A multicentre longitudinal study of flortaucipir (18F) in normal ageing, mild cognitive impairment and Alzheimer's disease dementia.

The advent of tau-targeted PET tracers such as flortaucipir (18F) (flortaucipir, also known as 18F-AV-1451 or 18F-T807) have made it possible to investigate the sequence of development of tau in relationship to age, amyloid-ß, and to the development of cognitive impairment due to Alzheimer's disease. Here we report a multicentre longitudinal evaluation of the relationships between baseline tau, tau change and cognitive change, using flortaucipir PET imaging. A total of 202 participants 50 years old or older, including 57 cognitively normal subjects, 97 clinically defined mild cognitive impairment and 48 possible or probable Alzheimer's disease dementia patients, received flortaucipir PET scans of 20 min in duration beginning 80 min after intravenous administration of 370 MBq flortaucipir (18F). On separate days, subjects also received florbetapir amyloid PET imaging, and underwent a neuropsychological test battery. Follow-up flortaucipir scans and neuropsychological battery assessments were also performed at 9 and 18 months. Fifty-five amyloid-ß+ and 90 amyloid-ß- subjects completed the baseline and 18-month study visits and had valid quantifiable flortaucipir scans at both time points. There was a statistically significant increase in the global estimate of cortical tau burden as measured by standardized uptake value ratio (SUVr) from baseline to 18 months in amyloid-ß+ but not amyloid-ß- subjects (least squared mean change in flortaucipir SUVr : 0.0524 ± 0.0085, P < 0.0001 and 0.0007 ± 0.0024 P = 0.7850, respectively), and a significant association between magnitude of SUVr increase and baseline tau burden. Voxel-wise evaluations further suggested that the regional pattern of change in flortaucipir PET SUVr over the 18-month study period (i.e. which regions exhibited the greatest change) also varied as a function of baseline global estimate of tau burden. In subjects with lower global SUVr, temporal lobe regions showed the greatest flortaucipir retention, whereas in subjects with higher baseline SUVr, parietal and frontal regions were increasingly affected. Finally, baseline flortaucipir and change in flortaucipir SUVr were both significantly (P < 0.0001) associated with changes in cognitive performance. Taken together, these results provide a preliminary characterization of the longitudinal spread of tau in Alzheimer's disease and suggest that the amount and location of tau may have implications both for the spread of tau and the cognitive deterioration that may occur over an 18-month period.

Relationships between flortaucipir PET tau binding and amyloid burden, clinical diagnosis, age and cognition.

The advent of tau-targeted positron emission tomography tracers such as flortaucipir (18F-AV-1451, also known as 18F-T807) have made it possible to investigate the sequence of development of tau and amyloid-ß in relationship to age, and to the development of cognitive impairment due to Alzheimer's disease. In this study, flortaucipir tau and florbetapir amyloid positron emission tomography were obtained for 217 subjects including 16 young and 58 older cognitively normal subjects, 95 subjects with mild cognitive impairment (Mini-Mental State Examination 24-30) and 48 subjects with clinically-defined possible or probable Alzheimer's disease (Mini-Mental State Examination >10). Images were evaluated visually and quantitatively by regional and voxel-based cortical to cerebellar standard uptake value ratios. For amyloid positron emission tomography positive (Aß+) subjects, flortaucipir neocortical standard uptake value ratio was significantly higher with more advanced clinical stage (Alzheimer's disease > mild cognitive impairment > older cognitively normal) and was significantly elevated for Aß+ mild cognitive impairment and Alzheimer's disease subjects relative to the respective Aß- subjects. In contrast, florbetapir Aß- older cognitively normal subjects showed an increase in flortaucipir standard uptake value ratios in mesial temporal lobe regions (amygdala, hippocampus/choroid plexus region of interest) compared to younger cognitively normal subjects, but no increased standard uptake value ratios in neocortical regions. Analysis of covariance with planned contrasts showed no differences in regional or composite posterior neocortical flortaucipir standard uptake value ratio as a function of diagnostic group among Aß- older cognitively normal or clinically diagnosed Alzheimer's disease or mild cognitive impairment subjects. The pattern of flortaucipir distribution among Aß+ subjects was reminiscent of the cross-sectional distribution of tau reported in post-mortem pathology studies, in that the most commonly affected regions were the inferior and lateral temporal lobes, the same regions where the first signs of increased retention appeared in Aß+ cognitively normal subjects. However, there was large variability in extent/density of flortaucipir tau binding among Aß+ subjects. Although high neocortical flortaucipir retention was consistently associated with an Aß+ florbetapir positron emission tomography scan, not all Aß+ subjects had elevated flortaucipir standard uptake value ratios. Finally, within the Aß+ group, increasing levels of flortaucipir tau binding were associated with increased cognitive impairment, as assessed by Mini-Mental State Examination and Alzheimer's Disease Assessment Scale. These results suggest development of tau beyond the mesial temporal lobe is associated with, and may be dependent on, amyloid accumulation. Further, the results are consistent with the hypothesis that cortical tau is associated with cognitive impairment.

Facile Route to 2-Fluoropyridines via 2-Pyridyltrialkylammonium Salts Prepared from Pyridine N-Oxides and Application to (18)F-Labeling.

Among known precursors for 2-[(18)F]fluoropyridines, pyridyltrialkylammonium salts have shown excellent reactivity; however, their broader utility has been limited because synthetic methods for their preparation suffer from poor functional group compatibility. In this paper, we demonstrate the regioselective conversion of readily available pyridine N-oxides into 2-pyridyltrialkylammonium salts under mild and metal-free conditions. These isolable intermediates serve as effective precursors to structurally diverse 2-fluoropyridines, including molecules relevant to PET imaging. In addition to providing access to nonradioactive analogues, this method has been successfully applied to (18)F-labeling in the radiosynthesis of [(18)F]AV-1451 ([(18)F]T807), a PET tracer currently under development for imaging tau.

Squaramide hydroxamate-based chemidosimeter responding to iron(III) with a fluorescence intensity increase.

The synthesis and in vitro evaluation of a squarate hydroxamate-coumarin conjugate, 12, as a chemodosimeter for Fe(III) and other oxidants, such as Cr(VI) and Ce(IV), is described. As 12 was originally designed to become a chelation-enhanced fluorescence (CHEF)-type sensor for Fe(III), the competence of the squarate diamide platform to relay a CHEF response was demonstrated using a zinc-binding, cyclen-substituted squarate coumarin amide. Due to a photo electron transfer process, 12 possesses a low fluorescence yield. Upon exposure of 12 to Fe(III) (or other oxidants), an irreversible 9-fold fluorescence intensity increase is observed as the result of an oxidation/hydrolysis reaction. The (aminomethyl)coumarin portion of 12 is oxidized to an iminocoumarin that hydrolyzes to produce a highly fluorescent coumarinaldehyde. Fe(III) acts as a catalyst in this transformation, thereby enhancing the sensitivity of the system for the detection of Fe(III) down to 1 ppm in aqueous buffer solution. The identities of the major reaction products between 12 and Fe(III) were proven by independent synthesis.

Glucosamine conjugates bearing N,N,O-donors: potential imaging agents utilizing the [M(CO)3]+ core (M = Re, Tc).

The design rationale, synthesis and radiolabeling evaluation of four glucosamine conjugated ligands for the [(99m)Tc(CO)(3)](+) core is described. The capability to bind the tricarbonyl core is initially demonstrated using the cold surrogate [Re(CO)(3)](+). The four compounds are competent chelates in binding [(99m)Tc(CO)(3)](+) as labeling studies show, with yields ranging from 79 to 96% and the resulting complexes showing stability in the presence of competing chelates for 24 h at 37 degrees C. The rhenium complexes were tested for hexokinase-catalysed phosphorylation, and the technetium complexes were tested for GLUT-1 mediated cell uptake--they showed a small amount of uptake but it was not glucose dependent, suggesting that it was not via the GLUT-1 transporters.

Pyridine-tert-nitrogen-phenol ligands: N,N,O-Type tripodal chelates for the [M(CO)3]+ core (M = Re, Tc).

The design rationale, synthesis, and preliminary radiolabeling evaluation of new N,N,O-type pyridyl- tert-nitrogen-phenol ligands for the [M(CO) 3] (+) core, where M = (99m)Tc or Re, are described. The capability of the ligands to bind this technetium core is initially demonstrated by using the cold surrogate [Re(CO) 3] (+). NMR studies of the relevant rhenium tricarbonyl complexes indicate the formation of either a monomeric or a possible dimeric complex with each phenolic O atom bridging between two metal centers. Labeling with [ (99m)Tc(CO) 3] (+) provided further insight into the differences in complex formation on the dilute, no carrier added, level compared to the macroscopic scale at which the Re (I) counterparts were made. These new tridentate, monoanionic ligands are competent chelates in binding the [ (99m)Tc(CO) 3] (+) core because radiolabeling yields ranged from 85 to 99% and the resulting complexes were stable to cysteine and histidine challenges for as long as 24 h.

Coumarin-based chemosensors for zinc(II): toward the determination of the design algorithm for CHEF-type and ratiometric probes.

The synthesis of a series of coumarin-based chemosensor assemblies for zinc is detailed, using established and novel synthetic pathways. Variations of the nature of the chelating unit (DPA or cyclen), position of the attachment point of the chelating unit (3- or 4-position), and nature of the 7-substituent (-OH, -OAc, or -NR2) on the coumarin play a crucial role in whether, and to what extent, a CHEF-type or ratiometric response of the chemosensor is observed. Solvent effects are also discussed. The chemosensors were shown to be competent for detecting zinc pools in cultured rat pituitary (GH3) and hepatoma (H4IIE) cell lines. The work further defines the design algorithms for zinc-selective CHEF-type and ratiometric chemosensors.

Illuminating zinc in biological systems.

Zinc is the second most abundant transition metal in the human body, fulfilling a multitude of biological roles, but the mechanisms underlying its physiology are poorly understood. The lack of knowledge is, in part, due to the hitherto limited techniques available to track zinc in biological systems. The recent emergence of a number of zinc-specific molecular sensors has provided a new tool to image zinc in live cells and tissue samples. This contribution highlights the concepts behind using zinc-specific fluorescent molecular sensors to gain information about zinc action in biological samples, and provides representative examples of images recorded.

DPA-substituted coumarins as chemosensors for zinc(II): modulation of the chemosensory characteristics by variation of the position of the chelate on the coumarin.

The sensory capabilities of two novel di(2-picolyl)amine (DPA)-substituted coumarins are described and it is shown that the variation of the point of attachment of the DPA group to the coumarin framework controls their sensing behavior: the 4-substituted system is a CHEF-type sensor that shows a significant increase in fluorescence intensity upon Zn(2+) binding, whereas the 3-substituted system is a ratiometric sensor.

Squaric acid N-hydroxylamides: synthesis, structure, and properties of vinylogous hydroxamic acid analogues.

The synthesis of squaric acid N-hydroxylamide esters 5 and amides 6 from dimethyl squarate 2a is described. These derivatives are analogues of the naturally occurring iron(III) chelator hydroxamic acid. On the basis of a comparative reactivity study, a concerted retro-Cope mechanism for the formation of the N-hydroxylamide esters 5 by reaction of dimethyl squarate with hydroxylamines is proposed. A preliminary iron(III) binding study of these hydroxamic acid analogues is presented, demonstrating binding of iron(III) to amides 6 in aqueous solutions, while the esters 5 did not show any sign of metal ion binding. 13C NMR spectroscopic data (chemical shift and spin-lattice relaxation time determination) of these and related derivatives delineate the resonance structures predominant in these molecules. The resonance structures of the derivatives rationalize their spectroscopic data, chemical reactivity, and iron(III) binding properties. Single-crystal X-ray structure analyses of squaric acid N-hydroxylamide ester 5b and squaric acid N-hydroxylamide amide 6c confirm their connectivity and provide structural evidence supporting the spectroscopically derived conclusions. The squaric acid N-hydroxylamides are potentially useful in the construction of chemosensors for iron(III).

Synthesis of a fluorescent chemosensor suitable for the imaging of zinc(II) in live cells.

The synthesis of a coumarin-cyclen conjugate-based zinc-specific chemosensor and its ability to sense Zn(2+) in vitro is described. Using fluorescence microscopy, the chemosensor was shown to be capable of imaging Zn(2+) in live rat pituitary tumour cells.