Graphical evaluation of blood-to-brain transfer constants from multiple-time uptake data. Generalizations

Spectral analysis of dynamic PET studies

We describe a new technique for the analysis of dynamic positron emission tomography (PET) studies in humans, where data consist of the time courses of label in tissue regions of interest and in arterial blood, following the administration of radiolabelled tracers. The technique produces a simple spectrum of the kinetic components which relate the tissue's response to the blood activity curve. From this summary of the kinetic components, the tissue's unit impulse response can be derived. The convolution of the arterial input function with the derived unit impulse response function gives the curve of best fit to the observed tissue data. The analysis makes no a priori assumptions regarding the number of compartments or components required to describe the time course of label in the tissue. Rather, it is based on a general linear model, presented here in a formulation compatible with its solution using standard computer algorithms. Its application is illustrated with reference to cerebral blood flow, glucose utilization, and ligand binding. The interpretation of the spectra, and of the tissue unit impulse response functions, are discussed in terms of vascular components, unidirectional clearance of tracer by the tissue, and reversible and irreversible phenomena. The significance of the number of components which can be identified within a given datum set is also discussed. The technique facilitates the interpretation of dynamic PET data and simplifies comparisons between regions and between subjects.

Imaging radiotracer model parameters in PET: a mixture analysis approach

Two methodologies for fitting radiotracer models on a pixel-wise basis to PET data are considered. The first method does parameter optimization for each pixel considered as a separate region of interest. The second method also does pixel-wise analysis but incorporates an additive mixture representation to account for heterogeneity effects induced by instrumental and biological blurring. Several numerical and statistical techniques including cluster analysis, constrained nonlinear optimization, subsampling, and spatial filtering are used to implement the methods. A computer simulation experiment, modeling a standard F-18 deoxyglucose (FDG) imaging protocol using the UW-PET scanner, is conducted to evaluate the statistical performance of the parametric images obtained by the two methods. The results obtained by mixture analysis are found to have substantially improved mean square error performance characteristics. The total computation time for mixture analysis is on the order of 0.7 s/pixel on a 16 MIPS workstation. This results in a total computation time of about 1 h per slice for a typical FDG brain study.

6-[18F]fluoro-L-dopa metabolism in living human brain: a comparison of six analytical methods

In 11 normal volunteers and six patients with Parkinson's disease, we compared six different analyses of dopaminergic function with L-3,4-dihydroxy-6-[18F]fluorophenylalanine (FDOPA) and positron emission tomography (PET). The caudate nucleus, putamen, and several reference regions were identified in PET images, using magnetic resonance imaging (MRI). The six analyses included two direct determinations of DOPA decarboxylase activity (k3D, k3*), the slope-intercept plot based on plasma concentration (K), two slope-intercept plots based on tissue content (k3r, k3s), and the striato-occipital ratio [R(T)]. For all analyses, the difference between two groups of subjects (normal volunteers and patients with Parkinson's disease) was larger in the putamen than in the caudate. For the caudate nucleus, the DOPA decarboxylase activity (k3D, k3*), tissue slope-intercept plots (kr3, ks3); and striato-occipital ratio [R(T)] analyses significantly discriminated between the normal volunteers and the patients with Parkinson's disease (p < 0.005) [with least significance for k3* (p < 0.05)], while the plasma slope-intercept plot (K) failed to do so. For the putamen, the values for k3D, k3*, K, k3r, k3s, and R(T) of normal volunteers were significantly higher than those of patients (p < 0.005) [with least significance for K (p < 0.025)]. Linear correlations were significant between k3D and k3s; k3D and k3r; k3D and R(T); and k3D and k3*, in this order of significance. We found no correlation between k3D and K values in the caudate nucleus.

Drug transport across the blood-brain barrier. II. Experimental techniques to study drug transport

This is part II of a review on the transport of drugs across the blood-brain barrier. In this part, the emphasis is on the various experimental techniques that can be used to characterize the blood-brain barrier transport of drugs. Generally speaking, three approaches can be distinguished: in vitro techniques using isolated brain capillaries, cerebrovascular endothelial cells in primary culture or endothelium-derived cell lines; in vivo techniques (both single-passage and multi-passage techniques) and in situ perfusion techniques. Each of these techniques has specific advantages and disadvantages associated with it. Therefore, in many instances, a combination of different approaches is needed to study the fundamental aspects of drug transport across the blood-brain barrier.

Bilateral fetal mesencephalic grafting in two patients with parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

BACKGROUND

Intracerebral transplantation of fetal dopaminergic neurons is a promising new approach for the treatment of Parkinson's disease. Patients with parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) have a relatively stable lesion limited to the nigrostriatal system, rendering them ideal candidates for transplantation. Improvement of motor function after neural grafting has previously been observed in nonhuman primates with MPTP-induced parkinsonism.

METHODS

We grafted human fetal tissue from the ventral mesencephalon (obtained six to eight weeks after conception) bilaterally to the caudate and putamen in two immunosuppressed patients with severe MPTP-induced parkinsonism, using a stereotaxic technique. The patients were assessed regularly with clinical rating scales, timed tests of motor performance, and [18F]fluorodopa positron-emission tomography during the 18 months before the operation and the 22 to 24 months after the operation.

RESULTS

Both patients had substantial, sustained improvement in motor function and became much more independent. Postoperatively, the second patient's maintenance dose of levodopa was decreased to 150 mg daily, which was 30 percent of the original dose. Striatal uptake of fluorodopa was unchanged 5 to 6 months postoperatively but was markedly and bilaterally increased at 12 to 13 and 22 to 24 months in both patients, closely paralleling the patients' clinical improvement. There were no serious complications.

CONCLUSIONS

Bilateral implantation of fetal mesencephalic tissue can induce substantial long-term functional improvement in patients with parkinsonism and severe dopamine depletion and is accompanied by increased uptake of fluorodopa by the striatum. The results in these patients resemble those obtained in MPTP-treated primates and suggest that this will be a useful model for the assessment of transplantation therapies in Parkinson's disease.

Positron emission tomography with 18F-dopa: interpretation and biological correlates in nonhuman primates

Positron emission tomography (PET) was carried out, with 18F-DOPA as a ligand, in normal control monkeys and "parkinsonian" monkeys who had been treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The following approaches were used in data analysis: ratio of 18F accumulation in specific to nonspecific brain areas and 18F-DOPA influx constant obtained using either the actual plasma 18F-DOPA or the 18F activity in a nonspecific brain area as the input function. The results from these analyses were compared to one another and to biological parameters relevant to dopaminergic function. The striatum/cortex ratio and the rate constant calculated from plasma 18F-DOPA appeared to be the most sensitive analytic techniques.

Compartmentation of hexokinase in rat heart. A critical factor for tracer kinetic analysis of myocardial glucose metabolism

Radiolabeled analogues of 2-deoxyglucose are widely used to trace glucose metabolism in cell cultures, whole organs, and intact animals, although kinetic differences in transport and phosphorylation between these compounds and glucose exist. The present studies were undertaken to determine the effects of insulin stimulation on the phosphorylation of 2-deoxyglucose compared to glucose in the intact, saline-perfused working rat heart. Rates of glucose utilization determined from tritiated glucose differed from rates estimated from the accumulation of [14C]2-deoxyglucose in a nonconstant manner when comparing rates in the absence or presence of physiologic levels of insulin (13 microU/ml). The fraction of monophosphorylated hexoses that was accounted for by [14C]2-deoxyglucose 6-phosphate was dramatically decreased in hearts perfused in the presence of insulin. Additionally, hexokinase activity associated with the mitochondrial fraction of tissue extracts was increased in hearts stimulated by insulin. While this redistribution of hexokinase to the mitochondria did not affect the apparent affinity constant for glucose, hexokinase bound to mitochondria exhibited an 8.5-fold decrease in the affinity for 2-deoxyglucose when compared with hexokinase present in the cytosolic fraction. The findings are consistent with an insulin-mediated preferential uptake and phosphorylation of glucose compared to deoxyglucose. The results also imply that the redistribution of hexokinase and the differential effect of insulin on its affinity for tracer and tracee are responsible for changes in the "lumped constant" (i.e., the correction factor used to equate 2-deoxyglucose to glucose uptake). These changes must be taken into account when regional myocardial glucose metabolism is assessed by the 2-deoxyglucose method.

The identification of presymptomatic parkinsonism: clinical and [18F]dopa positron emission tomography studies in an Irish kindred

An Irish kindred is described in which 5 of 10 siblings in the fourth or fifth decade of life developed an akinetic-rigid syndrome clinically indistinguishable from idiopathic Lewy body Parkinson's disease. Four of these patients were scanned by positron emission tomography (PET) with [18F]dopa after clinical diagnosis and in all, a profound impairment of tracer uptake into the striatum was recorded. The fifth patient was initially scanned at a time when he was asymptomatic and normal by clinical examination. His scan showed impaired tracer uptake, indicating a subclinical defect in the presynaptic nigrostriatal system. Within months of his scan, he too developed subtle symptoms and signs of parkinsonism although there was little further clinical progression or change in his PET scan over the following year. A clinically normal sibling was also scanned and found to have subclinical impairment of [18F]dopa uptake in the putamen. The 19-year-old daughter of an affected sibling had a mild postural tremor but no other symptoms or signs. The [18F]dopa uptake in her putamen fell at the borderline between normal and parkinsonian values. This study confirms that PET can identify preclinical parkinsonism in at-risk subjects. The finding of abnormalities in several clinically unaffected family members suggests that family studies based on clinical assessment alone may miss a significant number of subclinically affected individuals, leading to an underestimate of any genetic component to Parkinson's disease.

In vivo CT measurement of blood-brain transfer constant of iopamidol in human brain tumors

We have developed an in vivo method of measuring the blood-brain transfer constant (K) of iopamidol and the cerebral plasma volume (Vp) in brain tumors using a clinical X-ray CT scanner. In patient studies, Isovue 300 (iopamidol) was injected at a dosage of 1 ml/kg patient body weight. Serial CT scans of the tumor site and arterial blood samples from a radial artery were taken up to 48 min after injection. The leakage of iopamidol into the brain through the blood-brain barrier was modelled as an exchange process between two compartments, the intravascular plasma space and the tissue interstitial space. Using this model and the concentration measurements in blood plasma and tissue, quantitative estimates of K and Vp in brain tumors were obtained. In addition, distribution of the estimated values of K and Vp in tumors were displayed as false colour functional images overlaid on the conventional CT scan. In a study of twelve patients with anaplastic astrocytoma (n = 3), glioblastoma multiforme (n = 4) or metastases (n = 5) the mean K and Vp values in tumor were found to be 0.0273 +/- 0.0060 ml/min/g and 0.068 +/- 0.11 ml/g respectively. These values were significantly higher than those in grey or white matter in the contralateral 'normal' hemisphere (p less than 0.05). The functional images showed variations in K and Vp within the tumor which were difficult to perceive in the original contrast enhanced CT scans.

Fasting and lactate unmask insulin responsiveness in the isolated working rat heart

We have previously reported that the nutritional state in vivo results in differential insulin responses by the perfused heart in vitro. To further assess the effects of insulin on glucose uptake at physiological work loads, hearts from fed and fasted (16-20 h) rats were perfused with buffer containing 2-[18F]fluoro-2-deoxy-D-glucose (2-FDG) and glucose (10 mM) alone or plus lactate (10 mM) as a competing substrate, with insulin (10 mU/ml) added after a control period. When glucose was the only substrate, the addition of insulin decreased the fractional rate of 2-FDG uptake in hearts from either fed or fasted rats. The effect of insulin on increasing myocardial 2-FDG uptake was immediate and sustained only in hearts from fasted rats in the presence of lactate, despite no change in cardiac work. At the same time, the increase in 2-FDG uptake and phosphorylation was associated with an increase in the tissue content of glycogen in hearts from fasted rats. We conclude that lactate unmasks insulin sensitivity in heart muscle at physiological work loads but that this unmasking of insulin-mediated glucose uptake is dependent on the nutritional state of the animal. The glucose up as a result of insulin stimulation is preferentially utilized for glycogen repletion and does not enter the glycolytic pathway. This observation also suggests that myocardial glycogen synthesis in vitro is affected by the nutritional state in vivo and that lactate provides a substrate for oxidative phosphorylation while glucose is preferentially utilized for glycogen synthesis.

Errors introduced by tissue heterogeneity in estimation of local cerebral glucose utilization with current kinetic models of the [18F]fluorodeoxyglucose method

The effects of tissue heterogeneity on the estimation of regional cerebral glucose utilization (rCMRglc) in normal humans with [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) and positron emission tomography (PET) were compared with respect to the various kinetic models of the [18F]FDG method. The kinetic models were conventional homogeneous tissue models of the [18F]FDG method, with (4K Model) and without (3K Model) a rate constant to account for an apparent loss of [18F]2-fluoro-2-deoxy-D-glucose-6-phosphate ([18F]FDG-6-P), and a tissue heterogeneity model (TH Model). When either of the kinetic models designed for homogeneous tissues was applied to heterogeneous tissues, estimates of the rate constant for efflux of [18F]FDG from the tissue (k2*) and of the rate constant for phosphorylation of [18F]FDG (k3*) decreased as the duration of the experimental period was increased. When the 4K Model was used, estimates of the rate constant for the apparent dephosphorylation of [18F]FDG-6-P (k4*) were significantly greater than zero and fell with increasing duration of the experimental period. Although the TH Model included no term to describe an apparent dephosphorylation of [18F]FDG-6-P, the fit of the TH Model to the time course of total tissue radioactivity was at least as good as and often better than the fit of the 4K Model in the 120-min period following the pulse of [18F]FDG. Hence, the high estimates of k4* found in PET studies of less than or equal to 120 min can be explained as the consequence of measuring radioactivity in a heterogeneous tissue and applying a model designed for a homogeneous tissue; there remains no evidence of significant dephosphorylation of [18F]FDG-6-P in this time period. Furthermore, use of the 4K Model led to an overestimation of rCMRglc; whole-brain glucose utilization calculated with the 4K Model was greater than 20% higher than values usually obtained in normal humans by the model-independent Kety-Schmidt technique. rCMRglc was accurately estimated by the TH Model and, in experimental periods sufficiently long to minimize the effects of tissue heterogeneity, also by the original 3K Model of the deoxyglucose method.

Correlation between [5-3H]glucose and [U-14C]deoxyglucose as markers of glycolysis in reperfused myocardium

Studies were conducted in extracorporeally perfused, intact, working pig hearts to determine whether, in heart muscle, trace-labeled deoxyglucose serves as an accurate marker of glycolytic flux in reperfusion after exposures to mild to moderate regional ischemia. In the main study, two groups of hearts were compared, as distinguished by levels of glucose in the whole-blood perfusate (euglycemic hearts [group I], blood glucose of 7.4 +/- 0.2 mumol/ml, n = 7; hyperglycemic hearts [group II], blood glucose of 12.9 +/- 0.5 mumol/ml, n = 8). Both groups were subjected to a 60% reduction in anterior descending coronary flow for 30 minutes followed by reperfusion for 40 minutes. Modest and comparable regional mechanical stunning during reflow was noted in both groups. Glucose utilization, as estimated from the release of 3H2O from the steady-state infusion of [5-3H]glucose during aerobic perfusion, was modest but during reperfusion was noted to increase significantly above aerobic values in each of the two groups, with a doubling of rates in group II hearts compared with group I hearts (p less than 0.041 or p less than 0.090). Net lactate extraction was comparable in reflow in both groups, suggesting in this specific instance a preferential enhancement of glucose oxidation in hyperglycemic group II hearts. Shifts in accumulation of tissue radioactivity of [U-14C]2-deoxyglucose in reperfused myocardium were not able to track these trends. The variability of 14C-labeled radioactivity among animals was marked and essentially masked any ability to discern trends in glycolysis as described by tritiated glucose between the aerobic and reperfusion intervals. When the data were arrayed by linear regression analysis, the slopes derived from 14C-labeled deoxyglucose were either discordant or insensitive to those described by 3H-labeled glucose. Tissue glycogen levels were slow to recover in early reflow and at end reperfusion were still significantly depressed from aerobic levels. The present data indicate that coronary reperfusion and hyperglycemia have influence in determining glycolytic flux in myocardium. Labeled deoxyglucose, considered solely as a marker of exogenous glucose utilization, appears to be an insensitive agent in describing these events at conditions of relatively low glucose flux.

An absorptiometry method for the determination of arterial blood concentration of injected iodinated contrast agent

A single photon absorptiometry method to measure the arterial concentration of injected iodinated contrast agent was developed. A prototype absorptiometry unit was built which consists of either a square or circular cross section acrylic (polymethylmethacrylate) cuvette connected to an arterial catheter at one end and a paristaltic pump at the other via PE60 surgical tubing. At opposing ends of the length of the cuvette were a 0.4 GBq 125I source and a scintillation crystal/photomultiplier tube assembly. This assembly was connected to a single-channel analyser (SCA)/scaler unit to count the transmitted photons through the cuvette. The scaler was interfaced to an IBM PC and counts accumulated in preset time intervals were transmitted to the computer via a serial interface. Experiments were performed to calibrate the unit for measurement of blood concentration of contrast agent (Isovue 300) and to determine the dispersion characteristics of the unit. Deconvolution was used to correct the measured concentration waveform for the dispersion introduced by passage through the lead-in tubing and the cuvette. The precision of concentration measurements was determined to be between 5 and 10% using computer simulations and theoretical calculations. The method was used successfully in a number of patient and animal studies to measure the contrast concentration in blood following intravenous injection of contrast agent.

Glucose utilization in a patient with hepatoma and hypoglycemia. Assessment by a positron emission tomography

Tumor glucose use in patients with non-islet-cell tumors has been difficult to measure, particularly in hepatoma, because of hepatic involvement by neoplasm. We studied a patient with nonhepatic recurrence of hepatoma after successful liver transplantation. Tumor tissue contained messenger RNA for insulin-like growth factor-II (IGF-II), and circulating high molecular weight components and E-peptide of IGF-II were increased. Glucose use measured by isotope dilution with [3-3H]glucose was 7.94 mg/kg fat-free mass per min, and splanchnic glucose production was 0.93 mg/kg fat-free mass per min. Glucose uptake and glucose model parameters were independently measured in tissues by positron emission tomography with 18F-fluoro-2-deoxy-D-glucose. Glucose uptake by heart muscle, liver, skeletal muscle, and neoplasm accounted for 0.8, 14, 44, and 15% of total glucose use, respectively. Model parameters in liver and neoplasm were not significantly different, and glucose transport and phosphorylation were twofold and fourfold greater than in muscle. This suggests that circulating IGF-II-like proteins are partial insulin agonists, and that hypoglycemia in hepatoma with IGF-II production is predominantly due to glucose uptake by skeletal muscle and suppression of glucose production.

Glucose-free fatty acid cycle operates in human heart and skeletal muscle in vivo

Positron emission tomography permits noninvasive measurement of regional glucose uptake in vivo in humans. We employed this technique to determine the effect of FFA on glucose uptake in leg, arm, and heart muscles. Six normal men were studied twice under euglycemic hyperinsulinemic (serum insulin approximately 500 pmol/liter) conditions, once during elevation of serum FFA by infusions of heparin and Intralipid (serum FFA 2.0 +/- 0.4 mmol/liter), and once during infusion of saline (serum FFA 0.1 +/- 0.01 mmol/liter). Regional glucose uptake rates were measured using positron emission tomography-derived 18F-fluoro-2-deoxy-D-glucose kinetics and the three-compartment model described by Sokoloff (Sokoloff, L., M. Reivich, C. Kennedy, M. C. Des Rosiers, C. S. Patlak, K. D. Pettigrew, O. Sakurada, and M. Shinohara. 1977. J. Neurochem. 28: 897-916). Elevation of plasma FFA decreased whole body glucose uptake by 31 +/- 2% (1,960 +/- 130 vs. 2,860 +/- 250 mumol/min, P less than 0.01, FFA vs. saline study). This decrease was due to inhibition of glucose uptake in the heart by 30 +/- 8% (150 +/- 33 vs. 200 +/- 28 mumol/min, P less than 0.02), and in skeletal muscles; both when measured in femoral (1,594 +/- 261 vs. 2,272 +/- 328 mumol/min, 25 +/- 13%) and arm muscles (1,617 +/- 411 to 2,305 +/- 517 mumol/min, P less than 0.02, 31 +/- 6%). Whole body glucose uptake correlated with glucose uptake in femoral (r = 0.75, P less than 0.005), and arm muscles (r = 0.69, P less than 0.05) but not with glucose uptake in the heart (r = 0.04, NS). These data demonstrate that the glucose-FFA cycle operates in vivo in both heart and skeletal muscles in humans.

Uses and limitations of positron emission tomography in clinical pharmacokinetics/dynamics (Part II)

Positron emission tomography (PET) involves imaging the biodistribution and tissue localisation of small amounts of radiolabelled biomolecules or drugs. In Part I of this article, which appeared in the previous issue of the Journal, the applications of pharmacokinetics in PET were discussed in order to derive quantitative measures of physiological function. Part II examines the use of PET imaging as a tool to study the pharmacokinetics and pharmacodynamics of specific drugs.

Measurement of blood-brain barrier permeability using dynamic Gd-DTPA scanning--a comparison of methods

Two recently published methods of blood-brain barrier permeability measurement using Gd-DTPA scanning are compared by authors representing each group. The physiological models are reconciled. Results from both groups agree. These show that the transfer constant (the permeability surface area product per unit volume of tissue) of the defective blood-brain barrier in multiple sclerosis is in the range 1-12.10(-4) s-1.

Production of 6-[18F]fluoro-L-dopa and its metabolism in vivo--a critical review

This report critically appraises methods for the synthesis of 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine (6-FDOPA) that are based on labelling by non-regioselective electrophilic fluorination, regioselective fluorodemetalation or nucleophilic substitution. Recommendations for the standardization of labelling procedures, the optimization of radiochemical yield and the assurance of product quality and safety are given. Studies of the metabolism of 6-FDOPA in vivo are also reviewed to emphasize the importance of the biochemical component of the development of this tracer for positron emission tomography (PET).

Effects of endogenous dopamine on kinetics of [3H]N-methylspiperone and [3H]raclopride binding in the rat brain

Competition by endogenous dopamine with the binding of D2 dopamine receptor ligands may be important in the interpretation of positron emission tomography (PET) neuroreceptor studies. PET studies with N-methylspiperone (NMSP) have revealed increased D2 dopamine receptors in schizophrenia, whereas studies with raclopride (RAC) have not detected such differences. This may be due, at least in part, to differences in competition with endogenous dopamine for ligand binding. To determine effects of endogenous dopamine on in vivo receptor binding, adult male rats were preinjected with amphetamine and reserpine prior to [3H]NMSP or [3H]RAC. Striatal to cerebellar ratios of ligand binding were determined. To approximate the conditions of a PET study, a kinetic model was employed to examine effects of pharmacologically increasing brain dopamine levels (amphetamine pretreatment) on PET ligand binding. In these experiments, tail veins and arteries were cannulated and kinetic parameters determined from normalized integral plots in rats treated with amphetamine prior to radioligand injection. Both [3H]NMSP (43.5%) and [3H]RAC (41.5%) binding were significantly decreased after amphetamine pretreatment, whereas after reserpine pretreatment [3H]RAC binding was increased (52.7%). Kinetic studies revealed a marked resistance of [3H]NMSP to competition with endogenous dopamine released by amphetamine. In contrast, kinetic parameters of [3H]RAC were markedly reduced at all time intervals. This suggests significant differences in competition with endogenous dopamine by [3H]NMSP and [3H]RAC, determined kinetically. These findings may have important implications for the interpretation of PET neuroreceptor studies.

Effects of endogenous dopamine on measures of [18F]N-methylspiroperidol binding in the basal ganglia: comparison of simulations and experimental results from PET studies in baboons

The effect of endogenous dopamine on PET measures of radioligand binding is important to the measurement of receptor density (or availability) and neurotransmitter interactions in vivo. We recently reported that pretreatment with amphetamine, a drug which stimulates dopamine release, significantly reduced NMS binding in the baboon brain as determined by the product lambda k3 derived from the graphical analysis method for irreversible systems (lambda is the ratio of the forward to reverse plasma to tissue transport constants and k3 is proportional to receptor density) (Dewey et al.: Synapse 7:324-327, 1991). The purpose of this work is twofold: to evaluate the sensitivity and stability of the analysis method used for the NMS data and from simulation studies which include the competitive effects of dopamine on NMS binding to predict the effect of dopamine on the in vivo PET experiment. Using a measured plasma [18F]-NMS input function from a control study in a baboon, simulation data was numerically generated explicitly allowing competition between NMS and dopamine in the calculation. This data was analyzed using the same techniques as used for the experimental data and the results were compared to in vitro calculations. The following conclusions were reached: 1) The effect of dopamine on specific binding was found to be greater in vivo than in vitro because the in vitro equilibrium experiment is controlled only by the relative Kd's of tracer and dopamine while the in vivo experiment also depends upon the halftime of tracer in tissue which is controlled by the tissue-to-plasma transport constant; 2) Experimental evidence from rodent studies (Seeman et al.: Synapse 3:96-97, 1989) and the agreement between PET studies (Wong et al.: Science 234:1558-1563, 1986a) and postmortem human studies (Seeman et al.: Science 225:728-731, 1984) in schizophrenics suggest that NMS is not likely to be affected by normal levels of endogenous dopamine. From the calculations reported here the effective in vivo Kd of dopamine for the NMS binding site would have to be on the order of or greater than 100 nM, assuming a synaptic dopamine concentration of 20 nM, in order that this concentration of dopamine have little effect on NMS binding.

Measurements of blood-brain barrier permeability in patients undergoing radiotherapy and chemotherapy for primary cerebral lymphoma

Positron emission tomography (PET) has been used to measure changes in regional blood-brain barrier (BBB) permeability in patients with primary cerebral lymphoma undergoing radiotherapy and chemotherapy. The method employed is to measure the rate of wash-out of a radioactive tracer (68Ga-EDTA) from blood into brain tissue using time-sequence PET imaging. Preliminary studies carried out on patients with more common primary cerebral tumours show that time-activity data are reproducible to approximately 10%. Measurements made in 2 patients with primary cerebral lymphoma treated with initial chemotherapy showed significant changes in permeability in the region of the tumour. Within 5 weeks of the start of treatment, permeability values reached the levels of normal brain. No changes in BBB permeability in normal brain were seen immediately after radiotherapy.

Quantitative evaluation of regional substrate metabolism in the human heart by positron emission tomography

Meaning interpretation of metabolic images obtained by positron emission tomography for evaluation of cardiac disease requires a knowledge of the normal variation in regional myocardial substrate metabolism. Recent studies with fluorine-18 (F-18) fluorodeoxyglucose suggest inhomogeneity of myocardial glucose metabolism in the normal human heart, which may relate to substrate availability. Therefore, quantitative evaluation of myocardial oxidative metabolism and glucose metabolism, as derived by dynamic positron emission tomography with carbon-11 (C-11) acetate and F-18 fluorodeoxyglucose, was performed in nine healthy male volunteers. All were studied under tightly controlled metabolic conditions of hyperinsulinemic-euglycemic clamping with and without a concurrent lipid emulsion infusion. Significant inhomogeneity of regional glucose metabolism was noted although it was less than that described under fasting conditions. Glucose utilization was 13% lower in the septum compared with the lateral wall both without and with lipid infusion (0.34 vs. 0.39 mumol/g per min, respectively, p less than 0.05; and 0.33 vs. 0.38 mumol/g per min, respectively, (p less than 0.05). Relatively decreased septal glucose utilization could not be explained by decreased metabolic demand because C-11 clearance constants were marginally higher in the septum than in the lateral wall in both studies (0.055 vs. 0.054 per min, respectively, p = NS; and 0.061 vs. 0.056 per min, respectively, p less than 0.05). Relatively decreased septal glucose utilization could reflect regional variation in substrate use and possible preferential free fatty acid utilization by the septum. These data provide a useful framework for assessing altered cardiac metabolism in disease and support standardization of metabolic conditions during metabolic imaging with positron emission tomography.

Dopa-responsive dystonia: [18F]dopa positron emission tomography

The syndrome of dopa-responsive dystonia comprises a minority of patients with dystonia, yet it is of considerable diagnostic importance because patients respond dramatically to L-dopa therapy. Benefits from this treatment are lasting, and the problems associated with long-term L-dopa therapy in patients with Parkinson's disease are generally absent. It has been suggested that this condition is due to a defect in the dopamine synthetic pathway, which is bypassed when patients are treated with L-dopa. We have studied [18F]dopa uptake in 6 patients with classic dopa-responsive dystonia (5 familial patients and 1 sporadic patient), aged 18 to 66 years. Data have been analyzed according to a graphic approach, calculating an influx constant for each region studied. We have also studied a seventh, clinically atypical, patient with juvenile dystonia-parkinsonism. Similar data have been calculated for a group of 10 healthy control subjects and 10 patients with Parkinson's disease. The 6 patients with typical dopa-responsive dystonia had a modest but significant reduction in the uptake of tracer into both caudate and putamen, which indicates a defect in the decarboxylation, vesicular uptake, and storage of [18F]dopa. This argues against the proposition that dopa-responsive dystonia is due to an inherited defect of tyrosine hydroxylase alone. In the atypical patient, however, we found a greater reduction of [18F]dopa uptake into both caudate and putamen, comparable with that in patients with Parkinson's disease.

Dihydroergotamine as a pharmacologic euglycemic clamp in the surgically traumatized rabbit

We assessed the utility of a pharmacologic euglycemic clamp technique by examining the metabolic and hemodynamic changes brought about by administration of dihydroergotamine (DHE) prior to anesthesia and operative trauma in the rabbit. New Zealand white rabbits received an intramuscular injection of 0.15 mg DHE/kg body weight 20 minutes before general anesthesia and minor surgical trauma, which consisted of carotid artery and jugular vein catheterizations followed by femoral artery and vein cannulations. Arterial blood was sampled every 20 minutes and assayed for glucose, lactate, nonesterified fatty acids (NEFA), and insulin. Rates of hindlimb skeletal muscle glucose uptake (Rg) and blood flow were determined 2 hours after the initial administration of DHE. DHE did not produce any effects on heart rate, blood pressure, blood flow, and Rg when compared with control animals. Liver glycogen levels were significantly higher after DHE treatment (140 +/- 31.4 v 34 +/- 9.6 mumol/g dry weight, P less than .01). Those animals receiving DHE had significantly lower and more stable plasma glucose levels than untreated animals (ranges, 5 to 9 v 7 to 22 mumol/mL plasma) and circulating NEFA were also lower and less variable (ranges, 0.1 to 1.0 v 0.1 to 2.0 muEq/mL plasma). The results show that DHE prevents stress-induced hyperglycemia in vivo in the rabbit without altering glucose uptake by skeletal muscle. The technique provides control over circulating glucose levels during the study of skeletal muscle glucose uptake without apparent negative physiologic effects.

Quantification of human opiate receptor concentration and affinity using high and low specific activity [11C]diprenorphine and positron emission tomography

[11C]Diprenorphine, a weak partial opiate agonist, and positron emission tomography were used to obtain noninvasive regional estimates of opiate receptor concentration (Bmax) and affinity (Kd) in human brain. Different compartmental models and fitting strategies were compared statistically to establish the most reliable method of parameter estimation. Paired studies were performed in six normal subjects using high (769-5,920 Ci/mmol) and low (27-80 Ci/mmol) specific activity (SA) [11C]diprenorphine. Two subjects were studied a third time using high SA [11C]diprenorphine after a pretreatment with 1-1.5 mg/kg of the opiate antagonist naloxone. After the plasma radioactivity was corrected for metabolites, the brain data were analyzed using a three-compartment model and nonlinear least-squares curve fitting. Linear differential equations were used to describe the high SA (low receptor occupancy) kinetics. The k3/k4 ratio varied from 1.0 +/- 0.2 (occipital cortex) to 8.6 +/- 1.6 (thalamus). Nonlinear differential equations were used to describe the low SA (high receptor occupancy) kinetics and the curve fits provided the konf2 product. The measured free fraction of [11C]diprenorphine in plasma (f1) was 0.30 +/- 0.03, the average K1/k2 ratio from the two naloxone studies was 1.1 +/- 0.2, and the calculated free fraction of [11C]diprenorphine in the brain (f2) was 0.3. Using the paired SA studies, the estimated kinetic parameters, and f2, separate estimates of Bmax and Kd were obtained. Bmax varied from 2.3 +/- 0.5 (occipital cortex) to 20.6 +/- 7.3 (cingulate cortex) nM. The average Kd (eight brain regions) was 0.85 +/- 0.17 nM.

Kinetic analysis of transport and opioid receptor binding of [3H](-)-cyclofoxy in rat brain in vivo: implications for human studies

[3H]Cyclofoxy (CF: 17-cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta-fluoromorp hinan) is an opioid antagonist with affinity to both mu and kappa subtypes that was synthesized for quantitative evaluation of opioid receptor binding in vivo. Two sets of experiments in rats were analyzed. The first involved determining the metabolite-corrected blood concentration and tissue distribution of CF in brain 1 to 60 min after i.v. bolus injection. The second involved measuring brain washout for 15 to 120 s following intracarotid artery injection of CF. A physiologically based model (Sawada et al., 1990a) and a classical compartmental pharmacokinetic model (Wong et al., 1986a) were compared. The models included different assumptions for transport across the blood-brain barrier (BBB); estimates of nonspecific tissue binding and specific binding to a single opiate receptor site were found to be essentially the same with both models. The nonspecific binding equilibrium constant varied modestly in different brain structures (Keq = 3-9), whereas the binding potential (BP) varied over a much broader range (BP = 0.6-32). In vivo estimates of the opioid receptor dissociation constant were similar for different brain structures (KD = 2.1-5.2 nM), whereas the apparent receptor density (Bmax) varied between 1 (cerebellum) and 78 (thalamus) pmol/g of brain. The receptor dissociation rate constants in cerebrum (k4 = 0.08-0.16 min-1; koff = 0.16-0.23 min-1) and brain vascular permeability (PS = 1.3-3.4 ml/min/g) are sufficiently high to achieve equilibrium conditions within a reasonable period of time. Graphical analysis (Patlak and Blasberg, 1985) of the data is inappropriate due to the high tissue-loss rate constant (kb = 0.03-0.07 min-1) for CF in brain. From these findings, CF should be a very useful opioid receptor ligand for the estimation of the receptor binding parameters in human subjects using [18F]CF and positron emission tomography.

Positron emission tomography studies of brain receptors

Probing the regional distribution and affinity of receptors in the brain, in vivo, in human and non human primates has become possible with the use of selective ligands labelled with positron emitting radionuclides and positron emission tomography (PET). After describing the techniques used in positron emission tomography to characterize a ligand receptor binding and discussing the choice of the label and the limitations and complexities of the in vivo approach, the results obtained in the PET studies of various neurotransmission systems: dopaminergic, opiate, benzodiazepine, serotonin and cholinergic systems are reviewed.

The nigrostriatal dopaminergic pathway in Wilson's disease studied with positron emission tomography

Movement disorders, including Parkinsonism, are prominent features of neurological Wilson's disease (WD). This suggests there may be dysfunction of the nigrostriatal dopaminergic pathway. To explore this possibility, five patients were studied using positron emission tomography (PET) with 18F-6-fluorodopa (6FD), and magnetic resonance imaging (MRI). We calculated striatal 6FD uptake rate constants by a graphical method and compared the results with those of 18 normal subjects. It was found that four patients with symptoms all had abnormally low 6FD uptake, and the one asymptomatic patient had normal uptake. PET evidence for nigrostriatal dopaminergic dysfunction was present even after many years of penicillamine treatment. It is concluded that the nigrostriatal dopaminergic pathway is involved in neurological WD.

Effect of Huntington's and Alzheimer's diseases on the transport of nicotinic acid or nicotinamide across the human blood-brain barrier

The cerebral uptake of 11C-nicotinic acid (11C-NAC) and 11C-nicotinamide (11C-NAM) was quantified by the use of PET. Based on the amount of activity injected, the PET images showed a low cerebral uptake of 11C-NAC, while 11C-NAM was clearly visualized in the cortical areas. This discrepancy was found to be the result of the binding of 11C-NAC to the red blood cells by a factor of 5 to 20 above that for 11C-NAM. 11C-NAM was better extracted by the cerebrum than 11C-NAC, as shown by the mean values of the cortical tissue/plasma ratio of 1.9 for 11C-NAC and 5 for 11C-NAM at 30 min. post-injection. An analysis of Patlak-Gjedde plot curves revealed a metabolic compartment for 11C-NAC and 11C-NAM with similar values of about 0.02 l/min for the accumulation constant K3. This was indicative of a slower transport rate for 11C-NAC. A significant finding of the study was the increasing ratio of activity concentrations in red blood cells versus the concentrations in plasma (over time). There were no significant differences between the data from normal volunteers and patients with Huntington's or Alzheimer's disease.

Striatal function in normal aging: implications for Parkinson's disease

Central to several current theories of the etiology of Parkinson's disease is the premise that the nigrostriatal dopaminergic system degenerates with normal aging. Much of the evidence for this assertion has come from postmortem neurochemical studies. We have used L-6-[18F] fluoro-Dopa and positron emission tomography in 26 healthy volunteers (age range, 27-76 years) to examine striatal and frontal cortical tracer uptake. Data have been analyzed by using a graphical approach to calculate an influx constant (Ki) for L-6-[18F]fluoro-Dopa uptake into the caudate, putamen, and medial frontal cortex of each subject. In the population studied, there was no decline in Ki with age for any of these structures. A series of physiological measurements made on the older subjects also showed few significant changes with age. The positron emission tomographic findings demonstrate preservation of nigrostriatal dopaminergic function in normal aging. The pathological process causing Parkinson's disease may operate closer to the time of presentation than has been suggested.

Kinetic analysis of 3-quinuclidinyl 4-[125I]iodobenzilate transport and specific binding to muscarinic acetylcholine receptor in rat brain in vivo: implications for human studies

Radioiodinated R- and S-Quinuclidinyl derivatives of RS-benzilate (R- and S-125IQNB) have been synthesized for quantitative evaluation of muscarinic acetylcholine receptor binding in vivo. Two sets of experiments were performed in rats. The first involved determining the metabolite-corrected blood concentration and tissue distribution of tracer R-IQNB (active enantiomer) and S-IQNB (inactive enantiomer) in brain 1 min to 26 h after intravenous injection. The second involved the measurement of brain tissue washout over a 2-min period after loading the brain by an intracarotid artery injection of the ligands. Various pharmacokinetic models were tested, which included transport across the blood-brain barrier (BBB), nonspecific binding, low-affinity binding, and high-affinity binding. Our analysis demonstrated that the assumptions of rapid equilibrium across the BBB and rapid nonspecific binding are incorrect and result in erroneous estimates of the forward rate constant for binding at the high-affinity receptor sites (k3). The estimated values for influx across the BBB (K1), the steady-state accumulation rate in cerebrum (K), and the dissociation rate constant at the high-affinity site (k4) of R-IQNB were independent of the specific compartmental model used to analyze these data (K1 approximately 0.23 ml/min/g, K approximately 0.13 ml/min/g, and k4 approximately 0.0019 min-1 for caudate). In contrast, the estimated values of k3 and the efflux rate constant (k2) varied over a 10-fold range between different compartmental models (k3 approximately 2.3-22 min-1 and k2 approximately 1.6-16 min-1 in caudate), but their ratios were constant (k3/k2 approximately 1.4). Our analysis demonstrates that the estimates of k3 (and derived values such as the binding potential) are model dependent, that the rate of R-IQNB accumulation in cerebrum depends on transport across the BBB as well as the rate of binding, and that uptake in cerebrum is essentially irreversible during the first 360 min after intravenous administration. Graphical analysis was consistent with compartmental analysis of the data and indicated that steady-state uptake of R-IQNB in cerebrum is established within 1-5 min after intravenous injection. We propose a new approach to the analysis of R-IQNB time-activity data that yields reliable quantitative estimates of k3, k4, and the nonspecific binding equilibrium constant (Keq) by either compartmental or graphical analysis. The approach is based on determining the free unbound fraction of radiolabeled ligand in blood and an estimate of K1.(ABSTRACT TRUNCATED AT 400 WORDS)

Striatal binding of 11C-NMSP studied with positron emission tomography in patients with persistent tardive dyskinesia: no evidence for altered dopamine D2 receptor binding

Dopamine D2 receptor binding characteristics were studied by positron emission tomography (PET) using N-11C-methyl spiperone as receptor ligand in patients on longterm treatment with neuroleptic drugs and in control subjects. Eight of the patients had symptoms of tardive dyskinesia whereas three patients did not have any symptoms. Control subjects comprised 5 healthy volunteers and 7 patients with pituitary tumors. All patients had been free of neuroleptic drugs for at least 4 weeks. The time dependent regional radioactivity in the striatum was measured and the receptor binding rate, k3, proportional to receptor number, Bmax and association rate for the receptor was calculated in relation to the cerebellum. The lack in difference in k3 values between TD patients, neuroleptic treated patients without TD and control subjects throws doubt on the hypothesis that changes in striatal D2 dopamine receptor number or binding affinity is an etiological mechanism for persistent TD.

Differing patterns of striatal 18F-dopa uptake in Parkinson's disease, multiple system atrophy, and progressive supranuclear palsy

Using positron emission tomography (PET), we studied regional striatal 18F-dopa uptake in 16 patients with L-dopa-responsive Parkinson's disease (PD), 18 patients with multiple system atrophy, and 10 patients with progressive supranuclear palsy. Results were compared with those of 30 age-matched normal volunteers. The patients with PD showed significantly reduced mean uptake of 18F-dopa in the caudate and putamen compared to controls, but while function in the posterior part of the putamen was severely impaired (45% of normal), function in the anterior part of the putamen and in the caudate was relatively spared (62% and 84% of normal). Mean 18F-dopa uptake in the posterior putamen was depressed to similar levels in all patients. Unlike patients with PD, the patients with progressive supranuclear palsy showed equally severe impairment of mean 18F-dopa uptake in the anterior and posterior putamen. Caudate 18F-dopa uptake was also significantly lower in patients with progressive supranuclear palsy than in patients with PD, being depressed to the same level as that in the putamen. Mean 18F-dopa uptake values in the anterior putamen and caudate in patients with multiple system atrophy lay between PD and progressive supranuclear palsy levels. Locomotor disability of individual patients with PD or multiple system atrophy correlated with decline in striatal 18F-dopa uptake, but this was not the case for the patients with progressive supranuclear palsy. We conclude that patients with PD have selective nigral pathological features with relative preservation of the dopaminergic function in the anterior putamen and caudate, whereas there is progressively more extensive nigral involvement in multiple system atrophy and progressive supranuclear palsy.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitation of blood-brain barrier defect by magnetic resonance imaging and gadolinium-DTPA in patients with multiple sclerosis and brain tumors

In this study quantitation of the degree of deficiency of the blood-brain barrier (BBB) in patients with multiple sclerosis or brain tumors, by using MRI, is shown to be possible. As a measure of permeability of the BBB to Gadolinium-DTPA (Gd-DTPA) the flux per unit of distribution volume per unit of brain mass was used. This quantity was found by introducing the longitudinal relaxation rate (R1) as a measure of concentration of Gd-DTPA in the brain tissue in the mathematical model for the transcapillary transport over the BBB. High accordance between the observed data points and the model was found, and the results were comparable to results obtained from similar studies using positron emission tomography. The improved possibility of quantitating the defect of the BBB by MRI may give new information about pathogenesis or etiology, and leads to improved methods in monitoring the efficacy of treatments in intracranial diseases.

Graphical analysis of reversible radioligand binding from time-activity measurements applied to [N-11C-methyl]-(-)-cocaine PET studies in human subjects

A graphical method of analysis applicable to ligands that bind reversibly to receptors or enzymes requiring the simultaneous measurement of plasma and tissue radioactivities for multiple times after the injection of a radiolabeled tracer is presented. It is shown that there is a time t after which a plot of integral of t0ROI(t')dt'/ROI(t) versus integral of t0Cp(t')dt'/ROI(t) (where ROI and Cp are functions of time describing the variation of tissue radioactivity and plasma radioactivity, respectively) is linear with a slope that corresponds to the steady-state space of the ligand plus the plasma volume,.Vp. For a two-compartment model, the slope is given by lambda + Vp, where lambda is the partition coefficient and the intercept is -1/[kappa 2(1 + Vp/lambda)]. For a three-compartment model, the slope is lambda(1 + Bmax/Kd) + Vp and the intercept is -[1 + Bmax/Kd)/k2 + [koff(1 + Kd/Bmax)]-1) [1 + Vp/lambda(1 + Bmax/Kd)]-1 (where Bmax represents the concentration of ligand binding sites and Kd the equilibrium dissociation constant of the ligand-binding site complex, koff (k4) the ligand-binding site dissociation constant, and k2 is the transfer constant from tissue to plasma). This graphical method provides the ratio Bmax/Kd from the slope for comparison with in vitro measures of the same parameter. It also provides an easy, rapid method for comparison of the reproducibility of repeated measures in a single subject, for longitudinal or drug intervention protocols, or for comparing experimental results between subjects. Although the linearity of this plot holds when ROI/Cp is constant, it can be shown that, for many systems, linearity is effectively reached some time before this. This analysis has been applied to data from [N-methyl-11C]-(-)-cocaine ([11C]cocaine) studies in normal human volunteers and the results are compared to the standard nonlinear least-squares analysis. The calculated value of Bmax/Kd for the high-affinity binding site for cocaine is 0.62 +/- 0.20, in agreement with literature values.

Metabolic stability of 3-O-methyl-D-glucose in brain and other tissues

3-O-Methyl-D-glucose (methylglucose) is often used to study blood-brain barrier transport and the distribution spaces of hexoses in brain. A critical requirement of this application is that it not be chemically converted in the tissues. Recent reports of phosphorylation of methylglucose by yeast and heart hexokinase have raised questions about its metabolic stability in brain. Therefore, we have re-examined this question by studying the metabolism of methylglucose by yeast hexokinase and rat brain homogenates in vitro and rat brain, heart, and liver in vivo. Commercial preparations of yeast hexokinase did convert methylglucose to acidic products, but only when the enzyme was present in very large amounts. Methylglucose was not phosphorylated by brain homogenates under conditions that converted 97% of [U-14C]glucose to ionic derivatives. When [14C]methylglucose, labeled in either the methyl or glucose moiety, was administered to rats by an intravenous pulse or a programmed infusion that maintained the arterial concentration constant and total 14C was extracted from the tissues 60 min later, 97-100% of the 14C in brain, greater than 99% of the 14C in plasma, and greater than 90% of that in heart and liver were recovered as unmetabolized [14C]methylglucose. Small amounts of 14C in brain (1-3%), heart (3-6%), and liver (4-7%) were recovered in acidic products. Plasma glucose levels ranging from hypoglycemia to hyperglycemia had little influence on the degree of this conversion. The distribution spaces for methylglucose were found to be 0.52 in brain and heart and 0.75 in liver.

A two-compartment description and kinetic procedure for measuring regional cerebral [11C]nomifensine uptake using positron emission tomography

S-[11C]Nomifensine (S-[11C]NMF) is a positron-emitting tracer suitable for positron emission tomography, which binds to both dopaminergic and noradrenergic reuptake sites in the striatum and the thalamus. Modelling of the cerebral distribution of this drug has been hampered by the rapid appearance of glucuronide metabolites in the plasma, which do not cross the blood--brain barrier. To date, [11C]NMF uptake has simply been expressed as regional versus nonspecific cerebellar activity ratios. We have calculated a "free" NMF input curve from red cell activity curves, using the fact that the free drug rapidly equilibrates between red cells and plasma, while glucuronides do not enter red cells. With this free [11C]NMF input function, all regional cerebral uptake curves could be fitted to a conventional two-compartment model, defining tracer distribution in terms of [11C]NMF regional volume of distribution. Assuming that the cerebellar volume of distribution of [11C]NMF represents the nonspecific volume of distribution of the tracer in striatum and thalamus, we have calculated an equilibrium partition coefficient for [11C]NMF between freely exchanging specific and nonspecific compartments in these regions, representing its "binding potential" to dopaminergic or noradrenergic uptake sites (or complexes). This partition coefficient was lower in the striatum when the racemate rather than the active S-enantiomer of [11C]NMF was administered. In the striatum of patients suffering from Parkinson's disease and multiple-system atrophy, the specific compartmentation of S-[11C]NMF was significantly decreased compared with that of age-matched volunteers.

Grafts of fetal dopamine neurons survive and improve motor function in Parkinson's disease

Neural transplantation can restore striatal dopaminergic neurotransmission in animal models of Parkinson's disease. It has now been shown that mesencephalic dopamine neurons, obtained from human fetuses of 8 to 9 weeks gestational age, can survive in the human brain and produce marked and sustained symptomatic relief in a patient severely affected with idiopathic Parkinson's disease. The grafts, which were implanted unilaterally into the putamen by stereotactic surgery, restored dopamine synthesis and storage in the grafted area, as assessed by positron emission tomography with 6-L-[18F]fluorodopa. This neurochemical change was accompanied by a therapeutically significant reduction in the patient's severe rigidity and bradykinesia and a marked diminuation of the fluctuations in the patient's condition during optimum medication (the "on-off" phenomenon). The clinical improvement was most marked on the side contralateral to the transplant.

Optimal duration of experimental period in measurement of local cerebral glucose utilization with the deoxyglucose method

The time course and magnitude of the effects of product loss on the measurement of local cerebral glucose utilization (LCGU) by the 2-[14C]deoxyglucose (DG) method were studied by determination of LCGU in 38 rats with 25-120 min experimental periods after a [14C]DG pulse and in 45 rats with experimental periods of 2.5-120 min during which arterial plasma [14C]DG concentrations (C*P) were maintained constant. LCGU was calculated by the operational equation, which assumes no product loss, with the original set of rate constants and with a new set redetermined in the rats used in the present study; in each case the rate constants were those specific to the structure. Data on local tissue 14C concentrations and C*P were also plotted according to the multiple time/graphic evaluation technique ("Patlak Plot"). The results show that with both pulse and constant arterial inputs of [14C]DG the influence of the rate constants is critical early after onset of tracer administration but diminishes with time and becomes relatively minor by 30 min. After a [14C]DG pulse calculated LCGU remains constant between 25 and 45 min, indicating a negligible effect of product loss during that period; at 60 min it begins to fall and declines progressively with increasing time, indicating that product loss has become significant. When C*P is maintained constant, calculated LCGU does not change significantly over the full 120 min. The "Patlak Plots" reinforced the conclusions drawn from the time courses of calculated LCGU; evidence for loss of product was undetectable for at least 45 min after a pulse of [14C]DG and for at least 60 min after onset of a constant arterial input of [14C]DG.

Where have we got to with neuroreceptor mapping of the human brain?

In the past two decades, tritiated radioligand receptor binding, a tool commonly used to investigate the site of action of drugs in laboratory animals, has provided a vast body of information on neuropharmacology and neurobiology. Several neurological and psychiatric diseases have been related to neurotransmitter and receptor disorders. In order to study ligand interactions with receptors in vivo in humans, new tracers capable of carrying a gamma-emitting radionuclide to the receptor have been designed. Emission computerized tomography (ECT) techniques such as positron (PET) or single photon emission tomography (SPET) allow monitoring of the time-course of regional tissue concentration of these radiolabelled ligands. PET and SPET each have their inherent advantages and drawbacks. The cyclotron-based technology of PET is a demanding and expensive technique that, to date, is still mainly reserved for research purposes. It is hoped that once the scientific basis of a physiopathological study is established using PET, diagnostic information might be provided by the more readily available SPET technology. The purpose of this article is to review the current state of receptor-binding gamma-emitting radioligands and to present the clinical potential of these new kinds of radiopharmaceuticals in clinical investigation.

Models for in vivo kinetic interactions of dopamine D2-neuroreceptors and 3-(2'-[18F]fluoroethyl)spiperone examined with positron emission tomography

The in vivo tracer kinetics of 3-(2'-[18F]fluoroethyl)spiperone (FESP) in the caudate/striatum and cerebellar regions of the human and monkey brain were studied with positron emission tomography (PET). The minimal model configuration that can describe the kinetics was determined statistically. Three two-compartment model configurations were found to be suitable for describing the kinetics in caudate/striatum and cerebellum: (1) a nonlinear model (five parameters) applicable to studies using nontracer (partially saturating) quantities of FESP in monkey striatum, (2) a linear four-parameter model applicable to the caudate/striatal and cerebellar kinetics in human and monkey studies with tracer quantities of FESP, and (3) a linear three-parameter model derived from the four-parameter model by assuming irreversible binding applicable to tracer studies of the human caudate. In the human studies, when the caudate kinetics (n = 4) were fit by model 2 (with four parameters), the value of the in vivo ligand dissociation constant kd was found to be 0.0015 +/- 0.0032/min. The three-parameter model (model 3) was found to fit the data equally well: this model is equivalent to model 2 with kd set to zero. In the monkey studies, it was found that for short (90 min) studies using tracer quantities of FESP, model 2 fit the striatal kinetics better than model 3. The parameters estimated using model 2 (four parameters) were in better agreement with those estimated by the nonlinear model (model 1) than those estimated using model 3 (three parameters). The use of a graphical approach gives estimates of the plasma-tissue fractional transport rate constant K1 and the net uptake constant K3 comparable to estimates using model 3 for both human and monkey studies.

Nigrostriatal function in humans studied with positron emission tomography

The dopamine depletion that is characteristic of Parkinson's disease has been hypothesized to result from the combination of environmentally induced subclinical damage to the substantia nigra and the age-related loss of additional nigral neurons. Essential to this hypothesis is the existence of deteriorating function in the nigrostriatal pathway with advancing age. The present study was undertaken with [18F]6-fluoro-L-dopa and positron emission tomography to determine in vivo the effects of age on the nigrostriatal pathway in a series of 10 asymptomatic subjects (age range, 22-80 years; mean, 49.8 years). A graphical approach was used in the analysis of multiple-time tracer-uptake data to establish the presence of a compartment with unidirectional uptake and to calculate the rate constant, K, for uptake of [18F]6-fluoro-L-dopa from blood to striatum during steady-state, an index of the functional integrity of nigrostriatal nerve endings. There was a significant linear relationship between K and age (r = 0.80, p less than 0.005) with a decrease of 53.3% over the age range studied. These results demonstrate the application of a unidirectional transfer model to the analysis of [18F]6-fluoro-L-dopa and positron emission tomography data and provide in vivo confirmation of an age-related impairment of nigrostriatal function.

Multicompartmental analysis of [11C]-carfentanil binding to opiate receptors in humans measured by positron emission tomography

[11C]-Carfentanil is a high affinity opiate agonist that can be used to localize mu opiate receptors in humans by positron emission tomography (PET). A four-compartment model was used to obtain quantitative estimates of rate constants for receptor association and dissociation. PET studies were performed in five normal subjects in the absence and presence of 1 mg/kg naloxone. Arterial plasma concentration of [11C]-carfentanil and its labeled metabolites were determined during each PET study. The value of k3/k4 = Bmax/kD was determined for each subject in the presence and absence of naloxone. There was a significant reduction in the value of k3/k4 from 3.4 +/- 0.92 to 0.26 +/- 0.13 in the thalamus (p less than 0.01) and from 1.8 +/- 0.33 to 0.16 +/- 0.065 in the frontal cortex (p less than 0.001). Mean values of frontal cortex/occipital cortex and thalamus/occipital cortex ratios were determined for the interval 35-70 min after injection when receptor binding is high relative to nonspecific binding. The relationship between the measured region/occipital cortex values and the corresponding values of k3/k4 in the presence and absence of naloxone was: regions/occipital cortex = 0.95 + 0.74 (k3/k4) with r = 0.98 (n = 20). Simulation studies also demonstrated a linear relationship between the thalamus/occipital cortex or frontal cortex/occipital cortex ratio and k3/k4 for less than twofold increases or decreases in k3/k4. Simulation studies in which thalamic blood flow was varied demonstrated no significant effect on the region/occipital cortex ratio at 35-70 min for a twofold increase or fourfold decrease in blood flow. Therefore, the region/occipital cortex ratio can be used to quantitate changes in k3/k4 when tracer kinetic modeling is not feasible.

Brain slice glucose utilization

The metabolism of 2-deoxyglucose has been studied in 540 micron and 1,000 micron hypothalamic brain slices. Slice 2-deoxyglucose (2DG) and 2-deoxyglucose-6-phosphate (2DG6P) levels were measured after tissue homogenization and perchloric acid extraction. By analyzing the uptake and washout kinetics with nonlinear least-squares methods, we have determined the rate constants for three-, four-, or five-parameter kinetic models and obtained a value for the in vitro lumped constant (LC). The kinetic analysis reveals a small, slowly decaying, 2DG component that is not predicted by any of the models. If this component is treated as a separate, parallel compartment, then the four- and five-parameter models are essentially equivalent. To compare our data to prior in vivo data, we combined 2DG and 2DG6P to produce Ci*, the total slice radioactivity, and analyzed the first 45 min of uptake. These data were fit best by a three-parameter model and the slowly decaying pool was not identified. Calculation of glucose utilization from total tissue radioactivity, measured by whole slice homogenization and by image analysis of autoradiograms, showed excellent correlation between the two methods. Image analysis of radioactivity in the suprachiasmatic nucleus, which is present in these slices, revealed a spontaneous diurnal variation in in vitro glucose utilization in close quantitative agreement with prior in vivo measurements. The kinetic analysis of the 1,000 micron slice was qualitatively similar to that of the 540 micron slice but revealed an increase in the LC and a large decrease in k1 as well as the expected large increase in the hexokinase rate constant, k3. Overall, in vitro glucose utilization increased by about 60%. These results are consistent with our prior studies of the 1,000 micron slice and support our interpretation that the 1,000 micron slice is an excellent in vitro model for brain ischemia without infarction.

Quantitative assessment of blood-brain barrier permeability in multiple sclerosis using 68-Ga-EDTA and positron emission tomography

Fifteen patients with definite multiple sclerosis were examined with high volume delayed (HVD) CT scan and positron emission tomography (PET) using 68-Ga-EDTA as a tracer. The passage of 68-Ga-EDTA across the blood-brain barrier (BBB) was measured by using multiple graphical analysis. This method permits the simultaneous calculation of a blood to brain influx constant Ki (ml/g-1 min-1) and of the plasma volume Vp (ml/g-1). Focal areas of abnormal CT enhancement and pathological accumulation of 68-Ga-EDTA were visualised in four patients who were all examined during a clinical exacerbation of the disease. The mean Ki value measured in these areas was 12.5, SD 3.4 indicating a moderate but significant increase of BBB permeability compared with the value found in normal tissue (3.2, SD 0.9). No parallel increase in Vp values was found in these pathological areas. Quantitative data obtained with PET seems to provide further insight into the study of BBB function in multiple sclerosis.

Quantification of neuroreceptors in the living human brain. I. Irreversible binding of ligands

A first step in the quantification of receptor density in the living human brain is the measurement of the binding of a labeled ligand to the receptor in question. In the present study, we determined the rate of binding of 11C-labeled N-methylspiperone (NMSP) to the D2 dopamine receptor in 11 normal volunteers, using a three-compartment model to relate the time integral of the measured plasma concentration to the distribution of the tracer in the caudate nucleus. The plasma concentrations of NMSP were separated from the contaminating metabolites by the ratio of radioactivities in cerebellum and blood plasma. Plasma concentrations calculated in this way agreed with plasma concentrations determined by HPLC. The rate of binding of labeled NMSP to its receptors (k3) was defined as the product of the bimolecular association rate (kon) and the quantity of available receptors (B'max) and calculated as the ratio between the steady-state rate of accumulation and the volume of distribution of labeled NMSP in the caudate nucleus. The average value of k3 in the 11 normal volunteers was 0.065 min-1. The fractional clearance of labeled NMSP from the caudate nucleus (k2) was 0.070 min-1 and thus close to the value of k3. We also examined several indexes of binding based on ratios between different regions in brain. The indexes required that binding be negligible compared to the efflux of labeled NMSP (i.e., k2 much greater than k3) and therefore yielded incorrectly low values of k3. Thus, the only accurate approach used measured plasma concentrations to estimate transfer constants at steady state and yielded the absolute rate of binding k3. The approach is applicable to other irreversibly bound ligands.