# Practical Feasibility Assessment of Tau Propagation Therapeutic Hypotheses
Based on my analysis of the targets, existing clinical data, and drug development realities, here's a comprehensive assessment:
## 1. FYN-Mediated Extracellular Vesicle Release Inhibition
### Druggability Assessment: **MODERATE**
**Target Profile:** FYN is a well-validated kinase target with a defined ATP-binding pocket suitable for small molecule inhibitors.
**Existing Chemical Matter:**
- **Saracatinib (AZD0530)** - Dual Src/Abl kinase inhibitor
- **Clinical Trial:** NCT02167256 - Phase IIa completed in mild AD patients (100mg and 125mg daily doses)
- **Tool Compounds:** PP2, SU6656, dasatinib (multi-kinase inhibitors)
**Competitive Landscape:**
- **AstraZeneca** previously developed saracatinib but discontinued AD development
- **Limited current competition** in FYN-specific AD space
- Cancer FYN programs exist but focus different indications
**Safety Concerns:**
- **Major Issue:** FYN is essential for T-cell activation, synaptic plasticity, and oligodendrocyte development
- **Previous Clinical Data:** Saracatinib showed acceptable safety profile in cancer patients
- **CNS-specific risks:** Potential cognitive impairment, seizure risk
**Development Timeline & Cost:**
- **Advantage:** Existing safety data accelerates development
- **Timeline:** 4-6 years to Phase II readout
- **Cost:** $50-80M through Phase II
- **Challenge:** Need FYN-selective compounds vs. pan-Src inhibition
**Verdict:** Moderate feasibility but requires significant selectivity improvements over existing compounds.
---
## 2. HSP90-Dependent Tau Conformational Stabilization
### Druggability Assessment: **HIGH** (but wrong direction)
**Target Profile:** HSP90 is highly druggable with multiple validated binding sites.
**Existing Chemical Matter:**
- **HSP90 Inhibitors:** 17-AAG (geldanamycin analog), ganetespib, luminespib
- **HSP90 Activators:** Very limited - BGP-15 (weak activator), heat shock response inducers
- **Problem:** The hypothesis requires HSP90 activation, but most validated compounds are inhibitors
**Competitive Landscape:**
- **Synta Pharmaceuticals:** Ganetespib (discontinued)
- **Novartis:** Multiple HSP90 inhibitor programs
- **Critical Issue:** Literature shows HSP90 **inhibition**, not activation, may be therapeutic
**Safety Concerns:**
- **HSP90 inhibitors:** Significant toxicity (hepatotoxicity, cardiac issues, ocular toxicity)
- **HSP90 activation:** Unknown safety profile, potential for promoting other misfolded proteins
**Development Timeline & Cost:**
- **Timeline:** 8-12 years (no validated activators exist)
- **Cost:** $150-300M (requires novel mechanism)
- **Major Hurdle:** Contradicts existing literature showing HSP90 promotes tau pathology
**Verdict:** Poor feasibility due to mechanistic contradiction with existing evidence.
---
## 3. APOE-Mediated Tau Clearance Enhancement
### Druggability Assessment: **CHALLENGING**
**Target Profile:** APOE is a secreted protein, difficult to drug directly.
**Existing Chemical Matter:**
- **APOE Mimetics:** CN-105 (ApoE-derived peptide) - showed promise in preclinical models
- **APOE4 Structure Correctors:** Very early research stage
- **Gene Therapy Approaches:** APOE2/E3 overexpression vectors in development
**Competitive Landscape:**
- **CNS Pharmaceuticals:** CN-105 development (limited progress)
- **Multiple biotech companies** exploring APOE-targeted approaches
- **Broad interest** but limited clinical success
**Safety Concerns:**
- **APOE modulation:** Risk of disrupting lipid metabolism
- **Cardiovascular risks:** APOE is critical for cholesterol transport
- **CNS-specific delivery challenges**
**Development Timeline & Cost:**
- **Timeline:** 10-15 years (novel mechanism)
- **Cost:** $200-400M
- **Challenge:** No validated small molecule approach
**Verdict:** Long-term potential but currently lacks viable drug development pathway.
---
## 4. GSK3β-Dependent Tau Phosphorylation Cycling Modulation
### Druggability Assessment: **HIGH**
**Target Profile:** Well-validated kinase with multiple successful inhibitor programs.
**Existing Chemical Matter:**
- **Tideglusib (NP031112):** Irreversible GSK3β inhibitor
- **Lithium:** Non-selective but clinically validated GSK3 inhibitor
- **LY2090314, 9-ING-41:** Selective GSK3 inhibitors in clinical development
**Clinical Trial History:**
- **NCT00948259:** Tideglusib Phase I in AD patients (completed)
- **Outcome:** **FAILED** - Tideglusib showed no efficacy in Phase II AD trials
- **Multiple GSK3 inhibitors** have failed in AD clinical development
**Competitive Landscape:**
- **Noscira/Zeltia:** Tideglusib (discontinued for AD)
- **ActiNeuro:** Alternative GSK3 programs
- **Market reality:** Significant skepticism after multiple failures
**Safety Concerns:**
- **Established profile:** Tideglusib showed acceptable safety
- **Mechanism-based risks:** Metabolic disruption, potential oncogenic effects
- **Temporal control:** No validated technology for time-selective inhibition
**Development Timeline & Cost:**
- **Timeline:** 5-7 years (existing safety data)
- **Cost:** $80-150M
- **Major Challenge:** Overcoming previous clinical failures
**Verdict:** Technically feasible but faces significant commercial and scientific skepticism.
---
## 5. CD63-Targeted Exosome Cargo Selectivity
### Druggability Assessment: **VERY LOW**
**Target Profile:** Tetraspanin membrane protein - historically difficult to target.
**Existing Chemical Matter:**
- **No validated CD63 inhibitors**
- **Exosome inhibitors:** GW4869 (nSMase2 inhibitor), DMA (broad exosome inhibitor)
- **No selective cargo modulators exist**
**Competitive Landscape:**
- **Extremely limited:** No major pharma programs
- **Academic interest** in exosome biology but limited translation
- **No clinical-stage compounds**
**Safety Concerns:**
- **Unknown:** No clinical data for CD63 modulation
- **Broad exosome disruption:** Likely significant toxicity
- **Essential cellular function disruption**
**Development Timeline & Cost:**
- **Timeline:** 15+ years (no starting chemical matter)
- **Cost:** $300-500M (novel target class)
- **Challenge:** No proof-of-concept for selective cargo modulation
**Verdict:** Currently not feasible for drug development.
---
## 6. Synaptic VDAC1-Mediated Mitochondrial Tau Trafficking
### Druggability Assessment: **MODERATE**
**Target Profile:** Ion channel - druggable but challenging selectivity.
**Existing Chemical Matter:**
- **VDAC modulators:** Erastin, DIDS (research tools only)
- **No selective VDAC1 compounds**
- **Mitochondrial-targeted approaches:** SS peptides, TPP+ conjugates
**Competitive Landscape:**
- **Very limited commercial interest**
- **Academic research** on VDAC in cancer
- **No CNS-focused programs**
**Safety Concerns:**
- **Critical target:** VDAC1 essential for mitochondrial function
- **Potential for severe metabolic toxicity**
- **No clinical safety data**
**Development Timeline & Cost:**
- **Timeline:** 12-15 years (novel mechanism)
- **Cost:** $250-400M
- **Challenge:** Proving tau actually translocates via VDAC1
**Verdict:** Poor feasibility due to questionable biological premise and safety risks.
---
## Overall Ranking and Recommendations
### **Tier 1 - Potentially Feasible (with caveats):**
1. **GSK3β Modulation** - Established druggability but needs new approach after previous failures
2. **FYN Inhibition** - Good chemical starting point but requires selectivity improvements
### **Tier 2 - Long-term Potential:**
3. **APOE Enhancement** - Compelling biology but lacks viable drug development approach
### **Tier 3 - Not Currently Feasible:**
4. **HSP90 Activation** - Contradicts existing evidence
5. **CD63 Targeting** - No chemical starting point
6. **VDAC1 Modulation** - Questionable biology and safety risks
## Key Recommendations for Pursuit:
**For GSK3β (most viable):**
- Focus on brain-penetrant, reversible inhibitors
- Develop biomarkers to distinguish responder populations
- Consider combination approaches with tau immunotherapy
- Budget: $100-200M over 7-10 years
**For FYN (second choice):**
- Develop FYN-selective vs. pan-Src inhibitors
- Focus on synaptic compartment-selective delivery
- Extensive preclinical safety in cognitive models
- Budget: $80-150M over 6-8 years
**Critical Success Factors:**
- All programs require robust biomarkers for tau propagation
- CNS penetration and selectivity are paramount
- Consider combination strategies rather than monotherapy
- Early human PET imaging studies essential for proof-of-mechanism
The field would benefit most from focusing resources on the top two targets while developing better tools to validate the biological premises underlying the other hypotheses.