nct02605785-tau-pet-psp

Overview

The Molecular Anatomic Imaging Analysis of Tau in Progressive Supranuclear Palsy (NCT02605785) is an active imaging study investigating tau burden in the brains of PSP patients using advanced PET imaging techniques. This study represents a critical effort to characterize the spatial distribution, magnitude, and clinical correlates of tau pathology in vivo, providing essential insights into disease pathogenesis and enabling the development of tau-targeted therapeutics.

Tau PET imaging has revolutionized our ability to visualize protein pathology in living patients. While historically limited to post-mortem neuropathological examination, modern tau PET tracers allow researchers and clinicians to observe the distribution and progression of tau deposits in the living brain. In PSP, this capability is particularly valuable because tau pathology is the defining neuropathological feature of the disease, and understanding its in vivo distribution is crucial for diagnosis, prognosis, and therapeutic development.

Trial Information

Study Objectives

Primary Goals

The study aims to comprehensively characterize tau burden in PSP patients through:

1. Quantifying Regional Tau Deposition: Measure tau PET signal across brain regions known to be affected in PSP, including the basal ganglia, brainstem, and cerebellar nuclei. This provides quantitative data on the magnitude of tau pathology in each region.

2. Characterizing Distribution Patterns: Define the spatial pattern of tau accumulation and compare it to known neuropathological patterns. The characteristic distribution of tau in PSP—affecting subcortical structures preferentially—distinguishes it from other tauopathies.

3. Correlating with Clinical Features: Establish relationships between tau burden and clinical manifestations, including:

   • Motor symptoms (postural instability, vertical gaze palsy, bradykinesia)

   • Cognitive impairment (executive dysfunction, behavioral changes)

   • Disease severity and duration

4. Understanding Selective Neuronal Vulnerability: Investigate why specific neuronal populations are preferentially affected in PSP, using tau PET signal as a proxy for pathological burden.

Secondary Objectives

• Disease Progression Biomarkers: Establish whether tau PET signal correlates with disease progression rate, potentially enabling prediction of clinical course.

• Therapeutic Target Validation: Provide data to support anti-tau therapeutic development by confirming target engagement and demonstrating that tau burden can be modulated.

• Differential Diagnosis: Refine the diagnostic utility of tau PET for distinguishing PSP from other parkinsonian disorders, particularly Parkinson’s disease, corticobasal syndrome, and multiple system atrophy.

Scientific Context

Tau PET Imaging in PSP

Tau PET imaging has emerged as a critical tool for visualizing tau pathology in vivo. Unlike amyloid PET, where multiple FDA-approved tracers exist, tau PET tracers have been developed more recently and continue to evolve^{1Tau PET methodology in neurodegenerationOpen reference}. Key tracers used in PSP research include:

Available Tracers

4R-Tau Selective Tracers

Traditional tau PET tracers like flortaucipir were developed for Alzheimer’s disease and show high affinity for the paired helical filament (PHF) tau found in AD. However, PSP is characterized by different tau filament structures—predominantly straight filaments composed of 4-repeat (4R) tau isoforms. This structural difference limits the binding of some traditional tracers to PSP tau pathology

Novel tracers specifically designed for 4R tauopathies, such as PI-2620, show improved binding to PSP tau deposits. These 4R-selective tracers bind to the distinct conformations of 4R tau filaments, enabling more sensitive detection of tau pathology in PSP patients.

Tau Pathology in PSP

Progressive Supranuclear Palsy is classified as a 4-repeat (4R) tauopathy, meaning it is characterized by accumulation of tau protein isoforms containing four microtubule-binding repeats. This distinguishes PSP from Alzheimer’s disease, which involves both 3R and 4R tau, and from Pick’s disease, which involves only 3R tau.

Affected Brain Regions

In PSP, 4R tau accumulates in specific anatomical regions^{2Tau distribution in PSP brainOpen reference}:

Subcortical Structures:

• Globose basal ganglia neurons: The most severely affected population, particularly in the globus pallidus

• Subthalamic nucleus: Very early and severe involvement

• Brainstem nuclei: Substantia nigra, red nucleus, oculomotor nucleus

• Cerebellar nuclei: Dentate nucleus and other cerebellar nuclei

Cortical Regions:

• Motor and premotor cortex (in some variants)

• Frontal cortex (particularly in PSP with cortical involvement)

Cell Types Affected:

• Neurons: Primary tau accumulation in neurons, particularly in subcortical nuclei

• Astrocytes: Tufted astrocytes with tau pathology (a PSP hallmark)

• Oligodendrocytes: Oligodendroglial tau coils

Tau Strain Diversity

Emerging evidence suggests that tau aggregates in PSP may exist as distinct “strains” with different conformational properties

• Variable clinical presentations

• Differential tracer binding

• Potential for different propagation mechanisms

Understanding Selective Neuronal Vulnerability

One of the most intriguing aspects of PSP is the selective vulnerability of specific neuronal populations. Certain brain regions and cell types are preferentially affected, while others are relatively spared. Tau PET imaging provides an in vivo window into this selective vulnerability

Vulnerable Regions

• Subthalamic nucleus: Among the earliest and most affected regions

• Globus pallidus internus: Severe tau burden

• Brainstem nuclei: Severe involvement

• Certain cortical pyramidal neurons: Variable involvement

Relatively Spared Regions

• Hippocampus: Much less affected than in AD

• Posterior cingulate: Relatively spared

• Sensory and visual cortices: Generally spared

This selective pattern suggests that intrinsic properties of vulnerable neurons—such as connectivity, metabolism, or protein homeostasis mechanisms—make them particularly susceptible to tau pathology.

Imaging Methodology

PET Acquisition and Processing

The study employs standardized PET imaging protocols:

Image Acquisition:

• 90-minute dynamic PET acquisition following tracer injection

• Attenuation correction using CT or transmission scans

• Reconstruction with ordered subset expectation maximization (OSEM)

Image Processing

• Motion correction for head movement during acquisition

• Frame-to-frame alignment

• Co-registration to structural MRI for precise region-of-interest definition

• Partial volume correction to account for limited spatial resolution

Quantification Approaches

Tau PET data are quantified using several approaches:

1. Standardized Uptake Value (SUV): Regional tracer uptake normalized to injected dose and body weight

2. SUVR (SUV Ratio): SUV in target region divided by a reference region (e.g., cerebellar gray matter)

3. Distribution Volume Ratio (DVR): Kinetic analysis approach using Logan graphical method

4. Binding Potential: Full kinetic modeling with arterial input function

Reference Region Selection

Choice of reference region is critical for PSP tau PET:

Cerebellar Gray Matter: Commonly used as reference, assuming relatively low tau pathology in this region. However, some tau accumulation occurs in cerebellar nuclei in PSP.

Cortical Reference: Some studies use mean cortical uptake as reference.

White Matter: Sometimes used, but may contain tau in PSP.

Clinical Correlates and Significance

Tau Burden and Clinical Features

Tau PET imaging enables investigation of the relationship between tau pathology and clinical manifestations

Motor Symptoms

• Postural instability: Correlation with globus pallidus and subthalamic nucleus tau

• Vertical gaze palsy: Correlation with brainstem tau burden

• Bradykinesia: Associations with basal ganglia tau

Cognitive Impairment

• Executive dysfunction: Correlation with prefrontal cortex and caudate tau

• Behavioral changes: Associations with frontal and limbic system tau

• Dementia severity: Overall tau burden correlates with global cognitive impairment

Diagnostic Utility

Tau PET has significant diagnostic value in differentiating parkinsonian disorders

Disease Progression

Longitudinal tau PET studies demonstrate that tau burden increases over time in PSP

• Clinical progression rate

• Development of new symptoms

• Conversion from mild to severe disease stages

This progressive nature makes tau PET a promising biomarker for:

• Tracking disease progression

• Monitoring therapeutic efficacy

• Enriching clinical trials with rapidly progressing patients

Clinical Significance

This imaging study contributes to understanding the spatial distribution of tau pathology in PSP, which is essential for:

1. Diagnostic Accuracy

Tau PET provides objective, in vivo evidence of tau pathology, supporting clinical diagnosis. This is particularly valuable in:

• Early disease stages when clinical features are less specific

• Distinguishing PSP from other parkinsonian disorders

• Identifying atypical presentations

2. Disease Monitoring

Tau PET enables non-invasive monitoring of disease progression:

• Quantifies rate of tau accumulation

• Identifies regions of most rapid progression

• Provides objective measures for clinical trials

3. Therapeutic Development

The study supports anti-tau therapeutic development by:

• Validating tau as a therapeutic target

• Enabling patient selection based on tau burden

• Providing biomarkers for target engagement

• Monitoring treatment effects on tau pathology

4. Understanding Pathogenesis

Tau PET data illuminate disease mechanisms:

• Selective neuronal vulnerability patterns

• Tau propagation along neural circuits

• Relationship between tau and clinical symptoms

Related Mechanisms and Pathways

Tau Pathology in 4R Tauopathies

The molecular mechanisms underlying tau pathology in PSP involve:

flowchart TD
    A["MAPT Gene"] --> B["4R Tau Isoforms"]
    B --> C["Tau Hyperphosphorylation"]
    C --> D["Aggregation"]
    D --> E["Filament Formation"]
    E --> F["Neurofibrillary Tangles"]
    F --> G["Neuronal Dysfunction"]
    G --> H["Cell Death"]

    I["Genetic Risk - H1 Haplotype"] --> B
    J["Oligomeric Tau"] --> D
    K["Tau Spreading"] --> F

Key Pathways

• Tau Phosphorylation Pathway

• 4R Tauopathy Mechanisms

• PSP Tau Aggregate Specificity

• Tau Propagation in PSP

• PSP Neuropathology

Regional Vulnerabilities

• Basal Ganglia in PSP

• Subthalamic Nucleus

• Brainstem Nuclei

• Cerebellar Nuclei

Related Pages

• Tau PET in CBS/PSP

• PI-2620 Tau PET in PSP

• Tau Pathology in PSP

• Progressive Supranuclear Palsy Neuropathology

• PSP Overview

• Biomarkers for PSP

• 4R Tauopathies Overview

See Also

• Alzheimer’s Disease - Tau PET

• Parkinson’s Disease - Neuroimaging

• Corticobasal Syndrome - Tau PET

External Links

• ClinicalTrials.gov - NCT02605785

• PubMed - Tau PET in Neurodegeneration

• ACTRN - PSP Tau Imaging