Tau PET in CBS/PSP

Tau PET Imaging in Corticobasal Syndrome and Progressive Supranuclear Palsy

Introduction

Tau positron emission tomography (PET) imaging using radiotracers like flortaucipir (also known as AV-1451, 18F-AV-1451, or T807) enables in vivo visualization of tau pathology in neurodegenerative diseases.[@lowe2024][@malpetti2024] This page focuses on tau PET findings in corticobasal syndrome (CBS), corticobasal degeneration (CBD), and progressive supranuclear palsy (PSP) — collectively known as 4R tauopathies due to their predominance of 4-repeat tau isoforms.[@stamelou2023][@armstrong2013]

Flortaucipir Binding Characteristics

Mechanism of Action

Flortaucipir is a PET radiotracer that binds to paired helical filament (PHF) tau in neurofibrillary tangles. It was originally developed for Alzheimer's disease where it shows high affinity for AD-type tau pathology. However, its binding to 4R tau in CBS and PSP is more limited due to different tau filament structures [@lowe2024].

Binding Patterns in 4R Tauopathies

In CBS and PSP, flortaucipir binding is generally lower than in AD and shows distinct regional patterns [@malpetti2024][@chen2024]:

Progressive Supranuclear Palsy (PSP)

• Binding in the globus pallidus is a characteristic finding[@respondek2023][@passamonti2017]
• Midbrain and brainstem uptake[@respondek2023][@passamonti2017]
• Variable cortical binding (generally low)
• Subcortical predominance

Tau PET Imaging in Corticobasal Syndrome and Progressive Supranuclear Palsy

Introduction

Tau positron emission tomography (PET) imaging using radiotracers like flortaucipir (also known as AV-1451, 18F-AV-1451, or T807) enables in vivo visualization of tau pathology in neurodegenerative diseases.[@lowe2024][@malpetti2024] This page focuses on tau PET findings in corticobasal syndrome (CBS), corticobasal degeneration (CBD), and progressive supranuclear palsy (PSP) — collectively known as 4R tauopathies due to their predominance of 4-repeat tau isoforms.[@stamelou2023][@armstrong2013]

Flortaucipir Binding Characteristics

Mechanism of Action

Flortaucipir is a PET radiotracer that binds to paired helical filament (PHF) tau in neurofibrillary tangles. It was originally developed for Alzheimer's disease where it shows high affinity for AD-type tau pathology. However, its binding to 4R tau in CBS and PSP is more limited due to different tau filament structures [@lowe2024].

Binding Patterns in 4R Tauopathies

In CBS and PSP, flortaucipir binding is generally lower than in AD and shows distinct regional patterns [@malpetti2024][@chen2024]:

Progressive Supranuclear Palsy (PSP)

• Binding in the globus pallidus is a characteristic finding[@respondek2023][@passamonti2017]
• Midbrain and brainstem uptake[@respondek2023][@passamonti2017]
• Variable cortical binding (generally low)
• Subcortical predominance

• Often asymmetric, reflecting the asymmetric nature of the disease[@shimizu2024][@josephs2024]
• Variable cortical uptake (can be focal)[@shimizu2024][@josephs2024]
• Basal ganglia binding[@shimizu2024][@josephs2024]
• Lower overall than AD but higher than PSP in some cases

Differential Diagnostic Value

PSP vs. Alzheimer's Disease

| Region | PSP | AD | Utility |
|--------|-----|-----|---------|
| Global cortical | Low | High | Excellent |
| Braak stage regions | Absent | Present | Excellent |
| Globus pallidus | Elevated | Absent | Good |
| Brainstem | Elevated | Absent | Moderate |

CBD vs. AD

| Feature | CBD | AD | Utility |
|---------|-----|-----|---------|
| Asymmetry | Marked | Mild | Excellent |
| Posterior cingulate | Variable | High | Good |
| Cortical binding | Focal/regional | Diffuse | Moderate |

CBD vs. PSP

| Feature | CBD | PSP | Utility |
|---------|-----|-----|---------|
| Asymmetry | Marked | Mild | Good |
| Globus pallidus | Variable | Elevated | Moderate |
| Brainstem | Variable | Elevated | Moderate |

Clinical Applications

Diagnostic Workup

Tau PET can aid in differential diagnosis when the clinical presentation is ambiguous [@chen2024][@hglinger2017]:

Disease Monitoring

Tau PET may have potential for tracking disease progression:[@josephs2024][@boxer2017]

• Changes in binding over time
• Correlation with clinical measures
• Utility as a biomarker in clinical trials

Limitations

• Lower sensitivity for 4R tau compared to AD tau[@malpetti2024][@chen2024]
• Off-target binding in the choroid plexus and basal ganglia[@lowe2024][@nicastro2024]
• Partial volume effects in small brain structures[@hglinger2017]
• Variable uptake across individuals and disease stages[@chen2024][@josephs2024]

Other Tau PET Tracers

Emerging Tracers

Several second-generation tau PET tracers are under development:

| Tracer | Target | Status |
|--------|--------|--------|
| 18F-MK-6240 | PHF-tau | Research |
| 18F-PMB | 3R/4R tau | Research |
| 18F-JNJ-311 | 3R/4R tau | Research |
| 18F-PI-2620 | 4R tau | Research |

4R-Specific Tracers

Tracers specifically designed for 4R tauopathies are in development and may improve detection in CBS/PSP [@boellaard2024][@koga2024].

PI-2620 vs Flortaucipir: Comparison for 4R Tauopathies

PI-2620 (18F-PI-2620, RO-948)

PI-2620 (also known as RO-948) is a second-generation tau PET tracer designed with higher specificity for 4R tau isoforms found in CBS and PSP [@koga2024].

Key Advantages over Flortaucipir:

• Higher 4R tau affinity: Specifically developed to bind 4-repeat tau with higher affinity than flortaucipir
• Reduced off-target binding: Less non-specific binding in basal ganglia and brainstem regions
• Better signal-to-noise: Improved clearance kinetics in off-target regions

• PSP: Elevated binding in globus pallidus and midbrain[@koga2024]
• CBD: Asymmetric cortical uptake pattern
• Higher sensitivity for 4R tau pathology vs flortaucipir

Direct Comparison

| Feature | Flortaucipir | PI-2620 |
|----------|--------------|---------|
| Primary target | 3R/4R PHF-tau | 4R tau (optimized) |
| 4R tau specificity | Moderate | High |
| Off-target (basal ganglia) | High | Low |
| PSP signal | Moderate | Higher |
| CBD signal | Low-Moderate | Higher |
| Clinical availability | FDA approved | Research phase |

Clinical Implementation Guidance

Differential Diagnosis (CBS vs PSP):

• PI-2620 may provide better discrimination between CBS and PSP due to differential 4R tau binding patterns
• Both tracers show subcortical predominance, but PI-2620 shows stronger signal in target regions
• Pattern analysis (asymmetric CBD vs symmetric PSP) remains applicable with both tracers

• Flortaucipir: FDA approved for AD, insurance coverage varies for off-label use in 4R tauopathies
• PI-2620: Research use only, not FDA approved, clinical trials available

Rubric Scoring

Diagnostic Sensitivity (0-10)

• Score: 5/10 — Moderate sensitivity for detecting tau pathology in CBS/PSP
• Flortaucipir shows limited but detectable binding in 4R tauopathies
• Sensitivity is lower than for AD

Diagnostic Specificity (0-10)

• Score: 7/10 — Good specificity for distinguishing from AD
• Pattern differences (subcortical vs. cortical) aid differentiation
• Off-target binding can confound interpretation

Clinical Utility (0-10)

• Score: 6/10 — Moderate clinical utility
• Primarily a research tool at present
• Limited availability and high cost
• Requires specialized interpretation

Evidence Quality (0-10)

• Score: 7/10 — Good evidence base
• Multiple published studies
• Ongoing research in 4R tauopathies

Effect Size (0-10)

• Score: 5/10 — Moderate effect size
• Binding differences between diseases are detectable but not large
• Significant overlap between groups

Summary

Tau PET imaging with flortaucipir provides valuable insights into tau pathology in CBS and PSP, though its sensitivity is lower than for AD.[@lowe2024][@chen2024] The characteristic patterns—subcortical predominance in PSP and asymmetric cortical/basal ganglia involvement in CBD—aid in differential diagnosis.[@shimizu2024][@respondek2023][@passamonti2017] While primarily a research tool, tau PET shows promise for improving diagnostic accuracy and potentially for monitoring disease progression in clinical trials.[@josephs2024][@boxer2017]

CBS/PSP Cross-Link Hub

Diseases

• Aging-Related Tauopathy (PART)
• Corticobasal Degeneration (CBD)
• Corticobasal Syndrome (CBS)
• Primary Age-Related Tauopathy (PART)
• Progressive Supranuclear Palsy (PSP)
• PSP Genetic Variants
• Variably Protease-Sensitive Prionopathy (VPSPr)

Treatments

• Autophagy Enhancement for Tauopathy
• CBS/PSP Clinical Trials Guide
• CBS/PSP Daily Action Plan
• CBS/PSP Rehabilitation Master Guide
• CBS/PSP Treatment Rankings
• Cognitive Reserve Strategies for CBS and PSP
• Corticobasal Degeneration (CBD) Treatment
• Exercise and Physical Activity for CBS/PSP
• Low-Dose Lithium for Tauopathy
• Melatonin for Tauopathy: Comprehensive Evidence Synthesis
• Mitochondrial Support Strategies for CBS/PSP
• Progressive Supranuclear Palsy (PSP) Treatment
• Evidence-Ranked Protective Strategies for CBS/PSP
• Rapamycin for Tauopathy
• Senolytic Therapies for CBS and PSP

Mechanisms

• 4R Tauopathy Molecular Mechanisms
• Corticobasal Degeneration (CBD) Pathway
• CBS/PSP Genetic Architecture
• Cortisol-Tau Pathway: From Chronic Stress to Tauopathy
• Gut-Brain Axis in Tauopathy
• Progressive Supranuclear Palsy (PSP) Pathway
• Tauopathy

Biomarkers

• CSF Biomarkers for Corticobasal Syndrome and Progressive Supranuclear Palsy
• Imaging Biomarkers for Corticobasal Syndrome and Progressive Supranuclear Palsy
• Plasma Biomarkers for Corticobasal Syndrome and Progressive Supranuclear Palsy
• Biomarkers for Corticobasal Degeneration
• DTI White Matter Changes in CBS/PSP
• MRI Atrophy Patterns in CBS/PSP
• Biomarkers for Progressive Supranuclear Palsy
• Corticobasal Syndrome
• Corticobasal Degeneration
• Progressive Supranuclear Palsy
• 4R Tauopathy Mechanisms
• CBS/PSP Genetic Architecture
• Cortisol Tau Pathway
• Gut Brain Axis Tauopathy
• CBS/PSP Imaging Biomarkers
• CBS/PSP CSF Biomarkers
• CBS/PSP Plasma Biomarkers
• PSP Biomarkers
• CBD Biomarkers
• Tau PET in CBS/PSP
• MRI Atrophy Patterns in CBS/PSP
• DTI White Matter Changes in CBS/PSP
• CBS/PSP Treatment Rankings
• CBS/PSP Daily Action Plan
• CBS/PSP Rehabilitation Guide
• CBS/PSP Clinical Trials Guide
• Exercise for CBS/PSP
• Protective Strategies for CBS/PSP
• Cognitive Reserve for CBS/PSP
• Rapamycin for Tauopathy
• Lithium for Tauopathy
• Melatonin for Tauopathy
• Autophagy Enhancement for Tauopathy

Allen Brain Atlas Resources

• [Allen Brain Atlas - Gene Expression](https://human.brain-map.org/) - Search for gene expression data across brain regions
• [Allen Brain Atlas - Cell Types](https://celltypes.brain-map.org/) - Explore neuronal cell type taxonomy

External Links

• [Tau PET tracers - Wikipedia](https://en.wikipedia.org/wiki/Tau_protein_imaging)
• [AV-1451 (Flortaucipir) - Wikipedia](https://en.wikipedia.org/wiki/Flortaucipir)
• [UC Berkeley Tau Imaging Lab](https://memory.berkeley.edu/)
• [AlzForum: Tau PET Tracers](https://www.alzforum.org/tags/tau-pet)
• [NCBI: Tau PET in PSP](https://pubmed.ncbi.nlm.nih.gov/29779802/)
• [MJFF: Tau PET Imaging](https://www.michaeljfox.org/tau-pet-imaging)

References