SPECT quantification of [123I]iomazenil binding to benzodiazepine receptors in nonhuman primates: I. Kinetic modeling of single bolus experiments

Graphic plot analysis for estimating binding potential of translocator protein (TSPO) in positron emission tomography studies with [¹⁸F]FEDAA1106

PURPOSE

[(18)F]FEDAA1106 is expected to be used for evaluating the regional density of the peripheral benzodiazepine receptor (also called TSPO) in several neurodegenerative disorders. Regarding the quantification, direct binding potential (BP(ND)) has been reported to be preferable because of the variation of nondisplaceable distribution volume (V(ND)) among individuals. However, the precise calculation of BP(ND) is difficult in small regions or at voxel levels due to noise. Recently, a new graphical analysis (GA) was proposed to estimate V(ND) in a direct way. In this paper, we evaluated two types of GA for reliable quantification of BP(ND) in PET study with [(18)F]FEDAA1106 using computer simulations and human data.

METHODS

In the simulations, time-activity curves were generated with various rate constants and noise levels, and the errors of BP(ND) estimated by GA were analyzed by comparing with true values calculated from rate constants given for the simulations. Thereafter, in a human study with [(18)F]FEDAA1106 for healthy volunteers, BP(ND) was estimated by two types of GA for region-of-interest (ROI) data. Parametric images of BP(ND) were generated by two types of GA with or without wavelet-denoising.

RESULTS

Simulations showed that BP(ND) by GA was well correlated with true values, despite an underestimation. GA reduced unreasonable estimates compared with a conventional nonlinear least-square fitting (NLS), although larger variation of BP(ND) estimates was observed. In a ROI-based analysis of data obtained in a human study, BP(ND)s estimated by GA were well correlated with those generated by NLS, though they were underestimated. Parametric BP(ND) images by GA could be improved with wavelet-denoising.

CONCLUSION

Graphical analysis could provide BP(ND) values with high stability and simple calculation in both ROI-based and voxel-based analyses of [(18)F]FEDAA1106 data.

High vascular delivery of EGF, but low receptor binding rate is observed in AsPC-1 tumors as compared to normal pancreas

PURPOSE

Cellular receptor targeted imaging agents present the potential to target extracellular molecular expression in cancerous lesions; however, the image contrast in vivo does not reflect the magnitude of overexpression expected from in vitro data. Here, the in vivo delivery and binding kinetics of epidermal growth factor receptor (EGFR) was determined for normal pancreas and AsPC-1 orthotopic pancreatic tumors known to overexpress EGFR.

PROCEDURES

EGFR in orthotopic xenograft AsPC-1 tumors was targeted with epidermal growth factor (EGF) conjugated with IRDye800CW. The transfer rate constants (k(e), K₁₂, k₂₁, k₂₃, and k₃₂) associated with a three-compartment model describing the vascular delivery, leakage rate and binding of targeted agents were determined experimentally. The plasma excretion rate, k (e), was determined from extracted blood plasma samples. K₁₂, k₂₁, and k₃₂ were determined from ex vivo tissue washing studies at time points ≥ 24 h. The measured in vivo uptake of IRDye800CW-EGF and a non-targeted tracer dye, IRDye700DX-carboxylate, injected simultaneously was used to determined k₂₃.

RESULTS

The vascular exchange of IRDye800CW-EGF in the orthotopic tumor (K₁₂ and k₂₁) was higher than in the AsPC-1 tumor as compared to normal pancreas, suggesting that more targeted agent can be taken up in tumor tissue. However, the cellular associated (binding) rate constant (k₂₃) was slightly lower for AsPC-1 pancreatic tumor (4.1 × 10(-4) s(-1)) than the normal pancreas (5.5 × 10(-4) s(-1)), implying that less binding is occurring.

CONCLUSIONS

Higher vascular delivery but low cellular association in the AsPC-1 tumor compared to the normal pancreas may be indicative of low receptor density due to low cellular content. This attribute of the AsPC-1 tumor may indicate one contributing cause of the difficulty in treating pancreatic tumors with cellular targeted agents.

[11C]flumazenil binding is increased in a dose-dependent manner with tiagabine-induced elevations in GABA levels

Evidence indicates that synchronization of cortical activity at gamma-band frequencies, mediated through GABA-A receptors, is important for perceptual/cognitive processes. To study GABA signaling in vivo, we recently used a novel positron emission tomography (PET) paradigm measuring the change in binding of the benzodiazepine (BDZ) site radiotracer [¹¹C]flumazenil associated with increases in extracellular GABA induced via GABA membrane transporter (GAT1) blockade with tiagabine. GAT1 blockade resulted in significant increases in [¹¹C]flumazenil binding potential (BPND) over baseline in the major functional domains of the cortex, consistent with preclinical studies showing that increased GABA levels enhance the affinity of GABA-A receptors for BDZ ligands. In the current study we sought to replicate our previous results and to further validate this approach by demonstrating that the magnitude of increase in [¹¹C]flumazenil binding observed with PET is directly correlated with tiagabine dose. [¹¹C]flumazenil distribution volume (VT) was measured in 18 healthy volunteers before and after GAT1 blockade with tiagabine. Two dose groups were studied (n = 9 per group; Group I: tiagabine 0.15 mg/kg; Group II: tiagabine 0.25 mg/kg). GAT1 blockade resulted in increases in mean (± SD) [¹¹C]flumazenil VT in Group II in association cortices (6.8±0.8 mL g−1 vs. 7.3±0.4 mL g−1;p = 0.03), sensory cortices (6.7±0.8 mL g−1 vs. 7.3±0.5 mL g−1;p = 0.02) and limbic regions (5.2±0.6 mL g−1 vs. 5.7±0.3 mL g−1;p = 0.03). No change was observed at the low dose (Group I). Increased orbital frontal cortex binding of [¹¹C]flumazenil in Group II correlated with the ability to entrain cortical networks (r = 0.67, p = 0.05) measured via EEG during a cognitive control task. These data provide a replication of our previous study demonstrating the ability to measure in vivo, with PET, acute shifts in extracellular GABA.

Tiagabine increases [11C]flumazenil binding in cortical brain regions in healthy control subjects

Accumulating evidence indicates that synchronization of cortical neuronal activity at gamma-band frequencies is important for various types of perceptual and cognitive processes and that GABA-A receptor-mediated transmission is required for the induction of these network oscillations. In turn, the abnormalities in GABA transmission postulated to play a role in psychiatric conditions such as schizophrenia might contribute to the cognitive deficits seen in this illness. We measured the ability to increase GABA in eight healthy subjects by comparing the binding of [(11)C]flumazenil, a positron emission tomography (PET) radiotracer specific for the benzodiazepine (BDZ) site, at baseline and in the presence of an acute elevation in GABA levels through the blockade of the GABA membrane transporter (GAT1). Preclinical work suggests that increased GABA levels enhance the affinity of GABA-A receptors for BDZ ligands (termed 'GABA shift'). Theoretically, such an increase in the affinity of GABA-A receptors should be detected as an increase in the binding of a GABA-A BDZ-receptor site-specific PET radioligand. GAT1 blockade resulted in significant increases in mean (+/- SD) [(11)C]flumazenil-binding potential (BP(ND)) over baseline in brain regions representing the major functional domains of the cerebral cortex: association cortex +15.2+/-20.2% (p=0.05), sensory cortex +13.5+/-15.5% (p=0.03) and limbic (medial temporal lobe, MTL) +16.4+/-20.2% (p=0.03). The increase in [(11)C]flumazenil-BP(ND) was not accounted for by differences in the plasma-free fraction (f(P); paired t-test p=0.24) or changes in the nonspecific binding (pons V(T), p=0.73). Moreover, the ability to increase GABA strongly predicted (r=0.85, p=0.015) the ability to entrain cortical networks, measured through EEG gamma synchrony during a cognitive control task in these same subjects. Although additional studies are necessary to further validate this technique, these data provide preliminary evidence of the ability to measure in vivo, with PET, acute fluctuations in extracellular GABA levels and provide the first in vivo documentation of a relationship between GABA neurotransmission and EEG gamma-band power in humans.

Comparison of noninvasive quantification methods of in vivo vesicular acetylcholine transporter using [123I]-IBVM SPECT imaging

Dementia with Lewy Body and Alzheimer's disease exhibit degeneration of the cholinergic neurons, and currently, the primary target of treatment is the cholinergic neurotransmitter system. [(123)I]-IBVM is a highly selective radioligand for in vivo visualization of the vesicular acetylcholine transporter (VAChT) using single photon emission computed tomography. This study compares different noninvasive methods using the occipital cortex as a reference region for the quantification of [(123)I]-IBVM binding in six older, healthy volunteers: two kinetic analyses based on one-tissue (1TCM) or two-tissue compartment model (2TCM), one linear and one multilinear analysis, and a simplified peak equilibrium analysis. Time-activity curves were well described by a 1TCM for all regions. The 2TCM converged reliably only in the striatum. Goodness of fit was not improved by using a 2TCM as compared with a 1TCM. The multilinear analysis gave binding potentials similar to the 1TCM while being more robust. The peak equilibrium method might prove to be a useful simplified analysis. The binding potentials obtained with reference region methods strongly correlated with results from invasive blood-sampling analysis. Noninvasive quantification of [(123)I]-IBVM data provides reliable estimates of VAChT binding, which is most valuable to study neurodegenerative diseases with specific cholinergic alteration.

The PET radioligand [11C]MePPEP binds reversibly and with high specific signal to cannabinoid CB1 receptors in nonhuman primate brain

The cannabinoid CB(1) receptor is one of the most abundant G protein-coupled receptors in the brain and is a promising target of therapeutic drug development. Success of drug development for neuropsychiatric indications is significantly enhanced with the ability to directly measure spatial and temporal binding of compounds to receptors in central compartments. We assessed the utility of a new positron emission tomography (PET) radioligand to image CB(1) receptors in monkey brain. [(11)C]MePPEP ((3R,5R)-5-(3-methoxy-phenyl)-3-((R)-1-phenyl-ethylamino)-1-(4-trifluoromethyl-phenyl)-pyrrolidin-2-one) has high CB(1) affinity (K(b)=0.574+/-0.207 nM) but also moderately high lipophilicity (measured LogD(7.4)=4.8). After intravenous injection of [(11)C]MePPEP, brain activity reached high levels of almost 600% standardized uptake value (SUV) within 10-20 min. The regional uptake was consistent with the distribution of CB(1) receptors, with high radioactivity in striatum and cerebellum and low in thalamus and pons. Injection of pharmacological doses of CB(1)-selective agents confirmed that the tracer doses of [(11)C]MePPEP reversibly labeled CB(1) receptors. Preblockade or displacement with two CB(1) selective agents (ISPB; (4-(3-cyclopentyl-indole-1-sulfonyl)-N-(tetrahydro-pyran-4-ylmethyl)-benzamide) and rimonabant) showed that the majority (>89%) of brain uptake in regions with high receptor densities was specific and reversibly bound to CB(1) receptors in the high binding regions. [(11)C]MePPEP was rapidly removed from arterial plasma. Regional brain uptake could be quantified as distribution volume relative to the concentration of parent radiotracer in plasma. The P-glycoprotein (P-gp) inhibitor DCPQ ((R)-4-[(1a,6,10b)-1,1-dichloro-1,1a,6,10b-tetrahydrodibenzo[a,e]cyclopropa[c]cyclohepten-6-yl]-[(5-quinolinyloxy)methyl]-1-piperazineethanol) did not significantly increase brain uptake of [(11)C]MePPEP, suggesting it is not a substrate for this efflux transporter at the blood-brain barrier. [(11)C]MePPEP is a radioligand with high brain uptake, high specific signal to CB(1) receptors, and adequately fast washout from brain that allows quantification with (11)C (half-life=20 min). These promising results in monkey justify studying this radioligand in human subjects.

Analysis of neuroreceptor PET-data based on cytoarchitectonic maximum probability maps: a feasibility study

Three-dimensional maximum probability maps (MPMs) of cytoarchitectonically defined cortical regions based on postmortem histological studies have recently been made available in the stereotaxic reference space of the Montreal Neurological Institute (MNI) single subject template. This permits the use of cytoarchitectonic maps for the analysis of functional in vivo datasets, including neuroreceptor positron emission tomography (PET) studies. In this feasibility study, we used 5-hydroxytryptamine 2A (5-HT2A) receptor PET to test the applicability of maximum cytoarchitectonic probability maps for quantitative analysis. As the outcome parameter, we extracted local distribution volume ratios (DVRs) from 19 cytoarchitectonically defined volumes of interest (VOIs) per hemisphere from five healthy subjects. The experimental design included a forward ('PET to atlas' normalization) and a backward ('atlas to PET' normalization) procedure to double-check the stability of transformation and overlay. Resulting DVRs were compared with receptor densities (RDs) obtained from postmortem [3H]ketanserin autoradiography of multiple areas. Correlations between the bi-directional normalization procedures (r = 0.89; 38 VOIs) as well as between in vivo and vitro data (nine VOIs; r = 0.64 and r = 0.47 for forward and backward procedure, respectively) suggest that the implementation of cytoarchitectonic maximum probability maps is a promising method for an accurate and observer-independent analysis of neuroreceptor PET data.

Decreased prefrontal 5-HT2A receptor binding in subjects at enhanced risk for schizophrenia

The brain serotonin-2A receptor (5-HT(2A)R) has been implicated in both the pathology of schizophrenia and the therapeutic action of atypical antipsychotics. However, little is known about the 5-HT(2A)R status before the onset of schizophrenia and before the exposure to antipsychotics. We used [18F] altanserin and positron emission tomography (PET) in a pilot study of 6 individuals suspected to be at elevated risk for schizophrenia and seven age-matched controls to test the hypothesis that regional 5-HT(2A)R binding is altered in the prodromal stages of schizophrenia. Distribution volume ratios (DVRs) as a proxy for 5-HT(2A)R availability were significantly reduced in prefrontal cortex regions of at-risk subjects, implicating early abnormalities of serotonergic neurotransmission that antecede the onset of schizophrenia.

The new PET imaging agent [11C]AFE is a selective serotonin transporter ligand with fast brain uptake kinetics

A new positron emission tomography (PET) radioligand for the serotonin transporter (SERT), [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-(2-fluoroethyl)phenylamine ([(11)C]AFE, 12), was synthesized and evaluated in vivo in rats and baboons. [(11)C]AFE (12) was prepared from its monomethylamino precursor 11 by reaction with high specific activity [(11)C]methyl triflate. Radiochemical yield was 32+/-17% based on [(11)C]methyl triflate (n=6) and specific activity was 1670+/-864 Ci/mmol at end of synthesis (EOS, n=6). Binding assays indicated that AFE displays high affinity for SERT (K(i)=1.80 nM for hSERT) and lower affinity for norepinephrine transporter (K(i)=946 nM for hNET) or dopamine transporter (K(i)>10,000 nM for hDAT). In addition, AFE displays negligible binding affinities for other serotonin and dopamine receptors, indicating an excellent binding selectivity in vitro. Biodistribution studies in rats indicated that [(11)C]AFE enters the brain readily and localizes in regions known to contain high concentrations of SERT, such as the thalamus, hypothalamus, frontal cortex and striatum. Moreover, such binding in SERT-rich brain regions is reduced significantly by pretreatment with either citalopram or the cold compound itself, but not by nisoxetine or GBR 12935, thus demonstrating that [(11)C]AFE binding in the rat brain is saturable, specific and selective for the SERT. Imaging experiments in baboons indicated that the uptake pattern of [(11)C]AFE is consistent with the known distribution of SERT in the baboon brain, with high levels of radioactivity detected in the midbrain and thalamus, moderate levels in the hippocampus and striatum and low levels in the cortical regions. The uptake kinetics of [(11)C]AFE in the baboon brain is rapid, with activity in the midbrain and thalamus peaking at 15-40 min postinjection. Pretreatment of the baboon with citalopram (4 mg/kg) 20 min before radioactivity injection reduced the binding of [(11)C]AFE in all SERT-containing brain regions to the level in the cerebellum. Kinetic analysis revealed that in all brain regions examined, [(11)C]AFE specific-to-nonspecific partition coefficients (V(3)'') are similar to those of [(11)C]McN5652 and [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-fluorophenylamine ([(11)C]AFA), but lower than those of [(11)C]2-[2-[[(dimethylamino)methyl]phenyl]thio]-5-fluoromethylphenylamine ([(11)C]AFM) or [(11)C]DASB. In summary, [(11)C]AFE appears to be a PET radioligand with fast brain uptake kinetics and can be used for the visualization and quantification of SERT in vivo.

Quantification of brain phosphodiesterase 4 in rat with (R)-[11C]Rolipram-PET

OBJECTIVE

Phosphodiesterase 4 (PDE4) catabolizes the second messenger 3', 5'-cyclic adenosine monophosphate and may play a critical role in brain diseases. Our aim was to quantify PDE4 in rats with positron emission tomography (PET).

METHODS

High (n = 6) and low specific activity (SA) (n = 2) higher affinity ((R)-[(11)C]rolipram) and high SA lower affinity ((S)-[(11)C]rolipram) (n = 2) enantiomers were intravenously administered to Sprague-Dawley rats. Brain data were acquired using the ATLAS PET scanner and reconstructed using the 3D-ordered subset expectation maximization algorithm. Arterial samples were taken to measure unmetabolized [(11)C]rolipram. Total distribution volumes (V(T)') were calculated using a 1-tissue compartment (1C) and an unconstrained 2-tissue compartment (2C) model.

RESULTS

High SA R experiments showed later and greater brain uptake, and slower washout than low SA R and S experiments. In all regions and in all experiments, the 2C model gave significantly better fitting than the 1C model. The poor fitting by the latter caused underestimation of V(T)' by 19-31%. The 2C model identified V(T)' reasonably well with coefficients of variation less than 10%. V(T)' values by this model were 16.4-29.2 mL/cm(3) in high SA R, 2.9-3.5 in low SA R, and 3.1-3.7 in S experiments.

CONCLUSIONS

Specific binding of (R)-[(11)C]rolipram was accurately measured in living rats. In high SA R experiments, approximately 86% of V(T)' was specific binding. Distribution and changes of PDE4 in animal models can now be studied by measuring V(T)' of high SA (R)-[(11)C]rolipram.

A PET imaging agent with fast kinetics: synthesis and in vivo evaluation of the serotonin transporter ligand [11C]2-[2-dimethylaminomethylphenylthio)]-5-fluorophenylamine ([11C]AFA)

A new serotonin transporter (SERT) ligand, [11C]2-[2-(dimethylaminomethylphenylthio)]-5-fluorophenylamine (10, [11C]AFA), was synthesized and evaluated as a candidate PET radioligand in pharmacological and pharmacokinetic studies. As a PET radioligand, AFA (8) can be labeled with either C-11 or F-18. In vitro, AFA displayed high affinity for SERT (Ki 1.46 +/- 0.15 nM) and lower affinity for norepinephrine transporter (NET, Ki 141.7 +/- 47.4 nM) or dopamine transporter (DAT, Ki > 10,000 nM). [11C]AFA (10) was prepared from its monomethylamino precursor 9 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 43 +/- 20% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 2,129 +/- 1,369 Ci/mmol at end of synthesis (EOS, n = 10). Biodistribution studies in rats indicated that [11C]AFA accumulated in brain regions known to contain high concentrations of SERT. Binding in SERT-rich brain regions was reduced significantly by pretreatment with either the cold compound 8 or with the selective serotonin reuptake inhibitor (SSRI) citalopram, but not by the selective norepinephrine reuptake inhibitor nisoxetine, thus underlining its in vivo binding selectivity and specificity for SERT. Imaging experiments in baboons demonstrated that the uptake pattern of [11C]AFA in the baboon brain is consistent with the known distribution of SERT, with highest activity levels in the midbrain and thalamus, followed by striatum, hippocampus, and cortical regions. Activity levels in the baboon brain peaked at 15-40 min after radioligand injection, indicating a fast uptake kinetics for [11C]AFA. Pretreatment of the baboon with citalopram (4 mg/kg) significantly reduced the specific binding of [11C]AFA in all SERT-containing brain regions. Kinetic analysis revealed that the regional equilibrium specific to non-specific partition coefficients (V3") of [11C]AFA are similar to those of [11C]McN5652, but lower than those of [11C]AFM or [11C]DASB. In summary, [11C]AFA appears to be an appropriate PET radioligand with a fast brain uptake kinetics:

Effects of reduced endogenous 5-HT on the in vivo binding of the serotonin transporter radioligand11C-DASB in healthy humans

Although abnormal serotonin (5-HT) function is implicated in a range of mental disorders, there is currently no method to directly assess 5-HT synaptic levels in the living human brain. The in vivo binding of some dopamine (DA) radioligands such as (11)C-raclopride is affected by fluctuations in endogenous DA, thus providing an indirect measure of DA presynaptic activity. Attempts to identify a serotonergic radiotracer with similar properties have proved unsuccessful. Here, we investigated in humans the effects of reduced synaptic 5-HT on the in vivo binding of the 5-HT transporter (SERT) radioligand (11)C-DASB, using Positron Emission Tomography (PET) and the rapid tryptophan depletion (RTD) technique. Eight (8) subjects (5M, 3F) were scanned with (11)C-DASB under control and reduced endogenous 5-HT conditions, in a within-subject, double-blind, counterbalanced, crossover design. Regional distribution volumes (V(T)) were calculated using kinetic modeling and metabolite-corrected arterial input function. (11)C-DASB specific binding was estimated as binding potential (BP) and specific to nonspecific equilibrium partition coefficient (V(")(3)), using the cerebellum as reference region. RTD caused small but significant mean reductions in (11)C-DASB V(T) (-6.1%) and BP (-4.5%) across brain regions, probably explained by a concomitant reduction in (11)C-DASB plasma free fraction (f(1)) of similar magnitude. No significant change in (11)C-DASB V(")(3) was observed between control and reduced 5-HT conditions. Nor was there a significant relationship between the magnitude of tryptophan depletion and change in BP and V(")(3) across individual subjects. These results suggest that (11)C-DASB in vivo binding is not affected by reductions in endogenous 5-HT.

Estimation of kinetic parameters in graphical analysis of PET imaging data

Positron emission tomography (PET) imaging is a useful tool for quantifying various aspects of the distribution of neuroreceptors throughout the human brain in vivo. A typical analysis consists of applying a pharmacokinetic model to the data, estimating the parameters of the model using non-linear least squares methods, then taking the appropriate function of estimated model parameters as a final estimate of the parameter(s) of interest. As an alternative for fitting these models, it has been shown previously that taking a particular transformation of the data results in two variables that have a linear relationship, and that the slope of this linear relationship is the parameter of primary interest. However, estimating the slope using ordinary least squares (OLS) regression results in a large negative bias. By rearranging the terms in the relationship, the problem may be reformed to allow direct application of standard estimation principles. Estimators resulting from this approach are shown via simulation to have better estimation properties as compared to the OLS estimators.

A bolus/infusion paradigm for the novel NMDA receptor SPET tracer [123i]CNS 1261

We have previously performed quantitative kinetic modeling of [(123)I]CNS 1261, a new SPET ligand for the MK801 intrachannel site of the NMDA receptor. We now report a bolus-infusion protocol, which eliminates the need for arterial blood sampling. Dynamic SPET scanning and venous blood sampling were performed in 7 healthy volunteers. Good agreement was obtained between kinetic and equilibrium analysis. SPET scanning with a bolus-infusion protocol is a valid method to estimate the total volume of distribution for [(123)I]CNS 1261 in clinical populations.

Quantification of nicotinic acetylcholine receptors in human brain using [123I]5-I-A-85380 SPET

The purpose of this study was to assess the utility of a new single-photon emission tomography ligand, [123I]5-iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380), to measure regional nAChR binding in human brain. Six healthy nonsmoker subjects (two men and four women, age 33 +/- 15 years) participated in both a bolus (dose: 317 +/- 42 MBq) and a bolus plus constant infusion (dose of bolus: 98 +/- 32 MBq, B/I=6.7 +/- 2.6 h, total dose: 331 +/- 55 MBq) study. The study duration was 5-8 h and 14 h in the former and the latter, respectively. Nonlinear least-squares compartmental analysis was applied to bolus studies to calculate total (VT') and specific (VS') distribution volumes. A two-tissue compartment model was applied to identify VS'. VT' was also calculated in B/I studies. In bolus studies, VT' was well identified by both one- and two-tissue compartment models, with a coefficient of variation of less than 5% in most regions. The two-compartment model gave VT' values of 51, 22, 27, 32, 20, 19, 20, and 17 ml cm(-3) in thalamus, cerebellum, putamen, pons, and frontal, parietal, temporal, and occipital cortices, respectively. The two-compartment model did not identify VS' well. B/I studies provided poor accuracy of VT' measurement, possibly due to deviations from equilibrium conditions. These results demonstrate the feasibility of quantifying high-affinity type nAChRs using [123I]5-I-A-85380 in humans and support the use of VT' measured by bolus studies.

Comparative evaluation in nonhuman primates of five PET radiotracers for imaging the serotonin transporters: [11C]McN 5652, [11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM

The recent introduction of a number of new radiotracers suitable for imaging the serotonin transporters (SERT) has radically changed the field of SERT imaging. Whereas, until recently, only one selective SERT radiotracer was available ([11C]McN 5652) for SERT imaging with positron emission tomography (PET), several new C-11-labeled radiotracers of the -dimethyl-2-(arylthio)benzylamine class have been described as appropriate imaging agents for the SERT. The aim of this study was to conduct a comparative evaluation of four of the most promising agents in this class ([11C]ADAM, [11C]DASB, [11C]DAPA, and [11C]AFM) with the reference tracer [11C]McN 5652 under standardized experimental conditions. This evaluation included in vitro measurements of affinity and lipophilicity, and in vivo PET imaging experiments in baboons. In vitro, DASB displayed significantly lower affinity for SERT than the other four tracers. In the blood, [11C]DASB and [11C]AFM display faster clearance and higher free fractions. Brain uptake was analyzed with kinetic modeling using a one-tissue compartment model and the metabolite-corrected arterial input function. The kinetic uptake of [11C]DASB was significantly faster compared with the other compounds, and the scan duration required to derive time-independent estimates of regional distribution volumes was shorter. [11C]DAPA exhibited the slowest brain kinetic. Regional-specific-to-nonspecific equilibrium partition coefficient (V3") was the highest for [11C]AFM, followed by [11C]DASB and [11C]DAPA, which in turn provided higher V3" values than [11C]ADAM and [11C]McN 5652. From these experiments, two ligands emerged as superior radiotracers that provide a significant improvement over [11C]McN 5652 for PET imaging of SERT: [11C]DASB, because it enables the measurement of SERT availability in a shorter scanning time, and [11C]AFM, because its higher signal-to-noise ratios provide a more reliable measurement of SERT availability in brain regions with relatively low density of SERT, such as in the limbic system.

Synthesis and pharmacological characterization of a new PET ligand for the serotonin transporter: [11C]5-bromo-2-[2-(dimethylaminomethylphenylsulfanyl)]phenylamine ([11C]DAPA)

A new PET radioligand for the serotonin transporter (SERT), [11C]-5-bromo-2-[2-(dimethylaminomethylphenylsulfanyl)]phenylamine ([11C]DAPA (10), was synthesized and evaluated in vivo in rats and baboons. [11C]DAPA (10) was prepared from its monomethylamino precursor 8 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 24 +/- 5% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 1,553 +/- 939 Ci/mmol at end of synthesis (EOS, n = 10). Binding assays indicated that [11C]DAPA displays high affinity (Ki 1.49 +/- 0.28 nM for hSERT) and good selectivity for the SERT in vitro. Biodistribution studies in rats indicated that [11C]DAPA enters into the brain readily and localizes in brain regions known to contain high concentrations of SERT, such as the thalamus, hypothalamus, frontal cortex and striatum. Moreover, such binding in SERT-rich regions of the brain are blocked by pretreatment with either the selective serotonin reuptake inhibitor (SSRI) citalopram and by the cold compound itself, demonstrating that [11C]DAPA binding in the rat brain is saturable and specific to SERT. Imaging experiments in baboons indicated that [11C]DAPA binding is consistent with the known distribution of SERT in the baboon brain, with highest levels of radioactivity detected in the midbrain and thalamus, intermediate levels in the hippocampus and striatum, and lower levels in the cortical regions. Pretreatment of the baboon with citalopram 10 min before radioactivity injection blocked the binding of [11C]DAPA in all brain regions that contain SERT. Kinetic analysis revealed that, in all brain regions examined, [11C]DAPA specific to nonspecific distribution volume ratios (V(3)") are higher than [11C](+)-McN 5652 and similar to [11C]DASB. In summary, [11C]DAPA appears to be a promising radioligand suitable for the visualization of SERT in vivo using PET.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Prefrontal dopamine D1 receptors and working memory in schizophrenia

Studies in nonhuman primates documented that appropriate stimulation of dopamine (DA) D1 receptors in the dorsolateral prefrontal cortex (DLPFC) is critical for working memory processing. The defective ability of patients with schizophrenia at working memory tasks is a core feature of this illness. It has been postulated that this impairment relates to a deficiency in mesocortical DA function. In this study, D1 receptor availability was measured with positron emission tomography and the selective D1 receptor antagonist [11C]NNC 112 in 16 patients with schizophrenia (seven drug-naive and nine drug-free patients) and 16 matched healthy controls. [11C]NNC 112 binding potential (BP) was significantly elevated in the DLPFC of patients with schizophrenia (1.63 +/- 0.39 ml/gm) compared with control subjects (1.27 +/- 0.44 ml/gm; p = 0.02). In patients with schizophrenia, increased DLPFC [11C]NNC 112 BP was a strong predictor of poor performance at the n-back task, a test of working memory. These findings confirm that alteration of DLPFC D1 receptor transmission is involved in working memory deficits presented by patients with schizophrenia. Increased D1 receptor availability observed in patients with schizophrenia might represent a compensatory (but ineffective) upregulation secondary to sustained deficiency in mesocortical DA function.

Positron emission tomography quantification of [(11)C]-DASB binding to the human serotonin transporter: modeling strategies

[(11) C]-DASB, namely [(11) C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile, is a new highly selective radioligand for the in vivo visualization of the serotonin transporter (SERT) using positron emission tomography (PET). The current study evaluates different kinetic modeling strategies for quantification of [(11)C]-DASB binding in five healthy humans. Kinetic analyses of tissue data were performed with a one-tissue (1CM) and a two-tissue (2CM) compartment model. Time-activity curves were well described by a 1CM for all regions. A 2CM model with four parameters failed to converge reliably. Reliable fits of the data were obtained only if no more than three parameters were allowed to vary. However, even then, the rate constants k(3) and k(4) were estimated with poor precision. Only the ratio k(3)/k(4) was stable. Goodness of fit was not improved by using a 2CM as compared with a 1CM. The minimal study duration required to obtain stable k(3)/k(4) estimates was 80 minutes. For routine use of [(11)C]-DASB, several simplified methods using the cerebellum as a reference region to estimate nonspecific binding were also evaluated. The transient equilibrium, the linear graphical analysis, the ratio of target to reference region, and the simplified reference tissue methods all gave binding potential values consistent with those obtained with the 2CM. The suitability of [(11)C]-DASB for research on the SERT using PET is thus supported by the observations that tissue data can be described using a kinetic analysis and that simplified quantitative methods, using the cerebellum as reference, provide reliable estimates of SERT binding parameters.

Models and methods for derivation of in vivo neuroreceptor parameters with PET and SPECT reversible radiotracers

The science of quantitative analysis of PET and SPECT neuroreceptor imaging studies has grown considerably over the past decade. A number of methods have been proposed in which receptor parameter estimation results from fitting data to a model of the underlying kinetics of ligand uptake in the brain. These approaches have come to be collectively known as model-based methods and several have received widespread use. Here, we briefly review the most frequently used methods and examine their strengths and weaknesses. Kinetic modeling is the most direct implementation of the compartment models, but with some tracers accurate input function measurement and good compartment configuration identification can be difficult to obtain. Other methods were designed to overcome some particular vulnerability to error of classical kinetic modeling, but introduced new vulnerabilities in the process. Reference region methods obviate the need for arterial plasma measurement, but are not as robust to violations of the underlying modeling assumptions as methods using the arterial input function. Graphical methods give estimates of V(T) without the requirement of compartment model specification, but provide a biased estimator in the presence of statistical noise. True equilibrium methods are quite robust, but their use is limited to experiments with tracers that are suitable for constant infusion. In conclusion, there is no universally "best" method that is applicable to all neuroreceptor imaging studies, and carefully evaluation of model-based methods is required for each radiotracer.

CPD - education and self-assessment: functional imaging in epilepsy

Functional imaging plays a growing role in the clinical assessment and research investigation of patients with epilepsy. This article reviews the literature on functional MRI (fMRI) investigation of EEG activity, fMRI evaluation of cognitive and motor functions, magnetic resonance spectroscopy (MRS), single photon emission computed tomography (SPECT) and positron emission tomography (PET) in epilepsy. The place of these techniques in clinical evaluation and their contribution to a better neurobiological understanding of epilepsy are discussed.

Modeling of receptor ligand data in PET and SPECT imaging: a review of major approaches

Over the past decade, a number of new kinetic modeling techniques have been developed for PET and SPECT ligands. This article will review commonly used modeling solutions for reversible positron-emission tomography (PET) and single photon emission computed tomography (SPECT) radioligands, with an emphasis on noninvasive methods. All of the modeling approaches in PET and SPECT assume a compartmental system and derive parameters that describe the compartmental system. These parameters will be defined, and their relationship to analogous parameters in pharmacology will be discussed. Then the major approaches are presented under the categories of graphical or mathematical as well as invasive or noninvasive.

Similarity and robustness of PET and SPECT binding parameters for benzodiazepine receptors

The single photon emission computed tomography (SPECT) radiotracer [123I]iomazenil is used to assess benzodiazepine receptor binding parameters. These measurements are relative indices of benzodiazepine receptor concentration (B'max). To evaluate the ability of such indices in accurately accessing the B'max the authors compared them with absolute values of B'max, measured using positron emission tomography (PET). The authors performed SPECT, PET, and magnetic resonance imaging (MRI) studies on a group composed of seven subjects. For SPECT studies, the authors administered a single injection of [123I]iomazenil and estimated the total and specific distribution volumes (DV(T SPECT), DV(S SPECT)) and the binding potential (BP) using unconstrained (BP(SPECT)) and constrained (BP(C SPECT)) compartmental models. For PET studies, the authors used a multiinjection approach with [11C]flumazenil and unlabeled flumazenil to estimate absolute values of receptor concentration, B'max, and some other binding parameters. The authors studied the correlation of different binding parameters with B'max. To study the robustness of the binding parameter measurements at the pixel level, the authors applied a wavelet-based filter to improve signal-to-noise ratio of time-concentration curves, and the calculated kinetic parameters were used to build up parametric images. For PET data, the B'max and the DV(PET) were highly correlated (r = 0.988). This confirms that it is possible to use the DV(PET) to access benzodiazepine receptor density. For SPECT data, the correlation between DV(SPECT) estimated using a two- and three-compartment model was also high (r = 0.999). The DV(T SPECT) and BP(C SPECT) parameters estimated with a constrained three-compartment model or the DV(T''SPECT) parameter estimated with a two-compartment model were also highly correlated to the B'max parameter estimated with PET. Finally, the robustness of the binding parameters allowed the authors to build pixel-by-pixel parametric images using SPECT data.

Equilibrium modeling of 5-HT(2A) receptors with [18F]deuteroaltanserin and PET: feasibility of a constant infusion paradigm

[(18)F]Altanserin has emerged as a promising positron emission tomography (PET) ligand for serotonin-2A (5-HT(2A)) receptors. The deuterium substitution of both of the 2'-hydrogens of altanserin ([(18)F]deuteroaltanserin) yields a metabolically more stable radiotracer with higher ratios of parent tracer to radiometabolites and increased specific brain uptake than [(18)F]altanserin. The slower metabolism of the deuterated analog might preclude the possibility of achieving stable plasma and brain activities with a bolus plus constant infusion within a reasonable time frame for an (18)F-labeled tracer (T(1/2) 110 min). Thus, the purpose of this study was to test the feasibility in human subjects of a constant infusion paradigm for equilibrium modeling of [(18)F]deuteroaltanserin with PET. Seven healthy male subjects were injected with [(18)F]deuteroaltanserin as a bolus plus constant infusion lasting 10 h postinjection. PET acquisitions and venous blood sampling were performed throughout the infusion period. Linear regression analysis revealed that time-activity curves for both specific brain uptake and plasma [(18)F]deuteroaltanserin concentration stabilized after about 5 h. This permitted equilibrium modeling and estimation of V(')(3) (ratio of specific uptake to total plasma parent concentration) and the binding potential V(3) (ratio of specific uptake to free plasma parent concentration). Cortical/cerebellar ratios were increased by 26% relative to those we previously observed with [(18)F]altanserin using similar methodology in a somewhat older subject sample. These results demonstrate feasibility of equilibrium imaging with [(18)F]deuteroaltanserin and suggest that it may be superior to [(18)F]altanserin as a PET radioligand.

Validation and reproducibility of measurement of 5-HT1A receptor parameters with [carbonyl-11C]WAY-100635 in humans: comparison of arterial and reference tisssue input functions

Serotonin 5-HT(1A) receptors are implicated in the pathophysiology of neuropsychiatric conditions. The goal of this study was to evaluate methods to derive 5-HT(1A) receptor parameters in the human brain with positron emission tomography (PET) and [carbonyl-(11)C]WAY 100635. Five healthy volunteer subjects were studied twice. Three methods of analysis were used to derive the binding potential (BP), and the specific to nonspecific equilibrium partition coefficient (k3/k4). Two methods, kinetic analysis based on a three compartment model and graphical analysis, used the arterial plasma time-activity curves as the input function to derive BP and k3/k4. A third method, the simplified reference tissue model (SRTM), derived the input function from uptake data of a region of reference, the cerebellum, and provided only k3/k4. All methods provided estimates of regional 5-HT(1A) receptor parameters that were highly correlated. Results were consistent with the known distribution of 5-HT(1A) receptors in the human brain. Compared with kinetic BP, graphical analysis slightly underestimated BP, and this phenomenon was mostly apparent in small size-high noise regions. Compared with kinetic k3/k4, the reference tissue method underestimated k3/k4 and the underestimation was apparent primarily in regions with high receptor density. Derivation of BP by both kinetic and graphical analysis was highly reliable, with an intraclass correlation coefficient (ICC) of 0.84 +/- 0.14 (mean +/- SD of 15 regions) and 0.84 +/- 0.19, respectively. In contrast, the reliability of k3/k4 was lower, with ICC of 0.53 +/- 0.28, 0.47 +/- 0.28, and 0.55 +/- 0.29 for kinetic, graphical, and reference tissue methods, respectively. In conclusion, derivation of BP by kinetic analysis using the arterial plasma input function appeared as the method of choice because of its higher test-retest reproducibility, lower vulnerability to experimental noise, and absence of bias.

Measurement of striatal and extrastriatal dopamine D1 receptor binding potential with [11C]NNC 112 in humans: validation and reproducibility

To evaluate the postulated role of extrastriatal D1 receptors in human cognition and psychopathology requires an accurate and reliable method for quantification of these receptors in the living human brain. [11C]NNC 112 is a promising novel radiotracer for positron emission tomography imaging of the D1 receptor. The goal of this study was to develop and evaluate methods to derive D1 receptor parameters in striatal and extrastriatal regions of the human brain with [11C]NNC 112. Six healthy volunteers were studied twice. Two methods of analysis (kinetic and graphical) were applied to 12 regions (neocortical, limbic, and subcortical regions) to derive four outcome measures: total distribution volume, distribution volume ratio, binding potential (BP), and specific-to-nonspecific equilibrium partition coefficient (k3/k4). Both kinetic and graphic analyses provided BP and k3/k4 values in good agreement with the known distribution of D1 receptors (striatum > limbic regions = neocortical regions > thalamus). The identifiability of outcome measures derived by kinetic analysis was excellent. Time-stability analysis indicated that 90 minutes of data collection generated stable outcome measures. Derivation of BP and k3/k4 by kinetic analysis was highly reliable, with intraclass correlation coefficients (ICCs) of 0.90+/-0.06 (mean +/- SD of 12 regions) and 0.84+/-0.11, respectively. The reliability of these parameters derived by graphical analysis was lower, with ICCs of 0.72+/-0.17 and 0.58+/-0.21, respectively. Noise analysis revealed a noise-dependent bias in the graphical but not the kinetic analysis. In conclusion, kinetic analysis of [11C]NNC 112 uptake provides an appropriate method with which to derive D1 receptor parameters in regions with both high (striatal) and low (extrastriatal) D1 receptor density.

Applications of SPECT imaging of dopaminergic neurotransmission in neuropsychiatric disorders

Single photon emission computed tomography (SPECT) tracers selective for pre- and post-synaptic targets have allowed measurements of several aspects of dopaminergic (DA) neurotransmission. In this article, we will first review our DA transporter imaging in Parkinson's disease. We have developed the in vivo dopamine transporter (DAT) imaging with [123I]beta-CIT ((1R)-2beta-Carbomethoxy-3beta-(4-iodophenyl)tropane). This method showed that patients with Parkinson's disease have markedly reduced DAT levels in striatum, which correlated with disease severity and disease progression. Second, we applied DA imaging techniques in patients with schizophrenia. Using amphetamine as a releaser of DA, we observed the enhanced DA release, which was measured by imaging D2 receptors with [123I]IBZM (iodobenzamide), in schizophrenics. Further we developed the measurement of basal synaptic DA levels by AMPT (alpha-methyl-paratyrosine)-induced unmasking of D2 receptors. Finally, we expanded our techniques to the measurement of extrastriatal DA receptors using [123I]epidepride. The findings suggest that SPECT is a useful technique to measure DA transmission in human brain and may further our understanding of the pathophysiology of neuropsychiatric disorders.

Kinetic modeling of benzodiazepine receptor binding with PET and high specific activity [(11)C]Iomazenil in healthy human subjects

Quantitation of the PET benzodiazepine receptor antagonist, [(11)C]Iomazenil, using low specific activity radioligand was recently described. The purpose of this study was to quantitate benzodiazepine receptor binding in human subjects using PET and high specific activity [(11)C]Iomazenil. Six healthy human subjects underwent PET imaging following a bolus injection of high specific activity (>100 Ci/mmol) [(11)C]iomazenil. Arterial samples were collected at multiple time points after injection for measurement of unmetabolized total and nonprotein-bound parent compound in plasma. Time activity curves of radioligand concentration in brain and plasma were analyzed using two and three compartment model. Kinetic rate constants of transfer of radioligand between plasma, nonspecifically bound brain tissue, and specifically bound brain tissue compartments were fitted to the model. Values for fitted kinetic rate constants were used in the calculation of measures of benzodiazepine receptor binding, including binding potential (the ratio of receptor density to affinity), and product of BP and the fraction of free nonprotein-bound parent compound (V(3)'). Use of the three compartment model improved the goodness of fit in comparison to the two compartment model. Values for kinetic rate constants and measures of benzodiazepine receptor binding, including BP and V(3)', were similar to results obtained with the SPECT radioligand [(123)I]iomazenil, and a prior report with low specific activity [(11)C]Iomazenil. Kinetic modeling using the three compartment model with PET and high specific activity [(11)C]Iomazenil provides a reliable measure of benzodiazepine receptor binding. Synapse 35:68-77, 2000. Published 2000 Wiley-Liss, Inc.

[123I]IPCIT and [123I]beta-CIT as SPECT tracers for the dopamine transporter: a comparative analysis in nonhuman primates

[123I]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([123I]-CIT) and its isopropylester analog [123I]PCIT, both of which are phenyltropane derivatives of cocaine with high affinity for the dopamine (DA) transporter, were compared using single photon emission computed tomography in nonhuman primates. Although IPCIT is significantly more selective for the DA transporter than beta-CIT, striatal distribution volumes of specifically bound tracer were similar for both tracers. Compartmental modeling results were compared with a simple peak equilibrium method used previously by this group. The peak equilibrium method is shown to overestimate striatal distribution volumes, primarily due to a difference in the calculated time of peak specific uptake.

Quantitation of benzodiazepine receptor binding with PET [11C]iomazenil and SPECT [123I]iomazenil: preliminary results of a direct comparison in healthy human subjects

Although positron emission tomography (PET) and single photon emission computed tomography (SPECT) are increasingly used for quantitation of neuroreceptor binding, almost no studies to date have involved a direct comparison of the two. One study found a high level of agreement between the two techniques, although there was a systematic 30% increase in measures of benzodiazepine receptor binding in SPECT compared with PET. The purpose of the current study was to directly compare quantitation of benzodiazepine receptor binding in the same human subjects using PET and SPECT with high specific activity [11C]iomazenil and [123I]iomazenil, respectively. All subjects were administered a single bolus of high specific activity iomazenil labeled with 11C or 123I followed by dynamic PET or SPECT imaging of the brain. Arterial blood samples were obtained for measurement of metabolite-corrected radioligand in plasma. Compartmental modeling was used to fit values for kinetic rate constants of transfer of radioligand between plasma and brain compartments. These values were used for calculation of binding potential (BP = Bmax/Kd) and product of BP and the fraction of free non-protein-bound parent compound (V3'). Mean values for V3' in PET and SPECT were as follows: temporal cortex 23+/-5 and 22+/-3 ml/g, frontal cortex23+/-6 and 22+/-3 ml/g, occipital cortex 28+/-3 and 31+/-5 ml/g, and striatum 4+/-4 and 7+/-4 ml/g. These preliminary findings indicate that PET and SPECT provide comparable results in quantitation of neuroreceptor binding in the human brain.

PET studies of binding competition between endogenous dopamine and the D1 radiotracer [11C]NNC 756

NNC 756 ((+)-8-chloro-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-2,3,4,5- tetrahydro-1H-3-benzazepine) is a new high affinity dopamine (DA) D1 receptor antagonist. Labeled with C-11, it has been used as a PET radiotracer to visualize D1 receptors both in striatal and extrastriatal areas, such as the prefrontal cortex. The goal of this study was to evaluate several methods for derivation of D1 receptor binding potential (BP) with [11C]NNC 756 in baboons, and to use these methods to assess the vulnerability of [11C]NNC 756 binding to competition by endogenous DA. A three-compartment model provided a good fit to PET data acquired following a single bolus injection. BP values obtained with this analysis were in good agreement with values derived from in vitro studies. BP values measured following injection of the potent DA releaser amphetamine (1 mg/kg, n=2) were similar to values measured under control conditions. Kinetic parameters derived from single bolus experiments were used to design a bolus plus continuous infusion administration protocol aimed at achieving a state of sustained binding equilibrium. Injection of amphetamine during sustained equilibrium did not affect [11C]NNC 756 binding. Similar results were observed with another D1 radiotracer, [11C]SCH 23390. Doses of amphetamine used in this study are known to reduce by 20-40% the binding potential of several D2 receptors radiotracers. Therefore, the absence of displacement of [11C]NNC 756 by an endogenous DA surge may indicate important differences between D1 and D2 receptors in vivo, such as differences in proportion of high affinity states not occupied by DA at baseline. These findings may also imply that a simple binding competition model is inadequate to account for the effects of manipulation of endogenous DA levels on the in vivo binding of radiolabeled antagonists.

Stability of [123I]IBZM SPECT measurement of amphetamine-induced striatal dopamine release in humans

Binding competition between endogenous dopamine (DA) and the D2 receptor radiotracer [123I]IBZM allows measurement of the change in synaptic DA following amphetamine challenge with SPECT in the living human brain. Previous investigations using this technique in healthy subjects have shown that the magnitude of amphetamine effect on [123I]IBZM binding potential (BP) is small (range between 5 to 15% decrease), and that a large between-subject variability in this effect is observed. Therefore, it was unclear how much of the apparent between-subject variability was due to a low signal-to-noise ratio in the measurement, vs. true between-subject differences in the magnitude of the response. The goals of this investigation were to test the within-subject reproducibility and reliability of amphetamine-induced decrease in [123I]IBZM BP with a test/retest paradigm, and to establish the presence or absence of tolerance or sensitization to single administration ofi.v. amphetamine. Six healthy male subjects, never previously exposed to psychostimulants, twice underwent measurement of striatal amphetamine-induced DA release (between-measurement interval 16 +/- 10 days) using SPECT and the [123I]IBZM constant infusion technique. Results demonstrated an excellent within-subject reproducibility of amphetamine-induced DA release: amphetamine-induced decreases in [123I]IBZM BP were significant on each day, and had an intraclass correlation coefficient (ICC) of 0.89. Moreover, values from the second experiment were not significantly different from first experiment, suggesting the absence of either sensitization or tolerance to the effect of amphetamine on DA release in these experimental conditions. The subjective activation, as rated by the subjects on analog scales, was also highly reproducible. In conclusion, this scanning technique provides a reliable measurement of amphetamine-induced reduction of [123I]IBZM BP and enables detection of between-subject differences that appear stable over time.

Pharmacokinetic analysis of benzodiazepine receptor binding of [123I]iomazenil in human brain

PURPOSE

The purpose of this study is to evaluate the central benzodiazepine (BZP) receptor binding of iomazenil (IMZ) by pharmacokinetic analysis and to establish a methodology for the diagnosis of CNS disorders with abnormalities in BZP receptor binding.

METHODS

BZP receptor binding of IMZ was analyzed kinetically using plasma concentration-time profiles and dynamic single photon emission computed tomography (SPECT) data obtained after the intravenous administration of IMZ to patients with neuropsychiatric disease. The analysis was based on a 3-compartmental model including the processes of both blood-brain barrier (BBB) transport and BZP receptor binding.

RESULTS

Hydrolized metabolite of IMZ was detected in plasma, indicating the need for separation by HPLC. The BBB influx clearance and the receptor binding potential of IMZ in the medial temporal region was reduced in the epileptic patient.

CONCLUSIONS

Our findings suggest the possibility of detecting the epileptic focus by using our method.

A method for the quantification of benzodiazepine receptors by using 123I-iomazenil and SPECT with one scan and one blood sampling

Iodine-123-iomazenil (Iomazenil) is a ligand of central type benzodiazepine receptors for single photon emission computed tomography (SPECT). Previously we reported a simple, table look-up method for quantification of its binding potential (BP) by using two SPECT scans and calibrated standard input function with one blood sampling. This method is based on a two-compartment model (K1: influx rate constant; k2: efflux rate constant; Vd (= K1/k2): the total distribution volume corresponding BP), and requires two SPECT scans for calculating both K1 and Vd values. If the K1 value in the two-compartment model can be assumed to be constant, the radioactivity of one SPECT scan at 180 min after injection can be considered to tabulate as a function of Vd for a given K1 value and a given input function, and a table look-up procedure provides the corresponding Vd value. The purpose of this study was to develop a simple, autoradiographic method for quantification of BP by using one SPECT scan and calibrated standard input function with one blood sampling. SPECT studies were performed on 14 patients. A dynamic SPECT scan was initiated following an intravenous bolus injection of Iomazenil. A static SPECT scan was performed at 180 min after the injection. Frequent blood sampling from the brachial artery was performed on all subjects to determine the arterial input function. Simulation studies revealed that errors in calculated Vd values were around +/-10-15% for varied K1 values. A good correlation was observed between total distribution volume values calculated by three-compartment model analysis and those calculated by the present method (r = 0.90), supporting the validity of this method. The present method is simple and applicable for clinical use, and will be able to provide images of BP.

Preclinical evaluation of [123I]R93274 as a SPECT radiotracer for imaging 5-HT2A receptors

Studies in rodents have suggested that the radioiodinated 5-HT2A receptor antagonist [123I]R93274 (123-iodine-N-[(3-p-fluorophenyl-1-propyl)-4-methyl-4-piperidinyl]-4-ami no- 5-iodo-2-methoxybenzamide) (Kd = 0.1 nM) might be a promising single photon emission computerized tomography (SPECT) radiotracer to image 5-HT2A receptors in the living human brain. In this study, we characterized the brain uptake of [123I]R93274 in baboons. Highest brain uptake was observed in cortical areas, while lower uptake was observed in the striatum and the cerebellum. Injection of pharmacological doses of the 5-HT2A receptor antagonist ketanserin resulted in reduction of cortical and striatal radioactivities to the level observed in the cerebellum. Injection of the selective dopamine D2 receptor antagonist raclopride did not affect [123I]R93274 brain uptake. Quantification of 5-HT2A receptors was achieved by measuring the binding potential of 5-HT2A receptors for [123I]R93274 (the binding potential is the product of the density and affinity of available receptors). Regional binding potential values were derived with a three-compartmental kinetic analysis of the time-activity curves in the brain and plasma. Binding potential values of 93 +/- 34 ml/g, 71 +/- 35 ml/g and 31 +/- 11 ml/g were measured in the occipital, temporal and striatal regions, respectively. Similar values were derived using a noncompartmental graphical analysis. These values were in accordance with the in vitro regional distribution of 5-HT2A receptors in primate brain. In conclusion, [123I]R93274 allows visualization and quantification of 5-HT2A receptors in the baboon brain with SPECT.

Delayed image of iodine-123 iomazenil as a relative map of benzodiazepine receptor binding: the optimal scan time

"Delayed" single-photon emission tomograpic (SPET) images after an intravenous bolus injection of iodine-123 iomazenil have been used as a relative map of benzodiazepine receptor binding. We determined the optimal scan time for obtaining such a map and assessed the errors of the map. SPET and blood data from six healthy volunteers and five patients were used. A three-compartment kinetic model was employed in simulation studies and analyses of actual data. The simulation studies suggested that, in the normal brain, the scan time at which a single SPET image best represented the relative receptor binding was 3.0-3.5 h post-injection. This finding was supported by actual data from the volunteers. The simulation studies also suggested that the optimal scan time was not greatly changed by the variability of the input functions, and that the error in the SPET image contrast in the vicinity of the optimal scan time was not increased by changes in the tracer kinetics in the entire brain. The SPET image contrast in the patients at 3.0 h post-injection agreed well with the reference receptor binding estimated by kinetic analysis, with a mean error of 3.6%. These findings support the use of a single SPET image after bolus injection of [123I]iomazenil as a relative map of benzodiazepine receptor binding. For this purpose, a SPET scan time of 3.0-3.5 h post-injection is recommended.

A simple method for the quantification of benzodiazepine receptors using iodine-123 iomazenil and single-photon emission tomography

Iodine-123 iomazenil (Iomazenil) is a ligand for central type benzodiazepine receptors that is suitable for single-photon emission tomography (SPET). The purpose of this study was to develop a simple method for the quantification of its binding potential (BP). The method is based on a two-compartment model (K1, influx rate constant; k2', efflux rate constant; VT' (=K1/k2'), the total distribution volumes relative to the total arterial tracer concentration), and requires two SPET scans and one blood sampling. For a given input function, the radioactivity ratio of the early to delayed scans can be considered to tabulate as a function of k2', and a table look-up procedure provides the corresponding k2' value, from which K1 and VT' values are then calculated. The arterial input function is obtained by calibration of the standard input function by the single blood sampling. SPET studies were performed on 14 patients with cerebrovascular diseases, dementia or brain tumours (mean age+/-SD, 56.0+/-12.2). None of the patients had any heart, renal or liver disease. A dynamic SPET scan was performed following intravenous bolus injection of Iomazenil. A static SPET scan was performed at 180 min after injection. Frequent blood sampling from the brachial artery was performed on all subjects for determination of the arterial input function. Two-compartment model analysis was validated for calculation of the VT' value of Iomazenil. Good correlations were observed between VT' values calculated by three-compartment model analysis and those calculated by the present method, in which the scan time combinations (early scan/delayed scan) used were 15/180 min, 30/180 min or 45/180 min (all combinations: r=0.92), supporting the validity of this method. The present method is simple and applicable for clinical use.

Tracer kinetic modelling of receptor data with mathematical metabolite correction

Quantitation of metabolic processes with dynamic positron emission tomography (PET) and tracer kinetic modelling relies on the time course of authentic ligand in plasma, i.e. the input curve. The determination of the latter often requires the measurement of labelled metabolites, a laborious procedure. In this study we examined the possibility of mathematical metabolite correction, which might obviate the need for actual metabolite measurements. Mathematical metabolite correction was implemented by estimating the input curve together with kinetic tissue parameters. The general feasibility of the approach was evaluated in a Monte Carlo simulation using a two tissue compartment model. The method was then applied to a series of five human carbon-11 iomazenil PET studies. The measured cerebral tissue time-activity curves were fitted with a single tissue compartment model. For mathematical metabolite correction the input curve following the peak was approximated by a sum of three decaying exponentials, the amplitudes and characteristic half-times of which were then estimated by the fitting routine. In the simulation study the parameters used to generate synthetic tissue time-activity curves (K1-k4) were refitted with reasonable identifiability when using mathematical metabolite correction. Absolute quantitation of distribution volumes was found to be possible provided that the metabolite and the kinetic models are adequate. If the kinetic model is oversimplified, the linearity of the correlation between true and estimated distribution volumes is still maintained, although the linear regression becomes dependent on the input curve. These simulation results were confirmed when applying mathematical metabolite correction to the [11C]iomazenil study. Estimates of the distribution volume calculated with a measured input curve were linearly related to the estimates calculated using mathematical metabolite correction with correlation coefficients >0.990. However, the slope of the regression line displayed considerable variability among the subjects (0.33-0.95), demonstrating that absolute quantitation of the distribution volume was impaired. Mathematical metabolite correction is a feasible method and may prove useful in cases where actual metabolite data cannot be obtained. The potential for absolute quantitation seems limited, but the method allows the quantitative assessment of regional ratios of receptor measures.

Carbon-11 and iodine-123 labelled iomazenil: a direct PET-SPET compari son

The benzodiazepine receptor ligand iomazenil was labelled with carbon-11 to allow a direct positron emission tomography/single-photon emission tomography (PET/SPET) comparison with the well-known iodine-123 labelled compound. Imaging showed the same regional distribution for both modalities. Blood sample activity was corrected for metabolites by extraction with chloroform and high-performance liquid chromatographic analysis. Metabolism is very fast: 5min after application more than 85% of the plasma activity is present as hydrophilic metabolites. Kinetic methods were used to obtain regional estimates of transport rate constants and receptor concentrations. A three-compartment model was employed which gave transport rate constants for brain uptake (K1) and the distribution volume for the specifically receptor bound compartment (DVS). K1 varied from 0.32 to 0.50ml/min per gram for the cortical regions, cerebellum, thalamus and striatum for PET and SPET. Mean DVS-PET and DVS-SPET values were, respectively, 23+/-5 and 31+/-5ml/g for the occipital cortex, 11+/-3 and 15+/-2ml/g for the cerebellum, 7+/-2 and 11+/-3ml/g for the thalamus, 5+/-3 and 10+/-3ml/g for the striatum, and 3+/-2 and 3+/-1ml/g for the pons. These values correlated very well individually. The coefficient of variation of the SPET parameters was quite comparable to that of the PET parameters, especially after 180min (PET 90min) study duration. Thus quantitative benzodiazepine receptor information can be obtained from dynamic SPET imaging in the same way as with PET.

Synthesis and PET imaging of the benzodiazepine receptor tracer [N-methyl-11C]iomazenil

The central benzodiazepine receptor tracer [N-methyl-11C]iomazenil (Ro 16-0154) was synthesized by alkylation of the desmethyl precursor noriomazenil with [11C]methyl iodide. The [11C]CH3I (prepared by reduction of [11C]CO2 with LiA1H4 followed by reaction with HI) was reacted with noriomazenil in N,N-dimethylformamide and Bu4N+OH- for 1 min at 80 degrees C and purified by HPLC (C18, 34% CH3CN/H2O 7 mL/min). The product was obtained with synthesis time 35 +/- 5 min (mean +/- SD, n = 7), radiochemical yield (EOB) 36 +/- 16%, radiochemical purity 99 +/- 1%, and specific activity 5100 +/- 2800 mCi/mumol. Absorbed radiation doses were calculated from previously acquired human biodistribution data. The urinary bladder wall received the highest dose (0.099 mGy/MBq) for 4.8 h voiding interval and the effective dose equivalent was 0.015 mSv/MBq. After i.v. injection of [11C]iomazenil in an adult baboon or healthy human volunteer, radioactivity accumulated in the cortex with time-activity curves in agreement with results obtained with [11C]flumazenil PET and [123I]iomazenil SPECT studies. The count rate was sufficient to obtain quantitative images up to 2 h post-injection with a 14 mCi injection. These results suggest that [11C]iomazenil will be a useful agent for measuring benzodiazepine receptors in vivo by positron emission tomography.

Central benzodiazepine receptor imaging and quantitation with single photon emission computerised tomography: SPECT

This review discusses the current use of single photon emission computerised tomography (SPECT) for central benzodiazepine receptor imaging and quantitation. The general principles underlying SPECT imaging and receptor quantitation methods such as the kinetic, pseudo-equilibrium and steady-state (tracer infusion and bolus) approaches are described. The advantages and practical drawbacks of these techniques are highlighted.

Comparison of [123I]beta-CIT and [123I]IPCIT as single-photon emission tomography radiotracers for the dopamine transporter in nonhuman primates

Single-photon emission tomographic (SPET) imaging with the radiotracer [123I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane ([123I]beta-CIT) has been reported to be a useful in vivo measure of dopamine (DA) transporters. However, in addition to its high DA transporter affinity, beta-CIT also binds with high affinity to serotonin (5-HT) transporters. 2 beta-Carboisopropoxy-3 beta-(4-iodophenyl)tropane (IPCIT) has been demonstrated by in vitro studies to have higher selectivity for the DA transporter. We compared [123I]beta-CIT and [123I]IPCIT SPET imaging and plasma metabolite analyses in baboons to evaluate the potential advantages of [123I]IPCIT for quantitative in vivo measurements of DA transporter densities. Both tracers had low levels (2% of total plasma 123I activity) of lipophilic radiolabeled metabolites at 420 min. [123I]IPCIT had significantly higher binding to plasma proteins. The average percent free (nonprotein bound) [123I]beta-CIT and [123I]IPCIT were 52% +/- 7% and 14% +/- 6%, respectively. Region of interest uptake data were normalized by injected dose and body weight. Consistent with the high density of 5-HT transporters in the midbrain and the lower 5-HT transporter affinity of IPCIT, the normalized peak specific midbrain uptake of [123I]beta-CIT (1.7 +/- 0.5) was higher than that of [123I]IPCIT (0.4 +/- 0.2). Consistent with its greater lipophilicity, [123I]IPCIT had higher nonspecific uptake, such that normalized cerebellar uptake of [123I]IPCIT was about twice that of [123I]beta-CIT. The ratio of specific to nonspecific uptake in striatum was greater for [123I]beta-CIT compared to [123I]IPCIT; however, striatal binding potentials and distribution volumes were not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)

Graphical, kinetic, and equilibrium analyses of in vivo [123I] beta-CIT binding to dopamine transporters in healthy human subjects

The in vivo kinetics of the dopamine (DA) transporter probe 123I-labeled 2 beta-carboxymethoxy-3 beta-(4-iodophenyl) tropane ([123I] beta-CIT) in striatum was investigated with single-photon emission computerized tomography (SPECT) in five healthy human subjects. The aim of this study was to derive an adequate measure of the DA transporter density that would not be affected by regional cerebral blood flow or peripheral clearance of the tracer. SPECT data were acquired on the day of injection (day 1) from 0 to 7 h and on the following day (day 2) from 19 to 25 h. Arterial sampling on day 1 was used to measure the input function. Graphical, kinetic, and equilibrium analyses were evaluated. Graphical analysis of day 1 data, with the assumption of negligible dissociation of the tracer-receptor complex (k4 = 0), was found to be blood flow-dependent. A three-compartment kinetic analysis of day 1 data were performed using a three (k4 = 0)- and a four (k4 > 0)-parameter model. The three-parameter model estimated the konBmax product at 0.886 +/- 0.087 min-1. The four-parameter model gave a binding potential (BP) of 476 ml g-1, a value consistent with in vitro measurements. The stability of the regional uptake on day 2 allowed direct measurement of the specific to nonspecific equilibrium partition coefficient (V3" = k3/k4 = 6.66 +/- 1.54). Results of day 1 kinetic analysis and day 2 equilibrium analysis were well correlated among subjects. Simulations indicated that the error associated with the day 2 equilibrium analysis was acceptable for plasma tracer terminal half-lives > 10 h. We propose the equilibrium analysis on day 2 as the method of choice for clinical studies since it does not require multiple scans or the measurement of the arterial plasma tracer concentrations.

In vivo quantification of dopamine D2 receptor parameters in nonhuman primates with [123I]iodobenzofuran and single photon emission computerized tomography

[123I]Iodobenzofuran ([123I]IBF) is a new single photon emission computed tomography (SPECT) tracer for visualization of the dopamine D2 receptors. A tracer constant infusion paradigm was developed to measure the binding potential, density (Bmax) and affinity (KD) of the dopamine D2 receptor in baboons. Three baboons underwent both a single bolus and a constant infusion study. For the single bolus experiment, the striatal binding potential (134 +/- 24 ml g-1, mean +/- S.D.) was derived by kinetic analysis. For the constant infusion experiments, the striatal binding potential (127 +/- 16 ml g-1) was derived by equilibrium analysis. The two sets of experiments thus provided consistent data. Low specific activity constant infusion experiments were performed to measure KD (0.08 nM) and Bmax (12.7 nM). In vitro experiments carried out at 37 degrees C with [125I]IBF on rat striatal homogenate membranes provided results in agreement with the SPECT data. These studies suggested the feasibility of quantitation of dopamine D2 receptor parameters with [123I]IBF SPECT imaging.

SPECT quantification of [123I]iomazenil binding to benzodiazepine receptors in nonhuman primates: II. Equilibrium analysis of constant infusion experiments and correlation with in vitro parameters

In vivo benzodiazepine receptor equilibrium dissociation constant, KD, and maximum number of binding sites, Bmax, were measured by single photon emission computerized tomography (SPECT) in three baboons. Animals were injected with a bolus followed by a constant i.v. infusion of the high affinity benzodiazepine ligand [123I]iomazenil. Plasma steady-state concentration and receptor-ligand equilibrium were reached within 2 and 3 h, respectively, and were sustained for the duration (4-9 h) of the experiments (n = 15). At the end of the experiments, a receptor saturating dose of flumazenil (0.2 mg/kg) was injected to measure nondisplaceable activity. Experiments were carried out at various levels of specific activity, and Scatchard analysis was performed for derivation of the KD (0.59 +/- 0.09 nM) and Bmax (from 126 nM in the occipital region to 68 nM in the striatum). Two animals were killed and [125I]iomazenil Bmax and KD were measured at 22 and 37 degrees C on occipital homogenate membranes. In vitro values of Bmax (114 +/- 33 nM) and 37 degrees C KD (0.66 +/- 0.16 nM) were in good agreement with in vivo values measured by SPECT. This study demonstrates that SPECT can be used to quantify central neuroreceptors density and affinity.