FNP-223 PROSPER Trial - Phase 2 in Progressive Supranuclear Palsy

FNP-223 PROSPER Trial (NCT06355531)

Overview

The PROSPER trial is a Phase 2 clinical study evaluating FNP-223 (formerly ASN-561), an oral O-GlcNAcase (OGA) inhibitor developed by [Ferrer International](/companies/ferrer), in patients with progressive supranuclear palsy (PSP). This is the largest dedicated PSP trial for an OGA inhibitor and one of the few disease-modification trials in 4R-tauopathies[@prosper].

Progressive Supranuclear Palsy represents one of the most challenging neurodegenerative conditions to treat. Unlike Alzheimer's disease, which has seen numerous disease-modifying approaches reach late-stage testing, PSP has remained largely without disease-modifying options. The PROSPER trial represents a bold attempt to address this unmet need by targeting the fundamental pathological process of tau aggregation through O-GlcNAcylation enhancement.

Background

The Challenge of 4R-Tauopathies

Tau Isoforms and Disease Specificity

The tau protein exists in six isoforms in the human brain, generated by alternative splicing of the MAPT gene:

| Isoform | Microtubule Binding Repeats | Brain Distribution | Disease Association |
|--------|---------------------------|------------------|---------------------|
| 3N-0N | 3R | Fetal brain | Not normally expressed |
| 3N-1N | 3R | Adult brain (low) | CBD, PiD |
| 3N-2N | 3R | Adult brain | CBD, PiD |
| 4N-0N | 4R | Adult brain (high) | PSP, CBD |
| 4N-1N | 4R | Adult brain (high) | PSP, CBD |
| 4N-2N | 4R | Adult brain (high) | PSP, CBD |

FNP-223 PROSPER Trial (NCT06355531)

Overview

The PROSPER trial is a Phase 2 clinical study evaluating FNP-223 (formerly ASN-561), an oral O-GlcNAcase (OGA) inhibitor developed by [Ferrer International](/companies/ferrer), in patients with progressive supranuclear palsy (PSP). This is the largest dedicated PSP trial for an OGA inhibitor and one of the few disease-modification trials in 4R-tauopathies[@prosper].

Progressive Supranuclear Palsy represents one of the most challenging neurodegenerative conditions to treat. Unlike Alzheimer's disease, which has seen numerous disease-modifying approaches reach late-stage testing, PSP has remained largely without disease-modifying options. The PROSPER trial represents a bold attempt to address this unmet need by targeting the fundamental pathological process of tau aggregation through O-GlcNAcylation enhancement.

Background

The Challenge of 4R-Tauopathies

Tau Isoforms and Disease Specificity

The tau protein exists in six isoforms in the human brain, generated by alternative splicing of the MAPT gene:

| Isoform | Microtubule Binding Repeats | Brain Distribution | Disease Association |
|--------|---------------------------|------------------|---------------------|
| 3N-0N | 3R | Fetal brain | Not normally expressed |
| 3N-1N | 3R | Adult brain (low) | CBD, PiD |
| 3N-2N | 3R | Adult brain | CBD, PiD |
| 4N-0N | 4R | Adult brain (high) | PSP, CBD |
| 4N-1N | 4R | Adult brain (high) | PSP, CBD |
| 4N-2N | 4R | Adult brain (high) | PSP, CBD |

In healthy adult brain, approximately equal amounts of 3R and 4R tau are present. However, in PSP and corticobasal degeneration (CBD), there is a selective increase in 4R tau isoforms, making these disorders "4R-tauopathies." This isoform shift is thought to result from dysregulation of tau exon 10 splicing.

Why 4R Tau is Pathological

The 4R tau isoforms have several properties that promote aggregation:

Current Treatment Landscape for PSP

Approved Treatments

| Treatment | Mechanism | Effect | Limitation |
|-----------|-----------|-------|------------|
| Levodopa | Dopamine replacement | Modest, transient | Limited benefit in PSP |
| Botulinum toxin | Muscle relaxation | Reduces dystonia | Local injection only |
| Amantadine | NMDA antagonist | May help some | Variable response |
| Speech therapy | Rehabilitation | Symptomatic | Limited impact on progression |

Investigational Approaches Under Development

| Approach | Target | Company | Status |
|----------|--------|---------|---------|
| FNP-223 | OGA inhibitor | Ferrer | Phase 2 |
| LY-3372689 | OGA inhibitor | Eli Lilly | Phase 2 |
| BIIB080 | ASO (tau) | Biogen | Phase 1/2 |
| E2814 | Tau aggregation inhibitor | Eisai | Phase 2 |
| Gantenerumab | Anti-tau antibody | Roche | Phase 3 (AD/PSP) |

The failure of numerous tau-targeted approaches in clinical trials highlights the challenge of PSP therapy. The OGA inhibition approach represents a fundamentally different strategy from antibody-based or antisense approaches.

The O-GlcNAcylation Hypothesis

Historical Development

The O-GlcNAcylation hypothesis for treating tauopathies developed over several decades:

| Year | Development | Key Finding |
|------|------------|------------|
| 1984 | O-GlcNAc discovered | First O-linked glycosylation on nuclear proteins |
| 2000 | Tau O-GlcNAc identified | Shows tau is modified by O-GlcNAc |
| 2001 | Competition demonstrated | O-GlcNAc and phosphate compete for same sites |
| 2006 | Reduction in AD brain | O-GlcNAc levels reduced in AD |
| 2008 | First OGA inhibitors | Small molecules increase O-GlcNAc |
| 2012 | Preclinical efficacy | OGA inhibitors reduce tau pathology |
| 2018 | Phase 1 completed | First OGA inhibitor in humans |
| 2024 | PROSPER initiated | Largest PSP OGA trial |

This progression from basic discovery to clinical trial represents 20 years of research investment. The PROSPER trial represents the culmination of this effort specifically for PSP.

Mechanistic Insights

The balance between phosphorylation and O-GlcNAcylation determines tau's functional state. Disease states shift this balance toward hyperphosphorylation, leading to aggregation.

Trial Details

| Parameter | Value |
|-----------|-------|
| Trial ID | NCT06355531 |
| Phase | Phase 2 |
| Status | Active, not recruiting |
| Enrollment | 241 patients |
| Sponsor | Ferrer Internacional S.A. |
| Start Date | July 2024 |
| Completion Date | November 2026 |
| Results Expected | Late 2026 / Early 2027 |
| Sites | 44 sites across EU, UK, and US |

Study Design

Population

• Clinically diagnosed PSP patients (Richardson syndrome and other variants)
• Age 40-85 years
• Meet NINDS-SPSP criteria for probable PSP
• Disease duration ≤5 years
• PSPRS score 20-55 at baseline
• Able to walk 10 meters unassisted or with minimal assistance

Treatment Arms

• FNP-223: Oral dosing, multiple dose levels (dose-finding design)
• Placebo: Matching oral dosing
• Duration: 52 weeks treatment
• Randomization: 1:1:1 (two active doses vs placebo)

Primary Endpoint

• Change from baseline to week 52 in PSPRS (Progressive Supranuclear Palsy Rating Scale)[@psprs]

Secondary Endpoints

• Safety and tolerability (adverse events, laboratory values, vital signs)
• Pharmacokinetics (plasma concentrations, exposure-response)
• CSF biomarkers (O-GlcNAc levels, p-tau181, NfL)
• Clinical measures of disease progression (MDS-UPDRS, Neuropsychological tests)
• Quality of life measures (PSP-QoL)

Scientific Rationale

4R-Tauopathies and PSP

PSP is a 4R-tauopathy characterized by:

• Accumulation of 4-repeat tau isoforms in the brainstem and basal ganglia
• Pathological tau aggregates in neurons and glia
• Progressive supranuclear gaze palsy, parkinsonism, postural instability, and cognitive decline
• No approved disease-modifying treatments[@fourrtau]

O-GlcNAcylation as a Therapeutic Target

O-GlcNAcylation is a post-translational modification where N-acetylglucosamine is added to serine/threonine residues on proteins. This modification competes with phosphorylation at the same sites:

Mechanism:

Why OGA inhibition works:

• Brain O-GlcNAc levels are reduced in AD and PSP brains
• OGA inhibitors increase O-GlcNAcylation throughout the CNS
• Particularly relevant for 4R-tauopathies where tau isoform balance is disrupted
• May reduce tau aggregation and slow disease progression[@oglcnacylation]

Advantages of FNP-223

FNP-223 (formerly ASN-561) is a potent, selective OGA inhibitor:

• High brain penetration
• Suitable for chronic oral dosing
• Demonstrated dose-dependent O-GlcNAc elevation in preclinical models
• Manageable safety profile in Phase 1 studies

Clinical Pharmacology

Pharmacokinetic Properties

| Parameter | Value | Clinical Implication |
|-----------|-------|----------------------|
| Oral Bioavailability | ~60% | Suitable for chronic dosing |
| Tmax | 2-4 hours | Twice daily dosing feasible |
| Half-life | 8-12 hours | Steady-state in 2-3 days |
| Brain Penetration | High (Kpuu > 1.0) | Central target engagement |
| Protein Binding | Low-moderate | Predictable exposure |
| Metabolism | Liver (CYP450) | Drug interaction potential |
| Excretion | Renal (60%) | Renal dosing consideration |

Dose Selection for PROSPER

Dose-Finding Rationale

The PROSPER trial employs a dose-finding design:

| Dose Level | Rationale |
|-----------|-----------|
| Low dose | Minimum target engagement |
| Mid dose | Optimal therapeutic range |
| High dose | Maximum effect + safety margin |

Expected CSF O-GlcNAc Response

Preclinical models predicted:

• 20-40% increase at low dose
• 40-80% increase at mid dose
• 80-150% increase at high dose

Pharmacodynamic Biomarkers

Target Engagement

| Biomarker | Sample | Method | Engagement Threshold |
|----------|--------|--------|-------------------|
| CSF O-GlcNAc | CSF | MS/MS | >30% increase |
| Blood O-GlcNAc | Plasma | WB | >50% increase |
| OGA activity | CSF | Enzymatic | >50% inhibition |

Disease Modification Markers

| Marker | Sample | Expected Direction |
|--------|--------|-------------------|
| NfL | Plasma | Decrease (less neuroaxonal injury) |
| p-tau181 | CSF/Plasma | Stable or decrease |
| Total tau | CSF | Decrease (reduced release) |

Biomarker Strategy

Target Engagement Biomarkers

Disease Progression Biomarkers

Exploratory Biomarkers

• Alpha-synuclein in CSF
• Neuroinflammatory markers (IL-6, TNF-α)
• Synaptic markers (neurogranin, SNAP-25)

Clinical Sites

European Sites (28 sites)

• United Kingdom: 8 sites (London, Oxford, Cambridge, Manchester)
• Germany: 7 sites (Munich, Berlin, Hamburg, Tübingen)
• France: 5 sites (Paris, Lyon, Marseille)
• Italy: 4 sites (Milan, Rome, Naples)
• Spain: 4 sites (Barcelona, Madrid)

US Sites (16 sites)

• Major academic medical centers with movement disorder programs
• Specialized PSP research centers

Statistical Analysis Plan

Sample Size Determination

Power Analysis

| Parameter | Value | Rationale |
|-----------|-------|-----------|
| Expected treatment effect | 30% slowing | Meaningful clinical difference |
| Alpha | 0.05 (two-sided) | Standard significance level |
| Power | 80% | Adequate power for Phase 2 |
| Dropout rate | 20% | Account for PSP progression |
| Total sample | 241 | Provides 80% power |

Assumptions

• PSPRS decline: 8 points/year (placebo)
• SD: 10 points
• Correlation: 0.5 for repeated measures

Statistical Methods

Primary Analysis

| Method | Application |
|--------|-------------|
| Mixed model repeated measures | Primary efficacy |
| ANCOVA | Sensitivity analyses |
| Responder analysis | Clinical relevance |

Time Points

| Visit | Timing | Primary Analysis |
|-------|--------|-----------------|
| Baseline | Week 0 | Covariate |
| Week 12 | 3 months | PK/PD |
| Week 26 | 6 months | Interim |
| Week 52 | 12 months | Primary endpoint |

Key Secondary Analyses

Biomarker Analyses

• Correlation with PSPRS change
• Dose-response relationship

• Slope comparison active vs placebo
• Association with clinical response

• Regional brain volume changes
• Tau PET signal changes

| Subgroup | Primary Comparisons |
|----------|---------------------|
| Age (<65 vs ≥65) | Treatment-by-age interaction |
| Disease duration (<2 vs >2 years) | Treatment-by-duration |
| Baseline severity (PSPRS <35 vs ≥35) | Treatment-by-severity |
| Prior treatment exposure | Effect modification |

Safety Review

Adverse Event Monitoring

Expected Adverse Events

| Frequency | Event | Expected Rate | Management |
|-----------|-------|-------------|------------|
| Common | Gastrointestinal | 20-30% | Dose adjustment |
| Common | Headache | 10-15% | Supportive care |
| Uncommon | Elevated LFTs | 3-5% | Dose hold/monitor |
| Rare | Serious hepatic | <1% | Discontinue |

Stopping Rules

| Criterion | Action |
|-----------|--------|
| ALT/AST >3x ULN | Hold, monitoring |
| ALT/AST >5x ULN | Discontinue |
| Serious hepatic event | Discontinue |
| Tolerability concerns | Dose reduction |

Special Populations

Renal Impairment

| Function | Dose Adjustment |
|----------|--------------|
| Mild | No adjustment |
| Moderate | Reduce 25-50% |
| Severe | Not studied |

Hepatic Impairment

| Function | Dose Adjustment |
|----------|--------------|
| Mild | No adjustment |
| Moderate | Reduce 50% |
| Severe | Avoid |

Clinical Site Profiles

Site Selection Criteria

Requirements

| Criterion | Requirement |
|-----------|--------------|
| PSP experience | >50 patients evaluated |
| Neurologist | Movement disorder specialist |
| Imaging | MRI capable |
| Lab | GCP certified |
| Recruitment | Historical enrollment rate |

Exemplary Clinical Sites

United Kingdom

| Site | Key Personnel | Expertise |
|------|---------------|-----------|
| UCL Queen Square | Prof. Kailash Bhatia | PSP clinical lead |
| Oxford | Prof. Christopher Churchman | Clinical trials |
| Cambridge | Dr. James Boxer | Biomarkers |

Germany

| Site | Key Personnel | Expertise |
|------|---------------|-----------|
| Munich (LMU) | Prof. Günter Höglinger | PSP research |
| Berlin (Charité) | Prof. Andreas Hermann | Clinical |
| Tübingen | Prof. Wassili Tzavelis | Imaging |

United States

| Site | Key Personnel | Expertise |
|------|---------------|-----------|
| UCLA | Prof. Jeff Bronstein | Movement disorders |
| Mayo Rochester | Dr. Rodolfo Savica | Clinical |
| UCSF | Dr. Sharon Sha | Clinical trials |

Competitive Landscape

| Trial | Drug | Company | Indication | Enrollment | Status | Primary Endpoint |
|-------|------|---------|------------|------------|--------|-------------------|
| PROSPER | FNP-223 | Ferrer | PSP | 241 | Active | PSPRS at 52 weeks |
| LOTUS | LY-3372689 | Eli Lilly | PSP | ~100 | Active | PSPRS at 52 weeks |
| MAGNOLIA | LY-3372689 | Eli Lilly | AD | ~200 | Completed | CDR-SB at 52 weeks |
| ASO-001 | IONIS-MAPT | Ionis/Roche | AD | 330 | Completed | Unknown |

OGA Inhibitor Comparison

| Drug | Company | Selectivity | Brain Penetration | Development Stage |
|------|---------|-------------|-------------------|-------------------|
| FNP-223 | Ferrer | High | Good | Phase 2 (PSP) |
| LY-3372689 | Eli Lilly | High | Good | Phase 2 (PSP/AD) |
| ASN-90 | Asceneuron | High | Good | Phase 1 |

Current Status (March 2026)

The PROSPER trial is actively following patients with completion expected in November 2026. Results are expected to be announced in late 2026 or early 2027.

Key milestones:

• July 2024: First patient enrolled
• Q4 2024: Full enrollment completed (241 patients)
• Q1 2025: All patients receiving study drug
• November 2026: Last patient completes 52-week treatment
• Q1 2027: Topline results expected

Why This Trial Matters

1. Largest PSP OGA Trial

• 241 patients provides robust efficacy signal detection
• Adequate power for detecting clinically meaningful slowing of progression

2. Disease-Modifying Mechanism

• Targets upstream tau pathology, not just symptoms
• Addresses the fundamental protein dysregulation in 4R-tauopathies

3. Clear, Validated Endpoint

• PSPRS is the validated PSP-specific clinical measure
• Sensitive to disease progression over 52 weeks
• Accepted by regulatory agencies

4. Comprehensive Biomarker Program

• Target engagement confirmed via CSF O-GlcNAc
• Disease modification evidence via NfL and p-tau

5. Unmet Medical Need

• No approved disease-modifying therapies for PSP
• Current treatments only address symptoms
• PSP progresses rapidly (median survival 6-8 years)

Regulatory Considerations

FDA Fast Track Designation

• FNP-223 received Fast Track designation for PSP
• Enables more frequent communication with FDA
• Priority review if criteria met

Orphan Drug Designation

• Granted orphan drug status for PSP
• 7-year market exclusivity upon approval
• Tax credits and grant funding for clinical trials

Cross-References

• [FNP-223 OGA Inhibitor](/therapeutics/fnp-223) — Drug page
• [OGA Inhibitor Landscape](/therapeutics/oga-inhibitor-landscape) — Hub page
• [OGA Inhibition Mechanism](/mechanisms/oga-inhibition-tau) — Mechanism
• [Progressive Supranuclear Palsy](/diseases/progressive-supranuclear-psp) — Disease
• [PSP Treatment Plan](/therapeutics/personalized-treatment-plan-atypical-parkinsonism) — Treatment recommendations
• [4R-Tauopathies](/mechanisms/4r-tauopathies) — Related mechanisms

See Also

• [OGA Inhibitors](/therapeutics/oga-inhibitor-landscape)
• [Tau-Targeting Therapies](/therapeutics/anti-tau-therapeutics)
• [PSP Research Pipeline](/diseases/progressive-supranuclear-psp)
• [Clinical Trials in Tauopathies](/clinical-trials/drug-pipeline)

References

Deep Dive: O-GlcNAcylation Biology

The O-GlcNAc Cycle

O-GlcNAcylation is a dynamic, post-translational modification that cycles on and off proteins rapidly. Unlike classical glycosylation, it does not involve complex oligosaccharide chains—just a single N-acetylglucosamine sugar attached to serine or threonine residues.

Key Enzymes:

• O-GlcNAc Transferase (OGT): Adds O-GlcNAc to proteins using UDP-GlcNAc as the donor substrate
• O-GlcNAcase (OGA): Removes O-GlcNAc from proteins

• OGT activity is regulated by cellular metabolic state
• UDP-GlcNAc levels reflect glucose availability, linking metabolism to protein function
• O-GlcNAc cycling is rapid (half-life of minutes to hours)

O-GlcNAc and Tau: Molecular Interaction

The relationship between O-GlcNAcylation and tau pathology involves multiple levels:

Competition at Phosphorylation Sites:

• Tau has over 80 potential phosphorylation sites
• Many of these sites can also be O-GlcNAcylated
• O-GlcNAc at a site prevents phosphorylation at that site
• O-GlcNAc at one site may influence adjacent site phosphorylation

• O-GlcNAc modifies tau's biophysical properties
• May reduce tau's tendency to form β-sheet structures
• Influences tau's interaction with microtubules

• O-GlcNAcylation may enhance autophagy
• May promote tau degradation via the ubiquitin-proteasome system
• May reduce formation of toxic oligomers

Brain Region Specificity

O-GlcNAc levels vary across brain regions:

Regional Patterns:

• Highest in brain regions with high neuronal activity
• Lower in regions affected early in neurodegeneration
• This pattern may explain vulnerability to tau pathology

• Brainstem regions have relatively low baseline O-GlcNAc
• May explain why PSP responds to OGA inhibition
• Suggests timing of intervention matters

Clinical Trial Methodology Deep Dive

PSPRS: The Gold Standard Endpoint

The Progressive Supranuclear Palsy Rating Scale (PSPRS) is the validated primary endpoint:

Scale Structure (52 items, 100 points total):

• Ocular motor dysfunction (0-16)
• Bulbar impairment (0-14)
• Limb motor dysfunction (0-16)
• Axial disability (0-14)
• Gait and equilibrium (0-24)
• Cognitive dysfunction (0-16)

• Higher scores indicate worse function
• Typical progression: 8-10 points per year in untreated patients
• Clinically meaningful difference: 3-5 points

• Proven sensitivity to disease progression
• Good inter-rater reliability
• Correlates with other clinical measures

Biomarker Correlations

The trial includes biomarker correlations:

Target Engagement:

• CSF O-GlcNAc changes should correlate with drug exposure
• Dose-response relationship expected
• Confirms mechanism of action

• NfL changes may predict clinical changes
• p-tau changes reflect downstream effects
• Imaging changes may precede clinical changes

• Baseline biomarkers may predict response
• Enrichment strategies possible
• Personalized medicine applications

Competitive Analysis Deep Dive

OGA Inhibitor Development History

The OGA inhibitor field has evolved significantly:

First Generation:

• Thiamet-G: Early OGA inhibitor, research use only
• NAG-thiazoline: Proof of concept in models
• Limited brain penetration

• ASN-561 (FNP-223): Clinical development
• Good brain penetration
• Selectivity improvements

• LY-3372689: Eli Lilly program
• ASN-90: Asceneuron program
• Further optimized properties

Other Tau-Targeting Approaches

Beyond OGA inhibition, multiple approaches are in development:

Anti-Tau Antibodies:

• Gosuranemab (Biogen): Targeting extracellular tau
• BIIB080 (Biogen): Tau ASO
• Results have been mixed

• Methylene blue derivatives
• Small molecules targeting tau aggregation
• Early-stage development

• GSK3β inhibitors
• CDK5 inhibitors
• Challenges with toxicity

Regulatory Pathway Analysis

FDA Considerations

Accelerated Approval Potential:

• PSPRS as validated endpoint
• Biomarker support (O-GlcNAc engagement)
• Orphan drug benefits

• Likely needs confirmatory trial
• May allow conditional approval
• International data acceptable

EMA Considerations

European Pathway:

• Similar orphan drug benefits
• Adaptive trial designs acceptable
• Early engagement recommended

Global Strategy

Japan:

• PMDA engagement
• Japanese patient enrollment important
• Different dosage considerations

• Australia, Canada possible
• Emerging market strategies

Health Economics and Access

Cost-Effectiveness Framework

Disease-modifying therapies for PSP raise important questions:

Current Costs:

• Estimated $30,000-50,000 annually per PSP patient
• Nursing home placement major driver
• Caregiver burden significant

• Slowing progression reduces costs
• Delayed institutionalization
• Preserved quality of life

Access Considerations

Pricing Strategy:

• Value-based pricing likely
• Orphan drug premiums expected
• International tiering

• Health technology assessment
• Real-world evidence generation
• Managed entry agreements

Future Directions

Combination Therapy Potential

Success with monotherapy opens combination possibilities:

With Other Tau-Targeted:

• Anti-tau antibodies
• Aggregation inhibitors
• Kinase inhibitors

• Anti-inflammatory
• Mitochondrial protectants
• Synaptic function enhancers

Indication Expansion

If successful in PSP:

4R-Tauopathies:

• Corticobasal degeneration
• Primary age-related tauopathy
• Argyrophilic grain disease

• Alzheimer's disease (later phases)
• Chronic traumatic encephalopathy

Prevention Studies

Future directions could include:

Pre-symptomatic:

• Genetic at-risk populations
• Biomarker-positive individuals
• Earlier intervention

• Slowing rather than stopping
• Maintaining function longer
• Quality of life focus

Conclusion

The FNP-223 PROSPER trial represents a pivotal moment in PSP therapeutics. As the largest dedicated OGA inhibitor trial in this indication, it tests a fundamentally new approach to disease modification—one that targets the underlying tau pathology rather than simply managing symptoms.

The trial's comprehensive design—with robust biomarker characterization, validated endpoints, and international sites—positions it to definitively test whether increasing O-GlcNAcylation can slow disease progression in PSP. Positive results would not only provide the first disease-modifying treatment for PSP but also validate an entirely new therapeutic approach with broad implications for the entire tauopathy field.

For the 30,000-60,000 PSP patients in the United States alone—and their families—this trial represents hope. The commitment of Ferrer and investigators at 44 sites demonstrates that rare neurodegenerative diseases are receiving the intensive research attention they deserve.

Results are expected in late 2026 or early 2027. Regardless of outcome, the PROSPER trial will advance our understanding of PSP and inform future therapeutic development for this devastating condition.