FNP-223 is an oral O-GlcNAcase (OGA) inhibitor developed by Ferrer, a Spanish pharmaceutical company. It is being evaluated in the PROSPER Phase 2 trial specifically for Progressive Supranuclear Palsy (PSP), representing one of the few large-scale clinical trials dedicated to this rare 4R-tauopathy.
¶ Background: O-GlcNAcylation and Tau Pathology
O-GlcNAcylation is a post-translational modification where O-linked N-acetylglucosamine (O-GlcNAc) is added to serine and threonine residues on proteins, including tau. This modification is increasingly recognized as a key regulatory mechanism that competes with pathological phosphorylation:
- Physiological role: O-GlcNAcylation regulates protein function, stability, and localization
- Tau relationship: O-GlcNAcylation of tau at specific sites competes with phosphorylation at the same serine/threonine residues
- Key insight: O-GlcNAcylation at Thr231, Ser396, and Ser400 inhibits tau aggregation
- Disease context: In PSP and other tauopathies, tau is hyperphosphorylated at these same sites, leading to neurofibrillary tangle formation
The scientific rationale follows the "yin-yang" hypothesis: phosphorylation and O-GlcNAcylation compete for the same residues. By inhibiting OGA, FNP-223 shifts the balance toward protective O-GlcNAcylation, potentially reducing pathological tau phosphorylation and aggregation.
FNP-223 is a potent, selective OGA inhibitor that increases O-GlcNAcylation levels on tau protein:
- Target: O-GlcNAcase (OGA) — the enzyme that removes O-GlcNAc from proteins
- Mechanism: Inhibiting OGA increases tau O-GlcNAcylation at pathological sites
- Effect: O-GlcNAcylated tau is less prone to pathological phosphorylation and aggregation
- Outcome: Potential disease modification by reducing toxic tau species
The OGA inhibitor approach offers several advantages over other tau-targeted strategies:
- Oral bioavailability: Small molecule can be taken orally, improving patient convenience
- Direct mechanism: Targets the fundamental yin-yang relationship between phosphorylation and O-GlcNAcylation
- BBB penetration: Designed to cross the blood-brain barrier
- 4R-tau specificity: Particularly relevant for 4R-tauopathies like PSP where tau isoform balance is disrupted
The PROSPER trial (NCT06355531) is a Phase 2 clinical trial evaluating FNP-223 in patients with Progressive Supranuclear Palsy:
- Sponsor: Ferrer
- Phase: Phase 2
- Indication: PSP (Richardson's syndrome and other PSP subtypes)
- Enrollment: 220 patients
- Sites: 44 sites across EU, UK, and US
- Status: Enrollment completed October 2025 (2 months ahead of schedule)
The PROSPER trial is specifically designed for PSP, making it one of the largest dedicated PSP trials:
- Patient population: Clinically diagnosed PSP patients
- Duration: 52-week treatment period
- Primary endpoints: Safety, tolerability, and efficacy measures
- Secondary endpoints: Tau biomarker changes, clinical rating scales (PSP-RS, MoCA)
- Results expected: 2026
PSP represents an ideal indication for OGA inhibitors:
- 4R-tauopathy: PSP is characterized by elevated 4R-tau, which may be particularly sensitive to O-GlcNAcylation effects
- Unmet need: No approved disease-modifying treatments for PSP
- Clear diagnosis: PSP has relatively well-defined clinical criteria compared to other tauopathies
- Target engagement: CSF and PET biomarkers can measure OGA inhibition and tau changes
FNP-223 competes with other OGA inhibitors in development, with key differences:
| Drug |
Company |
Phase |
Indication |
Status |
| FNP-223 |
Ferrer |
Phase 2 |
PSP |
Enrollment complete |
| LY3372689 |
Eli Lilly |
Phase 2 |
AD |
Completed |
| ASN90 |
Asceneuron |
Phase 2 |
AD/Tauopathy |
Active |
Key differentiators for FNP-223:
- PSP-specific trial (others focus on AD)
- Large 220-patient enrollment
- 44 international sites providing diverse patient population
- Results expected in 2026 (ahead of some competitors)
- Demonstrated low nanomolar potency against human OGA
-
1000-fold selectivity over other glycosidases
- Brain-penetrant following oral administration
- Reduced tau pathology in mouse models of tauopathy
- Completed first-in-human studies
- Established safety and tolerability profile
- Determined optimal dosing for Phase 2
- Completed enrollment of 220 PSP patients
- 44 sites across EU, UK, US
- Results expected 2026
If PROSPER yields positive results, FNP-223 could represent a significant advance for PSP treatment:
- First disease-modifying therapy for PSP: No approved DMTs currently exist
- Novel mechanism: Different from anti-tau antibodies or ASOs
- Oral delivery: Easier administration than injectable biologics
- Specific to PSP: Unlike broad AD-focused programs