XTR006 (also known as [18F]MK-6240 or Florquinitau) is a next-generation PET radiopharmaceutical designed to detect brain neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein in living patients. The Phase 3 clinical trial (NCT07115238) evaluates the efficacy and safety of XTR006 PET visual assessment for detecting tau pathology in elderly subjects across the Alzheimer's disease spectrum.
This trial represents a critical advancement in Alzheimer's disease diagnostics, as tau PET imaging provides the unique ability to visualize the pathological hallmark that most closely correlates with cognitive impairment. Unlike amyloid plaques, which can be present decades before symptoms, tau pathology tracks closely with clinical disease severity and progression.
| Parameter |
Value |
| NCT Number |
NCT07115238 |
| Trial Name |
XTR006 PET for Detection of Brain NFTs |
| Drug Name |
XTR006 ([18F]MK-6240, Florquinitau) |
| Target |
Neurofibrillary tangles (hyperphosphorylated tau) |
| Phase |
Phase 3 |
| Status |
Recruiting |
| Sponsor |
Sinotau Pharmaceutical Group |
| Estimated Enrollment |
354 subjects |
| Start Date |
2024 |
| Completion Date |
2027 |
This is a multicenter, Phase 3 clinical trial evaluating the efficacy and safety of PET visual assessment using XTR006 injection for detection of brain neurofibrillary tangles in elderly subjects.
The trial enrolls participants across three distinct diagnostic categories:
| Characteristic |
Criteria |
| Clinical Dementia Rating (CDR) |
0 |
| MMSE Score |
≥28 |
| Amyloid PET |
Negative |
| tau PET |
Expected negative |
| Characteristic |
Criteria |
| Diagnosis |
2011 NIA-AA MCI criteria |
| Amyloid PET |
Positive |
| tau PET |
May be positive or negative |
| Cognitive concern |
Subjective or objective |
| Characteristic |
Criteria |
| Diagnosis |
2014 IWG-2 typical AD criteria |
| Amyloid PET |
Positive |
| tau PET |
Expected positive |
| Functional impairment |
Present |
All subjects receive:
- Single intravenous injection of 4-6 mCi XTR006
- Followed by 10 ml saline flush
- PET imaging at standardized timepoints (90-120 minutes post-injection)
- Brain uptake assessment using standardized protocols
- Age: ≥50 years
- Tolerance: Able to tolerate PET/MRI scanning
- Consent: Written informed consent obtained
- Contraception: For women of childbearing potential
- Diagnosis: One of the three specified populations
- Atypical dementia: Atypical AD, frontotemporal lobar degeneration (FTLD), or Lewy body dementia
- Psychiatric illness: Active psychiatric illness affecting participation
- Structural abnormalities: Significant structural brain abnormalities
- Claustrophobia: Unable to undergo MRI/PET scanning
- Substance abuse: Current alcohol or drug abuse
- Allergy: Known drug allergy to study medication
- Breastfeeding: Currently breastfeeding
- Recent radiation: Previous participation in other PET studies
Tau protein is a microtubule-associated protein that stabilizes neuronal cytoskeleton. In Alzheimer's disease, tau becomes hyperphosphorylated, leading to:
- Monomer aggregation: Hyperphosphorylated tau monomers
- Oligomer formation: Toxic soluble oligomers
- Paired helical filament (PHF) assembly: insoluble aggregates
- Neurofibrillary tangles: Insoluble intracellular inclusions
The progression of tau pathology follows a predictable pattern known as Braak staging:
- Stage I-II: Transentorhinal region (early, clinically silent)
- Stage III-IV: Limbic region (correlates with MCI)
- Stage V-VI: Isocortical region (correlates with dementia)
XTR006 ([18F]MK-6240) is a second-generation tau PET tracer with optimized properties:
- High affinity for PHFs: Specifically binds to paired helical filaments in NFTs
- AD tau selectivity: Preferential binding to 3R/4R tau characteristic of Alzheimer's disease
- Optimal kinetics: Rapid brain uptake (5-10 minutes) and appropriate clearance (90-120 min optimal imaging window)
- Low off-target binding: Reduced non-specific binding to melanin, monoamine oxidase, and other structures compared to first-generation tracers
| Property |
First-Generation (Flortaucipir) |
Second-Generation (MK-6240) |
| Off-target binding |
Higher (choroid plexus, basal ganglia) |
Lower |
| Kinetic profile |
Slower clearance |
Faster clearance |
| Imaging window |
80-120 min |
90-120 min |
| Signal-to-background |
Moderate |
Higher |
| Specificity |
Good |
Excellent |
Tau PET imaging with XTR006 enables:
- In vivo tau visualization: Seeing NFT distribution in living patients for the first time
- Diagnostic confirmation: Supporting AD diagnosis in clinically uncertain cases
- Differential diagnosis: Distinguishing AD from other neurodegenerative dementias
- Disease staging: Mapping regional tau burden that corresponds to clinical severity
Tau burden correlates with:
- Clinical disease stage: Higher tau = more severe dementia
- Rate of progression: Elevated tau predicts faster decline
- Treatment response: Baseline tau affects anti-amyloid therapy response
- Life expectancy: Tau burden predicts survival in AD
Tau PET is becoming essential for therapeutic trials:
- Patient selection: Selecting tau-positive patients for anti-tau trials
- Target engagement: Demonstrating drug reaches tau pathology
- Dose selection: Optimizing dosing based on tau occupancy
- Biomarker validation: Correlating tau PET with other biomarkers
Standardized acquisition includes:
- Injection: 4-6 mCi [18F]MK-6240 IV
- Uptake period: 90-120 minutes post-injection
- Scan duration: 20-30 minutes
- Attenuation correction: CT or transmission scan
- Reconstruction: OSEM or filtered back-projection
¶ SUVR (Standardized Uptake Value Ratio)
- Calculation: Target region SUV / Reference region SUV
- Reference region: Cerebellum gray matter (typically)
- Regions analyzed:
- Trained readers: Nuclear medicine physicians
- Binary read: Positive vs. negative for tau
- Regional assessment: Pattern analysis (Braak-like staging)
- Clinical cutoffs: Established SUVR thresholds
Positive tau PET的特征:
- Elevated SUVR in temporal regions
- Regional pattern consistent with Braak staging
- Contrast with negative amyloid status (in controls)
Negative tau PET的特征:
- SUVR close to reference region
- No focal cortical uptake
- Pattern consistent with healthy aging
The NIA-AA Research Framework uses biomarker-based classification:
| Biomarker Category |
Marker |
Positive Indicator |
| A (Amyloid) |
Amyloid PET or CSF Aβ |
Aβ42↓ or amyloid PET+ |
| T (Tau) |
Tau PET or CSF p-tau |
Tau PET+ or p-tau↑ |
| N (Neurodegeneration) |
MRI, FDG PET, or CSF t-tau |
Hip atrophy, hypometabolism, t-tau↑ |
Clinical phenotypes by ATN status:
- AD: A+T+N+ (typical AD)
- AD pathologic change: A+T-N-
- Alzheimer's disease with suspected non-AD pathology: A+T+N-
XTR006 PET provides the T (tau) component for this framework.
While amyloid-beta accumulation is considered the upstream trigger in the amyloid cascade hypothesis, tau pathology is the proximate cause of neuronal dysfunction:
| Feature |
Amyloid-beta |
Tau |
| Location |
Extracellular plaques |
Intracellular NFTs |
| Correlation with symptoms |
Weak |
Strong |
| Spreading |
Less predictable |
Braak staging |
| Therapeutic target |
Anti-amyloid antibodies |
Anti-tau therapies |
Tau PET imaging provides unique value:
- Mechanistic insight: Direct visualization of the pathologic process driving cognitive decline
- Clinical correlation: Tau burden explains clinical symptoms better than amyloid
- Therapeutic targeting: Anti-tau therapies require tau PET for development
- Patient stratification: Tau status predicts response to various therapies
¶ Available and Development Tracers
| Tracer |
Other Names |
Company |
Target |
Status |
| [18F]Flortaucipir |
AV-1451, Tauvid |
Avid Radiopharmaceuticals |
3R/4R tau (AD) |
FDA approved |
| [18F]MK-6240 |
Florquinitau, XTR006 |
Sinotau/Merck |
PHF-tau (AD) |
Phase 3 |
| [18F]PI-2620 |
- |
Roche/Genentech |
4R tau (PSP, CBD) |
Phase 2/3 |
| [18F]PM-PBB3 |
- |
Life Molecular |
3R/4R tau |
Research |
| [18F]RO948 |
- |
Roche |
PHF-tau (AD) |
Phase 2 |
- Flortaucipir: First FDA-approved tau PET, established clinical utility
- MK-6240: Improved off-target binding profile, better kinetics
- PI-2620: 4R tau binding, useful for PSP/CBD
- PM-PBB3: Broader tau isoform binding, research use only
- Effective dose: ~3-4 mSv per scan
- Comparison: Similar to CT angiography
- Risk: Minimal for diagnostic indication
Reported adverse events are typically mild:
- Injection site reactions: Rare
- Nausea: Uncommon
- Headache: Occasionally reported
- Dizziness: Rare
- Pregnancy: Contraindicated due to radiation
- Severe renal impairment: May affect tracer clearance
- Allergy to tracer components: Absolute contraindication
With 354 participants across three groups:
- Power: >90% to establish sensitivity and specificity
- Precision: Confidence intervals ±5% for each estimate
- Balance: ~118 subjects per diagnostic group
- Sensitivity: Ability to detect tau pathology in patients with AD
- Specificity: Ability to correctly identify tau-negative controls
- Timeframe: 12 months of enrollment and follow-up
- Brain uptake patterns (SUVR) across diagnostic groups
- Correlation with clinical measures (MMSE, CDR)
- Safety and tolerability assessment
XTR006 approval would enable:
- Enhanced diagnosis: Better diagnostic accuracy for AD
- Clinical staging: In vivo disease stage assessment
- Trial enrichment: Improved clinical trial design
- Therapeutic monitoring: Track treatment response
¶ Competitive Landscape
Tau PET is becoming essential for AD clinical care:
- Approved: Flortaucipir (Tauvid) - 2020 FDA approval
- In development: MK-6240, PI-2620, others
- Research use: PM-PBB3, RO948
Combined amyloid and tau PET provides comprehensive in vivo pathology assessment:
| Amyloid Status |
Tau Status |
Interpretation |
| Negative |
Negative |
Normal aging or other dementia |
| Positive |
Negative |
Preclinical/prodromal AD |
| Positive |
Positive |
Alzheimer's disease dementia |
Tau PET correlates with CSF measures:
- p-tau181: Moderate correlation with cortical tau PET
- Total tau: Correlates with neurodegeneration markers
- Neurofilament light chain: Correlates with disease severity
Emerging blood tests complement PET:
- p-tau217: Highly correlated with tau PET
- p-tau181: Good correlation with tau PET
- NfL: Correlates with disease progression
- Flortaucipir (Tauvid): FDA approved (2020) for tau PET in AD
- Limited availability: Access restricted to specialized centers
- Phase 2: Completed, demonstrating safety and efficacy
- Phase 3: Ongoing (NCT07115238)
- Potential approval: 2028-2029 if successful