cGMP production of the radiopharmaceutical [18 F]MK-6240 for PET imaging of human neurofibrillary tangles

Late Life Depression is Not Associated With Alzheimer-Type Tau: Preliminary Evidence From a Next-Generation Tau Ligand PET-MR Study

OBJECTIVE

To investigate whether tau accumulation is higher in late life depression (LLD) compared to non-depressed cognitively unimpaired (CU) older adults. To situate these findings in the neurodegeneration model of LLD by assessing group differences in tau and grey matter volume (GMV) between LLD, non-depressed CU and mild cognitive impairment due to Alzheimer's Disease (MCI).

DESIGN

Monocentric, cross-sectional study.

SETTING

University Psychiatric hospital, memory clinic and outpatient neurology practice.

PARTICIPANTS

A total of 102 adults over age 60, of whom 19 currently depressed participants with LLD, 19 with MCI and 36 non-depressed CU participants completed neuropsychological testing and tau PET-MR imaging.

MEASUREMENTS

PET-MRI: 18F-MK-6240 tracer SUVR for tau assessment; 3D T1-weighted structural MRI derived GMV in seven brain regions (temporal, cingulate, prefrontal and parietal regions); amyloid PET to assess amyloid positivity; Neuropsychological test scores: MMSE, RAVLT, GDS, MADRS. ANCOVA and Spearman's rank correlations to investigate group differences in tau and GMV, and correlations with neuropsychological test scores respectively.

RESULTS

Compared to non-depressed CU participants, LLD patients showed lower GMV in temporal and anterior cingulate regions but similar tau accumulation and amyloid positivity rate. In contrast, MCI patients had significantly higher tau accumulation in all regions. Tau did not correlate with any neuropsychological test scores in LLD.

CONCLUSION

Our findings suggest AD-type tau is not higher in LLD compared to non-depressed, cognitively unimpaired older adults and appears unlikely to contribute to lower gray matter volume in LLD, further underscoring the need to distinguish major depressive disorder from depressive symptoms occurring in early AD.

Tau Imaging in Late Traumatic Brain Injury: A [18F]MK-6240 Positron Emission Tomography Study

Epidemiological studies have identified prior traumatic brain injury (TBI) as a risk factor for developing Alzheimer's disease (AD). Neurofibrillary tangles (NFTs) are common to AD and chronic traumatic encephalopathy following repetitive mild TBI. However, it is unclear if a single TBI is sufficient to cause accumulation of NFTs. We performed a [18F]MK-6240 positron emission tomography (PET) imaging study to assess NFTs in patients who had sustained a single TBI at least 2 years prior to study inclusion. Fourteen TBI patients (49 ± 20 years; 5 M/9 F; 8 moderate-severe, 1 mild-probable, 5 symptomatic-possible TBI) and 40 demographically similar controls (57 ± 19 years; 19 M/21 F) underwent simultaneous [18F]MK-6240 PET and magnetic resonance imaging (MRI) as well as neuropsychological assessment including the Cambridge Neuropsychological Test Automated Battery (CANTAB). A region-based voxelwise partial volume correction was applied, using parcels obtained by FreeSurfer v6.0, and standardized uptake value ratios (SUVR) were calculated relative to the cerebellar gray matter. Group differences were assessed on both a voxel- and a volume-of-interest-based level and correlations of [18F]MK-6240 SUVR with time since injury as well as with clinical outcomes were calculated. Visual assessment of TBI images did not show global or focal increases in tracer uptake in any subject. On a group level, [18F]MK-6240 SUVR was not significantly different in patients versus controls or between subgroups of moderate-severe TBI versus less severe TBI. Within the TBI group, One Touch Stockings problem solving and spatial working memory (executive function), reaction time (attention), and Mini-Mental State Examination (MMSE) (global cognition) were associated with [18F]MK-6240 SUVR. We found no group-based increase of [18F]MK-6240 brain uptake in patients scanned at least 2 years after a single TBI compared with healthy volunteers, which suggests that no NFTs are building up in the first years after a single TBI. Nonetheless, correlations with cognitive outcomes were found that warrant further investigation.

Head-to-head comparison of [18F]-Flortaucipir, [18F]-MK-6240 and [18F]-PI-2620 postmortem binding across the spectrum of neurodegenerative diseases

We and others have shown that [18F]-Flortaucipir, the most validated tau PET tracer thus far, binds with strong affinity to tau aggregates in Alzheimer's (AD) but has relatively low affinity for tau aggregates in non-AD tauopathies and exhibits off-target binding to neuromelanin- and melanin-containing cells, and to hemorrhages. Several second-generation tau tracers have been subsequently developed. [18F]-MK-6240 and [18F]-PI-2620 are the two that have garnered most attention. Our recent data indicated that the binding pattern of [18F]-MK-6240 closely parallels that of [18F]-Flortaucipir. The present study aimed at the direct comparison of the autoradiographic binding properties and off-target profile of [18F]-Flortaucipir, [18F]-MK-6240 and [18F]-PI-2620 in human tissue specimens, and their potential binding to monoamine oxidases (MAO). Phosphor-screen and high resolution autoradiographic patterns of the three tracers were studied in the same postmortem tissue material from AD and non-AD tauopathies, cerebral amyloid angiopathy, synucleopathies, transactive response DNA-binding protein 43 (TDP-43)-frontotemporal lobe degeneration and controls. Our results show that the three tracers show nearly identical autoradiographic binding profiles. They all strongly bind to neurofibrillary tangles in AD but do not seem to bind to a significant extent to tau aggregates in non-AD tauopathies pointing to their limited utility for the in vivo detection of non-AD tau lesions. None of them binds to lesions containing β-amyloid, α-synuclein or TDP-43 but they all show strong off-target binding to neuromelanin and melanin-containing cells, as well as weaker binding to areas of hemorrhage. The autoradiographic binding signals of the three tracers are only weakly displaced by competing concentrations of selective MAO-B inhibitor deprenyl but not by MAO-A inhibitor clorgyline suggesting that MAO enzymes do not appear to be a significant binding target of any of them. These findings provide relevant insights for the correct interpretation of the in vivo behavior of these three tau PET tracers.

Measurement of Cerebral Perfusion Indices from the Early Phase of [18F]MK6240 Dynamic Tau PET Imaging

6-(fluoro-18F)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine ([18F]MK6240) has high affinity and selectivity for hyperphosphorylated tau and readily crosses the blood-brain barrier. This study investigated whether the early phase of [18F]MK6240 can be used to provide a surrogate index of cerebral perfusion. Methods: Forty-nine subjects who were cognitively normal (CN), had mild cognitive impairment (MCI), or had Alzheimer's disease (AD) underwent paired dynamic [18F]MK6240 and [11C]Pittsburgh compound B (PiB) PET, as well as structural MRI to obtain anatomic information. Arterial blood samples were collected in a subset of 24 subjects for [18F]MK6240 scans to derive metabolite-corrected arterial input functions. Regional time-activity curves were extracted using atlases available in the Montreal Neurologic Institute template space and using FreeSurfer. The early phase of brain time-activity curves was analyzed using a 1-tissue-compartment model to obtain a robust estimate of the rate of transfer from plasma to brain tissue, K 1 (mL⋅cm-3⋅min-1), and the simplified reference tissue model 2 was investigated for noninvasive estimation of the relative delivery rate, R 1 (unitless). A head-to-head comparison with R 1 derived from [11C]PiB scans was performed. Grouped differences in R 1 were evaluated among CN, MCI, and AD subjects. Results: Regional K 1 values suggested a relatively high extraction fraction. R 1 estimated noninvasively from simplified reference tissue model 2 agreed well with R 1 calculated indirectly from the blood-based compartment modeling (r = 0.99; mean difference, 0.024 ± 0.027), suggesting that robust estimates were obtained. R 1 measurements obtained with [18F]MK6240 correlated strongly and overall agreed well with those obtained from [11C]PiB (r = 0.93; mean difference, -0.001 ± 0.068). Statistically significant differences were observed in regional R 1 measurements among CN, MCI, and AD subjects, notably in the temporal and parietal cortices. Conclusion: Our results provide evidence that the early phase of [18F]MK6240 images may be used to derive a useful index of cerebral perfusion. The early and late phases of a [18F]MK6240 dynamic acquisition may thus offer complementary information about the pathophysiologic mechanisms of the disease.

The Association of Age-Related and Off-Target Retention with Longitudinal Quantification of [18F]MK6240 Tau PET in Target Regions

6-(fluoro-18F)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine ([18F]MK6240) tau PET tracer quantifies the brain tau neurofibrillary tangle load in Alzheimer disease. The aims of our study were to test the stability of common reference region estimates in the cerebellum over time and across diagnoses and evaluate the effects of age-related and off-target retention on the longitudinal quantification of [18F]MK6240 in target regions. Methods: We assessed reference, target, age-related, and off-target regions in 125 individuals across the aging and Alzheimer disease spectrum with longitudinal [18F]MK6240 SUVs and SUV ratios (SUVRs) (mean ± SD, 2.25 ± 0.40 y of follow-up). We obtained SUVR from meninges, exhibiting frequent off-target retention with [18F]MK6240. Additionally, we compared tracer uptake between 37 cognitively unimpaired young (CUY) (mean age, 23.41 ± 3.33 y) and 27 cognitively unimpaired older (CU) adults (amyloid-β-negative and tau-negative, 58.50 ± 9.01 y) to identify possible nonvisually apparent, age-related signal. Two-tailed t testing and Pearson correlation testing were used to determine the difference between groups and associations between changes in region uptake, respectively. Results: Inferior cerebellar gray matter SUV did not differ on the basis of diagnosis and amyloid-β status, cross-sectionally and over time. [18F]MK6240 uptake significantly differed between CUY and CU adults in the putamen or pallidum (affecting ∼75% of the region) and in the Braak II region (affecting ∼35%). Changes in meningeal and putamen or pallidum SUVRs did not significantly differ from zero, nor did they vary across diagnostic groups. We did not observe significant correlations between longitudinal changes in age-related or meningeal off-target retention and changes in target regions, whereas changes in all target regions were strongly correlated. Conclusion: Inferior cerebellar gray matter was similar across diagnostic groups cross-sectionally and stable over time and thus was deemed a suitable reference region for quantification. Despite not being visually perceptible, [18F]MK6240 has age-related retention in subcortical regions, at a much lower magnitude but topographically colocalized with significant off-target signal of the first-generation tau tracers. The lack of correlation between changes in age-related or meningeal and target retention suggests little influence of possible off-target signals on longitudinal tracer quantification. Nevertheless, the age-related retention in the Braak II region needs to be further investigated. Future postmortem studies should elucidate the source of the newly reported age-related [18F]MK6240 signal, and in vivo studies should further explore its impact on tracer quantification.

Dosimetry and efficacy of a tau PET tracer [18F]MK-6240 in Japanese healthy elderly and patients with Alzheimer's disease

OBJECTIVE

A new tau PET tracer [¹⁸F]MK-6240 has been developed; however, its dosimetry and pharmacokinetics have been published only for a European population. This study investigated the safety, radiation dosimetry, pharmacokinetics and biodistribution of [¹⁸F]MK-6240 in Japanese elderly subjects. Also, the pattern and extent of brain retention of [¹⁸F]MK-6240 in Japanese healthy elderly subjects and patients with Alzheimer's disease (AD) were investigated. These Japanese results were compared with previous reports on non-Japanese.

METHODS

Three healthy elderly subjects and three AD patients were enrolled. Dynamic whole-body PET scans were acquired for up to 232 min after starting injection of [¹⁸F]MK-6240 (370.4 ± 27.0 MBq) for the former, while a dynamic brain scan was performed from 0 to 75 min post injection for the latter. For both groups, brain PET scans were conducted from 90 to 110 min post injection. Sequential venous blood sampling was performed to measure the radioactivity concentration in the whole blood and plasma as well as the percentages of parent [¹⁸F]MK-6240 and radioactive metabolites in plasma. Organ doses and effective doses were estimated using the OLINDA Ver.2 software. Standardized uptake value ratios (SUVRs) and distribution volume ratios (DVRs) by Logan reference tissue model (LRTM) were measured in eight brain regions using the cerebellar cortex as the reference. Blood tests, urine analysis, vital signs and electrocardiography were performed for safety assessments.

RESULTS

No adverse events were observed. The highest radiation doses were received by the gallbladder (257.7 ± 74.9 μGy/MBq) and the urinary bladder (127.3 ± 11.7 μGy/MBq). The effective dose was 26.8 ± 1.4 μSv/MBq. The parent form ([¹⁸F]MK-6240) was metabolized quickly and was less than 15% by 35 min post injection. While no obvious accumulation was found in the brain of healthy subjects, focal accumulation of [¹⁸F]MK-6240 was observed in the cerebral cortex of AD patients. Regional SUVRs of the focal lesions in AD patients increased gradually over time, and the difference of SUVRs between healthy subjects and AD patients became large and stable at 90 min after injection. High correlations of SUVR and DVR were observed (p < 0.01).

CONCLUSION

The findings supported safety and efficacy of [¹⁸F]MK-6240 as a tau PET tracer for Japanese populations. Even though the number of subjects was limited, the radiation dosimetry profiles, pharmacokinetics, and biodistribution of [¹⁸F]MK-6240 were consistent with those for non-Japanese populations.

TRIAL REGISTRATION

Japan Pharmaceutical Information Center ID, JapicCTI-194972.

Spatial decrease of synaptic density in amnestic mild cognitive impairment follows the tau build-up pattern

Next to amyloid and tau, synaptic loss is a key pathological hallmark in Alzheimer's disease, closely related to cognitive dysfunction and neurodegeneration. Tau is thought to cause synaptic loss, but this has not been experimentally verified in vivo. In a 2-year follow-up study, dual tracer PET-MR was performed in 12 amnestic MCI patients using ¹⁸F-MK-6240 for tau and ¹¹C-UCB-J for SV2A as a proxy for synaptic density. Tau already accumulated in the neocortex at baseline with progression in Braak V/VI at follow-up. While synaptic loss was limited to limbic regions at baseline, it followed the specific tau pattern to stage IV/V regions two years later, indicating that tau spread might drive synaptic vulnerability. Moreover, synaptic density changes correlated to changes in cognitive function. This study shows for the first time in vivo that synaptic loss regionally follows tau accumulation after two years, providing a disease-modifying window of opportunity for (combined) tau-targeting therapies.

Impact of meningeal uptake and partial volume correction techniques on [18F]MK-6240 binding in aMCI patients and healthy controls

[¹⁸F]MK-6240 is a second-generation tau PET-tracer to quantify neurofibrillary tangles in-vivo. However, individually variable levels of meningeal uptake induce spill-in-effects into the cortex, complicating [¹⁸F]MK-6240 PET quantification. Group SUVR differences between age-matched HC subgroups with varying extracerebral uptake (EC-low/mixed/high), and between aMCI and each HC subgroup were assessed without and with partial volume correction (PVC). Both Müller-Gartner (MG-)PVC and region-based voxelwise (RBV-)PVC, with the latter also correcting for extracerebral spill-in-effects, were implemented. Between HC groups, where no differences are to be expected, HC EC-high showed spill-in differences compared to HC EC-low when no PVC was applied while for MG-PVC, differences were reduced and, for RBV-PVC, no statistically significant differences were observed. Between aMCI and HC, cortical SUVR differences were statistically significant, both without and with PVC, but modulated by the varying meningeal uptake in HC subgroups when no PVC was applied. After applying PVC, correlations to clinical parameters improved and effect sizes between HC and aMCI increased, independent of the HC-subgroup. Therefore, appropriate PVC with correction for extracerebral spill-in-effects is recommended to minimize the impact of varying meningeal uptake on cortical differences between HC and aMCI.

Near-Infrared Fluorescent Probes as Imaging and Theranostic Modalities for Amyloid-Beta and Tau Aggregates in Alzheimer's Disease

A person suspected of having Alzheimer's disease (AD) is clinically diagnosed for the presence of principal biomarkers, especially misfolded amyloid-beta (Aβ) and tau proteins in the brain regions. Existing radiotracer diagnostic tools, such as PET imaging, are expensive and have limited availability for primary patient screening and pre-clinical animal studies. To change the status quo, small-molecular near-infrared (NIR) probes have been rapidly developed, which may serve as an inexpensive, handy imaging tool to comprehend the dynamics of pathogenic progression in AD and assess therapeutic efficacy in vivo. This Perspective summarizes the biochemistry of Aβ and tau proteins and then focuses on structurally diverse NIR probes with coverages of their spectroscopic properties, binding affinity toward Aβ and tau species, and theranostic effectiveness. With the summarized information and perspective discussions, we hope that this paper may serve as a guiding tool for designing novel in vivo imaging fluoroprobes with theranostic capabilities in the future.

MicroPET Imaging Assessment of Brain Tau and Amyloid Deposition in 6 × Tg Alzheimer’s Disease Model Mice

Alzheimer’s disease (AD) is characterized by the deposition of extracellular amyloid plaques and intracellular accumulation of neurofibrillary tangles (NFT). Amyloid beta (Aβ) and tau imaging are widely used for diagnosing and monitoring AD in clinical settings. We evaluated the pathology of a recently developed 6 × Tg − AD (6 × Tg) mouse model by crossbreeding 5 × FAD mice with mice expressing mutant (P301L) tau protein using micro-positron emission tomography (PET) image analysis. PET studies were performed in these 6 × Tg mice using [¹⁸F]Flutemetamol, which is an amyloid PET radiotracer; [¹⁸F]THK5351 and [¹⁸F]MK6240, which are tau PET radiotracers; moreover, [¹⁸F]DPA714, which is a translocator protein (TSPO) radiotracer, and comparisons were made with age-matched mice of their respective parental strains. We compared group differences in standardized uptake value ratio (SUVR), kinetic parameters, biodistribution, and histopathology. [¹⁸F]Flutemetamol images showed prominent cortical uptake and matched well with 6E10 staining images from 2-month-old 6 × Tg mice. [¹⁸F]Flutemetamol images showed a significant correlation with [¹⁸F]DPA714 in the cortex and hippocampus. [¹⁸F]THK5351 images revealed prominent hippocampal uptake and matched well with AT8 immunostaining images in 4-month-old 6 × Tg mice. Moreover, [¹⁸F]THK5351 images were confirmed using [¹⁸F]MK6240, which revealed significant correlations in the cortex and hippocampus. Uptake of [¹⁸F]THK5351 or [¹⁸F]MK6240 was highly correlated with [¹⁸F]Flutemetamol in 4-month-old 6 × Tg mice. In conclusion, PET imaging revealed significant age-related uptake of Aβ, tau, and TSPO in 6 × Tg mice, which was highly correlated with age-dependent pathology.

Preclinical Safety Evaluation and Human Dosimetry of [18F]MK-6240, a Novel PET Tracer for Imaging Neurofibrillary Tangles

PURPOSE

[¹⁸F]MK-6240 is a selective, high-affinity positron emission tomography tracer for imaging neurofibrillary tangles, a key pathological signature that correlates with cognitive decline in Alzheimer disease. This report provides safety information from preclinical toxicology studies and first-in-human whole-body biodistribution and dosimetry studies of [¹⁸F]MK-6240 for its potential application in human brain imaging studies.

PROCEDURES

MK-6240 was administered intravenously (IV) in a 7-day rat toxicity study at × 50, × 100, and × 1000 dose margins relative to projected highest clinical dose of 0.333 μg/kg. The IV formulation of MK-6240 for clinical use and the formulation used in the 7-day rat toxicity study was tested for hemolysis potential in human and Wistar rat whole blood. Sequential whole-body positron emission tomography scans were performed in three healthy young subjects after IV bolus injection of 180 ± 0.3 MBq [¹⁸F]MK-6240 to characterize organ biodistribution and estimate whole-body radiation exposure (effective dose).

RESULTS

MK-6240 administered IV in a 7-day rat toxicity study did not show any test article-related changes. The no-observed-adverse-effect level in rats was ≥ 333 μg/kg/day which provides a margin 1000-fold over an anticipated maximum clinical dose of 0.333 μg/kg. Additionally, the MK-6240 formulation was not hemolytic in human or Wistar rat blood. [¹⁸F]MK-6240 activity was widely distributed to the brain and the rest of the body, with organ absorbed doses largest for the gall bladder (202 μGy/MBq). The average (±SD) effective dose was 29.4 ± 0.6 μSv/MBq, which is in the typical range for F-18 radiolabeled ligands.

CONCLUSIONS

Microdoses of [¹⁸F]MK-6240 are safe for clinical positron emission tomography imaging studies. Single IV administration of 185 MBq (5 mCi) [¹⁸F]MK-6240 is anticipated to result in a total human effective dose of 5.4 mSv and thus allows multiple positron emission tomography scans of the same subject per year.

Test-retest characteristic of [18F]MK-6240 quantitative outcomes in cognitively normal adults and subjects with Alzheimer's disease

[¹⁸F]MK-6240 is a selective, high-affinity PET radiotracer for imaging neurofibrillary tangles (NFT) in Alzheimer's disease (AD). Herein, we report test-retest (T-RT) reproducibility of [¹⁸F]MK-6240 in AD and healthy volunteers (HV). Twelve subjects with AD and three cognitively normal HV were enrolled in the study and dynamically scanned for 150 min with [¹⁸F]MK-6240 under a T-RT protocol. Two radioactivity doses were investigated: 165 ± 3 MBq (n = 6) and 300 ± 40 MBq (n = 9). Serial arterial blood samples were taken for each scan to obtain metabolite-corrected input functions. Following intravenous administration of [¹⁸F]MK-6240, the tracer rapidly partitioned into the brain and its heterogenous distribution pattern was consistent with known NFT pathology in AD. In contrast, uptake in HV was low and uniform across the brain parenchyma. Across all subjects, average T-RT variabilities in NFT-rich regions were ∼21%, ∼14% and ∼6% for various quantitative metrics: total distribution volume (V_T), binding potential (BP_ND), and standardized uptake ratio (SUVR_90-120), respectively. No significant differences in SUVR T-RT variability were observed between the high and low injected radioactivity groups (5.6% and 6.1%, respectively). This work suggests [¹⁸F]MK-6240 has adequate SUVR T-RT characteristics supporting the use of this outcome in future studies.

Characterization of MK6240, a tau PET tracer, in autopsy brain tissue from Alzheimer's disease cases

Purpose

MK6240 is a second-generation tau PET tracer designed to detect the neurofibrillary tangles in the brains of patients with Alzheimer’s disease (AD). The aim of the study was to characterize ³H-MK6240 in AD and control brain tissue and to compare its binding properties with those of first-generation tau PET tracers.

Methods

Saturation binding assays with ³H-MK6240 were carried out in the temporal and parietal cortices of AD brains to determine the maximum number of binding sites (Bmax) and the dissociation constants (Kd) at these sites. Competitive binding assays were carried out between ³H-MK6240 and unlabelled MK6240, AV-1451 (aka T807, flortaucipir) and THK5117, and between ³H-THK5351 and unlabelled MK6240. Regional binding studies with ³H-MK6240 were carried out in homogenates from six AD and seven control brains and, using autoradiography, on large frozen sections from two AD brains and one control brain.

Results

The saturation binding assays gave Bmax and Kd values of 59.2 fmol/mg and 0.32 nM in the temporal cortex and 154.7 fmol/mg and 0.15 nM in the parietal cortex. The competitive binding assays revealed two binding sites with affinities in the picomolar and nanomolar range shared by ³H-MK6240 and all the tested unlabelled compounds. There were no binding sites in common between ³H-THK5351 and unlabelled MK6240. Regional binding of ³H-MK6240 was significantly higher in AD brain tissue than in controls. Binding in brain tissue from AD patients with early-onset AD was significantly higher than in brain tissue from patients with late-onset AD. Binding of ³H-MK6240 was not observed in off-target regions. Autoradiography showed high regional cortical binding in the two AD brains and very low binding in the control brain.

Conclusions

³H-MK6240 has a high binding affinity for tau deposits in AD brain tissue but also has different binding characteristics from those of the first-generation tau tracers. This confirms the complexity of tau tracer binding on tau deposits with different binding affinities for different binding sites.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-05035-y) contains supplementary material, which is available to authorized users.

In vivo synaptic density loss is related to tau deposition in amnestic mild cognitive impairment

OBJECTIVE

To investigate in vivo whether synaptic loss and neurofibrillary tangle load spatially overlap and correlate with clinical symptoms in patients with amnestic mild cognitive impairment (aMCI).

METHODS

In this cross-sectional study, 10 patients with aMCI and 10 healthy controls underwent triple PET-MRI with ¹¹C-UCB-J (synaptic vesicle protein 2A), ¹⁸F-MK-6240 (tau deposition), and ¹¹C-Pittsburgh compound B (β-amyloid) and neuropsychological assessment. Gray matter atrophy was assessed by voxel-based morphometry with T1-weighted MRIs. Voxel-wise and volume-of-interest analyses were conducted on PET data. The interrelationship of synaptic density and tau deposition was investigated. We also investigated correlations of ¹⁸F-MK-6240 and ¹¹C-UCB-J binding with cognitive performance.

RESULTS

Compared to controls, patients with aMCI showed a decreased ¹¹C-UCB-J binding mainly in substructures of the medial temporal lobe (MTL; 48%-51%, p _cluster = 0.02). Increased ¹⁸F-MK6240 binding in the same region was observed (42%-44%, p _cluster = 0.0003), spreading to association cortices. In the MTL, higher ¹⁸F-MK-6240 binding inversely related to lower ¹¹C-UCB-J binding (p = 0.02, r = -0.76). Decreased performance on cognitive tests was associated with both increased ¹⁸F-MK-6240 and decreased ¹¹C-UCB-J binding in the hippocampus (p < 0.01, r > 0.7), although in a multivariate analysis only ¹⁸F-MK-6240 binding was significantly related to cognitive performance.

CONCLUSIONS

Patients with aMCI have high tau deposition and synaptic density loss mainly in key regions known to be involved in early cognitive impairment, indicating that these are interrelated in the MTL, while tau binding had already spread toward association cortices. Longitudinal data are needed to provide further insight into the temporal aspects of this relationship.

A concisely automated synthesis of TSPO radiotracer [18 F]FDPA based on spirocyclic iodonium ylide method and validation for human use

Fluorine-18 labeled N,N-diethyl-2-(2-(4-(2-fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide ([18 F]FDPA) is a potent and selective radiotracer for positron-emission tomography (PET) imaging of the translocator protein 18 kDa (TSPO). Our previous in vitro and in vivo evaluations have proven that this tracer is promising for further human translation. Our study addresses the need to streamline the automatic synthesis of this radiotracer to make it more accessible for widespread clinical evaluation and application. Here, we successfully demonstrate a one-step radiolabeling of [18 F]FDPA based on a novel spirocyclic iodonium ylide (SCIDY) precursor using tetra-n-butyl ammonium methanesulfonate (TBAOMs), which has demonstrated the highest radiochemical yields and molar activity from readily available [18 F]fluoride ion. The nucleophilic radiofluorination was completed on a GE TRACERlab FX2 N synthesis module, and the formulated [18 F]FDPA was obtained in nondecay corrected (n.d.c) radiochemical yields of 15.6 ± 4.2%, with molar activities of 529.2 ± 22.5 GBq/μmol (14.3 ± 0.6 Ci/μmol) at the end of synthesis (60 minutes, n = 3) and validated for human use. This methodology facilitates efficient synthesis of [18 F]FDPA in a commercially available synthesis module, which would be broadly applicable for routine production and widespread clinical PET imaging studies.

Evaluation of pharmacokinetic modeling strategies for in-vivo quantification of tau with the radiotracer [18F]MK6240 in human subjects

PURPOSE

[18F]MK6240 was developed for PET imaging of tau aggregates, which are implicated in Alzheimer's disease. The goal of this work was to evaluate the kinetics of [18F]MK6240 and to investigate different strategies for in-vivo quantification of tau aggregates in humans.

METHODS

Thirty-five subjects, consisting of 18 healthy controls (CTRL), 11 subjects with mild cognitive impairment (MCI) and six with Alzheimer's Disease (AD), underwent dynamic [18F]MK6240 PET scans. Arterial blood measurements were collected in 16 subjects (eight CTRLs, six MCIs and two AD) to measure whole blood and plasma concentration time courses. Radiometabolite analysis was performed on a subset of plasma samples. Various compartmental model configurations as well as the Logan and multilinear analysis (MA1) graphical methods with arterial plasma input function were tested. Simplified reference tissue methods were investigated, including Logan distribution volume ratio (DVR), multilinear reference tissue method (MRTM2), and static SUV ratio using the cerebellum as a reference region.

RESULTS

Whole blood:plasma ratio stabilized to 0.66 ± 0.01 after 15 min. Percent parent in plasma (%PP) followed a single exponential and ranged from 0 to 10% at 90 min. [18F]MK6240 in gray matter peaked quickly (SUV > 2 at ~3 min). The preferred compartmental model was a reversible two-tissue compartment model, with the blood contribution included as a model parameter (2T4k1v). Compartmental and graphical analysis methods with arterial input functions yielded concordant results, but rapid metabolism raised challenges for blood-based quantification. MCI and AD subjects demonstrated a broad range of VT as compared to CTRL subjects. DVR from MRTM2 and Logan reference tissue methods correlated with DVR calculated indirectly from compartmental modeling, but underestimation was observed in data sets with very high binding (DVR > 3). SUVR also underestimated indirect DVR from blood-based analyses in high binding regions, although a non-linear relationship was exhibited.

CONCLUSIONS

[18F]MK6240 exhibited a wide dynamic range of uptake, with binding patterns in MCI/AD subjects consistent with neurofibrillary tau deposition patterns. Linearized reference tissue methods using an estimated average tissue-to-plasma efflux constant [Formula: see text] and static SUVR agreed well with blood-based methods for most data sets; however, discrepancies were noted in the highest binding cases. Caution should therefore be exercised in application of simplified methods to such data sets, and in quantitative interpretation of corresponding outcomes.

Autoradiography validation of novel tau PET tracer [F-18]-MK-6240 on human postmortem brain tissue

[F-18]-MK-6240, a novel tau positron emission tomography (PET) tracer recently discovered for the in vivo detection of neurofibrillary tangles, has the potential to improve diagnostic accuracy in the detection of Alzheimer disease. We have examined regional and substrate-specific binding patterns as well as possible off-target binding of this tracer on human brain tissue to advance towards its validation. We applied [F-18]-MK-6240 phosphor screen and high resolution autoradiography to postmortem samples from patients with a definite pathological diagnosis of Alzheimer disease, frontotemporal lobar degeneration-tau (Pick's disease, progressive supranuclear palsy and corticobasal degeneration), chronic traumatic encephalopathy, frontotemporal lobar degeneration-Tar DNA-binding protein 43 (TDP-43), dementia with Lewy bodies, cerebral amyloid angiopathy and elderly controls free of pathologic changes of neurodegenerative disease. We also directly compared the binding properties of [F-18]-MK-6240 and [F-18]-AV-1451 in human tissue, and examined potential nonspecific binding of both tau tracers to monoamine oxidases (MAO) by using autoradiography in the presence of selective monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) inhibitors. Our data indicate that MK-6240 strongly binds to neurofibrillary tangles in Alzheimer disease but does not seem to bind to a significant extent to tau aggregates in non-Alzheimer tauopathies, suggesting that it may have a limited utility for the in vivo detection of these pathologies. There is no evidence of binding to lesions containing β-amyloid, α-synuclein or TDP-43. In addition, we identified MK-6240 strong off-target binding to neuromelanin and melanin-containing cells, and some weaker binding to areas of hemorrhage. These binding patterns are nearly identical to those previously reported by our group and others for [F-18]-AV-1451. Of note, [F-18]-MK-6240 and [F-18]-AV-1451 autoradiographic binding signals were only weakly displaced by competing concentrations of selective MAO-B inhibitor deprenyl but not by MAO-A inhibitor clorgyline, suggesting that MAO enzymes do not appear to be a significant binding target of any of these two tracers. Together these novel findings provide relevant insights for the correct interpretation of in vivo [F-18]-MK-6240 PET imaging.

Dementia spectrum disorders: lessons learnt from decades with PET research

The dementia spectrum encompasses a range of disorders with complex diagnosis, pathophysiology and limited treatment options. Positron emission tomography (PET) imaging provides insights into specific neurodegenerative processes underlying dementia disorders in vivo. Here we focus on some of the most common dementias: Alzheimer's disease, Parkinsonism dementias including Parkinson's disease with dementia, dementia with Lewy bodies, progressive supranuclear palsy and corticobasal syndrome, and frontotemporal lobe degeneration. PET tracers have been developed to target specific proteinopathies (amyloid, tau and α-synuclein), glucose metabolism, cholinergic system and neuroinflammation. Studies have shown distinct imaging abnormalities can be detected early, in some cases prior to symptom onset, allowing disease progression to be monitored and providing the potential to predict symptom onset. Furthermore, advances in PET imaging have identified potential therapeutic targets and novel methods to accurately discriminate between different types of dementias in vivo. There are promising imaging markers with a clinical application on the horizon, however, further studies are required before they can be implantation into clinical practice.

Brain Imaging of Alzheimer Dementia Patients and Elderly Controls with 18F-MK-6240, a PET Tracer Targeting Neurofibrillary Tangles

¹⁸F-MK-6240 (¹⁸F-labeled 6-(fluoro)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine) is a highly selective, subnanomolar-affinity PET tracer for imaging neurofibrillary tangles (NFTs). Plasma kinetics, brain uptake, and preliminary quantitative analysis of ¹⁸F-MK-6240 in healthy elderly (HE) subjects, subjects with clinically probable Alzheimer disease (AD), and subjects with amnestic mild cognitive impairment were characterized in a study that is, to our knowledge, the first to be performed on humans. Methods: Dynamic PET scans of up to 150 min were performed on 4 cognitively normal HE subjects, 4 AD subjects, and 2 amnestic mild cognitive impairment subjects after a bolus injection of 152-169 MBq of ¹⁸F-MK-6240 to evaluate tracer kinetics and distribution in brain. Regional SUV ratio (SUVR) and distribution volume ratio were determined using the cerebellar cortex as a reference region. Total distribution volume was assessed by compartmental modeling using radiometabolite-corrected input function in a subgroup of 6 subjects. Results: ¹⁸F-MK-6240 had rapid brain uptake with a peak SUV of 3-5, followed by a uniformly quick washout from all brain regions in HE subjects; slower clearance was observed in regions commonly associated with NFT deposition in AD subjects. In AD subjects, SUVR between 60 and 90 min after injection was high (approximately 2-4) in regions associated with NFT deposition, whereas in HE subjects, SUVR was approximately 1 across all brain regions, suggesting high tracer selectivity for binding NFTs in vivo. ¹⁸F-MK-6240 total distribution volume was approximately 2- to 3-fold higher in neocortical and medial temporal brain regions of AD subjects than in HE subjects and stabilized by 60 min in both groups. Distribution volume ratio estimated by the Logan reference tissue model or compartmental modeling correlated well (R ² > 0.9) to SUVR from 60 to 90 min for AD subjects. Conclusion: ¹⁸F-MK-6240 exhibited favorable kinetics and high binding levels to brain regions with a plausible pattern for NFT deposition in AD subjects. In comparison, negligible tracer binding was observed in HE subjects. This pilot study suggests that simplified ratio methods such as SUVR can be used to quantify NFT binding. These results support further clinical development of ¹⁸F-MK-6240 for potential application in longitudinal studies.

PET/MR Imaging: New Frontier in Alzheimer's Disease and Other Dementias

Alzheimer's disease (AD) is the most common form of dementia; a progressive neurodegenerative disease that currently lacks an effective treatment option. Early and accurate diagnosis, in addition to quick elimination of differential diagnosis, allows us to provide timely treatments that delay the progression of AD. Imaging plays an important role for the early diagnosis of AD. The newly emerging PET/MR imaging strategies integrate the advantages of PET and MR to diagnose and monitor AD. This review introduces the development of PET/MR imaging systems, technical considerations of PET/MR imaging, special considerations of PET/MR in AD, and the system's potential clinical applications and future perspectives in AD.

Discovery of PET radiopharmaceuticals at the academia-industry interface

Project-specific collaborations between academia and pharmaceutical partners are a growing phenomenon within molecular imaging and in particular in the positron emission tomography (PET) radiopharmaceutical community. This cultural shift can be attributed in part to decreased public funding in academia in conjunction with the increased reliance on outsourcing of chemistry, radiochemistry, pharmacology and molecular imaging studies by the pharmaceutical industry. This account highlights some of our personal experiences working with industrial partners to develop new PET radiochemistry methodologies for drug discovery and neuro-PET research studies. These symbiotic academic-industrial partnerships have not only led to novel radiotracers for new targets but also to the application of new carbon-11 and fluorine-18 labeling methodologies and technologies to label previously unprecedented compounds for in vivo evaluations.