FOXO4-DRI Peptide

FOXO4-DRI is a D-retro-inverso (DRI) peptide designed to disrupt the FOXO4-p53 protein-protein interaction within senescent cells. The DRI modification uses D-amino acids assembled in reverse order, creating a peptide with protease resistance while maintaining the topological presentation of side chains necessary for target binding. Senescent cells accumulate with age and secrete a pro-inflammatory cocktail known as the senescence-associated secretory phenotype (SASP). The FOXO4-p53 complex normally protects senescent cells from apoptosis. FOXO4-DRI was designed to competitively disrupt this protective interaction. For laboratory research use only.

FOXO4-DRI is a synthetic D-retro-inverso (DRI) peptide engineered to selectively disrupt the protein-protein interaction between Forkhead box O4 (FOXO4) and the tumor suppressor p53 (TP53) within senescent cells. The peptide's sequence is derived from the FOXO4 region that binds p53, but it is synthesized entirely from D-amino acids in reverse order. This DRI design preserves the side-chain topology required for target binding while conferring near-complete resistance to proteolytic degradation, dramatically extending intracellular half-life compared with conventional L-peptides.

FOXO4-p53 Interaction Disruption

In senescent cells, FOXO4 accumulates in the nucleus and binds activated, phosphorylated p53, sequestering it away from mitochondria. This sequestration is thought to be a key adaptation that allows senescent cells to resist p53-driven apoptosis while maintaining the senescence-associated secretory phenotype (SASP). By competitively binding FOXO4, FOXO4-DRI displaces endogenous p53, releasing it to translocate to the mitochondria where it triggers intrinsic apoptosis via mitochondrial outer membrane permeabilization, cytochrome c release, and caspase-9/caspase-3 activation [1].

Selective Senolytic Activity

Because non-senescent cells contain comparatively little nuclear FOXO4 and do not depend on FOXO4-p53 sequestration for survival, FOXO4-DRI exhibits substantial selectivity for senescent cells over proliferating or quiescent cells in preclinical models. This selectivity distinguishes the compound from broad cytotoxic senolytics and from kinase-inhibitor senolytics such as dasatinib + quercetin or navitoclax (ABT-263), which act on BCL-2/BCL-xL anti-apoptotic family proteins [2].

Cell-Penetrating Modification

The peptide incorporates an N-terminal cell-penetrating sequence (derived from FOXO4's own nuclear localization region in some constructs, or fused HIV-TAT-like residues in others) that facilitates plasma membrane translocation and nuclear delivery. Once internalized, the DRI architecture resists ubiquitin-proteasome and lysosomal degradation, allowing sustained occupancy of the FOXO4 binding pocket.

Downstream Consequences

Following p53 release, senescent cells undergo apoptosis with characteristic phosphatidylserine externalization, caspase activation, and DNA fragmentation. Clearance of senescent cells in vivo has been associated with reduced SASP cytokine output (IL-6, IL-8, MCP-1), decreased systemic inflammation, and improvements in tissue homeostasis markers in aged and chemotherapy-treated rodent models [1]. The mechanism is fundamentally distinct from BCL-2 family inhibitors and from autophagy-modulating senolytics, making FOXO4-DRI a mechanistically unique tool compound for dissecting senescent-cell survival pathways.

FOXO4-DRI is a relatively large, modified synthetic peptide designed as a D-retro-inverso analog of the FOXO4 region that interacts with p53. The full sequence comprises a cell-penetrating segment fused to the FOXO4-derived binding sequence, with all residues in D-configuration arranged in reverse order relative to the parent L-peptide. Key physicochemical characteristics are summarized below.

Full Name	FOXO4 D-Retro-Inverso peptide (FOXO4-DRI)
Synonyms	FOXO4-p53 disruptor peptide, ProxofimĀ (developmental designation in some references)
Molecular Formula	C218H335N61O73 (approximate, varies with exact construct)
Molecular Weight	~4,827 g/mol
CAS Number	2074718-87-9
Sequence Design	D-retro-inverso peptide derived from FOXO4 forkhead domain p53-interacting region, N-terminally fused to a cell-penetrating sequence
Approximate Length	~40 amino acid residues
Stereochemistry	All D-amino acids in reverse sequence order (retro-inverso)
Origin / Developer	Peter L.J. de Keizer laboratory, Erasmus University Medical Center (2017)
Key Modifications	D-amino acid substitution throughout; reversed sequence; cell-penetrating N-terminal extension
Physical Form	Lyophilized white to off-white powder
Solubility	Soluble in bacteriostatic water and sterile saline; DMSO can aid initial solubilization at higher concentrations
Purity	≥98% by HPLC
Net Charge (approx., pH 7)	Strongly cationic due to multiple D-arginine and D-lysine residues, contributing to membrane translocation

The D-retro-inverso architecture is the defining structural feature of FOXO4-DRI. Compared with the parent L-peptide, the DRI analog displays markedly improved stability against serum and intracellular proteases, longer functional half-life, and reduced immunogenicity in rodent models. The cationic character imparted by D-arginine and D-lysine residues supports nonspecific membrane translocation, while the engineered binding face engages the FOXO4 forkhead domain with sufficient affinity to displace p53 in cellular assays. Researchers should note that exact molecular formula and mass can vary slightly between published constructs depending on the cell-penetrating segment used; values reported here reflect the most commonly cited Baar et al. (2017) construct.

FOXO4-DRI is supplied as a lyophilised white powder. Because of its high molecular weight (~4,827 g/mol), 48 amino acid length, and D-amino acid composition, careful reconstitution practices are essential to preserve the integrity of the peptide and ensure consistent experimental results.

Recommended Reconstitution Protocol

1. Equilibrate to room temperature: Remove the vial from -20°C storage and allow it to reach room temperature (approximately 20-25 minutes) before opening. This prevents condensation from forming inside the vial, which could compromise the lyophilised cake.
2. Centrifuge briefly: Pulse-spin the vial at low speed (1-2 seconds) to ensure all lyophilised material settles at the bottom.
3. Select appropriate solvent: FOXO4-DRI is typically dissolved in sterile bacteriostatic water or sterile 0.9% saline. For research applications requiring vehicle compatibility with cell culture, sterile water or PBS (pH 7.4) is often used. Some protocols use a small fraction of DMSO (≤5%) to aid initial dissolution, followed by dilution into aqueous buffer.
4. Calculate concentration: For a 10 mg vial reconstituted in 2 mL of solvent, the resulting concentration is 5 mg/mL. Adjust volume according to your experimental concentration requirements.
5. Add solvent slowly: Inject the solvent down the side wall of the vial, not directly onto the peptide cake. This minimises mechanical stress.
6. Gentle mixing: Allow the peptide to dissolve passively for 1-2 minutes, then gently swirl or invert. Do NOT vortex or shake aggressively — mechanical agitation can damage the peptide and induce aggregation.
7. Inspect the solution: The reconstituted solution should be clear and colourless. Any visible particulates indicate incomplete dissolution or aggregation; the solution may be filtered through a 0.22 μm low-protein-binding syringe filter if downstream applications require sterility.

Compound-Specific Handling Notes

• D-amino acid stability: The D-retro-inverso configuration confers exceptional resistance to enzymatic degradation, but the peptide is still susceptible to chemical degradation from oxidation, hydrolysis, and aggregation. Always handle under clean, low-moisture conditions.
• Avoid repeated freeze-thaw: Prepare working aliquots immediately after reconstitution to minimise freeze-thaw cycles, which can promote aggregation in high-MW peptides.
• pH sensitivity: The peptide is most stable in slightly acidic to neutral aqueous solutions (pH 5-7). Avoid strongly basic conditions, which can accelerate degradation.
• Surface adsorption: Use low-protein-binding tubes (e.g., polypropylene with low-binding treatment) for storage of dilute solutions to minimise loss to plastic surfaces.
• Protect from light: Although FOXO4-DRI does not contain highly photolabile residues, general best practice for peptide research includes protection from prolonged light exposure.

All handling should be performed in a clean laboratory setting using appropriate PPE. FOXO4-DRI is intended for in vitro research and preclinical animal studies only and is not approved for human or veterinary use.

Proper storage is critical to maintain the structural integrity and biological activity of FOXO4-DRI. The peptide's 48-residue length and D-amino acid composition provide enhanced enzymatic stability relative to native L-peptides, but it remains susceptible to physical and chemical degradation if storage guidelines are not followed.

Lyophilised Powder Storage

• Long-term storage: Store the lyophilised powder at -20°C in a sealed, desiccated environment. Under these conditions, FOXO4-DRI is stable for approximately 24 months from date of manufacture.
• Short-term storage: The lyophilised peptide may be stored at 2-8°C (standard refrigeration) for up to 30 days without significant degradation.
• Transit conditions: Brief exposure to ambient temperature during shipping (up to 7 days) does not significantly affect lyophilised peptide integrity. Upon receipt, transfer immediately to -20°C storage.
• Desiccation: Always store with desiccant to prevent moisture uptake, which can promote hydrolysis and aggregation.

Reconstituted Solution Storage

• 2-8°C (refrigerated): Reconstituted FOXO4-DRI may be stored at 2-8°C for up to 7-14 days for short-term experimental use.
• -20°C or -80°C (frozen aliquots): For longer-term storage of reconstituted material, prepare single-use aliquots in low-protein-binding tubes and store at -20°C (up to 1 month) or -80°C (up to 3 months).
• Avoid freeze-thaw cycles: Each freeze-thaw cycle increases the risk of peptide aggregation and loss of activity. Aliquot immediately upon reconstitution.

Compound-Specific Stability Notes

• D-amino acid resistance: The D-retro-inverso architecture confers strong protease resistance, but does not protect against chemical hydrolysis, oxidation of any methionine residues, or aggregation driven by hydrophobic interactions.
• High molecular weight risk: At ~4,827 g/mol and 48 residues, FOXO4-DRI is more prone to aggregation than smaller peptides. Maintain dilute working concentrations and avoid mechanical stress.
• pH stability window: Optimal stability is observed in slightly acidic to neutral buffers (pH 5-7). Strongly alkaline conditions accelerate degradation.
• Container compatibility: Glass or low-protein-binding polypropylene containers are recommended. Standard polypropylene at high dilutions may result in surface adsorption losses.

Following these storage guidelines preserves the structural and functional integrity of FOXO4-DRI for reproducible research applications. For research use only.