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Andarine (code name S4, chemical designations GTx-007, S-40503) is a synthetic nonsteroidal selective androgen receptor modulator (SARM). Thanks to stable physicochemical properties, high bioavailability and precise tissue targeting, andarine(S4) solution formulation has become a core reagent for biomedical research and mechanistic investigations into musculoskeletal disorders. This compound is characterised by tissue-selective activation of the androgen receptor (AR): it exerts potent agonistic effects on AR in skeletal muscle and bone tissues, yet acts only as a weak partial agonist on AR in reproductive tissues including the prostate and mammary glands. Accordingly, it fundamentally avoids adverse effects such as reproductive tissue hyperplasia and hormonal disturbance commonly induced by conventional steroidal androgens.
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Andarine COA
Subclassification of Downstream Signalling Pathways (Cell Biological Perspective)
The biological functions of andarine(S4) solution are fully mediated by AR-dependent downstream signalling cascades. Unlike systemic non-selective activation triggered by traditional androgens, its key advantage lies in eliciting tissue-specific effects of anabolism promotion, degeneration suppression and low off-target toxicity via differential regulation of multiple intracellular signalling cascades in target cells. From a cell biological viewpoint, upon entering the organism, S4 penetrates the phospholipid bilayer of cell membranes and specifically binds to ligand-free androgen receptors in the cytoplasm.
This binding triggers conformational changes, receptor dimerisation and nuclear translocation, subsequently initiating two categories of downstream signalling: transcription-dependent and transcription-independent pathways, which separately govern cellular gene expression, proliferation and differentiation, energy metabolism and apoptosis. Marked disparities exist in the activation intensity of signalling pathways and expression levels of downstream targets across distinct target cells (skeletal muscle cells, osteoblasts, adipocytes, germ cells), ultimately generating its unique tissue-selective biological activity. The downstream signalling regulatory mechanisms are elaborated below from three dimensions: core pathways, auxiliary regulatory pathways and cell-type-specific pathways.
Core AR Transcription-Dependent Signalling Pathway (Primary Effector Pathway)
The AR transcription-dependent pathway serves as the primary cascade through which Andarine (S4) exerts anabolic actions. Widely distributed in target cells such as skeletal muscle cells and osteoblasts, it governs cell-specific gene transcription and functional remodelling, forming the fundamental molecular basis for muscle growth and bone metabolic remodelling. In the inactive state, cytoplasmic AR binds molecular chaperones including heat shock proteins HSP90 and HSP70 to maintain a stable folded conformation and remains quiescent.
After S4 solution enters cells, it competitively binds the AR ligand-binding domain with high affinity (Ki ≈ 7.5 nM), displacing chaperone proteins and inducing precise conformational rearrangement of AR to expose nuclear localisation signals. Two activated AR monomers then undergo homodimerisation to form a stable S4–AR dimeric complex, which rapidly translocates into the nucleus.
Nuclear activated AR dimers specifically recognise androgen response elements (AREs) within the promoter regions of target genes. Following binding, they recruit complexes comprising histone acetyltransferases and transcriptional coactivators to remodel chromatin accessibility and initiate specific transcription of downstream target genes.
In skeletal muscle cells, this pathway preferentially activates transcription of myogenic regulatory factor family genes, including core regulators MyoD, myogenin and MEF2C. MyoD drives proliferation and activation of skeletal muscle satellite cells to exit quiescence, facilitating satellite cell multiplication and fusion into muscle fibres to realise muscle fibre hyperplasia. Myogenin orchestrates terminal differentiation of myocytes, modulates synthesis of structural proteins such as actin and myosin, enlarges cross-sectional areas of muscle fibres and inhibits muscular atrophy.
Meanwhile, the pathway downregulates transcriptional expression of atrophy-related genes (MuRF1, Atrogin-1), reduces ubiquitin-mediated degradation of muscle proteins and sustains dynamic equilibrium between muscle protein synthesis and breakdown.
Within osteoblasts, the S4-mediated AR transcriptional pathway specifically upregulates expression of master osteogenic transcription factors Runx2 and Osterix. As a central regulator of osteogenic differentiation, Runx2 directs lineage commitment of osteoprogenitor cells and promotes synthesis and deposition of bone matrix proteins such as type I collagen, osteocalcin and osteonectin.
Osterix synergistically controls osteoblast maturation and mineralisation, elevating trabecular bone density and cortical bone thickness. In parallel, the pathway indirectly suppresses secretion of receptor activator of nuclear factor κB ligand (RANKL), attenuates proliferative activity of osteoclasts and reduces bone resorption, ultimately establishing a positive bone metabolic balance and ameliorating pathological states including osteoporosis and bone loss.Compared with full agonists such as testosterone, S4 exhibits prominent tissue selectivity when activating this pathway.
In prostate epithelial cells, AR dimers fail to efficiently recruit transcriptional activation complexes and only induce weak transcriptional responses, without provoking aberrant proliferation of prostate cells. This property circumvents reproductive adverse effects of conventional androgens at the molecular level.
Transcription-Independent Rapid Signalling Pathway (Auxiliary Regulatory Pathway)
Beyond the canonical AR nuclear transcription cascade, andarine(S4) solution initiates rapid transcription-independent cellular responses via membrane and cytoplasmic signalling axes.
This pathway operates independently of gene transcription and protein synthesis; it can be activated within minutes, primarily regulating cellular energy homeostasis, calcium ion balance and kinase cascades to augment anabolic effects, acting as an indispensable supplement to S4 biological activity.Its activation mechanism is as follows: a small fraction of S4 directly binds membrane androgen receptors (mARs) on the cell surface.
No nuclear translocation is required, and multiple intracellular kinase signalling cascades are rapidly triggered.
The most prominent cascade is the PI3K/Akt/mTOR signalling axis, a pivotal pathway governing cellular protein synthesis, proliferation and metabolism. Upon mAR activation by S4, phosphatidylinositol 3-kinase (PI3K) is rapidly activated, catalysing phosphorylation of membrane-bound PIP2 into PIP3. PIP3 recruits cytoplasmic Akt kinases and facilitates their phosphorylation and activation. On one hand, activated Akt directly phosphorylates the downstream mTORC1 complex to stimulate mTOR signalling, further enhancing activities of ribosomal S6 protein kinase (S6K1) and eukaryotic translation initiation factor 4E (eIF4E). This accelerates translational synthesis of proteins in skeletal muscle cells and osteoblasts and rapidly elevates cellular anabolic efficiency.
On the other hand, phosphorylated Akt inhibits the TSC1/TSC2 complex, relieving its negative modulation of mTOR and sustaining persistent pathway activation. Additionally, Akt suppresses activity of FoxO transcription factors, preventing FoxO nuclear translocation and subsequent activation of atrophy-associated genes, further diminishing muscle protein breakdown and bidirectionally amplifying muscle anabolism.
Cell-Type-Specific Disparities in Downstream Signalling Regulation (Core Mechanism Underpinning Targeting)
The tissue-selective pharmacological activity of andarine(S4) solution essentially originates from differential activation of downstream signalling cascades driven by variable AR expression abundance and auxiliary regulatory factors across distinct cell types. This constitutes its defining cell biological feature that distinguishes it from conventional steroidal hormones. Signalling regulatory disparities in three categories of target cells are detailed below.
Skeletal muscle cells and osteoblasts (highly responsive anabolic target cells)These cells express high levels of AR and are enriched with cell-specific transcriptional cofactors including MyoD and Runx2. Upon AR binding, S4 efficiently triggers both transcriptional and non-transcriptional pathways. The transcriptional cascade sustains long-term expression of structural protein and differentiation factor genes to drive cell proliferation, differentiation and tissue remodelling. The non-transcriptional pathway rapidly initiates mTOR-mediated protein synthesis to maintain real-time positive metabolic balance. Collectively, pronounced muscle hyperplasia and elevated bone density are achieved without cytotoxicity or risk of excessive cell proliferation.
Prostate and mammary epithelial cells (low-responsive inert target cells)Although AR is expressed in these cells, intracellular transcriptional coactivators exist at very low levels alongside abundant transcriptional repressors. The complex formed by S4-bound AR cannot effectively associate with ARE sites on target genes and barely initiates transcription of proliferation-related genes. Furthermore, activation of non-transcriptional kinase cascades remains negligible and fails to induce abnormal cell proliferation. Unlike testosterone, which robustly activates proliferative pathways and causes tissue hyperplasia in such cells, the weak activation of reproductive cell signalling pathways by S4 enables complete avoidance of typical adverse effects induced by androgenic pharmaceuticals.
Adipocytes and immune cells (regulatory target cells)In adipocytes, S4 downregulates expression of lipogenic genes (FAS, ACC) via AR-mediated downstream signalling, activates lipolytic cascades and suppresses adipocyte proliferation and lipid accumulation to optimise body composition. In immune cells, it moderately inhibits the NF-κB inflammatory signalling pathway, lowers release of pro-inflammatory cytokines and alleviates the chronic inflammatory microenvironment within muscle and bone tissues, generating favourable conditions for tissue repair and anabolism.
The development of Andarine (S4) occurred during the rapid advancement of selective androgen receptor modulator technology in the 1990s. It was jointly developed by US biopharmaceutical firm GTx, Inc. and a research team at the University of Tennessee, ranking among the world's first nonsteroidal SARM compounds capable of tissue-selective modulation.Steroidal androgens widely adopted in clinical practice at that time exerted potent anabolic effects yet lacked tissue selectivity.
Long-term administration frequently induced severe side effects including benign prostatic hyperplasia, endocrine disruption and hepatic injury, limiting clinical applicability. To tackle this industry challenge, research teams focused on structural variations of the androgen receptor and strived to develop nonsteroidal compounds capable of differentially activating AR across various tissues.
In 1998, researchers performed high-throughput compound screening on an arylpropionamide compound library and identified the high-affinity AR lead compound S-40503, later known as Andarine. In the subsequent two years, structural optimisation, in vitro cellular assays and validation in animal models confirmed its partial agonist properties and tissue-selective pharmacological effects.
The compound was formally designated GTx-007 (generic name Andarine, abbreviated S4).After 2000, the compound entered preclinical research. Extensive animal experiments verified its capacity to efficiently ameliorate muscular atrophy and increase bone mineral density without significant prostate stimulation. Clinical development of S4 was later suspended due to higher priority assigned to research on the analogue ostarine.
Nevertheless, thanks to its excellent targeted regulatory characteristics, it has become a core reagent for investigating androgen signalling pathways within life science research. Its solution formulation has also gained widespread popularity owing to superior chemical stability.
FAQ
1. What is the impact of solvent selection on the long-term stability of the solution?
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The impact is significant. Although S4 can be dissolved in DMSO and ethanol, the differences in dielectric constant and redox potential among different solvent systems can lead to different degradation pathways for it. For example, in DMSO, the S4 solution must be strictly protected from oxygen exposure; otherwise, DMSO may slowly oxidize to dimethyl sulfoxide during long-term storage, changing the polarity environment of the solution and thereby inducing the isomerization of S4. In the alcohol-based system, attention should be paid to the risk of ester exchange reaction, and it is usually recommended to use degassed solvents and store them under nitrogen atmosphere.
2. Does the material of the packaging container have any adsorption effect on S4?
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There is a container adsorption effect, especially when the concentration of S4 is low. The aromatic rings and hydrophobic groups in the molecular structure of S4 may cause it to form non-specific binding with certain plastic materials (such as untreated polypropylene or polyethylene). Although glass containers adsorb less, it is necessary to be aware that the free silanol groups may have hydrogen bonding effects on S4. It is recommended to use inner-lined tubes that have been silanized or specific low adsorption consumables for packaging and storage.
3. Is the generation of photodegradation products wavelength-dependent?
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It is highly specific. The degradation of S4 is not only affected by the intensity of light, but also related to specific wavelengths. Studies have shown that ultraviolet light (especially the UV-B band) can induce [2+2] cycloaddition reactions in S4, mainly generating dimeric impurities; while visible light (especially the blue light region) is more inclined to catalyze its optical isomerization. Therefore, storing the S4 solution requires not only avoiding light, but also it is recommended to use brown containers that can block specific wavelengths (such as borosilicate glass with UV-cut properties).
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