Mapping of central dopamine synthesis in man,using positron emission tomography with <Emphasis Type="SmallCaps">l</Emphasis>-[β-<Superscript>11</Superscript>C]DOPA

Objective To estimate the presynaptic function of the central dopaminergic system, positron emission tomography measurement of the endogenous dopamine synthesis rate was performed with l-[β-¹¹C]DOPA. In the present study, we developed a simple method for calculating an indicator of the dopamine synthesis rate with l-[β-¹¹C]DOPA on a voxel-by-voxel basis for parametric mapping. Methods After intravenous injection of l-[β-¹¹C]DOPA, dynamic scanning was performed on ten healthy men for 89 min. The dopamine synthesis ratio was calculated on a voxel-by-voxel basis as the ratio of the area under the time-activity curves of brain regions to the reference brain region, that is, occipital cortex. The overall uptake rate constant as an indicator of dopamine synthesis was also calculated by kinetic and graphical analyses. Results The dopamine synthesis ratio calculated by the present method was in good agreement with the indicators of dopamine synthesis calculated by kinetic and graphical analyses, although a systemic underestimation was observed, especially when the integration interval was set in the early phase of the scan duration. In particular, underestimations were prominent in brain regions with relatively lower influx rate constant K ₁. Conclusions By this method, regional dopamine synthesis could be estimated on a voxel-by-voxel basis. This method does not need an arterial input function and should prove to be useful for clinical research.     

18F]Fluoro-beta-fluoromethylene-m-tyrosine derivatives show stereo, geometrical, and regio specificities as in vivo central dopaminergic probes in monkeys.

Stereo (D and L), geometrical (E and Z), and regiospecific (2-, 4-, and 6-[18F]fluoro) analogs of beta-fluoromethylene-m-tyrosine (FMMT) have been investigated in adult vervet monkeys (Cercopithecus aethiops sabaeus, n = 12) in vivo with positron emission tomography (PET). Brain transport through the blood-brain barrier and central aromatic amino acid decarboxylase (AAAD)-mediated decarboxylation rates were established. Results show strict structural dependency of the kinetic behavior of radiofluorinated FMMT analogs, with the E-isomer exhibiting a higher specificity over the (Z) geometrical counterpart for central dopaminergic structures. The 6-[18F]fluoro substituted L-(E)-FMMT was also favored over the 2- and 4-[18F]fluorosubstituted isomers in terms of their ability to localize in the same brain areas. The role of PET in drug development is also exemplified in this work.     

In vivo 13C nuclear magnetic resonance investigations of choline metabolism in rabbit brain

The metabolism of choline in rabbit brain was studied by the noninvasive approach of in vivo 13C NMR spectroscopy. 13C-Enriched precursors were introduced into the brain. Surgery of the head skin was avoided through controlled localization of the surface coil. For long-term accumulation studies in brain, repeated subcutaneous injections proved to be advantageous over other forms of application. The resorption kinetics was calculated to be zero order which suggests slow delivery from the subcutaneous depots. Choline metabolism was studied by two approaches: [N-13CH3]choline and S-[13CH3]methionine were administered separately to adult and myelinating rabbits (Days 5 to 32), respectively, over 4 weeks. [N-13CH3]Choline and the 13CH3 group of methionine were incorporated into lecithin and sphingomyelin of brain myelin. In vivo kinetic studies of the turnover of these labeled structures were carried out. Choline and methionine are readily transported through the blood-brain barrier and utilized by the myelinating brain for the biosynthesis of its phospholipids.     

The effects of carbidopa administration on 6-[18F]fluoro-L-dopa kinetics in positron emission tomography.

Carbidopa (L-alpha-hydrazino-alpha-methyl-b-(3,4-dihydroxyphenyl) propionic acid is a known inhibitor of aromatic amino acid decarboxylase. In both humans and monkeys, we studied the effects of carbidopa on plasma and brain kinetics of 6-[18F]fluoro-L-DOPA (FDOPA), an analog of L-DOPA used for PET studies of the central dopaminergic system. Pretreatment with carbidopa resulted in increases in the plasma levels of FDOPA and 3-O-methyl-6-[18F]fluoro-L-DOPA (3-OMFD). Total striatal and cerebellar activities measured with PET were also increased. Furthermore, increases observed in the specific striatal activity (striatum minus cerebellum total activity) were correlated with increases in the plasma FDOPA curve. Carbidopa pretreatment did not affect the influx rate constant (K) for FDOPA from plasma to striatum in humans as determined by Patlak graphical analysis. Thus, an increase in measured striatal tomographic activity was secondary to the increase in plasma FDOPA levels rather than as a result of changes in the FDOPA influx rate constant.     

Ratio of dopamine synthesis capacity to D2 receptor availability in ventral striatum correlates with central processing of affective stimuli

PURPOSE: Dopaminergic neurotransmission in the ventral striatum may interact with limbic processing of affective stimuli, whereas dorsal striatal dopaminergic neurotransmission can affect habitual processing of emotionally salient stimuli in the pre-frontal cortex. We investigated the dopaminergic neurotransmission in the ventral and dorsal striatum with respect to central processing of affective stimuli in healthy subjects. METHODS: Subjects were investigated with positron emission tomography and [(18)F]DOPA for measurements of dopamine synthesis capacity and [(18)F]DMFP for estimation of dopamine D2 receptor binding potential. Functional magnetic resonance imaging was used to assess the blood-oxygen-level-dependent (BOLD) response to affective pictures, which was correlated with the ratio of [(18)F]DOPA net influx constant [Formula: see text]/[(18)F]DMFP-binding potential (BP_ND) in the ventral and dorsal striatum. RESULTS: The magnitude of the ratio in the ventral striatum was positively correlated with BOLD signal increases elicited by negative versus neutral pictures in the right medial frontal gyrus (BA10), right inferior parietal lobe and left post-central gyrus. In the dorsal striatum, the ratio was positively correlated with BOLD signal activation elicited by negative versus neutral stimuli in the left post-central gyrus. The BOLD signal elicited by positive versus neutral stimuli in the superior parietal gyrus was positively correlated with the dorsal and ventral striatal ratio. CONCLUSIONS: The correlations of the ratio in the ventral and dorsal striatum with processing of affective stimuli in the named cortical regions support the hypothesis that dopamine transmission in functional divisions of the striatum modulates processing of affective stimuli in specific cortical areas.     

Investigation of transport mechanism and uptake kinetics of O-(2-[18F]fluoroethyl)-L-tyrosine in vitro and in vivo.

The aim of the study was to investigate the transport mechanism and uptake kinetics of the new 18F-labeled amino acid O-(2-[18F]fluoroethyl)-L-tyrosine (L-[18F]FET) and D-[18F]FET in human SW 707 colon carcinoma cells and the in vivo biodistribution of this tracer in SW 707 tumor-bearing mice. METHODS: SW 707 cells were incubated with L- and D-[18F]FET under physiologic amino acid concentrations with and without the competitive transport inhibitors 2-amino-2 norbornane-carboxylic acid and a-(methylamino)isobutyric acid plus serine. For the investigation of the transport capacity, unlabeled L-FET was added to the samples. In addition, xenotransplanted mice were injected intravenously with L-[18F]FET; killed 10, 30, 60 and 120 min after injection; and the radioactivity concentration in different organs was measured in a gamma counter. RESULTS: The in vitro kinetic experiments showed a fast initial uptake of L-[18F]FET into the cells up to 6 min, followed by a nearly constant tracer concentration. The accumulation factor, calculated as the ratio between intracellular and extracellular tracer concentration, ranged from 3.0 to 5.0. In comparison, D-[18F]FET did not accumulate in the cells. Washing the cells in medium at 37 degrees C, after a 30-min incubation with L-[F-18]FET, led to a rapid decrease of radioactivity, which demonstrates the bidirectional transport. In addition, experiments with increasing concentrations of unlabeled L-FET indicated a linear correlation between L-FET uptake rate and the extracellular concentration. Results of transport inhibition experiments with the specific competitive inhibitors demonstrated that the uptake of L-FET into SW 707 cells was caused mainly (>80%) by the transport system L. In the in vivo studies, the half-life (t1/2 beta) of L-[18F]FET in the plasma was determined to be 94 min and the uptake into the brain increased to 120 min with a brain-to-blood ratio of 0.86. The xenotransplanted tumor showed higher uptake of L-[18F]FET (>6 %ID/g) at 30 and 60 min than all other organs, except the pancreas. The tumor-to-blood ratio reached about 2 between 30 and 120 min. CONCLUSION: L-[18F]FET, which is transported by the specific amino acid transport system L, seems to be a potential amino acid tracer for tumor imaging and therapy monitoring with PET.     

Characterization of technetium-99m-L,L-ECD for brain perfusion imaging, Part 1: Pharmacology of technetium-99m ECD in nonhuman primates

Technetium-99m ethyl cysteinate dimer ([99mTc]ECD) is a neutral, lipophilic complex which rapidly crosses the blood-brain barrier. Brain retention and tissue metabolism of [99mTc]ECD is dependent upon the stereochemical configuration of the complex. While both L,L and D,D enantiomers are extracted by the brain, only the L,L but not the D,D form, is metabolized and retained in the monkey brain (4.7% injected dose initially, T 1/2 greater than 24 hr). Dynamic single photon emission computed tomography imaging studies in one monkey indicates 99mTc-L,L-ECD to be distributed in a pattern consistent with regional cerebral blood flow for up to 16 hr postinjection. Dual-labeled 99mTc-L,L-ECD and [14C]iodoantipyrine autoradiography studies performed 1 hr after administration show cortical gray to white matter ratios of both isotopes to be equivalent (approximately 4-5:1). These data suggest that 99mTc-L,L-ECD will be useful for the scintigraphic assessment of cerebral perfusion in humans.     

18F-FDOPA kinetics in brain tumors.

L-3,4-Dihydroxy-6-(18)F-fluoro-phenyl-alanine ((18)F-FDOPA) is an amino acid analog used to evaluate presynaptic dopaminergic neuronal function. Evaluation of tumor recurrence in neurooncology is another application. Here, the kinetics of (18)F-FDOPA in brain tumors were investigated. METHODS: A total of 37 patients underwent 45 studies; 10 had grade IV, 10 had grade III, and 13 had grade II brain tumors; 2 had metastases; and 2 had benign lesions. After (18)F-DOPA was administered at 1.5-5 MBq/kg, dynamic PET images were acquired for 75 min. Images were reconstructed with iterative algorithms, and corrections for attenuation and scatter were applied. Images representing venous structures, the striatum, and tumors were generated with factor analysis, and from these, input and output functions were derived with simple threshold techniques. Compartmental modeling was applied to estimate rate constants. RESULTS: A 2-compartment model was able to describe (18)F-FDOPA kinetics in tumors and the cerebellum but not the striatum. A 3-compartment model with corrections for tissue blood volume, metabolites, and partial volume appeared to be superior for describing (18)F-FDOPA kinetics in tumors and the striatum. A significant correlation was found between influx rate constant K and late uptake (standardized uptake value from 65 to 75 min), whereas the correlation of K with early uptake was weak. High-grade tumors had significantly higher transport rate constant k(1), equilibrium distribution volumes, and influx rate constant K than did low-grade tumors (P < 0.01). Tumor uptake showed a maximum at about 15 min, whereas the striatum typically showed a plateau-shaped curve. Patlak graphical analysis did not provide accurate parameter estimates. Logan graphical analysis yielded reliable estimates of the distribution volume and could separate newly diagnosed high-grade tumors from low-grade tumors. CONCLUSION: A 2-compartment model was able to describe (18)F-FDOPA kinetics in tumors in a first approximation. A 3-compartment model with corrections for metabolites and partial volume could adequately describe (18)F-FDOPA kinetics in tumors, the striatum, and the cerebellum. This model suggests that (18)F-FDOPA was transported but not trapped in tumors, unlike in the striatum. The shape of the uptake curve appeared to be related to tumor grade. After an early maximum, high-grade tumors had a steep descending branch, whereas low-grade tumors had a slowly declining curve, like that for the cerebellum but on a higher scale.     

The transport of tyrosine into the human brain as determined with L-[1-11C]tyrosine and PET

An alteration of dopaminergic transmission in the brain has been proposed for schizophrenia. To explore this, the rate constant for the intransport of L-tyrosine across the blood-brain barrier in healthy controls and in patients with schizophrenia (DSM-III-R) was determined with PET and L-[1-11C] tyrosine as the tracer. Kinetics for tyrosine transport were determined according to a two-compartment model using radioactivity data of arterial blood and brain tissue sampled between 1 and 3.5 min after a bolus injection of L-[1-11C] tyrosine. Radioactivity was measured every second in the blood and in 10-sec intervals in the brain tissue. In the normal controls the brain intransport rate constant for tyrosine was 0.052 ml/g/min with an influx rate of 2.97 nmol/g/min. The patients had a similar intransport rate constant (0.045 ml/g/min) but a lower influx rate of tyrosine 1.95 nmol/g/min (p less than 0.05). The patients' tyrosine concentrations in the blood were lower. For data sampled between 5 and 25 min, the net accumulation rate of tyrosine into the brain was 0.015 ml/g/min in the controls which did not differ to the patients' rate. However, the net utilization of tyrosine was lower in the patients (0.672 nmol/g/min) than in the controls (0.883 nmol/g/min) despite similar tissue concentrations of tyrosine.     

Postinjection L-phenylalanine increases basal ganglia contrast in PET scans of 6-18F-DOPA

The sensitivity of 18F-DOPA positron emission tomography for imaging presynaptic dopamine systems is limited by the amount of specific-to-nonspecific accumulation of radioactivity in brain. In rhesus monkeys, we have been able to increase this ratio by taking advantage of the lag time between 18F-DOPA injection and the formation of its main metabolite, the amino acid 18F-fluoromethoxydopa, the entrance of which into brain is responsible for most of the brain's nonspecific radioactivity. By infusing an unlabeled amino acid, L-phenylalanine, starting 15 min after 18F-DOPA administration, we preferentially blocked the accumulation of 18F-fluoromethoxydopa by preventing its entrance into brain through competition at the large neutral amino acid transport system of the blood-brain barrier. This method appears as reliable as the original and more sensitive, as demonstrated by the comparison of normal and MPTP-treated animals under both conditions.     

Investigation of iodine-123-labelled amino acid derivatives for imaging cerebral gliomas: uptake in human glioma cells and evaluation in stereotactically implanted C6 glioma rats

In developing iodine-123-labelled amino acid derivatives for imaging cerebral gliomas by single-photon emission tomography (SPET), we compared p-[123I]iodo-L-phenylalanine (IPA), L-[123I]iodo-1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (ITIC) and L-3-[123I]iodo-alpha-methyltyrosine (IMT) with regard to their uptake in human glioblastoma T99 and T3868 cells, and thereafter studied the mechanisms promoting the cellular uptake. The potential of the 123I-iodinated agents for use as SPET radiopharmaceuticals was evaluated in healthy experimental rats as well as in rats with stereotactically implanted C6 gliomas. The radiopharmaceutical uptake into glioblastoma cells was rapid, temperature and pH dependent, and linear during the first 5 min. Equilibrium was reached after 15-20 min, except in the case of ITIC, the initial uptake of which gradually decreased from 15 min onwards. The radioactivity concentration in glioma cells following 30-min incubation at 37 degrees C (pH 7.4) varied from 11% to 35% of the total activity per million cells (ITIC < IMT < or = IPA). Competitive inhibition experiments using alpha-(methylamino)-isobutyric acid and 2-amino-2-norbornane-carboxylic acid, known as specific substrates for systems A and L, respectively, as well as representative amino acids preferentially transported by system ASC, indicated that IPA, like IMT, is predominantly mediated by the L and ASC transport systems, while no significant involvement of the A transport system could be demonstrated. By contrast, none of the three principal neutral amino acid transport systems (A, L and ASC) appear to be substantially involved in the uptake of ITIC into glioblastoma cells. Analysis of uptake under conditions that change the cell membrane potential, i.e. in high K+ medium, showed that the membrane potential plays an important role in ITIC uptake. Alteration of the mitochondrial activity by means of valinomycin or nigericin induces a slight increase or decrease in the radiopharmaceutical uptake, suggesting a minor contribution of the mitochondria in the uptake. IPA, IMT and ITIC passed the blood-brain barrier, and thereafter showed efflux from the brain. The radioactivity concentration in healthy rat brain 15 min following intravenous injection varied from 0.07% (ITIC) to 0.27% ID/g (IPA). In comparison, the brain uptake in the stereotactically implanted C6 glioma rats was substantially higher (up to 1.10% ID/g 15 min p.i.), with tumour-to-background ratios greater than 4. These data indicate that IPA and ITIC, like IMT, exhibit interesting biological characteristics which hold promise for in vivo brain tumour investigations by SPET.     

Increased amino acid transport into brain tumors measured by PET of L-(2-18F)fluorotyrosine

The uptake of L-(2-18F)fluorotyrosine (F-Tyr), a newly synthetized amino acid tracer, was studied in 15 patients with various brain tumors by dynamic PET. The higher F-Tyr accumulation in tumors (mean 27% above contralateral tissue) was associated with two-fold transport rates into tumors, while the rate constants describing irreversible incorporation were decreased. The increased F-Tyr transport was not correlated to 68Ga-EDTA accumulation and cannot be explained by disruption of the blood-brain barrier. Kinetic analysis of 2-(18F)-fluoro-deoxy-glucose accumulation in the same patients demonstrated that increased metabolic rates in tumors are mainly caused by altered phosphorylation rates while transport of glucose is less affected. Since F-Tyr transport rates clearly separated tumors from normal tissue and since F-Tyr accumulation was related to tumor grade, PET studies of F-Tyr uptake are of clinical value for diagnosis and classification of brain tumors.     

Effects of dopamine on the in vivo binding of dopamine D2 receptor radioligands in rat striatum

The effects of moderate changes in extracellular dopamine concentrations on the in vivo binding of specific dopaminergic D2 radioligands with different affinities and kinetics were investigated in rats. Either [125I]NCQ298 (Kd = 19 pM), or [25I]iodolisuride (Kd = 0.27 nM) or [3H]raclopride (Kd = 1.5 nM) were administered intravenously (IV) to animals 1 h after the intraperitoneal (IP) injection of either alpha-methyl-p-tyrosine (AMPT) (250 mg/kg) or nomifensine (15 mg/kg), or saline. The kinetics of radioactivity concentration in the striatum, cerebellum, and plasma were measured for up to 4 h after [125I]NCQ298 or [125I]iodolisuride injection and up to 1.5 h after [3H]raclopride injection. For each tracer, the striatum-to-cerebellum radioactivity concentration ratios (S/C) and the binding potential (BP), calculated as the association to dissociation binding rate constant ratios (k3/k4), were assessed and related to the changes in extracellular dopamine concentration induced by drug treatments. Results show that S/C and BP of [3H]raclopride were significantly diminished by pretreatment with nomifensine, a drug that increases extracellular dopamine concentration. Nomifensine pretreatment induced no changes in the in vivo binding indexes of the high affinity [125I]NCQ298 and a slight but not significant decrease of the binding indexes of 125I]iodolisuride. Treatment with AMPT, which induced a 40% reduction in dopamine concentration, did not change [125I]NCQ298 binding indexes but slightly increased those of [3H]raclopride and [125I]iodolisuride. In conclusion, the change of dopamine concentration induces modification of radiotracer kinetics. Thus, the combined use of tracers with high and low affinities could allow us to obtain information both on receptor density and neurotransmitter release in vivo. However, as indicated by the [3H]raclopride study with AMPT, small changes in the concentration of intrasynaptic dopamine cannot be easily detected.     

Automatic synthesis of L-[beta-11C]amino acids using an immobilized enzyme column.

We have developed a system for the automatic synthesis of L-[beta-11C]amino acids for i.v. injection by means of enzyme-mediated reactions from 11CO2 via 11CH3I and D,L-[beta-11C]alanine as labeled intermediates. This system, which incorporates an ultrafilter cartridge sterilized by electron beam irradiation and a column packed with immobilized enzymes, was effective for eliminating enzymes and endotoxins that may contaminate the product. Using this system, 1.3 +/- 0.5 GBq of 5-hydroxy-L-[beta-11C]tryptophan with a radiochemical purity of 97.1 +/- 0.6% and a specific activity of 39.6 +/- 8 GBq/mumol a pH value of 4 could be obtained in about 32 min (n = 3, at EOS). No endotoxin, enzyme, or bacteria was detected in the product. L-[beta-11C]dihydroxyphenylalanine (L-[beta-11C]DOPA) was also synthesized using this system.     

Synthesis of a 11C-labeled novel,quinuclidine based ligand for the 5-HT3 receptor

L-683,877, a high affinity, 5-HT₃ selective receptor ligand has been labeled with ¹¹C for use in PET studies to measure regional brain kinetics of L-683,877 and to determine if [¹¹C]L-683,877 can be used for serotonin 5-HT₃ receptor imaging. [¹¹C]L-683,877 was prepared by reacting [¹¹C]methyl iodide with the desmethyl, borane-protected precursor, L-686,472, in DMF in the presence of tetrabutyl ammonium hydroxide. The average specific activity of [¹¹C]L-683,877 was 2700 Ci/mmol and the average radiochemical yield (decay uncorrected) was 20% at the end of synthesis (22 min from EOB).     

Evaluation of the skeletal kinetics of fluorine-18-fluoride ion with PET.

To evaluate the feasibility of quantitatively assessing regional skeletal fluoride uptake in humans in focal and generalized bone disease, we investigated the skeletal kinetics of [18F]fluoride ion with dynamic PET imaging. Dynamic image sets were acquired over a 60-min interval in a multiplane PET device, and input functions (plasma 18F time-activity curves) were measured directly from arterialized blood and, in some cases, determined from image-derived left ventricular cavity activity measurements. Our results indicate: 1. A steady-state ratio of [18F]fluoride ion concentration in plasma to whole blood greater than unity (1.23 for plasma to directly assayed whole blood and 1.44 for plasma to left ventricular cavity imaged concentrations. This concentration difference produces a scaling factor that must be considered when using image derived or directly measured input functions. 2. The preferred tracer kinetic model configuration for [18F]fluoride ion skeletal kinetics is a three compartment model that includes a "bound" and "unbound" bone [18F]fluoride ion compartment. 3. The rate constant for forward transport of [18F]fluoride ion from plasma to the extravascular space of bone (K1) and the regional blood volume parameter generate estimates of bone blood flow and vascular volume, respectively, that are in the physiologic range of reported for mammals. Estimates of the uptake constant for fluoride in bone, using nonlinear regression (KNLR = 0.0360 +/- 0.0064 ml/min/ml), are in very good agreement with an estimate of the same parameter obtained with Patlak graphical analysis (KPAT = 0.0355 +/- 0.0061 ml/min/ml). 4. Generating parametric images of KPAT facilitates quantification of regional bone [18F]fluoride ion kinetics. The method is computationally practical, and, with either the parametric imaging approach or with standard region of interest analysis, can be used to generate quantitative estimates of fluoride uptake (a "bone metabolic index") in focal skeletal regions or in more generalized distributions.     

Synthesis of L-[β-11C]amino acids using immobilized enzymes

L-[β-¹¹C]-3,4-dihydroxyphenylalanine(L-[β-¹¹C]DOPA) and L-[β-¹¹C]-5-hydroxytryptophan(L-[β-¹¹C]-5-HTP) were synthesized in one step with immobilized thermostable enzymes (alanine racemase, D-amino acid oxidase, and β-tyrosinase or tryptophanase) on an aminopropyl-CPG carrier in a single column and by passing D,L-[3-¹¹C]alanine through the column with coenzymes and other substrates. L-[β-¹¹C]DOPA and L-[β-¹¹C]-5-HTP could be obtained at yields of 53% and 60%, respectively, by optimizing the amounts and the ratios of the enzymes used, the reaction temperature, the pH, and the flow rate. Moreover, the same immobilized enzyme column could be used repeatedly.     

Evaluation of [1-11C]-alpha-aminoisobutyric acid for tumor detection and amino acid transport measurement: spontaneous canine tumor studies

Alpha-aminoisobutyric acid (AIB), or alpha-methyl alanine, is a nonmetabolized amino acid transported into cells, particularly malignant cells, predominantly by the 'A' amino acid transport system. Since it is not metabolized, [1-11C]-AIB can be used to quantify A-type amino acid transport into cells using a relatively simple compartmental model and quantitative imaging procedures (e.g. positron tomography). The tissue distribution of [1-11C]-AIB was determined in six dogs bearing spontaneous tumors, including lymphosarcoma, osteogenic sarcoma, mammary carcinoma, and adenocarcinoma. Quantitative imaging with tissue radioassay confirmation at necropsy showed poor to excellent tumor localization. However, in all cases the concentrations achieved appear adequate for amino acid transport measurement at known tumor locations. The observed low normal brain (due to blood-brain barrier exclusion) and high (relative to brain) tumor concentrations of [1-11C]-AIB suggest that this agent may prove effective for the early detection of human brain tumors.     

In vivo labeling of endothelin receptors with [(11)C]L-753,037: studies in mice and a dog.

Endothelin (ET) is a potent mammalian vasoconstrictive peptide and a pressor agent. Its 3 isoforms, ET-1, ET-2, and ET-3, mediate several physiologic actions in several organ systems, binding to 2 major receptor subtypes: ET(A) and ET(B). This study was undertaken to evaluate [(11)C]L-753,037 [(+)-(5S,6R,7R)-2-butyl-7-[2-((2S)-2-carboxy-propyl)-4-methoxyphenyl]-5-(3,4-methylenedioxyphenyl)cyclopenteno [1,2-beta]pyridine-6-carboxylate), a new mixed ET receptor A and B antagonist, as a tracer for in vivo labeling of ET receptors in mice and a dog. METHODS: [(11)C]L-753,037 was synthesized, purified, and formulated from a normethyl precursor, L-843,974, and [(11)C]H(3)I. The tracer was studied for its in vivo kinetics, biodistribution, and ET receptor binding characteristics in mice. In the dog, PET imaging was performed to evaluate binding of [(11)C]L-753,037 to ET receptors in the heart. Specificity of binding was studied in the heart with the selective ET(A) antagonist L-753,164. RESULTS: Kinetic studies in mice showed highest tracer uptake at 5 min after injection in liver (25.0 percentage injected dose per gram [%ID/g]), kidneys (18.7 %ID/g), lungs (15.2 %ID/g), and heart (5.6 %ID/g). Initial high uptake in liver, lungs, and kidneys was followed by rapid washout during the next 10 min and a very slow clearance during the time of observation (2 h after injection). By contrast, the radioactivity in the heart remained constant over 2 h. Administration of both ET(A) (L-753,164) and mixed ET(A)/ET(B) (L-753,137) receptor antagonists resulted in dose-dependent inhibition of [(11)C]L-753,037 binding in mouse heart, lungs, and kidneys but not in the liver. Radioactivity in the brain was very low, indicating that the tracer does not cross the blood-brain barrier. In the dog, a dynamic PET study of the heart showed high tracer accumulation at 55-95 min after injection. Injection of L-753,164 at 30 min before [(11)C]L-753,037 administration led to a significant reduction in tracer binding. [(11)C]methyl triphenyl phosphonium was used as a tracer for reference images of the dog heart muscle. CONCLUSION: The results suggest that [(11)C]L-753,037 binds to ET receptors in vivo and is, therefore, a promising candidate for investigation of these receptors and their occupancy by ET receptor antagonists using PET.     

Quantification of protein synthesis in the human brain using L-[1-11C]-leucine PET: incorporation of factors for large neutral amino acids in plasma and for amino acids recycled from tissue.

The rate of incorporation of exogenous amino acids into brain proteins is indicative of the protein synthesis rate (PSR). The objective of this study was to assess the effect of plasma concentrations of leucine and large neutral amino acids (LNAAs) on the unidirectional uptake rate constant (Kcplx) of l-[1-(11)C]-leucine in the brain and to estimate the amino acid pool recycled from tissue. METHODS: Twenty-seven healthy adult volunteers (11 men and 16 women; age range, 20-50 y) underwent dynamic l-[1-(11)C]-leucine PET with arterial blood sampling. Data were analyzed with a standard 2-tissue-compartment model yielding the unidirectional uptake rate of plasma leucine into tissue (Kcplx = K(1)k(3)/(k(2) + k(3))) and the fraction of leucine originating from exogenous sources (lambda = k(2)/(k(2) + k(3))). PSR in brain was calculated as PSR = [Kcplx/lambda] x leucine. RESULTS: The mean plasma concentration of the sum of all LNAAs was 13% higher in men (981 +/- 86 micromol/L) than in women (850 +/- 76 micromol/L, P = 0.012), whereas the plasma leucine concentration was found to be similar in both sexes (men, 64 +/- 20 micromol/L; women, 58 +/- 21 micromol/L, P = 0.57). The whole-brain value for lambda was determined to be 0.64 +/- 0.03 and did not show a sex difference (P = 0.66). Whole-brain Kcplx values were significantly higher in women (0.0162 +/- 0.0024) than in men (0.0121 +/- 0.0031, P = 0.011); however, after normalization of the Kcplx to a standard plasma concentration of the sum of all LNAAs (Kcplx'), the Kcplx' was similar between the sexes (P = 0.21), as was the PSR' (1.24 +/- 0.49 micromol/L/min in men; 1.29 +/- 0.62 micromol/L/min in women, P = 0.87). No relationship between plasma leucine and Kcplx (r = -0.13, P = 0.63) was observed. Finally, there was a significant correlation between the PSR and the Kcplx derived using Patlak graphical analysis (rho = 0.65, P < 0.001). CONCLUSION: We conclude that both the Kcplx macroparameter and the PSR are stable indices of brain protein synthesis and are appropriate measures for testing altered protein synthesis in neurologic disorders.