18F-PM-PBB3 PET Study in Tauopathy (NCT03625128)

Trial Overview

| Field | Value |
|-------|-------|
| NCT Number | NCT03625128 |
| Status | Completed |
| Phase | Phase 1 |
| Sponsor | To be verified (Taiwan/Japan collaboration) |
| Study Type | Interventional |
| Intervention | 18F-PM-PBB3 PET imaging |
| Conditions | PSP, CBD, AD, healthy controls |
| Sites | Japan, Taiwan |
| Tracer Name | 18F-APN-1607, 18F-PM-PBB3 |

Scientific Rationale

The Need for 4R Tau-Specific PET Tracers

Positron Emission Tomography (PET) imaging of tau pathology has revolutionized our ability to visualize and quantify neurodegeneration in vivo. However, existing tau PET tracers were designed primarily for Alzheimer's disease, where 3R/4R tau (paired helical filaments) predominates.

Limitations of First-Generation Tau Tracers:

AD-centric design: Optimized for 3R/4R tau in AD

Off-target binding: Bind to monoamine oxidase and other proteins

Limited 4R specificity: Poor discrimination between 3R/4R and 4R tauopathies

Signal overflow: Signal in AD brains can saturate, limiting quantification

Why 4R Tau Imaging Matters

Progressive Supranuclear Palsy (PSP) and Corticobasal Degeneration (CBD) are characterized by:

• 4R tau predominance: Straight filaments instead of paired helical filaments
• Distinct regional distribution: Brainstem and basal ganglia involvement
• Different therapeutic implications: 4R-specific treatments require 4R-specific biomarkers

PM-PBB3: A Second-Generation Tau Tracer

...

Trial Overview

| Field | Value |
|-------|-------|
| NCT Number | NCT03625128 |
| Status | Completed |
| Phase | Phase 1 |
| Sponsor | To be verified (Taiwan/Japan collaboration) |
| Study Type | Interventional |
| Intervention | 18F-PM-PBB3 PET imaging |
| Conditions | PSP, CBD, AD, healthy controls |
| Sites | Japan, Taiwan |
| Tracer Name | 18F-APN-1607, 18F-PM-PBB3 |

Scientific Rationale

The Need for 4R Tau-Specific PET Tracers

Positron Emission Tomography (PET) imaging of tau pathology has revolutionized our ability to visualize and quantify neurodegeneration in vivo. However, existing tau PET tracers were designed primarily for Alzheimer's disease, where 3R/4R tau (paired helical filaments) predominates.

Limitations of First-Generation Tau Tracers:

AD-centric design: Optimized for 3R/4R tau in AD

Off-target binding: Bind to monoamine oxidase and other proteins

Limited 4R specificity: Poor discrimination between 3R/4R and 4R tauopathies

Signal overflow: Signal in AD brains can saturate, limiting quantification

Why 4R Tau Imaging Matters

Progressive Supranuclear Palsy (PSP) and Corticobasal Degeneration (CBD) are characterized by:

• 4R tau predominance: Straight filaments instead of paired helical filaments
• Distinct regional distribution: Brainstem and basal ganglia involvement
• Different therapeutic implications: 4R-specific treatments require 4R-specific biomarkers

PM-PBB3: A Second-Generation Tau Tracer

18F-PM-PBB3 (also known as 18F-APN-1607) represents a second-generation tau PET tracer specifically developed for 4R tauopathies:

• High affinity for 4R tau: Selective binding to straight filaments
• Low off-target binding: Reduced binding to MAO-B and other proteins
• Optimal kinetics: Fast brain uptake and clearance
• Quantifiable signal: Suitable for quantitative analysis

Mechanism of Action

Tracer Chemistry

PM-PBB3 is a small molecule radioligand that:

Crosses the blood-brain barrier: Lipophilic structure enables CNS penetration

Binds selectively to tau filaments: High affinity for aggregated tau

Emits positrons: 18F label enables PET detection

Clears rapidly: Favorable kinetics for repeated imaging

Binding Characteristics

| Property | 18F-PM-PBB3 |
|----------|-------------|
| Ki for 4R tau | ~1-5 nM |
| Ki for 3R/4R tau | ~5-10 nM |
| Off-target (MAO-B) | Low |
| Brain uptake | High |
| Clearance | Moderate |

PET Imaging Protocol

Typical PET imaging with 18F-PM-PBB3 involves:

• Radiotracer injection: 185-370 MBq (5-10 mCi) IV
• Dynamic acquisition: 90-120 minutes
• Reconstruction: Standard filtered back-projection or iterative methods
• Quantification: Standardized uptake value (SUVR), distribution volume (VT)

Trial Design

Study Objectives

Primary: Evaluate safety and tolerability

Secondary:

• Assess radiation dosimetry
• Compare uptake patterns in different tauopathies
• Correlate PET signal with clinical severity

Study Population

• PSP patients: Probable PSP Richardson syndrome
• CBD patients: Clinically diagnosed CBD
• AD patients: Probable Alzheimer's disease
• Healthy controls: Age-matched individuals without neurological disease

Imaging Assessments

• Baseline MRI: Structural imaging for atrophy assessment
• 18F-PM-PBB3 PET: Tau tracer imaging
• 18F-FDG PET: Metabolic imaging (optional)
• Clinical rating scales: MDS-UPDRS, PSPRS, MMSE

Eligibility Criteria

Inclusion Criteria

Age: Typically 50-85 years

Diagnosis: Probable PSP, CBD, or AD per established criteria

Capacity: Able to provide informed consent

Tolerance: Able to lie still for PET imaging

Exclusion Criteria

Contraindications: MRI/PET contraindications

Other neurological conditions: Unless healthy control

Psychiatric conditions: Active psychosis

Radiotracer allergies: Previous adverse reactions

Outcome Measures

Primary Outcomes

| Measure | Description |
|---------|-------------|
| Adverse events | Safety monitoring |
| Radiation dosimetry | Effective dose estimation |
| Vital signs | Post-injection monitoring |

Secondary Outcomes

| Measure | Description |
|---------|-------------|
| SUVR | Standardized uptake value ratio |
| Regional binding | Tau distribution in specific brain regions |
| Clinical correlation | PET signal vs. clinical scores |

Clinical Significance

Disease-Specific Imaging

18F-PM-PBB3 enables differentiation between tauopathies:

PSP Pattern:

• Brainstem: Pontine and midbrain involvement
• - Basal ganglia: Globus pallidus, subthalamic nucleus
• - Cerebellum: Dentate nucleus

CBD Pattern:

• Motor cortex: Primary motor and premotor areas
• - Parietal lobe: Posterior regions
• - Basal ganglia: Variable involvement

AD Pattern:

• - Entorhinal cortex: Early involvement
• - Hippocampus: Memory structures
• - Posterior cingulate: Default mode network
• - Cortical association areas

Clinical Utility

Diagnostic Accuracy

• Differentiate 4R tauopathies from 3R/4R tauopathies
• Support clinical diagnosis
• Identify atypical presentations

Patient Stratification

• Select patients for clinical trials
• Enrich biomarker-positive populations
• Exclude non-tauopathy mimics

Therapeutic Monitoring

• Track disease progression
• Monitor treatment response
• Validate biomarker endpoints

Comparison with Other Tau PET Tracers

| Tracer | Target | Primary Use | 4R Specificity |
|--------|--------|-------------|---------------|
| 18F-AV-1451 (Flortaucipir) | 3R/4R tau | AD | Low |
| 18F-GTP-1 | 3R/4R tau | AD | Low |
| 18F-PM-PBB3 | 4R tau | PSP/CBD | High |
| PI-2620 | 4R tau | PSP/CBD | High |

Cross-References

• [Progressive Supranuclear Palsy](/diseases/progressive-supranuclear-palsy)
• [Corticobasal Degeneration](/diseases/corticobasal-degeneration)
• [Tau PET Imaging](/mechanisms/tau-pet-imaging)
• [4R Tauopathies](/mechanisms/4r-tauopathies)
• [Tau Protein](/proteins/tau)
• [Neuroimaging in Neurodegeneration](/mechanisms/neuroimaging-neurodegeneration)

References

[ClinicalTrials.gov - NCT03625128](https://clinicaltrials.gov/study/NCT03625128)

[PM-PBB3 Development - PubMed](https://pubmed.ncbi.nlm.nih.gov/)

[18F-APN-1607 in PSP - PubMed](https://pubmed.ncbi.nlm.nih.gov/)

[Tau PET Tracer Comparison - PubMed](https://pubmed.ncbi.nlm.nih.gov/)

Detailed Background: Tau PET Imaging

History of Tau PET Development

First Generation Tracers (2010s)

The first tau PET tracers were developed primarily for Alzheimer's disease, where 3R/4R tau in the form of paired helical filaments (PHFs) is the hallmark pathology:

• 11C-PBB3 (2012): First human tau PET tracer
• 18F-AV-1451/Flortaucipir (2013): FDA-approved for AD tau imaging
• 18F-GTP-1 (2014): Another AD-focused tracer
• 18F-MK-6240 (2015): High-affinity AD tracer

These tracers achieved remarkable success in visualizing AD tau pathology but showed limited utility in 4R tauopathies.

Second Generation Tracers (2015-present)

Recognizing the need for 4R-specific tracers, second-generation compounds were developed:

• 18F-PM-PBB3: Specifically optimized for 4R tau (straight filaments)
• PI-2620: Another 4R-selective tracer
• 18F-JNJ-311: Currently in development
• RO-948: High-affinity tracer with 4R specificity

Tau Filament Structure

Paired Helical Filaments (PHFs) - AD

• Composed of 3R and 4R tau isoforms
• Characteristic 80nm periodicity
• In the neocortex, found in neurons and glia

Straight Filaments (SFs) - 4R Tauopathies

• Composed predominantly of 4R tau
• No periodic structure
• Predominant in PSP, CBD, and other 4R tauopathies

The structural differences between PHFs and SFs provide the basis for selective tracer binding.

Binding Site Characterization

PM-PBB3 binds to a distinct site on tau filaments:

• Tangle core binding: Recognizes the aggregated filament core
• Conformational selectivity: Prefers the conformation of 4R tau filaments
• Stoichiometry: Multiple binding sites per filament
• Selectivity index: >10-fold preference for 4R over 3R/4R tau

Technical Considerations

PET Quantification Methods

Standardized Uptake Value (SUV)

• Simplest metric: tissue activity / injected dose / body weight
• SUVR (ratio to reference region) corrects for blood flow

Distribution Volume (VT)

• Kinetic modeling approach
• Requires arterial input function
• More accurate but invasive

Parametric Imaging

• Pixel-by-pixel quantification
• Enables detailed spatial analysis

Reference Regions

Selection of appropriate reference region is critical:

• For AD: Cerebellar cortex (low tau burden)
• For PSP: Pons or white matter (different pattern)
• For CBD: Variable selection

Clinical Applications

Diagnostic Workup

18F-PM-PBB3 PET can assist in:

Differential diagnosis

• PSP vs. PD
• CBD vs. AD
• Atypical parkinsonism vs. Lewy body disease

Disease staging

• Regional tau burden correlates with clinical severity
• Can track disease progression

Biomarker stratification

• Enrich clinical trials with biomarker-positive patients
• Exclude tau-negative mimics

Therapeutic Development

Tau PET enables:

Patient selection

• Confirm tauopathy diagnosis
• Exclude non-tau conditions

Target engagement

• Measure drug binding to tau
• Validate mechanism of action

Treatment response

• Track tau burden over time
• Identify disease modification

Limitations and Challenges

Signal Quantification

• Partial volume effect: Atrophy leads to underestimation
• Background signal: Non-specific binding
• Floor effects: Low signal in early disease

Off-Target Binding

• MAO-B binding: First-generation tracers
• Bone uptake: Some tracers
• White matter binding: Variable

Accessibility

• Cyclotron production: Required for 18F
• Radiochemistry: Specialized facilities
• Cost: Higher than traditional imaging

Regulatory Status

Current Approvals

• Flortaucipir (AV-1451): FDA-approved for AD tau imaging
• 18F-PM-PBB3: Available in Japan, under development elsewhere

Clinical Trial Use

18F-PM-PBB3 is currently used in:

• Diagnostic studies
• Clinical trials for 4R tauopathies
• Natural history studies
• Treatment response monitoring

Future Directions

Next-Generation Tracers

Ongoing research focuses on:

Improved kinetics: Faster clearance

Enhanced specificity: Higher 4R selectivity

Better quantification: Reduced partial volume effects

Broader accessibility: Simplified production

Combined Modalities

• PET/MRI: Structural-functional correlation
• PET/CT: Attenuation correction
• Hybrid markers: Tau + amyloid + neurodegeneration

Quantification Standards

• Harmonization: Cross-site standardization
• Centiloid scaling: Unified quantification
• SUVR cutoffs: Diagnostic thresholds

Page updated: 2026-03-27

Tauopathies: Classification and Clinical Features

Overview of Tauopathies

Tauopathies are a group of neurodegenerative diseases characterized by abnormal accumulation of the tau protein in the brain. They represent a spectrum of conditions with overlapping clinical presentations but distinct pathological features.

Classification by Tau Isoform

| Type | Isoforms | Representative Diseases |
|------|----------|------------------------|
| 3R/4R tauopathies | Both 3R and 4R | Alzheimer's disease, Chronic traumatic encephalopathy |
| 4R tauopathies | Primarily 4R | PSP, CBD, CBD, AGD, Perthes |
| 3R tauopathies | Primarily 3R | Pick's disease |

Progressive Supranuclear Palsy (PSP)

Clinical Features

• Vertical supranuclear gaze palsy
• Postural instability with falls
• Akinesia and rigidity
• Dysarthria and dysphagia
• Cognitive decline

Neuropathology

• Neurofibrillary tangles (4R tau)
• Tufted astrocytes
• Globose neurofibrillary tangles in brainstem
• Neuronal loss in subthalamic nucleus, globus pallidus

MRI Findings

• Midbrain atrophy ("hummingbird sign")
• Superior cerebellar peduncle atrophy
• Third ventricle dilation
• Frontal lobe atrophy

Corticobasal Degeneration (CBD)

Clinical Features

• Asymmetric rigidity
• Apraxia
• Alien limb phenomenon
• Myoclonus
• Cortical sensory loss

Neuropathology

• Cortical and basal ganglia tau pathology
• Astrocytic plaques
• Neuronal loss and gliosis

MRI Findings

• Asymmetric frontal and parietal atrophy
• Contralateral caudate head atrophy
• Callosal atrophy

Alzheimer's Disease (AD)

Clinical Features

• Memory impairment
• Progressive cognitive decline
• Behavioral changes
• Motor features in later stages

Neuropathology

• 3R/4R tau neurofibrillary tangles
• Amyloid-beta plaques
• Neuritic plaques
• Braak staging (I-VI)

MRI Findings

• Hippocampal atrophy
• Posterior cingulate involvement
• Posterior cortical atrophy

PET Imaging Biomarkers in Tauopathies

Amyloid PET

While not directly related to tau, amyloid PET provides important diagnostic information:

11C-PiB (Pittsburgh Compound B)

• First amyloid PET tracer
• High specificity for amyloid plaques
• Short half-life limits use

18F-Tracers

• 18F-Florbetapir (Amyvid)
• 18F-Florbetaben (Neuraceq)
• 18F-Flutemetamol (Vizamyl)

Interpretation

• Positive: High likelihood of AD
• Negative: Rules out AD as primary cause

FDG PET

18F-Fluorodeoxyglucose PET measures cerebral metabolism:

AD Pattern

• Posterior cingulate hypometabolism
• Temporoparietal hypometabolism

PSP Pattern

• Brainstem hypometabolism
• Frontal cortex hypometabolism
• Cerebellar hypometabolism

CBD Pattern

• Asymmetric frontal/parietal hypometabolism
• Basal ganglia hypometabolism

Tau PET Tracers in Clinical Practice

Interpretation Guidelines

Regional patterns matter more than global burden

Use appropriate reference regions

Consider partial volume effects

Correlate with clinical findings

Reporting Standards

• SUVR thresholds for positivity
• Regional cutoff values
• Standardized reporting templates
• Integration with MRI findings

Radiopharmaceutical Chemistry

18F Production

Cyclotron Production

• 18F produced via 18O(p,n)18F reaction
• Target: [18O]water
• Typical yield: 1-5 Ci

Quality Control

• Radionuclide purity
• Chemical purity
• Radiochemical purity
• Sterility
• Endotoxin testing

Formulation

Formulation Steps

18F capture on cartridge

Elution with acetonitrile

Reaction vessel transfer

Synthesis

Purification

Formulation

Sterile filtration

Quality Requirements

• pH: 5.5-8.0
• Osmolality: isotonic
• Radiochemical purity: >95%
• Chemical purity: per pharmacopeia

Future Perspectives

Theranostic Applications

The future of tau PET imaging includes:

Personalized medicine

• Patient-specific treatment selection
• Prognostic information
• Risk stratification

Treatment monitoring

• Early response assessment
• Dose optimization
• Disease modification detection

Trial enrichment

• Biomarker-positive populations
• Stratified randomization
• Surrogate endpoints

Artificial Intelligence Integration

AI/ML applications in tau PET:

• Automated quantification
• Pattern recognition
• Diagnostic support
• Progression modeling
• Treatment response prediction

Multi-Modal Imaging

Future approaches will integrate:

• PET/MRI hybrid systems
• Simultaneous acquisition
• Integrated biomarkers
• Personalized imaging protocols

Page updated: 2026-03-27