We sought to develop (11)C-labeled ligands for sensitive imaging of brain peripheral benzodiazepine receptors (PBR) in vivo. Two aryloxyanilides with high affinity for PBR were identified and synthesized, namely, N-acetyl- N-(2-methoxycarbonylbenzyl)-2-phenoxyaniline ( 3, PBR01) and N-(2-methoxybenzyl)- N-(4-phenoxypyridin-3-yl)acetamide ( 10, PBR28). 3 was hydrolyzed to 4, which was esterified with [ (11)C]iodomethane to provide [ (11)C] 3. The O-desmethyl analogue of 10 was converted into [ (11)C] 10 with [ (11)C]iodomethane. [ (11)C] 3 and [ (11)C] 10 were each injected into monkey to assess their brain kinetics with positron emission tomography (PET). After administration of either radioligand there was moderately high brain uptake of radioactivity. Receptor blocking and displacement experiments showed that a high proportion of this radioactivity was bound specifically to PBR. In monkey and rat, 3 and 10 were rapidly metabolized by ester hydrolysis and N-debenzylation, respectively, each to a single polar radiometabolite. [ (11)C] 3 and [ (11)C] 10 are effective for imaging PBR in monkey brain. [ (11)C] 10 especially warrants further evaluation in human subjects.

Objectives-Peripheral benzodiazepine receptors (PBRs) are upregulated on activated microglia and are thereby biomarkers of neuroinflammation. We developed a PET ligand with an aryloxyanilide structure, [O-methyl-11 C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine ([ 11 C] PBR28), to image PBRs. The objectives of the current study were to evaluate kinetics of brain uptake, and the influence of the peripheral binding on the arterial input function in rhesus monkey.Methods-Brain (baseline: n=6, blocking: n=1) and whole-body PET imaging (baseline: n=3, blocking: n=1) of [ 11 C]PBR28 were performed with the measurement of radiometabolite-corrected arterial input function in all brain and two whole body scans.Results-Saturating doses of nonradioactive PBR ligands markedly increased [ 11 C]PBR28 in plasma (∼400% increase) and brain (∼200%) at 2 min by displacing radioligand from PBRs in peripheral organs. Brain uptake of radioactivity peaked in baseline scans at ∼40 min after injection of [ 11 C]PBR28 and was high (∼300% standardized uptake value). The images showed no receptorfree region that could be used for reference tissue analysis. Thus, quantitation of receptor density required measurement of parent radioligand in arterial plasma. Nondisplaceable uptake was estimated from the blocked scans and was only ∼5% of total distribution volume measured under baseline conditions. Distribution volume of [ 11 C]PBR28 was stably determined within 110 min of scanning.Conclusions-Regional brain uptake of [ 11 C]PBR28 in monkey could be quantified as a value proportional to the density of receptors -namely, as equilibrium distribution volume. [ 11 C]PBR28 had high levels of specific binding in brain and should provide a sensitive measure of changes in PBRs.

Bicycloalkyl groups have been previously described as phenyl group bioisosteres. This article describes the synthesis of new building blocks allowing their introduction into complex molecules, and explores their use as a means to modify the physicochemical properties of drug candidates and improve the quality of imaging agents. In particular, the replacement of an aromatic ring with a bicyclo[1.1.1]pentane-1,3-diyl (BCP) group improves aqueous solubility by at least 50-fold, and markedly decreases nonspecific binding (NSB) as measured by CHI(IAM), the chromatographic hydrophobicity index on immobilized artificial membranes. Structural variations with the bicyclo[2.2.2]octane-1,4-diyl group led to more lipophilic molecules and did not show the same benefits regarding NSB or solubility, whereas substitutions with cubane-1,4-diyl showed improvements for both parameters. These results confirm the potential advantages of both BCP and cubane motifs as bioisosteric replacements for optimizing para-phenyl-substituted molecules.

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Elevated levels of peripheral benzodiazepine receptors (PBR) are associated with activated microglia in their response to inflammation. Hence, PBR imaging in vivo is valuable for investigating brain inflammatory conditions. Sensitive, easily prepared, and readily available radioligands for imaging with positron emission tomography (PET) are desirable for this purpose. We describe a new 18F-labeled PBR radioligand, namely [18F]N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([18F]9). [18F]9 was produced easily through a single and highly efficient step, the reaction of [18F]fluoride ion with the corresponding bromo precursor, 8. Ligand 9 exhibited high affinity for PBR in vitro. PET showed that [18F]9 was avidly taken into monkey brain and gave a high ratio of PBR-specific to nonspecific binding. [18F]9 was devoid of defluorination in rat and monkey and gave predominantly polar radiometabolite(s). In rat, a low level radiometabolite of intermediate lipophilicity was identified as [18F]2-fluoro-N-(2-phenoxyphenyl)acetamide ([18F]11). [18F]9 is a promising radioligand for future imaging of PBR in living human brain.

Carboxylic esters were successfully labeled with one of two short-lived positron-emitters, carbon-11 or fluorine-18, within a hydrodynamically-driven micro-reactor. The non-radioactive methyl ester 4a was obtained at room temperature; its yield increased with higher substrate concentration and with reduced infusion rate. Radioactive methyl ester 4b was obtained from the reaction of 1 (10 mM) with 2b in 56% decay-corrected radiochemical yield (RCY) at an infusion rate of 10 µL min −1 , and when the infusion rate was reduced to 1 µL min −1 , the RCY increased to 88%. The synthesis of the non-radioactive fluoroethyl ester 5a from 1 and 3a required heating of the micro-reactor on a heating block at 80 °C (14-17% RCY), whilst the corresponding radioactive 5b from 1 and 3b was obtained in 10% RCY. The radioactive 'peripheral' benzodiazepine receptor ligand 7b was obtained from the reaction of acid 6 with labeling agent 2b in 45% RCY at an infusion rate of 10 µL min −1 . When the infusion rate was reduced to 1 µL min −1 , the RCY increased to 65%. The results exemplify a new methodology for producing radiotracers for imaging with positron emission tomography that has many potential advantages, including a requirement for small quantities of substrates, enhanced reaction, rapid reaction optimisation and easy product purification.

Peripheral benzodiazepine receptors (PBRs) are upregulated on activated microglia and are, thereby, biomarkers of cellular inflammation in brain. We recently developed two PET ligands with an aryloxyanilide structure to image PBRs and now evaluate the kinetics of these radiotracers in monkey to determine whether they are suitable to explore in human. Baseline and receptor-blocking scans were performed with [(11)C]PBR01 and [(18)F]PBR06 in conjunction with serial measurements of the arterial plasma concentration of parent radiotracer separated from radiometabolite. We used brain and plasma data with compartmental modeling to calculate regional brain distribution volume, which is equal to the ratio at equilibrium of the concentration of radioligand in brain to that of plasma. The distribution volume of [(11)C]PBR01 was inaccurately estimated in the baseline scans, possibly because of the short half-life of (11)C or the presence of radiometabolite in brain. In contrast, the distribution volume of [(18)F]PBR06 was stably determined within 200 min of scanning, and nondisplaceable uptake was only approximately 10% of total brain uptake. [(18)F]PBR06 is promising for use in human because brain activity could be quantified with standard compartmental models and showed higher ratios ( approximately 10:1) of specific to nonspecific uptake. A critical factor for human use will be whether the tracer has adequately fast wash out from brain relative to the half-life of the radionuclide to obtain stable values of distribution volume.

This study evaluated 18 F-labeled IMPY [6-iodo-2-(4′-N,N-dimethylamino)phenylimidazo[1,2-a]pyridine] derivatives as agents for imaging β-amyloid plaque with positron emission tomography (PET). The precursor for radiolabeling and reference compounds was synthesized in up to five steps from commercially accessible starting materials. One of the two N-methyl groups of IMPY was substituted with either a 3-fluoropropyl (FPM-IMPY) or a 2-fluoroethyl (FEM-IMPY) group. FPM-IMPY and FEM-IMPY were found to have moderate affinity for Aβ-aggregates with K i = 27 ± 8 and 40 ± 5 nM, respectively. A "one-pot" method for 18 F-2-fluoroethylation and 18 F-3-fluoropropylation of the precursor was developed. The overall decaycorrected radiochemical yields were 26-51%. In PET experiments with normal mouse, high uptake of activity was obtained in the brain after iv injection of each probe: 6.4% ID/g for