SS-20
SS-20 (Phe-D-Arg-Phe-Lys-NH2), Szeto-Schiller mitochondria-targeted peptide
Written by Aaron CuhaReviewed Sep 2026
Also known as: SS20, Szeto-Schiller peptide 20
In plain English, from Aaron
This is how I would explain it to a friend. The evidence, with the numbers, is further down the page.
SS-20 is a four amino acid peptide that protects mitochondria during a heart attack or a blocked blood supply. Its close cousin became a real approved drug. This one never left the animal lab. The interesting part is that it cannot mop up free radicals at all, and it still works in rats.
What people take it for
- Protecting the heart and kidneys under stress.
- More energy, on the theory that better mitochondria means more fuel.
- Nerve pain from chemotherapy.
- General anti aging, since aging and mitochondria get linked together.
What the trials actually showed
No human has been given it and no trial is registered. Everything below is animals. In rats with a blocked heart artery, the damaged area was 47.1 percent of the tissue at risk on this peptide, against 59.9 percent with nothing. Its cousin, the one that can mop up free radicals, did slightly worse at 53.9 percent. Eight rats per group. In rat kidneys it stretched how long the organ could go without blood from 30 minutes to 45. In mice on a chemotherapy drug it stopped the nerve pain and brought the nerve fibres back to normal. The reason it works is that it sits on a fat in the mitochondria and keeps one key protein from falling apart. In a starved rat kidney the damaged form of that protein rose 17 times, and this peptide stopped that.
What people report
Reports, not trial results
The good
- There is nothing to report. This peptide is barely sold and almost nobody has used it.
- Its cousin is now a real prescription drug, which is the only human track record this chemistry has.
The bad
- No human safety data at all. The animal studies measured whether it worked, not whether it was safe.
- The lab that made it holds a financial stake, and nearly all the research comes from that one group.
- Its cousin went through real trials and mostly missed the target, which is not a good sign for the family.
- Anything sold under this name is unregulated with no purity testing.
Where these come from: All of it is published animal work from one research group and its partners. There are no user reports to speak of, and no trial data of any kind.
My bottom line
The rat data is clean and the mechanism is genuinely interesting. But the company owned both peptides and picked the other one. That one ran real trials and mostly came up short. I would not be the first human to try this.
An opinion, not a finding. I am a coach, not a doctor.
Overview
The sibling of SS-31, the peptide that became the approved drug elamipretide. SS-20 cannot scavenge free radicals at all, which made it the control that proved a point, and it works anyway in rats and mice. There is no human study and no registered trial.
SS-20 is Phe-D-Arg-Phe-Lys-NH2, one of the Szeto-Schiller peptides developed at Weill Cornell. Its interest comes from what it lacks.
SS-31, which became elamipretide, carries a dimethyltyrosine that scavenges reactive oxygen species. SS-20 does not have it and cannot scavenge. The two peptides were therefore used against each other to ask whether the cardioprotection seen in this family came from antioxidant activity or from something else.
In rats with a 60 minute coronary artery ligation, both peptides at 3 milligrams per kilogram intraperitoneally shrank the infarct: 53.9 percent of the area at risk with SS-31 and 47.1 percent with SS-20, against 59.9 percent in controls. Both reduced lipid peroxidation and arrhythmia severity. The peptide that cannot scavenge did at least as well as the one that can.
What SS-20 does instead emerged later. It selectively targets cardiolipin in the inner mitochondrial membrane and protects the methionine 80 to iron ligation of cytochrome c. In ischaemic rat kidney mitochondria that ligation is disrupted, methionine-sulfoxide cytochrome c rises 17-fold, the cristae collapse, and cytochrome c is lost with proteolytic degradation of OPA1. SS-20 prevented all of it.
The practical results follow from that. Pretreatment 30 minutes before warm ischaemia raised rat kidney ischaemia tolerance from 30 to 45 minutes and reduced later interstitial fibrosis. In mice given weekly oxaliplatin, continuous SS-20 prevented neuropathic pain and returned intraepidermal nerve fibre loss to normal levels. In MPTP-treated mice, a Parkinson's model, SS-20 protected dopaminergic neurons significantly despite having no scavenging ability.
Every one of those is an animal. SS-20 never entered clinical development. Its sibling did.
Mechanism of action
Selective targeting of cardiolipin in the inner mitochondrial membrane, where it preserves the methionine 80 to iron ligation of cytochrome c. Ischaemia disrupts that ligation, producing a globin-like pentacoordinated heme that inhibits electron transport and turns cytochrome c into an oxygenase, which peroxidises cardiolipin and drives OPA1 degradation and cytochrome c release. By protecting the ligation, SS-20 keeps electron transport efficient and improves coupling of oxidative phosphorylation, which is why its effects look antioxidant without any scavenging chemistry: it reduces reactive oxygen species production rather than mopping them up.
Reading the research record
The useful comparison is with what happened to its sibling. SS-31, elamipretide, went all the way through clinical development and received accelerated approval in the United States in 2025 for Barth syndrome, on an open-label uncontrolled extension, after randomised placebo-controlled trials in other mitochondrial conditions missed their primary endpoints. That is the ceiling this chemistry has demonstrated in humans so far: an approval in an ultra-rare disease on uncontrolled evidence, alongside negative randomised results in more common conditions.
SS-20 did not go down that road at all. Read against its sibling's record, the absence of SS-20 trials is not a gap waiting to be filled with good news. A developer holding both peptides chose one, took it through randomised trials that largely failed, and left the other in animals.
One structural note about this literature: the SS peptides are licensed to a commercial developer and the senior author, coauthors and Cornell hold financial interests, disclosed in the papers themselves. That does not invalidate the findings. It does mean essentially all of the SS-20 literature comes from one laboratory and its collaborators.
The evidence, charted
Fig. 1 · evidence composition
0of 5 citations (0%) are in people
Counted from the citation list on this page. The Human count is the same number shown in the badge at the top. Both understate any literature larger than the sources we cite.
Fig. 2 · evidence over time
Evidence spans 4 distinct years, 2007 to 2018, counted from the citation list on this page. The newest citation on file is from 2018, more than five years ago; the published record may have gone quiet.
Fig. 3 · legal status at a glance
US
Not approved
UK
Not approved
AU
Not approved
CA
Not approved
Not approved in any of the four jurisdictions shown. A jurisdiction's classification is a regulatory fact, not a verdict on the science; see Legal status below for the full text and any notes.
Fig. 4 · dose response
No human dose response curve exists
We draw this figure where the data supports it. For this compound in humans it does not, so the panel stays empty rather than borrowing an animal curve and implying it transfers.
Key studies & citations
- Animal2007
Potent mitochondria-targeted peptides reduce myocardial infarction in rats.
Rats, 8 per group, randomised to SS-31, SS-20 or placebo at 3 milligrams per kilogram intraperitoneally 30 minutes before a 60 minute left anterior descending ligation and again 5 minutes before reperfusion. Infarct area as a fraction of area at risk was 53.9 percent with SS-31 and 47.1 percent with SS-20 against 59.9 percent in controls. Both reduced lipid peroxidation and arrhythmia severity, with Lambeth scores of 5 and 3 against 13 in controls. Since SS-20 does not scavenge reactive oxygen species, the authors conclude it most likely reduces their production instead. Rats. The PubMed record carries a Clinical Trial publication type tag, which is a database artefact: the study is in rats.
Coronary Artery Disease - In vitro2015
Disruption of cytochrome c heme coordination is responsible for mitochondrial injury during ischemia.
The mechanism paper, using mitochondria isolated from ischaemic rat kidneys. Ischaemia raised methionine-sulfoxide cytochrome c 17-fold and caused dramatic cristae collapse with OPA1 degradation and cytochrome c loss. SS-20 selectively targets cardiolipin, protects the methionine 80 to iron ligation, and prevented all of those changes. The authors disclose that the SS peptides are licensed to Stealth Peptides Inc and that they and Cornell hold financial interests.
Biochimica et Biophysica Acta - Animal2015
Improving mitochondrial bioenergetics under ischemic conditions increases warm ischemia tolerance in the kidney.
Rats. Pretreatment with SS-20 thirty minutes before warm ischaemia raised tolerated ischaemia time from 30 to 45 minutes. It reduced cytoskeletal breakdown, cell swelling and mitochondrial matrix swelling, preserved cristae, improved state 3 respiration and respiratory control ratio in isolated kidney mitochondria, and reduced later interstitial fibrosis when given after ischaemia. A surgical-window result in rats, not a chronic kidney disease treatment in people.
American Journal of Physiology, Renal Physiology - Animal2018
Protective Effect of a Mitochondria-Targeted Peptide against the Development of Chemotherapy-Induced Peripheral Neuropathy in Mice.
Mice given weekly oxaliplatin developed neuropathic pain and lost intraepidermal nerve fibres in the hind paw. Continuous SS-20 prevented the pain and returned nerve fibre density to normal levels. The authors note that SS-31 was the inventor's compound and declare that competing interest. Mice, prevention rather than treatment, and no human follow-up has been located.
ACS Chemical Neuroscience - Animal2009
Mitochondria targeted peptides protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine neurotoxicity.
MPTP-treated mice, a Parkinson's model. SS-31 gave dose-dependent complete protection against loss of striatal dopamine and of tyrosine hydroxylase positive neurons in the substantia nigra. SS-20, which the paper states has no intrinsic ability to scavenge reactive oxygen species, also showed significant neuroprotection. Both prevented MPP+ induced inhibition of oxygen consumption, ATP production and mitochondrial swelling in isolated mitochondria. Mice and cultured cells.
Antioxidants and Redox Signaling
Frequently asked questions
Has SS-20 been tested in humans?
No. There is no human study and no registered clinical trial. All of the work is in rats, mice and isolated mitochondria.
How is SS-20 different from SS-31?
SS-31, which became elamipretide, carries a dimethyltyrosine that scavenges reactive oxygen species. SS-20 does not have it and cannot scavenge. SS-20 works by binding cardiolipin and protecting the methionine 80 to iron ligation of cytochrome c, which reduces how many reactive oxygen species get produced in the first place.
If SS-20 works without scavenging, is the antioxidant story wrong?
That is close to the point the rat infarct study was designed to make. Both peptides cut infarct size and lipid peroxidation, and the one that cannot scavenge did at least as well. The authors' reading is that reducing production matters more than mopping up.
Why was SS-31 developed and not SS-20?
That is a company decision, not a published comparison, so any answer is inference. What is on the record is that SS-31 went through randomised placebo-controlled trials that missed their primary endpoints and then received accelerated approval in 2025 for Barth syndrome on an uncontrolled open-label extension, while SS-20 stayed in animals.
Is SS-20 safe?
No human safety data exists. The animal studies report efficacy endpoints, not toxicology programmes, and nothing has been characterised at any human dose.