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.

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.

[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).

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.

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.

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.

An optimized radiosynthesis of [18 F]FNDP, a positron emission tomography radiotracer for imaging soluble epoxide hydrolase (sEH).

In this concise practitioner protocol, the radiochemical synthesis of [18 F]FNDP suitable for human positron emission tomography studies is described and the 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 requirements.

Synthesis and quality control of [(18) F]T807 for tau PET imaging.

The detailed synthesis and quality control of [(18) F]T807, radiotracer for tau protein aggregate imaging, are described. The radiotracer synthesis was accomplished in an average of 48 min with an average specific activity at end-of-synthesis of over 4.4 TBq/µmole (120 Ci/µmole) and an average radiochemical yield of 32%. Compliance with all standard US Pharmacopeia Chapter <823> acceptance tests was observed.

An improved synthesis of the radiolabeled prostate-specific membrane antigen inhibitor, [(18) F]DCFPyL.

The radiosynthesis of [(18) F]DCFPyL on 2 distinct automated platforms with full regulatory compliant quality control specifications is described. The radiotracer synthesis was performed on a custom-made radiofluorination module and the Sofie Biosciences ELIXYS. The radiofluorination module synthesis was accomplished in an average of 66 minutes from end of bombardment with an average specific activity at end of synthesis (EOS) of 4.4 TBq/µmol (120 Ci/µmol) and an average radiochemical yield of 30.9% at EOS. The ELIXYS synthesis was completed in an average of 87 minutes with an average specific activity of 2.2 TBq/µmol (59.3 Ci/µmol) and an average radiochemical yield of 19% at EOS. Both synthesis modules produced large millicurie quantities of [(18) F]DCFPyL while conforming to all standard US Pharmacopeia Chapter <823> acceptance testing criteria.

Determination of [11C]rifampin pharmacokinetics within Mycobacterium tuberculosis-infected mice by using dynamic positron emission tomography bioimaging.

Information about intralesional pharmacokinetics (PK) and spatial distribution of tuberculosis (TB) drugs is limited and has not been used to optimize dosing recommendations for new or existing drugs. While new techniques can detect drugs and their metabolites within TB granulomas, they are invasive, rely on accurate resection of tissues, and do not capture dynamic drug distribution in the tissues of interest. In this study, we assessed the in situ distribution of (11)C-labeled rifampin in live, Mycobacterium tuberculosis-infected mice that develop necrotic lesions akin to human disease. Dynamic positron emission tomography (PET) imaging was performed over 60 min after injection of [(11)C]rifampin as a microdose, standardized uptake values (SUV) were calculated, and noncompartmental analysis was used to estimate PK parameters in compartments of interest. [(11)C]rifampin was rapidly distributed to all parts of the body and quickly localized to the liver. Areas under the concentration-time curve for the first 60 min (AUC0-60) in infected and uninfected mice were similar for liver, blood, and brain compartments (P > 0.53) and were uniformly low in brain (10 to 20% of blood values). However, lower concentrations were noted in necrotic lung tissues of infected mice than in healthy lungs (P = 0.03). Ex vivo two-dimensional matrix-assisted laser desorption ionization (MALDI) imaging confirmed restricted penetration of rifampin into necrotic lung lesions. Noninvasive bioimaging can be used to assess the distribution of drugs into compartments of interest, with potential applications for TB drug regimen development.

Microwave-assisted radiosynthesis of [(18) F]ASEM, a radiolabeled α7-nicotinic acetylcholine receptor antagonist.

An improvement of the original radiochemical synthesis of [(18) F]ASEM, an α7-nicotinic acetylcholinergic receptor radioligand, is reported. The new procedure utilizes microwave-assisted radiofluorination. In addition, a new preparative HPLC method was developed to eliminate a chemical impurity in the final product. Quality control procedures were also enhanced to improve detection of product with enhanced resolution of potential impurities. [(18) F]ASEM was produced in 20.1 ± 8.9% non-decay corrected (NDC) yield with an average synthesis time of 57 min and an average specific radioactivity of 856 ± 332 GBq/µmol (23 ± 9 Ci/µmol).

A microwave radiosynthesis of the 4-[18F]-fluorobenzyltriphenylphosphonium ion.

The 4-[(18)F]-fluorobenzyltriphenylphosphonium cation was synthesized by a series of microwave reactions from no carrier added [(18)F]-fluoride. The microwave procedure reduced the quantity of reagents used and synthesis time when compared with the original synthesis. In addition, problematic solid phase extraction, sodium borohydride reduction by column and inconsistent yields with excessive precipitate formation during the bromination step were eliminated. The 4-[(18)F]-fluorobenzyltriphenylphosphonium cation was produced radiochemically pure in 8.3% yield with a specific radioactivity of 534.5 ± 371.4 GBq/µmole at end of synthesis.

Recent PET radioligands with optimal brain kinetics for imaging nicotinic acetylcholine receptors.

Cerebral neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in various neurophysiological processes and in the pathophysiology and/or treatment strategies of various disorders. Positron emission tomography (PET) imaging of nAChR and, especially, the most prominent cerebral subtype α4ß2-nAChR is important in smoking, epilepsy, attention deficit hyperactivity disorder, depression, schizophrenia, cognition, behavior, memory, and in research involving aging, cognitive impairments, and dementia. Most human α4ß2-nAChR PET imaging has been performed with 2-[(18) F]FA, but slow brain kinetics is the substantial drawback of 2-[(18) F]FA that precludes widespread PET imaging research of nAChR in humans. Development of a better PET radioligand for α4ß2-nAChR was a focus of substantial investigation that has been thoroughly reviewed (up to 2009) previously. This article attempts to summarize the peer-reviewed publications of the most recent development and preclinical studies of novel α4ß2-nAChR PET radioligands with improved brain kinetics and first human studies with one of these radioligands ([(18) F]AZAN).

6-O-(2-[18F]Fluoroethyl)-6-O-Desmethyl-Diprenorphine ([18F]FE-DPN) Preferentially Binds to Mu Opioid Receptors In Vivo.

PURPOSE: 6-O-(2-[18F]Fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) is regarded as a non-selective opioid receptor radiotracer. PROCEDURE: Here, we report the first characterization of [18F]FE-DPN synthesized from the novel precursor, 6-O-(2-tosyloxyethoxy)-6-O-desmethyl-3-O-trityl-diprenorphine (TE-TDDPN), using a one-pot, two-step nucleophilic radiosynthesis to image opioid receptors in rats and mice using positron emission tomography. RESULTS: We also show that [18F]FE-DPN and [3H]DPN exhibit negligible brain uptake in mu opioid receptor (MOR) knockout mice. CONCLUSIONS: Taken together with prior findings, our results suggest that [18F]FE-DPN and [3H]DPN preferentially bind to MOR in rodents in vivo.

Multi-timepoint imaging with PSMA-targeted [18F]F-Florastamin PET/CT: lesion detection and comparison to conventional imaging.

OBJECTIVE: The aims of this study were to investigate the utility of [18F]F-Florastamin, a novel prostate-specific membrane antigen (PSMA)-targeted PET radiotracer with facile radiochemistry, relative to the conventional imaging for the detection of sties of disease and evaluate the effect of multi-timepoint imaging with [18F]F-Florastamin PET on lesion detectability. METHODS: Eight prostate cancer patients with known or suspected recurrence who underwent [18F]F-Florastamin PET/CT at 1-h and 2-h imaging time-points were included in this prospective pilot study. [18F]F-Florastamin PET images were interpreted visually and quantitatively at both time points and compared with CIM. RESULTS: [18F]F-Florastamin PET was superior to CT in the detection of active osseous metastases and small-sized metastatic lymph nodes that do not fall under the anatomic imaging size criteria for metastasis. Multi-timepoint imaging showed a significant reduction in the blood pool, bone marrow and muscular uptake, and increase in liver uptake over time. There is a significant improvement in tumor-to-background ratio (TBR) at the 2-h imaging time-point (P = 0.04). The mean percentage change in TBR at 2-h was 21% (SD = 0.31). CONCLUSIONS: [18F]F-Florastamin is a promising new radioligand for PSMA-targeted PET with suitable lesion detectability and high TBR at both time points.

Pilot imaging of the colony stimulating factor 1 receptor in the brains of virally-suppressed individuals with HIV.

OBJECTIVE: Neuroimmune activation is a putative driver of cognitive impairment in people with HIV (PWH), even in the age of modern antiretroviral therapy. Nevertheless, imaging of the microglial marker, the 18 kDa translocator protein (TSPO), with positron emission tomography (PET) in treated PWH has yielded inconclusive findings. One potential reason for the varied TSPO results is a lack of cell-type specificity of the TSPO target. DESIGN: [ 11 C]CPPC, 5-cyano- N -(4-(4-[ 11 C]methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl) furan-2-carboxaminde, is a radiotracer for use with PET to image the colony stimulating factor 1 receptor (CSF1R). The CSF1R is expressed on microglia and central nervous system macrophages, with little expression on other cell types. We used [ 11 C]CPPC PET in virally-suppressed- (VS)-PWH and HIV-uninfected individuals to estimate the effect sizes of higher CSF1R in the brains of VS-PWH. METHODS: Sixteen VS-PWH and 15 HIV-uninfected individuals completed [ 11 C]CPPC PET. [ 11 C]CPPC binding (V T ) in nine regions was estimated using a one-tissue compartmental model with a metabolite-corrected arterial input function, and compared between groups. RESULTS: Regional [ 11 C]CPPC V T did not significantly differ between groups after age- and sex- adjustment [unstandardized beta coefficient ( B ) = 1.84, standard error (SE) = 1.18, P  = 0.13]. The effect size was moderate [Cohen's d  = 0.56, 95% confidence interval (CI) -0.16, 1.28), with strongest trend of higher V T in VS-PWH in striatum and parietal cortex (each P  = 0.04; Cohen's d  = 0.71 and 0.72, respectively). CONCLUSIONS: A group difference in [ 11 C]CPPC V T was not observed between VS-PWH and HIV-uninfected individuals in this pilot, although the observed effect sizes suggest the study was underpowered to detect regional group differences in binding.