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

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

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FNP-223 PROSPER Trial (NCT06355531)

Overview

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FNP-223 PROSPER Trial (NCT06355531)

Overview

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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"]. The trial represents a significant milestone in the development of disease-modifying therapies for PSP, a fatal neurodegenerative disorder with no approved treatments that modify its underlying course.

The PROSPER trial enrolled 241 patients across 44 sites in the European Union, United Kingdom, and United States, making it the largest PSP-specific clinical trial conducted to date. The 52-week treatment duration and rigorous endpoint assessment using the Progressive Supranuclear Palsy Rating Scale (PSPRS) provide a robust framework for evaluating whether OGA inhibition can slow disease progression in this devastating condition. Results are expected in late 2026 or early 2027, potentially marking a turning point in PSP therapeutics.

Background

Progressive Supranuclear Palsy: An Unmet Medical Need

Progressive supranuclear palsy is a rare but devastating neurodegenerative disorder characterized by the accumulation of abnormal tau protein in the brain. First described by John Steele, Jerzy Richardson, and John Olszewski in 1964, PSP has become recognized as one of the most common atypical parkinsonian syndromes, affecting approximately 5-6 per 100,000 individuals worldwide. The disease typically presents in the sixth or seventh decade of life, with patients surviving an average of 6-8 years after symptom onset[@hoglinger2017].

Core Clinical Features

The classic presentation of PSP, known as Richardson syndrome, includes:

Vertical supranuclear gaze palsy: Downgaze impairment is typically earliest and most prominent, affecting downward eye movements specifically. Patients complain of difficulty reading, walking down stairs, or looking at their feet. This sign is highly specific to PSP and helps distinguish it from other parkinsonian disorders.
Postural instability and falls: Beginning early in the disease course, patients experience frequent backward falls, often within the first year of symptom onset. This results from axial rigidity and the loss of righting reflexes. Falls are a major cause of morbidity, leading to fractures, head trauma, and loss of independence.
Parkinsonism: Symmetric bradykinesia and rigidity, typically without tremor. The "cockroach gait" pattern—wide-based, shuffling steps with sudden starts and stops—is characteristic. Unlike Parkinson's disease, dopamine responsiveness is minimal and transient in most PSP patients.
Cognitive dysfunction: Frontal executive dysfunction is prominent early, including slowed thinking, impaired planning and judgment, and behavioral changes. Progressive aphasia and memory deficits develop as the disease advances, ultimately leading to global dementia.

Pathological Basis

The neuropathology of PSP involves:

Neurofibrillary tangles composed of hyperphosphorylated 4-repeat tau protein
Tau-positive glial inclusions (tufted astrocytes and coiled bodies)
Neuronal loss and gliosis in the basal ganglia, brainstem, and cerebellar nuclei
Characteristic tufted astrocytes that are highly specific to PSP

The distribution of tau pathology correlates with the clinical syndrome, affecting:

Substantia nigra pars compacta (causing parkinsonism)
Superior colliculus (causing gaze palsy)
Globus pallidus and subthalamic nucleus (causing axial symptoms)
Frontal cortex (causing cognitive impairment)

The Tau Phosphorylation Problem

Tau protein, normally involved in microtubule stabilization, becomes pathological in PSP through excessive phosphorylation. This hyperphosphorylation causes tau to detach from microtubules, aggregate into neurofibrillary tangles, and lose its normal biological function. The balance between tau phosphorylation and dephosphorylation is regulated by a network of kinases and phosphatases. In PSP, this balance is disrupted, favoring pathological phosphorylation.

Key kinases implicated in tau hyperphosphorylation include:

GSK-3β: Glycogen synthase kinase-3 beta, one of the most important tau kinases
CDK5: Cyclin-dependent kinase 5, activated in neurodegeneration
MAPK: Mitogen-activated protein kinases

Conversely, protein phosphatases such as PP2A normally dephosphorylate tau. The activity of these phosphatases is reduced in neurodegenerative conditions, contributing to the accumulation of phosphorylated tau species.

O-GlcNAcylation: A Natural Counterbalance to Tau Pathology

O-GlcNAcylation is a dynamic post-translational modification that adds a single N-acetylglucosamine sugar to serine and threonine residues on proteins. Unlike complex glycosylation, O-GlcNAcylation is reversible and regulates numerous cellular processes, including transcription, signal transduction, and protein degradation. Critically, O-GlcNAcylation and phosphorylation compete for the same sites on many proteins, creating a Yin-Yang relationship between these modifications[@liu2009].

Mechanistic Basis

On tau protein specifically:

O-GlcNAcylation occurs at many of the same serine and threonine residues that are phosphorylated in pathological tau
When a site is O-glycosylated, it cannot be simultaneously phosphorylated
O-GlcNAcylation therefore functions as a protective modification that prevents tau hyperphosphorylation
In Alzheimer's disease and other tauopathies, tau O-GlcNAcylation is reduced, correlating with increased phosphorylation
Increasing O-GlcNAcylation via OGA inhibition reduces tau pathology in preclinical models

The landmark studies by Yuzwa et al. established that:

Thiamet-G, a potent OGA inhibitor, increased tau O-GlcNAcylation in mouse brain
This reduced tau phosphorylation at multiple epitopes
Treated mice showed improved cognitive performance in memory tasks
The effect was dose-dependent and reversible upon drug withdrawal

O-GlcNAc Levels in Disease

Post-mortem studies have demonstrated:

Reduced total O-GlcNAc levels in AD brain compared to age-matched controls
Direct correlation between O-GlcNAc and tau phosphorylation status (inverse relationship)
Similar patterns observed in PSP and other tauopathies
This suggests that enhancing O-GlcNAcylation could be broadly therapeutic across tauopathies

OGA Inhibition as a Therapeutic Strategy

O-GlcNAcase (OGA) is the enzyme responsible for removing O-GlcNAc modifications from proteins. Inhibiting OGA therefore increases global O-GlcNAcylation, including on tau protein. This pharmacological approach has several advantages:

Target engagement: OGA inhibition directly increases O-GlcNAc levels, providing a measurable pharmacodynamic effect

Blood-brain barrier penetration: Several OGA inhibitors have demonstrated good CNS exposure

Disease-modifying potential: By addressing upstream tau pathology, OGA inhibition may slow disease progression rather than merely treating symptoms

Broad applicability: The mechanism is relevant to multiple tauopathies, not just PSP

The development of OGA inhibitors has progressed through several generations:

First generation: Thiamet-G (academic compound, not developed clinically)
Second generation: ASN-561/FNP-223 (Ferrer), MK-8719 (Merck)
Third generation: LY3372689 (Eli Lilly), others in development

Trial Details

| Parameter | Value |
|-----------|-------|
| Trial ID | NCT06355531 |
| Phase | Phase 2 |
| Status | Active, not recruiting |
| Enrollment | 220 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 |
| Treatment Duration | 52 weeks |
| Primary Endpoint | Change in PSPRS from baseline to Week 52 |

Study Design

Population

The PROSPER trial enrolled patients meeting clinical criteria for PSP:

Diagnostic Criteria

Clinically diagnosed PSP patients (Richardson syndrome and other variants)
Age 40-85 years
Meet NINDS-SPSP criteria for probable PSP
Disease duration 1-5 years
PSPRS score 20-69 at baseline (moderate disease severity)

Key Inclusion Criteria

Ability to ambulate (with or without assistance, excluding wheelchair-bound)
MMSE score ≥ 20 (adequate cognitive function to comply with assessments)
Stable medication regimen for Parkinson's symptoms for ≥ 30 days
Ability to undergo repeated lumbar punctures for CSF sampling

Key Exclusion Criteria

Previous treatment with OGA inhibitors
Significant medical conditions that could affect outcomes
Active participation in another clinical trial
Inability to comply with study procedures

Treatment Arms

The trial employs a randomized, double-blind, placebo-controlled design:

FNP-223: Oral dosing at multiple dose levels (specific doses not publicly disclosed)
Placebo: Matching oral dosing
Duration: 52 weeks of treatment
Randomization: Approximately 2:1 active:placebo ratio (to maximize exposure to active drug)

Endpoints

Primary Endpoint

Change from baseline to week 52 in PSPRS (Progressive Supranuclear Palsy Rating Scale)

The PSPRS is a validated 36-item scale measuring:

Mentation (7 items, max 28 points)
Bulbar (6 items, max 24 points)
Ocular and manual (5 items, max 20 points)
Gait and midline (8 items, max 32 points)
Total score: 100 points (higher = more severe)

Secondary Endpoints

Safety and tolerability (adverse events, laboratory parameters)
Pharmacokinetics (plasma and CSF concentrations)
CSF biomarkers:
O-GlcNAc levels (target engagement)
Phosphorylated tau (p-tau181, p-tau217)
Neurofilament light chain (NfL) - marker of neuronal injury
Clinical measures:
MDS-UPDRS
Clinical Global Impression of Change
Frontal Assessment Battery

Exploratory Endpoints

Tau PET imaging (in subset of patients)
Blood O-GlcNAc levels (peripheral biomarker)
Genetic stratification (MAPT haplotype, other tau-related genes)

Scientific Rationale

Why PSP is a Compelling Target for OGA Inhibition

PSP presents several characteristics that make it particularly suitable for OGA inhibitor therapy:

1. 4R-Tauopathy

Unlike Alzheimer's disease, which features a mixture of 3-repeat and 4-repeat tau isoforms, PSP is a pure 4R-tauopathy. The 4-repeat tau isoform has more serine/threonine residues available for O-GlcNAcylation than 3R tau, potentially making it more responsive to OGA inhibition. Furthermore, the imbalance between 4R and 3R tau in PSP may relate to dysregulated post-translational modifications.

2. Early Intervention Opportunity

The 1-5 year disease duration window required for trial entry represents early-stage disease where tau pathology may be more modifiable. In advanced PSP, extensive neuronal loss may be irreversible. Early intervention could preserve remaining neurons and slow the cascade of degeneration.

3. Clear Mechanistic Rationale

The OGA inhibition mechanism directly addresses the core pathological process in PSP—tau hyperphosphorylation. This is distinct from approaches that merely treat symptoms (like dopaminergic medications) or attempt more indirect neuroprotection.

4. Biomarker Availability

The presence of established fluid biomarkers (NfL, p-tau) and imaging markers (tau PET) enables assessment of both target engagement and disease progression, critical for proving mechanistic efficacy.

Clinical Evidence Supporting OGA Inhibition

Preclinical Studies

Multiple preclinical studies have demonstrated OGA inhibitor efficacy in tauopathy models:

Yuzwa et al. (2008): Thiamet-G reduced tau phosphorylation and improved cognition in JNPL3 tau transgenic mice[@yuzwa2006]
Moreno et al. (2017): OGA inhibitor treatment reduced tau phosphorylation and improved memory in rTg4510 tauopathy mice[@moreno2017]
Aharon et al. (2019): Selective OGA inhibition prevented tau pathology and cognitive decline in multiple models[@aharon2019]

Clinical Evidence from Other Tauopathies

The most advanced clinical program for OGA inhibition has been in Alzheimer's disease:

MAGNOLIA (Eli Lilly, LY3372689): Phase 2 study in early AD
LOTUS (Eli Lilly, LY3372689): Phase 2 study in PSP

The MAGNOLIA study demonstrated:

Dose-dependent increase in CSF O-GlcNAc (target engagement)
Reduction in CSF p-tau181 with treatment
Generally well-tolerated safety profile

This clinical data supports the biological rationale for OGA inhibition and informs the PROSPER trial design.

Biomarker Strategy

The PROSPER trial incorporates an extensive biomarker program to demonstrate mechanism of action and track disease progression.

Target Engagement Biomarkers

CSF O-GlcNAc Levels

Direct measurement of O-GlcNAc modification on proteins in CSF
Expected to increase with effective OGA inhibition
Serves as a pharmacodynamic marker confirming target engagement
Allows dose-response characterization

Blood O-GlcNAc

Peripheral blood mononuclear cell (PBMC) O-GlcNAc levels
Less invasive than lumbar puncture for repeated sampling
Correlates with CNS O-GlcNAc changes in preclinical models

Disease Progression Biomarkers

Neurofilament Light Chain (NfL)

Marker of axonal injury and neuronal death
Elevated in PSP compared to controls
Correlates with disease severity and progression rate
Change in NfL over time may predict clinical progression

Phosphorylated Tau (p-tau181, p-tau217)

CSF and plasma p-tau reflect tau pathology burden
In PSP, p-tau levels are elevated compared to healthy controls
May serve as surrogate marker for treatment effect on tau pathology

Tau PET Imaging

Advanced imaging to visualize regional tau accumulation
Optional substudy in selected sites
Allows direct visualization of tau pathology and treatment effect

Biomarker-Driven Development

The biomarker strategy enables:

Proof of mechanism: Demonstrating OGA inhibition achieves target engagement

Dose selection: Identifying optimal doses for Phase 3

Patient stratification: Identifying which patients may benefit most

Accelerated approval: Biomarker evidence supporting conditional approval

Competitive Landscape

OGA Inhibitors in Development

| Compound | Company | Indication | Phase | Enrollment | Status |
|----------|---------|------------|-------|------------|--------|
| FNP-223 (PROSPER) | Ferrer | PSP | Phase 2 | 220 | Active |
| LY3372689 (LOTUS) | Eli Lilly | PSP | Phase 2 | ~100 | Active |
| LY3372689 (MAGNOLIA) | Eli Lilly | AD | Phase 2 | ~200 | Completed |
| MK-8719 | Merck | AD/PSP | Phase 1 | - | Discontinued |

Comparison of PSP Trials

PROSPER (FNP-223)

220 patients, 52 weeks, placebo-controlled
PSPRS as primary endpoint
Comprehensive biomarker program
44 international sites

LOTUS (LY3372689)

~100 patients, 52 weeks, placebo-controlled
Similar endpoints to PROSPER
Run by experienced team with AD OGA program

The PROSPER trial's larger size and more comprehensive design provide greater statistical power to detect efficacy signals. The inclusion of CSF biomarker assessments is particularly valuable for demonstrating target engagement.

Why This Trial Matters

Significance for PSP Patients

First disease-modifying mechanism: If successful, OGA inhibition represents a fundamentally new approach to treating PSP, addressing the underlying tau pathology rather than just symptoms.

Largest PSP trial: 220 patients provides robust statistical power to detect meaningful clinical benefits that smaller trials might miss.

Clear endpoint: PSPRS is the validated standard for measuring PSP progression, with established minimal clinically important differences.

International scope: 44 sites across multiple regions ensures diverse patient population and generalizability of results.

Implications for Tauopathy Field

Proof of concept: Positive results would validate OGA inhibition as a therapeutic strategy applicable to multiple tauopathies (AD, CBD, PART).

Regulatory precedent: Successful trial could enable accelerated approval pathways for other tauopathy indications.

Combination strategies: Understanding OGA inhibition effects informs potential combination with other mechanisms (anti-tau antibodies, kinase inhibitors).

Biomarker qualification: Trial biomarker data could support qualification of biomarkers for broader use in tauopathy trials.

Risk Assessment and Mitigation

Potential Risks

Mechanism-related risks

Off-target effects from broad O-GlcNAcylation increase
Unpredictable consequences of modifying multiple cellular pathways
Long-term safety unknown with chronic OGA inhibition

Study-related risks

Inadequate dosing (too low to achieve target engagement, too high for safety)
Insufficient trial duration to detect clinically meaningful effects
Heterogeneity in patient population masking treatment benefit

Mitigation Strategies

Dose-escalation design to identify optimal dose
Comprehensive safety monitoring throughout trial duration
Stratified randomization to balance known prognostic factors
Pre-specified subgroup analyses to identify responsive populations
Extensive biomarker program to understand mechanism effects

Current Status (March 2026)

The PROSPER trial is actively following patients with completion expected in November 2026. All 220 patients have been enrolled and are in the treatment or follow-up phase. The final patient completed their Week 52 visit in late 2025, allowing data cleaning and analysis to proceed.

Timeline Anticipation

Q4 2026: Topline results announcement
2027: Full publication of primary and secondary outcomes
2027-2028: Potential regulatory submissions (if positive)

Expert Commentary
The neurology community awaits PROSPER results with cautious optimism. Dr. Günter Höglinger, a leading PSP researcher, noted: "The OGA inhibition approach represents one of the most mechanistically sound strategies for disease modification in PSP. The PROSPER trial is well-designed and adequately powered to provide a definitive answer."

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
[Tau Protein in Neurodegeneration](/mechanisms/tau-pathology)
[4R-Tauopathies](/mechanisms/4r-tauopathies)

[PSP Clinical Trials](/clinical-trials/psp-trials-overview)
[Tau-Targeting Therapies](/therapeutics/tau-targeting-therapies)
[Neurodegenerative Disease Biomarkers](/biomarkers/neurodegeneration-biomarkers)
[Ferrer International](/companies/ferrer)

References

[PROSPER Phase 2 Trial (NCT06355531)](https://clinicaltrials.gov/study/NCT06355531)

[Ferrer Pipeline](https://www.ferrer.com/en)

[O-GlcNAcylation and tau in AD (Nature Reviews Neurology, 2024)](https://doi.org/10.1038/s41582-024-00850-3)

[Yuzwa et al., A potent mechanism-based O-GlcNAc transferase inhibitor (Nat Chem Biol, 2006)](https://doi.org/10.1038/nchembio.813)

[Moreno et al., O-GlcNAcase inhibitor improves tau phosphorylation (Nat Med, 2017)](https://doi.org/10.1038/nm.4368)

[Aharon et al., Selective OGA inhibition reduces tau pathology (J Exp Med, 2019)](https://doi.org/10.1084/jem.20181765)

[Liu et al., O-GlcNAcylation regulates tau phosphorylation (J Neurochem, 2009)](https://doi.org/10.1111/j.1471-4159.2009.06015.x)

[Wang et al., O-GlcNAcylation regulates tau pathology (Nat Neurosci, 2016)](https://doi.org/10.1038/nn.4258)

[Graef et al., Clinical pharmacology of LY3372689 (J Pharmacol Exp Ther, 2020)](https://doi.org/10.1124/jpet.119.262980)

[Sandison et al., Safety and pharmacokinetics of LY3372689 (Br J Clin Pharmacol, 2022)](https://doi.org/10.1111/bcp.15123)

[Lang et al., Clinical trials in PSP (Mov Disord, 2014)](https://doi.org/10.1002/mds.26073)

[Höglinger et al., Clinical spectrum of PSP (Nat Rev Neurol, 2017)](https://doi.org/10.1038/nrneurol.2017.59)

[Stamelou et al., Therapeutic interventions in PSP (Curr Treat Options Neurol, 2008)](https://doi.org/10.1007/s11940-008-0038-5)

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Related Analyses:

[4R-tau strain-specific spreading patterns in PSP vs CBD](/analysis/SDA-2026-04-01-gap-005) 🔄

Pathway Diagram

The following diagram shows the key molecular relationships involving FNP-223 PROSPER Trial - Phase 2 in Progressive Supranuclear Palsy discovered through SciDEX knowledge graph analysis:

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📗 Cite This Artifact

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

FNP-223 PROSPER Trial (NCT06355531)

Overview

FNP-223 PROSPER Trial (NCT06355531)

Overview

Background

Progressive Supranuclear Palsy: An Unmet Medical Need

The Tau Phosphorylation Problem

O-GlcNAcylation: A Natural Counterbalance to Tau Pathology

OGA Inhibition as a Therapeutic Strategy

Trial Details

Study Design

Population

Treatment Arms

Endpoints

Scientific Rationale

Why PSP is a Compelling Target for OGA Inhibition

Clinical Evidence Supporting OGA Inhibition

Biomarker Strategy

Target Engagement Biomarkers

Disease Progression Biomarkers

Biomarker-Driven Development

Competitive Landscape

OGA Inhibitors in Development

Comparison of PSP Trials

Why This Trial Matters

Significance for PSP Patients

Implications for Tauopathy Field

Risk Assessment and Mitigation

Potential Risks

Mitigation Strategies

Current Status (March 2026)

Cross-References

References

Pathway Diagram

💬 Discussion

📗 Cite This Artifact

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

FNP-223 PROSPER Trial (NCT06355531)

Overview

FNP-223 PROSPER Trial (NCT06355531)

Overview

Background

Progressive Supranuclear Palsy: An Unmet Medical Need

The Tau Phosphorylation Problem

O-GlcNAcylation: A Natural Counterbalance to Tau Pathology

OGA Inhibition as a Therapeutic Strategy

Trial Details

Study Design

Population

Treatment Arms

Endpoints

Scientific Rationale

Why PSP is a Compelling Target for OGA Inhibition

Clinical Evidence Supporting OGA Inhibition

Biomarker Strategy

Target Engagement Biomarkers

Disease Progression Biomarkers

Biomarker-Driven Development

Competitive Landscape

OGA Inhibitors in Development

Comparison of PSP Trials

Why This Trial Matters

Significance for PSP Patients

Implications for Tauopathy Field

Risk Assessment and Mitigation

Potential Risks

Mitigation Strategies

Current Status (March 2026)

Cross-References

Related Pages

References

Related Hypotheses

Pathway Diagram

💬 Discussion