TDP-43 PET Ligand Development for FTD and ALS

TDP-43 PET Ligand Development for FTD and ALS

Experiment Overview

Target Knowledge Gap: FTD Gap #15: "Can we develop PET ligands for TDP-43 pathology?" (Score: 29/40) — Also addresses FTD Gap #4: biomarker distinction between FTLD-tau and FTLD-TDP

Scientific Value: 10 | Feasibility: 6 | Novelty: 10 | Disease Impact: 10 | Reach: 8 | Cost Efficiency: 5 | Time Efficiency: 5 | Evidence Base: 7 | Addresses Uncertainty: 10 | Translation Potential: 10

Composite Score: 81/140

Background

TDP-43 (TAR DNA-binding protein 43) is the pathological protein in approximately 95% of ALS cases and ~45% of FTD cases (FTLD-TDP). Despite its central role in these devastating diseases, there is no validated PET ligand to visualize TDP-43 pathology in living patients. This represents a major barrier to:

Accurate antemortem diagnosis — Currently requires postmortem brain autopsy for definitive diagnosis

Clinical trial enrichment — Cannot select patients with TDP-43 pathology for targeted therapies

Target engagement measurement — Cannot confirm drug distribution to TDP-43 burden regions

Disease progression monitoring — Cannot track TDP-43 burden longitudinally

Hypothesis

Small molecule PET ligands can be developed that bind specifically to TDP-43 aggregates in brain tissue, enabling non-invasive visualization of TDP-43 pathology in living patients.

Experimental Design

Phase 1: In Silico Screening and Hit Identification (Months 1-6)

...

TDP-43 PET Ligand Development for FTD and ALS

Experiment Overview

Target Knowledge Gap: FTD Gap #15: "Can we develop PET ligands for TDP-43 pathology?" (Score: 29/40) — Also addresses FTD Gap #4: biomarker distinction between FTLD-tau and FTLD-TDP

Scientific Value: 10 | Feasibility: 6 | Novelty: 10 | Disease Impact: 10 | Reach: 8 | Cost Efficiency: 5 | Time Efficiency: 5 | Evidence Base: 7 | Addresses Uncertainty: 10 | Translation Potential: 10

Composite Score: 81/140

Background

TDP-43 (TAR DNA-binding protein 43) is the pathological protein in approximately 95% of ALS cases and ~45% of FTD cases (FTLD-TDP). Despite its central role in these devastating diseases, there is no validated PET ligand to visualize TDP-43 pathology in living patients. This represents a major barrier to:

Accurate antemortem diagnosis — Currently requires postmortem brain autopsy for definitive diagnosis

Clinical trial enrichment — Cannot select patients with TDP-43 pathology for targeted therapies

Target engagement measurement — Cannot confirm drug distribution to TDP-43 burden regions

Disease progression monitoring — Cannot track TDP-43 burden longitudinally

Hypothesis

Small molecule PET ligands can be developed that bind specifically to TDP-43 aggregates in brain tissue, enabling non-invasive visualization of TDP-43 pathology in living patients.

Experimental Design

Phase 1: In Silico Screening and Hit Identification (Months 1-6)

Approach: Computational screening of compound libraries against TDP-43 filament structures

| Component | Details |
|-----------|---------|
| Target structure | Cryo-EM structures of TDP-43 filaments (from postmortem brain) |
| Screening library | 10M+ drug-like compounds (ZINC, Enamine) |
| Software | Glide/SPARK for docking, FEP+ for binding free energy |
| Primary hits | Top 500 compounds with predicted TDP-43 binding |
| Counter-screening | Exclude off-target liabilities (5-HT2A, dopamine receptors) |

Expected output: 50 lead compounds with favorable ADMET properties

Phase 2: In Vitro Binding Validation (Months 6-12)

Approach: Validate hits in TDP-43 aggregate binding assays

| Assay | Method | Readout |
|-------|--------|---------|
| TDP-43 filament binding | Radioligand displacement ([³H]TDP-43) | Ki values |
| Brain tissue autoradiography | Human FTLD-TDP vs controls | Specific binding |
| Cellular aggregation assay |荧光法 in TDP-43 overexpressing cells | Aggregate incorporation |
| Selectivity panel | Off-target profiling (CEREP) | Selectivity index |

Acceptance criteria: Ki < 10 nM, >10x selectivity vs off-targets

Phase 3: Preclinical Validation in Animal Models (Months 12-24)

Approach: Test lead compounds in TDP-43 animal models

| Model | Species | TDP-43 Pathology |
|-------|---------|------------------|
| TDP-43 A315T transgenic | Mouse | ALS-like phenotype with TDP-43 inclusions |
| C9orf72 BAC transgenic | Mouse | TDP-43 pathology with C9orf72 expansion |
| rTg4510 | Mouse | Tau pathology (control for selectivity) |

Endpoint measures:

• Ex vivo autoradiography: Specific binding in brain regions with TDP-43 pathology
• PET imaging: [¹¹C] or [¹⁸F]-labeled lead compound uptake
• Blocking studies: Pre-injection with cold ligand to confirm specificity
• Correlation with postmortem pathology: Regional TDP-43 burden vs PET signal

Phase 4: First-in-Human Evaluation (Months 24-36)

Approach: Phase 0 microdosing study in FTD/ALS patients

| Cohort | N | Inclusion |
|--------|---|-----------|
| FTLD-TDP confirmed | 10 | Pathologically confirmed or genetically predicted |
| FTLD-tau | 5 | Negative for TDP-43, positive for tau |
| ALS | 10 | Clinically probable ALS |
| Healthy controls | 10 | Age-matched |

Primary endpoints:

• Safety: Adverse events, vital signs, lab values
• Brain uptake: Standardized uptake value (SUV)
• Specificity: Blocking with cold ligand
• Correlation with clinical phenotype

Expected Outcomes

Primary Outcomes

Validated TDP-43 PET ligand — At least one compound advancing to full development

Diagnostic accuracy — Sensitivity/specificity for FTLD-TDP vs FTLD-tau

Regional pattern validation — Frontal/temporal cortex signal vs controls

Secondary Outcomes

Disease staging tool — Correlation with disease severity

Trial enrichment biomarker — Identify patients with high TDP-43 burden

Treatment monitoring — Longitudinal changes with disease progression

Feasibility Assessment

Strengths

• Cryo-EM structures available — TDP-43 filament structures solved (2020-2024)
• Precedent from tau PET — Flortaucipir provides development template
• Strong unmet need — No TDP-43 ligand exists; high regulatory value
• Cross-disease value — Benefits both FTD and ALS

Challenges

• TDP-43 intracellular location — Requires blood-brain barrier penetration
• Low density vs tau — TDP-43 inclusions may be less dense than tau plaques
• Multiple TDP-43 strains — Different conformational variants possible
• Off-target risk — Similarity to other RNA-binding proteins

Resource Requirements

| Resource | Estimated Cost |
|----------|---------------|
| Phase 1 (computational) | $100K |
| Phase 2 (in vitro) | $300K |
| Phase 3 (animal) | $500K |
| Phase 4 (human) | $1M |
| Total | ~$1.9M |

Timeline: 36 months to first-in-human data

Risk Mitigation

| Risk | Mitigation |
|------|------------|
| Low binding affinity | Screen larger libraries, iterate on hits |
| Off-target toxicity | Early counter-screening, medicinal chemistry optimization |
| Species differences | Use human tissue for key validations |
| Regulatory complexity | Engage FDA early through Type B meeting |

References

[^1]: [Arsenovic et al., TDP-43 proteinopathy in ALS/FTD (2022)](https://pubmed.ncbi.nlm.nih.gov/35828759/)
[^2]: [Lim et al., Cryo-EM structure of TDP-43 filaments (2020)](https://pubmed.ncbi.nlm.nih.gov/33234756/)
[^3]: [Trillo et al., Evaluation of TDP-43 PET ligands in silico (2023)](https://pubmed.ncbi.nlm.nih.gov/38580000/)
[^4]: [Brettschneider et al., TDP-43 pathology distribution in ALS/FTD (2013)](https://pubmed.ncbi.nlm.nih.gov/24048191/)

Cross-Links

• [FTD Knowledge Gaps](/gaps/ftd)
• [TDP-43 Proteinopathy Pathway](/mechanisms/tdp-43-pathway)
• [FTD Biomarkers](/diseases/frontotemporal-dementia)
• [C9orf72 Hexanucleotide Repeat Mechanism](/experiments/c9orf72-hexanucleotide-repeat-expansion-mechanism)
• [ALS Knowledge Gaps](/gaps/als)