Novel reference region model reveals increased microglial and reduced vascular binding of 11C-(R)-PK11195 in patients with Alzheimer's disease.

11C-(R)-PK11195 is a PET radiotracer for the quantification of peripheral benzodiazepine binding sites (PBBSs). The PBBS is a consistent marker of activated microglia, and 11C-(R)-PK11195 has been used to image microglial activity in the diseased brain and in neoplasia. However, the PBBS is also expressed in the brain vasculature (endothelium and smooth muscles), and no evidence, to our knowledge, exists of a change in the vascular PBBS in pathologic brains or of such a change having an effect on the quantification of 11C-(R)-PK11195 binding. To investigate this issue, we have used a modified reference-tissue model (SRTMV) that accounts for tracer vascular activity both in reference and target tissues and applied it for the estimation of binding potential (BP) in a cohort of patients with Alzheimer's disease (AD). METHODS: A total of 10 patients with AD and 10 age-matched healthy subjects who underwent a 11C-(R)-PK11195 scan were considered in the analysis. The time-activity curves of 11 regions of interest were extracted using the Hammersmith maximum probability atlas. BPs were first estimated using the standard simplified reference-tissue model (SRTM) with the reference tissue computed with a supervised selection algorithm. Subsequently, we applied an SRTMV that models PBBS vascular activity using an additional linear term for both target (VbT) and reference (VbR) regions accounting for vascular tracer activity (C(B)), whereas C(B) was extracted directly from the images. VbR was fixed to 5%, and R1, k2, BP, and VbT were estimated. PBBS density in the vasculature was also assessed by immunocytochemistry on a separate cohort of young and elderly controls and 3 AD postmortem brains. RESULTS: The inclusion of a vascular component in the SRTM increased BPs in all subjects, but the amount of the increase was different (about 11.9% in controls and 16.8% in patients with AD). In addition, average VbT values derived using the SRTMV were 4.22% for controls but only 2.87% in patients with AD. Immunochemistry showed reduced PBBS expression in AD due to vascular fibrosis. CONCLUSION: The reduction of VbT in AD can be interpreted as a consequence of 2 independent but concurring phenomena. The vascular fibrosis in the AD brain causes the well-documented decrease of the size of lumens and the reduction of blood volume. At the same time, the fibrotic process determines the loss of vascular PBBS, particularly in smooth muscles, as here documented by immunochemistry. The inclusion of the additional vascular component in the SRTM effectively models these 2 concurrent processes and amplifies the BP in AD more than in controls because of the decrease in tracer binding to the vasculature in the disease cohort.     

Human biodistribution and radiation dosimetry of 11C-(R)-PK11195, the prototypic PET ligand to image inflammation

Purpose  

The positron emission tomography (PET) radiotracer ¹¹C-(R)-PK11195 allows the in vivo imaging in humans of the translocator protein 18 kDa (TSPO), previously called peripheral benzodiazepine receptor (PBR), a marker of inflammation. Despite its widespread use, the radiation burden associated with ¹¹C-(R)-PK11195 in humans is not known. To examine this, we performed dynamic whole-body imaging with PET and ¹¹C-(R)-PK11195 in healthy humans.     

11C-DPA-713: a novel peripheral benzodiazepine receptor PET ligand for in vivo imaging of neuroinflammation.

The induction of neuroinflammatory processes, characterized by upregulation of the peripheral benzodiazepine receptor (PBR) expressed by microglial cells, is well correlated with neurodegenerative diseases and with acute neuronal loss. The continually increasing incidence of neurodegenerative diseases in developed countries has become a major health problem, for which the development of diagnostic and follow-up tools is required. Here we investigated a new PBR ligand suitable for PET to monitor neuroinflammatory processes as an indirect hallmark of neurodegeneration. METHODS: We compared PK11195, the reference compound for PBR binding sites, with the new ligand DPA-713 (N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl]acetamide), using a small-animal dedicated PET camera in a model of neuroinflammation in rats. Seven days after intrastriatal injection of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), a PET scan was performed using (11)C-PK11195 or (11)C-DPA-713. Immunohistochemistry for neuronal (NeuN), astrocyte (glial fibrillary acidic protein), and microglial (CD11) specific markers as well as (3)H-PK11195 autoradiographic studies were then correlated with the imaging data. RESULTS: Seven days after a unilateral injection of AMPA in the striatum, (11)C-DPA-713 exhibits a better contrast between healthy and damaged brain parenchyma than (11)C-PK11195 (2.5-fold +/- 0.14 increase vs. 1.6-fold +/- 0.05 increase, respectively). (11)C-DPA-713 and (11)C-PK11195 exhibit similar brain uptake in the ipsilateral side, whereas, in the contralateral side, (11)C-DPA-713 uptake was significantly lower than (11)C-PK11195. Modeling of the data using the simplified reference tissue model shows that the binding potential was significantly higher for (11)C-DPA-713 than for (11)C-PK11195. CONCLUSION: (11)C-DPA-713 displays a higher signal-to-noise ratio than (11)C-PK11195 because of a lower level of unspecific binding that is likely related to the lower lipophilicity of (11)C-DPA-713. Although further studies in humans are required, (11)C-DPA-713 represents a suitable alternative to (11)C-PK11195 for PET of PBR as a tracer of neuroinflammatory processes induced by neuronal stress.     

Reference and target region modeling of [11C]-(R)-PK11195 brain studies.

PET with [(11)C]-(R)-PK11195 is currently the modality of choice for the in vivo imaging of microglial activation in the human brain. In this work we devised a supervised clustering procedure and a new quantification methodology capable of producing binding potential (BP) estimates quantitatively comparable with those derived from plasma input with robust quantitative implementation at the pixel level. METHODS: The new methodology uses predefined kinetic classes to extract a gray matter reference tissue without specific tracer binding and devoid of spurious signals (in particular, blood pool and muscle). Kinetic classes were derived from an historical database of 12 healthy control subjects and from 3 patients with Huntington's disease. BP estimates were obtained using rank-shaping exponential spectral analysis (RS-ESA) (both plasma and reference input) and the simplified reference tissue model (SRTM). Comparison between plasma- derived BPs and those produced with the new reference methodology was performed using 6 additional healthy control subjects. Reliability of the new methodology was performed on 4 test-retest studies of patients with Alzheimer's disease. RESULTS: The new algorithm selected reference voxels in gray matter tissue avoiding regions with specific binding located, in particular, in the venous and arterial circulation. Using the new reference, BP values obtained using a plasma input and a reference input were in excellent agreement and highly correlated (r = 0.811, P < 10(-5)) when calculated with RS-ESA and less so (r = 0.507, P < 0.005) when SRTM was used. In the production of parametric maps, SRTM was used with the new reference extraction, resulting in test-retest variability (10.6%; mean ICC = 0.878) that was superior to that obtained using the previous unsupervised clustering approach (mean ICC = 0.596). CONCLUSION: Reference region modeling combined with supervised reference tissue extraction produces a robust and reproducible quantitative assessment of [(11)C]-(R)-PK11195 studies in the human brain.     

In vivo imaging of microglial activation using a peripheral benzodiazepine receptor ligand: [11C]PK-11195 and animal PET following ethanol injury in rat striatum

OBJECTIVE: To investigate whether [(11)C]PK-11195, a specific peripheral benzodiazepine receptors (PBRs) ligand for positron emission tomography (PET), can show activated microglia in a rat brain injury model. METHODS: On day 1, ethanol was injected into the rat's right striatum (ST) using a stereotaxic operative procedure. On day 3, head magnetic resonance imaging (MRI) scans for surgically treated rats were performed to evaluate ethanol injury morphologically. On day 4, dynamic PET scans (17 injured rats and 7 non-injured controls) were performed for 60 min with an animal PET scanner under chloral hydrate anesthesia following a bolus injection of [(11)C]PK-11195 through tail vein. Because PBRs are present throughout the brain, there is no suitable receptor-free reference region. The reference tissue model may not be applicable because of low target to background ratio for low affinity of [(11)C]PK-11195 to PBRs. We evaluated the PBRs binding with regions of interest (ROIs)-based approach to estimate total distribution volume (V). We used an integral from 0 min to 60 min (V (60)) as an estimate of V. On the coronal PET image, ROIs were placed on bilateral ST. Differences in right/left ST V (60) ratios between lesioned and unlesioned control rats were compared using unpaired t tests. Immunohistochemical staining was performed for confirming the presence of activated microglia following decapitation on the PET experiment day. RESULTS: The right/left ST V (60) ratios in lesioned rats (1.07 +/- 0.08) were significantly higher than those in unlesioned control rats (1.00 +/- 0.06, P < 0.05). On immunohistochemical staining, activated microglia were exclusively observed in the injured right ST but not in the noninjured left ST of the injury rats and the bilateral ST of the non-injured control rats. CONCLUSIONS: These results suggest that [(11)C]PK-11195 PET imaging would be a useful tool for evaluating microglial activation in a rat brain injury model.     

Evaluation of [11C]-DAA1106 for imaging and quantification of neuroinflammation in a rat model of herpes encephalitis

IntroductionMany neurological and psychiatric disorders are associated with neuroinflammation. Positron emission tomography (PET) with [¹¹C]-PK11195 can be used to study neuroinflammation in these disorders. However, [¹¹C]-PK11195 may not be sensitive enough to visualize mild neuroinflammation. As a potentially more sensitive PET tracer for neuroinflammation, [¹¹C]-N-(2,5-dimethoxybenzyl)-N-(4-fluoro-2-phenoxyphenyl)-acetamide (DAA1106) was evaluated in a rat model of herpes encephalitis.MethodsMale Wistar rats were intranasally inoculated with HSV-1 (HSE) or phosphate-buffered saline (control). At Day 6 or Day 7 after inoculation, small-animal [¹¹C]-DAA1106 PET scans were acquired, followed by ex vivo biodistribution. Arterial blood sampling was performed for quantification of uptake.ResultsIn HSE rats, a significantly higher ex vivo, but not in vivo, uptake of [¹¹C]-DAA1106 was found in almost all examined brain areas (24–71%, P<.05), when compared to control rats. Pretreatment with unlabeled PK11195 effectively reduced [¹¹C]-DAA1106 uptake in HSE rats (54–84%; P<.001). The plasma and brain time–activity curves showed rapid uptake of [¹¹C]-DAA1106 into tissue. The data showed a good fit to the Logan analysis but could not be fitted to a two-tissue compartment model.Conclusions[¹¹C]-DAA1106 showed a high and specific ex vivo uptake in the encephalitic rat brain. However, neuroinflammation could not be demonstrated in vivo by [¹¹C]-DAA1106 PET. Quantification of the uptake of [¹¹C]-DAA1106 using plasma sampling is not optimal, due to rapid tissue uptake, slow tissue clearance and low plasma activity.     

Two activated stages of microglia and PET imaging of peripheral benzodiazepine receptors with [<Superscript>11</Superscript>C]PK11195 in rats

Objective  

The transition of microglia from the normal resting state to the activated state is associated with an increased expression of peripheral benzodiazepine receptors (PBR). The extent of PBR expression is dependent on the level of microglial activation. A PBR ligand, [¹¹C]PK11195, has been used for imaging of the activation of microglia in vivo. We evaluated whether [¹¹C]PK11195 PET can indicate differences of microglial activation between no treatment and lipopolysaccharide (LPS) treatment in a rat artificial injury model of brain inflammation.     

PET scanning of macrophages in patients with scleroderma fibrosing alveolitis

RationaleAssessment of disease activity in fibrosing alveolitis due to systemic sclerosis (FASSc) is difficult without using invasive investigation. A repeatable noninvasive method of assessing disease at a cellular level such as with positron emission tomography (PET) could be of great value in evaluating high-resolution changes in the pathological process.ObjectivesTo investigate whether the level of inflammatory cell traffic and lung density in FASSc, imaged in vivo by PET, is different to controls and whether they are associated with changes in pulmonary function indices.MethodsWe used PET to measure lung density and tissue uptake of ¹¹C-[R]-PK11195, a ligand that binds to receptors found in abundance in macrophages. Fifteen patients with FASSc were compared to seven controls.ResultsA trend of reduced uptake of ¹¹C-[R]-PK11195 was observed in FASSc patients (P=.09) and correlated inversely with lung density (r=?.62; P<.05), which was significantly elevated in FASSc [0.35±0.02 vs. 0.23±0.02 g/cc (mean±S.E.M.); P<.005].ConclusionThese results demonstrate that inflammatory cell traffic and lung density can be imaged in vivo in FASSc using PET, and that this approach might be of potential value in understanding, in situ, components of pathogenesis that may have value for prognosis.     

Nuclear imaging of neuroinflammation: a comprehensive review of [11C]PK11195 challengers

Neurodegenerative, inflammatory and neoplastic brain disorders involve neuroinflammatory reactions, and a biomarker of neuroinflammation would be useful for diagnostic, drug development and therapy control of these frequent diseases. In vivo imaging can document the expression of the peripheral benzodiazepine receptor (PBR)/translocator protein 18 kDa (TSPO) that is linked to microglial activation and considered a hallmark of neuroinflammation. The prototype positron emission tomography tracer for PBR, [¹¹C]PK11195, has shown limitations that until now have slowed the clinical applications of PBR imaging. In recent years, dozens of new PET and SPECT radioligands for the PBR have been radiolabelled, and several have been evaluated in imaging protocols. Here we review the new PBR ligands proposed as challengers of [¹¹C]PK11195, critically analyze preclinical imaging studies and discuss their potential as neuroinflammation imaging agents.     

Evaluation of CLINDE as potent translocator protein (18 kDa) SPECT radiotracer reflecting the degree of neuroinflammation in a rat model of microglial activation

Background  The translocator protein (TSPO; 18 kDa), the new name of the peripheral-type benzodiazepine receptor, is localised in mitochondria of glial cells and expressed in very low concentrations in normal brain. Their expression rises after microglial activation following brain injury. Accordingly, TSPO are potential targets to evaluate neuroinflammatory changes in a variety of CNS disorders. Purpose  To date, only a few effective tools are available to explore TSPO by SPECT. We characterised here 6-chloro-2-(4′iodophenyl)-3-(N,N-diethyl)-imidazo[1,2-a]pyridine-3-acetamide or CLINDE in a rat model with different stages of excitotoxic lesion. Methods  Excitotoxicity was induced in male Wistar rats by unilateral intrastriatal injection of different amounts of quinolinic acid (75, 150 or 300 nmol). Six days later, two groups of rats (n = 5–6/group) were i.v. injected with [¹²⁵I]-CLINDE (0.4 MBq); one group being pre-injected with PK11195 (5 mg/kg). Brains were removed 30 min after tracer injection and the radioactivity of cerebral areas measured. Complementary ex vivo autoradiography, in vitro autoradiography ([³H]-PK11195) and immunohistochemical studies (OX-42) were performed on brain sections. Results  In the control group, [¹²⁵I]-CLINDE binding was significantly higher (p < 0.001) in lesioned than that in intact side. This binding disappeared in rats pre-treated with PK11195 (p < 0.001), showing specific binding of CLINDE to TSPO. Ex vivo and in vitro autoradiographic studies and immunohistochemistry were consistent with this, revealing a spatial correspondence between radioactivity signal and activated microglia. Regression analysis yielded a positive relation between the ligand binding and the degree of neuroinflammation. Conclusion  These results demonstrate that CLINDE is suitable for TSPO in vivo SPECT imaging to explore their involvement in neurodegenerative disorders associated with microglial activation. An erratum to this article can be found at     

Detection of secondary thalamic degeneration after cortical infarction using cis-4-18F-fluoro-D-proline.

The amino acid cis-4-(18)F-fluoro-D-proline (D-cis-(18)F-FPro) exhibits preferential uptake in the brain compared with its L-isomer, but the clinical potential of the tracer is as yet unknown. In this study we explored the cerebral uptake of D-cis-(18)F-FPro in rats with focal cortical infarctions. METHODS: Focal cortical infarctions were induced in different areas of the cortex of 20 Fisher CDF rats by photothrombosis (PT). At variable time points after PT (1 d to 4 wk), the rats were injected intravenously with D-cis-(18)F-FPro. For comparison, 12 rats were injected simultaneously with (3)H-deoxyglucose ((3)H-DG), 3 rats were injected with (3)H-methyl-L-methionine ((3)H-MET), and 2 rats were injected with (3)H-PK11195. Within 2 h after injection of the tracers, coronal cryosections of the brains were produced and evaluated by dual-tracer autoradiography. Lesion-to-brain ratios (L/B ratios) were calculated by dividing the maximal uptake in areas with increased tracer uptake by the mean uptake in normal brain tissue. Histologic slices were stained by toluidine blue and by immunostainings for glial fibrillary acidic protein (GFAP), CD68 for macrophages, and CD11b for microglia. RESULTS: Prominent uptake of D-cis-(18)F-FPro was found in ipsilateral thalamic nuclei (TN) and partially in the corpus striatum starting at 3 d after infarction with increasing L/B ratios up to 4 wk (mean L/B ratio +/- SD, 6.7 +/- 3.5). The involved TN varied with the site of the cortical lesion corresponding to their thalamocortical projections connecting them with their specific target region in the cerebral cortex. The TN were positive for CD11b and GFAP from day 7 onward, whereas uptake of (3)H-DG, (3)H-MET, and (3)H-PK11195 and immunostaining for CD68 were similar to that of normal brain. Furthermore, increased uptake of D-cis-(18)F-FPro was found in the area of the cortical infarctions (mean L/B ratio +/- SD, 12.1 +/- 8.1). From day 5 onward, the pattern of uptake was congruent with that of immunostaining for CD11b and CD68 but was different from that of GFAP. CONCLUSION: D-cis-(18)F-FPro appears to be a sensitive PET tracer for detection of secondary degeneration of TN after cortical injury. The uptake mechanisms of D-cis-(18)F-FPro remain to be elucidated, but the relationship to microglial activation suggests a diagnostic potential in various brain diseases.     

In vivo changes in microglial activation and amyloid deposits in brain regions with hypometabolism in Alzheimer��s disease

Purpose

Amyloid ?? protein (A??) is known as a pathological substance in Alzheimer??s disease (AD) and is assumed to coexist with a degree of activated microglia in the brain. However, it remains unclear whether these two events occur in parallel with characteristic hypometabolism in AD in vivo. The purpose of the present study was to clarify the in vivo relationship between A?? accumulation and neuroinflammation in those specific brain regions in early AD.

Methods

Eleven nootropic drug-na?ve AD patients underwent a series of positron emission tomography (PET) measurements with [¹¹C](R)PK11195, [¹¹C]PIB and [¹⁸F]FDG and a battery of cognitive tests within the same day. The binding potentials (BPs) of [¹¹C](R)PK11195 were directly compared with those of [¹¹C]PIB in the brain regions with reduced glucose metabolism.

Results

BPs of [¹¹C](R)PK11195 and [¹¹C]PIB were significantly higher in the parietotemporal regions of AD patients than in ten healthy controls. In AD patients, there was a negative correlation between dementia score and [¹¹C](R)PK11195 BPs, but not [¹¹C]PIB, in the limbic, precuneus and prefrontal regions. Direct comparisons showed a significant negative correlation between [¹¹C](R)PK11195 and [¹¹C]PIB BPs in the posterior cingulate cortex (PCC) (p?<?0.05, corrected) that manifested the most severe reduction in [¹⁸F]FDG uptake.

Conclusion

A lack of coupling between microglial activation and amyloid deposits may indicate that A?? accumulation shown by [¹¹C]PIB is not always the primary cause of microglial activation, but rather the negative correlation present in the PCC suggests that microglia can show higher activation during the production of A?? in early AD.     

Visualising neuroinflammation in post-stroke patients: a comparative PET study with the TSPO molecular imaging biomarkers [11C]PK11195 and [11C]vinpocetine

With the main objective of comparing the prospective diagnostic power of two 11C-labelled molecular imaging biomarkers with affinity for TSPO and used for the visualisation of activated microglia after a stroke, we measured with positron emission tomography (PET) in four post-stroke patients the regional brain uptake and binding potential of [11C]vinpocetine and [11C]PK11195. Percentage standard uptake values (%SUV) and binding potential (BPND) were used as outcome measures. The total peak brain uptake value and average global brain uptake value were higher for [11C]vinpocetine than for [11C]PK11195. The regional %SUV values were significantly higher for [11C]vinpocetine than for [11C]PK11195 in the hemispheres as well as in almost all standard brain regions. The %SUV values of [11C]vinpocetine were higher in the peri-infarct zone than in the ischaemic core, however, the difference did not prove to be significant. There was basically no difference in %SUV values between the ischaemic core and the peri-infarct zone for [11C]PK11195. The BPND values for [11C]vinpocetine were higher in all standard regions than those for [11C]PK11195, but the difference was not significant between them. The BPND values of [11C]vinpocetine were higher in the peri-infarct zone than in the ischaemic core, however, the difference did not prove to be significant. A comparative analysis of the two ligands indicates that [11C]vinpocetine shows a number of favourable characteristics over [11C]PK11195, but to demonstrate that it may serve as a prospective molecular imaging biomarker of microglia activation in post-stroke patients, further studies are required.     

In vivo imaging of neuroinflammation: a comparative study between [18F]PBR111, [11C]CLINME and [11C]PK11195 in an acute rodent model

Purpose

The key role of neuroinflammation in acute and chronic neurological disorders has stimulated the search for specific radiotracers targeting the peripheral benzodiazepine receptor (PBR)/18 kDa translocator protein (TSPO), a hallmark of neuroinflammation. Here we evaluate the new radiotracer for positron emission tomography (PET) [¹⁸F]PBR111 in a rodent model of acute inflammation and compare it with [¹¹C]CLINME, an ¹¹C-labelled tracer of the same chemical family, and with the isoquinolinic carboxamide [¹¹C]PK11195.

Methods

We studied radiometabolites by HPLC, in vitro binding by autoradiography and in vivo brain kinetics as well as in vivo specificity of binding using PET imaging.

Results

We show that this radiotracer has a high in vitro specificity for PBR/TSPO versus central benzodiazepine receptors, as reflected by the drastic reduction of its binding to target tissue by addition of PK11195 or PBR111, while addition of flumazenil does not affect binding. Only intact [¹⁸F]PBR111 is detected in brain up to 60 min after i.v. injection, and PET imaging shows an increased uptake in the lesion as compared to the contralateral side as early as 6 min after injection. Administration of an excess of PK11195 and PBR111, 20 min after [¹⁸F]PBR111 administration, induces a rapid and complete displacement of [¹⁸F]PBR111 binding from the lesion. Modelling of the PET data using the simplified reference tissue model showed increased binding potential (BP) in comparison to [¹¹C]PK11195.

Conclusion

[¹⁸F]PBR111 is a metabolically stable tracer with a high specific in vitro and in vivo binding to TSPO. In addition, considering the longer half-life of ¹⁸F over ¹¹C, these results support [¹⁸F]PBR111 as a promising PET tracer of the PBR/TSPO for neuroinflammation imaging.     

Neonatal periventricular leukomalacia: real-time sonographic diagnosis with CT correlation

The utility of real-time sonography in the diagnosis of neonatal periventricular leukomalacia (PVL) has been described only recently. Six cases are reported of PVL diagnosed by serial real-time scanning. The sonographic findings were correlated with the computed tomographic findings and the clinical history. In five of six infants in whom scanning was performed, characteristic multiseptated periventricular cavitations developed 2-3 weeks after birth or later. A transition from normal to increased periventricular echogenicity was often observed before the development of the periventricular cavitations in nonhemorrhagic PVL. The parenchymal abnormality demonstrated by sonography correlated well with an abnormal neurologic outcome. It is suggested that serial real-time scanning be performed in neonates whose history suggests the possibility of hypoxic-ischemic brain injury. Nonspecific predictors of PVL include seizures, apnea, disturbed mental status, abnormal muscle tone, and leg weakness.     

Optimizing an online SPE-HPLC method for analysis of (R)-[11C]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide [(R)-[11C]PK11195] and its metabolites in humans

(R)-[11C]PK11195 is used as a positron emission tomography tracer for activated microglia in several neurological disorders. Quantification of specific binding requires a metabolite-corrected plasma input function. In this study, a high-performance liquid chromatography (HPLC) procedure with online solid phase extraction was modified for analyzing (R)-[11C]PK11195 plasma samples, yielding total sample recoveries of more than 98%. When applied to human studies, the use of two HPLC systems enabled analysis of up to seven plasma samples under regular conditions. Online radioactivity detection was compared with offline sample measurements of HPLC profiles. Offline measurements provided the most reliable results especially for late plasma samples. In 10 patients, an average decrease of parent compound from 94.6% at 2.5 min to 45.2% at 1 h after administration was observed.     

Synthesis and evaluation of11C-PK 11195 forin vivo study of peripheral-type benzodiazepine receptors using position emission tomography

The biodistribution of 3H-PK 11195, an antagonist of the peripheral-type benzodiazepine receptors, was studied in mice. High accumulations of radioactivity in the heart, lung, spleen, kidney and adrenal were observed after intravenous injection of tracer amounts of 3H-PK 11195 into the mice. The radioactivity in the heart, lung, spleen, kidney and adrenal was significantly decreased by the coadministration of carrier PK 11195, which indicated that PK 11195 specifically binds to the receptors. No radioactive metabolites were observed in the heart, lung and brain 20 min after intravenous administration of 3H-PK 11195. The accumulation of 3H-PK 11195 in the lung was not affected by pretreatment with either alpha-methyl benzylamine or imipramine, suggesting that 3H-PK 11195 specifically binds to the receptors. The ratios of radioactivity of the kidney, adrenal and spleen to blood increased as a function of time, whereas that of the lung and heart rapidly reached to a steady state. 11C-PK 11195 was synthesized by the N-methylation of desmethyl precursor yielding more than 100 mCi with high specific activity (more than 1.4 Ci/mumol). The labeling and purification procedure was completed within 23 min after the end of bombardment (EOB). The 11C-PK 11195 solution for injection seems to have a high potential for the in vivo study of the peripheral-type benzodiazepine receptors in the living human by means of positron emission tomography (PET).     

Preclinical in vitro & in vivo evaluation of [11C]SNAP-7941 – the first PET tracer for the melanin concentrating hormone receptor 1

IntroductionDue to its involvement in a variety of pathologies (obesity, diabetes, gut inflammation and depression), the melanin concentrating hormone receptor 1 (MCHR1) is a new target for the treatment of these lifestyle diseases. We previously presented the radiosynthesis of [¹¹C]SNAP-7941, the first potential PET tracer for the MCHR1.MethodsWe herein present its in vitro and in vivo evaluation, including binding affinity, plasma stability, stability against liver mircrosomes and carboxylesterase, lipohilicity, biodistribution, in vivo metabolism and small-animal PET.Results[¹¹C]SNAP-7941 evinced high stability against liver microsomes, carboxylesterase and in human plasma. The first small-animal PET experiments revealed a 5 fold increased brain uptake after Pgp/BCRP inhibition. Therefore, it can be assumed that [¹¹C]SNAP-7941 is a Pgp/BCRP substrate. No metabolites were found in brain.ConclusionOn the basis of these experiments with healthy rats, the suitability of [¹¹C]SNAP-7941 for the visualisation of central and peripheral MCHR1 remains speculative.     

Application of carbon-11 labelled nicotine in the measurement of human cerebral blood flow and other physiological parameters

Using positron emission tomography (PET), we measured the regional cerebral blood flow (rCBF) in five normal human subjects after intravenous injection of carbon-11 labelled (R)nicotine. The rCBF of the same subjects was measured by PET using the C¹⁵O₂ inhalation steady-state method. The distribution of¹¹C activity in the brain after injection of¹¹C-(R)nicotine was almost equivalent to the CBF image obtained with the C¹⁵O₂ inhalation stead-state method. The kinetics of¹¹C-(R)nicotine in the brain was analysed using a two-compartment model consisting of vascular and brain tissue compartments. The rCBF values obtained with¹¹C-(R)nicotine were higher than with C¹⁵O₂ gas. The relatively long fixed distribution of¹¹C-(R)nicotine with a short uptake period allows stimulation studies by measurement of CBF to be performed with better photon flux and a longer imaging time than are possible with H₂ ¹⁵O.     

Estimation of the 18F-FDG input function in mice by use of dynamic small-animal PET and minimal blood sample data.

Derivation of the plasma time-activity curve in murine small-animal PET studies is a challenging task when tracers that are sequestered by the myocardium are used, because plasma time-activity curve estimation usually involves drawing a region of interest within the area of the reconstructed image that corresponds to the left ventricle (LV) of the heart. The small size of the LV relative to the resolution of the small-animal PET system, coupled with spillover effects from adjacent myocardial pixels, makes this method reliable only for the earliest frames of the scan. We sought to develop a method for plasma time-activity curve estimation based on a model of tracer kinetics in blood, muscle, and liver. METHODS: Sixteen C57BL/6 mice were injected with (18)F-FDG, and approximately 15 serial blood samples were taken from the femoral artery via a surgically inserted catheter during 60-min small-animal PET scans. Image data were reconstructed by use of filtered backprojection with CT-based attenuation correction. We constructed a 5-compartment model designed to predict the plasma time-activity curve of (18)F-FDG by use of data from a minimum of 2 blood samples and the dynamic small-animal PET scan. The plasma time-activity curve (TACp) was assumed to have 4 exponential components (TAC(P)=A(1)e(lambda(1)t)+A(2)e(lambda(2)t)+A(3)e(lambda(3)t)-(A(1)+A(2)+A(3))e(lambda(4)t)) based on the serial blood samples. Using Bayesian constraints, we fitted 2-compartment submodels of muscle and liver to small-animal PET data for these organs and simultaneously fitted the input (forcing) function to early small-animal PET LV data and 2 blood samples (approximately 10 min and approximately 1 h). RESULTS: The area under the estimated plasma time-activity curve had an overall Spearman correlation of 0.99 when compared with the area under the gold standard plasma time-activity curve calculated from multiple blood samples. Calculated organ uptake rates (Patlak K(i)) based on the predicted plasma time-activity curve had a correlation of approximately 0.99 for liver, muscle, myocardium, and brain when compared with those based on the gold standard plasma time-activity curve. The model was also able to accurately predict the plasma time-activity curve under experimental conditions that resulted in different rates of clearance of the tracer from blood. CONCLUSION: We have developed a robust method for accurately estimating the plasma time-activity curve of (18)F-FDG by use of dynamic small-animal PET data and 2 blood samples.