Development of a new toolbox for mouse PET-CT brain image analysis fully based on CT images and validation in a PD mouse model.

Automatic analysis toolboxes are popular in brain image analysis, both in clinical and in preclinical practices. In this regard, we proposed a new toolbox for mouse PET-CT brain image analysis including a new Statistical Parametric Mapping-based template and a pipeline for image registration of PET-CT images based on CT images. The new templates is compatible with the common coordinate framework (CCFv3) of the Allen Reference Atlas (ARA) while the CT based registration step allows to facilitate the analysis of mouse PET-CT brain images. From the ARA template, we identified 27 volumes of interest that are relevant for in vivo imaging studies and provided binary atlas to describe them. We acquired 20 C57BL/6 mice with [18F]FDG PET-CT, and 12 of them underwent 3D T2-weighted high-resolution MR scans. All images were elastically registered to the ARA atlas and then averaged. High-resolution MR images were used to validate a CT-based registration pipeline. The resulting method was applied to a mouse model of Parkinson's disease subjected to a test-retest study (n = 6) with the TSPO-specific radioligand [18F]VC701. The identification of regions of microglia/macrophage activation was performed in comparison to the Ma and Mirrione template. The new toolbox identified 11 (6 after false discovery rate adjustment, FDR) brain sub-areas of significant [18F]VC701 uptake increase versus the 4 (3 after FDR) macro-regions identified by the Ma and Mirrione template. Moreover, these 11 areas are functionally connected as found by applying the Mouse Connectivity tool of ARA. In conclusion, we developed a mouse brain atlas tool optimized for PET-CT imaging analysis that does not require MR. This tool conforms to the CCFv3 of ARA and could be applied to the analysis of mouse brain disease models.

Identification of imaging biomarkers for the assessment of tumour response to different treatments in a preclinical glioma model.

PURPOSE: Hypoxia-inducible factor 1α (HIF-1α) activity is one of the major players in hypoxia-mediated glioma progression and resistance to therapies, and therefore the focus of this study was the evaluation of HIF-1α modulation in relation to tumour response with the purpose of identifying imaging biomarkers able to document tumour response to treatment in a murine glioma model. METHODS: U251-HRE-mCherry cells expressing Luciferase under the control of a hypoxia responsive element (HRE) and mCherry under the control of a constitutive promoter were used to assess HIF-1α activity and cell survival after treatment, both in vitro and in vivo, by optical, MRI and positron emission tomography imaging. RESULTS: This cell model can be used to monitor HIF-1α activity after treatment with different drugs modulating transduction pathways involved in its regulation. After temozolomide (TMZ) treatment, HIF-1α activity is early reduced, preceding cell cytotoxicity. Optical imaging allowed monitoring of this process in vivo, and carbonic anhydrase IX (CAIX) expression was identified as a translatable non-invasive biomarker with potential clinical significance. A preliminary in vitro evaluation showed that reduction of HIF-1α activity after TMZ treatment was comparable to the effect of an Hsp90 inhibitor, opening the way for further elucidation of its mechanism of action. CONCLUSION: The results of this study suggest that the U251-HRE-mCherry cell model can be used for the monitoring of HIF-1α activity through luciferase and CAIX expression. These cells can become a useful tool for the assessment and improvement of new targeted tracers for potential theranostic procedures.

Genetic and morphological diversity of Moenkhausia oligolepis (Characiformes: Characidae) populations in the tributaries of the Araguaia River, Brazil: implications for taxonomy and conservation.

Molecular genetic assessments that consider ecological information, in addition to endogamy levels, genetic diversity, and the genetic differentiation among species and populations, are particularly important for the conservation of biological diversity. Prime candidates for conservation genetic review are those subject to human use, including harvests for the ornamental fish trade. Colorful South American tetra, such as Moenkhausia oligolepis and M. forestii, are good examples of fish species that are widely collected and exported worldwide. This study aimed to evaluate the population-specific characteristics of M. oligolepis and M. forestii by comparing morphometric and molecular analyses based on ISSR markers, to provide information that would facilitate the sustainable management of these 2 species. Seventy-two specimens were collected from the Araguaia-Tocantins and Paraguay River Basins in Brazil. All specimens were measured and analyzed using ISSR markers. Population-exclusive bands were found among the 86 detected bands, while morphometric clusters reflected the geographical distribution of individuals. Correlated genetic and morphological variation supported the presence of 3 distinct groups from tributaries of the Araguaia and Mortes Rivers. Using the same techniques, all M. oligolepis populations were isolated from M. forestii. This study on Moenkhausia presents an interesting example that could be used to construct a framework of South American ichthyodiversity, and reinforces the necessity of habitat conservation to prevent the loss of biological diversity.

Validation of an engineered cell model for in vitro and in vivo HIF-1α evaluation by different imaging modalities.

PURPOSE: The aim of this study was to characterize a cell-based model for the molecular study of hypoxia-inducible factor (HIF)-1α activity, in the context of hypoxia, by means of different imaging techniques. PROCEDURES: Engineered U251-HRE glioma cells were used to analyze the molecular mechanisms underlying HIF-1α activity in vitro in relation to luciferase expression. The same cells were orthotopically implanted in mice to evaluate tumor progression and hypoxia induction by bioluminescence imaging, fluorescence imaging, positron emission tomography (PET), and magnetic resonance imaging (MRI). RESULTS: In vitro analyses highlighted the relationship between HIF-1α and luciferase activity in hypoxic conditions and after pharmacological treatments in U251-HRE cells. Through in vivo studies, it was possible to assess hypoxia establishment in relation to tumor growth by optical imaging, PET and MRI. CONCLUSIONS: The findings of this study indicate that the U251-HRE orthotopic murine model can be used to reliably evaluate processes modulating HIF-1α activity, using both molecular and preclinical non-invasive imaging techniques.

Therapeutic antibody targeting of Notch1 in T-acute lymphoblastic leukemia xenografts.

T-acute lymphoblastic leukemia (T-ALL) is characterized by several genetic alterations and poor prognosis in about 20-25% of patients. Notably, about 60% of T-ALL shows increased Notch1 activity, due to activating NOTCH1 mutations or alterations in the FBW7 gene, which confer to the cell a strong growth advantage. Therapeutic targeting of Notch signaling could be clinically relevant, especially for chemotherapy refractory patients. This study investigated the therapeutic efficacy of a novel anti-Notch1 monoclonal antibody by taking advantage of a collection of pediatric T-ALL engrafted systemically in NOD/SCID mice and genetically characterized with respect to NOTCH1/FBW7 mutations. Anti-Notch1 treatment greatly delayed engraftment of T-ALL cells bearing Notch1 mutations, including samples derived from poor responders or relapsed patients. Notably, the therapeutic efficacy of anti-Notch1 therapy was significantly enhanced in combination with dexamethasone. Anti-Notch1 treatment increased T-ALL cell apoptosis, decreased proliferation and caused strong inhibitory effects on Notch-target genes expression along with complex modulations of gene expression profiles involving cell metabolism. Serial transplantation experiments suggested that anti-Notch1 therapy could compromise leukemia-initiating cell functions. These results show therapeutic efficacy of Notch1 blockade for T-ALL, highlight the potential of combination with dexamethasone and identify surrogate biomarkers of the therapeutic response.

In vivo microglia activation in very early dementia with Lewy bodies, comparison with Parkinson's disease.

BACKGROUND: Reactive microgliosis, hallmark of neuroinflammation, may contribute to neuronal degeneration, as shown in several neurodegenerative diseases. We in vivo evaluated microglia activation in early dementia with Lewy bodies, still not reported, and compared with early Parkinson's disease, to assess possible differential pathological patterns. METHODS: We measured the [(11)C]-PK11195 binding potentials with Positron Emission Tomography, using a simplified reference tissue model, as marker of microglia activation, and cerebral spinal fluid protein carbonylation levels, as marker of oxidative stress. Six dementia with Lewy bodies and 6 Parkinson's disease patients within a year from the onset, and eleven healthy controls were included. Clinical diagnosis was confirmed at a 4-year follow-up. RESULTS: In dementia with Lewy bodies as well as in Parkinson's disease, we found significant (p < 0.001) [(11)C]-PK11195 binding potential increases in the substantia nigra and putamen. Patients with Lewy bodies dementia had extensive additional microglia activation in several associative cortices. This was evident also at a single subject level. Significant increase of Cerebral Spinal Fluid protein carbonylation was shown in both patients' groups. CONCLUSIONS: [(11)C]-PK11195 Positron Emission Tomography imaging revealed neuroinflammation in dementia with Lewy bodies and Parkinson's disease, mirroring, even at a single subject level, the common and the different topographical distribution of neuropathological changes, yet in the earliest stages of the disease process. Focusing on those events that characterize parkinsonisms and Parkinson's disease may be the key to further advancing the understanding of pathogenesis and to taking these mechanisms forward as a means of defining targets for neuroprotection.

From cancer metabolism to new biomarkers and drug targets.

Great interest is presently given to the analysis of metabolic changes that take place specifically in cancer cells. In this review we summarize the alterations in glycolysis, glutamine utilization, fatty acid synthesis and mitochondrial function that have been reported to occur in cancer cells and in human tumors. We then propose considering cancer as a system-level disease and argue how two hallmarks of cancer, enhanced cell proliferation and evasion from apoptosis, may be evaluated as system-level properties, and how this perspective is going to modify drug discovery. Given the relevance of the analysis of metabolism both for studies on the molecular basis of cancer cell phenotype and for clinical applications, the more relevant technologies for this purpose, from metabolome and metabolic flux analysis in cells by Nuclear Magnetic Resonance and Mass Spectrometry technologies to positron emission tomography on patients, are analyzed. The perspectives offered by specific changes in metabolism for a new drug discovery strategy for cancer are discussed and a survey of the industrial activity already going on in the field is reported.

Quinolinic acid induced neurodegeneration in the striatum: a combined in vivo and in vitro analysis of receptor changes and microglia activation.

PURPOSE: Huntington's disease (HD) is a progressive neurodegenerative disorder, which is characterised by prominent neuronal cell loss in the basal ganglia with motor and cognitive disturbances. One of the most well-studied pharmacological models of HD is produced by local injection in the rat brain striatum of the excitotoxin quinolinic acid (QA), which produces many of the distinctive features of this human neurodegenerative disorder. Here, we report a detailed analysis, obtained both in vivo and in vitro of this pharmacological model of HD. MATERIALS AND METHODS: By combining emission tomography (PET) with autoradiographic and immunocytochemical confocal laser techniques, we quantified in the QA-injected striatum the temporal behavior (from 1 to 60 days from the excitotoxic insult) of neuronal cell density and receptor availability (adenosine A(2A) and dopamine D(2) receptors) together with the degree of microglia activation. RESULTS: Both approaches showed a loss of adenosine A(2A) and dopamine D(2) receptors paralleled by an increase of microglial activation. CONCLUSION: This combined longitudinal analysis of the disease progression, which suggested an impairment of neurotransmission, neuronal integrity and a reversible activation of brain inflammatory processes, might represent a more quantitative approach to compare the differential effects of treatments in slowing down or reversing HD in rodent models with potential applications to human patients.

SPET in psychopharmacology.

Positron emission tomography (PET) and single photon emission tomography (SPET) permit the in vivo assessment of human biological processes with methods originally developed for preclinical studies. This opportunity is of primary interest in biological psychiatry and in psychoactive drugs development. Most of research efforts have focused on the use of PET field, and several biological finding on central nervous system (CNS) diseases have been published. Despite methodological limitations, the feasibility and usefulness of SPET for research, including full quantification, kinetic modeling and tracer validation, has been clearly demonstrated. The aim of this paper is to review advantages, limitations and results of SPET in psychopharmacology.

In vivo imaging of adenosine A2A receptors in rat and primate brain using [11C]SCH442416.

PURPOSE: The aim of this study was to evaluate the suitability of [11C]SCH442416 for the in vivo imaging of adenosine A2A receptors. METHODS: In rats and Macaca nemestrina, we evaluated the time course of the cerebral distribution of [11C]SCH442416. Furthermore, in rats we investigated the rate of metabolic degradation, the inhibitory effects of different drugs acting on adenosine or dopamine receptors and the modification induced by the intrastriatal administration of quinolinic acid (QA). RESULTS: The rate of metabolic degradation of [11C]SCH442416 in rats was slow; 60 min after tracer injection, more than 40% of total plasma activity was due to unmetabolised [11C]SCH442416. At the time of maximum uptake, radioactive metabolites represented only 6% of total extractable activity in the cerebellum and less than 1% in the striatum. In the striatum, the region with the highest expression of A2A receptors, the in vivo uptake of [11C]SCH442416 was significantly reduced only by drugs acting on A2A receptors or by QA, a neurotoxin that selectively reduces the number of intrastriatal GABAergic neurons. Position emission tomography (PET) studies in monkeys indicated that the tracer rapidly accumulates in brain, reaching maximum uptake between 5 and 10 min. Twenty minutes after the injection, radioactivity concentration in the striatum was two times that in the cerebellum. CONCLUSION: The specificity of binding, the rank order of regional distribution in the brain of rats and M. nemestrina, the good signal to noise ratios and the low amount of radioactive metabolites in brain and periphery indicate that [11C]SCH442416 is a promising tracer for the in vivo imaging of A2A adenosine receptors using PET.

Evaluation of three quinoline-carboxamide derivatives as potential radioligands for the in vivo pet imaging of neurodegeneration.

The peripheral-type benzodiazepine receptors (PBRs) are only minimally expressed in normal brain parenchyma, where they are primarily localized in glial cells. Their basal expression rises in different neurodegenerative disorders, due to the presence of infiltrating inflammatory cells and activated microglia. [11C]PK11195, a selective PBR antagonist, has been used for the in vivo PET monitoring of neurodegeneration in clinical observations. We recently developed and labeled with carbon-11 three new carboxamide derivatives: [11C]VC193M, [11C]VC195 and [11C]VC198M. Aim of this study was to evaluate these ligands for the in vivo measuring of PBRs expression in neurodegenerations and compare their kinetic behavior with that of the reference tracer [11C]PK11195. Radioligands were evaluated in a preclinical model of Huntington's disease consisting in the monolateral striatal injection of quinolinic acid (QA). Activated microglia and astrocytic gliosis was present only within the affected striatum. A concomitant increase in radioactivity accumulation was observed for all the tracers examined (P<0.01). Among the new compounds, [11C]VC195 showed higher levels of lesioned/unlesioned striatum ratios (3.28+/-0.44), in comparison with [11C]VC193M and [11C]VC198M (2.69+/-0.53 and 1.52+/-0.36, respectively), but slightly inferior to that observed for [11C]PK11195 (3.76+/-1.41).In conclusion, the results of the study indicate that [11C]VC195 is a promising candidate for in vivo PET monitoring of neurodegenerative processes but its in vivo behavior overlap that of [11C]PK11195.

Ethanol inhibits the binding of substance P to rat brain cortex NK1 receptors.

The binding of 125I-labeled substance P (SP) to rat brain cortex membranes has been studied under control conditions and in the presence of ethanol. The binding of SP at low concentrations (20-1000 pM) gave two components, one with a KD value of 80 pM and another one with a KD of 500 pM. The higher-affinity component is due to NK1 receptors, as confirmed by the inhibition of the SP binding by the rodent NK1 specific agonist [Sar9 Met(O2)11]SP. Ethanol (1.7 mM) added to the binding assays inhibited by more than 50% the specific binding at a very low SP concentration (20 pM); however, it had no effect at SP concentrations ranging from 50 to 120 pM. This suggests a decrease by ethanol of the affinity of SP to the NK1 receptors involved in this binding component. The ethanol effect disappeared at [EtOH] < or = 0.17 mM.

The feasibility of statistical parametric mapping for the analysis of positron emission tomography studies using 11C-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)-tropane in patients with movement disorders.

Movement disorders, including Parkinson's disease and parkinsonian syndromes, e.g. progressive supranuclear palsy, multiple system atrophy, and Lewy body dementia, may be difficult to differentiate among each other at an early stage, since they may share similar clinical features and response to dopaminergic drugs. As new tracers for imaging the dopamine transporters become available, the use of positron emission tomography (PET) for the differential diagnosis of movement disorders is gaining clinical relevance. Visual interpretation is generally used for PET image analysis. However, the use of some form of less subjective analysis is desirable in order to detect subtle changes that may be difficult to identify by visual interpretation and to achieve an operator independent analysis. To this end this study was aimed at assessing the feasibility of using statistical parametric mapping (SPM) for the clinical evaluation of single PET scans performed with 2-beta-carbomethoxy-3-beta-(4-fluorophenyl)-tropane ( C-beta-CIT-FE). Eleven healthy volunteers and five patients with movement disorders (Parkinson's disease, essential tremor, PSP and Lewy body dementia) were included in this study. Each subject underwent a PET study after i.v. injection of C-beta-CIT-FE. The PET images of C-beta-CIT-FE distribution acquired between 60 and 90 min were spatially fitted into the Talairach and Tournoux space. A template of normal C-beta-CIT-FE distribution was derived from studies in the 11 normal control subjects. Different patterns of reduction of the uptake of the tracer were detected in the basal ganglia of the five patients, in relation to each pathological condition. The patterns of distribution were all consistent with the severity and type of disease. The results of this study demonstrate the feasibility of differentiating among different states of dopaminergic impairment, due to Parkinson's disease and parkinsonian syndromes, by using PET scans with C-beta-CIT-FE and by using the SPM procedure for analysis of the data.

New perspectives on neurochemical effects of amantadine in the brain of parkinsonian patients: a PET - [(11)C]raclopride study.

Amantadine, is a non competitive NMDA receptors antagonist that has been proved beneficial in Parkinson's disease. However its mechanism of action at therapeutic doses is still under discussion. Aim of this study was to evaluate the effect of repeated administration of amantadine on striatal dopaminergic system by measuring [(11)C]raclopride binding to striatal D(2) dopamine receptors, in patients with moderate idiopathic Parkinson's disease. Eight patients completed the study undergoing a PET scan, before and after 10-14 days treatment with Amantadine (200 mg/day). Patients were on treatment with L-DOPA, which was suspended 1 night before each PET scans, and free from dopaminergic agonists, anticholinergic and antidepressants. Amantadine treatment significantly increased [(11)C-]Raclopride binding (caudate: 10% p = 0.04; putamen 11% p = 0.01). A slight reduction (-7.3%, p = 0.062) of UPDRS total scores was also observed. The increased availability of striatal D(2) receptors, is likely to be caused by drug induced modification of receptors expression. This hypothesis is consistent with previous experiments, indicating an increase in striatal D(2) receptors in rats treated with amantadine or other non competitive NMDA antagonists and suggests that the neo-synthesis of D(2) receptors may represent a reinforcing mechanism of drug efficacy.

Striatal dopaminergic denervation in early and late onset Parkinson's disease assessed by PET and the tracer [11C]FECIT: preliminary findings in one patient with autosomal recessive parkinsonism (Park2).

Neuroimaging studies of striatal dopamine transporters (DAT) have shown that this measurement is a specific marker of dopaminergic degeneration in patients with Parkinson's disease. However, little data is available in subjects with early disease onset, particularly in those with autosomal recessive parkinsonism. We measured striatal DAT binding in 10 patients with early onset PD (onset <40 years) and in 10 with late onset PD (onset >50 years) using PET and the tracer [(11)C]FECIT. One early onset subject presented a mutation in the parkin gene consistent with autosomal recessive parkinsonism. Data were compared with those of 15 control subjects. We found a comparable decrement of striatal DAT binding in early and late onset PD. Loss was widespread and bilateral in the patient carrying the Park2 mutation, suggesting a different pattern of denervation in these individuals.

A PET study with [11-C]raclopride in Parkinson's disease: preliminary results on the effect of amantadine on the dopaminergic system.

Amantadine has been proved to be beneficial in Parkinson's disease. Although it is still uncertain which neurochemical events are modified at therapeutic doses, an increase in dopaminergic tone secondary to NMDA receptor blockade and a direct inhibition of the glutamatergic overactivity have been suggested to be involved in its clinical effects. The aim of this study was to evaluate the effects of amantadine on the dopaminergic system by measuring the in vivo binding of [11-C]raclopride to D2 dopamine receptors in the basal ganglia of 6 patients with idiopathic Parkinson's disease. Each patient underwent a PET study, before and after 14 days of treatment with amantadine (200 mg/day). Repeated treatment with therapeutic doses of amantadine induced a moderate increase in the in vivo binding of [11C]raclopride in the putamen of PD patients. This observation indicates that in PD patients, 200 mg/day amantadine does not produce an increase in extracellular levels of dopamine sufficiently to inhibit raclopride binding or that, if present, is it masked by a concurrent increase in receptor availability, as recently reported in rat striatum.

The status of dopamine nerve terminals in Parkinson's disease and essential tremor: a PET study with the tracer [11-C]FE-CIT.

Neuroimaging studies of the striatal dopamine transporter (DAT) are useful in the assessment of the dopaminergic system in Parkinson's disease (PD). We used positron emisson tomography (PET) and the tracer [11C]FE-CIT to measure DAT binding in the caudate nucleus and putamen of 31 patients with PD, 5 with essential tremor and 8 healthy control subjects. Of the patients with PD, 17 were drug naive, while the others were either on levodopa or dopamine agonist monotherapy. DAT binding was significantly reduced in the caudate nucleus and to a greater extent in the putamen of PD patients compared to both healthy controls and essential tremor individuals. No overlap was observed between putamen values in PD and normals. No differences were found between controls and essential tremor subjects. These data confirm that measurements of DAT binding can provide an accurate and highly sensitive measure of degeneration in the dopamine system in PD.

Acute effect of 3-(4-acetamido)-butyrril-lorazepam (DDS2700) on brain function assessed by PET at rest and during attentive tasks.

The aim of this study was to assess, by positron emission tomography (PET), the effect on cerebral functional activity of a new lorazepam-gamma-aminobutyric acid (GABA) conjugate [3-(4-acetamido)-butyrril lorazepam (DDS2700)]. Ten healthy volunteers were studied by PET and [18F]fluoro-deoxy-D-glucose ([18F]FDG) under baseline conditions and following the administration of DDS2700. Regional cerebral blood flow (rCBF) was measured by PET and 15O-water in three additional participants while they performed attentive tasks, before and after drug administration. DDS2700 induced a decrease in the regional cerebral metabolic rate of glucose (rCMRglu) in the thalamus (-17%), cerebellum (-11%) and caudate nucleus (-8%). The observed effects on glucose metabolism were probably related to the subjective sedation and tiredness reported by the participants. During the attentive tasks, rCBF increased in frontal and temporal regions associated with attentional processing of auditory material. These circuits were no longer active after DDS2700 administration, while rCBF increased in cingulate cortex, occipitoparietal regions, pons and cerebellum. These drug-induced activations might be directly related to intervening sleepiness and to the consequent effort in keeping attention focused on the tasks. The effects of DDS2700 on glucose metabolism at rest, and on rCBF during activation conditions, indicate a drug action on cerebral networks involved in alertness, vigilance and attention maintenance. PET assessment by [18F]FDG and water may provide complementary information in pharmacodynamic studies.

Labeling and evaluation of N-[11C]methylated quinoline-2-carboxamides as potential radioligands for visualization of peripheral benzodiazepine receptors.

The novel quinoline-2-carboxamide derivatives N-[methyl-11C]-3-methyl-4-phenyl-N-(phenylmethyl)quinoline-2-carboxamide ([11C]4), (+/-)-N-[methyl-11C]-3-methyl-N-(1-methylpropyl)-4-phenylquinoline-2-carboxamide ([11C]5), and (+/-)-N-[methyl-11C]-3-methyl-4-(2-fluorophenyl)-N-(1-methylpropyl)quinoline-2-carboxamide ([11C]6) were labeled with carbon-11 (t1/2 = 20.4 min, beta+ = 99.8%) as potential radioligands for the noninvasive assessment of peripheral benzodiazepine type receptors (PBR) in vivo with positron emission tomography (PET). The radiosynthesis consisted of N-methylation of the desmethyl precursors 3-methyl-4-phenyl-N-(phenylmethyl)quinoline-2-carboxamide (4a), (+/-)-3-methyl-N-(1-methylpropyl)-4-phenylquinoline-2-carboxamide (5a), and (+/-)-4-(2-fluorophenyl)-3-methyl-N-(1-methylpropyl)quinoline-2-carboxamide (6a) with either [11C]methyl iodide or [11C]methyl triflate in the presence of tetrabutylammonium hydroxide or potassium hydroxide in dimethylformamide. The radioligands [11C]4, [11C]5, and [11C]6 were synthesized with over 99% radiochemical purity in 30 min, 30 +/- 5% radiochemical yield, calculated at the end of synthesis (EOS) non-decay-corrected, and 2.5 +/- 1.2 Ci/micromol of specific radioactivity. Inhibition studies in rats following intravenous pre-administration of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195, 1) showed high specific binding to PBR of [11C]4, [11C]5, and [11C]6 in heart, lung, kidney, adrenal gland, spleen, and brain. The biological data suggest that [11C]5, [11C]6, and particularly [11C]4 are promising radioligands for PBR imaging in vivo with PET.