FOXO4-DRI: A Peptide Whose Design Is the Whole Point

Most research peptides are interesting for what they bind. FOXO4-DRI is interesting for how it was built.

The design reverses a natural sequence and rebuilds it from D-amino acids. That combination keeps the side chains pointing roughly where they were while giving proteases nothing they recognise. The name carries the design: D-retro-inverso.

The target is a protein-protein interaction rather than a receptor. FOXO4 holds p53 in the nucleus of senescent cells, and the peptide competes for that contact [1]. Kimera supplies FOXO4-DRI as 10 mg of lyophilised powder.

Everything below reports findings from cells and animal models. This material is for research use only, not for human or veterinary use.

What FOXO4-DRI is

The molecule is a large synthetic peptide, roughly 5.4 kilodaltons, made of two functional parts: a FOXO4-derived segment and a cationic cell-permeability segment [2].

Identity and physical data

Two segments doing different jobs

The FOXO4-derived region provides the interaction surface. Its cationic region carries the peptide across membranes, which a 5.4 kilodalton molecule cannot do on its own.

A 2025 NMR study found that both segments contribute to binding, not just the FOXO4-derived one [2]. The delivery half is part of the pharmacology, which is unusual and worth carrying into any analogue design.

Naming across the record

Three names reach the same compound. FOXO4-DRI is the common one, proxofim is the assigned nonproprietary name, and papers sometimes write FOXO4 D-Retro-Inverso in full [9].

Search all three. A search for FOXO4-DRI alone misses papers filed under the expanded form, and the CAS number 2460055-10-9 catches supplier records that use neither (PubChem).

What D-retro-inverso means

Two changes that cancel

Take a peptide sequence and reverse it. Then build every residue from the D enantiomer instead of the L. Those two changes together approximately restore the original arrangement of side chains along the backbone, while the backbone itself now runs the other way.

The result presents a similar surface to a binding partner and looks nothing like a substrate to a protease. A 2021 review collects recent applications of that strategy across several target classes [14].

What it buys and what it costs

Protease resistance is the gain. A D-peptide survives in serum for hours where its L counterpart survives for minutes, which is why the approach exists at all.

The cost is that the mimicry is approximate. Backbone hydrogen bonding differs, and a retro-inverso peptide is not guaranteed to reproduce the parent’s affinity. For this compound the 2025 structural work is what settled that question experimentally [2].

Two PubChem records, one CAS number

What the records show

Searching the name returns two compound records. Both carry formula C228H388N86O64, both give 5358 g/mol, and both list CAS 2460055-10-9. Their InChIKeys share a first block, WVZCDZFJLXBWHG, and differ in the stereochemistry block: XXZPGMBKSA against QLCXAPFOSA.

Why that matters more here than usual

A stereochemistry difference is exactly what a D-retro-inverso design is about. For this compound the stereo block is the defining feature, not a detail. Two records disagreeing on it is a reason to check what a given supplier or paper actually used.

How the discrepancy probably arose

Large peptide records enter public databases from several sources, and stereochemistry is the field most often lost or guessed in translation. A 46-residue all-D sequence has to be specified residue by residue, and a single transcription slip changes the block.

That is speculation about the cause. The fact is not speculative: two records exist, they disagree, and the disagreement sits in the field that defines this molecule.

The practical rule

Cite the InChIKey in full, including the stereo block, rather than the name or the CAS alone. Both records are reachable through the same search, and a formula match proves nothing about chirality.

The mechanism

FOXO4 keeps p53 in the nucleus

The 2017 discovery paper identified FOXO4 as a pivot in senescent cell viability [1]. In senescent cells, FOXO4 interacts with p53 and holds it where it does not trigger apoptosis. Displace that interaction and p53 leaves the nucleus, and the cell dies through a cell-intrinsic route [1].

The structural picture arrived in 2025

Solution NMR gave structural models of the p53 transactivation domain bound to the FOXO4 forkhead domain, and separately bound to FOXO4-DRI [2]. Both partners are intrinsically disordered, and they form a transiently folded complex rather than a rigid one (PubMed).

The same work reported that p53 phosphorylation increases affinity for both FOXO4 and the peptide [2]. That detail connects the mechanism to cell state, since phosphorylation of p53 rises under the stress conditions that drive senescence.

Selectivity comes from cell state

Nothing in the design targets senescent cells specifically. Selectivity emerges because the FOXO4-p53 interaction matters for viability in senescent cells and not in quiescent ones [1]. The peptide is selective for a situation rather than for a cell type, which is a different claim and a more fragile one.

Why a protein-protein target is harder

Receptors have pockets. Protein-protein interfaces are flat, wide and shallow, which is why small molecules struggle against them and why a peptide is a reasonable tool for the job.

FOXO4-DRI works on an interface between two disordered regions, which is harder still [2]. There is no stable pocket to occupy, and the complex it forms is transient rather than fixed. That the approach works at all is the interesting part.

The founding experiments

What the 2017 paper tested

The Cell paper ran three settings [1]. It neutralised doxorubicin-induced chemotoxicity in mice. It restored fitness, fur density and renal function in fast-ageing XpdTTD/TTD mice. And it produced comparable restoration in naturally aged mice.

A commentary in the same issue framed the result as rejuvenation by therapeutic elimination of senescent cells [13]. That reading is the one the field mostly adopted.

What the endpoints do and do not show

Fur density and renal function are organism-level readouts. They report that something changed across a whole animal, which is what a senolytic claim needs, and they say nothing about which cells died.

Pairing them with a cellular marker closes that gap. The 2017 work did pair them, and later studies vary in whether they bothered [1]. An organism-level improvement without a senescent-cell measurement is an observation, not a mechanism.

The design of that study

Fast-ageing mutants and naturally aged animals answer different questions, and running both is a stronger design than either alone. Renal function and fur density are crude endpoints, and they are also hard to fake.

Where other groups took it

Reproductive tissue

A 2020 study reported that FOXO4-DRI alleviated age-related testosterone secretion insufficiency by targeting senescent Leydig cells in aged mice [4]. A 2024 follow-up from an overlapping group reported improved spermatogenesis through reduced senescence-associated secretory phenotype output from the same cell type [5].

Cartilage and cell expansion

In 2021 a different group used FOXO4-DRI to remove senescent cells selectively from in vitro expanded human chondrocytes [6]. That is a tissue-engineering application rather than a therapeutic one: expansion drives senescence, and senescent cells reduce the quality of the expanded population.

Fibrosis

Three studies applied it to fibrosis models. One reported radiosensitisation of non-small cell lung cancer alongside reduced radiation-induced pulmonary fibrosis [7]. Two others targeted bleomycin-induced pulmonary fibrosis in mice, one through myofibroblast targeting [8] and one through extracellular matrix production in fibroblasts [9].

What the tissue spread shows

Six tissues, at least six groups, one shared mechanism claim. That breadth is the strongest argument in this literature: independent laboratories reproduced senescent-cell killing in preparations that share little except the target axis.

Breadth is not depth, though. Most of these studies measure senescent-cell markers and a tissue endpoint, and few of them run the concentration controls that would separate the intended mechanism from membrane activity. Read them as convergent evidence for the phenomenon rather than as confirmation of the mechanism.

Other cell types

A 2025 study reported apoptosis in keloid senescent fibroblasts through nuclear exclusion of phosphorylated p53 [10]. Another 2025 study examined endothelial cell senescence through p53 signalling [11]. A 2024 study implicated senescence in hyperoxic bronchopulmonary dysplasia and used senolytic tools in that model [15].

The counter-result

Clearing senescent cells made a model worse

A 2023 Circulation study is the most important paper in this article for anyone planning experiments [3]. It examined senescent cells in pulmonary arterial hypertension patients and in animal models. Clearance then followed by three independent methods: a p16 promoter-driven suicide gene, senolytic drugs including this peptide, and p16 inactivation.

Clearance promoted pulmonary hypertension development and progression rather than preventing it [3].

Why one negative result carries this much weight

Three clearance methods agreeing rules out a compound-specific artefact. The result says senescent cells are not uniformly harmful, and that removing them is context-dependent.

Any experiment using a senolytic in a new tissue should treat direction of effect as an open question. A senolytic is a tool for removing a cell population, not a tool that improves tissues by default.

Senolysis as a field

FOXO4-DRI sits among several senolytic strategies with different mechanisms. A 2022 study characterised the ionophore nigericin as a senolytic acting through multiple homeostasis-maintaining systems [16], which is a different route to the same cellular outcome.

A 2026 review covers retro-inverso peptide senolytics aimed at the FOXO4-p53 axis in the context of brain ageing [12]. Reviews of that kind inherit the evidence base they summarise, and this one inherits both the positive tissue results and the pulmonary counter-result.

Designing an experiment with FOXO4-DRI

Establish senescence before treating

The selectivity argument depends on the cells being senescent [1]. Confirm that with markers, not with passage number alone: p16 and p21 expression, senescence-associated beta-galactosidase, and absence of proliferation together make the case.

Decide what senescent means in your system

Senescence is a family of states rather than one state. Replicative senescence, oncogene-induced senescence and therapy-induced senescence differ in their markers and in their dependence on the FOXO4-p53 axis.

State which one the model produces. A compound that works against therapy-induced senescence has not been shown to work against the replicative kind, and the founding paper used specific inducers rather than a general claim [1].

Include a non-senescent arm

Run proliferating and quiescent cells in parallel at the same concentrations. Selectivity is a comparison, and a single-population experiment cannot demonstrate it.

Watch the concentration range

The cationic segment is membrane-active at high concentrations, independent of the FOXO4 sequence. A scrambled peptide of matched composition and charge separates that effect from the intended one, and it is the control this compound most needs.

Expect tissue-specific direction

The pulmonary hypertension result means direction of effect is not portable between tissues [3]. Treat a new tissue as a new question, and power the study to detect harm as well as benefit.

Physicochemical properties and handling

Net peptide content is not optional here

A 5.4 kilodalton peptide purified by reversed-phase chromatography carries trifluoroacetate counterion and water. Both add mass that is not peptide.

Weigh the vial contents and you overestimate the peptide, sometimes by a substantial fraction. Ask for net peptide content on the certificate and calculate concentrations from that number.

The cationic segment makes it sticky

Arginine-rich sequences adsorb to glass and to plastic, and they bind nonspecifically to negatively charged surfaces including some filters. Low-binding tubes help, and filtering a dilute working solution is a good way to lose most of it.

Aliquot before the first freeze

A 10 mg vial reconstituted once and refrozen repeatedly loses material to each cycle, and a cationic peptide loses more than most because adsorption compounds the damage.

Split the reconstituted stock the same day into single-use volumes. Record the diluent and the date on each tube rather than in a notebook, since a peptide solution with no date is one nobody can defend in a methods section.

Trifluoroacetate has its own effects

TFA is not inert in cell culture at the levels a peptide salt can introduce. For sensitive assays, ask whether an acetate salt is available, or account for the counterion in a vehicle control.

Analytical characterization

Mass on a large peptide

At 5358 g/mol the compound produces a multiply charged envelope in electrospray rather than a single dominant ion. Deconvolution gives the neutral mass, and the envelope itself is a rough purity indicator: a clean preparation gives a regular charge series.

Mass cannot see chirality

D and L peptides of the same sequence have identical formulas and identical masses. No mass spectrometer distinguishes a D-retro-inverso peptide from its L parent, which is precisely the property the two PubChem records highlight.

Chiral confirmation needs a different experiment. Hydrolysis followed by chiral derivatisation and chromatography reports the D or L composition of the released amino acids directly.

Reading the charge envelope

A large peptide gives a series of charge states rather than one peak, and the shape of that series carries information. A regular, symmetric envelope suggests a homogeneous preparation. Ragged series with satellite peaks point at truncation products or adducts.

Deconvolute before comparing against 5358, and check whether the deconvoluted mass sits within a few units of the expected value rather than exactly on it. Sodium and potassium adducts shift large peptides by amounts a small-molecule chemist would find alarming.

Purity and identity together

Reversed-phase HPLC gives purity, amino acid analysis gives composition, and tandem mass spectrometry gives sequence order. Batch documentation for catalogue material sits in the certificate of analysis database. For a compound defined by stereochemistry, ask whether chiral analysis was part of the release testing.

Where FOXO4-DRI sits among peptide research compounds

Three of these target receptors or enzymes. This one targets a protein-protein interaction, which is a harder class of target and a different kind of tool. Further reading sits in the peptides research library, including the Epithalon article and the Oxytocin article.

What this literature does not establish

That senolysis helps by default

The pulmonary hypertension result shows the opposite in one model, confirmed by three clearance methods [3]. Direction of effect is tissue-dependent and has to be measured rather than assumed.

Human evidence

No controlled human trial results appear in the indexed literature. Everything above is cells and animals.

That the two segments can be separated

The 2025 structures show the cationic delivery segment participating in target binding [2]. That means the usual assumption, that a cell-penetrating tag is inert cargo, does not hold here. Nobody has published a version with a different delivery segment to test how much that matters.

That the design is optimal

The 2025 structures show the cationic segment participating in binding [2]. Nobody has published a systematic optimisation around that finding, so the current sequence reflects a 2017 design rather than a refined one.

Which stereochemical record is authoritative

Two PubChem entries share a CAS and differ in stereochemistry. Until the public record resolves that, the InChIKey a supplier states is the identity claim worth checking. A certificate without chiral analysis does not close the question.

Frequently asked questions

What does DRI stand for?

D-retro-inverso. The sequence runs backwards and every residue is a D-amino acid, which approximately restores the original side-chain arrangement while defeating proteases [14].

What does it actually target?

The interaction between FOXO4 and the p53 transactivation domain [1][2]. Displacing it causes p53 nuclear exclusion and apoptosis in senescent cells.

Why is it selective for senescent cells?

Because the interaction matters for viability in that state, not because the peptide recognises a senescence marker [1]. Selectivity depends on cell state rather than on targeting.

Has clearing senescent cells ever made things worse?

Yes. A 2023 Circulation study reported that senescent cell clearance promoted pulmonary hypertension in models, using three independent clearance methods [3].

How do I confirm the peptide is the D form?

Hydrolyse and run chiral analysis on the released amino acids. Mass spectrometry cannot answer the question.

Is FOXO4-DRI the same as proxofim?

Yes. Proxofim is the nonproprietary name for the same peptide, and both map to CAS 2460055-10-9.

Why does my working solution seem weak?

Adsorption or counterion mass, or both. Use low-binding tubes, avoid filtering dilute solutions, and calculate concentrations from net peptide content rather than vial mass.

References

1. Baar MP, Brandt RMC, Putavet DA, et al. Targeted apoptosis of senescent cells restores tissue homeostasis in response to chemotoxicity and aging. Cell. 2017;169(1):132-147.e16. PubMed DOI
2. Bourgeois B, Spreitzer E, Platero-Rochart D, et al. The disordered p53 transactivation domain is the target of FOXO4 and the senolytic compound FOXO4-DRI. Nat Commun. 2025;16(1):5672. PubMed DOI
3. Born E, Lipskaia L, Breau M, et al. Eliminating senescent cells can promote pulmonary hypertension development and progression. Circulation. 2023;147(8):650-666. PubMed DOI
4. Zhang C, Xie Y, Chen H, et al. FOXO4-DRI alleviates age-related testosterone secretion insufficiency by targeting senescent Leydig cells in aged mice. Aging (Albany NY). 2020;12(2):1272-1284. PubMed DOI
5. Li Y, Zhang C, Cheng H, et al. FOXO4-DRI improves spermatogenesis in aged mice through reducing senescence-associated secretory phenotype secretion from Leydig cells. Exp Gerontol. 2024;195:112522. PubMed DOI
6. Huang Y, He Y, Makarcyzk MJ, et al. Senolytic peptide FOXO4-DRI selectively removes senescent cells from in vitro expanded human chondrocytes. Front Bioeng Biotechnol. 2021;9:677576. PubMed DOI
7. Meng J, Li Y, Wan C, et al. Targeting senescence-like fibroblasts radiosensitizes non-small cell lung cancer and reduces radiation-induced pulmonary fibrosis. JCI Insight. 2021;6(23):e146334. PubMed DOI
8. Han X, Yuan T, Zhang J, et al. FOXO4 peptide targets myofibroblast and ameliorates bleomycin-induced pulmonary fibrosis in mice through the ECM-receptor interaction pathway. J Cell Mol Med. 2022;26(11):3269-3280. PubMed DOI
9. Liu Y, Hou Q, Wang R, et al. FOXO4-D-retro-inverso targets extracellular matrix production in fibroblasts and ameliorates bleomycin-induced pulmonary fibrosis in mice. Naunyn Schmiedebergs Arch Pharmacol. 2023;396(10):2393-2403. PubMed DOI
10. Kong YX, Li ZS, Liu YB, et al. FOXO4-DRI induces keloid senescent fibroblast apoptosis by promoting nuclear exclusion of upregulated p53-serine 15 phosphorylation. Commun Biol. 2025;8(1):299. PubMed DOI
11. Hu Z, Li F, Hu C, et al. FOXO4-DRI regulates endothelial cell senescence via the p53 signaling pathway. Front Bioeng Biotechnol. 2025;13:1729166. PubMed DOI
12. Alameen AAM, Al-Kuraishy HM, Fawzy MN, et al. Targeting the FOXO4-p53 axis by retro-inverso peptide senolytic agents: a pharmacological strategy to mitigate brain aging and cognitive decline. Naunyn Schmiedebergs Arch Pharmacol. 2026;399(10):14659-14676. PubMed DOI
13. Krimpenfort P, Berns A. Rejuvenation by therapeutic elimination of senescent cells. Cell. 2017;169(1):3-5. PubMed DOI
14. Doti N, Mardirossian M, Sandomenico A, et al. Recent applications of retro-inverso peptides. Int J Mol Sci. 2021;22(16):8677. PubMed DOI
15. Jing X, Jia S, Teng M, et al. Cellular senescence contributes to the progression of hyperoxic bronchopulmonary dysplasia. Am J Respir Cell Mol Biol. 2024;70(2):94-109. PubMed DOI
16. Deryabin PI, Shatrova AN, Borodkina AV. Targeting multiple homeostasis-maintaining systems by ionophore nigericin is a novel approach for senolysis. Int J Mol Sci. 2022;23(22):14251. PubMed DOI