Error analysis for PET measurement of dopamine D2 receptor occupancy by antipsychotics with [11C]raclopride and [11C]FLB 457

Dopamine D₂ receptor occupancy by antipsychotic drugs has been measured with positron emission tomography (PET) by comparing the binding potential (BP) values before and after drug administration. This occupancy has been found to be related to clinical effects and side effects. In this study, we evaluated the uncertainty of the quantitative analysis for estimating the dopamine D₂ receptor occupancy by antipsychotics in simulation and human studies of [¹¹C]raclopride and for the high affinity ligand [¹¹C]FLB 457. Time–activity curves of [¹¹C]raclopride and [¹¹C]FLB 457 were simulated, and the reliability of BP estimated by a simplified reference tissue model and the calculated occupancy were investigated for various noise levels, BP values, and scan durations. Then, in the human PET study with and without antipsychotics, the uncertainty of BP and occupancy estimates and the scan duration required for a reliable estimation were investigated by a bootstrap approach. Reliable and unbiased estimates of [¹¹C]raclopride BP_ND could be obtained with recording as short as 32 min, with the relative standard deviation (SD) of the striatal occupancy remaining less than 10%. Conversely, in [¹¹C]FLB 457 studies, the mean value increased and SD of the temporal cortex and thalamus exceeded 10% when the scan duration was shorter than 60 min. These results demonstrated that dopamine D₂ receptor occupancy by antipsychotics can be estimated precisely with an optimal scan duration with [¹¹C]raclopride and [¹¹C]FLB 457.     

Strategies for the generation of parametric images of [C]PIB with plasma input functions considering discriminations and reproducibility

Pittsburgh compound B or [¹¹C]PIB is an amyloid imaging agent which shows a clear differentiation between subjects with Alzheimer's disease (AD) and controls. However the observed signal difference in other forms of dementia such as dementia with Lewy bodies (DLB) is smaller, and mild cognitively impaired (MCI) subjects and some healthy elderly normals may show intermediate levels of [¹¹C]PIB binding. The cerebellum, a commonly used reference region for non-specific tracer uptake in [¹¹C]PIB studies in AD may not be valid in Prion disorders or monogenic forms of AD. The aim of this work was to: 1—compare methods for generating parametric maps of [¹¹C]PIB retention in tissue using a plasma input function in respect of their ability to discriminate between AD subjects and controls and 2—estimate the test–retest reproducibility in AD subjects. 12  AD subjects (5 of which underwent a repeat scan within 6 weeks) and 10 control subjects had 90 minute [¹¹C]PIB dynamic PET scans, and arterial plasma input functions were measured. Parametric maps were generated with graphical analysis of reversible binding (Logan plot), irreversible binding (Patlak plot), and spectral analysis. Between group differentiation was calculated using Student's t-test and comparisons between different methods were made using p values. Reproducibility was assessed by intraclass correlation coefficients (ICC). We found that the 75 min value of the impulse response function showed the best group differentiation and had a higher ICC than volume of distribution maps generated from Logan and spectral analysis. Patlak analysis of [¹¹C]PIB binding was the least reproducible.     

PET Mapping of Extrastriatal D2-like Dopamine Receptors in the Human Brain Using an Anatomic Standardization Technique and [C]FLB 457

The Computerized Brain Atlas (CBA) transforms PET images of individual subjects into a standard brain anatomy. We have previously applied this to PET images with [¹¹C]raclopride and confirmed that the D2 dopamine receptors in the striatum can be evaluated accurately with a standard brain anatomy. There is growing evidence that extrastriatal D2 receptors, in spite of their low density, have pathophysiological significance for schizophrenia. We used the CBA to explore the extrastriatal distribution of D2 receptors in 13 healthy subjects using [¹¹C]FLB 457, a substituted benzamide with very high affinity for D2 and D3 receptors. There was good agreement between the specific binding ratios from CBA quantification of standardized images and those from region-of-interest analyses of original images. The highest levels of binding were observed in the putamen and caudate nucleus, followed by the globus pallidus and nucleus accumbens. Besides the basal ganglia, the hypothalamus and nucleus ruber also showed high levels of binding. Intermediate levels were found in the substantia nigra, nucleus subthalami, amygdala, and thalamus. Interestingly, there was very heterogeneous binding among the thalamic nuclei. The anterior and mediodorsal nuclei showed relatively high binding. The cerebral cortices showed lower levels with significant regional differences. Binding was highest in the temporal cortex and hippocampus followed by the anterior cingulate gyrus, and the parietal and frontal cortices, but was lowest in the occipital cortex. The use of CBA for analysis of [¹¹C]FLB 457 binding makes it possible to build a normal database for the extrastriatal D2 receptors in the living human brain. The heterogeneous distribution of D2 receptors provides an attractive opportunity for new research on the pathophysiology and drug treatment of schizophrenia.     

Quantitative evaluation of changes in binding potential with a simplified reference tissue model and multiple injections of [C]raclopride

Positron emission tomography (PET) with [¹¹C]raclopride is widely used to investigate temporal changes in the dopamine D₂ receptor system attributed to the dopamine release. The simplified reference tissue model (SRTM) can be used to determine the binding potential (BP_ND) value using the time–activity curve (TAC) of the reference region as input function. However, in assessing temporal changes in BP_ND using the SRTM, multiple [¹¹C]raclopride PET scans are required, and a second scan must be performed after the disappearance of the [¹¹C]raclopride administered in the first scan. In this study, we have developed an extended multiple-injection SRTM to estimate the BP_ND change, from a single PET scan with multiple injections of [¹¹C]raclopride, and we have validated this approach by performing numerous simulations and studies on monkeys. In the computer simulations, TACs were generated for dual injections of [¹¹C]raclopride, in which binding conditions changed during the scans, and the BP_ND values before, and after, the second injection were estimated by the proposed method. As a result, the reduction in BP_ND was correlated, either with the integral of released dopamine, or with the administered mass of raclopride. This method was applied to studies on monkeys, and was capable of determining two identical BP_ND values when there were no changes in binding conditions. The BP_ND after the second injection decreased when binding conditions changed due to an increase in administered raclopride. An advantage of the proposed method is the shortened scan period for the quantitative assessment of the BP_ND change for neurotransmitter competition studies.     

In vivo quantification of dopamine transporters in mice with unilateral 6-OHDA lesions using [11C]methylphenidate and PET

Quantification of the binding of [¹¹C]methylphenidate to the dopamine transporter (DAT) using positron emission tomography (PET) is often used to evaluate the integrity of dopaminergic neurons in the striatal regions of the brain. Over the past decade, many genetically engineered mouse models of human disease have been developed and have become particularly useful for the study of disease onset and progression over time. Quantitative imaging of small structures such as the mouse brain is especially challenging. Thus, the aims of this study were (1) to evaluate the accuracy of quantifying DAT binding using in vivo PET and (2) to examine the impact of different methodologies.

Methods

Eight mice were scanned with [¹¹C]methylphenidate under true or transient equilibrium conditions using a bolus and constant infusion protocol or a bolus injection protocol to evaluate the accuracy of the Logan graphical approach for [¹¹C]methylphenidate imaging in mice. Displacement with unlabeled methylphenidate (0.1, 3 and 10 mg/kg) was used to verify specific binding. In a second experiment, 30 mice were lesioned by injection of 6-hydroxydopamine (6-OHDA) at doses of 0, 2 or 4 μg (n = 10) into the right striatum to assess the dose-dependent correlation between the PET signal and dopaminergic degeneration. In addition, we performed test-retest experiments and used ex vivo autoradiography (AR) to validate the effect of partial volume on the accuracy of the [¹¹C]methylphenidate PET quantification in the mouse striatum.

Results

The binding potentials (BP_ND) calculated from the Logan graphical analysis under transient equilibrium conditions (1.03 ± 0.1) were in excellent agreement with those calculated at true equilibrium (1.07 ± 0.1). Displacement of specific binding with 0.1, 3 and 10 mg/kg methylphenidate resulted in 38%, 77% and 81% transporter occupancy in the striatum. Intra-striatal injections of 6-OHDA caused a dose-dependent decrease in the specific binding of [¹¹C]methylphenidate to the DAT in the striatum. The BP_ND was reduced by 49% and 61% after injection with 2 and 4 μg of 6-OHDA, respectively. The test-retest reproducibility was 6% in the healthy striatum and 27% in the lesioned striatum. In addition, only a small (15%) difference was found between the [¹¹C]methylphenidate DVR-1 values determined by PET and AR on the healthy side, and no differences were observed on the lesioned side.

Conclusion

The present work demonstrates for the first time that [¹¹C]methylphenidate PET is useful for the quantification of striatal dopamine transporters at the dopaminergic nerve terminals in the mouse striatum; therefore, this marker may be used as a biomarker in genetically engineered mouse models of neurodegenerative disorders. However, only changes resulting in greater than 10% differences in BP_ND values can reliably be detected in vivo.     

Cerebral decreases in opioid receptor binding in patients with central neuropathic pain measured by [11C]diprenorphine binding and PET.

Central neuropathic pain (CNP) is pain resulting from damage to the central nervous system. Up till now, it has not been possible to identify a common lesion or pharmacological deficit in these patients. This preliminary study in a group of patients with CNP with predominantly post-stroke pain, demonstrates that there is significantly less opioid receptor binding in a number of cortical and sub-cortical structures that are mostly, but not exclusively, within the medial pain system in patients compared to age-matched pain-free controls. The reductions in opioid receptor binding within the medial system were observed mainly in the dorsolateral (Brodman area 10) and anterior cingulate (Brodman area 24 with some extension into area 23) and insula cortices and the thalamus. There were also reductions in the lateral pain system within the inferior parietal cortex (Brodman area 40). These changes in binding could not be accounted for by the cerebral lesions shown by CT or MRI, which were outside the areas of reduced binding and the human pain system. To our knowledge this is the first systematic demonstration of a reduction in opioid receptor-binding capacity in neurones within the human nociceptive system in patients with CNP. This may be a key common factor resulting in undamped nociceptor activity within some of the structures that are predominantly within the medial nociceptive system. If confirmed, these findings may explain why certain patients with CNP require high doses of synthetic opiates to achieve optimum analgesia. The findings also raise the possibility of new pharmacological approaches to treatment.     

Activating endogenous visceral pain modulation: A comparison of heterotopic stimulation methods in healthy controls

All sensory input underlies modulation by endogenous central nervous system pathways. Dysfunctional endogenous pain modulation has been demonstrated in central sensitization and in several pain syndromes, including Irritable Bowel Syndrome (IBS) Activation of endogenous visceral pain modulation by heterotopic stimulation was compared using different methods. Rectal electrical or distension pain alone or with simultaneous (i.e. heterotopic) noxious hand or foot cold stimulation were investigated in randomized sequence in 14 male and 1 female healthy subjects.Mean pain intensities on a visual analogue scale of 0–100 (95% CI) during tonic rectal electrical and distension stimulation alone were 64 (52–76) and 55 (39–71), respectively. Rectal distension pain decreased by 36% (18–55) with simultaneous hand and by 45% (24–66) with simultaneous foot cold pain. Rectal electrical pain decreased by 45% (29–61) during hand and by 46% (28–64) during foot cold pain. Facilitation, i.e. increased rectal pain during heterotopic stimulation was observed in only 1 of 60 stimulation runs.Potent and consistent activation of endogenous visceral pain inhibition was achieved with heterotopic cold pain limb stimulation. Somato-visceral convergence did not affect the effectiveness of induction of endogenous visceral pain inhibition in healthy subjects, as hand and foot heterotopic stimulation resulted in similar pain inhibition. Pain facilitation, as shown earlier in IBS patients, was not evident in healthy controls.     

Multi-Scale hierarchical generation of PET parametric maps: application and testing on a [11C]DPN study

We propose a general approach to generate parametric maps. It consists in a multi-stage hierarchical scheme where, starting from the kinetic analysis of the whole brain, we then cascade the kinetic information to anatomical systems that are akin in terms of receptor densities, and then down to the voxel level. A-priori classes of voxels are generated either by anatomical atlas segmentation or by functional segmentation using unsupervised clustering. Kinetic properties are transmitted to the voxels in each class using maximum a posteriori (MAP) estimation method. We validate the novel method on a [¹¹C]diprenorphine (DPN) test-retest data-set that represents a challenge to estimation given [¹¹C]DPN's slow equilibration in tissue.The estimated parametric maps of volume of distribution (V_T) reflect the opioid receptor distributions known from previous [¹¹C]DPN studies. When priors are derived from the anatomical atlas, there is an excellent agreement and strong correlation among voxel MAP and ROI results and excellent test-retest reliability for all subjects but one. Voxel level results did not change when priors were defined through unsupervised clustering.This new method is fast (i.e. 15 min per subject) and applied to [¹¹C]DPN data achieves accurate quantification of V_T as well as high quality V_T images. Moreover, the way the priors are defined (i.e. using an anatomical atlas or unsupervised clustering) does not affect the estimates.     

Imaging the Dopamine System with <Emphasis Type="Italic">In Vivo</Emphasis> [<Superscript>11</Superscript>C]raclopride Displacement Studies: Understanding the True Mechanism

Measuring changes in dopamine (DA) levels in humans using radioligand-displacement studies and positron emission tomography (PET) has provided important empirical findings in diseases and normal neurophysiology. These studies are based on the assumption that DA exerts a competitive inhibition on D₂-radioligand binding. However, the transfer of this hypothesis to a proven mechanism has not been fully achieved yet and an accumulating number of studies challenge it. In addition, new evidence suggests that DA exerts a noncompetitive inhibition on D₂-radioligand binding under amphetamine conditions. This article reviews the theoretical basis for the DA competition hypothesis, the in vivo and in vitro evidences supporting a noncompetitive action of DA on D₂-radioligand binding under amphetamine conditions, and discusses possible mechanisms underlying this noncompetitive interaction. Finally, we propose that such noncompetitive interactions may have important implications for how one interprets findings obtained from radioligand-displacement PET studies in neuropsychiatric diseases, especially in schizophrenia in which a dysregulation of the DA-promoted internalization of D₂ receptors was recently suggested.     

¹⁸F]FDOPA uptake in the raphe nuclei complex reflects serotonin transporter availability. A combined [¹⁸F]FDOPA and [¹¹C]DASB PET study in Parkinson's disease

Brain uptake of [¹⁸F]FDOPA, measured with PET, reflects the activity of aromatic amino acid decarboxylase, an enzyme largely expressed in monoaminergic nerve terminals. This enzyme catalyzes a number of decarboxylation reactions including conversion of l-dopa into dopamine and 5-hydroxytryptophan into serotonin. For more than 20 years [¹⁸F]FDOPA PET has been used to assess dopaminergic nigrostriatal dysfunction in patients with Parkinson's disease (PD). More recently, however, [¹⁸F]FDOPA PET has also been employed as a marker of serotoninergic and noradrenergic function in PD patients. In this study, we provide further evidence in support of the view that [¹⁸F]FDOPA PET can be used to evaluate the distribution and the function of serotoninergic systems in the brain.Eighteen patients with PD were investigated with both [¹⁸F]FDOPA and [¹¹C]DASB PET, the latter being a marker of serotonin transport (SERT) availability. We then assessed the relationship between measurements of the two tracers within brain serotoninergic structures. [¹⁸F]FDOPA uptake in the median raphe nuclei complex of PD patients was significantly correlated with SERT availability in the same structure. Trends towards significant correlations between [¹⁸F]FDOPA Ki values and [¹¹C]DASB binding values were also observed in the hypothalamus and the anterior cingulate cortex, suggesting a serotoninergic contribution to [¹⁸F]FDOPA uptake in these regions. Conversely, no correlations were found in brain structures with mixed dopaminergic, serotoninergic and noradrenergic innervations, or with predominant dopaminergic innervation.These findings provide evidence that [¹⁸F]FDOPA PET represents a valid marker of raphe serotoninergic function in PD and supports previous studies where [¹⁸F]FDOPA PET has been used to assess serotoninergic function in PD.     

Extended characterisation of the serotonin 2A (5-HT2A) receptor-selective PET radiotracer 11C-MDL100907 in humans: quantitative analysis, test-retest reproducibility, and vulnerability to endogenous 5-HT tone

Introduction

Scanning properties and analytic methodology of the 5-HT_2A receptor-selective positron emission tomography (PET) tracer ¹¹C-MDL100907 have been partially characterised in previous reports. We present an extended characterisation in healthy human subjects.

Methods

64 ¹¹C-MDL100907 PET scans with metabolite-corrected arterial input function were performed in 39 healthy adults (18-55 years). 12 subjects were scanned twice (duration 150 min) to provide data on plasma analysis, model order estimation, and stability and test-retest characteristics of outcome measures. All other scans were 90 min duration. 3 subjects completed scanning at baseline and following 5-HT_2A receptor antagonist medication (risperidone or ciproheptadine) to provide definitive data on the suitability of the cerebellum as reference region. 10 subjects were scanned under reduced 5-HT and control conditions using rapid tryptophan depletion to investigate vulnerability to competition with endogenous 5-HT. 13 subjects were scanned as controls in clinical protocols. Pooled data were used to analyse the relationship between tracer injected mass and receptor occupancy, and age-related decline in 5-HT_2A receptors.

Results

Optimum analytic method was a 2-tissue compartment model with arterial input function. However, basis function implementation of SRTM may be suitable for measuring between-group differences non-invasively and warrants further investigation. Scan duration of 90 min achieved stable outcome measures in all cortical regions except orbitofrontal which required 120 min. Binding potential (BP_P and BP_ND) test-retest variability was very good (7-11%) in neocortical regions other than orbitofrontal, and moderately good (14-20%) in orbitofrontal cortex and medial temporal lobe. Saturation occupancy of 5-HT_2A receptors by risperidone validates the use of the cerebellum as a region devoid of specific binding for the purposes of PET. We advocate a mass limit of 4.6 μg to remain below 5% receptor occupancy. ¹¹C-MDL100907 specific binding is not vulnerable to competition with endogenous 5-HT in humans. Paradoxical decreases in BP_ND were found in right prefrontal cortex following reduced 5-HT, possibly representing receptor internalisation. Mean age-related decline in brain 5-HT_2A receptors was 14.0 ± 5.0% per decade, and higher in prefrontal regions.

Conclusions

Our data confirm and extend support for ¹¹C-MDL100907 as a PET tracer with very favourable properties for quantifying 5-HT_2A receptors in the human brain.     

White matter glucose metabolism during intracortical electrostimulation: a quantitative [(18)F]Fluorodeoxyglucose autoradiography study in the rat

[(18)F]Fluorodeoxyglucose (FDG) autoradiography was used to analyze the effects of intracortical electrostimulation on local cerebral metabolic rate for glucose (LCMRglu). The hindleg area in rat brains was electrically stimulated with different frequencies (0, 0.1, 0.25, 0.5, 0.75, 1 stimulus trains per second, two animals per condition). The major result was a strong positive correlation between stimulation frequency and LCMRglu in the callosal fibers originating in the stimulated cortical area. At the highest stimulation frequency callosal LCMRglu was 50.01 micromol/min/100 g compared to 27.87 micromol/min/100 g at baseline. LCMRglu in gray and white matter control areas was stable across conditions. Direct injection of FDG in the stimulated cortex failed to produce increased callosal uptake, excluding the possibility that FDG uptake in the corpus callosum is related to axonal diffusion. Although several previous autoradiographic studies have demonstrated alterations in LCMRglu in white matter, correlations between neural activity and LCMRglu have never been explicitly addressed. Changes in white matter metabolism most likely reflect changes of electrical fiber activity and thus the presented results bear important implications for brain imaging studies.     

A short-scan method for k(3) estimation with moderately reversible PET ligands: application of irreversible model to early-phase PET data

Long dynamic scans (60-120 min) are often required for estimating the k₃ value, an index of receptor density, by positron emission tomography (PET). However, the precision of k₃ is usually low in kinetic analyses for reversible PET ligands compared with irreversible ligands. That is largely due to unstable estimation of the dissociation rate constant, k₄. We propose a novel ‘3P+’ method for estimating k₃ of moderately reversible ligands, where a 3-parameter model without k₄ is applied to early-phase PET data to obtain a good model-fit of k₃ estimation. By using [¹¹C] Pittsburgh compound B (PIB) (k₄ = 0.018/min) as an example of a moderately reversible ligand, the 3P+ method simulation with a 28 min PET scan yielded less than 3% k₃ relative bias with a +100% k₃ change. In [¹¹C]PIB PET scans of 15 normal controls (NC) and nine patients with Alzheimer's disease (AD), the 3P+ method provided a precise k₃ estimate (mean SE of 13.6% in parietal cortex; covariance matrix method). The results revealed linear correlations (r = 0.964) of parietal k₃ values in 24 subjects between 28 minute 3P+ method and conventional 90 minute 4-parameter method. A good separation of k₃ between NC and AD groups (P < 0.001; t-test) was replicated in 28 minute 3P+ method. The short-scan 3P+ method may be a practical alternative method for analyzing reversible ligands.     

脑内占位病变18F-FET PET显像初步研究与18F-FDG PET对照

目的:通过O-(2-18F-氟代乙基)-L-酪氨酸(18F-FET)和18F-氟脱氧葡萄糖(18F-FDG)2种PET显像剂的对照研究,探讨18F-FET能否起到弥补18F-FDG脑显像不足的作用。方法:14例脑占位病变患者(13例胶质瘤,1例鼻咽癌放疗后出现脑占位病变)1周内行18F-FET和18F-FDG PET2次脑显像。半定量分析:用感兴趣区(ROI)法计算标准摄取值(SUV)及T/NT。结果:①12例脑胶质瘤,无论术前原发灶或术后残存/复发18F-FET显像均显示病灶,T/NT=2.34±0.70,阳性率100%;18F-FDG显像,2例摄取高于皮层,病灶显示清晰,10例病灶摄取低于正常皮层,T/NT=0.59±0.18,阳性率75%;②1例胶质瘤术后疑有复发者,18F-FET及18F-FDG均未见异常,随访10个月,临床无进展;③1例鼻咽癌患者放射治疗后左颞叶占位病变,18F-FET图像病灶呈环形,周边高摄取,中心减低,T/NT=1.86;18F-FDG显像也显示为轻度环形摄取,T/NT=0.77。结论:①18F-FET显像不论肿瘤恶性程度如何均可清晰检出病灶,优于18F-FDG;②放射治疗后坏死1例,18F-FET为高摄取,其鉴别肿瘤和放射治疗后坏死的能力尚待进一步研究。     

First Evaluation of [11C]R116301 as an In Vivo Tracer of NK1 Receptors in Man

Purpose  NK1 receptors have been implicated in various neuropsychiatric and other disorders. R116301 is a selective NK1 receptor antagonist. In this pilot study, [¹¹C]R116301 was evaluated as a potential positron emission tomography (PET) ligand for the NK1 receptor. Procedures  Two dynamic PET studies were performed in three normal volunteers before and after a blocking dose of aprepitant. Data were analyzed using striatum to cerebellum standardized uptake value (SUV) ratios. Results  Baseline SUV ratios at 60–90 min after injection ranged from 1.22 to 1.70. Following aprepitant administration, this specific signal was completely blocked. Aprepitant administration did not significantly affect uptake in cerebellum, confirming the absence of NK1 receptors in cerebellum. Conclusion  These preliminary results indicate that [¹¹C]R116301 has potential as a radioligand for in vivo assessment of NK1 receptors in the human brain.     

Quantification of the glycine transporter 1 in rhesus monkey brain using [18F]MK-6577 and a model-based input function

Background

Glycine transporter 1 (GlyT1) inhibitors have emerged as potential treatments for schizophrenia due to their potentiation of NMDA receptor activity by modulating the local concentrations of the NMDA co-agonist glycine. [¹⁸F]MK-6577 is a potent and selective GlyT1 inhibitor PET tracer. Although differences in ligand kinetics can be expected between non-human primates and humans, the tracer pre-clinical evaluation can provide valuable information supporting protocol design and quantification in the clinical space. The main objective of this work was to evaluate the in vivo kinetics of [¹⁸F]MK-6577 in rhesus monkey brain. Additionally, a method for estimating the tracer input function from the tracer brain tissue kinetics and venous sampling was validated. This technique was applied for determination of the dose-occupancy relationship of a GlyT1 inhibitor in monkey brain.

Methods

Compartmental and Logan graphical analysis were utilized for quantification of the [¹⁸F]MK-6577 binding using the measured tracer arterial input function. The stability of the tracer volume of distribution relative to scan length was assessed. The proposed model-based input function method takes advantage of the agreement between the tracer concentration in arterial and venous plasma from ~ 5 min. The approach estimates the initial peak of the input curve by adding a gamma like function term to the measured venous curve. The parameters of the model function were estimated by simultaneously fitting several brain time activity curves to a compartmental model.

Results

Good agreement was found between the model-based and the measured arterial plasma curve and the corresponding distribution volumes. The Logan analysis was the preferred method of analysis providing reliable and stable volume of distribution and occupancy results using a 90 and possibly 60 min scan length.

Conclusion

The model-based input function method and Logan analysis are well suited for quantification of [¹⁸F]MK-6577 binding and GlyT1 occupancy in monkey brain.     

Direct Site-Specific Radiolabeling of an Affibody Protein with 4-[18F]Fluorobenzaldehyde via Oxime Chemistry

PURPOSE: In this study, we introduce a methodology for preparing 18F-labeled Affibody protein, specifically 18F-Anti-HER2 dimeric Affibody (14 kDa), for in vivo imaging of HER2neu with positron emission tomography (PET). PROCEDURES: We have used 4-[18F]fluorobenzaldehyde as a synthon to prepare 18F-Anti-HER2 Affibody. Aminooxy-functionalized Affibody (Anti-HER2-ONH2) was incubated with 4-[18F]fluorobenzaldehyde in ammonium acetate buffer at pH 4 in the presence of methanol at 70 degrees C for 15 min. The resulting 18F-labeled Affibody molecule was evaluated as a PET probe in xenograft models expressing HER2. RESULTS: We have successfully prepared 18F-Anti-HER2 dimeric Affibody (14 kDa), N-(4-[18F]fluorobenzylidine)oxime-Anti-HER2 Affibody, [18F]FBO-Anti-HER2, in 26-30% radiochemical yields (decay corrected). High-contrast small-animal PET images with relatively moderate tumor uptake (1.79 +/- 0.40% ID/g) were observed for the 18F-Anti-HER2 Affibody. CONCLUSION: Site-specific 18F-labeled Affibody against HER2 has been synthesized via chemoselective oxime formation between an aminooxy-functionalized Affibody and 18F-fluorobenzaldehyde. The results have implications for radiolabeling of other affibodies and macromolecules and should also be important for advancing Affibody imaging with PET.     

Comparison of [<Superscript>14</Superscript>C]FMAU, [<Superscript>3</Superscript>H]FEAU, [<Superscript>14</Superscript>C]FIAU,and [<Superscript>3</Superscript>H]PCV for Monitoring Reporter Gene Expression of Wild Type and Mutant Herpes Simplex Virus Type 1 Thymidine Kinase in Cell Culture

Purpose To assess the optimal reporter probe/reporter gene combination for monitoring herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression, we compared the cellular uptake of 1-(2′-fluoro-2′-deoxy-d-arabinofuranosyl)-5-methyluracil (FMAU), 2′-fluoro-2′-deoxyarabinofuranosyl-5-ethyluracil (FEAU), 2′-fluoro-2′-deoxy-β-d-arabinofuranosyl-5-iodouracil (FIAU) and penciclovir (PCV) in both HSV1-tk and HSV1-sr39tk expressing cells. Procedures For stably transfected cell studies, C6 rat glioma cells, C6 HSV1-tk transfectant, C6 mutant HSV1-sr39tk transfectant, rat Morris hepatoma cells (MH3924A), and MH3924A HSV1-tk transfectant cells were used. For adenoviral infection studies, C6 rat glioma cells were exposed to serial titers of AdCMV–HSV1-tk, AdCMV–HSV1-sr39tk, or AdCMV–fluc for 24 hours. These cells were incubated with [¹⁴C]FMAU, [³H]FEAU, [¹⁴C]FIAU, and [³H]PCV, and cellular uptake of radioactivity was measured. Results [³H]FEAU exhibited the highest or second highest accumulation and the most selectivity regardless of the mode of gene transfer for both HSV1-tk and mutant HSV1-sr39tk reporter genes. Conclusion This combination of high accumulation and high selectivity for both HSV1-tk and HSV1-sr39tk makes suitably radiolabeled FEAU a promising candidate as a radiotracer for imaging HSV1-tk/HSV1-sr39tk gene expression in living subjects.     

Reduced serotonin transporter binding in the insular cortex in patients with obsessive–compulsive disorder: A [C]DASB PET study

The serotonin transporter (5-HTT) and other markers of the serotonergic system have been of interest in the pathophysiology of obsessive-compulsive disorder (OCD). Previous studies using single photon emission computed tomography (SPECT) with [¹²³I]β-CIT or positron emission tomography (PET) with [¹¹C]McN5652 have not shown consistent findings about 5-HTT in OCD patients. The aim of the present study was to investigate 5-HTT binding using [¹¹C]DASB, which has higher selectivity or specific binding-to-nonspecific binding ratios for 5-HTT compared to the aforementioned radioligands. Four drug-naive and 6 drug-free patients with OCD who were free of comorbid depression and 18 gender and age-matched healthy subjects underwent PET scans with [¹¹C]DASB. The severity of OCD was assessed by Yale–Brown Obsessive–Compulsive Scale (Y-BOCS) (mean ± SD: 22 ± 7.6, range: 7–32). The binding potential (BP_ND) of [¹¹C]DASB was calculated using a two-parameter multilinear reference tissue model (MRTM2). The parametric images of BP_ND were analyzed using a statistical parametric mapping system. Significant reductions of BP_ND were observed in the right posterior and left anterior insular cortices in patients with OCD compared to controls. Region-of-interest analysis has also confirmed significant reduction of BP_ND in the insular cortex. Although significantly reduced BP_ND in the orbitofrontal cortex was also observed in patients with OCD compared to controls, this finding should be considered with caution because of the very low 5-HTT binding in the region. On the other hand, no significant correlation was observed between the Y-BOCS score and BP_ND. The change in [¹¹C]DASB binding in the insular cortex suggests that dysfunction of the serotonergic system in the limbic area might be involved in the pathophysiology of OCD.     

Quantification of cerebral A1 adenosine receptors in humans using [F]CPFPX and PET: an equilibrium approach

The cerebral A(1) adenosine receptor (A(1)AR) has recently become accessible for in vivo imaging using the selective A(1)AR ligand [(18)F]CPFPX and PET. For broad application in neurosciences, imaging at distribution equilibrium is advantageous to quantify stimulus-dependent changes in receptor availability and to avoid arterial blood sampling. Here we propose a bolus/infusion (B/I) protocol to assess the total distribution volume (DV(t)) of [(18)F]CPFPX under equilibrium conditions. Employing a bolus-to-infusion ratio of 0.8 h, (near) equilibrium conditions were attained within 60 min. The regional DV(t)' given by arterial and venous equilibrium analyses agreed well with conventional two-tissue compartment model analyses (r(2) > 0.94 and r(2) > 0.84, respectively) and Logan's graphical analyses (r(2) = 1.0 and r(2) > 0.93, respectively) (n = 4 healthy volunteers). The mean regional DV(t)' values of these equilibrium analyses and of venous equilibrium analyses in additional seven volunteers demonstrated excellent agreement with the results of earlier bolus studies (r(2) > 0.98). Error simulations show that minor deviations from true equilibrium are associated with negligible to small DV(t) errors. In conclusion, [(18)F]CPFPX shows suitable characteristics for A(1)AR quantification by B/I PET scanning. Carefully standardized venous equilibrium analyses may substitute arterial analyses and thus considerably enhance applicability of A(1)AR PET in clinical routine.