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, 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[1]. 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.
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[2].
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:
The distribution of tau pathology correlates with the clinical syndrome, affecting:
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:
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 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[3].
Mechanistic Basis
On tau protein specifically:
The landmark studies by Yuzwa et al. established that:
O-GlcNAc Levels in Disease
Post-mortem studies have demonstrated:
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:
The development of OGA inhibitors has progressed through several generations:
| 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 |
The PROSPER trial enrolled patients meeting clinical criteria for PSP:
Diagnostic Criteria
Key Inclusion Criteria
Key Exclusion Criteria
The trial employs a randomized, double-blind, placebo-controlled design:
Primary Endpoint
The PSPRS is a validated 36-item scale measuring:
Secondary Endpoints
Exploratory Endpoints
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.
Preclinical Studies
Multiple preclinical studies have demonstrated OGA inhibitor efficacy in tauopathy models:
Clinical Evidence from Other Tauopathies
The most advanced clinical program for OGA inhibition has been in Alzheimer's disease:
The MAGNOLIA study demonstrated:
This clinical data supports the biological rationale for OGA inhibition and informs the PROSPER trial design.
The PROSPER trial incorporates an extensive biomarker program to demonstrate mechanism of action and track disease progression.
CSF O-GlcNAc Levels
Blood O-GlcNAc
Neurofilament Light Chain (NfL)
Phosphorylated Tau (p-tau181, p-tau217)
Tau PET Imaging
The biomarker strategy enables:
| 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 |
PROSPER (FNP-223)
LOTUS (LY3372689)
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.
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.
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.
Mechanism-related risks
Study-related risks
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
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."
Höglinger GU, et al. Clinical spectrum of PSP: a review. Nat Rev Neurol. 2017. ↩︎
Liu F, et al. O-GlcNAcylation regulates tau phosphorylation and toxicity. J Neurochem. 2009. ↩︎
Yuzwa SA, et al. A potent mechanism-based O-GlcNAc transferase inhibitor. Nat Chem Biol. 2006. ↩︎
Moreno JA, et al. Oral administration of an O-GlcNAcase inhibitor improves tau phosphorylation and cognition in tau transgenic mice. Nat Med. 2017. ↩︎
Aharon A, et al. Selective inhibition of O-GlcNAcase reduces tau phosphorylation and prevents cognitive deficits. J Exp Med. 2019. ↩︎