2min after injection of 20-25MBq [(18)F]FPTC. SUV values ranged from 0.4 to 0.6 and were not reduced by propranolol. Cerebral distribution volume of the tracer (calculated from a Logan plot) was increased rather than decreased after propranolol treatment. CONCLUSION: 'Click chemistry' was successfully applied to the synthesis of [(18)F]FPTC resulting in high radiochemical yields. [(18)F]FPTC showed specific binding in vitro, but not in vivo. Based on the logP value and its ability to block [(125)I]ICYP binding to C6 cells, FPTC may be a lead to suitable cerebral ß-AR ligands.

Synthesis of [18F]RGD-K5 by catalyzed [3 + 2] cycloaddition for imaging integrin αvß3 expression in vivo.

In the last few years click chemistry reactions, and in particular copper-catalyzed cycloadditions have been used extensively for the preparation of new bioconjugated molecules such as (18)F-radiolabeled radiopharmaceuticals for positron emission tomography (PET). This study is focused on the synthesis of the Siemens imaging biomarker [(18)F]RGD-K5. This cyclic peptide contains an amino acid sequence which is a well known binding motif for integrin αvß3 involved in cellular adhesion to the extracellular matrix. We developed an improved "click" chemistry method using Cu(I)-Monophos as catalyst to conjugate [(18)F]fluoropentyne to the RGD-azide precursor yielding [(18)F]RGD-K5. A comparison is made with the registered Siemens method with respect to synthesis, purification and quality control. [(18)F]RGD-K5 was obtained after 75 min overall synthesis time with an overall radiochemical yield of 35% (EOB). The radiochemical purity was >98% and the specific radioactivity was 100-200 GBq/µmol at the EOS.

One pot 'click' reactions: tandem enantioselective biocatalytic epoxide ring opening and [3+2] azide alkyne cycloaddition.

Halohydrin dehalogenase (HheC) can perform enantioselective azidolysis of aromatic epoxides to 1,2-azido alcohols which are subsequently ligated to alkynes producing chiral hydroxy triazoles in a one-pot procedure with excellent enantiomeric excess.

Phosphoramidite accelerated copper(i)-catalyzed [3 + 2] cycloadditions of azides and alkynes.

Monodentate phosphoramidite ligands are used to accelerate the copper(i)-catalyzed 1,3-dipolar cycloaddition of azides and alkynes (CuAAC) rapidly yielding a wide variety of functionalized 1,4-disubstituted-1,2,3-triazoles; Cu(i) and Cu(ii) salts both function as the copper source in aqueous solution to provide excellent yields.

Copper-free 'click': 1,3-dipolar cycloaddition of azides and arynes.

Arynes formed through fluoride-promoted ortho-elimination of o-(trimethylsilyl)aryl triflates can undergo [3 + 2] cycloaddition with various azides to form substituted benzotriazoles. The rapid reaction times and mild conditions make this an attractive variation of the classical 'click' reaction of azides and alkynes.

Characterization of nucleobase-amino acid stacking interactions utilized by a DNA repair enzyme.

The present work characterizes the gas-phase stacking interactions between four aromatic amino acid residues (histidine, phenylalanine, tyrosine, and tryptophan) and adenine or 3-methyladenine due to the proposed utilization of these interactions by enzymes that repair DNA alkylation damage. The MP2 potential energy surfaces of the stacked dimers are considered as a function of four variables (vertical displacement, angle of rotation, horizontal displacement, and tilt angle) using a variety of basis sets. It is found that the maximum stacking interaction energy decreases with the amino acid according to TRP > TYR approximately HIS > PHE for both nucleobases. However, the magnitude of the stacking interaction significantly increases upon alkylation (by 50-115%). Comparison of the stacking energies calculated using our surface scans to those estimated from experimental crystal structures indicates that the stacking interactions within the active site of 3-methyladenine DNA glycosylase can account for 65-75% of the maximum possible stacking interaction between the relevant molecules. The decrease in stacking in the crystal structure arises due to significant differences in the relative orientations of the nucleobase and amino acid. Nevertheless, alkylation is found to significantly increase the stacking energy when the crystal structure geometries are considered. Our calculations provide computational support for suggestions that alkylation enhances the stacking interactions within the active site of DNA repair enzymes, and they give a measure of the magnitude of this enhancement. Our results suggest that alkylation likely plays a more important role in substrate identification and removal than the nature of the aromatic amino acid that interacts with the substrate via stacking interactions.