Akihiro Takano

Dual tracer tau PET imaging reveals different molecular targets for 11C-THK5351 and 11C-PBB3 in the Alzheimer brain

PurposeSeveral tau PET tracers have been developed, but it remains unclear whether they bind to the same molecular target on the heterogeneous tau pathology. In this study we evaluated the binding of two chemically different tau-specific PET tracers (11C-THK5351 and 11C-PBB3) in a head-to-head, in vivo, multimodal design.MethodsNine patients with a diagnosis of mild cognitive impairment or probable Alzheimer’s disease and cerebrospinal fluid biomarker evidence supportive of the presence of Alzheimer’s disease brain pathology were recruited after thorough clinical assessment. All patients underwent imaging with the tau-specific PET tracers 11C-THK5351 and 11C-PBB3 on the same day, as well as imaging with the amyloid-beta-specific tracer 11C-AZD2184, a T1-MRI sequence, and neuropsychological assessment.ResultsThe load and regional distribution of binding differed between 11C-THK5351 and 11C-PBB3 with no statistically significant regional correlations observed between the tracers. The binding pattern of 11C-PBB3, but not that of 11C-THK5351, in the temporal lobe resembled that of 11C-AZD2184, with strong correlations detected between 11C-PBB3 and 11C-AZD2184 in the temporal and occipital lobes. Global cognition correlated more closely with 11C-THK5351 than with 11C-PBB3 binding. Similarly, cerebrospinal fluid tau measures and entorhinal cortex thickness were more closely correlated with 11C-THK5351 than with 11C-PBB3 binding.ConclusionThis research suggests different molecular targets for these tracers; while 11C-PBB3 appeared to preferentially bind to tau deposits with a close spatial relationship to amyloid-beta, the binding pattern of 11C-THK5351 fitted the expected distribution of tau pathology in Alzheimer’s disease better and was more closely related to downstream disease markers.

Test-retest reproducibility of [11C]-l-deprenyl-D2 binding to MAO-B in the human brain

Background[11C]-l-deprenyl-D2 is a positron emission tomography (PET) radioligand for measurement of the monoamine oxidase B (MAO-B) activity in vivo brain. The estimation of the test-retest reproducibility is important for accurate interpretation of PET studies.ResultsWe performed two [11C]-l-deprenyl-D2 scans for six healthy subjects and evaluated the test-retest variability of this radioligand. MAO-B binding was quantified by two tissue compartment model (2TCM) with three rate constants (K1, k2, k3) using metabolite-corrected plasma radioactivity. The λk3 defined as (K1/k2)u2009×u2009k3 was also calculated. The correlation between MAO-B binding and age, and the effect of partial volume effect correction (PVEc) for the reproducibility were also estimated. %difference of k3 was 2.6% (medial frontal cortex) to 10.3% (hippocampus), and that of λk3 was 5.0% (thalamus) to 9.2% (cerebellum). Mean %difference of all regions were 5.3 and 7.0% in k3 and λk3, respectively. All regions showed below 10% variabilities except the hippocampus in k3 (10.3%). Intraclass correlation coefficient (ICC) of k3 was 0.78 (hippocampus) to 0.98 (medial frontal cortex), and that of λk3 was 0.78 (hippocampus) to 0.95 (thalamus). Mean ICC were 0.94 and 0.89 in k3 and λk3, respectively. The highest positive correlation with age was observed in the hippocampus, as ru2009=u20090.75 in k3 and 0.76 in λk3. After PVEc, mean %difference were 5.6 and 7.2% in k3 and λk3, respectively. Mean ICC were 0.92 and 0.90 for k3 and λk3, respectively. These values were almost the same as those before PVEc.ConclusionsThe present results indicate that k3 and λk3 of [11C]-l-deprenyl-D2 are reliable parameters for test-retest reproducibility with healthy subjects both before and after PVEc. The studies with patients of larger sample size are required for further clinical applications.

In Vivo Visualization of Beta Cells by Targeting of GPR44

GPR44 expression has recently been described as highly β-cell selective in the human pancreas and constitutes a tentative surrogate imaging biomarker in diabetes. A radiolabeled small-molecule GPR44 antagonist, [11C]AZ12204657, was evaluated for visualization of β-cells in pigs and nonhuman primates by positron emission tomography as well as in immunodeficient mice transplanted with human islets under the kidney capsule. In vitro autoradiography of human and animal pancreatic sections from subjects without and with diabetes, in combination with insulin staining, was performed to assess β-cell selectivity of the radiotracer. Proof of principle of in vivo targeting of human islets by [11C]AZ12204657 was shown in the immunodeficient mouse transplantation model. Furthermore, [11C]AZ12204657 bound by a GPR44-mediated mechanism in pancreatic sections from humans and pigs without diabetes, but not those with diabetes. In vivo [11C]AZ12204657 bound specifically to GPR44 in pancreas and spleen and could be competed away dose-dependently in nondiabetic pigs and nonhuman primates. [11C]AZ12204657 is a first-in-class surrogate imaging biomarker for pancreatic β-cells by targeting the protein GPR44.

Potential Effect of Prolonged Sevoflurane Anesthesia on the Kinetics of [11C]Raclopride in Non-human Primates

PurposePositron emission tomography (PET) in non-human primates (NHP) is commonly performed under anesthesia, with sevoflurane being a widely used inhaled anesthetic. PET measurement in NHP can be repeated, and a difference in radioligand kinetics has previously been observed between the first and second PET measurement on the same day using sevoflurane anesthesia. In this study, we evaluated the effect of prolonged sevoflurane anesthesia on kinetics and binding potential (BPND) of [11C]raclopride in NHP.ProceduresThree cynomolgus monkeys underwent two to three PET measurements with [11C]raclopride under continuous sevoflurane anesthesia on the same day. The concentration of sevoflurane was adjusted according to the general conditions and safety parameters of the NHP. Time to peak (TTP) radioactivity in the striatum was estimated from time-activity curves (TACs). The BPND in the striatum was calculated by the simplified reference tissue model using the cerebellum as reference region.ResultsIn each NHP, the TTP became shorter in the later PET measurements than in the first one. Across all measurements (nxa0=xa08), concentration of sevoflurane correlated with TTP (Spearman’s ρxa0=xa0−xa00.79, pxa0=xa00.03), but not with BPND (ρxa0=xa0−xa00.25, pxa0=xa00.55).ConclusionsThese data suggest that sevoflurane affects the shape of TACs but has no evident effect on BPND in consecutive PET measurements.

Characterization of [11C]PXT012253 as a PET Radioligand for mGlu4 Allosteric Modulators in Nonhuman Primates

PurposeModulation of presynaptic metabotropic glutamate receptor 4 (mGlu4) by an allosteric ligand has been proposed as a promising therapeutic target in Parkinson’s disease and levodopa-induced dyskinesia. A positron emission tomography (PET) ligand for an allosteric site of mGlu4 may provide evidence that a clinical drug candidate reaches and binds the target. A carbon-11-labeled PET radioligand binding an allosteric site of mGlu4, [11C]PXT012253, has been recently developed. Here, we describe the detailed characterization of this novel radiolabeled mGlu4 ligand in nonhuman primates.Procedures[11C]PXT012253 binding in the brain of cynomolgus monkeys, under the baseline and blocking conditions with the structurally different mGlu4 allosteric ligand PXT002331, currently in clinical trials for Parkinson’s disease, was quantified with compartment and graphical modeling approaches using a radiometabolite-corrected plasma input function. Whole-body biodistribution of [11C]PXT012253 was then assessed using PET/x-ray computed tomography to estimate the human effective doses of [11C]PXT012253 for further clinical studies.Results[11C]PXT012253 displayed binding in mGlu4-expressing regions in the brain of cynomolgus monkeys. Brain regional time-activity curves of [11C]PXT012253 were well described in the two-tissue compartment model (2TC). Total distribution volume was stably estimated using Logan plot and multilinear analysis (MA1) although 2TC showed unstable values in some cases. Competition with PXT002331 showed high specific binding in the total distribution volume. Whole-body PET showed high accumulation of [11C]PXT012253 in the liver, kidney, heart, and brain in the initial phase. The radioligand was excreted through both the gastrointestinal and the urinary tracts. Effective dose of [11C]PXT012253 was estimated to be 0.0042xa0mSv/MBq.Conclusions[11C]PXT012253 was shown to be a promising PET radioligand for mGlu4 allosteric modulators in the monkey brain. MA1 would be the choice of quantitative method. Further development of [11C]PXT012253 in human subjects is warranted.

Serotonin and norepinephrine transporter occupancy of tramadol in non-human primate using positron emission tomography

Abstract Background Tramadol, a centrally acting analgesic drug, has relatively high affinity to serotonin transporter and norepinephrine transporter in addition to μ-opioid receptor. Based on this characteristic, tramadol is expected to have an antidepressant effect. Methods Positron emission tomography measurements with [11C]MADAM and [18F]FMeNER-D2 were performed at baseline and after i.v. administration of 3 different doses (1, 2, and 4 mg/kg) of tramadol using 6 cynomolgus monkeys. The relationship between dose and occupancy for serotonin transporter and norepinephrine transporter was estimated. Results Tramadol occupied similarly both serotonin transporter (40%–72%) and norepinephrine transporter (7%–73%) in a dose-dependent manner. The Kd was 2.2 mg/kg and 2.0 mg/kg for serotonin transporter and norepinephrine transporter, respectively. Conclusions Both serotonin transporter and norepinephrine transporter of in vivo brain were blocked at >70% at a clinically relevant high dose of tramadol. This study suggests tramadol has potential antidepressant effects through the inhibition of serotonin transporter and norepinephrine transporter in the brain.

The brain-penetrant clinical ATM inhibitor AZD1390 radiosensitizes and improves survival of preclinical brain tumor models

Preclinical data highlight AZD1390 as a potentially powerful new therapy to enhance brain tumor patient responses to radiotherapy. Poor survival rates of patients with tumors arising from or disseminating into the brain are attributed to an inability to excise all tumor tissue (if operable), a lack of blood-brain barrier (BBB) penetration of chemotherapies/targeted agents, and an intrinsic tumor radio-/chemo-resistance. Ataxia-telangiectasia mutated (ATM) protein orchestrates the cellular DNA damage response (DDR) to cytotoxic DNA double-strand breaks induced by ionizing radiation (IR). ATM genetic ablation or pharmacological inhibition results in tumor cell hypersensitivity to IR. We report the primary pharmacology of the clinical-grade, exquisitely potent (cell IC50, 0.78 nM), highly selective [>10,000-fold over kinases within the same phosphatidylinositol 3-kinase–related kinase (PIKK) family], orally bioavailable ATM inhibitor AZD1390 specifically optimized for BBB penetration confirmed in cynomolgus monkey brain positron emission tomography (PET) imaging of microdosed 11C-labeled AZD1390 (Kp,uu, 0.33). AZD1390 blocks ATM-dependent DDR pathway activity and combines with radiation to induce G2 cell cycle phase accumulation, micronuclei, and apoptosis. AZD1390 radiosensitizes glioma and lung cancer cell lines, with p53 mutant glioma cells generally being more radiosensitized than wild type. In in vivo syngeneic and patient-derived glioma as well as orthotopic lung-brain metastatic models, AZD1390 dosed in combination with daily fractions of IR (whole-brain or stereotactic radiotherapy) significantly induced tumor regressions and increased animal survival compared to IR treatment alone. We established a pharmacokinetic-pharmacodynamic-efficacy relationship by correlating free brain concentrations, tumor phospho-ATM/phospho-Rad50 inhibition, apoptotic biomarker (cleaved caspase-3) induction, tumor regression, and survival. On the basis of the data presented here, AZD1390 is now in early clinical development for use as a radiosensitizer in central nervous system malignancies.

The metabotropic glutamate receptor 5 radioligand [ 11 C]AZD9272 identifies unique binding sites in primate brain

&NA; The metabotropic glutamate receptor 5 (mGluR5) is a target for drug development and for imaging studies of the glutamate system in neurological and psychiatric disorders. [11C]AZD9272 is a selective mGluR5 PET radioligand that is structurally different from hitherto applied mGluR5 radioligands. In the present investigation we compared the binding patterns of radiolabeled AZD9272 and other mGluR5 radioligands in the non‐human primate (NHP) brain. PET studies were undertaken using [11C]AZD9272 and the commonly applied mGluR5 radioligand [11C]ABP688. Autoradiography studies were performed in vitro using [3H]AZD9272 and the standard mGluR5 radioligands [3H]M‐MTEP and [3H]ABP688 in NHP tissue. Competition binding studies were undertaken in vivo and in vitro using different mGluR5 selective compounds as inhibitors. In comparison to other mGluR5 radioligands radiolabeled AZD9272 displayed a distinct regional distribution pattern with high binding in ventral striatum, midbrain, thalamus and cerebellum. While the binding of [11C]AZD9272 was almost completely inhibited by the structurally unique mGluR5 compound fenobam (2.0 mg/kg; 98% occupancy), it was only partially inhibited (46% and 20%, respectively) by the mGluR5 selective compounds ABP688 and MTEP, at a dose (2.0 mg/kg) expected to saturate the mGluR5. Autoradiography studies using [3H]AZD9272 confirmed a distinct pharmacologic profile characterized by preferential sensitivity to fenobam. The distinctive binding in ventral striato‐pallido‐thalamic circuits and shared pharmacologic profile with the pro‐psychotic compound fenobam warrants further examination of [11C]AZD9272 for potential application in psychiatric neuroimaging studies. HighlightsmGluR5 radioligand [11C]AZD9272 was characterized in the non‐human primate brain.Binding distribution of [11C]AZD9272 differs from that of other mGluR5 radioligands.Unique AZD9272 binding sites are found in ventral striatum, thalamus and midbrain.AZD9272 can be fully displaced from its binding sites by fenobam.mGluR5 specific MPEP‐like ligands induce only partial inhibition of AZD9272 binding.

Identification of a Novel Positron Emission Tomography (PET) Ligand for Imaging β-Site Amyloid Precursor Protein Cleaving Enzyme 1 (BACE-1) in Brain

Alzheimers disease (AD) is characterized by accumulation of β-amyloid (Aβ) plaques and neurofibrillary tau tangles in the brain. β-Site amyloid precursor protein cleaving enzyme 1 (BACE1) plays a key role in the generation of Aβ fragments via extracellular cleavage of the amyloid precursor protein (APP). We became interested in developing a BACE1 PET ligand to facilitate clinical assessment of BACE1 inhibitors and explore its potential in the profiling and selection of patients for AD trials. Using a set of PET ligand design parameters, compound 3 (PF-06684511) was rapidly identified as a lead with favorable in vitro attributes and structural handles for PET radiolabeling. Further evaluation in an LC-MS/MS cold tracer study in rodents revealed high specific binding to BACE1 in brain. Upon radiolabeling, [18F]3 demonstrated favorable brain uptake and high in vivo specificity in nonhuman primate (NHP), suggesting its potential for imaging BACE1 in humans.

Increased Brain Exposure of an Alpha-Synuclein Fibrillization Modulator by Utilization of an Activated Ester Prodrug Strategy

Previous work in our laboratories has identified a series of peptidomimetic 2-pyridone molecules as modulators of alpha-synuclein (α-syn) fibrillization in vitro. As a first step toward developing molecules from this scaffold as positron emission tomography imaging agents, we were interested in evaluating their blood-brain barrier permeability in nonhuman primates (NHP) in vivo. For this purpose, 2-pyridone 12 was prepared and found to accelerate α-syn fibrillization in vitro. Acid 12, and its acetoxymethyl ester analogue 14, were then radiolabeled with 11C ( t1/2 = 20.4 min) at high radiochemical purity (>99%) and high specific radioactivity (>37 GBq/μmol). Following intravenous injection of each compound in NHP, a 4-fold higher radioactivity in brain was observed for [11C]14 compared to [11C]12 (0.8 vs 0.2 SUV, respectively). [11C]14 was rapidly eliminated from plasma, with [11C]12 as the major metabolic product observed by radio-HPLC. The presented prodrug approach paves the way for future development of 2-pyridones as imaging biomarkers for in vivo imaging of α-synuclein deposits in brain.