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.

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 ∼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.

We sought to develop (11)C-labeled ligands for sensitive imaging of brain peripheral benzodiazepine receptors (PBR) in vivo. Two aryloxyanilides with high affinity for PBR were identified and synthesized, namely, N-acetyl- N-(2-methoxycarbonylbenzyl)-2-phenoxyaniline ( 3, PBR01) and N-(2-methoxybenzyl)- N-(4-phenoxypyridin-3-yl)acetamide ( 10, PBR28). 3 was hydrolyzed to 4, which was esterified with [ (11)C]iodomethane to provide [ (11)C] 3. The O-desmethyl analogue of 10 was converted into [ (11)C] 10 with [ (11)C]iodomethane. [ (11)C] 3 and [ (11)C] 10 were each injected into monkey to assess their brain kinetics with positron emission tomography (PET). After administration of either radioligand there was moderately high brain uptake of radioactivity. Receptor blocking and displacement experiments showed that a high proportion of this radioactivity was bound specifically to PBR. In monkey and rat, 3 and 10 were rapidly metabolized by ester hydrolysis and N-debenzylation, respectively, each to a single polar radiometabolite. [ (11)C] 3 and [ (11)C] 10 are effective for imaging PBR in monkey brain. [ (11)C] 10 especially warrants further evaluation in human subjects.

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.