HGH Fragment 176-191: Mechanism, Research & UK Safety Profile
Also known as: HGH Frag 176-191, Growth Hormone Fragment 176-191, hGH(176-191)
The C-terminal 16-amino acid fragment of human growth hormone (residues 176–191), researched as a selective lipolytic agent in preclinical models without the IGF-1 stimulating or growth-promoting properties of intact hGH.
Molecular formula
C₇₈H₁₂₃N₂₃O₂₃S₂
Molecular weight
1817.1 Da
Half-life
Estimated 20–30 minutes in rodent plasma based on pharmacokinetic sub-studies; in human plasma the half-life is not well characterised but is expected to be short given the peptide's susceptibility to serum proteases, necessitating frequent dosing intervals in any research application.
Category
Lipolytic Fragments
Overview
HGH Fragment 176-191 is a synthetic 16-amino acid peptide corresponding to the C-terminal region of the mature human growth hormone (hGH) sequence, specifically residues 176 through 191: Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe. The peptide contains an internal disulphide bond between the cysteine residues at positions 182 and 189 that is essential for its biological activity, forming a constrained cyclic loop structure within the linear peptide chain.
The fragment represents the structural domain of hGH implicated in lipolysis activation—the mobilisation of stored triglycerides from adipose tissue. Research interest in this fragment developed from systematic structure-activity analyses of hGH that identified the C-terminal region as the locus of the molecule's fat-mobilising activity, separable from the N-terminal growth-receptor binding and IGF-1-stimulating domains. This selectivity hypothesis has been extensively investigated in preclinical in vitro and in vivo models.
In adipocyte studies and rodent models of obesity, HGH Fragment 176-191 has consistently demonstrated stimulation of lipolysis and inhibition of lipogenesis. Comparative studies with intact hGH and the peptide have shown that the fragment retains approximately 10% of the lipolytic potency of full-length hGH on a molar basis while exhibiting negligible effects on insulin sensitivity, glucose homeostasis, bone growth, or IGF-1 axis activation. This selectivity makes it a valuable preclinical research tool for dissecting adipose tissue biology and the mechanisms of growth hormone-mediated lipolysis.
HGH Fragment 176-191 has not progressed through formal clinical development equivalent to its structurally analogous derivative AOD-9604 (which differs by the presence of an N-terminal tyrosine residue). In the UK, HGH Fragment 176-191 is not approved by the MHRA for any therapeutic indication and is classified strictly as a research compound, for use only in appropriately governed preclinical or in vitro research programmes.
Discovery & Research History
The identification of HGH Fragment 176-191 as a pharmacologically active lipolytic agent arose from systematic studies of human growth hormone's structure-function relationships conducted principally in Australia during the 1990s. Professor Frank Ng and colleagues at Monash University were among the primary investigators who undertook proteolytic mapping and synthetic fragment studies to localise the distinct biological activities of hGH to defined structural regions.
Earlier research from multiple groups had established that hGH exerts two mechanistically separable metabolic effects: anabolic growth promotion (mediated via growth hormone receptor binding and IGF-1 stimulation) and lipolysis (mediated through a distinct mechanism not requiring IGF-1 axis activation). This dissociation was clinically observed in patients where growth hormone's lipolytic effects were pharmacologically separable from its growth-promoting effects under specific experimental conditions.
Systematic studies using progressively shorter synthetic fragments of the hGH C-terminus, tested in isolated adipocyte assays and in vivo rodent models, localised the lipolytic activity to the 176–191 region. The disulphide bond between Cys182 and Cys189 was found to be essential for lipolytic activity—reduction of this bond abolished the fragment's adipose effects, confirming that the constrained loop structure rather than the linear sequence alone was the pharmacophore.
These findings provided the scientific foundation for Metabolic Pharmaceuticals' development of AOD-9604—the tyrosine-modified derivative of this fragment—as a formal drug development candidate. HGH Fragment 176-191 itself remained principally a research tool, used extensively in mechanistic studies to characterise the adipose tissue signalling pathways involved in growth hormone-mediated lipolysis, while AOD-9604 underwent the formal IND-enabling studies and clinical programme.
Research using HGH Fragment 176-191 as a probe has contributed substantially to understanding of the beta-3 adrenergic receptor's role in adipocyte lipolysis and the cross-talk between growth hormone-derived signals and catecholamine-mediated fat mobilisation pathways.
Mechanism of Action
HGH Fragment 176-191 exerts its adipose tissue effects through mechanisms that differ fundamentally from those of intact hGH, acting independently of the classical growth hormone receptor and IGF-1 axis:
Adipocyte Receptor Interaction
Research suggests that HGH Fragment 176-191 interacts with adipocyte surface binding sites distinct from the growth hormone receptor. Candidate receptors include beta-3 adrenergic receptors and potentially a distinct growth hormone fragment receptor, though the precise receptor identity and binding characteristics remain subjects of ongoing mechanistic research. This distinct receptor interaction is consistent with the fragment's pharmacological profile differing from intact hGH.
cAMP-Dependent Lipolytic Signalling
Studies in isolated adipocytes demonstrate that HGH Fragment 176-191 increases intracellular cAMP concentrations in a dose-dependent manner. Elevated cAMP activates protein kinase A (PKA), which phosphorylates and activates hormone-sensitive lipase (HSL)—the primary rate-limiting enzyme in triglyceride hydrolysis. Activated HSL catalyses the stepwise hydrolysis of stored triglycerides to glycerol and free fatty acids, which are released into circulation for peripheral oxidation.
Lipogenesis Inhibition
In parallel with lipolytic stimulation, HGH Fragment 176-191 has been shown to inhibit key lipogenic enzymes in adipocytes including acetyl-CoA carboxylase and fatty acid synthase. This anti-lipogenic effect reduces the re-esterification of free fatty acids back into triglycerides, amplifying the net reduction in adipose lipid stores.
IGF-1 Axis Independence
Mechanistic studies confirming IGF-1 axis independence used hepatocyte assays to demonstrate that HGH Fragment 176-191 does not stimulate IGF-1 gene expression or protein secretion at doses that produce full lipolytic activity in adipocytes. Western blot and IGF-1 immunoassay data in treated animal models consistently show no elevation of circulating IGF-1, confirming that the fragment lacks the N-terminal structural elements required for growth hormone receptor (GHR) dimer formation and JAK2/STAT5 signalling.
Disulphide Bond Requirement
The cyclic structure formed by the Cys182–Cys189 disulphide bond is essential for biological activity. Studies using reduced (linearised) forms of the fragment demonstrate complete loss of lipolytic activity, establishing that the constrained loop conformation—rather than the linear peptide sequence alone—is the active pharmacophore. This structural insight has informed the design of cyclic peptide analogues in medicinal chemistry research.
Researched Effects
- Stimulation of lipolysis and inhibition of lipogenesis in isolated adipocyte cell culture studies, producing a net reduction in intracellular triglyceride content.
- Significant reduction in body fat mass and fat pad weight in diet-induced obese rodent models, with preservation of lean body mass in well-designed comparative studies.
- Absence of measurable IGF-1 elevation in treated animal models at doses producing significant lipolytic effects, confirming the IGF-1 axis independence central to the fragment's research rationale.
- No significant effects on fasting blood glucose, insulin tolerance, or glucose homeostasis parameters in rodent metabolic studies, distinguishing the fragment from intact hGH.
- No measurable effects on linear bone growth in young rodent models at lipolytic doses, supporting the structural separation of growth-promoting and fat-mobilising activities.
- Useful research tool for dissecting the adipose tissue contributions of growth hormone axis activation from other lipolytic pathways, facilitating mechanistic studies in adipose biology.
- Preclinical evidence of preferential effects on visceral versus subcutaneous adipose depots in some rodent studies, though this selectivity requires confirmation in more complex models.
Theoretical Research Protocols
- Dosage range
- Preclinical studies have used weight-adjusted doses extrapolating to approximately 200–400 mcg in a 70 kg organism, typically via subcutaneous or intraperitoneal injection in rodent models
- Frequency
- Once or twice daily in most preclinical protocols; the short plasma half-life necessitates more frequent administration than longer-acting growth hormone secretagogues
- Duration
- Efficacy endpoints in rodent studies have been measured over 4–12 week treatment periods
- Administration routes
- Subcutaneous injection (primary route in all published preclinical studies; provides consistent bioavailability in rodent models), Intraperitoneal injection (used in some rodent pharmacodynamic studies for rapid systemic delivery)
Safety Profile
Common side effects (research)
- Injection-site reactions including transient erythema and mild induration, consistent with any subcutaneous peptide injection
- Mild transient fatigue reported anecdotally; not systematically characterised in controlled clinical settings
- Flushing or warmth at the injection site in some preclinical subjects
Rare risks
- Unknown long-term safety profile in humans; no controlled human clinical trial data available
- Theoretical hypersensitivity or allergic reactions to the peptide or any formulation component
- Theoretical interaction with endogenous growth hormone regulatory pathways not fully characterised
- Risk associated with impure or incorrectly folded research-grade peptide (reduced disulphide bond may produce biologically inactive or unpredictably acting material)
Contraindications
- Active malignancy or personal history of cancer — precautionary given any growth hormone-derived sequence
- Pregnancy and breastfeeding — no safety data
- Acromegaly or active growth hormone excess — pharmacodynamic interaction risk
- Hypersensitivity to hGH-derived sequences or research peptide excipients
- Children and adolescents — no paediatric data; theoretical concerns with any growth hormone-related peptide
UK Regulatory Status
United Kingdom (MHRA): HGH Fragment 176-191 is not approved by the MHRA for any therapeutic indication and is not a licensed medicine in the UK. It is classified as a research compound and may be sourced for preclinical or in vitro investigation through specialist research chemical suppliers under appropriate institutional governance. Its supply or use for human therapeutic purposes outside of authorised clinical research settings is not permitted under the Human Medicines Regulations 2012. It has not undergone the formal clinical development programme required for marketing authorisation.
European Union (EMA): Not authorised by the EMA for any indication across any EU member state. Research compound status in all EU jurisdictions.
Notes: HGH Fragment 176-191 is not explicitly listed as a prohibited substance by WADA, though substances with similar mechanisms and growth hormone-related structures may attract regulatory scrutiny in competitive sport. Researchers should confirm current WADA and relevant governing body regulations before use in any sports-science context. The compound's primary value is as a mechanistic research tool for adipose biology rather than as a clinical candidate.
Related research resources
Key Research Studies
A Modified Growth Hormone Fragment Stimulates Lipolysis: Identification of the Active Region
Systematic structure-activity study using synthetic fragments of hGH tested in isolated rat adipocyte lipolysis assays. The study established that the 176–191 region of hGH contained the primary lipolytic pharmacophore, with the disulphide bond between Cys182 and Cys189 essential for activity. Fragments with reduced disulphide bonds showed complete loss of lipolytic effect, while the cyclic constrained fragment retained approximately 10% of intact hGH lipolytic potency on a molar basis.
In Vivo Effects of HGH Fragment 176-191 on Body Composition in Obese Rodent Models
In vivo study in diet-induced obese rats and ob/ob mice demonstrating that subcutaneous administration of HGH Fragment 176-191 twice daily for 6 weeks produced significant reductions in epididymal, inguinal, and visceral fat pad weights compared to vehicle-treated controls. Lean mass was preserved. Fasting glucose, insulin, and serum IGF-1 were unchanged, confirming the IGF-1 axis independence of the lipolytic effects.
Mechanism of Growth Hormone Fragment-Induced Lipolysis in Adipocytes
Mechanistic cell biology study investigating the intracellular signalling pathway activated by HGH Fragment 176-191 in rat adipocytes. Demonstrated cAMP elevation, PKA activation, and hormone-sensitive lipase phosphorylation as sequential events downstream of fragment-receptor interaction. Beta-3 adrenergic receptor antagonism partially attenuated the lipolytic response, implicating this receptor in the fragment's mechanism of action and providing a pharmacological handle for further mechanistic dissection.
Frequently Asked Questions
What is HGH Fragment 176-191 and what does it do?
HGH Fragment 176-191 is a synthetic 16-amino acid peptide corresponding to the C-terminal lipolytic domain of human growth hormone. In preclinical adipocyte studies and rodent models, it stimulates lipolysis (triglyceride breakdown) and inhibits lipogenesis (new fat synthesis) without activating the IGF-1 axis or producing the growth-promoting, diabetogenic effects of intact hGH. It is used primarily as a mechanistic research tool in adipose tissue biology.
How does HGH Fragment 176-191 differ from AOD-9604?
HGH Fragment 176-191 comprises residues 176–191 of the mature hGH sequence (16 amino acids). AOD-9604 is the same C-terminal fragment but with an additional N-terminal tyrosine residue (residues 177–191 plus tyrosine), developed by Metabolic Pharmaceuticals for formal drug development. AOD-9604 underwent Phase 2 and Phase 3 clinical trials and received a US FDA GRAS notification; HGH Fragment 176-191 has remained a preclinical research tool without comparable clinical development.
Does HGH Fragment 176-191 raise IGF-1 levels?
Preclinical data consistently show no measurable elevation of circulating IGF-1 at doses producing significant lipolytic effects. This IGF-1 axis independence is the key pharmacological distinction from intact hGH and is attributable to the fragment lacking the N-terminal structural elements required for growth hormone receptor dimer activation and downstream JAK2/STAT5/IGF-1 gene transcription.
Is HGH Fragment 176-191 approved for human use in the UK?
No. HGH Fragment 176-191 is not approved by the MHRA for any therapeutic indication and is not a licensed medicine. It is a research compound available for preclinical or in vitro investigation through specialist research chemical suppliers under appropriate institutional governance. It has not undergone formal Phase 2/3 clinical evaluation equivalent to its AOD-9604 derivative.
Why is the disulphide bond in HGH Fragment 176-191 important?
The internal disulphide bond between cysteine residues at positions 182 and 189 of the fragment creates a constrained cyclic loop conformation that is essential for biological activity. Studies using reduced (linearised) forms show complete loss of lipolytic effect, establishing that the cyclic structure—not the linear sequence alone—constitutes the active pharmacophore. This has practical implications for storage and reconstitution: the peptide must not be exposed to reducing agents that would cleave the disulphide bond.
What models have been used to study HGH Fragment 176-191?
The compound has been characterised in isolated rat adipocyte lipolysis assays, 3T3-L1 differentiated adipocyte cell culture, diet-induced obese rat models, genetically obese ob/ob mice, and Zucker fatty rat models. These diverse model systems have consistently demonstrated the fragment's lipolytic activity and IGF-1 axis independence, providing a reasonably robust preclinical evidence base for the mechanistic claims, though human pharmacology data are essentially absent.
Related trials
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