18 F-PI-2620 is a PET tracer with high binding affinity for aggregated tau, a key pathologic feature of Alzheimer disease (AD) and other neurodegenerative disorders. Preclinically, 18 F-PI-2620 binds to both 3-repeat and 4-repeat tau isoforms. The purpose of this firstin-humans study was to evaluate the ability of 18 F-PI-2620 to detect tau pathology in AD patients using PET imaging, as well as to assess the safety and tolerability of this new tau PET tracer. Methods: Participants with a clinical diagnosis of probable AD and healthy controls (HCs) underwent dynamic 18 F-PI-2620 PET imaging for 180 min. 18 F-PI-2620 binding was assessed visually and quantitatively using distribution volume ratios (DVR) estimated from noninvasive tracer kinetics and SUV ratio (SUVR) measured at different time points after injection, with the cerebellar cortex as the reference region. Time-activity curves and SUVR were assessed in AD and HC subjects, as well as DVR and SUVR correlations and effect size (Cohen's d) over time. Results: 18 F-PI-2620 showed peak brain uptake around 5 min after injection and fast washout from nontarget regions. In AD subjects, focal asymmetric uptake was evident in temporal and parietal lobes, precuneus, and posterior cingulate cortex. DVR and SUVR in these regions were significantly higher in AD subjects than in HCs. Very low background signal was observed in HCs. 18 F-PI-2620 administration was safe and well tolerated. SUVR time-activity curves in most regions and subjects achieved a secular equilibrium after 40 min after injection. A strong correlation (R 2. 0.93) was found between noninvasive DVR and SUVR for all imaging windows starting at more than 30 min after injection. Similar effect sizes between AD and HC groups were obtained across the different imaging windows. 18 F-PI-2620 uptake in neocortical regions significantly correlated with the degree of cognitive impairment. Conclusion: Initial clinical data obtained in AD and HC subjects demonstrated a high image quality and excellent signal-to-noise ratio of 18 F-PI-2620 PET for imaging tau deposition in AD subjects. Noninvasive quantification using DVR and SUVR for 30-min imaging windows between 30 and 90 min after injection-for example, 45-75 min-provides robust and significant discrimination between AD and HC subjects. 18 F-PI-2620 uptake in expected regions correlates strongly with neurocognitive performance.

disease (HD) striatal neuron loss precedes and predicts motor signs or symptoms. Current imaging biomarkers lack adequate sensitivity for assessing the early stages of HD. Developing an imaging biomarker for HD spanning the time of onset of motor signs remains a major unmet research need. Intracellular proteins whose expression is altered by the mutant huntingtin protein may be superior markers for early HD stages.OBJECTIVE To evaluate whether [ 18 F] ]fluoroethoxy)phenyl)-7methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione), a novel phosphodiesterase 10 positron emission tomography (PET) ligand, is a sensitive marker for striatal changes in early HD. DESIGN, SETTING, AND PARTICIPANTSA cohort of individuals with HD, including premanifest (pre-HD) or manifest with motor signs (mHD), underwent clinical assessments, genetic determination, [ 18 F]MNI-659 PET imaging, and brain magnetic resonance imaging. Age-matched healthy volunteers (HVs) also received clinical assessments and PET and magnetic resonance imaging. MAIN OUTCOMES AND MEASURESBinding potentials (BPnds) were estimated for brain regions of interest, specifically within the basal ganglia, and compared between participants with HD and the HVs and correlated with markers of HD severity and atrophy of basal ganglia nuclei.RESULTS Eleven participants with HD (8 mHD and 3 pre-HD) and 9 HVs participated. Ten of 11 HD participants had known huntingtin CAG repeat length, allowing determination of a burden of pathology (BOP) score. One individual with HD declined CAG determination. All participants with mHD had relatively early-stage disease (4 with stage 1 and 4 with stage 2) and a Unified Huntington's Disease Rating Scale (UHDRS) total Motor subscale score of less than 50. The HD cohort had significantly lower striatal [ 18 F]MNI-659 uptake than did the HV cohort (mean, −48.4%; P < .001). The HD cohort as a whole had a reduction in the basal ganglia BPnd to approximately 50% of the level in the HVs (mean, −47.6%; P < .001). The 3 pre-HD participants had intermediate basal ganglia BPnds. Striatal [ 18 F]MNI-659 uptake correlated strongly with the severity of disease measured by the clinical scale (UHDRS Motor subscale; R = 0.903; P < .001), the molecular marker (BOP; R = 0.908; P < .001), and regional atrophy (R = 0.667; P < .05). CONCLUSIONS AND RELEVANCEAs a promising striatal imaging biomarker, [ 18 F]MNI-659 is potentially capable of assessing the extent of disease in early mHD. Furthermore, [ 18 F]MNI-659 may identify early changes in medium spiny neurons and serve as a marker to predict conversion to mHD. Additional studies with larger, stratified cohorts of patients with HD and prospective studies of individuals with pre-HD are warranted.

Longitudinal data in this small cohort of participants with early HD support [(18)F]MNI-659 PET imaging of PDE10 as a useful biomarker to track HD disease progression.

18 F-AV-1451 is currently the most widely used of several experimental tau PET tracers. The objective of this study was to evaluate 18 F-AV-1451 binding with full kinetic analysis using a metabolitecorrected arterial input function and to compare parameters derived from kinetic analysis with SUV ratio (SUVR) calculated over different imaging time intervals. Methods: 18 F-AV-1451 PET brain imaging was completed in 16 subjects: 4 young healthy volunteers (YHV), 4 aged healthy volunteers (AHV), and 8 Alzheimer disease (AD) subjects. Subjects were imaged for 3.5 h, with arterial blood samples obtained throughout. PET data were analyzed using plasma and reference tissue-based methods to estimate the distribution volume, binding potential (BP ND ), and SUVR. BP ND and SUVR were calculated using the cerebellar cortex as a reference region and were compared across the different methods and across the 3 groups (YHV, AHV, and AD). Results: AD demonstrated increased 18 F-AV-1451 retention compared with YHV and AHV based on both invasive and noninvasive analyses in cortical regions in which paired helical filament tau accumulation is expected in AD. A correlation of R 2 . 0.93 was found between BP ND (130 min) and SUVR-1 at all time intervals. Cortical SUVR curves reached a relative plateau around 1.0-1.2 for YHV and AHV by approximately 50 min, but increased in AD by up to approximately 20% at 110-130 min and approximately 30% at 160-180 min relative to 80-100 min. Distribution volume (130 min) was lower by 30%-35% in the YHV than AHV. Conclusion: Our data suggest that although 18 F-AV-1451 SUVR curves do not reach a plateau and are still increasing in AD, an SUVR calculated over an imaging window of 80-100 min (as currently used in clinical studies) provides estimates of paired helical filament tau burden in good correlation with BP ND , whereas SUVR sensitivity to regional cerebral blood changes needs further investigation.

We have identified a F-18-labeled tracer ([F]MNI-1126) that exhibits comparable in vivo characteristics and specificity for SV2A to [C]UCB-J in non-human primates, which makes [F]MNI-1126 a promising PET radiotracer for imaging SV2A in human trials.

Phosphodiesterase (PDE) 10A is an enzyme involved in the regulation of cyclic adenosine monophosphate and cyclic guanosine monophosphate and is highly expressed in medium-sized spiny neurons of the striatum, making it an attractive target for novel therapies for a variety of neurologic and psychiatric disorders that involve striatal function. Potential ligands for PET imaging of PDE10A have been reported. Here, we report the first-in-human characterization of 2 new PDE10A radioligands, 2-(2-(3-(1-(2-fluoroethyl)-1H-indazol-6-yl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ( 18 F-MNI-654) and 2-(2-(3-(4-(2-fluoroethoxy)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ( 18 F-MNI-659), with the goal of selecting the best one for use in future studies interrogating pathophysiologic changes in neuropsychiatric disorders and aiding pharmaceutical development targeting PDE10A. Methods: Eleven healthy volunteers participated in this study ( 18 F-MNI-654 test-retest, 2 men; 18 F-MNI-659 test-retest, 4 men and 1 woman; 18 F-MNI-659 dosimetry, 2 men and 2 women). Brain PET images were acquired over 5.5 h for 18 F-MNI-654 and over 3.5 h for 18 F-MNI-659, and pharmacokinetic modeling with plasma-and reference-region (cerebellar cortex)-based methods was performed. Whole-body PET images were acquired over 6 h for 18 F-MNI-659 and radiation dosimetry estimated with OLINDA. Results: Both radiotracers were similarly metabolized, with about 20% of intact parent remaining at 120 min after injection. PET time-activity data demonstrated that 18 F-MNI-654 kinetics were much slower than 18 F-MNI-659 kinetics. For 18 F-MNI-659, there was good agreement between the Logan and simplified reference tissue models for nondisplaceable binding potential (BP ND ), supporting noninvasive quantification, with test-retest variability less than 10% and intraclass correlation greater than 0.9. The 18 F-MNI-659 effective dose was estimated at 0.024 mSv/MBq. Conclusion: PET imaging in the human brain with 2 novel PDE10A 18 F tracers is being reported. Noninvasive quantification of 18 F-MNI-659 with the simplified reference tissue model using the cerebellum as a reference is possible. In addition, 18 F-MNI-659 kinetics are fast enough for a good estimate of BP ND with 90 min of data, with values around 3.0 in the basal ganglia. Finally, 18 F-MNI-659 dosimetry is favorable and consistent with values reported for other PET radiotracers currently used in humans.

Motor symptoms in Parkinson disease (PD) are caused by a loss of dopamine input from the substantia nigra to the striatum. Blockade of adenosine 2A (A 2A ) receptors facilitates dopamine D 2 receptor function. In phase 2 clinical trials, A 2A antagonists (istradefylline, preladenant, and tozadenant) improved motor function in PD. We developed a new A 2A PET radiotracer, 18 F-MNI-444, and used it to investigate the relationship between plasma levels and A 2A occupancy by preladenant and tozadenant in nonhuman primates (NHP). Methods: A series of 20 PET experiments was conducted in 5 adult rhesus macaques. PET data were analyzed with both plasma-input (Logan graphical analysis) and reference-region-based (simplified reference tissue model and noninvasive Logan graphical analysis) methods. Whole-body PET images were acquired for radiation dosimetry estimates. Human pharmacokinetic parameters for tozadenant and preladenant were used to predict A 2A occupancy in humans, based on median effective concentration (EC 50 ) values estimated from the NHP PET measurements. Results: 18 F-MNI-444 regional uptake was consistent with A 2A receptor distribution in the brain. Selectivity was demonstrated by dose-dependent blocking by tozadenant and preladenant. The specific-to-nonspecific ratio was superior to that of other A 2A PET radiotracers. Pharmacokinetic modeling predicted that tozadenant and preladenant may have different profiles of A 2A receptor occupancy in humans. Conclusion: 18 F-MNI-444 appears to be a better PET radiotracer for A 2A imaging than currently available radiotracers. Assuming that EC 50 in humans is similar to that in NHP, it appears that tozadenant will provide a more sustained A 2A receptor occupancy than preladenant in humans at clinically tested doses. Par kinson disease (PD) has a prevalence of 1.6% in individuals over the age of 65 y (1) and a lifetime risk of 6.7% from age 45 to 100 y (2). Motor symptoms, which include tremor, bradykinesia, and rigidity, emerge when there is a loss of more than 50% of dopamine neurons in the substantia nigra (SN) (3,4). Loss of these neurons reduces dopamine input to the striatum, where dopamine binds to both D 1 and D 2 receptors. Most striatal D 1 receptors are localized in the so-called direct pathway, whereas most striatal D 2 receptors are localized in medium spiny neurons that project to the globus pallidus pars externa (indirect pathway). Adenosine signals via 4 different G-protein-coupled receptors: A 1 , A 2A , A 2B , and A 3 (5). A 2A receptors are predominantly expressed in striatum, with lower levels present in cortex and thalamus and even lower in cerebellum (5-9). A 2A receptors may play a role in inflammation (10) and could therefore be important in a variety of neurologic diseases, including multiple sclerosis, in which A 2A receptor density is increased (11). In PD, A 2A receptors may be important because they form heterodimers with D 2 receptors in the striatum (5,12), and agonists of A 2A (e.g., adenosine) reduce the affinity of D 2 for dopamin...

18 F-PI-2620 is a next-generation tau PET tracer that has demonstrated ability to image the spatial distribution of suspected tau pathology. The objective of this study was to assess the tracer biodistribution, dosimetry, and quantitative methods of 18 F-PI-2620 in the human brain. Full kinetic modeling to quantify tau load was investigated. Noninvasive kinetic modeling and semiquantitative methods were evaluated against the full tracer kinetics. Finally, the reproducibility of PET measurements from test and retest scans was assessed. Methods: Three healthy controls (HCs) and 4 Alzheimer disease (AD) subjects underwent 2 dynamic PET scans, including arterial sampling. Distribution volume ratio (DVR) was estimated using full tracer kinetics (reversible 2-tissue-compartment [2TC] model and Logan graphical analysis [LGA]) and noninvasive kinetic models (noninvasive LGA [NI-LGA] and the multilinear reference tissue model [MRTM2]). SUV ratio (SUVR) was determined at different imaging windows after injection. The correlation between DVR and SUVR, effect size (Cohen's d), and test-retest variability (TRV) were evaluated. Additionally, 6 HCs received 1 tracer administration and underwent whole-body PET for dosimetry calculation. Organ doses and the whole-body effective dose were calculated using OLINDA 2.0. Results: A strong correlation was found across different kinetic models (R 2 . 0.97) and between DVR(2TC) and SUVR between 30 and 90 min, with an R 2 of more than 0.95. Secular equilibrium was reached at around 40 min after injection in most regions and subjects. TRV and effect size for SUVR across different regions were similar at 30-60 min (TRV, 3.8%; Cohen's d, 3.80), 45-75 min (TRV, 4.3%; Cohen's d, 3.77) and 60-90 min (TRV, 4.9%; Cohen's d, 3.73) and increased at later time points. Elimination was via the hepatobiliary and urinary systems. The whole-body effective dose was 33.3 ± 2.1 μSv/MBq for an adult female and 33.1 ± 1.4 μSv/MBq for an adult male, with a 1.5-h urinary bladder voiding interval. Conclusion: 18 F-PI-2620 exhibits fast kinetics, suitable dosimetry, and low TRV. DVR measured using the 2TC model with arterial sampling correlated strongly with DVR measured by NI-LGA, MRTM2, and SUVR. SUVR can be used for 18 F-PI-2620 PET quantification of tau deposits, avoiding arterial blood sampling. Static 18 F-PI-2620 PET scans between 45 and 75 min after injection provide excellent quantification accuracy, a large effect size, and low TRV.