Evaluation of [18F]fluoroestradiol and ChRERα as a gene expression PET reporter system in rhesus monkey brain.

Positron emission tomography (PET) reporter systems are a valuable means of estimating the level of expression of a transgene in vivo. For example, the safety and efficacy of gene therapy approaches for the treatment of neurological and neuropsychiatric disorders could be enhanced via the monitoring of exogenous gene expression levels in the brain. The present study evaluated the ability of a newly developed PET reporter system [18F]fluoroestradiol ([18F]FES) and the estrogen receptor-based PET reporter ChRERα, to monitor expression levels of a small hairpin RNA (shRNA) designed to suppress choline acetyltransferase (ChAT) expression in rhesus monkey brain. The ChRERα gene and shRNA were expressed from the same transcript via lentivirus injected into monkey striatum. In two monkeys that received injections of viral vector, [18F]FES binding increased by 70% and 86% at the target sites compared with pre-injection, demonstrating that ChRERα expression could be visualized in vivo with PET imaging. Post-mortem immunohistochemistry confirmed that ChAT expression was significantly suppressed in regions in which [18F]FES uptake was increased. The consistency between PET imaging and immunohistochemical results suggests that [18F]FES and ChRERα can serve as a PET reporter system in rhesus monkey brain for in vivo evaluation of the expression of potential therapeutic agents, such as shRNAs.

Investigating the Mechanism of Aluminum Fluoride Chelation.

Aluminum fluoride (AlF) complexes have been used over the past decade to incorporate [18F]fluoride into large biomolecules in a highly selective fashion by using relatively facile conditions. However, despite their widespread usage, there are a large number of variations in the reaction conditions, without a definitive discussion provided on the mechanism to understand how these changes would alter the end result. Herein, we report a detailed mechanistic investigation of the reaction, using a mixture of theoretical studies, fluorine-19 and fluorine-18 chemistry, and the consequences it has on the efficient clinical translation of AlF-containing imaging agents.

Evaluation of Tetrazine Tracers for Pretargeted Imaging within the Central Nervous System.

The pretargeting approach separates the biological half-life of an antibody from the physical half-life of the radioisotope label, providing a strategy for reducing the radiation burden. A widely explored pretargeting approach makes use of the bioorthogonal click reaction between tetrazines (Tzs) and trans-cyclooctenes (TCOs), combining the targeting specificity of monoclonal antibodies (mAbs) with the rapid clearance and precise reaction of Tzs and TCOs. Such a strategy can allow for the targeting and imaging (e.g., by positron emission tomography (PET)) of molecular markers, which cannot be addressed by solely relying on small molecules. Tz derivatives that undergo inverse electron-demand Diels-Alder (IEDDA) reactions with an antibody bearing TCO moieties have been investigated. This study describes the synthesis and characterization of 11 cold Tz imaging agent candidates. These molecules have the potential to be radiolabeled with 18F or 3H, and with the former label, they could be of use as imaging tracers for positron emission tomography studies. Selection was made using a multiparameter optimization score for the central nervous system (CNS) PET tracers. Novel tetrazines were tested for their pH-dependent chemical stability. Those which turned out to be stable in a pH range of 6.5-8 were further characterized in in vitro assays with regard to their passive permeability, microsomal stability, and P-glycoprotein transport. Furthermore, selected Tzs were examined for their systemic clearance and CNS penetration in a single-dose pharmacokinetic study in rats. Two tetrazines were successfully labeled with 18F, one of which showed brain penetration in a biodistribution study in mice. Another Tz was successfully tritium-labeled and used to demonstrate a bioorthogonal click reaction on a TCO-modified antibody. As a result, we identified one Tz as a potential fluorine-18-labeled CNS-PET agent and a second as a 3H-radioligand for an IEDDA-based reaction with a modified brain-penetrating antibody.

First-in-human imaging using [11C]MDTC: a radiotracer targeting the cannabinoid receptor type 2.

PURPOSE: We report findings from the first-in-human study of [11C]MDTC, a radiotracer developed to image the cannabinoid receptor type 2 (CB2R) with positron emission tomography (PET). METHODS: Ten healthy adults were imaged according to a 90-min dynamic PET protocol after bolus intravenous injection of [11C]MDTC. Five participants also completed a second [11C]MDTC PET scan to assess test-retest reproducibility of receptor-binding outcomes. The kinetic behavior of [11C]MDTC in human brain was evaluated using tissue compartmental modeling. Four additional healthy adults completed whole-body [11C]MDTC PET/CT to calculate organ doses and the whole-body effective dose. RESULTS: [11C]MDTC brain PET and [11C]MDTC whole-body PET/CT was well-tolerated. A murine study found evidence of brain-penetrant radiometabolites. The model of choice for fitting the time activity curves (TACs) across brain regions of interest was a three-tissue compartment model that includes a separate input function and compartment for the brain-penetrant metabolites. Regional distribution volume (VT) values were low, indicating low CB2R expression in the brain. Test-retest reliability of VT demonstrated a mean absolute variability of 9.91%. The measured effective dose of [11C]MDTC was 5.29 µSv/MBq. CONCLUSION: These data demonstrate the safety and pharmacokinetic behavior of [11C]MDTC with PET in healthy human brain. Future studies identifying radiometabolites of [11C]MDTC are recommended before applying [11C]MDTC PET to assess the high expression of the CB2R by activated microglia in human brain.

[18F]FNDP PET neuroimaging test-retest repeatability and whole-body dosimetry in humans.

PURPOSE: Soluble epoxide hydrolase (sEH) is an enzyme that shapes immune signaling through its role in maintaining the homeostasis of polyunsaturated fatty acids and their related byproducts. [18F]FNDP is a radiotracer developed for use with positron emission tomography (PET) to image sEH, which has been applied to imaging sEH in the brains of healthy individuals. Here, we report the test-retest repeatability of [18F]FNDP brain PET binding and [18F]FNDP whole-body dosimetry in healthy individuals. METHODS: Seven healthy adults (4 men, 3 women, ages 40.1 ± 4.6 years) completed [18F]FNDP brain PET on two occasions within a period of 14 days in a test-retest study design. [18F]FNDP regional total distribution volume (VT) values were derived from modeling time-activity data with a metabolite-corrected arterial input function. Test-retest variability, mean absolute deviation, and intraclass correlation coefficient (ICC) were investigated. Six other healthy adults (3 men, 3 women, ages 46.0 ± 7.0 years) underwent [18F]FNDP PET/CT for whole-body dosimetry, which was acquired over 4.5 h, starting immediately after radiotracer administration. Organ-absorbed doses and the effective dose were then estimated. RESULTS: The mean test-retest difference in regional VT (ΔVT) was 0.82 ± 5.17%. The mean absolute difference in regional VT was 4.01 ± 3.33%. The ICC across different brain regions ranged from 0.92 to 0.99. The organs with the greatest radiation-absorbed doses included the gallbladder (0.081 ± 0.024 mSv/MBq), followed by liver (0.077 ± 0.018 mSv/MBq) and kidneys (0.063 ± 0.006 mSv/MBq). The effective dose was 0.020 ± 0.003 mSv/MBq. CONCLUSION: These data support a favorable test-retest repeatability of [18F]FNDP brain PET regional VT. The radiation dose to humans from each [18F]FNDP PET scan is similar to that of other 18F-based PET radiotracers.

An optimized radiosynthesis of [18 F]DK222, a PET radiotracer for imaging PD-L1.

A radiochemical synthesis of [18 F]DK222, a peptide binder of programmed death ligand 1 protein, suitable for human PET studies is described, and results from validation productions are presented. The high specific activity radiotracer product is prepared as a sterile, apyrogenic solution that conforms to current Good Manufacturing Practice (cGMP) requirements. In addition, the production is extended to use a commercial synthesizer platform (General Electric FASTlab 2).

RO6807936 as a novel positron emission tomography (PET) radiotracer for in vitro and in vivo visualization and quantification of beta-site amyloid precursor protein cleaving enzyme (BACE1) in the rodent and baboon brain.

The beta-site amyloid precursor protein cleaving enzyme (BACE1) is responsible for initiating the generation of beta-amyloid, the major constituent of amyloid plaques in Alzheimer's disease (AD). The purpose of this study was to develop a specific BACE1 radioligand for visualization of the distribution pattern and quantification of the BACE1 protein in the rodent and monkey brain both in vitro by autoradiography and in vivo by positron emission tomography (PET). The BACE1 inhibitor RO6807936 originating from an in-house chemical drug optimization program was selected based on its PET tracer-like physicochemical properties and a favorable pharmacokinetic profile. Saturation binding analysis of [3 H]RO6807936 revealed specific and high-affinity binding (KD = 2.9 nM) and a low Bmax value (4.3 nM) of the BACE1 protein in native rat brain membranes. [3 H]RO6807936 binding showed a ubiquitous distribution on rat brain slices in vitro with higher levels in the CA3 pyramidal cell layer and the granule cell layer of the hippocampus. In a next step, RO6807936 was successfully radiolabeled with carbon-11 and showed acceptable uptake in the baboon brain as well as a widespread and rather homogeneous distribution consistent with rodent data. In vivo blockade studies with a specific BACE1 inhibitor reduced uptake of the tracer to homogenous levels across brain regions and demonstrated specificity of the signal. Our data warrant further profiling of this PET tracer candidate in humans to investigate BACE1 expression in normal individuals and those with AD and as an imaging biomarker for target occupancy studies in clinical drug trials.

PET/CT imaging of CSF1R in a mouse model of tuberculosis.

PURPOSE: Macrophages represent an essential means of sequestration and immune evasion for Mycobacterium tuberculosis. Pulmonary tuberculosis (TB) is characterized by dense collections of tissue-specific and recruited macrophages, both of which abundantly express CSF1R on their outer surface. 4-Cyano-N-(5-(1-(dimethylglycyl)piperidin-4-yl)-2',3',4',5'-tetrahydro-[1,1'-biphenyl]-2-yl)-1H-imidazole-2-carboxamide (JNJ-28312141) is a reported high affinity, CSF1R-selective antagonist. We report the radiosynthesis of 4-cyano-N-(5-(1-(N-methyl-N-([11C]methyl)glycyl)piperidin-4-yl)-2',3',4',5'-tetrahydro-[1,1'-biphenyl]-2-yl)-1H-imidazole-2-carboxamide ([11C]JNJ-28312141) and non-invasive detection of granulomatous and diffuse lesions in a mouse model of TB using positron emission tomography (PET). METHODS: Nor-methyl-JNJ-28312141 precursor was radiolabeled with [11C]iodomethane to produce [11C]JNJ-28312141. PET/CT imaging was performed in the C3HeB/FeJ murine model of chronic pulmonary TB to co-localize radiotracer uptake with granulomatous lesions observed on CT. Additionally, CSF1R, Iba1 fluorescence immunohistochemistry was performed to co-localize CSF1R target with reactive macrophages in infected and healthy mice. RESULTS: Radiosynthesis of [11C]JNJ-28312141 averaged a non-decay-corrected yield of 18.7 ± 2.1%, radiochemical purity of 99%, and specific activity averaging 658 ± 141 GBq/µmol at the end-of-synthesis. PET/CT imaging in healthy mice showed hepatobiliary [13.39-25.34% ID/g, percentage of injected dose per gram of tissue (ID/g)] and kidney uptake (12.35% ID/g) at 40-50 min post-injection. Infected mice showed focal pulmonary lesion uptake (5.58-12.49% ID/g), hepatobiliary uptake (15.30-40.50% ID/g), cervical node uptake, and renal uptake (11.66-29.33% ID/g). The ratio of infected lesioned lung/healthy lung uptake is 5.91:1, while the ratio of lesion uptake to adjacent infected radiolucent lung is 2.8:1. Pre-administration of 1 mg/kg of unlabeled JNJ-28312141 with [11C]JNJ-28312141 in infected animals resulted in substantial blockade. Fluorescence microscopy of infected and uninfected whole lung sections exclusively co-localized CSF1R staining with abundant Iba1 + macrophages. Healthy lung exhibited no CSF1R staining and very few Iba1 + macrophages. CONCLUSION: [11C]JNJ-28312141 binds specifically to CSF1R + macrophages and delineates granulomatous foci of disease in a murine model of pulmonary TB.

PET imaging of microglia by targeting macrophage colony-stimulating factor 1 receptor (CSF1R).

While neuroinflammation is an evolving concept and the cells involved and their functions are being defined, microglia are understood to be a key cellular mediator of brain injury and repair. The ability to measure microglial activity specifically and noninvasively would be a boon to the study of neuroinflammation, which is involved in a wide variety of neuropsychiatric disorders including traumatic brain injury, demyelinating disease, Alzheimer's disease (AD), and Parkinson's disease, among others. We have developed [11C]CPPC [5-cyano-N-(4-(4-[11C]methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide], a positron-emitting, high-affinity ligand that is specific for the macrophage colony-stimulating factor 1 receptor (CSF1R), the expression of which is essentially restricted to microglia within brain. [11C]CPPC demonstrates high and specific brain uptake in a murine and nonhuman primate lipopolysaccharide model of neuroinflammation. It also shows specific and elevated uptake in a murine model of AD, experimental allergic encephalomyelitis murine model of demyelination and in postmortem brain tissue of patients with AD. Radiation dosimetry in mice indicated [11C]CPPC to be safe for future human studies. [11C]CPPC can be synthesized in sufficient radiochemical yield, purity, and specific radioactivity and possesses binding specificity in relevant models that indicate potential for human PET imaging of CSF1R and the microglial component of neuroinflammation.

An improved radiosynthesis of [18 F]FAraG, a PET radiotracer for imaging T-cell activation.

In this concise practitioner protocol, the radiochemical synthesis of 2'-deoxy-2'-[18 F]fluoro-9-ß-d-arabinofuranosylguanine ([18 F]FAraG) suitable for human positron emission tomography (PET) studies is described and the results from validation productions are presented. The high specific activity (sometimes referred to as molar activity) radiotracer product is prepared as a sterile, apyrogenic solution that conforms to current Good Manufacturing Practice (cGMP) requirements established by the U.S. Food and Drug Administration.

First-in-human neuroimaging of soluble epoxide hydrolase using [18F]FNDP PET.

PURPOSE: Soluble epoxide hydrolase (sEH) is an enzyme with putative effect on neuroinflammation through its influence on the homeostasis of polyunsaturated fatty acids and related byproducts. sEH is an enzyme that metabolizes anti-inflammatory epoxy fatty acids to the corresponding, relatively inert 1,2-diols. A high availability or activity of sEH promotes vasoconstriction and inflammation in local tissues that may be linked to neuropsychiatric diseases. We developed [18F]FNDP to study sEH in vivo with positron emission tomography (PET). METHODS: Brain PET using bolus injection of [18F]FNDP followed by emission imaging lasting 90 or 180 min was completed in healthy adults (5 males, 2 females, ages 40-53 years). The kinetic behavior of [18F]FNDP was evaluated using a radiometabolite-corrected arterial plasma input function with compartmental or graphical modeling approaches. RESULTS: [18F]FNDP PET was without adverse effects. Akaike information criterion favored the two-tissue compartment model (2TCM) in all ten regions of interest. Regional total distribution volume (VT) values from each compartmental model and Logan analysis were generally well identified except for corpus callosum VT using the 2TCM. Logan analysis was assessed as the choice model due to stability of regional VT values from 90-min data and due to high correlation of Logan-derived regional VT values with those from the 2TCM. [18F]FNDP binding was higher in human cerebellar cortex and thalamus relative to supratentorial cortical regions, which aligns with reported expression patterns of the epoxide hydrolase 2 gene in human brain. CONCLUSION: These data support further use of [18F]FNDP PET to study sEH in human brain.

Osteopontin/secreted phosphoprotein-1 behaves as a molecular brake regulating the neuroinflammatory response to chronic viral infection.

BACKGROUND: Osteopontin (OPN) as a secreted signaling protein is dramatically induced in response to cellular injury and neurodegeneration. Microglial inflammatory responses in the brain are tightly associated with the neuropathologic hallmarks of neurodegenerative disease, but understanding of the molecular mechanisms remains in several contexts poorly understood. METHODS: Micro-positron emission tomography (PET) neuroimaging using radioligands to detect increased expression of the translocator protein (TSPO) receptor in the brain is a non-invasive tool used to track neuroinflammation in living mammals. RESULTS: In humanized, chronically HIV-infected female mice in which OPN expression was knocked down with functional aptamers, uptake of TSPO radioligand DPA-713 was markedly upregulated in the cortex, olfactory bulb, basal forebrain, hypothalamus, and central grey matter compared to controls. Microglia immunoreactive for Iba-1 were more abundant in some HIV-infected mice, but overall, the differences were not significant between groups. TSPO+ microglia were readily detected by immunolabeling of post-mortem brain tissue and unexpectedly, two types of neurons also selectively stained positive for TSPO. The reactive cells were the specialized neurons of the cerebellum, Purkinje cells, and a subset of tyrosine hydroxylase-positive neurons of the substantia nigra. CONCLUSIONS: In female mice with wild-type levels of osteopontin, increased levels of TSPO ligand uptake in the brain was seen in animals with the highest levels of persistent HIV replication. In contrast, in mice with lower levels of osteopontin, the highest levels of TSPO uptake was seen, in mice with relatively low levels of persistent infection. These findings suggest that osteopontin may act as a molecular brake regulating in the brain, the inflammatory response to HIV infection.

Correction to: Osteopontin/secreted phosphoprotein-1 behaves as a molecular brake regulating the neuroinflammatory response to chronic viral infection.

An amendment to this paper has been published and can be accessed via the original article.

The Relationship of Varenicline Agonism of α4ß2 Nicotinic Acetylcholine Receptors and Nicotine-Induced Dopamine Release in Nicotine-Dependent Humans.

INTRODUCTION: Cigarette smoking continues to be one of the most important behavioral causes of morbidity and mortality in the world. Varenicline, an α4ß2 nicotinic acetylcholine receptor (nAChR) partial agonist, has been shown to increase smoking quit rates compared with nicotine-based products. This human laboratory, double-blind, placebo-controlled study examined varenicline and placebo effects on α4ß2-nAChRs occupancy, nicotine-induced change in [11C]raclopride non-displaceable binding potential (BPND), and behavioral measures of cigarette smoking, nicotine craving, and withdrawal. METHODS: Current nicotine dependent daily smokers (N = 17) were randomized to varenicline 1 mg twice daily or placebo for 13 days. Using positron emission tomography), we characterized α4ß2-nAChRs occupancy using [18F]AZAN and dopamine receptor binding using [11C]raclopride as well as behavioral measures of cigarettes smoked, craving, and nicotine withdrawal. RESULTS: Varenicline compared with placebo resulted in significant reductions in [18F]AZAN BPND in multiple brain regions including thalamus, midbrain, putamen, and ventral striatum. Following administration of a controlled-dose nicotine cigarette, dopamine release was significantly suppressed in the ventral striatum in the varenicline-treated compared with the placebo group. There was a significant relationship between α4ß2-nAChRs BPND measured in thalamus during the [18F]AZAN scan and nicotine-induced change in raclopride BPND in the ventral striatum. CONCLUSION: This is the first human study to demonstrate a direct relationship between the extent of varenicline occupancy of α4ß2-nAChRs and the magnitude of dopamine release following nicotine use. IMPLICATIONS: It has remained unclear how nicotinic receptor blockade through partial agonist medications such as varenicline promotes smoking cessation. One hypothesized mechanism is downstream dampening of the mesolimbic reward dopamine system. For the first time in human smokers, we observed a direct relationship between the extent of varenicline blockade of α4ß2-nACh nicotinic receptors and the magnitude of dopamine release following smoking. This has mechanistic and therapeutic implications for improving smoking cessation interventions.

Radiosynthesis and validation of [Carboxy-11 C]4-Aminobenzoic acid ([11 C]PABA), a PET radiotracer for imaging bacterial infections.

In this practitioner protocol, the radiochemical synthesis of [11 C] PABA is described in detail, and a quality control summary of three validation productions is presented. The results indicate that the radiotracer product can be produced in good radiochemical yield (14% at end-of-synthesis (EOS)) at high specific activity (molar activity 11 Ci/µmole EOS; 407 GBq/µmole) and high chemical and radiochemical purity as a sterile, pyrogen-free solution suitable for injection conforming to current Good Manufacturing Practice (cGMP) requirements.

Radiosynthesis and validation of [5-cyano-N-(4-(4-[11 C]methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl) furan-2-carboxamide] ([11 C]CPPC), a PET radiotracer for imaging CSF1R, a microglia-specific marker.

In this practitioner protocol, the radiochemical synthesis of [11 C]CPPC is described in detail, and a quality control summary of three validation productions is presented. The results indicate that the radiotracer product can be produced in good radiochemical yield (> 60 mCi (2.22 GBq) at end-of-synthesis (EOS)), at high specific activity (molar activity > 11,435 mCi/µmole (423 GBq/µmole) at EOS) and high chemical and radiochemical purity. The entire production conforms to current Good Manufacturing Practice (cGMP) requirements. The final product is formulated as a sterile, pyrogen-free solution suitable for human injection.

The distribution of the alpha7 nicotinic acetylcholine receptor in healthy aging: An in vivo positron emission tomography study with [18F]ASEM.

Altered function of the alpha7 nicotinic acetylcholine receptor (α7-nAChR) is implicated in several neuropsychiatric diseases. Nevertheless, studies of the human cerebral α7-nAChR even in healthy aging are limited in number and to postmortem tissue. METHODS: The distribution of the cerebral α7-nAChR was estimated in nine brain regions in 25 healthy volunteers (ages 21-86 years; median 57 years, interquartile range 52 years) using [18F]ASEM with positron emission tomography (PET) imaging. Regional total distribution volume (VT) measurements were calculated using the Logan method from each subject's 90 min dynamic PET data and their metabolite-corrected plasma input function. Spearman's rank or Pearson's correlation analysis was used depending on the normality of the data. Correlation between age and regional 1) volume relative to intracranial volume (volume ratio) and 2) [18F]ASEM VT was tested. Correlation between regional volume ratio and [18F]ASEM VT was also evaluated. Finally, the relationship between [18F]ASEM VT and neuropsychological measures was investigated in a subpopulation of 15 elderly healthy participants (those 50 years of age and older). Bonferroni correction for multiple comparisons was applied to statistical analyses. RESULTS: A negative correlation between tissue volume ratio and age was observed in six of the nine brain regions including striatum and five cortical (temporal, occipital, cingulate, frontal, or parietal) regions. A positive correlation between [18F]ASEM VT and age was observed in all nine brain regions of interest (ROIs). There was no correlation between [18F]ASEM VT and volume ratio in any ROI after controlling for age. Regional [18F]ASEM VT and neuropsychological performance on each of eight representative subtests were not correlated among the well-performing subpopulation of elderly healthy participants. CONCLUSIONS: Our results suggest an increase in cerebral α7-nAChR distribution over the course of healthy aging that should be tested in future longitudinal studies. The preservation of the α7-nAChR in the aging human brain supports the development of therapeutic agents that target this receptor for use in the elderly. Further study of the relationship between α7-nAChR availability and cognitive impairment over aging is needed.

Imaging glial activation in patients with post-treatment Lyme disease symptoms: a pilot study using [11C]DPA-713 PET.

The pathophysiology of post-treatment Lyme disease syndrome (PTLDS) may be linked to overactive immunity including aberrant activity of the brain's resident immune cells, microglia. Here we used [11C]DPA-713 and positron emission tomography to quantify the 18 kDa translocator protein, a marker of activated microglia or reactive astrocytes, in the brains of patients with post-treatment Lyme disease symptoms of any duration compared to healthy controls. Genotyping for the TSPO rs6971 polymorphism was completed, and individuals with the rare, low affinity binding genotype were excluded. Data from eight brain regions demonstrated higher [11C]DPA-713 binding in 12 patients relative to 19 controls. [11C]DPA-713 PET is a promising tool to study cerebral glial activation in PTLDS and its link to cognitive symptoms.

Brain PET Imaging of α7-nAChR with [18F]ASEM: Reproducibility, Occupancy, Receptor Density, and Changes in Schizophrenia.

Background: The α7 nicotinic acetylcholine receptor increasingly has been implicated in normal brain physiology, as well as in neuropsychiatric disorders. The highly cortical distribution of α7 nicotinic acetylcholine receptor suggests a role in cognition. Methods: We expanded the first-in-human PET imaging of α7 nicotinic acetylcholine receptor with [18F]ASEM from 5 to 21 healthy nonsmoking volunteers and added a feasibility study in 6 male patients with schizophrenia. Study aims included: (1) confirmation of test-retest reproducibility of [18F]ASEM binding, (2) demonstration of specificity by competition with DMXB-A, an α7 nicotinic acetylcholine receptor partial agonist, (3) estimation of [18F]ASEM binding potentials and α7 nicotinic acetylcholine receptor density in vivo in humans, and (4) demonstrating the feasibility of studying α7 nicotinic acetylcholine receptor as a target for schizophrenia. Results: Test-retest PET confirmed reproducibility (>90%) (variability ≤7%) of [18F]ASEM volume of distribution (VT) estimates in healthy volunteers. Repeated sessions of PET in 5 healthy subjects included baseline and effect of inhibition after oral administration of 150 mg DMXB-A. From reduction of binding potentials, we estimated the dose-dependent occupancy of α7 nicotinic acetylcholine receptor by DMXB-A at 17% to 49% for plasma concentrations at 60 to 200 nM DMXB-A. In agreement with evidence postmortem, α7 nicotinic acetylcholine receptor density averaged 0.67 to 0.82 nM and inhibitor affinity constant averaged 170 to 385 nM. Median VT in a feasibility study of 6 patients with schizophrenia was lower than in healthy volunteers in cingulate cortex, frontal cortex, and hippocampus (P = 0.02, corrected for multiple comparions, Mann-Whitney test). Conclusions: The current results confirm the reproducibility of [18F]ASEM VT estimates and the specificity of the tracer for α7 nicotinic acetylcholine receptor. Preliminary findings from our feasibility study of [18F]ASEM binding in patients with schizophrenia are suggestive and provide guidance for future studies with more subjects.

Peptide-Based 68Ga-PET Radiotracer for Imaging PD-L1 Expression in Cancer.

Tumors create and maintain an immunosuppressive microenvironment that promotes cancer cell escape from immune surveillance. The immune checkpoint protein programmed death-ligand 1 (PD-L1) is expressed in many cancers and is an important contributor to the maintenance of the immunosuppressive tumor microenvironment. PD-L1 is a prominent target for cancer immunotherapy. Guidance of anti-PD-L1 therapy is currently effected through measurement of PD-L1 through biopsy and immunohistochemistry. Here, we report a peptide-based imaging agent, [68Ga]WL12, to detect PD-L1 expression in tumors noninvasively by positron emission tomography (PET). WL12, a cyclic peptide comprising 14 amino acids, binds to PD-L1 with high affinity (IC50≈ 23 nM). Synthesis of [68Ga]WL12 provided radiochemical purity >99% after purification. Biodistribution in immunocompetent mice demonstrated 11.56 ± 3.18, 4.97 ± 0.8, 1.9 ± 0.1, and 1.33 ± 0.21 percentage of injected dose per gram (%ID/g) in hPD-L1, MDAMB231, SUM149, and CHO tumors, respectively, at 1 h postinjection, with high binding specificity noted with coinjection of excess, nonradiolabeled WL12. PET imaging demonstrated high tissue contrast in all tumor models tested.