YK-11: Steroidal SARM, Androgen Receptor Partial Agonism, Follistatin-Mediated Myostatin Inhibition, and Dual Anabolic-Osteogenic Activity in Research Models

YK-11: Steroidal SARM, Androgen Receptor Partial Agonism, Follistatin-Mediated Myostatin Inhibition, and Dual Anabolic-Osteogenic Activity in Research Models – research illustration Abstract & Overview YK-11 — also designated Myostine and formally named (17α,20E)-17,20-[(1-methoxyethylidene)bis(oxy)]-3-oxo-19-norpregna-4,20-diene-21-carboxylic acid methyl ester — is a synthetic steroidal selective androgen receptor modulator (SARM) first characterised by Kanno et al. at Toho University, Japan, in 2011 [1]. Unlike the majority of SARMs, which are non-steroidal small molecules, YK-11 is built upon a modified 19-norpregnane steroid scaffold, placing it in a structurally distinct subclass of androgen receptor (AR) modulators. Its molecular formula is C₂₅H₃₄O₆ with a molar mass of 430.54 g/mol (CAS: 1370003-76-1; PubChem CID: 119058028). YK-11 operates through a dual pharmacological mechanism that distinguishes it from both classical anabolic steroids and conventional non-steroidal SARMs. First, it functions as a gene-selective partial agonist of the androgen receptor, binding the receptor’s ligand-binding domain without inducing the N-terminal/C-terminal (N/C) interaction required for full AR transactivation, thereby activating a distinct subset of androgen-responsive genes [1][2]. Second, and uniquely, YK-11 induces the expression of follistatin (FST) in skeletal muscle cells — an effect not observed with dihydrotestosterone (DHT) — which in turn neutralises myostatin (GDF-8), the primary endogenous inhibitor of skeletal muscle mass [2]. This dual mechanism positions YK-11 as both a SARM and a functional myostatin inhibitor. “YK11 is a selective androgen receptor modulator (SARM), which activates AR without the N/C interaction… YK11 treatment of C2C12 cells, but not DHT, induced the expression of follistatin (Fst), and the YK11-mediated myogenic differentiation was reversed by anti-Fst antibody. These results suggest that the induction of Fst is important for the anabolic effect of YK11.” — Kanno Y et al., Biol Pharm Bull (2013) [2]. Preclinical research has further demonstrated that YK-11 promotes osteoblastic proliferation and differentiation via Akt signalling [3], attenuates sepsis-induced muscle wasting and reduces mortality in animal models through suppression of the TLR4/NF-κB/TGF-β inflammatory cascade [4], and has been investigated for its effects on hippocampal function and oxidative stress [5][6]. YK-11 has not received regulatory approval for human use and is classified as a designer drug and research compound. Molecular Identity and Structural Architecture YK-11 is built upon a 19-norpregnane steroidal backbone — the same core scaffold found in progestins such as norethisterone — with several key structural modifications that confer its unique pharmacological profile. The most distinctive feature is the 17α,20-ketal group: a (1-methoxyethylidene)bis(oxy) moiety bridging positions 17 and 20 of the steroid nucleus. This ketal group is the primary determinant of YK-11’s selective AR binding profile, as it sterically prevents the receptor from adopting the conformation required for the N/C interaction. The molecule also bears a 3-oxo-4-ene configuration (a conjugated enone in ring A, common to androgenic steroids) and a methyl ester at C-21, which contributes to oral bioavailability and metabolic stability [1]. The steroidal nature of YK-11 is pharmacologically significant. Most SARMs in research use — including RAD-140, LGD-4033, and Ostarine — are non-steroidal compounds that bind the AR through entirely different chemical scaffolds. YK-11’s steroid scaffold allows it to interact with the AR in a manner that more closely resembles natural androgens, yet the 17α,20-ketal modification fundamentally alters the receptor’s conformational response, producing a gene-selective activation pattern distinct from both testosterone and DHT. The compound is orally bioavailable with an estimated half-life of approximately 6 to 8 hours, necessitating multiple daily administrations in research protocols [1][2]. Mechanistic Rationale: Dual Pathway Anabolic Activity Androgen Receptor Partial Agonism and Gene-Selective Activation Upon cellular uptake, YK-11 binds to the ligand-binding domain (LBD/AF2) of the androgen receptor with high affinity. In contrast to full AR agonists such as DHT and testosterone, YK-11 binding does not induce the physical interaction between the receptor’s N-terminal activation function 1 (NTD/AF1) and its ligand-binding domain activation function 2 (LBD/AF2) — a conformational event known as the N/C interaction that is required for maximal AR transactivation. The absence of this interaction means that YK-11 activates only a subset of androgen-responsive genes, producing a tissue-selective anabolic profile that theoretically spares androgenic side effects associated with full AR agonism [1]. Despite being a partial agonist, YK-11 demonstrated greater anabolic potency than DHT in C2C12 murine

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