11C-PBR28 PET can detect the 18-kDa translocator protein (TSPO) expressed within macrophages. However, quantitative evaluation of the signal in brain tissue from donors with multiple sclerosis (MS) shows that PBR28 binds the TSPO with high affinity (binding affinity [Ki], ~4 nM), low affinity (Ki, ~200 nM), or mixed affinity (2 sites with Ki, ~4 nM and ~300 nM). Our study tested whether similar binding behavior could be detected in brain tissue from donors with no history of neurologic disease, with TSPO-binding PET ligands other than 11C-PBR28, for TSPO present in peripheral blood, and with human brain PET data acquired in vivo with 11C-PBR28. Methods The affinity of TSPO ligands was measured in the human brain post-mortem from donors with a history of MS (n = 13), donors without any history of neurologic disease (n = 20), and in platelets from healthy volunteers (n = 13). Binding potential estimates from thirty-five 11C-PBR28 PET scans from an independent sample of healthy volunteers were analyzed using a gaussian mixture model. Results Three binding affinity patterns were found in brains from subjects without neurologic disease in similar proportions to those reported previously from studies of MS brains. TSPO ligands showed substantial differences in affinity between subjects classified as high-affinity binders (HABs) and low-affinity binders (LABs). Differences in affinity between HABs and LABs are approximately 50-fold with PBR28, approximately 17-fold with PBR06, and approximately 4-fold with DAA1106, DPA713, and PBR111. Where differences in affinity between HABs and LABs were low (~4-fold), distinct affinities were not resolvable in binding curves for mixed-affinity binders (MABs), which appeared to express 1 class of sites with an affinity approximately equal to the mean of those for HABs and LABs. Mixed-affinity binding was detected in platelets from an independent sample (HAB, 69%; MAB, 31%), although LABs were not detected. Analysis of 11C-PBR28 PET data was not inconsistent with the existence of distinct subpopulations of HABs, MABs, and LABs. Conclusion With the exception of 11C-PK11195, all TSPO PET ligands in current clinical application recognize HABs, LABs, and MABs in brain tissue in vitro. Knowledge of subjects’ binding patterns will be required to accurately quantify TSPO expression in vivo using PET.

Neuroinflammation is a pathological hallmark of Alzheimer's 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 Alzheimer's 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 Alzheimer's, 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 Alzheimer's 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 Alzheimer's 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 Alzheimer's 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 Alzheimer's disease.

Alzheimer’s disease (AD) is associated with increase in brain of the 18 kDa translocator protein (TSPO), which is over-expressed in activated microglia and reactive astrocytes. Measuring the density of TSPO with PET typically requires absolute quantitation with arterial blood sampling, because a reference region devoid of TSPO does not exist in brain. We sought to determine whether a simple ratio method could substitute for absolute quantitation of binding with 11C-PBR28, a second generation radioligand for TSPO. Methods 11C-PBR28 PET imaging was performed in 21 healthy controls, 11 individuals with mild cognitive impairment (MCI), and 25 AD patients. Group differences in 11C-PBR28 binding were compared using two methods. First, the “gold standard” method of calculating total distribution volume (VT), using the two-tissue compartmental model with the arterial input function, corrected for plasma free fraction of radiotracer (fP). Second, a ratio of brain uptake in target regions to that in cerebellum—i.e., standardized uptake value ratio (SUVR). Results Using absolute quantitation, we confirmed that TSPO binding (VT/fP): 1) was greater in AD patients than in healthy controls in expected temporo-parietal regions, and 2) was not significantly different among the three groups in cerebellum. Using the cerebellum as a pseudo-reference region, the SUVR method detected greater binding in AD patients than controls in the same regions as absolute quantification and in one additional region, suggesting SUVR may have greater sensitivity. Coefficients of variation of SUVR measurements were about two-thirds lower than those of absolute quantification, and the resulting statistical significance was much higher for SUVR when comparing AD and healthy controls (e.g. P < 0.0005 for SUVR vs. P = 0.023 for VT/fP in combined middle and inferior temporal cortex). Conclusion To measure TSPO density in AD and control subjects, a simple ratio method SUVR can substitute for, and may even be more sensitive than, absolute quantitation. The SUVR method is expected to improve subject tolerability by allowing shorter scan time and not requiring arterial catheterization. In addition, this ratio method allows smaller sample sizes for comparable statistical significance because of the relatively low variability of the ratio values.

Ten percent of humans lack specific binding of [11C]PBR28 to 18 kDa translocator protein (TSPO), a biomarker for inflammation. “Non-binders” have not been reported using another TSPO radioligand, [11C]-(R)-PK 11195, despite its use for more than two decades. This study asked two questions: 1) What is the cause of non-binding to PBR28? 2) Why has this phenomenon not been reported using [11C]-(R)-PK 11195? Methods Five binders and five non-binders received whole-body imaging with both [11C]-(R)-PK 11195 and [11C]PBR28. In vitro binding was performed using leukocyte membranes from binders and non-binders and the tritiated versions of the ligand. Rhesus monkeys were imaged with [11C]-(R)-PK 11195 at baseline and after blockade of TSPOs. Results Using [11C]PBR28, uptake in all five organs with high densities of TSPO (lung, heart, brain, kidney, and spleen) was 50% to 75% lower in non-binders than in binders. In contrast, [11C]-(R)-PK 11195 distinguished binders and non-binders in only heart and lung. For the in vitro assay, [3H]PBR28 had more than ten-fold lower affinity to TSPO in non-binders than in binders. The in vivo specific binding of [11C]-(R)-PK 11195 in monkey brain was ∼80-fold lower than that reported for [11C]PBR28. Conclusions Based on binding of [3H]PK 11195 to leukocyte membranes, both binders and non-binders express TSPO. Non-binding to PBR28 is caused by its low affinity for TSPO in non-binders. Non-binding may be differentially expressed in organs of the body. The relatively low in vivo specific binding of [11C]-(R)-PK 11195 may have obscured its detection of non-binding in peripheral organs.

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 [(11)C]PBR28. In vitro binding to leukocytes and [(11)C]PBR28 brain imaging were performed in 27 human subjects with known TSPO genotype. Specific [(3)H]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 [(3)H]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.

Permeability-glycoprotein (P-gp), an efflux transporter in several organs, acts at the blood-brain barrier to protect the brain from exogenous toxins. P-gp almost completely blocks brain entry of the PET radiotracer 11 C-N-desmethyl-loperamide ( 11 C-dLop). We examined the ability of 11 C-dLop to quantify P-gp function in humans after increasing doses of tariquidar, an inhibitor of P-gp. Methods: Seventeen healthy volunteers had a total of 23 PET scans with 11 C-dLop at baseline and after increasing doses of tariquidar (2, 4, and 6 mg/kg intravenously). A subset of subjects received PET with 15 O-H 2 O to measure cerebral blood flow. Brain uptake of 11 C-dLop was quantified in 2 ways. Without blood data, uptake was measured as area under the time-activity curve in the brain from 10 to 30 min (AUC 10-30 ). With arterial blood data, brain uptake was quantified with compartmental modeling to estimate the rates of entry into (K 1 ) and efflux from (k 2 ) the brain. Results: Brain uptake of radioactivity was negligible at baseline and increased only slightly (;30%) after 2 mg of tariquidar per kilogram. In contrast, 4 and 6 mg of tariquidar per kilogram increased brain uptake 2-and 4-fold, respectively. Greater brain uptake reflected greater brain entry (K 1 ), because efflux (k 2 ) and cerebral blood flow did not differ between tariquidartreated and untreated subjects. In the subjects who received the highest dose of tariquidar (and had the highest brain uptake), regional values of K 1 correlated linearly with absolute cerebral blood flow, consistent with high single-pass extraction of 11 C-dLop. AUC 10-30 correlated linearly with K 1 . Conclusion: P-gp function at the blood-brain barrier in humans can be quantified using PET and 11 C-dLop. A simple measure of brain uptake (AUC 10-30 ) may be used as a surrogate of the fully quantified rate constant for brain entry (K 1 ) and thereby avoid arterial sampling. However, to dissect the function of P-gp itself, both brain uptake and the influx rate constant must be corrected for radiotracer delivery (blood flow).

The potential for positron emission tomography (PET) to detect neuroinflammation in vivo has sparked a remarkable interest in various disciplines of neuroscience. Early PET radioligands, such as [C]PK(R)-11195 for the 18-kDa translocator protein (TSPO) and [C]L-deprenyl for monoamine oxidase B, have been used in studies designed to clarify the role of neuroinflammation in a variety of psychiatric and neurological disorders. Recent years have witnessed the development of several second-generation PET radioligands for TSPO and radioligands to measure endogenous targets that are active in various stages of the inflammatory cascade, such as cyclooxygenase and arachidonic acid. Here, we discuss some of the biomarkers for neuroinflammation that are available for quantification with PET, as well as recent findings from studies where neuroinflammation has been assessed in neurodegenerative disorders. In addition, we highlight the challenges to accurate interpretation of PET studies of neuroinflammation.

IMPORTANCE Neuroinflammation may play a role in epilepsy. Translocator protein 18 kDa (TSPO), a biomarker of neuroinflammation, is overexpressed on activated microglia and reactive astrocytes. A preliminary positron emission tomographic (PET) imaging study using carbon 11 ([ 11 C])-labeled PBR28 in patients with temporal lobe epilepsy (TLE) found increased TSPO ipsilateral to seizure foci. Full quantitation of TSPO in vivo is needed to detect widespread inflammation in the epileptic brain. OBJECTIVES To determine whether patients with TLE have widespread TSPO overexpression using [ 11 C]PBR28 PET imaging, and to replicate relative ipsilateral TSPO increases in patients with TLE using [ 11 C]PBR28 and another TSPO radioligand, [ 11 C]DPA-713. DESIGN, SETTING, AND PARTICIPANTS In a cohort study from March 2009 through September 2013 at the Clinical Epilepsy Section of the National Institute of Neurological Disorders and Stroke, participants underwent brain PET and a subset had concurrent arterial sampling. Twenty-three patients with TLE and 11 age-matched controls were scanned with [ 11 C]PBR28, and 8 patients and 7 controls were scanned with [ 11 C]DPA-713. Patients with TLE had unilateral temporal seizure foci based on ictal electroencephalography and structural magnetic resonance imaging. Participants with homozygous low-affinity TSPO binding were excluded. MAIN OUTCOMES AND MEASURESThe [ 11 C]PBR28 distribution volume (V T ) corrected for free fraction (f P ) was measured in patients with TLE and controls using FreeSurfer software and T1-weighted magnetic resonance imaging for anatomical localization of bilateral temporal and extratemporal regions. Side-to-side asymmetry in patients with TLE was calculated as the ratio of ipsilateral to contralateral [ 11 C]PBR28 and [ 11 C]DPA-713 standardized uptake values from temporal regions.RESULTS The [ 11 C]PBR28 V T to f p ratio was higher in patients with TLE than in controls for all ipsilateral temporal regions (27%-42%; P < .05) and in contralateral hippocampus, amygdala, and temporal pole (approximately 30%-32%; P < .05). Individually, 12 patients, 10 with mesial temporal sclerosis, had asymmetrically increased hippocampal [ 11 C]PBR28 uptake exceeding the 95% confidence interval of the controls. Binding of [ 11 C]PBR28 was increased significantly in thalamus. Relative [ 11 C]PBR28 and [ 11 C]DPA-713 uptakes were higher ipsilateral than contralateral to seizure foci in patients with TLE ([ 11 C]PBR28: 2%-6%; [ 11 C]DPA-713: 4%-9%). Asymmetry of [ 11 C]DPA-713 was greater than that of [ 11 C]PBR28 (F = 29.4; P = .001).CONCLUSIONS AND RELEVANCE Binding of TSPO is increased both ipsilateral and contralateral to seizure foci in patients with TLE, suggesting ongoing inflammation. Anti-inflammatory therapy may play a role in treating drug-resistant epilepsy.