Two Binding Sites for [³H]PBR28 in Human Brain: Implications for TSPO PET Imaging of Neuroinflammation

Abstract

[11C]PBR28, a radioligand targeting the translocator protein (TSPO), does not produce a specific binding signal in approximately 14% of healthy volunteers. This phenomenon has not been reported for [11C]PK11195, another TSPO radioligand. We measured the specific binding signals with [3H]PK11195 and [3H]PBR28 in brain tissue from 22 donors. Overall, 23% of the samples did not generate a visually detectable specific autoradiographic signal with [3H]PBR28, although all samples showed [3H]PK11195 binding. There was a marked reduction in the affinity of [3H]PBR28 for TSPO in samples with no visible [3H]PBR28 autoradiographic signal ( K i=188±15.6 nmol/L), relative to those showing normal signal ( K i=3.4±0.5 nmol/L, P<0.001). Of this latter group, [3H]PBR28 bound with a two-site fit in 40% of cases, with affinities ( K i) of 4.0±2.4 nmol/L (high-affinity site) and 313±77 nmol/L (low-affinity site). There was no difference in Kd or Bmax for [3H]PK11195 in samples showing no [3H]PBR28 autoradiographic signal relative to those showing normal [3H]PBR28 autoradiographic signal. [3H]PK11195 bound with a single site for all samples. The existence of three different binding patterns with PBR28 (high-affinity binding (46%), low-affinity binding (23%), and two-site binding (31%)) suggests that a reduction in [11C]PBR28 binding may not be interpreted simply as a reduction in TSPO density. The functional significance of differences in binding characteristics warrants further investigation.

Quantification of the Specific Translocator Protein Signal of 18F-PBR111 in Healthy Humans: A Genetic Polymorphism Effect on In Vivo Binding

“…The TSPO is a nuclear encoded mitochondrial protein, present in high density in macrophages and microglia and elevated in a variety of neuroinflammatory brain diseases, such as Alzheimer disease, multiple sclerosis, stroke, and Huntington disease (2)(3)(4)(5). PK11195 ([1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide]), a selective antagonist for the TSPO, has been labeled with 11 C and used as a PET radioligand for more than 15 y to image neuroinflammation (6). However, the low brain extraction and poor signal-to-noise ratio of 11 C-(R)-PK11195 images have led to the search for improved TSPO PET imaging agents.…”

“…PK11195 ([1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide]), a selective antagonist for the TSPO, has been labeled with 11 C and used as a PET radioligand for more than 15 y to image neuroinflammation (6). However, the low brain extraction and poor signal-to-noise ratio of 11 C-(R)-PK11195 images have led to the search for improved TSPO PET imaging agents.…”

“…In the past 5 y, a substantial number (.50) of second-generation TSPO ligands have been proposed, including 18 F-PBR111, 11 C-PBR28, 18 F-FEPPA ( 18 F-N-2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide), and 11 C-DPA713 ( 11 C-N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethyl-pyrazolo [1,5-a]pyrimidin-3-yl]-acetamide) (7)(8)(9)(10). Although these ligands were claimed to have increased delivery across the blood-brain barrier or a higher signal-to-noise ratio than 11 C-(R)-PK11195, initial human studies have demonstrated considerable variability in their binding across subjects, including the presence of the so-called nonbinders, healthy subjects who demonstrated very little if any binding of 11 C-PBR28 (8).…”

“…Although these ligands were claimed to have increased delivery across the blood-brain barrier or a higher signal-to-noise ratio than 11 C-(R)-PK11195, initial human studies have demonstrated considerable variability in their binding across subjects, including the presence of the so-called nonbinders, healthy subjects who demonstrated very little if any binding of 11 C-PBR28 (8). We have previously demonstrated in vitro, in human tissue samples, that all second-generation ligands tested bind TSPO with either high affinity (high-affinity binders, HABs), or low affinity (low-affinity binders, LABs), and that some subjects have a mix of high-and low-affinity binding sites (mixed-affinity binders, MABs) (11,12). This finding has retrospectively explained the "nonbinders" phenomenon, as the affinity of 11 C-PBR28 to LABs was only 188 nM (11) and thus no specific binding in these subjects would be expected.…”

Imaging in Parkinson's Disease

“…Following CNS injury, microglia become activated overexpress the 18-kDa translocator protein (TSPO) that is involved in the release of proinflammatory cytokines during inflammation and is present at very low levels in the normal healthy CNS (Banati, 2002). The upregulation of TSPO expression can be detected in vivo with PET and selective radioligands Ikoma et al, 2007;Oh et al, 2011;Owen et al, 2010Owen et al, , 2014Su & Politis, 2012;Vas et al, 2008), and the most widely used TSPO radioligand to date is [ 11 C]PK11195 (Banati et al, 1999(Banati et al, , 2000Chauveau, Boutin, Van Camp, Dolle, & Tavitian, 2008). In early de novo PD patients, [ 11 C]PK11195 PET binding in the midbrain contralateral to the clinically affected side was significantly increased compared to a group of healthy controls and increased microglial activation was associated with putamen dopaminergic deficits and increased motor symptom severity (Ouchi et al, 2005).…”