Translocator Protein (TSPO) Positron Emission Tomography (PET) imaging is a powerful in vivo technique for visualizing and quantifying neuroinflammation in the living human brain. Originally known as the Peripheral Benzodiazepine Receptor (PBR), TSPO is an 18 kDa transmembrane protein located primarily on the outer mitochondrial membrane of [microglia](/cell-types/microglia-neuroinflammation), the resident immune cells of the central nervous system [1](https://pubmed.ncbi.nlm.nih.gov/28459611/). Following activation of microglia in response to neuronal injury, infection, or disease processes, TSPO expression increases substantially, making it a sensitive biomarker for detecting neuroinflammatory processes in neurodegenerative diseases [2](https://pubmed.ncbi.nlm.nih.gov/23274174/). [@tspo2017]
This mechanism page provides a comprehensive overview of TSPO biology, the development of PET radiotracers targeting TSPO, clinical applications in Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic Lateral Sclerosis (ALS), Frontotemporal Dementia (FTD), and Multiple Sclerosis (MS), as well as technical considerations and future directions for therapeutic monitoring. [@pet2013]
Translocator Protein (TSPO) Positron Emission Tomography (PET) imaging is a powerful in vivo technique for visualizing and quantifying neuroinflammation in the living human brain. Originally known as the Peripheral Benzodiazepine Receptor (PBR), TSPO is an 18 kDa transmembrane protein located primarily on the outer mitochondrial membrane of [microglia](/cell-types/microglia-neuroinflammation), the resident immune cells of the central nervous system [1](https://pubmed.ncbi.nlm.nih.gov/28459611/). Following activation of microglia in response to neuronal injury, infection, or disease processes, TSPO expression increases substantially, making it a sensitive biomarker for detecting neuroinflammatory processes in neurodegenerative diseases [2](https://pubmed.ncbi.nlm.nih.gov/23274174/). [@tspo2017]
This mechanism page provides a comprehensive overview of TSPO biology, the development of PET radiotracers targeting TSPO, clinical applications in Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic Lateral Sclerosis (ALS), Frontotemporal Dementia (FTD), and Multiple Sclerosis (MS), as well as technical considerations and future directions for therapeutic monitoring. [@pet2013]
The translocator protein (TSPO) is encoded by the TSPO gene (also known as PBR) located on chromosome 22q13.3 [3](https://pubmed.ncbi.nlm.nih.gov/21606672/). TSPO is highly conserved across species and is expressed ubiquitously in peripheral tissues including kidney, heart, lung, and adrenal glands, as well as in the brain where it is predominantly expressed by microglia [4](https://pubmed.ncbi.nlm.nih.gov/22951353/). Under physiological conditions, TSPO is present at low levels in the healthy brain, with baseline expression primarily restricted to microglia with minimal neuronal expression. [@tspo2012]
The protein functions as part of a complex that includes the voltage-dependent anion channel (VDAC) and the adenine nucleotide translocase (ANT) on the mitochondrial membrane, playing roles in cholesterol transport, heme synthesis, cell proliferation, and [apoptosis](/entities/apoptosis) [5](https://pubmed.ncbi.nlm.nih.gov/25557139/). The TSPO complex is part of the mitochondrial permeability transition pore (mPTP), though its exact physiological functions remain an area of active investigation. [@tspo2014]
Neuroinflammation is a hallmark of virtually all neurodegenerative diseases and is characterized by microglial activation, cytokine release, and immune cell recruitment [6](https://pubmed.ncbi.nlm.nih.gov/32044179/). Activated microglia adopt a spectrum of phenotypes, from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype, and TSPO expression increases significantly during microglial activation regardless of the specific phenotype [7](https://pubmed.ncbi.nlm.nih.gov/24345279/). [@tspo2016]
The increase in TSPO expression during neuroinflammation makes it an attractive target for PET imaging, as it allows for the visualization of microglial activation in vivo. TSPO PET signal correlates with histopathological measures of microglial density in post-mortem brain tissue [8](https://pubmed.ncbi.nlm.nih.gov/25209713/), providing validation for this imaging biomarker. [@neuroinflammation2020]
The first widely used TSPO PET tracer was [^11C]PK11195 (R-[^11C]PK11195), developed in the early 1980s [9](https://pubmed.ncbi.nlm.nih.gov/6886384/). PK11195 is a isoquinoline carboxamide with high affinity for TSPO (Kd ≈ 2-5 nM). Despite its widespread use, [^11C]PK11195 has several limitations: [@microglial2014]
Second-generation TSPO tracers were developed to address the limitations of [^11C]PK11195. Key examples include: [@development1983]
[^11C]PBR28 (N-acetyl-N-(2-[^11C]methoxybenzyl)-2-phenoxy-5-pyridinamine): Developed in 2008, PBR28 shows higher affinity for TSPO and improved brain penetration compared to PK11195 [10](https://pubmed.ncbi.nlm.nih.gov/18465950/). However, PBR28 binding is highly variable among individuals due to a TSPO polymorphism (see Technical Considerations below). [@cpbr2008]
[^11C]DPA-713 (N,N-diethyl-2-[^11C]acetamidobenzamide): Developed in 2008, DPA-713 demonstrates improved signal-to-noise ratio and lower non-specific binding compared to PK11195 [11](https://pubmed.ncbi.nlm.nih.gov/18558647/). DPA-713 also shows sensitivity to the TSPO polymorphism but to a lesser extent than PBR28. [@cdpa2008]
The development of fluorine-18 labeled TSPO tracers has revolutionized the field by enabling broader distribution due to the longer half-life of ^18F (110 minutes). Key third-generation tracers include: [@ffeppa2010]
[^18F]FEPPA (N-(5-fluoro-2-phenoxyphenyl)-N-[^18F]fluoroethyl-acetamide): Shows high affinity for TSPO and favorable pharmacokinetics [12](https://pubmed.ncbi.nlm.nih.gov/20471953/). [@fdpa2010]
[^18F]PBR06 and [^18F]PBR07: Hexadecyl esters of TSPO ligands with improved lipophilicity for brain entry. [@fge2013]
[^18F]DPA-714 (N,N-diethyl-2-[^18F]fluorobenzamide): The fluorine-18 analog of DPA-713, showing high uptake and specific binding to TSPO [13](https://pubmed.ncbi.nlm.nih.gov/20163763/). [@tspo2018]
[^18F]GE-180: A highly selective TSPO tracer with reduced sensitivity to the rs6971 polymorphism compared to earlier tracers [14](https://pubmed.ncbi.nlm.nih.gov/23651845/). [@relationship2017]
TSPO PET has been extensively studied in Alzheimer's Disease, where neuroinflammation is recognized as a key contributor to disease progression [15](https://pubmed.ncbi.nlm.nih.gov/29474812/). Studies consistently show increased TSPO binding in AD patients compared to healthy controls, particularly in: [@tspo2014a]
In Parkinson's Disease, TSPO PET imaging has revealed increased neuroinflammation in multiple brain regions [17](https://pubmed.ncbi.nlm.nih.gov/25034872/). Key findings include: [@tspo2017a]
TSPO PET studies in ALS demonstrate widespread increases in neuroinflammation that parallel disease progression [18](https://pubmed.ncbi.nlm.nih.gov/29474812/). Increased TSPO binding is observed in: [@tspo2011]
In Frontotemporal Dementia, TSPO PET reveals regional neuroinflammation that varies by clinical subtype [19](https://pubmed.ncbi.nlm.nih.gov/28751667/):
TSPO-PET imaging in Progressive Supranuclear Palsy (PSP) reveals a distinctive pattern of microglial activation that reflects the characteristic subcortical and brainstem pathology of the disease. Unlike [Alzheimer's disease](/diseases/alzheimers-disease) where cortical inflammation predominates, PSP shows preferential involvement of deep gray matter structures and brainstem nuclei.
TSPO-PET studies in PSP demonstrate increased binding in several key regions:
| Region | Signal Intensity | Clinical Correlation |
|--------|-----------------|---------------------|
| Globus pallidus | High | Falls, postural instability |
| Substantia nigra | High | Motor severity, disease duration |
| Pons | Moderate-high | Gait dysfunction, axial symptoms |
| Midbrain | High | Vertical gaze palsy, progression |
| Thalamus | Moderate | Cognitive involvement |
| Striatum | Moderate | Bradykinesia, rigidity |
| Cerebellar dentate nucleus | Moderate | Ataxia in PSP variant |
The pattern differs from [Parkinson's disease](/diseases/parkinsons-disease) where substantia nigra signal is more focal, whereas PSP shows more widespread subcortical involvement.
TSPO signal in PSP correlates with clinical measures:
| Feature | PSP | CBD | AD |
|---------|-----|-----|---|
| Primary region | Brainstem, basal ganglia | Frontoparietal cortex | Posterior cortex, limbic |
| Signal pattern | Subcortical > cortical | Asymmetric cortical | Cortical > subcortial |
| Intensity in globus pallidus | Very high | Moderate | Low |
| Intensity in midbrain | High | Low-moderate | Low |
| Temporal progression | Rapid (2-3 years) | Variable | Slow (8-10 years) |
PSP shows significantly higher TSPO binding in the [globus pallidus](/brain-regions/globus-pallidus) and midbrain compared to both CBD and AD, making TSPO-PET potentially useful for differential diagnosis.
Longitudinal TSPO-PET studies in PSP reveal:
Second-generation TSPO tracers provide improved signal-to-noise ratio:
TSPO-PET in PSP has several clinical trial applications:
TSPO PET is particularly valuable in Multiple Sclerosis for visualizing active inflammatory lesions [20](https://pubmed.ncbi.nlm.nih.gov/24879752/). Applications include:
CSF biomarkers provide direct measurement of central nervous system inflammatory processes. Key CSF inflammatory markers include:
| Biomarker | Source | Clinical Utility |
|-----------|--------|------------------|
| IL-1β | Microglia, [astrocytes](/entities/astrocytes) | Pro-inflammatory; elevated in AD, PD, MS |
| IL-6 | Multiple cell types | Pro-inflammatory; correlates with disease severity |
| TNF-α | Activated microglia | Pro-inflammatory; elevated in neurodegeneration |
| YKL-40 | Astrocytes, microglia | Microglial activation marker; AD, MS |
| [GFAP](/entities/gfap) | Astrocytes | Astrocyte activation; AD progression |
| NFL | [Neurons](/entities/neurons) | Axonal damage; disease progression marker |
| [β-amyloid](/proteins/amyloid-beta) 1-42 | Neurons | Reduced in AD; interacts with inflammation |
| Total tau | Neurons | Axonal injury; elevated in AD, CTE |
| Phospho-tau | Neurons | Tau pathology; AD specific |
Plasma biomarkers offer minimally invasive assessment but may be less specific to CNS processes:
TSPO PET provides unique spatial information that CSF and plasma biomarkers cannot offer:
The ideal approach combines TSPO PET with CSF/plasma biomarkers:
A single nucleotide polymorphism (SNP) in the TSPO gene (rs6971) dramatically affects binding of many TSPO PET tracers [21](https://pubmed.ncbi.nlm.nih.gov/21349847/). This SNP results in:
Clinical recommendation: Genotype patients before TSPO PET studies using second-generation tracers. For [^18F]GE-180, genotyping is less critical but still recommended for interpretation.
Beyond the rs6971 polymorphism, other factors affect TSPO binding:
TSPO PET data can be analyzed using several approaches:
TSPO PET has important limitations:
Despite significant progress, TSPO PET faces several challenges:
Novel Tracers: Next-generation TSPO tracers with:
Therapeutic Monitoring Applications:
For TSPO PET to become a routine clinical tool:
TSPO PET imaging represents a powerful tool for visualizing neuroinflammation in vivo in neurodegenerative diseases. The field has advanced from first-generation [^11C]PK11195 to third-generation ^18F-labeled tracers with improved pharmacokinetics and reduced polymorphism sensitivity. While significant challenges remain—including interpretation ambiguity and limited standardization—TSPO PET offers unique insights into the spatial distribution of microglial activation that complement CSF and plasma biomarkers. As novel tracers and combined imaging approaches emerge, TSPO PET is poised to become an essential tool for understanding neuroinflammation, developing anti-inflammatory therapies, and personalizing treatment for patients with neurodegenerative diseases.
Recent advances in TSPO PET imaging: