Sami S. Zoghbi

[sup 123 I] beta-CIT/SPECT imaging demonstrates bilateral loss of dopamine transporters in hemi-Parkinson's disease

Article abstract-We have used in vivo single-photon emission computed tomography (SPECT) of the dopamine transporter with 2 beta-carboxymethoxy-3 beta-(4-iodophenyl)tropane ([sup 123 I] beta-CIT) to investigate striatal dopamine transporter loss in patients with early Parkinsons disease (PD). Striatal uptake of [sup 123 I] beta-CIT was compared in eight early-PD patients with exclusively hemiparkinsonism and eight age- and sex-matched healthy subjects. [sup 123 I] beta-CIT striatal uptake was reduced by approximately 53% contralateral and by 38% ipsilateral to the clinically symptomatic side in the hemi-PD patients, compared with the mean striatal uptake in age- and sex-matched healthy subjects. The relative reduction in [sup 123 I] beta-CIT uptake in the hemi-PD patients was greater in the putamen than in the caudate. These data demonstrate that SPECT imaging of the dopamine transporter with [sup 123 I] beta-CIT can identify patients with PD at the onset of motor symptoms and suggest that this technique also may be useful in identifying individuals with developing dopaminergic pathology before onset of motor symptoms. NEUROLOGY 1996;46: 231-237

Graphical, Kinetic, and Equilibrium Analyses of in vivo [123I]β-CIT Binding to Dopamine Transporters in Healthy Human Subjects

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

MICRODIALYSIS AND SPECT MEASUREMENTS OF AMPHETAMINE-INDUCED DOPAMINE RELEASE IN NONHUMAN PRIMATES

The competition between endogenous transmitters and radiolabeled ligands for in vivo binding to neuroreceptors might provide a method to measure endogenous transmitter release in the living human brain with noninvasive techniques such as positron emission tomography (PET) or single photon emission computerized tomography (SPECT). In this study, we validated the measure of amphetamine‐induced dopamine release with SPECT in nonhuman primates. Microdialysis experiments were conducted to establish the dose‐response curve of amphetamine‐induced dopamine release and to document how pretreatment with the dopamine depleter alpha‐methyl‐para‐tyrosine (αMPT) affects this response. SPECT experiments were performed with two iodinated benzamides, [123I]IBZM and [123I]IBF, under sustained equilibrium condition. Both radio‐tracers are specific D2 antagonists, but the affinity of [123I]IBZM (KD = 0.4 nM) is lower than that of [123I]IBF (KD = 0.1 nM). With both tracers, we observed a prolonged reduction in binding to D2 receptors following amphetamine injection. [123I]IBZM binding to D2 receptors was more affected than [123I]IBF by high doses of amphetamine, indicating that a lower affinity increases the vulnerability of a tracer to endogenous competition. With [123I]IBZM, we observed an excellent correlation between reduction of D2 receptor binding measured with SPECT and peak dopamine release measured with microdialysis after various doses of amphetamine. Pretreatment with αMPT significantly reduced the effect of amphetamine on [123I]IBZM binding to D2 receptors, confirming that this effect was mediated by intrasynaptic dopamine release. Together, these results validate the use of this SPECT paradigm as a noninvasive measurement of intrasynaptic dopamine release in the living brain. Synapse 25:1–14, 1997.

Imaging D2 Receptor Occupancy by Endogenous Dopamine in Humans

The impact of endogenous dopamine on in vivo measurement of D2 receptors in humans was evaluated with single photon emission computerized tomography (SPECT), by comparing the binding potential (BP) of the selective D2 radiotracer [123I]IBZM before and after acute dopamine depletion. Dopamine depletion was achieved by administration of the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (AMPT), given orally at a dose of 1 g every six hours for two days. AMPT increased [123I]IBZM BP by 28 ± 16% (±SD, n = 9). Experiments in rodents suggested that this effect was due to removal of endogenous dopamine rather than D2 receptor upregulation. Synaptic dopamine concentration was estimated as 45 ± 25 nM, in agreement with values reported in rodents. The amplitude and the variability of the AMPT effect suggested that competition by endogenous dopamine introduces a significant error in measurement of D2 receptors in vivo with positron emission tomography (PET) or SPECT. However, these results also imply that D2 receptor imaging coupled with acute dopamine depletion might provide estimates of synaptic dopamine concentration in the living human brain.

Reversible and regionally selective downregulation of brain cannabinoid CB1 receptors in chronic daily cannabis smokers

Chronic cannabis (marijuana, hashish) smoking can result in dependence. Rodent studies show reversible downregulation of brain cannabinoid CB1 (cannabinoid receptor type 1) receptors after chronic exposure to cannabis. However, whether downregulation occurs in humans who chronically smoke cannabis is unknown. Here we show, using positron emission tomography imaging, reversible and regionally selective downregulation of brain cannabinoid CB1 receptors in human subjects who chronically smoke cannabis. Downregulation correlated with years of cannabis smoking and was selective to cortical brain regions. After ∼4 weeks of continuously monitored abstinence from cannabis on a secure research unit, CB1 receptor density returned to normal levels. This is the first direct demonstration of cortical cannabinoid CB1 receptor downregulation as a neuroadaptation that may promote cannabis dependence in human brain.

In vivo radioligand binding to translocator protein correlates with severity of Alzheimer’s disease

Neuroinflammation is a pathological hallmark of Alzheimers disease, but its role in cognitive impairment and its course of development during the disease are largely unknown. To address these unknowns, we used positron emission tomography with (11)C-PBR28 to measure translocator protein 18 kDa (TSPO), a putative biomarker for inflammation. Patients with Alzheimers disease, patients with mild cognitive impairment and older control subjects were also scanned with (11)C-Pittsburgh Compound B to measure amyloid burden. Twenty-nine amyloid-positive patients (19 Alzheimers, 10 mild cognitive impairment) and 13 amyloid-negative control subjects were studied. The primary goal of this study was to determine whether TSPO binding is elevated in patients with Alzheimers disease, and the secondary goal was to determine whether TSPO binding correlates with neuropsychological measures, grey matter volume, (11)C-Pittsburgh Compound B binding, or age of onset. Patients with Alzheimers disease, but not those with mild cognitive impairment, had greater (11)C-PBR28 binding in cortical brain regions than controls. The largest differences were seen in the parietal and temporal cortices, with no difference in subcortical regions or cerebellum. (11)C-PBR28 binding inversely correlated with performance on Folstein Mini-Mental State Examination, Clinical Dementia Rating Scale Sum of Boxes, Logical Memory Immediate (Wechsler Memory Scale Third Edition), Trail Making part B and Block Design (Wechsler Adult Intelligence Scale Third Edition) tasks, with the largest correlations observed in the inferior parietal lobule. (11)C-PBR28 binding also inversely correlated with grey matter volume. Early-onset (<65 years) patients had greater (11)C-PBR28 binding than late-onset patients, and in parietal cortex and striatum (11)C-PBR28 binding correlated with lower age of onset. Partial volume corrected and uncorrected results were generally in agreement; however, the correlation between (11)C-PBR28 and (11)C-Pittsburgh Compound B binding was seen only after partial volume correction. The results suggest that neuroinflammation, indicated by increased (11)C-PBR28 binding to TSPO, occurs after conversion of mild cognitive impairment to Alzheimers disease and worsens with disease progression. Greater inflammation may contribute to the precipitous disease course typically seen in early-onset patients. (11)C-PBR28 may be useful in longitudinal studies to mark the conversion from mild cognitive impairment or to assess response to experimental treatments of Alzheimers disease.

Kinetic Analysis in Healthy Humans of a Novel Positron Emission Tomography Radioligand to Image the Peripheral Benzodiazepine Receptor, a Potential Biomarker for Inflammation

The peripheral benzodiazepine receptor (PBR) is upregulated on activated microglia and macrophages and thereby is a useful biomarker of inflammation. We developed a novel PET radioligand, [(11)C]PBR28, that was able to image and quantify PBRs in healthy monkeys and in a rat model of stroke. The objective of this study was to evaluate the ability of [(11)C]PBR28 to quantify PBRs in brain of healthy human subjects. Twelve subjects had PET scans of 120 to 180 min duration as well as serial sampling of arterial plasma to measure the concentration of unchanged parent radioligand. One- and two-tissue compartmental analyses were performed. To obtain stable estimates of distribution volume, which is a summation of B(max)/K(D) and nondisplaceable activity, 90 min of brain imaging was required. Distribution volumes in human were only approximately 5% of those in monkey. This comparatively low amount of receptor binding required a two-rather than a one-compartment model, suggesting that nonspecific binding was a sizeable percentage compared to specific binding. The time-activity curves in two of the twelve subjects appeared as if they had no PBR binding-i.e., rapid peak of uptake and fast washout from brain. The cause(s) of these unusual findings are unknown, but both subjects were also found to lack binding to PBRs in peripheral organs such as lung and kidney. In conclusion, with the exception of those subjects who appeared to have no PBR binding, [(11)C]PBR28 is a promising ligand to quantify PBRs and localize inflammation associated with increased densities of PBRs.

Persistent Dopamine Functions of Neurons Derived from Embryonic Stem Cells in a Rodent Model of Parkinson Disease

The derivation of dopamine neurons is one of the best examples of the clinical potential of embryonic stem (ES) cells, but the long‐term function of the grafted neurons has not been established. Here, we show that, after transplantation into an animal model, neurons derived from mouse ES cells survived for over 32 weeks, maintained midbrain markers, and had sustained behavioral effects. Microdialysis in grafted animals showed that dopamine (DA) release was induced by depolarization and pharmacological stimulants. Positron emission tomography measured the expression of presynaptic dopamine transporters in the graft and also showed that the number of postsynaptic DA D2 receptors was normalized in the host striatum. These data suggest that ES cell‐derived neurons show DA release and reuptake and stimulate appropriate postsynaptic responses for long periods after implantation. This work supports continued interest in ES cells as a source of functional DA neurons.

A Genetic Polymorphism for Translocator Protein 18 Kda Affects both in Vitro and in Vivo Radioligand Binding in Human Brain to this Putative Biomarker of Neuroinflammation

Second-generation radioligands for translocator protein (TSPO), an inflammation marker, are confounded by the codominant rs6971 polymorphism that affects binding affinity. The resulting three groups are homozygous for high-affinity state (HH), homozygous for low-affinity state (LL), or heterozygous (HL). We tested if in vitro binding to leukocytes distinguished TSPO genotypes and if genotype could affect clinical studies using the TSPO radioligand [11C]PBR28. In vitro binding to leukocytes and [11C]PBR28 brain imaging were performed in 27 human subjects with known TSPO genotype. Specific [3H]PBR28 binding was measured in prefrontal cortex of 45 schizophrenia patients and 47 controls. Leukocyte binding to PBR28 predicted genotype in all subjects. Brain uptake was ~40% higher in HH than HL subjects. Specific [3H]PBR28 binding in LL controls was negligible, while HH controls had ~80% higher binding than HL controls. After excluding LL subjects, specific binding was 16% greater in schizophrenia patients than controls. This difference was insignificant by itself (P = 0.085), but was significant after correcting for TSPO genotype (P = 0.011). Our results show that TSPO genotype influences PBR28 binding in vitro and in vivo. Correcting for this genotype increased statistical power in our postmortem study and is recommended for in vivo positron emission tomography studies.

Imaging the function of P-glycoprotein with radiotracers: pharmacokinetics and in vivo applications.

P‐glycoprotein (P‐gp), an efflux transporter, controls the pharmacokinetics of various compounds under physiological conditions. P‐gp‐mediated drug efflux has been suggested as playing a role in various disorders, including multidrug‐resistant cancer and medication‐refractory epilepsy. However, P‐gp inhibition has had, to date, little or no clinically significant effect in multidrug‐resistant cancer. To enhance our understanding of its in vivo function under pathophysiological conditions, substrates of P‐gp have been radiolabeled and imaged using single‐photon emission computed tomography (SPECT) and positron emission tomography (PET). To accurately quantify P‐gp function, a radiolabeled P‐gp substrate should be selective for P‐gp, produce a large signal after P‐gp blockade, and generate few radiometabolites that enter the target tissue. Furthermore, quantification of P‐gp function via imaging requires pharmacological inhibition of P‐gp, which requires knowledge of P‐gp density at the target site. By meeting these criteria, imaging can elucidate the function of P‐gp in various disorders and improve the efficacy of treatments.