AR antagonist · Never approved · Discontinued at Phase 2
RU58841
a hair drug that vanished mid-trial
RU58841 is a topical androgen-receptor antagonist from the early 1990s with genuinely interesting preclinical data, two real human trials, and almost nothing to show for it. The results were never published. What fills that vacuum online is equal parts miracle story and horror story, and neither is supported by the evidence.
- Real preclinical signal: a competitive AR antagonist with dose-dependent activity in the hamster flank organ and qualitative hair regrowth in bald macaques.
- Two human trials, zero published results: a Phase 1 in 2002 and a 120-man Phase 2a in 2003, both unreported to this day.
- The documented reason it stopped is commercial: an early-1990s molecule whose exclusivity was nearly gone by the mid-2000s, passed through three companies. No documented safety failure.
- The internet is the only source left: 17 published papers, none a human safety study. The scary reported side effects are forum posts, not case reports.
Evidence review of the published RU58841 literature and trial-registry records · Reviewed by the Anagen Research Team · Last reviewed 3 July 2026
What RU58841 is
A topical antiandrogen, built to stay in the skin
RU58841 (research codes PSK-3841 and HMR-3841) is a nonsteroidal androgen-receptor antagonist. Roussel Uclaf patented it in the early 1990s as a topical, locally-acting antiandrogen for androgen-driven skin conditions such as acne, hair loss and hirsutism: it competes with dihydrotestosterone at the receptor in the skin, but was engineered to break down quickly once it reaches the bloodstream so it would not cause systemic antiandrogen effects such as feminization.
In the receptor-binding work, RU58841 behaves as a competitive AR antagonist with an in-vitro IC50 near 100 nM, similar to hydroxyflutamide, and an affinity for the human androgen receptor roughly 30 times higher than older nonsteroidal antiandrogens like flutamide (Battmann 1994; Pan 1998).
The 'soft drug' design is real but was characterized in animals, not people. In rats the parent compound has an elimination half-life on the order of an hour, and only about 1 percent converts to the longer-lived active systemic metabolite RU56279 (Cousty-Berlin 1994). Whether it actually stays local after topical use in humans was never published.
The preclinical case
Real signal, in old and small models
The preclinical package is genuinely interesting, but it is old, single-lab, and small. Ranked from the least to the most human-relevant model, this is the entire published efficacy case for RU58841.
| Study | Model | What it found | Evidence tier |
|---|---|---|---|
| Pan / Uno 1998 | in-vitro androgen receptor | competitive antagonist, IC50 ~100 nM (similar to hydroxyflutamide) | in vitro |
| Battmann 1994 | hamster flank organ | dose-dependent sebaceous-organ regression with no contralateral (systemic) effect | animal |
| Cousty-Berlin 1994 | rat pharmacokinetics | ~1 h half-life; only ~1% forms the active systemic metabolite (the soft-drug basis) | animal |
| Pan / Uno 1998 | bald stumptail macaque (n = 4) | terminal-hair regrowth over 5 months, reported qualitatively with no hair-count numbers | animal |
| de Brouwer 1997 | human balding-scalp grafts (ex vivo) | faster hair-growth rate (P = 0.0387) and more recycling, but no increase in hair number or diameter | ex vivo (human tissue) |
There is no published human efficacy trial. A frequently cited 'Diani 1992 RU58841 macaque' paper does not exist: that 1992 study tested finasteride and minoxidil, not RU58841.
The honest read
Read the last row carefully. The single most human-relevant study we have improved how fast existing hairs grew, but did not grow more hairs or thicker hairs. That is the high-water mark of the published human-tissue evidence.
The human trials
Two real trials. Zero published results.
RU58841 was not an internet compound that never reached people. It went into two registered human trials, run by ProStrakan under the code PSK-3841. Here is exactly what was tested, pulled from the trial registry.
| Phase 1 | Phase 2a | |
|---|---|---|
| Year completed | 2002 | 2003 |
| Registry ID | ISRCTN49873657 | ISRCTN71083772 |
| Concentration | 5% | 2.5% and 5% |
| Frequency | twice daily | once daily |
| Duration | 4 weeks | 6 months |
| Size | up to 60 men | 120 men |
| Primary measure | hormone safety, PK | hair counts, safety |
| Published results | none | none |
Between the two trials the regimen was changed: twice-daily dropped to once-daily, and a lower 2.5% arm was added alongside 5%. Both trials list no posted results and no shared patient data.
That dosing change is one of the few things the public record does show, and it is the opposite of a program panicking about a safety signal: they lowered the dose and the frequency, and lengthened the study to actually measure hair.
The vacuum at the center of the story
Both trials finished. Neither was ever published.
We asked Kyowa Kirin, who owns the data today, for it. They declined. So every claim about what these trials found, good or bad, is filling a space where the actual human data should be.
Why it stopped
The paper trail points at money, not safety
If a drug grows hair and is safe, the folk wisdom says it sails through trials, so a discontinued drug must have been dangerous. That intuition is wrong, and RU58841 is the cautionary tale.
The documented reason RU58841 was dropped is commercial, not clinical. Its core patent was filed in the early 1990s (US 5,411,981; French priority 1992), so by the mid-2000s little exclusivity remained, and it changed corporate hands three times in a decade: Roussel Uclaf to Hoechst Marion Roussel to ProSkelia to ProStrakan, then to Kyowa Kirin when it acquired ProStrakan in 2011. ProStrakan's own materials described the Phase 2a as 'not powered for efficacy' (a 2005 analyst note).
You do not spend the money to finish and publish trials on a drug you cannot own. A non-core topical dermatology asset with spent IP is exactly the kind of program that quietly gets shelved inside a specialty-pharma portfolio, regardless of whether it works.
The 'works about as well as finasteride' line that circulates online traces to that same 2005 analyst note paraphrasing the company, not to any peer-reviewed data. And the 'failed on safety' story has no primary source at all: no press release, no registry field, and no publication documents a safety signal. The adverse-event record was simply never posted, which is an absence of data, not evidence of harm.
The honest read
Safe, working drugs die all the time. Not because they hurt people, because there is no money in them.
RU58841 might be one of those. Or it might have been quietly dangerous. The maddening truth is the same either way: the only people who know are the ones who will not tell you.
The internet vs the evidence
A loud reputation on a quiet evidence base
Because the trial data is locked away, the internet became the record. It tells two stories, a miracle and a horror story, and the evidence supports neither. This is the asymmetry at the heart of RU58841: a very loud reputation built on a very quiet evidence base.
Reported online vs published in humans
What the internet reports
Reported gains
- “works as well as finasteride”
- endorsements from prominent hair-loss and men's-health influencers
- thicker hair and regrown hairlines in personal logs
Reported side effects
- heart and chest problems
- gynecomastia
- pulmonary fibrosis
- brain fog and low libido
What is published in humans
The one human-tissue study, de Brouwer 1997: hair grew faster, but there were no more hairs and no thicker hairs.
| Reported side effect | Where the claim comes from | Published human evidence |
|---|---|---|
| Heart / chest problems | forum anecdotes | none; the reports are self-described forum accounts, not documented case reports |
| Gynecomastia | forum anecdotes | none |
| Low libido / ED | forum anecdotes | none |
| Brain fog / mood | forum anecdotes | none |
| Pulmonary fibrosis | borrowed from the warning label of a different antiandrogen, nilutamide | none from any RU58841 case |
What you can actually buy today
An unregulated research chemical
There is one safety concern about RU58841 that does not require any trial data to make, and it is the most concrete of all.
RU58841 is not approved by the FDA or any regulator, and it is not a compounded prescription product. It is sold purely as a not-for-human-use research chemical. That means no pharmaceutical quality control, no chain of custody, and no medical monitoring.
Vendors may publish a certificate of analysis, but those are vendor-controlled and cannot be verified by the buyer, and no independent published analytical audit of market RU58841 exists. So a person using it is self-dosing an unapproved compound of unknown true purity and dose, on the basis of trial results no one has ever seen.
What you are actually buying
An unapproved research chemical, made with no medical oversight, on trial data no one has ever seen.
This page is educational and is not medical advice or a recommendation to use RU58841.
Frequently asked questions
RU58841, answered straight
Does RU58841 actually regrow hair?
There is no published human trial showing RU58841 regrows hair. The strongest human-relevant evidence is a 1997 ex-vivo study on balding-scalp grafts, which found hair grew faster but did not increase the number of hairs or their thickness (de Brouwer 1997). Macaque data from 1998 was qualitative. Efficacy in people has never been demonstrated in a published, controlled trial.
Why did RU58841 fail clinical trials?
It did not clearly fail; its trials were simply never reported. ProStrakan ran a Phase 1 in 2002 and a 120-man Phase 2a in 2003, and no results were ever published. The documented reason for stopping is commercial: an early-1990s molecule whose patent was nearly spent, that changed corporate hands three times, not a disclosed safety or efficacy failure.
Did RU58841 fail because it was unsafe?
There is no public evidence that RU58841 failed on safety. The trial adverse-event records were never posted, and no press release or publication documents a safety signal. The 'failed on safety' story circulates on forums but has no primary source, and an absence of posted data is not evidence of harm.
Is RU58841 safe to use?
Nobody knows from published human data. There are 17 papers on RU58841 and none is a human safety study. The internet-reported side effects such as heart problems, gynecomastia and pulmonary fibrosis trace to forum posts, not case reports, and the pulmonary-fibrosis claim was borrowed from a different drug's label (nilutamide).
Is RU58841 FDA-approved or available by prescription?
No. RU58841 is not approved by the FDA or any regulator, and it is not a compounded prescription product. It is sold only as a not-for-human-use research chemical with no pharmaceutical quality control, so its purity and dose cannot be independently verified.
Is RU58841 better than finasteride?
There is no head-to-head trial. The often-repeated 'works as well as finasteride' line traces to a single 2005 analyst note, not a published study. Finasteride and dutasteride have large controlled trials behind them; RU58841 has none in humans.
Limitations
What this page can and cannot support
RU58841 has almost no published human evidence. These constraints bound every claim above.
No published human efficacy trial exists. Every efficacy claim about RU58841 rests on animal or ex-vivo tissue data, or on anecdote.
The best human-tissue study measured growth rate, not hair count. de Brouwer 1997 found faster growth but no increase in hair number or diameter.
The macaque data is qualitative. Pan and Uno 1998 reported folliculogram patterns in four animals with no hair-count numbers.
The Phase 1 and Phase 2a results were never released. Any statement about what those trials actually found, including this page's, is bounded by that absence.
“Stays local” is a rodent property, not a human measurement. The soft-drug design was characterized in hamster and rat; systemic exposure after topical use in people was never published.
Market RU58841 is unregulated. No independent analytical audit of vendor material exists, so real-world purity and dose are unknown.
Sources & citations
Every claim, traced to its source
The published RU58841 literature plus the trial-registry records behind the timeline and the two trials above.
- 1Battmann T, et al. RU 58841, a new specific topical antiandrogen: a candidate of choice for the treatment of acne, androgenetic alopecia and hirsutism. PMID 8136306doi:10.1016/0960-0760(94)90250-x
- 2Cousty-Berlin D, et al. Preliminary pharmacokinetics and metabolism of novel non-steroidal antiandrogens in the rat: relation of their systemic activity to the formation of a common metabolite. PMID 7947350doi:10.1016/0960-0760(94)90114-7
- 3Pan HJ, Wilding G, Uno H, et al. Evaluation of RU58841 as an anti-androgen in prostate PC3 cells and a topical anti-alopecia agent in the bald scalp of stumptailed macaques. PMID 9798729doi:10.1385/endo:9:1:39
- 4de Brouwer B, Tetelin C, Leroy T, Bonfils A, Van Neste D A controlled study of the effects of RU58841, a non-steroidal antiandrogen, on human hair production by balding scalp grafts maintained on testosterone-conditioned nude mice. PMID 9415227doi:10.1046/j.1365-2133.1997.19382053.x
- 5Teutsch G, et al. Non-steroidal antiandrogens: synthesis and biological profile of high-affinity ligands for the androgen receptor. PMID 8136296doi:10.1016/0960-0760(94)90257-7
- 6Roussel Uclaf SA Phenylimidazolidines having antiandrogenic activity (US Patent 5,411,981; discloses RU58841). Filed 18 May 1993, granted 2 May 1995; French priority FR 2,693,461, 8 July 1992. https://patents.google.com/patent/US5411981A/en
- 7ProStrakan Pharmaceuticals PSK-3841 (RU58841) topical antiandrogen, Phase 1 in male pattern hair loss. https://www.isrctn.com/ISRCTN49873657
- 8ProStrakan Pharmaceuticals PSK-3841 (RU58841) topical antiandrogen, Phase 2a in male pattern hair loss. https://www.isrctn.com/ISRCTN71083772
- 9National Center for Advancing Translational Sciences (NCATS) RU-58841: development discontinued, never approved (Inxight Drugs database). https://drugs.ncats.io/
If you want an antiandrogen approach with data
Treatments that have finished their trials
RU58841 targets the androgen receptor. These options work on the same DHT pathway and, unlike RU58841, have published human results. All are prescription medicines used off-label for hair loss and are not FDA-approved for that use. A licensed clinician decides whether any of them is appropriate for you, and individual results vary.
