MK-4541

Overview

MK-4541 is a preclinical, orally active steroidal androgen-receptor (AR) ligand developed by Merck & Co. It is NOT a peptide, and — importantly — it is NOT a clean anabolic SARM. It appears in this peptide reference because it is discussed alongside peptides and SARMs in the "research chemical" community, not because it is a peptide itself.

Chemically, MK-4541 is a 4-azasteroid — the same structural class as finasteride and dutasteride — so it is a steroidal molecule, not a nonsteroidal SARM like ostarine or S-23. Mechanistically it is a dual/mixed AR modulator: it is anabolic (agonist) in muscle and bone but an AR antagonist (antiandrogenic) in the prostate, it is also a 5α-reductase inhibitor, and it strongly suppresses circulating testosterone. That antiandrogenic, testosterone-lowering profile is the opposite of what an anabolic "muscle-building" SARM is marketed to do.

Everything known about MK-4541 comes from in-vitro and animal studies. In cell studies it induced death of androgen-independent, AR-positive prostate-cancer cells while sparing normal cells; in castrated mice and a rat prostate xenograft it suppressed tumor growth while increasing lean body mass and muscle function. It was explored as a possible adjuvant to androgen-deprivation therapy for prostate cancer. It never reached human trials and is not an approved medicine anywhere.

Chemistry and structure

MK-4541 is a steroidal small molecule of the 4-azasteroid class — not an amino-acid chain (not a peptide), and not a nonsteroidal SARM.

The canonical SMILES was not returned by PubChem in this session and is listed as not established / not found in sources here rather than guessed.

Mechanism of action

• Dual/mixed AR modulator. MK-4541 is reported as a selective androgen receptor modulator with mixed agonistic and antagonistic effects: anabolic (agonist) in muscle and bone, but antagonist (antiandrogenic) in prostate tissue and prostate-cancer cells. This mixed profile distinguishes it from a "clean" anabolic SARM. [Animal]/[In vitro] (PMID 24565564; PMID 27106747)
• Potent AR antagonist. An AR antagonist IC50 ≈ 44 nM is reported — but this figure comes from vendor/secondary compound pages and has not been independently re-verified against the primary literature in this session. [In vitro]
• 5α-reductase inhibitor (4-azasteroid). As a 4-azasteroid (the same structural class as finasteride and dutasteride), MK-4541 is reported to inhibit 5α-reductase, with a 5α-reductase IC50 ≈ 82 nM — again a vendor/secondary-source figure, not primary-verified. [In vitro]
• Strong testosterone suppression (antigonadotropic). In intact male animals it reduced plasma testosterone concentrations and produced anti-androgen effects on seminal vesicles — i.e. it suppresses circulating androgens. [Animal] (PMID 27106747)
• Induces prostate-cancer cell death while sparing normal cells. From a screen of ~3,000 SARMs, MK-4541 was the sole compound that induced caspase-3 activity and cell death in androgen-independent, AR-positive prostate-cancer lines (22Rv1, LNCaP) without affecting AR-negative cells or AR-positive non-cancer cells. [In vitro] (PMID 24565564)
• Anti-tumor plus anabolic in vivo. It inhibited growth of R3327-G prostate tumors and reduced Ki67 expression; in castrated mice it increased lean body mass and muscle function and restored general activity — the basis for proposing it as an adjuvant to androgen-deprivation therapy. [Animal] (PMID 27106747)

Research and evidence

The evidence base is entirely preclinical (in-vitro and animal). There are no human trials of MK-4541 for any indication.

Human evidence in context. There is none. MK-4541 never entered a clinical trial, so there are no human studies establishing efficacy or safety for prostate cancer, muscle/bone, or any indication. The anti-tumor and anabolic findings rest on cell and rodent studies. Human pharmacokinetics — half-life, bioavailability, and metabolism — are not established / not found in sources, because no human data exist. Its developmental status after ~2020 has not been publicly disclosed.

Status and regulation

• Regulatory approval: None. MK-4541 is not approved by the FDA, the EMA, or any other regulator for any indication. Its highest phase of development is preclinical — it never reached human trials.
• Anti-doping: As a SARM-class anabolic agent, MK-4541 falls under the World Anti-Doping Agency (WADA) category Anabolic Agents — Other Anabolic Agents (SARMs), S1.2, prohibited at all times. WADA names only a subset of SARMs explicitly; whether MK-4541 is individually named on the list is not established here, but the class is prohibited. WADA sport-eligibility is a separate axis from legality.
• Market reality: MK-4541 is a research reagent only. Any consumer-facing "SARM" product is of unknown identity, purity, and potency (see Safety, below).

Safety

No human safety data of any kind. Because MK-4541 was never tested in humans, there is no dose, no toxicology, no long-term safety, and no drug-interaction profile. The absence of trials means adverse effects in humans are unquantified, not absent. The overall picture: MK-4541 is an antiandrogenic, testosterone-lowering steroidal research compound — not a muscle-building drug for healthy people — that sits within a drug class the FDA has flagged for liver injury, and, as an unregulated research chemical, carries the additional, separate hazards of contamination and mislabeling.

Legal status

How it compares

MK-4541 is often grouped with SARMs, but it is a fundamentally different kind of compound — steroidal and antiandrogenic — and none in this group is an approved physique/performance drug:

• Testolone (RAD-140) and Andarine (S-4) are nonsteroidal SARMs designed to be anabolic (AR agonists in muscle). MK-4541 is the near-opposite in intent: a steroidal 4-azasteroid that is an AR antagonist in the prostate, a 5α-reductase inhibitor, and a strong testosterone suppressor — it was studied against prostate cancer, not for muscle-building in healthy people.
• Cardarine (GW-501516) is frequently sold and stacked alongside SARMs, but it is not a SARM at all — it is a PPARδ agonist whose development was abandoned over a rodent cancer signal. Different target entirely.

The honest framing: MK-4541 is a preclinical, unapproved, steroidal research reagent with a dual antiandrogenic/anabolic profile and no human data — it is not a "milder, safer, or legal alternative to steroids," and it is not a clean anabolic SARM. See the muscle growth & hormone overview.

Common misconceptions

• "MK-4541 is a peptide." No. It is a steroidal small molecule — a 4-azasteroid (C22H31F3N2O3, 428.5 g/mol), the same structural class as finasteride/dutasteride. It is not a peptide. It is covered here only because it is discussed alongside peptides and SARMs.
• "It's a SARM, so it builds muscle." Not in the usual sense. MK-4541 is a dual/mixed modulator that is antiandrogenic in the prostate and strongly suppresses testosterone. Its anabolic effect on muscle was shown in castrated mice as part of a prostate-cancer strategy — it is not designed to build muscle in hormonally intact people.
• "SARMs like MK-4541 are a safe, legal alternative to steroids." No. MK-4541 is preclinical and never approved, has no human safety data, and its drug class is FDA-flagged for liver injury and testosterone suppression. It is prohibited in sport and exists only as a research reagent.
• "The IC50 numbers prove how it works in people." The reported AR antagonist IC50 (~44 nM) and 5α-reductase IC50 (~82 nM) come from vendor/secondary compound pages, are not primary-verified, and are in-vitro values — they say nothing about human effects, which have never been tested.
• "If it's labeled MK-4541, that's what's in it." Not reliably. In the JAMA 2017 analysis, only about half of products sold as SARMs contained the labeled compound; many contained a different drug or nothing active.

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This entry is educational and summarizes published research and public regulatory information. It is not medical advice, a recommendation, or a guide to obtaining or using any substance. Not legal advice — check your own jurisdiction. Last reviewed 2026-07-10.