Kinetic modelling of dissolution dynamic nuclear polarisation 13 C magnetic resonance spectroscopy data for analysis of pyruvate delivery and fate in tumours

Dissolution dynamic nuclear polarisation (dDNP) of ¹³ C-labelled pyruvate in magnetic resonance spectroscopy/imaging (MRS/MRSI) has the potential for monitoring tumour progression and treatment response. Pyruvate delivery, its metabolism to lactate and efflux were investigated in rat P22 sarcomas following simultaneous intravenous administration of hyperpolarised ¹³ C-labelled pyruvate (¹³ C₁ -pyruvate) and urea (¹³ C-urea), a nonmetabolised marker. A general mathematical model of pyruvate-lactate exchange, incorporating an arterial input function (AIF), enabled the losses of pyruvate and lactate from tumour to be estimated, in addition to the clearance rate of pyruvate signal from blood into tumour, K_ip , and the forward and reverse fractional rate constants for pyruvate-lactate signal exchange, k_pl and k_lp . An analogous model was developed for urea, enabling estimation of urea tumour losses and the blood clearance parameter, K_iu . A spectral fitting procedure to blood time-course data proved superior to assuming a gamma-variate form for the AIFs. Mean arterial blood pressure marginally correlated with clearance rates. K_iu equalled K_ip , indicating equivalent permeability of the tumour vasculature to urea and pyruvate. Fractional loss rate constants due to effluxes of pyruvate, lactate and urea from tumour tissue into blood (k_po , k_lo and k_uo , respectively) indicated that T₁ s and the average flip angle, θ, obtained from arterial blood were poor surrogates for these parameters in tumour tissue. A precursor-product model, using the tumour pyruvate signal time-course as the input for the corresponding lactate signal time-course, was modified to account for the observed delay between them. The corresponding fractional rate constant, k_avail , most likely reflected heterogeneous tumour microcirculation. Loss parameters, estimated from this model with different TRs, provided a lower limit on the estimates of tumour T₁ for lactate and urea. The results do not support use of hyperpolarised urea for providing information on the tumour microcirculation over and above what can be obtained from pyruvate alone. The results also highlight the need for rigorous processes controlling signal quantitation, if absolute estimations of biological parameters are required.

Synthesis, characterisation and Pickering emulsifier performance of poly(stearyl methacrylate)-poly(N-2-(methacryloyloxy)ethyl pyrrolidone) diblock copolymer nano-objects via RAFT dispersion polymerisation in n-dodecane

A near-monodisperse poly(stearyl methacrylate) macromolecular chain transfer agent (PSMA macro-CTA) was prepared via reversible addition-fragmentation chain transfer (RAFT) solution polymerisation in toluene. This PSMA macro-CTA was then utilised as a stabiliser block for the RAFT dispersion polymerisation of a highly polar monomer, N-2-(methacryloyloxy)ethyl pyrrolidone (NMEP), in n-dodecane at 90 °C. 1H NMR studies confirmed that the rate of NMEP polymerisation was significantly faster than that of a non-polar monomer (benzyl methacrylate, BzMA) under the same conditions. For example, when targeting a PSMA14-PNMEP100 diblock copolymer, more than 99% NMEP conversion was achieved within 30 min, whereas only 19% BzMA conversion was obtained on the same time scale for the corresponding PSMA14-PBzMA100 synthesis. The resulting PSMA-PNMEP diblock copolymer chains underwent polymerisation-induced self-assembly (PISA) during growth of the insoluble PNMEP block to form either spherical micelles, highly anisotropic worms or polydisperse vesicles, depending on the target DP of the PNMEP chains. Systematic variation of this latter parameter, along with the solids content, allowed the construction of a phase diagram which enabled pure morphologies to be reproducibly targeted. Syntheses conducted at 10% w/w solids led to the formation of kinetically-trapped spheres. A monotonic increase in particle diameter with PNMEP DP was observed for such PISA syntheses, with particle diameters of up to 462 nm being obtained for PSMA14-PNMEP960. Increasing the copolymer concentration to 15% w/w solids led to worm-like micelles, while vesicles were obtained at 27.5% w/w solids. High (≥95%) NMEP conversions were achieved in all cases and 3 : 1 chloroform/methanol GPC analysis indicated relatively high blocking efficiencies. However, relatively broad molecular weight distributions (Mw/Mn > 1.50) were observed when targeting PNMEP DPs greater than 150. This indicates light branching caused by the presence of a low level of dimethacrylate impurity. Finally, PSMA14-PNMEP49 spheres were evaluated as Pickering emulsifiers. Unexpectedly, it was found that either water-in-oil or oil-in-water Pickering emulsions could be obtained depending on the shear rate employed for homogenisation. Further investigation suggested that high shear rates lead to in situ inversion of the initial hydrophobic PSMA14-PNMEP49 spheres to form hydrophilic PNMEP49-PSMA14 spheres.

Human Kinetic Modeling of the 5HT6 PET Radioligand 11C-GSK215083 and Its Utility for Determining Occupancy at Both 5HT6 and 5HT2A Receptors by SB742457 as a Potential Therapeutic Mechanism of Action in Alzheimer Disease

Antagonism of 5-hydroxytrypamine-6 (5HT6) receptors is associated with procognitive effects in preclinical species, suggesting a therapeutic potential for this mechanism in Alzheimer disease (AD) and other cognitive diseases. In a phase 2 dose study, SB742457, a novel 5HT6 antagonist, showed increasing procognitive effects in patients with AD as the dose increased, with a procognitive signal in AD patients at a dose of 35 mg/d superior to the other doses tested (5 and 15 mg/d).

METHODS

In this article, we describe the quantification and pharmacologic selectivity of a new 5HT6 PET ligand ((11)C-GSK215083) in healthy volunteers and its use to measure occupancies achieved at various doses of SB742457.

RESULTS

Kinetic analysis of (11)C-GSK215083 uptake in the human brain demonstrated the multilinear model, MA2, to represent the method of choice when a blood input was available and the full tissue reference method when no input was available. Pharmacologic dissection of the in vivo (11)C-GSK215083-specific binding showed the ligand bound mostly the 5HT6 in the striatum (blocked by SB742457 but not by the selective 5-hydroxytryptamine-2A (5HT2A) antagonist ketanserin) and the 5HT2A in the frontal cortex (blocked by both ketanserin and SB742457). Repeated administration of SB742457 (3, 15, and 35 mg/d) saturated the 5HT6 receptors at all doses. In the cortex, 5HT2A receptor occupancy was 24% ± 6% (3 mg/d), 35% ± 4% (15 mg/d), and 58% ± 19% (35 mg/d; mean ± SD), suggesting a progressive engagement of 5HT2A as the dose increased.

CONCLUSION

Collectively, these data support the use of (11)C-GSK215083 as a 5HT6 clinical imaging tool and suggest that blocking both the 5HT6 and the 5HT2A receptors may be required for the optimal therapeutic action of SB742457 in AD.

Are block copolymer worms more effective Pickering emulsifiers than block copolymer spheres?

RAFT-mediated polymerisation-induced self-assembly (PISA) is used to prepare six types of amphiphilic block copolymer nanoparticles which were subsequently evaluated as putative Pickering emulsifiers for the stabilisation of n-dodecane-in-water emulsions. It was found that linear poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA-PHPMA) diblock copolymer spheres and worms do not survive the high shear homogenisation conditions used for emulsification. Stable emulsions are obtained, but the copolymer acts as a polymeric surfactant; individual chains rather than particles are adsorbed at the oil-water interface. Particle dissociation during emulsification is attributed to the weakly hydrophobic character of the PHPMA block. Covalent stabilisation of these copolymer spheres or worms can be readily achieved by addition of ethylene glycol dimethacrylate (EGDMA) during the PISA synthesis. TEM studies confirm that the resulting cross-linked spherical or worm-like nanoparticles survive emulsification and produce genuine Pickering emulsions. Alternatively, stabilisation can be achieved by either replacing or supplementing the PHPMA block with the more hydrophobic poly(benzyl methacrylate) (PBzMA). The resulting linear spheres or worms also survive emulsification and produce stable n-dodecane-in-water Pickering emulsions. The intrinsic advantages of anisotropic worms over isotropic spheres for the preparation of Pickering emulsions are highlighted. The former particles are more strongly adsorbed at similar efficiencies compared to spheres and also enable smaller oil droplets to be produced for a given copolymer concentration. The scalable nature of PISA formulations augurs well for potential applications of anisotropic block copolymer nanoparticles as Pickering emulsifiers.

Acute increases in synaptic GABA detectable in the living human brain: a [¹¹C]Ro15-4513 PET study

The inhibitory γ-aminobutyric acid (GABA) neurotransmitter system is associated with the regulation of normal cognitive functions and dysregulation has been reported in a number of neuropsychiatric disorders including anxiety disorders, schizophrenia and addictions. Investigating the role of GABA in both health and disease has been constrained by difficulties in measuring acute changes in synaptic GABA using neurochemical imaging. The aim of this study was to investigate whether acute increases in synaptic GABA are detectable in the living human brain using the inverse agonist GABA-benzodiazepine receptor (GABA-BZR) positron emission tomography (PET) tracer, [(11)C]Ro15-4513. We examined the effect of 15 mg oral tiagabine, which increases synaptic GABA by inhibiting the GAT1 GABA uptake transporter, on [(11)C]Ro15-4513 binding in 12 male participants using a paired, double blind, placebo-controlled protocol. Spectral analysis was used to examine synaptic α1 and extrasynaptic α5 GABA-BZR subtype availability in brain regions with high levels of [(11)C]Ro15-4513 binding. We also examined the test-retest reliability of α1 and a5-specific [(11)C]Ro15-4513 binding in a separate cohort of 4 participants using the same spectral analysis protocol. Tiagabine administration produced significant reductions in hippocampal, parahippocampal, amygdala and anterior cingulate synaptic α1 [(11)C]Ro15-4513 binding, and a trend significance reduction in the nucleus accumbens. These reductions were greater than test-retest reliability, indicating that they are not the result of chance observations. Our results suggest that acute increases in endogenous synaptic GABA are detectable in the living human brain using [(11)C]Ro15-4513 PET. These findings have potentially major implications for the investigation of GABA function in brain disorders and in the development of new treatments targeting this neurotransmitter system.

Unexpectedly high affinity of a novel histamine H(3) receptor antagonist, GSK239512, in vivo in human brain, determined using PET

BACKGROUND AND PURPOSE

This study aimed to investigate the relationship between the plasma concentration (PK) of the novel histamine H3 receptor antagonist, GSK239512, and the brain occupancy of H(3) receptors (RO) in healthy human volunteers.

EXPERIMENTAL APPROACH

PET scans were obtained after i.v. administration of the H(3) -specific radioligand [(11) C]GSK189254. Each subject was scanned before and after single oral doses of GSK239512, at 4 and 24 h after dose. PET data were analysed by compartmental analysis, and regional RO estimates were obtained by graphical analysis of changes in the total volumes of distribution of the radioligand, followed by a correction for occupancy by the high affinity radioligand. The PK/RO relationship was analysed by a population-modelling approach, using the average PK of GSK239512 during each scan.

KEY RESULTS

Following administration of GSK239512, there was a reduction in the brain uptake of [(11) C]GSK189254 in all regions, including cerebellum. RO at 4 h was higher than at 24 h, and the PK/RO model estimated a PK associated with 50% of RO of 0.0068 ng·mL(-1) . This corresponds to a free concentration of 4.50 × 10(-12 ) M (pK = 11.3).

CONCLUSIONS AND IMPLICATIONS

The affinity of GSK239512 for brain H3 receptors in humans in vivo is much higher than that expected from studies in vitro, and higher than that observed in PET studies in pigs. The study illustrates the utility of carrying out PET studies in humans early in drug development, providing accurate quantification of GSK239512 RO in vivo as a function of time and dose.

Non linear mixed effects analysis in PET PK-receptor occupancy studies

The characterisation of a pharmacokinetic-receptor occupancy (PK-RO) relationship derived from a PET study is typically modelled in a conventional non-linear least squares (NLLS) framework. In the present work, we explore the application of a non-linear mixed effects approach (NLME) and compare this with NLLS estimation (using both naive pooled data and two-stage approaches) in the context of a direct PK-RO relationship described by an Emax model, using simulated data sets. Target and reference tissue time-activity curves were simulated using a two-tissue compartmental model and an arterial plasma input function for a typical PET study (12 subjects in 3 dose groups with 3 scans each). A range of different PET scenarios was considered to evaluate the impact of between-subject variability and reference region availability. The PET outcome measures derived from the simulations were then used to estimate the parameters of the PK-RO model. The performance of the two approaches was compared in terms of parameters estimates (square mean error SME, root mean square error RMSE) and prediction of the exposure-occupancy relationship. In general, both NLME and NLLS estimation methods provided unbiassed and precise population estimates for the Emax model parameters, although a slight bias was observed for the individual-NLLS method due to a few outliers. The increased value of NLME over NLLS was most notable in the estimation of the between-subject variability (BSV), especially in the case of a more complex PK-RO model when no reference region was available (maximum SME and RMSE values related to BSV of EC₅₀ of 27.6% and 86.5% from NLME versus 264.6% and 689.5% from NLLS). Overall, the NLME approach provided a more robust estimation and produced less-biassed estimates of the population means and variances than either the NLLS approach for the simulations considered.

Kinetic modeling of hyperpolarized (13)C pyruvate metabolism in tumors using a measured arterial input function

Mathematical models are required to estimate kinetic parameters of [1-(13)C] pyruvate-lactate interconversion from magnetic resonance spectroscopy data. One- or two-way exchange models utilizing a hypothetical approximation to the true arterial input function (AIF), (e.g. an ideal 'box-car' function) have been used previously. We present a method for direct measurement of the AIF in the rat. The hyperpolarized [1-(13)C] pyruvate signal was measured in arterial blood as it was continuously withdrawn through a small chamber. The measured signal was corrected for T1 relaxation of pyruvate, RF pulses and dispersion of blood in the chamber to allow for the estimation of the direct AIF. Using direct AIF, rather than the commonly used box-car AIF, provided realistic estimates of the rate constant of conversion of pyruvate to lactate, kpl, the rate constant of conversion of lactate to pyruvate klp, the clearance rate constant of pyruvate from blood to tissue, Kip, and the relaxation rate of lactate T1la. Since no lactate signal was present in blood, it was possible to use a simple precursor-product relationship, with the tumor tissue pyruvate time-course as the input for the lactate time-course. This provided a robust estimate of kpl, similar to that obtained using a directly measured AIF.

Radiosynthesis and characterization of 11C-GSK215083 as a PET radioligand for the 5-HT6 receptor

The development of a PET radioligand for imaging 5-hydroxytryptamine (5-HT) 6 receptors in the brain would, for the first time, enable in vivo imaging of this target along with assessment of its involvement in disease pathophysiology. In addition, such a tool would assist in the development of novel drugs targeting the 5-HT6 receptor.

METHODS

On the basis of in vitro data, GSK215083 was identified as a promising 5-HT6 radioligand candidate and was radiolabeled with (11)C via methylation. The in vivo properties of (11)C-GSK215083 were evaluated first in pigs (to investigate brain penetration and specific binding), second in nonhuman primates (to confirm brain penetration, specific binding, selectivity, and kinetics), and third in human subjects (to confirm brain penetration and biodistribution).

RESULTS

(11)C-GSK215083 readily entered the brain in all 3 species, leading to a heterogeneous distribution (striatum > cortex > cerebellum) consistent with reported 5-HT6 receptor densities and distribution determined by tissue-section autoradiography in preclinical species and humans. In vivo saturation studies using escalating doses of GSK215083 in primates demonstrated saturable, dose-dependent binding to the 5-HT6 receptor in the striatum. Importantly, (11)C-GSK215083 also exhibited affinity for the 5-HT2A receptor; however, given the differential localization of these 2 receptors in the central nervous system, the discrete 5-HT6 binding properties of this radioligand were able to be determined.

CONCLUSION

These data demonstrate the utility of (11)C-GSK215083 as a promising PET radioligand for probing the 5-HT6 receptor in vivo in both preclinical and clinical species.

A multi-center randomized proof-of-concept clinical trial applying [¹⁸F]FDG-PET for evaluation of metabolic therapy with rosiglitazone XR in mild to moderate Alzheimer's disease

Here we report the first multi-center clinical trial in Alzheimer's disease (AD) using fluorodeoxyglucose positron emission tomography ([18F]FDG-PET) measures of brain glucose metabolism as the primary outcome. We contrasted effects of 12 months treatment with the PPARγ agonist Rosiglitazone XR versus placebo in 80 mild to moderate AD patients. Secondary objectives included testing for reduction in the progression of brain atrophy and improvement in cognition. Active treatment was associated with a sustained but not statistically significant trend from the first month for higher mean values in Kiindex and CMRgluindex, novel quantitative indices related to the combined forward rate constant for [18F]FDG uptake and to the rate of cerebral glucose utilization, respectively. However, neither these nor another analytical approach recently validated using data from the Alzheimer's Disease Neuroimaging Initiative indicated that active treatment decreased the progression of decline in brain glucose metabolism. Rates of brain atrophy were similar between active and placebo groups and measures of cognition also did not suggest clear group differences. Our study demonstrates the feasibility of using [18F]FDG-PET as part of a multi-center therapeutics trial. It suggests that Rosiglitazone is associated with an early increase in whole brain glucose metabolism, but not with any biological or clinical evidence for slowing progression over a 1 year follow up in the symptomatic stages of AD.

Evaluation of 11C-GSK189254 as a novel radioligand for the H3 receptor in humans using PET

The histamine H(3) receptor is implicated in the pathophysiology of several central nervous system disorders. N-methyl-6-(3-cyclobutyl-2,3,4,5-tetrahydro-1H-benzo[d]azepin-7-yloxy)-nicotamide (GSK189254) is a highly potent, selective, and brain-penetrant H(3) receptor antagonist. Previous studies in the pig using PET have shown that (11)C-GSK189254 uptake in H(3)-rich regions of the brain can be blocked by the selective H(3) antagonist ciproxifan. The purpose of the present study was to evaluate (11)C-GSK189254 as a PET radioligand for human studies and to determine the dose-receptor occupancy relationship of GSK189254 in the human brain.

METHODS

Dynamic PET scans were obtained in healthy subjects over 90 min after intravenous administration of approximately 370 MBq of (11)C-GSK189254. Blood samples were taken throughout the scans to derive the arterial plasma parent input function. Each subject was scanned twice, either with tracer alone (test-retest) or before and after a single oral dose of GSK189254 (10-100 microg). Data were analyzed by compartmental analysis, and regional receptor-occupancy estimates were obtained by graphical analysis of changes in the total volumes of distribution (V(T)) of the radioligand.

RESULTS

(11)C-GSK189254 readily entered the brain; its regional brain distribution reflected the known distribution of H(3) receptors, with high binding in the caudate and putamen, intermediate binding in cortical regions, and low binding in the cerebellum. GSK189254 displayed a high receptor affinity, and a marked reduction in V(T) was apparent at all the doses tested. The oral dose equaling 50% occupancy of the available receptor sites (ED(50)) was estimated as 4.33 microg. Additional data on plasma pharmacokinetics after oral dosing and the plasma free fraction gave a corresponding estimate of the free concentration of GSK189254 required to occupy 50% of the available receptor sites (EC(50)) (0.011 nM). The test-retest data showed reductions in regional V(T) on the second scan in all subjects. A nonlinear compartmental analysis of this effect demonstrated that this reduction was consistent with carryover of a tracer mass dose effect with an estimated in vivo apparent dissociation constant of 0.010 nM, close to the independent estimate of the plasma EC(50).

CONCLUSION

(11)C-GSK189254 can be used to quantify H(3) receptor availability in humans in vivo using PET but requires high specific activity; the possibility of tracer mass dose effects should be carefully analyzed.

Measuring drug occupancy in the absence of a reference region: the Lassen plot re-visited

Quantitative estimation of neuroreceptor occupancy by exogenous drugs using positron emission tomography is based on the reduction in the total volume of distribution (V(T)) of site-specific radioligands after drug administration. An estimate of the distribution volume of free and nonspecifically bound radioligand (V(ND)) is also required to distinguish specific from total binding. However, a true reference region, devoid of specific binding, is often not available. We present a transformation of a graphical method, originally introduced by Lassen, using regional estimates of V(T) alone to determine occupancy, together with an extension that does not require baseline data.

11C-GSK189254: a selective radioligand for in vivo central nervous system imaging of histamine H3 receptors by PET

The histamine H(3) receptor is a G-protein-coupled presynaptic auto- and heteroreceptor whose activation leads to a decrease in the release of several neurotransmitters including histamine, acetycholine, noradrenaline, and dopamine. H(3) receptor antagonists such as 6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254) can increase the release of these neurotransmitters and thus may offer potential therapeutic benefits in diseases characterized by disturbances of neurotransmission. The aim of this study was to synthesize and evaluate (11)C-labeled GSK189254 ((11)C-GSK189254) for imaging the histamine H(3) receptor in vivo by PET.

METHODS

GSK189254 exhibits high affinity (0.26 nM) and selectivity for the human histamine H(3) receptor. Autoradiography experiments were performed using (3)H-GSK189254 to evaluate its in vitro binding in porcine brain tissues. GSK189254 was labeled by N-alkylation using (11)C-methyl iodide in good yields, radiochemical purity, and specific activity. A series of PET experiments was conducted to investigate (11)C-GSK189254 binding in the porcine brain.

RESULTS

In vitro autoradiography demonstrated specific (3)H-GSK189254 binding in the porcine brain; therefore, (11)C-GSK189254 was evaluated in vivo in pigs and showed good brain penetration and high uptake in regions such as the striatum and cortices, known to contain high densities of the histamine H(3) receptors. The radioligand kinetics were reversible, and quantitative analysis was achieved with a 2-tissue-compartmental model yielding the distribution volume as the outcome measure of interest. The distribution volume was reduced to a homogeneous level in all regions after blocking by the coadministration of either unlabeled GSK189254 or ciproxifan, a structurally distinct histamine H(3) antagonist. Further coadministration studies allowed for the estimation of the radioligand affinity (0.1 nM) and the density of histamine H(3) receptor sites in the cerebellum (0.74 nM), cortex (2.05 nM), and striatum (2.65 nM).

CONCLUSION

These findings suggest that (11)C-GSK189254 possesses appropriate characteristics for the in vivo imaging of the histamine H(3) receptor by PET.

The dopaminergic basis of human behaviors: A review of molecular imaging studies

This systematic review describes human molecular imaging studies which have investigated alterations in extracellular DA levels during performance of behavioral tasks. Whilst heterogeneity in experimental methods limits meta-analysis, we describe the advantages and limitations of different methodological approaches. Interpretation of experimental results may be limited by regional cerebral blood flow (rCBF) changes, head movement and choice of control conditions. We revisit our original study of striatal DA release during video-game playing [Koepp, M.J., Gunn, R.N., Lawrence, A.D., Cunningham, V.J., Dagher, A., Jones, T., Brooks, D.J., Bench, C.J., Grasby, P.M., 1998. Evidence for striatal dopamine release during a video game. Nature 393, 266-268] to illustrate the potentially confounding influences of head movement and alterations in rCBF. Changes in [(11)C]raclopride binding may be detected in extrastriatal as well as striatal brain regions-however we review evidence which suggests that extrastriatal changes may not be clearly interpreted in terms of DA release. Whilst several investigations have detected increases in striatal extracellular DA concentrations during task components such as motor learning and execution, reward-related processes, stress and cognitive performance, the presence of potentially biasing factors should be carefully considered (and, where possible, accounted for) when designing and interpreting future studies.

Quantitative estimation of tissue blood flow rate

Tissue blood flow rate (F) is a critical parameter for assessing functional efficiency of a blood vessel network following angiogenesis. This chapter aims to provide the principles behind estimation of F and a practical approach to its determination in laboratory animals using small, readily diffusible, and metabolically inert radiotracers. The methods described require relatively nonspecialized equipment. However, the analytical descriptions apply equally to complementary techniques involving sophisticated noninvasive imaging. Two techniques are described for the quantitative estimation of F using the tissue uptake following intravenous administration of radioactive iodoantipyrine (or other suitable radiotracer). The tissue equilibration technique is the classical approach, and the indicator fractionation technique, which is simpler to perform, is a practical alternative in many cases. The experimental procedures and analytical methods for both techniques are given, as well as guidelines for choosing the most appropriate method.

Gray matter blood flow change is unevenly distributed during moderate isocapnic hypoxia in humans

Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H(2)(15)O positron emission tomography imaging to measure absolute blood flow in 50 regions of interest across the human brain (n = 5) during normoxia and moderate hypoxia. Pco(2) was kept constant ( approximately 44 Torr) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired Po(2) = 70 Torr), mean end-tidal Po(2) fell to 45 +/- 6.3 Torr (means +/- SD). Mean global CBF increased from normoxic levels of 0.39 +/- 0.13 to 0.45 +/- 0.13 ml/g during hypoxia. Increases in regional CBF were not uniform and ranged from 9.9 +/- 8.6% in the occipital lobe to 28.9 +/- 10.3% in the nucleus accumbens. Regions of interest that were better perfused during normoxia generally showed a greater regional CBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g., the putamen, brain stem, thalamus, caudate nucleus, nucleus accumbens, and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath-hold diving, and obstructive sleep apnea, and may have implications for functional brain imaging studies that involve hypoxia.

Consensus nomenclature for in vivo imaging of reversibly binding radioligands

An international group of experts in pharmacokinetic modeling recommends a consensus nomenclature to describe in vivo molecular imaging of reversibly binding radioligands.

Validation of a tracer kinetic model for the quantification of 5-HT(2A) receptors in human brain with [(11)C]MDL 100,907

The positron emission tomography (PET) ligand [(11)C]MDL 100,907 has previously been introduced to image the serotonin 2A (5-HT(2A)) receptor in human brain. The aim of this work was to contribute to the verification of the tracer kinetic modelling in human studies. Five healthy volunteers were scanned twice after intravenous bolus injection of approximately 370 MBq [(11)C]MDL 100,907 using dynamic PET. One scan was performed under baseline condition, the other scan commenced 90 mins after a single oral dose of 30 mg of the antidepressant mirtazapine, which binds to the 5-HT(2A) receptor. There did not appear to be radiolabelled metabolites of [(11)C]MDL 100,907 in human plasma, which are likely to cross the blood-brain barrier. Total volumes of distribution VD in 11 different brain regions were estimated using a reversible, two tissue, four rate constants compartment model with a variable fractional blood volume term and the metabolite-corrected plasma input function. There were no significant changes of the VD in the cerebellum between the baseline and the blocked scans confirming the cerebellum as a region devoid of displaceable binding. Regional estimates of binding potential were then obtained indirectly using the cerebellar VD and occupancies calculated. The mean occupancy with this clinically effective dose of mirtazapine was 60% without significant regional differences. This study confirmed the use of an arterial input kinetic model for the quantification of 5-HT(2A) receptor binding with [(11)C]MDL 100,907 and the use of the cerebellum as a reference region for the free and nonspecific binding.

Characterization of the SPECT 5-HT2A receptor ligand 123I-R91150 in healthy volunteers: Part 1--pseudoequilibrium interval and quantification methods

With the aim of characterizing radioiodinated 4-amino-N-1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]5-iodo-2-methoxybenzamide ((123)I-R91150) as a SPECT ligand for subtype 2A of the 5-hydroxytryptamine receptor (5-HT(2A)), tracer kinetic compartmental analyses were compared with the tissue ratio method (TR). The pseudoequilibrium interval after a single bolus injection was identified, and a reference database of specific uptake ratio (SUR) values was obtained. Within-scan and between-subject variability was also assessed.

METHODS

Nineteen healthy men (mean age +/- SD, 24.4 +/- 3.3 y) were included and separated into 2 groups. Dynamic scans with venous blood sampling from 0 to 470 min after a single bolus injection of (123)I-R91150 was completed for 7 of the 9 subjects included in group A, and in one of them compartmental modeling was performed with an arterial blood input function using 1-tissue-compartment (1TC) and 2-tissue-compartment (2TC) models. Binding potential (BP) using the simplified reference tissue model (SRTM) (BP(SRTM)) and SUR values using TR over time were also calculated. The 10 remaining subjects (group B) underwent a single scan at pseudoequilibrium with the aim of improving the precision of mean normal SUR estimates. Regions of interest in cortical regions and basal ganglia for specific uptake, and in cerebellum for nonspecific uptake, were manually drawn on each subject's MR images and translated to the corresponding SPECT slices after coregistration.

RESULTS

The 1TC model correlated well with the 2TC model (BP(2TC) = 1.04.BP(1TC) - 0.01, R(2) = 0.98), and both methods correlated with BP(SRTM) and SUR with little bias (BP(1TC) = 1.10 BP(SRTM) + 0.03, R(2) = 0.98; BP(2TC) = 1.15 BP(SRTM) + 0.01, R(2) = 0.98; BP(SRTM) = 0.99 SUR(mean) + 0.01, R(2) = 0.98). SUR values stabilized from 180 min after injection in most cortical regions, ranging from 0.51 +/- 0.10 in the orbitofrontal region to 0.27 +/- 0.09 in the parietal region. Within-scan and between-subject variability among regions ranged from 10% to 14.8%, and from 18.3% to 35.4%, respectively.

CONCLUSION

(123)I-R91150 distribution agrees with autoradiography results, showing highly specific binding in cortical regions. The correlations found among 1TC, 2TC, SRTM, and TR outcome measurements support the use of TR for quantification of 5-HT(2A) receptor binding with (123)I-R91150 SPECT and a simple protocol avoiding arterial blood sampling and serial scanning over time.

Characterization of the SPECT 5-HT2A receptor ligand 123I-R91150 in healthy volunteers: part 2--ketanserin displacement

As part of the radioiodinated 4-amino-N-1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]5-iodo-2-methoxybenzamide ((123)I-R91150) characterization study, ketanserin challenges were performed on healthy volunteers with the aim of assessing the specificity of (123)I-R91150 binding to subtype 2A of the 5-hydroxytryptamine receptor (5-HT(2A)), the sensitivity of (123)I-R91150 SPECT in measuring ligand displacement, the relationship between ketanserin plasma concentrations and (123)I-R91150 displacement, and the suitability of the cerebellum as a reference region for quantification.

METHODS

Dynamic SPECT was performed on 6 healthy men (mean age +/- SD, 21 +/- 0.89 y) from the time of (123)I-R91150 injection until 470 min afterward. Ketanserin was administered intravenously at 210 min after injection at 3 doses: 0.1 mg/kg (n = 2), 0.05 mg/kg (n = 2), and 0.015 mg/kg (n = 2). Blood samples for measurement of ketanserin plasma concentrations were drawn. MRI was performed on all subjects and coregistered to the SPECT data for region-of-interest drawing on cortical regions and cerebellum. The simplified reference tissue model (SRTM) was considered the gold standard for quantification, and results were compared with those obtained with the tissue ratio method (TR). The percentage (123)I-R91150 displacement was calculated with both methods as the percentage difference between baseline and postketanserin scans.

RESULTS

Depending on the cerebral regions with the maximum ketanserin dose studied, SRTM and TR mean displacements were 57.1%-95.4% and 71.9%-101.2%, respectively, for the 0.1 mg/kg dose; 51.7%-91.4% and 56.7%-102.8%, respectively, for the 0.05 mg/kg dose; and 7.7%-54.5% and 13.8%-47.0%, respectively, for the lowest dose, 0.015 mg/kg. A good correlation was found between the 2 methods. No ketanserin-induced displacement was observed in the cerebellum time-activity curves, supporting the use of the cerebellum as a reference region. The relationship between displacement and ketanserin plasma concentration fit with a rectangular hyperbola, with a 5.6 ng/mL concentration associated with 50% of the maximum displacement (EC(50)). EC(50) values calculated using occupancies derived both with SRTM and with TR were in good agreement.

CONCLUSION

(123)I-R91150 SPECT is sensitive enough to measure ketanserin dose-dependent displacement in cerebral regions rich in 5-HT(2A) receptors. These results support the selectivity of (123)I-R91150 for 5-HT(2A) receptors and its use as a SPECT ligand for measurements of drug-induced 5-HT(2A) receptor occupancy in humans.

GABA-benzodiazepine receptor function in alcohol dependence: a combined 11C-flumazenil PET and pharmacodynamic study

RATIONALE

Gamma-aminobutyric acid (GABA)-benzodiazepine receptor function is hypothesised to be reduced in alcohol dependence.

OBJECTIVES

We used positron emission tomography (PET) with [11C]flumazenil, a non-selective tracer for brain GABA-benzodiazepine (GABA-BDZ) receptor binding, to determine in vivo the relationship between BDZ receptor occupancy by an agonist, midazolam, and its functional effects.

METHODS

Abstinent male alcohol dependent subjects underwent [11C]flumazenil PET to measure occupancy of BDZ receptors by midazolam whilst recording its pharmacodynamic effects on behavioural and physiological measures. Rate constants describing the exchange of [11C]flumazenil between the plasma and brain compartments were derived from time activity curves.

RESULTS

A 50% reduction in electroencephalography (EEG)-measured sleep time was seen in the alcohol dependent group despite the same degree of occupancy by midazolam as seen in the control group. The effects of midazolam on other measures of benzodiazepine receptor function, increasing EEG beta1 power and slowing of saccadic eye movements, were similar in the two groups. No differences in midazolam or flumazenil metabolism were found between the groups.

CONCLUSIONS

In summary, our study suggests that alcohol dependence in man is associated with a reduced EEG sleep response to the benzodiazepine agonist, midazolam, which is not explained by reduced BDZ receptor occupancy, and is consistent with reduced sensitivity in this measure of GABA-BDZ receptor function in alcohol dependence. The lack of change in other functional measures may reflect a differential involvement of particular subtypes of the GABA-BDZ receptor.

Behaviour of [11C]R(-)- and [11C]S(+)-rolipram in vitro and in vivo, and their use as PET radiotracers for the quantificative assay of PDE4

Cyclic AMP (cAMP) is a continually produced nucleotide which is inactivated by hydrolysis to 5'AMP via phosphodiesterase 4 (PDE4) enzymes. Rolipram is a selective PDE4 inhibitor which exists in two enantiomeric forms, R(-) and S(+). Both of these enantiomers have previously been labelled with carbon-11 and used as positron emission tomography (PET) ligands for measuring PDE4 expression and function, and indirectly to explore the function of the cAMP second messenger, in vivo, using PET. The aim of these studies was to relate the in vitro affinities of the two rolipram enantiomers using standard pharmacological assays with the in vivo behaviour of the two enantiomers using PET. In vitro competition assays were performed using rat cortical membranes and [(3)H]R(-)- and [(3)H]S(+)-rolipram with increasing concentrations of either unlabelled R(-)- or S(+)-rolipram. In vivo, a series of PET studies were performed in the porcine brain using [(11)C]R(-)-rolipram with co-administration of increasing doses of either unlabelled R(-)- or S(+)-rolipram. Additional in vivo PET studies were performed using [(11)C]S(+)-rolipram with saturating doses of rolipram. In all studies, R(-)-rolipram exhibited a higher affinity for the PDE4 enzyme than S(+)-rolipram. The calculated affinity ratios were 7.97 from the in vitro studies; 12.5 from the in vivo studies using [(11)C]R(-)-rolipram; and 14.7 from the in vivo studies using [(11)C]S(+)-rolipram. To conclude, the in vitro affinities of R(-)- and S(+)-rolipram predict their apparent in vivo behaviour in the porcine brain, as measured by PET.

Quantification in positron emission tomography for research in pharmacology and drug development

Positron emission tomography (PET) is a quantitative in vivo tracer technique, enabling images of the distribution of biochemical, physiological and pharmacological functions in living tissue, at a resolution of a few millimetres. Applications include the imaging of blood flow rate, metabolic rate and neuroreceptor distribution and function. These applications are playing an increasing role in drug development. This brief article seeks to emphasize how these applications of PET need to rest on a solid quantitative foundation.

PET studies in drug development: Methodological considerations

Positron emission tomography (PET), as an in vivo pharmacological imaging tool in experimental medicine, is playing an increasing role in drug development. There are two areas where PET is particularly useful in this respect, namely biodistribution and drug occupancy studies. Radiotracer design, the properties of the molecular targets which can be studied and the quantitative estimation of pharmacological endpoints will be discussed in relation to these applications, with particular reference to studies in brain.:

On the logic of hypothesis testing in functional imaging

Statistics is nowadays the customary language of functional imaging. It is common to express an experimental setting as a set of null hypotheses over complex models and to present results as maps of p-values derived from sophisticated probability distributions. However, the growing interest in the development of advanced statistical algorithms is not always paralleled by similar attention to how these techniques may regiment the ways in which users draw inferences from their data. This article investigates the logical bases of current statistical approaches in functional imaging and probes their suitability to inductive inference in neuroscience. The frequentist approach to statistical inference is reviewed with attention to its two main constituents: Fisherian "significance testing" and Neyman-Pearson "hypothesis testing". It is shown that these conceptual systems, which are similar in the univariate testing case, dissociate into two quite different methods of inference when applied to the multiple testing problem, the typical framework of functional imaging. This difference is explained with reference to specific issues, like small volume correction, which are most likely to generate confusion in the practitioner. Further insight into this problem is achieved by recasting the multiple comparison problem into a multivariate Bayesian formulation. This formulation introduces a new perspective where the inferential process is more clearly defined in two distinct steps. The first one, inductive in form, uses exploratory techniques to acquire preliminary notions on the spatial patterns and the signal and noise characteristics. The (smaller) set of likely spatial patterns generated is then tested with newer data and a more rigorous multiple hypothesis testing technique (deductive step).

Parametrically defined cerebral blood vessels as non-invasive blood input functions for brain PET studies

A non-invasive alternative to arterial blood sampling for the generation of a blood input function for brain positron emission tomography (PET) studies is presented. The method aims to extract the dimensions of the blood vessel directly from PET images and to simultaneously correct the radioactivity concentration for partial volume and spillover. This involves simulation of the tomographic imaging process to generate images of different blood vessel and background geometries and selecting the one that best fits, in a least-squares sense, the acquired PET image. A phantom experiment was conducted to validate the method which was then applied to eight subjects injected with 6-[18F]fluoro-L-DOPA and one subject injected with [11C]CO-labelled red blood cells. In the phantom study, the diameter of syringes filled with an 11C solution and inserted into a water-filled cylinder were estimated with an accuracy of half a pixel (1 mm). The radioactivity concentration was recovered to 100 +/- 4% in the 8.7 mm diameter syringe, the one that most closely approximated the superior sagittal sinus. In the human studies, the method systematically overestimated the calibre of the superior sagittal sinus by 2-3 mm compared to measurements made in magnetic resonance venograms on the same subjects. Sources of discrepancies related to the anatomy of the blood vessel were found not to be fundamental limitations to the applicability of the method to human subjects. This method has the potential to provide accurate quantification of blood radioactivity concentration from PET images without the need for blood samples, corrections for delay and dispersion, co-registered anatomical images, or manually defined regions of interest.

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.

Rank-shaping regularization of exponential spectral analysis for application to functional parametric mapping

Compartmental models are widely used for the mathematical modelling of dynamic studies acquired with positron emission tomography (PET). The numerical problem involves the estimation of a sum of decaying real exponentials convolved with an input function. In exponential spectral analysis (SA), the nonlinear estimation of the exponential functions is replaced by the linear estimation of the coefficients of a predefined set of exponential basis functions. This set-up guarantees fast estimation and attainment of the global optimum. SA, however, is hampered by high sensitivity to noise and, because of the positivity constraints implemented in the algorithm, cannot be extended to reference region modelling. In this paper, SA limitations are addressed by a new rank-shaping (RS) estimator that defines an appropriate regularization over an unconstrained least-squares solution obtained through singular value decomposition of the exponential base. Shrinkage parameters are conditioned on the expected signal-to-noise ratio. Through application to simulated and real datasets, it is shown that RS ameliorates and extends SA properties in the case of the production of functional parametric maps from PET studies.

Analysis of D2 dopamine receptor occupancy with quantitative SPET using the high-affinity ligand [123I]epidepride: resolving conflicting findings

Recent studies of limbic cortical dopamine D(2) receptor occupancy by clozapine using high-affinity PET and SPET radioligands have produced conflicting findings. It has been suggested that these divergent findings are due to between-study differences in the method used to estimate D(2) receptor-binding potential. We compared different methods for estimating striatal and temporal cortical D(2) receptor occupancy with high-affinity tracers. In vivo experimental SPET data, obtained with [(123)I]epidepride were analysed with reference tissue kinetic modeling and with the ratio method, applied to data corresponding to short (60 min) and long (240 min) acquisition times. Dopamine D(2) receptor occupancy by the atypical antipsychotic drug risperidone was evaluated. Simulation experiments were also performed, comparing occupancy values obtained for different receptor densities in relation to different data acquisition times. The simulation results revealed that previously published data regarding errors in occupancy estimation by analysis of time activity data acquired for 60 min cannot be extrapolated to studies performed over 240 min. The ratio method provided accurate temporal cortical D(2) receptor occupancy values when applied to data from a late time period, but underestimated the occupancy with earlier data. In striatum, both the late data ratio method and reference tissue kinetic modeling using all data underestimated D(2) receptor occupancy. However, more accurate analyses of striatal D(2) occupancy still showed selective limbic/cortical occupancy by risperidone. Our results substantiate the previous [(123)I]epidepride findings of high temporal cortical occupancy by other atypical antipsychotic drugs and suggest that a potential source of conflicting findings might be short scanning times imposed by [(11)C]FLB 457, leading to underestimation of temporal cortical D(2) receptor occupancy by this method.

Kinetic modelling of [123I]CNS 1261--a potential SPET tracer for the NMDA receptor

N-(1-napthyl)-N'-(3-[(123)I]-iodophenyl)-N-methylguanidine ([(123)I]CNS 1261) is a novel SPET ligand developed for imaging the NMDA receptor intra-channel MK 801/PCP/ketamine site. Data was acquired in 7 healthy volunteers after bolus injection of [(123)I]CNS 1261. Kinetic modeling showed reversible tracer binding. Arterial and venous time-activity curves overlapped after 90 min. The rank order of binding was: Thalamus > striatum > cortical regions > white matter. This distribution concurs with [(11)C]-ketamine and [(18)F]-memantine PET studies. These data provide a methodological basis for further direct in vivo challenge studies.

On the undecidability among kinetic models: from model selection to model averaging

This article deals with the problem of model selection for the mathematical description of tracer kinetics in nuclear medicine. It stems from the consideration of some specific data sets where different models have similar performances. In these situations, it is shown that considerate averaging of a parameter's estimates over the entire model set is better than obtaining the estimates from one model only. Furthermore, it is also shown that the procedure of averaging over a small number of "good" models reduces the "generalization error," the error introduced when the model selected over a particular data set is applied to different conditions, such as subject populations with altered physiologic parameters, modified acquisition protocols, and different signal-to-noise ratios. The method of averaging over the entire model set uses Akaike coefficients as measures of an individual model's likelihood. To facilitate the understanding of these statistical tools, the authors provide an introduction to model selection criteria and a short technical treatment of Akaike's information-theoretic approach. The new method is illustrated and epitomized by a case example on the modeling of [11C]flumazenil kinetics in the brain, containing both real and simulated data.

A linear wavelet filter for parametric imaging with dynamic PET

This paper describes a new filter for parametric images obtained from dynamic positron emission tomography (PET) studies. The filter is based on the wavelet transform following the heuristics of a previously published method that are here developed into a rigorous theoretical framework. It is shown that the space-time problem of modeling a dynamic PET sequence reduces to the classical one of estimation of a normal multivariate vector of independent wavelet coefficients that, under least-squares risk, can be solved by straightforward application of well established theory. From the study of the distribution of wavelet coefficients of PET images, it is inferred that a James-Stein linear estimator is more suitable for the problem than traditional nonlinear procedures that are incorporated in standard wavelet filters. This is confirmed by the superior performance of the James-Stein filter in simulation studies compared to a state-of-the-art nonlinear wavelet filter and a nonstationary filter selected from literature. Finally, the formal framework is interpreted for the practitioner's point of view and advantages and limitations of the method are discussed.

Positron emission tomography compartmental models: a basis pursuit strategy for kinetic modeling

A kinetic modeling approach for the quantification of in vivo tracer studies with dynamic positron emission tomography (PET) is presented. The approach is based on a general compartmental description of the tracer's fate in vivo and determines a parsimonious model consistent with the measured data. The technique involves the determination of a sparse selection of kinetic basis functions from an overcomplete dictionary using the method of basis pursuit denoising. This enables the characterization of the systems impulse response function from which values of the systems macro parameters can be estimated. These parameter estimates can be obtained from a region of interest analysis or as parametric images from a voxel-based analysis. In addition, model order estimates are returned that correspond to the number of compartments in the estimated compartmental model. Validation studies evaluate the methods performance against two preexisting data led techniques, namely, graphical analysis and spectral analysis. Application of this technique to measured PET data is demonstrated using [11C]diprenorphine (opiate receptor) and [11C]WAY-100635 (5-HT1A receptor). Although the method is presented in the context of PET neuroreceptor binding studies, it has general applicability to the quantification of PET/SPECT radiotracer studies in neurology, oncology, and cardiology.

Positron emission tomography partial volume correction: estimation and algorithms

Partial volume effects in positron emission tomography (PET) lead to quantitative under- and over-estimations of the regional concentrations of radioactivity in reconstructed images and corresponding errors in derived functional or parametric images. The limited resolution of PET leads to "tissue-fraction" effects, reflecting underlying tissue heterogeneity, and "spillover" effects between regions. Addressing the former problem in general requires supplementary data, for example, coregistered high-resolution magnetic resonance images, whereas the latter effect can be corrected for with PET data alone if the point-spread function of the tomograph has been characterized. Analysis of otherwise homogeneous region-of-interest data ideally requires a combination of tissue classification and correction for the point-spread function. The formulation of appropriate algorithms for partial volume correction (PVC) is dependent on both the distribution of the signal and the distribution of the underlying noise. A mathematical framework has therefore been developed to accommodate both of these factors and to facilitate the development of new PVC algorithms based on the description of the problem. Several methodologies and algorithms have been proposed and implemented in the literature in order to address these problems. These methods do not, however, explicitly consider the noise model while differing in their underlying assumptions. The general theory for estimation of regional concentrations, associated error estimation, and inhomogeneity tests are presented in a weighted least squares framework. The analysis has been validated using both simulated and real PET data sets. The relations between the current algorithms and those published previously are formulated and compared. The incorporation of tensors into the formulation of the problem has led to the construction of computationally rapid algorithms taking into account both tissue-fraction and spillover effects. The suitability of their application to dynamic and static images is discussed.

Imaging the GABA-benzodiazepine receptor subtype containing the alpha5-subunit in vivo with [11C]Ro15 4513 positron emission tomography

There is evidence of marked variation in the brain distribution of specific subtypes of the GABA-benzodiazepine receptor and that particular subtypes mediate different functions. The alpha5-containing subtype is highly expressed in the hippocampus, and selective alpha5 inverse agonists (which decrease tonic GABA inhibition) are being developed as potential memory-enhancing agents. Evidence for such receptor localization and specialization in humans in vivo is lacking because the widely used probes for imaging the GABA-benzodiazepine receptors, [11C]flumazenil and [123I]iomazenil, appear to reflect binding to the alpha1 subtype, based on its distribution and affinity of flumazenil for this subtype. The authors characterized for positron emission tomography (PET) a radioligand from Ro15 4513, the binding of which has a marked limbic distribution in the rat and human brain in vivo. Competition studies in vivo in the rat revealed that radiolabeled Ro15 4513 uptake was reduced to nonspecific levels only by drugs that have affinity for the alpha5 subtype (flunitrazepam, RY80, Ro15 4513, L655,708), but not by the alpha1 selective agonist, zolpidem. Quantification of [11C]Ro15 4513 PET was performed in humans using a metabolite-corrected plasma input function. [11C]Ro15 4513 uptake was relatively greater in limbic areas compared with [11C]flumazenil, but lower in the occipital cortex and cerebellum. The authors conclude that [11C]Ro15 4513 PET labels in vivo the GABA-benzodiazepine receptor containing the alpha5 subtype in limbic structures and can be used to further explore the functional role of this subtype in humans.

In vivo metabolism and partitioning of 6-[18F]fluoro-L-meta-tyrosine in whole blood: a unified compartment model

Physiological quantification of dynamic PET data requires the determination of an input function, preferably from plasma. A compartmental model relating a parent radiotracer, its radiolabelled metabolites and their exchange between plasma and erythrocytes is presented. This model allows for the time course of radioactivity measured in whole blood to be transformed into the time course of the radiotracer in plasma. The utility of this approach is illustrated with blood data collected on 30 human subjects injected with 6-[18F]fluoro-L-meta-tyrosine (FmT), a pre-synaptic dopaminergic radiotracer. A three-compartment four-parameter model is shown to yield significantly better fits to the blood data than related lower and higher order models. This model is found to be robust to measurement noise, and yet sensitive to metabolic changes induced by pretreatment with carbidopa. For FmT, the between-subject variations are shown to be small enough to warrant the use of a population-based correction; tissue time-activity curves were simulated to verify that this correction does not significantly affect the precision and accuracy of the derived rate constants. The unified blood model can be adapted for radiotracers other than FmT as long as the blood partition ratio of the parent radiotracer differs from that of its metabolites and/or the rate at which they equilibrate between plasma and erythrocytes is different.

Implementation and application of a brain template for multiple volumes of interest

We present a region template and a protocol for transforming that template to define anatomical volumes of interest (VOIs) in the human brain without operator intervention, based on software contained in the SPM99 package (Statistical Parametric Mapping, Wellcome Department of Cognitive Neurology, London, UK). We used an MRI of a reference brain to create an anatomical template of 41 VOIs, covering the entire brain, that can be spatially transformed to fit individual brain scans. Modified software allows for the reslicing and adaptation of the transformed template to any type of coregistered functional data. Individually defined VOIs can be added. We present an assessment of the necessary spatial transformations and compare results obtained for scans acquired in two different orientations. To evaluate the spatial transformations, 11 landmarks distributed throughout the brain were chosen. Euclidean distances between repeat samples at each landmark were averaged across all landmarks to give a mean difference of 1.3 plus minus 1.0 mm. Average Euclidean distances between landmarks (MRI:transformed template) were 8.1 plus minus 3.7 mm in anterior-posterior commissure (ACPC) and 7.6 plus minus 3.7 mm in temporal lobe (TL) orientation. In this study, we use [(11)C]-flumazenil-(FMZ-)PET as an example for the application of the region template. Thirty-four healthy volunteers were scanned, 21 in standard ACPC orientation, 13 in TL orientation. All had high resolution MRI and FMZ-PET. The average coefficient of variation (CV) of FMZ binding for cortical regions was 0.15, comparable with CVs from manually defined VOIs. FMZ binding was significantly different in 6/19 anatomical areas in the control groups obtained in the different orientations, probably due to anisotropic voxel dimensions. This new template allows for the reliable and fast definition of multiple VOIs. It can be used for different imaging modalities and in different orientations. It is necessary that imaging data for groups compared are acquired in the same orientation.

Central benzodiazepine receptors in malformations of cortical development: A quantitative study

We calculated [(11)C]flumazenil volume of distribution ([(11)C]FMZ-V(d)) after correction for partial volume effect in 10 patients with malformations of cortical development (MCDs) and partial seizures, to quantify the GABA(A)-central benzodiazepine receptor complex. Abnormal grey matter and adjacent or overlying cortex were outlined individually and added to an individualized anatomical template for correction for partial volume effect. Nine of 10 patients showed single or multiple increases or decreases in [(11)C]FMZ-V(d) in or around MCDs. Two of three patients with band heterotopia showed multiple increases in the overlying cortex. In three of four patients with subependymal nodular heterotopia, nodules had lower [(11)C]FMZ-V(d) than the overlying cortex, which was normal. Decreases in [(11)C]FMZ-V(d) were found in two of three clefts and one of six adjacent regions in one schizencephalic patient; another had normal [(11)C]FMZ-V(d) in the thickened cortex itself but increases in all adjacent regions. Binding was reduced within focal cortical dysplasia but increased in adjacent cortex. [(11)C]FMZ-V(d) was normal within one patient's polymicrogyric cortex but increased in one of six adjacent volumes of interest. The localization of abnormalities correlated with EEG and clinical data in cortical MCDs. Flumazenil binding was decreased in some MCDs with increased grey matter volume and increased in some adjacent or overlying areas of normal-appearing cortex, suggesting functional abnormalities beyond MRI- detectable structural changes.

Extraction of 5-fluorouracil by tumor and liver: a noninvasive positron emission tomography study of patients with gastrointestinal cancer

Tumor and normal tissue pharmacokinetics of 5-Fluorouracil (5-FU) in patients can be determined with positron emission tomography scanning. However, the data obtained are of limited value because of the inability to distinguish catabolites (inactive species) from parent 5-FU and anabolites (cytotoxic species). In this paper, we have blocked 5-FU catabolism in one arm of a paired study with eniluracil, an inactivator of dihydropyrimidine dehydrogenase, enabling catabolite correction and calculation of tissue pharmacokinetic parameters to be achieved. Using this novel approach, we report for the first time that the net clearance of 5-[(18)F]FU from plasma into tumors (liver metastases and pancreatic tumor) of patients is low (K(I) = 0.0033 +/- 0.0005 ml plasma/ml tissue/min). In contrast, the initial (up to 10 min) clearance through catabolism in liver was high (K(I) = 0.7313 +/- 0.092 ml plasma/ml tissue/min). In the absence of eniluracil, catabolites in tumors accounted for 83% of total tumor exposure (range, 66-91%), whereas catabolites in liver accounted for 96% of total liver exposure (range, 94-98%). This study provides definitive evidence that the cytotoxicity of 5-FU in patients with gastrointestinal cancer could be compromised by its intrinsically low uptake by tumors, as well as decreased systemic availability through hepatic catabolism.

Positron emission tomography compartmental models

The current article presents theory for compartmental models used in positron emission tomography (PET). Both plasma input models and reference tissue input models are considered. General theory is derived and the systems are characterized in terms of their impulse response functions. The theory shows that the macro parameters of the system may be determined simply from the coefficients of the impulse response functions. These results are discussed in the context of radioligand binding studies. It is shown that binding potential is simply related to the integral of the impulse response functions for all plasma and reference tissue input models currently used in PET. This article also introduces a general compartmental description for the behavior of the tracer in blood, which then allows for the blood volume-induced bias in reference tissue input models to be assessed.

Neocortical abnormalities of [11C]-flumazenil PET in mesial temporal lobe epilepsy

OBJECTIVE

To characterize abnormalities in neocortical central benzodiazepine receptor (cBZR) binding in patients with mesial temporal lobe epilepsy (mTLE) with unilateral hippocampal sclerosis (HS) using [(11)C]-flumazenil-(FMZ) PET and complementary voxel-based and quantitative volume-of-interest (VOI) methods.

METHODS

The authors studied 13 control subjects and 15 patients with refractory mTLE and unilateral HS with [(11)C]-FMZ PET. Data were corrected for partial volume effect in the interactively outlined hippocampus and in 28 cortical VOI using an individualized template. A voxel-based analysis was also performed using statistical parametric mapping (SPM).

RESULTS

Fourteen patients with mTLE had reduced [(11)C]-FMZ volume distribution (V(d)) in the hippocampus ipsilateral to the EEG focus, extending into the amygdala in four. Five patients showed additional significant neocortical abnormalities of [(11)C]-FMZ binding: temporal neocortical increases (1), extratemporal decreases (2), extratemporal increases only (1), and temporal and extratemporal neocortical increases (1). Group VOI analysis revealed significant reductions only in the ipsilateral hippocampus. SPM showed decreased [(11)C]-FMZ-V(d) in the ipsilateral hippocampus in 13 of 15 patients, extending into the amygdala in eight. Five patients showed additional neocortical abnormalities: temporal neocortical increases only (3), extratemporal decreases (1), or both temporal neocortical and extratemporal decreases (1). Group analysis showed significant reductions in the ipsilateral hippocampus only.

CONCLUSIONS

A combination of VOI- and voxel-based analysis of [(11)C]-FMZ PET detected extrahippocampal changes of cBZR binding in eight of 15 patients with mTLE due to HS. The finding of abnormalities in patients who were thought to have unilateral HS only based on MRI suggests that more widespread abnormalities are present in HS.