FOXO4-DRI: A D-Retro-Inverso Peptide and What the Research Describes

FOXO4-DRI is unusual in a research catalogue for a reason that has nothing to do with what it is studied for. It is a 46-residue peptide in which every chiral residue is the D-enantiomer and the sequence runs backwards relative to the natural protein segment it mimics. That construction, called retro-inverso, is why the molecule exists in the form it does, why it costs what it does, and why a certificate for it has to be read differently from a certificate for an ordinary L-peptide. FOXO4-DRI is listed in the catalogue at 10 mg.

This guide covers the design as the 2017 paper by Baar and colleagues describes it, what retro-inverso means and why it resists proteolysis, the length and mass the catalogue records, what an all-D construction does to identity testing, and what a buyer is paying for at this price.

Everything said here about biological activity is reported from published work and named by model. The material is supplied for laboratory research only.

Where the sequence comes from

The 2017 Cell paper by Baar and colleagues describes FOXO4 as a factor that, in senescent cells, binds the transcription factor p53 and keeps it in the nucleus. The design idea was a peptide that competes for that interaction. The authors took the FOXO4 segment that contacts p53 and made two changes to it: they reversed the order of the residues and they built the chain from D-amino acids. The result is the D-retro-inverso peptide, DRI for short.

The paper reports its experiments in cultured senescent cells, principally human fibroblasts made senescent by irradiation, and in mice, including a fast-ageing mouse strain and naturally aged animals. The 2018 review by Bourgeois and Madl summarises the FOXO4-p53 axis as it was understood then, and a 2025 paper from Bourgeois and colleagues in Nature Communications maps the FOXO4 and FOXO4-DRI binding site to the disordered transactivation domain of p53, using purified proteins and nuclear magnetic resonance. A 2021 paper by Huang and colleagues reports the peptide's effect on senescent cells within cultures of expanded human chondrocytes. All of this is cell and animal work.

What retro-inverso means

Two operations are involved. Inverso means replacing each L-residue with its D-enantiomer. Retro means writing the sequence in reverse order. Done on its own, either operation scrambles the arrangement of side chains relative to the parent peptide. Done together, they approximately cancel: reversing the direction of the backbone and flipping the configuration at each alpha carbon leaves the side chains pointing roughly where they did in the original. Chorev and Goodman set this out in their 1995 review of retro peptides, and Fletcher and Campbell's 1998 review in Chemical Reviews covers the partially modified variants and their conformational behaviour.

What changes is the backbone. In a retro-inverso peptide the amide bonds run in the opposite direction to the parent, and the chiral centres are mirrored. The side-chain surface that a binding partner sees is similar; the backbone that an enzyme would have to cut is not.

Why D-residues resist proteolysis

Proteases are stereospecific. Their active sites evolved around L-amino acid substrates, and a chain built from D-residues does not fit. This is not a subtle effect: an all-D peptide is, for most proteases, simply not a substrate. The design intent in Baar and colleagues was a molecule persistent enough to compete for the FOXO4-p53 interaction inside cells, and the all-D construction is how the paper achieves that persistence. Whether that persistence translates into any particular outcome is a question the published models address, not the structure.

For a laboratory the practical point is different. The same resistance that makes the peptide stable in biological matrices makes it stable in solution during handling, so the usual worry about serum proteases in a culture medium is smaller here than for an L-peptide of the same length. Chemical degradation routes, which do not care about chirality, still apply.

Length, sequence and mass as the catalogue records them

The catalogue record gives 46 residues, the sequence ltlrkepaseiaqsileaysqngwanrrsggkrppprrrqrrkkrg with a free N-terminus and a free acid C-terminus, the formula C228H388N86O64, a molecular weight of 5358.06 g/mol, and CAS number 2460055-10-9. The sequence is written in lowercase, which is the convention for D-residues in one-letter notation. The glycines, written g, have no chiral centre, so "all-D" means every residue that can be D is D.

PubChem's entry for FOXO4-DRI, CID 167312269, records the same formula and a molecular weight of 5358 g/mol, so the two reference figures agree. Counting along the recorded sequence gives 10 arginines and 4 lysines against 3 glutamates and no aspartate, so the chain is strongly basic, and most of that charge sits in the C-terminal stretch rrrqrrkkrg. There is one tryptophan, no methionine and no cysteine.

That composition has two consequences on the bench. A strongly cationic peptide adsorbs to glass and some plastics at low working concentrations, and it behaves differently on reversed-phase columns from a neutral sequence of the same length. The single tryptophan is the only readily oxidisable residue, and the storage line in the record, lyophilised at -20°C protected from light and moisture, is the right response to it. How peptides degrade maps the routes to the residues that enable them.

The analytical consequence: mass spectrometry cannot see chirality

A D-amino acid has the same formula and the same mass as its L-enantiomer. Mass spectrometry measures mass. So an intact-mass result for FOXO4-DRI is identical to the result for a peptide of the same sequence built entirely from L-residues, and a fragmentation spectrum, which reads the order of residues, is identical too. Everything a mass spectrometer reports about this peptide is consistent with the correct all-D product and equally consistent with an all-L chain of the same sequence. Mass spectrometry and peptide identity explains what intact mass and fragmentation each confirm; neither confirms configuration.

Reversed-phase HPLC does not help either. Enantiomers have identical retention on an achiral column, which is what a standard purity method uses. A purity figure of 99 percent says the lot is 99 percent one compound; it does not say which hand that compound is.

Two things do fix chirality. The first is chiral amino acid analysis: the peptide is hydrolysed to its constituent amino acids and these are derivatised with a chiral reagent such as Marfey's reagent, which converts enantiomers into diastereomers that separate on an ordinary column. Bhushan and Brückner's 2004 review covers the method. The second is the synthesis record. The peptide is assembled one residue at a time by solid-phase peptide synthesis, and the chirality of the product is set by the protected amino acid building blocks that went into the synthesiser. A supplier who can show that the lot was built from D-amino acid building blocks has answered the question; a supplier who can only show HPLC and mass has not.

Retro-inverso adds one more wrinkle. A reversed sequence has exactly the same composition as the forward one, so intact mass cannot distinguish the retro peptide from the forward peptide either. Fragmentation can, because it reads order. That makes a fragmentation result more useful on this compound than on most, even though it still says nothing about chirality.

What a long all-D peptide costs and why

The catalogue lists FOXO4-DRI from $490 for 10 mg, which is $49 per milligram. Three things in the structure put it there, and what research peptides cost and why sets out the general case.

• Length. Forty-six couplings, each with a yield below 100 percent. At 99 percent per step the crude yield of full-length chain is about 63 percent; at 98 percent it is about 40 percent. The arithmetic is 0.99 and 0.98 raised to the 46th power. Every deletion sequence that forms along the way has to be separated out later.
• Building blocks. Protected D-amino acids are made at a fraction of the scale of their L-counterparts and cost correspondingly more per gram. A 46-mer consumes a lot of them, and the arginines, ten of them, carry the bulkiest and most expensive protecting groups in the set.
• Purification. A long, strongly basic, arginine-rich chain is a difficult purification target. Deletion sequences differ from the product by a single residue in a 5358 Da molecule, and the arginine-rich C-terminus broadens peaks on reversed-phase columns. Reaching the 99 percent purity stated in the record costs yield.

The comparison is not perfect, since the three are made at different scales, but the direction is what the chemistry predicts: price per milligram rises steeply with length, and the all-D building blocks add a premium on top.

What to check before ordering

• The sequence on the certificate, in full, with its notation. Lowercase letters or an explicit "D-" prefix should appear; a sequence printed in uppercase with no comment is a question to ask.
• Observed mass against 5358.06 g/mol on the average basis, remembering that this confirms composition only.
• Whether a fragmentation result is offered. On a retro-inverso peptide it is the only routine test that distinguishes the reversed sequence from the forward one.
• Whether the supplier can state that the lot was synthesised from D-amino acid building blocks, or offers chiral amino acid analysis. Neither HPLC nor mass answers this.
• Net peptide content. On a strongly basic 46-mer with 14 basic residues the trifluoroacetate counterion fraction is substantial, and the gravimetric 10 mg is not 10 mg of peptide.

What the published work does and does not say

The literature on FOXO4-DRI is a literature of cell culture and rodent models: irradiated human fibroblasts, expanded human chondrocytes in culture, fast-ageing and naturally aged mice, and purified protein work on the binding site. The catalogue record describes it as a tool compound in senolytic and cellular-ageing research, which is an accurate description of where it has been used. Nothing in that body of work licenses a statement about effects in people, and this guide makes none.

For a buyer, the useful summary is this: the design is well described, the structure is well defined, the two reference figures for its mass agree, and the one property that makes it what it is, chirality, is the one property a standard certificate does not test.