Microglial activation, tau and amyloid deposition in TREM2 p.R47H carriers and mild cognitive impairment patients: a multi-modal/multi-tracer PET/MRI imaging study with influenza vaccine immune challenge.

BACKGROUND: Microglia are increasingly understood to play an important role in the pathogenesis of Alzheimer's disease. The rs75932628 (p.R47H) TREM2 variant is a well-established risk factor for Alzheimer's disease. TREM2 is a microglial cell surface receptor. In this multi-modal/multi-tracer PET/MRI study we investigated the effect of TREM2 p.R47H carrier status on microglial activation, tau and amyloid deposition, brain structure and cognitive profile. METHODS: We compared TREM2 p.R47H carriers (n = 8; median age = 62.3) and participants with mild cognitive impairment (n = 8; median age = 70.7). Participants underwent two [18F]DPA-714 PET/MRI scans to assess TSPO signal, indicative of microglial activation, before and after receiving the seasonal influenza vaccination, which was used as an immune stimulant. Participants also underwent [18F]florbetapir and [18F]AV1451 PET scans to assess amyloid and tau burden, respectively. Regional tau and TSPO signal were calculated for regions of interest linked to Braak stage. An additional comparison imaging healthy control group (n = 8; median age = 45.5) had a single [18F]DPA-714 PET/MRI. An expanded group of participants underwent neuropsychological testing, to determine if TREM2 status influenced clinical phenotype. RESULTS: Compared to participants with mild cognitive impairment, TREM2 carriers had lower TSPO signal in Braak II (P = 0.04) and Braak III (P = 0.046) regions, despite having a similar burden of tau and amyloid. There were trends to suggest reduced microglial activation following influenza vaccine in TREM2 carriers. Tau deposition in the Braak VI region was higher in TREM2 carriers (P = 0.04). Furthermore, compared to healthy controls TREM2 carriers had smaller caudate (P = 0.02), total brain (P = 0.049) and white matter volumes (P = 0.02); and neuropsychological assessment revealed worse ADAS-Cog13 (P = 0.03) and Delayed Matching to Sample (P = 0.007) scores. CONCLUSIONS: TREM2 p.R47H carriers had reduced levels of microglial activation in brain regions affected early in the Alzheimer's disease course and differences in brain structure and cognition. Changes in microglial response may underlie the increased Alzheimer's disease risk in TREM2 p.R47H carriers. Future therapeutic agents in Alzheimer's disease should aim to enhance protective microglial actions.

Discriminatory ability of next-generation tau PET tracers for Alzheimer's disease.

A next generation of tau PET tracers for the imaging of Alzheimer's disease and other dementias has recently been developed. Whilst the new compounds have now entered clinical studies, there is limited information available to assess their suitability for clinical applications. Head-to-head comparisons are urgently needed to understand differences in the radiotracer binding profiles. We characterized the binding of the tau tracers PI2620, RO948, MK6240 and JNJ067 in human post-mortem brain tissue from a cohort of 25 dementia cases and age-matched controls using quantitative phosphorimaging with tritium-labelled radiotracers in conjunction with phospho-tau specific immunohistochemistry. The four radiotracers depicted tau inclusions composed of paired helical filaments with high specificity, both in cases with Alzheimer's disease and in primary tauopathy cases with concomitant Alzheimer's disease pathology. In contrast, cortical binding to primary tauopathy in cases without paired helical filament tau was found to be within the range of age-matched controls. Off-target binding to monoamine oxidase B has been overcome, as demonstrated by heterologous blocking studies in basal ganglia tissue. The high variability of cortical tracer binding within the Alzheimer's disease group followed the same pattern with each tracer, suggesting that all compounds are suited to differentiate Alzheimer's disease from other dementias.

Oxidation-Cyclisation of Biphenyl Thioethers to Dibenzothiophenium Salts for Ultrarapid 18F-Labelling of PET Tracers.

18F-labelled radiotracers are in high demand and play an important role for diagnostic imaging with positron emission tomography (PET). Challenges associated with the synthesis of the labelling precursors and the incorporation of [18F]fluoride with practical activity yields at batch scale are the main limitations for the development of new 18F-PET tracers. Herein, we report a high-yielding and robust synthetic method to access naked dibenzothiophenium salt precursors of complex PET tracers and their labelling with [18F]fluoride. C-S cross-coupling of biphenyl-2-thioacetate with aryl halides followed by sequential oxidation-cyclisation of the corresponding thioethers gives dibenzothiophenium salts in good to excellent yields. Labelling of neutral and electron-deficient substrates with [18F]fluoride is ultrarapid and occurs under mild conditions (1 min at 90 °C) with high activity yields. The method enables facile synthesis of complex and sensitive radiotracers, as exemplified by radiofluorination of three clinically relevant PET tracers [18F]UCB-J, [18F]AldoView and [18F]FNDP, and can accelerate the development and clinical translation of new 18F-radiopharmaceuticals.

Validation of a combined image derived input function and venous sampling approach for the quantification of [18F]GE-179 PET binding in the brain.

Blood-based kinetic analysis of PET data relies on an accurate estimate of the arterial plasma input function (PIF). An alternative to invasive measurements from arterial sampling is an image-derived input function (IDIF). However, an IDIF provides the whole blood radioactivity concentration, rather than the required free tracer radioactivity concentration in plasma. To estimate the tracer PIF, we corrected an IDIF from the carotid artery with estimates of plasma parent fraction (PF) and plasma-to-whole blood (PWB) ratio obtained from five venous samples. We compared the combined IDIF+venous approach to gold standard data from arterial sampling in 10 healthy volunteers undergoing [18F]GE-179 brain PET imaging of the NMDA receptor. Arterial and venous PF and PWB ratio estimates determined from 7 patients with traumatic brain injury (TBI) were also compared to assess the potential effect of medication. There was high agreement between areas under the curves of the estimates of PF (r = 0.99, p<0.001), PWB ratio (r = 0.93, p<0.001), and the PIF (r = 0.92, p<0.001) as well as total distribution volume (VT) in 11 regions across the brain (r = 0.95, p<0.001). IDIF+venous VT had a mean bias of -1.7% and a comparable regional coefficient of variation (arterial: 21.3 ± 2.5%, IDIF+venous: 21.5 ± 2.0%). Simplification of the IDIF+venous method to use only one venous sample provided less accurate VT estimates (mean bias 9.9%; r = 0.71, p<0.001). A version of the method that avoids the need for blood sampling by combining the IDIF with population-based PF and PWB ratio estimates systematically underestimated VT (mean bias -20.9%), and produced VT estimates with a poor correlation to those obtained using arterial data (r = 0.45, p<0.001). Arterial and venous blood data from 7 TBI patients showed high correlations for PF (r = 0.92, p = 0.003) and PWB ratio (r = 0.93, p = 0.003). In conclusion, the IDIF+venous method with five venous samples provides a viable alternative to arterial sampling for quantification of [18F]GE-179 VT.

Resolving the cellular specificity of TSPO imaging in a rat model of peripherally-induced neuroinflammation.

The increased expression of 18 kDa Translocator protein (TSPO) is one of the few available biomarkers of neuroinflammation that can be assessed in humans in vivo by positron emission tomography (PET). TSPO PET imaging of the central nervous system (CNS) has been widely undertaken, but to date no clear consensus has been reached about its utility in brain disorders. One reason for this could be because the interpretation of TSPO PET signal remains challenging, given the cellular heterogeneity and ubiquity of TSPO in the brain. The aim of the current study was to ascertain if TSPO PET imaging can be used to detect neuroinflammation induced by a peripheral treatment with a low dose of the endotoxin, lipopolysaccharide (LPS), in a rat model (ip LPS), and investigate the origin of TSPO signal changes in terms of their cellular sources and regional distribution. An initial pilot study utilising both [18F]DPA-714 and [11C]PK11195 TSPO radiotracers demonstrated [18F]DPA-714 to exhibit a significantly higher lesion-related signal in the intracerebral LPS rat model (ic LPS) than [11C]PK11195. Subsequently, [18F]DPA-714 was selected for use in the ip LPS study. Twenty-four hours after ip LPS, there was an increased uptake of [18F]DPA-714 across the whole brain. Further analyses of regions of interest, using immunohistochemistry and RNAscope Multiplex fluorescence V2 in situ hybridization technology, showed TSPO expression in microglia, monocyte derived-macrophages, astrocytes, neurons and endothelial cells. The expression of TSPO was significantly increased after ip LPS in a region-dependent manner: with increased microglia, monocyte-derived macrophages and astrocytes in the substantia nigra, in contrast to the hippocampus where TSPO was mostly confined to microglia and astrocytes. In summary, our data demonstrate the robust detection of peripherally-induced neuroinflammation in the CNS utilising the TSPO PET radiotracer, [18F]DPA-714, and importantly, confirm that the resultant increase in TSPO signal increase arises mostly from a combination of microglia, astrocytes and monocyte-derived macrophages.

Mapping Aldehyde Dehydrogenase 1A1 Activity using an [18 F]Substrate-Based Approach.

Aldehyde dehydrogenases (ALDHs) catalyze the oxidation of aldehydes to carboxylic acids. Elevated ALDH expression in human cancers is linked to metastases and poor overall survival. Despite ALDH being a poor prognostic factor, the non-invasive assessment of ALDH activity in vivo has not been possible due to a lack of sensitive and translational imaging agents. Presented in this report are the synthesis and biological evaluation of ALDH1A1-selective chemical probes composed of an aromatic aldehyde derived from N,N-diethylamino benzaldehyde (DEAB) linked to a fluorinated pyridine ring either via an amide or amine linkage. Of the focused library of compounds evaluated, N-ethyl-6-(fluoro)-N-(4-formylbenzyl)nicotinamide 4 b was found to have excellent affinity and isozyme selectivity for ALDH1A1 in vitro. Following 18 F-fluorination, [18 F]4 b was taken up by colorectal tumor cells and trapped through the conversion to its 18 F-labeled carboxylate product under the action of ALDH. In vivo positron emission tomography revealed high uptake of [18 F]4 b in the lungs and liver, with radioactivity cleared through the urinary tract. Oxidation of [18 F]4 b, however, was observed in vivo, which may limit the tissue penetration of this first-in-class radiotracer.

Ex vivo characterization of neuroinflammatory and neuroreceptor changes during epileptogenesis using candidate positron emission tomography biomarkers.

OBJECTIVE: Identification of patients at risk of developing epilepsy before the first spontaneous seizure may promote the development of preventive treatment providing opportunity to stop or slow down the disease. METHODS: As development of novel radiotracers and on-site setup of existing radiotracers is highly time-consuming and expensive, we used dual-centre in vitro autoradiography as an approach to characterize the potential of innovative radiotracers in the context of epilepsy development. Using brain slices from the same group of rats, we aimed to characterise the evolution of neuroinflammation and expression of inhibitory and excitatory neuroreceptors during epileptogenesis using translational positron emission tomography (PET) tracers; 18 F-flumazenil (18 F-FMZ; GABAA receptor), 18 F-FPEB (metabotropic glutamate receptor 5; mGluR5), 18 F-flutriciclamide (translocator protein; TSPO, microglia activation) and 18 F-deprenyl (monoamine oxidase B, astroglia activation). Autoradiography images from selected time points after pilocarpine-induced status epilepticus (SE; baseline, 24 and 48 hours, 5, 10 and 15 days and 6 and 12-14 weeks after SE) were normalized to a calibration curve, co-registered to an MRI-based 2D region-of-interest atlas, and activity concentration (Bq/mm2 ) was calculated. RESULTS: In epileptogenesis-associated brain regions, 18 F-FMZ and 18 F-FPEB showed an early decrease after SE. 18 F-FMZ decrease was maintained in the latent phase and further reduced in the chronic epileptic animals, while 18 F-FPEB signal recovered from day 10, reaching baseline levels in chronic epilepsy. 18 F-flutriciclamide showed an increase of activated microglia at 24 hours after SE, peaking at 5-15 days and decreasing during the chronic phase. On the other hand, 18 F-deprenyl autoradiography showed late astrogliosis, peaking in the chronic phase. SIGNIFICANCE: Autoradiography revealed different evolution of the selected targets during epileptogenesis. Our results suggest an advantage of combined imaging of inter-related targets like glutamate and GABAA receptors, or microglia and astrocyte activation, in order to identify important interactions, especially when using PET imaging for the evaluation of novel treatments.

Ring-Closing Synthesis of Dibenzothiophene Sulfonium Salts and Their Use as Leaving Groups for Aromatic 18F-Fluorination.

Herein, we report a novel intramolecular ring-closing reaction of biaryl thioethers that give access to highly functionalized dibenzothiophene sulfonium salts under mild conditions. The resulting precursors react regioselectively with [18F]fluoride to give [18F]fluoroarenes in predictable radiochemical yields. The strategy expands the available radiochemical space and provides superior labeling efficiency for clinically relevant PET tracers.

Characterization of tau positron emission tomography tracer [18F]AV-1451 binding to postmortem tissue in Alzheimer's disease, primary tauopathies, and other dementias.

INTRODUCTION: Aggregation of tau is a hallmark of many neurodegenerative diseases, and tau imaging with positron emission tomography (PET) may allow early diagnosis and treatment monitoring. We assessed binding of the PET tracer [18F]AV-1451 in a range of dementias. METHODS: Phosphorimaging was used to quantify binding to postmortem brain tissue from 33 patients with different, histopathologically characterized, neurodegenerative dementias. RESULTS: [18F]AV-1451 showed high specific binding in cases with Alzheimer's disease (AD), moderate binding in Pick's disease and frontotemporal dementia with parkinsonism-17, and low but displaceable binding in corticobasal degeneration, progressive supranuclear palsy, non-tau proteinopathies, and in controls without pathology. Tracer binding did not correlate with tau load within disease groups. DISCUSSION: [18F]AV-1451 binds to tau in AD, and some other tauopathies. However, evidence for a non-tau binding site and lack of correlation between tracer binding and antibody staining suggest that reliable quantification of tau load with this tracer is problematic.

A dual optical and nuclear imaging reagent for peptide labelling via disulfide bridging.

We report a concise approach to a multimodal imaging reagent for peptide labelling via disulfide bridging. The reagent is constructed using a one pot, three component, [3 + 2] cycloaddition of a fluorescent azide with a dithiomaleimide-alkyne, with concomitant incorporation of (125)I. The dithiomaleimide handle then enables site selective conjugation to a disulfide bond of a peptide whilst retaining the key structural bridging functionality, as exemplified on the therapeutic peptide octreotide.

A new model measuring bacterial phagocytosis and phagolysosomal oxidisation in humans using the intradermal injection of methylene blue-labelled Escherichia coli.

Phagocytosis is an important leucocyte function, however using existing models it cannot be measured in human tissues in vivo. To address this, we characterized a new phagocytosis model using intradermal methylene blue-labelled Escherichia coli injection (MBEC). Methylene blue (MB) is a licensed human medicine and bacterial stain potentially useful for labelling E. coli that are safe for human injection. Ex vivo co-culture of leucocytes with MBEC caused MB to transfer into neutrophils and macrophages by phagocytosis. During this, a 'red shift' in MB fluorescence was shown to be caused by phagolysosomal oxidisation. Hence, MBEC co-culture could be used to measure phagocytosis and phagolysosomal oxidisation in humans, ex vivo. In healthy volunteers, inflammatory exudate sampling using suction blisters 2-24h after intradermal MBEC injection showed that tissue-acquired neutrophils and monocytes contained more MB than their circulating counterparts, whereas blood and inflamed tissue T, B and NK cells were MBlo. This was validated with spectral flow cytometry by visualizing the MB emission spectrum in tissue-acquired neutrophils. Neutrophil MB emission spectra demonstrated more 'red shift' at 24h compared to earlier time-points, in-keeping with progressive phagolysosomal MB oxidisation in neutrophils over time in vivo. This new MBEC model can therefore measure bacterial phagocytosis and phagolysosomal oxidisation in human skin, in vivo. This has a number of important research applications, for example in studying human phagocyte biology, testing novel antimicrobials, and understanding why certain groups such as males, the elderly or those with diabetes, recent surgery or malnutrition are at increased risk of bacterial infection.

A one-pot three-component radiochemical reaction for rapid assembly of 125I-labeled molecular probes.

Nuclear imaging in conjunction with radioactive tracers enables noninvasive measurements of biochemical events in vivo. However, access to tracers remains limited due to the lack of methods for rapid assembly of radiolabeled molecules with the prerequisite biological activity. Herein, we report a one-pot, three-component, copper(II)-mediated reaction of azides, alkynes, and [(125)I]iodide to yield 5-[(125)I]iodo-1,2,3-triazoles. Using a selection of azides and alkynes in a combinatorial approach, we have synthesized a library of structurally diverse (125)I-labeled triazoles functionalized with bioconjugation groups, fluorescent dyes, and biomolecules. Our preliminary biological evaluation suggests that 5-[(125)I]iodo-1,2,3-triazoles are resistant to deiodination in vivo, both as small molecular probes and as antibody conjugates. The ability to incorporate radioactive iodide into triazoles directly from the parent azides and alkynes makes the method broadly applicable and offers the potential to rapidly assemble molecular probes from an array of structurally diverse, and readily available, building blocks.

Synthesis and evaluation of a 125I-labeled iminodihydroquinoline-derived tracer for imaging of voltage-gated sodium channels.

In vivo imaging of voltage-gated sodium channels (VGSCs) can potentially provide insights into the activation of neuronal pathways and aid the diagnosis of a number of neurological diseases. The iminodihydroquinoline WIN17317-3 is one of the most potent sodium channel blockers reported to date and binds with high affinity to VGSCs throughout the rat brain. We have synthesized a (125)I-labeled analogue of WIN17317-3 and evaluated the potential of the tracer for imaging of VGSCs with SPECT. Automated patch clamp studies with CHO cells expressing the Nav1.2 isoform and displacement studies with [(3)H]BTX yielded comparable results for the non-radioactive iodinated iminodihydroquinoline and WIN17317-3. However, the (125)I-labeled tracer was rapidly metabolized in vivo, and suffered from low brain uptake and high accumulation of radioactivity in the intestines. The results suggest that iminodihydroquinolines are poorly suited for tracer development.

Exploration of the structure-activity relationship of a novel tetracyclic class of TSPO ligands-potential novel positron emitting tomography imaging agents.

A series of novel TSPO ligands based on the tetracyclic class of translocator protein (TSPO) ligands first described by Okubo et al. was synthesised and evaluated as potential positron emitting tomography (PET) ligands for imaging TPSO in vivo. Fluorine-18 labelling of the molecules was achieved using direct radiolabelling or synthon based labelling approaches. Several of the ligands prepared have promising profiles as potential TSPO PET imaging ligands.

Chelator-accelerated one-pot 'click' labeling of small molecule tracers with 2-[¹8F]fluoroethyl azide.

2-[¹8F]Fluoroethyl azide ([¹8F]FEA) can readily be obtained by nucleophilic substitution of 2-azidoethyl-4-toluenesulfonate with [¹8F]fluoride (half-life 110 min), and has become widely used as a reagent for 'click' labeling of PET tracers. However, distillation of [18F]FEA is typically required, which is time-consuming and unpractical for routine applications. In addition, copper(I)-catalyzed cycloaddition of [¹8F]FEA with non-activated alkynes, and with substrates containing labile functional groups, can be challenging. Herein, we report a highly efficient and practical ligand-accelerated one-pot/two-step method for 'click' labeling of small molecule tracers with [¹8F]FEA. The method exploits the ability of the copper(I) ligand bathophenanthrolinedisulfonate to accelerate the rate of the cycloaddition reaction. As a result, alkynes can be added directly to the crude reaction mixture containing [¹8F]FEA, and as cyclisation occurs almost immediately at room temperature, the reaction is tolerant to labile functional groups. The method was demonstrated by reacting [¹8F]FEA with a series of alkyne-functionalized 6-halopurines to give the corresponding triazoles in 55-76% analytical radiochemical yield.

Evaluation of a 125I-labelled benzazepinone derived voltage-gated sodium channel blocker for imaging with SPECT.

Voltage-gated sodium channels (VGSCs) are a family of transmembrane proteins that mediate fast neurotransmission, and are integral to sustain physiological conditions and higher cognitive functions. Imaging of VGSCs in vivo holds promise as a tool to elucidate operational functions in the brain and to aid the treatment of a wide range of neurological diseases. To assess the suitability of 1-benzazepin-2-one derived VGSC blockers for imaging, we have prepared a (125)I-labelled analogue of BNZA and evaluated the tracer in vivo. In an automated patch-clamp assay, a diastereomeric mixture of the non-radioactive compound blocked the Na(v)1.2 and Na(v)1.7 VGSC isoforms with IC(50) values of 4.1 ± 1.5 µM and 0.25 ± 0.07 µM, respectively. [(3)H]BTX displacement studies revealed a three-fold difference in affinity between the two diastereomers. Iodo-destannylation of a tin precursor with iodine-125 afforded the two diastereomerically pure tracers, which were used to assess binding to VGSCs in vivo by comparing their tissue distributions in mice. Whilst the results point to a lack of VGSC binding in vivo, SPECT imaging revealed highly localized uptake in the interscapular region, an area typically associated with brown adipose tissue, which in addition to high metabolic stability of the iodinated tracer, demonstrate the potential of 1-benzazepin-2-ones for in vivo imaging.

Positron emission tomography imaging of drug-induced tumor apoptosis with a caspase-3/7 specific [18F]-labeled isatin sulfonamide.

Of the molecular biochemical alterations that occur during apoptosis, activation of caspases, notably caspase-3, is probably the most attractive for developing specific in vivo molecular imaging probes. We recently designed a library of isatin-5 sulfonamides and selected [18F]ICMT-11 for further evaluation on the basis of subnanomolar affinity for activated capsase-3, high metabolic stability, and facile radiolabeling. In this present study, we have demonstrated that [18F]ICMT-11 binds to a range of drug-induced apoptotic cancer cells in vitro and to 38C13 murine lymphoma xenografts in vivo by up to 2-fold at 24 h posttreatment compared to vehicle treatment. We further demonstrated that the increased signal intensity in tumors after drug treatment, detected by whole body in vivo microPET imaging, was associated with increased apoptosis. In summary, we have characterized [18F]ICMT-11 as a caspase-3/7 specific PET imaging radiotracer for the assessment of tumor apoptosis that could find utility in anticancer drug development and the monitoring of early responses to therapy.

Development of [18F]AldoView as the First Highly Selective Aldosterone Synthase PET Tracer for Imaging of Primary Hyperaldosteronism.

The purpose of this study was to synthesize a fluorine-18 labeled, highly selective aldosterone synthase (hCYP11B2) inhibitor, [18F]AldoView, and to assess its potential for the detection of aldosterone-producing adenomas (APAs) with positron emission tomography in patients with primary hyperaldosteronism (PHA). Using dibenzothiophene sulfonium salt chemistry, [18F]AldoView was obtained in high radiochemical yield in one step from [18F]fluoride. In mice, the tracer showed a favorable pharmacokinetic profile, including rapid distribution and clearance. Imaging in the adrenal tissue from patients with PHA revealed diffuse binding patterns in the adrenal cortex, avid binding in some adenomas, and "hot spots" consistent with aldosterone-producing cell clusters. The binding pattern was in good visual agreement with the antibody staining of hCYP11B2 and distinguished areas with normal and excessive hCYP11B2 expression. Taken together, [18F]AldoView is a promising tracer for the detection of APAs in patients with PHA.

Drug delivery, biodistribution and anti-EGFR activity: theragnostic nanoparticles for simultaneous in vivo delivery of tyrosine kinase inhibitors and kinase activity biosensors.

In vivo delivery of small molecule therapeutics to cancer cells, assessment of the selectivity of administration, and measuring the efficacity of the drug in question at the molecule level, are important ongoing challenges in developing new classes of cancer chemotherapeutics. One approach that has the potential to provide targeted delivery, tracking of biodistribution and readout of efficacy, is to use multimodal theragnostic nanoparticles to deliver the small molecule therapeutic. In this paper, we report the development of targeted theragnostic lipid/peptide/DNA lipopolyplexes. These simultaneously deliver an inhibitor of the EGFR tyrosine kinase, and plasmid DNA coding for a Crk-based biosensor, Picchu-X, which when expressed in the target cells can be used to quantify the inhibition of EGFR in vivo in a mouse colorectal cancer xenograft model. Reversible bioconjugation of a known analogue of the tyrosine kinase inhibitor Mo-IPQA to a cationic peptide, and co-formulation with peptides containing both EGFR-binding and cationic sequences, allowed for good levels of inhibitor encapsulation with targeted delivery to LIM1215 colon cancer cells. Furthermore, high levels of expression of the Picchu-X biosensor in the LIM1215 cells in vivo allowed us to demonstrate, using fluorescence lifetime microscopy (FLIM)-based biosensing, that EGFR activity can be successfully suppressed by the tyrosine kinase inhibitor, released from the lipopolyplexes. Finally, we measured the biodistribution of lipopolyplexes containing 125I-labelled inhibitors and were able to demonstrate that the lipopolyplexes gave significantly higher drug delivery to the tumors compared with free drug.

Evaluation of the kappa-opioid receptor-selective tracer [(11)C]GR103545 in awake rhesus macaques.

PURPOSE: The recent development in radiosynthesis of the (11)C-carbamate function increases the potential of [(11)C]GR103545, which for the last decade has been regarded as promising for imaging the kappa-opioid receptor (kappa-OR) with PET. In the present study, [(11)C]GR103545 was evaluated in awake rhesus macaques. Separate investigations were performed to clarify the OR subtype selectivity of this compound. METHODS: Regional brain uptake kinetics of [(11)C]GR103545 was studied 0-120 min after injection. The binding affinity and opioid subtype selectivity of [(11)C]GR103545 was determined in cells transfected with cloned human opioid receptors. RESULTS: In vitro binding assays demonstrated a high affinity of GR103545 for kappa-OR (K(i) = 0.02 +/- 0.01 nM) with excellent selectivity over mu-OR (6 x 10(2)-fold) and) delta-OR (2 x 10(4)-fold). PET imaging revealed a volume of distribution (V(T)) pattern consistent with the known distribution of kappa-OR, with striatum = temporal cortex > cingulate cortex > frontal cortex > parietal cortex > thalamus > cerebellum. CONCLUSION: [(11)C]GR103545 is selective for kappa-OR and holds promise for use to selectively depict and quantify this receptor in humans by means of PET.