Susan P. Hume

Simplified reference tissue model for PET receptor studies.

The reference tissue model allows for quantification of receptor kinetics without measuring the arterial input function, thus avoiding arterial cannulation and time-consuming metabolite measurements. The model contains four parameters, of which the binding potential (BP) is the parameter of interest. Although BP is robust, convergence rates are slow and the other parameters can have large standard errors. To overcome this problem, a simplified reference tissue containing only three parameters was developed. This new three-parameter model was compared with the previous four-parameter model using a variety of PET studies: [11C]SCH 23390 (D1 receptor) and [11C]raclopride (D2 receptor) in humans, and [11C]SCH 23390, [11C]raclopride and [11C]RTI-121 (dopamine transporter) in rats. The BP values obtained from both models were essentially the same for all cases. In addition, the three-parameter model was insensitive to starting values, produced stable results for the other parameters (small standard errors), and converged rapidly. In conclusion, for the ligands tested the three-parameter model is a better choice, combining increased convergence rate with increased stability.

Parametric imaging of ligand-receptor binding in PET using a simplified reference region model.

A method is presented for the generation of parametric images of radioligand-receptor binding using PET. The method is based on a simplified reference region compartmental model, which requires no arterial blood sampling, and gives parametric images of both the binding potential of the radioligand and its local rate of delivery relative to the reference region. The technique presented for the estimation of parameters in the model employs a set of basis functions which enables the incorporation of parameter bounds. This basis function method (BFM) is compared with conventional nonlinear least squares estimation of parameters (NLM), using both simulated and real data. BFM is shown to be more stable than NLM at the voxel level and is computationally much faster. Application of the technique is illustrated for three radiotracers: [11C]raclopride (a marker of the D2 receptor), [11C]SCH 23390 (a marker of the D1 receptor) in human studies, and [11C]CFT (a marker of the dopamine transporter) in rats. The assumptions implicit in the model and its implementation using BFM are discussed.

Tracer Kinetic Modeling of the 5-HT1AReceptor Ligand [carbonyl-11C]WAY-100635 for PET

[Carbonyl-11C]WAY-100635 is a promising PET radioligand for the 5-HT1A receptor, having demonstrated more favorable characteristics for in vivo imaging than the previously available [O-methyl-11C]WAY-100635. The current study evaluates different tracer kinetic modelling strategies for the quantification of 5-HT1A receptor binding in human brain. Mathematical modelling of the carbonyl-labeled radiotracer is investigated using compartmental structures, including both plasma input and reference tissue approaches. Furthermore, the application of basis function methods allows for the investigation of parametric imaging, providing functional maps of both delivery and binding of the radioligand. Parameter estimates of binding from normal volunteers indicate a low intra- versus a high intersubject variability. It is concluded that a simplified reference tissue approach may be used to quantify 5-HT1A binding either in terms of ROI data or as parametric images.

Exquisite delineation of 5-HT1A receptors in human brain with PET and [carbonyl-11C]WAY-100635

The 5-HT1A receptor antagonist, WAY-100635 [N-(2-(4-(2-methoxyphenyl)- 1-piperazinyl)ethyl)-N-(2-pyridyl) cyclohexanecarboxamide], was labelled in its carbonyl group with carbon-11 (t1/2 = 20.4 min), injected intravenously into healthy male volunteers and studied with positron emission tomography (PET). The acquired data provide exquisite delineation of 5-HT1A receptors in brain, with the ratio of radioactivity uptake in receptor-rich regions, such as medial temporal cortex, to that in receptor-devoid cerebellum reaching 25 by 60 min after radioligand injection. Application of biomathematical modelling to the data revealed high values (7.8) for binding potential, a measure of Bmax/Kp, in receptor-rich regions. Only very polar radioactive metabolites were present in plasma, a finding consistent with the low level of nonspecific binding seen in cerebellum. [carbonyl-11C]WAY-100635 is concluded to be far superior to the previously reported [0-methyl-11C]WAY-100635 as a radioligand for PET studies of 5-HT1A receptors in human brain.

The design and physical characteristics of a small animal positron emission tomograph

A small diameter positron emission tomography, designed specifically for small animal studies, was constructed from existing, commercially available, bismuth germanate (BGO) detectors and electronics. The scanner consists of 16 BGO detector blocks arranged to give a tomograph with a diameter of 115 mm and an axial field of view (FOV) of 50 mm. Each block is cut to produce eight (axial) by seven (radial) individual detector elements. The absence of interplane septa enables the acquisition of 3D data sets consisting of 64 sinograms. A 2D data set of 15 sinograms, consisting of eight direct and seven adjacent cross planes, can be extracted from the 3D data set. Images are reconstructed from the 2D sinograms using a conventional filtered backprojection algorithm. Two methods of normalization were investigated, based on either a rotating 68Ge rod source, or a uniform 68Ge plane source, with a uniform cylindrical 18F phantom. Attenuation of the emitted photons was estimated using a rotating 68Ge rod source. The transaxial resolution of the tomograph was measured as 2.3 mm full width at half maximum (FWHM) and 5.6 mm full width at tenth maximum (FWTM) at the centre of the FOV, degrading to 6.6 mm (radial) and 4.4 mm (tangential) FWHM and 10.4 mm (radial) and 14.4 mm (tangential) FWTM at 40.0 mm from the centre of the FOV. The axial slice width was 4.3 mm FWHM, 10.3 mm FWTM at the centre of the transaxial field of view and 4.4 mm FWHM, 10.6 mm FWTM at 20.0 mm from the centre of the FOV. A scatter fraction of 31.0% was measured at 250-850 keV, for an 18F line source centred in a 60 mm diameter, water-filled phantom, reducing to 20.4% and 13.8% as the lower energy discrimination was increased to 380 keV and 450 keV, respectively. The count rate performance was measured using a noise equivalent count rate method, and the linearity of the dead time correction was confirmed over the count rates encountered during routine scanning. In 2D mode, the absolute sensitivity of the tomograph was measured as 9948 counts s-1 MBq-1 at 250-850 keV, 8284 counts s-1 MBq-1 at 380-850 keV and 6280 counts s-1 MBq-1 at 450-850 keV.

Synthesis of the enantiomers of [N-methyl-11C]PK 11195 and comparison of their behaviours as radioligands for PK binding sites in rats

The enantiomers of [N-methyl-11C]PK 11195, a radioligand for PET studies of PK (peripheral benzodiazepine) binding sites, have been prepared from the newly synthesized N-desmethyl-enantiomers by 11C-methylation with N.C.A. [11C]iodomethane. The brain uptake and retention of each enantiomer was compared with that of the racemic radioligand after i.v. administration into normal rats and into rats with focal cortical lesions. No significant differences in the uptakes of the enantiomers were observed in regions devoid of PK binding sites. However, the R-enantiomer was retained to a significantly greater extent than the S-enantiomer in olfactory bulbs-tubercles, which contain some PK binding sites, and also in 9-day-old focal cortical lesions, which are greatly enriched in PK binding sites associated with macrophage infiltration. The observed differences are consistent with the approximately 2-fold greater affinity of the R-enantiomer for PK binding sites reported in vitro and imply that the use of this enantiomer would have advantages over the use of the racemate currently used for PET studies.

The potential of high-resolution positron emission tomography to monitor striatal dopaminergic function in rat models of disease

The use of a recently commissioned small-diameter, high-resolution positron emission tomography (PET) to obtain a measure of specific binding of 3 carbon-11 labelled ligands in rat striatum is described. Using cerebellum as a reference tissue, compartmental modelling was used to obtain individual estimates of striatal binding potential (defined as the ratio of rate constants to and from the specifically bound compartment) for [11C]raclopride (D2 receptors), [11C]SCH 23390 (D1 receptors) and [11C]RTI-121 (dopamine transporter). The coefficients of variation in control, anaesthetized rats were of the order of 10%. Using two models of human disease, namely striatal injection of ibotenic acid to produce postsynaptic cell loss as in Huntingtons disease, and 6-hydroxydopamine injection into substantia nigra pars compacta to mimic dopaminergic terminal loss in Parkinsons disease, marked reductions in binding potential were observed for the corresponding pre- or postsynaptic markers. When the regions of interest are so small as to be of the order of the spatial resolution of the system, factor such as spill over and partial volume negate absolute quantification of tissue radioactivity. Nevertheless, the use of PET to monitor relative changes in dopaminergic integrity should be considered as a viable complement to established in vivo microdialysis and post mortem techniques.

Compartmental Analysis of Diprenorphine Binding to Opiate Receptors in the Rat in vivo and its Comparison with Equilibrium Data in vitro

The regional binding of the opiate receptor ligand diprenorphine has been examined in rat brain both in vivo and in vitro. The time course of total label in specific brain regions was followed up to 2 h after intravenous bolus injection of [3H]diprenorphine, with or without a pulse chase of unlabelled diprenorphine at 30 min. In addition, total label was measured 30 min after injection of labelled diprenorphine at nontracer concentrations over a range of specific activities. Total data sets for each region were fitted simultaneously to a compartmental model to give estimates of maximal binding capacity (Bmax), the second-order apparent association rate constant, and the first-order dissociation rate constant of the receptor-ligand complex. The model incorporated the use of a reference region with low specific binding (cerebellum). The binding of diprenorphine to rat brain homogenates was measured in vitro under equilibrium conditions at 37°C, pH 7.4, in the presence and absence of naloxone, to give corresponding regional estimates of Bmax and the half-saturation constant Kd The results showed a close correlation between in vitro and in vivo regional estimates of Bmax over a wide range. There were no significant interregional differences either in Kd in vitro or in the Kd derived from the in vivo analysis, although in vitro and in vivo estimates differed by an order of magnitude. This work was carried out as part of a validation study with a view to the application of the compartmental model to data obtained in vivo in humans using positron emission tomography, when successive studies over a range of specific activities are not feasible. Restriction of the rat data to tracer alone and pulse chase protocols showed that the compartmental model gave regional estimates of the combined forward rate constant consistent with estimates obtained using the complete data set.

First delineation of 5-HT1A receptors in human brain with PET and [11C]WAY-100635

The selective 5-HT1A receptor radioligand, [11C]WAY-100635 ([11C]N-2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2- pyridyl)cyclohexanecarboxamide), has been injected intravenously into healthy male volunteers and studied by PET (positron emission tomography). The results provide the first delineation of 5-HT1A receptors in living human brain and demonstrate the potential to use [11C]WAY-100635 for the study of central 5-HT1A receptors in patients with psychiatric and neurological disorders and for the investigation of the pharmacology of drugs acting on the central nervous system.

Imaging the GABA-Benzodiazepine Receptor Subtype Containing the α5-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 α5-containing subtype is highly expressed in the hippocampus, and selective α5 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 α1 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 α5 subtype (flunitrazepam, RY80, Ro15 4513, L655,708), but not by the α1 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 α5 subtype in limbic structures and can be used to further explore the functional role of this subtype in humans.