HGH Fragment 176-191

1. Overview

HGH Fragment 176-191 is a synthetic peptide consisting of the 16 C-terminal amino acids of human growth hormone, corresponding to residues 176 through 191 of the full 191-amino acid hGH molecule. Its sequence is Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe, with a stabilizing intramolecular disulfide bond between the two cysteine residues (corresponding to Cys182 and Cys189 of native hGH). The molecular weight is approximately 1,817 Da [1][7][15].

The identification of this fragment as the minimal lipolytic domain of growth hormone was achieved through systematic structure-function mapping by Professor Frank M. Ng and colleagues at Monash University, Melbourne, Australia, during the early 1990s. Wu and Ng published the foundational 1993 finding that the synthetic C-terminal peptide hGH(177-191) possessed antilipogenic activity identical to intact growth hormone when tested on isolated adipocytes, while completely lacking the growth-promoting and diabetogenic effects of the full hormone [1]. Natera, Jiang, and Ng (1994) extended this to in vivo models, demonstrating chronic weight loss in obese ob/ob mice [2].

It is essential to distinguish HGH Fragment 176-191 from its closely related derivative, AOD-9604 (Anti-Obesity Drug 9604). AOD-9604 consists of the same core 176-191 sequence but with an N-terminal tyrosine residue substituted for the native leucine at position 176, making it a 16-residue peptide with modified N-terminus (Tyr-hGH 177-191). This single amino acid modification was introduced by the Monash University group to improve peptide stability and oral bioavailability [5][10]. Critically, all six human clinical trials (over 890 participants total) were conducted exclusively with the AOD-9604 (tyrosine-modified) form [11]. The unmodified HGH Fragment 176-191 has never been tested in human clinical trials, meaning that human safety and efficacy data cannot be directly extrapolated to this peptide.

HGH Fragment 176-191 circulates extensively in the research peptide market and is commonly promoted for fat loss applications based on the preclinical data generated for the hGH C-terminal domain. It has no regulatory approval for any therapeutic use in any jurisdiction.

Type

Synthetic peptide fragment of hGH C-terminus

Sequence

Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (16 aa)

Molecular Weight

~1,817 Da

Parent Protein

Human Growth Hormone (residues 176-191 of 191-aa hGH)

Disulfide Bond

Cys182-Cys189 (intramolecular, corresponds to Cys7-Cys14 in fragment)

Half-life

Not well characterized; rapidly degraded in serum

FDA Status

Not approved for any indication; research compound only

WADA Status

Prohibited at all times (S0: Non-Approved Substances)

2. Molecular Structure

HGH Fragment 176-191 retains the structural features of the C-terminal region of native human growth hormone. The three-dimensional solution structure of this peptide domain was determined by Ogru et al. (2000) using two-dimensional proton NMR spectroscopy [7]. Key structural features include:

Disulfide bridge: The two cysteine residues (corresponding to Cys182 and Cys189 of full-length hGH) form an intramolecular disulfide bond, creating a constrained cyclic loop spanning residues Cys7 through Cys14 within the fragment numbering system. This cyclic region is essential for biological activity [7].

Beta-turn structures: Within the constrained cyclic core, the peptide adopts type I beta-turns at Ser8-Val9-Glu10-Gly11 and Ser12-Cys13-Gly14-Phe15, preceded by loop-like structures. These turns create a defined three-dimensional pharmacophore responsible for the lipolytic interaction [7].

Lack of GHR binding capacity: The hGH C-terminal region contains only a partial overlap with one of the two GH receptor binding sites (Site 2) of the full hormone and completely lacks the other (Site 1). Since GH receptor activation requires sequential binding through both sites to induce receptor dimerization, the fragment physically cannot activate the GH receptor, explaining the complete absence of growth-promoting, IGF-1-stimulating, and diabetogenic effects [6][8][16].

Comparison with AOD-9604

The structural difference between HGH Fragment 176-191 and AOD-9604 is a single N-terminal amino acid substitution. HGH Fragment 176-191 retains the native leucine (or in some descriptions, begins at position 176 with the native phenylalanine) from the hGH sequence, while AOD-9604 substitutes tyrosine at this position. The tyrosine modification enhances resistance to proteolytic degradation and improves oral bioavailability, and comparative preclinical studies indicate that AOD-9604 demonstrates stronger lipolytic activity per unit dose [5][10]. Both peptides share the same cyclic core structure and disulfide bond topology.

3. Mechanism of Action

The lipolytic mechanism of HGH Fragment 176-191 operates through the beta-3 adrenergic receptor (beta3-AR) pathway, entirely independent of GH receptor signaling [9]:

1. The fragment upregulates beta3-AR messenger RNA expression on adipocyte cell surfaces, restoring receptor levels that are characteristically depressed in obese animals to levels approaching those found in lean controls [9]
2. Enhanced beta3-AR signaling activates adenylyl cyclase, increasing intracellular cyclic AMP (cAMP) concentrations
3. Elevated cAMP activates protein kinase A (PKA)
4. PKA phosphorylates and activates hormone-sensitive lipase (HSL), which catalyzes the hydrolysis of stored triglycerides into free fatty acids and glycerol [5]
5. Released free fatty acids are mobilized for mitochondrial beta-oxidation

Simultaneously, the fragment inhibits acetyl-CoA carboxylase, the rate-limiting enzyme in de novo lipogenesis (fat synthesis), reducing the conversion of carbohydrate substrates into new fat stores [5]. This dual action -- stimulating fat breakdown while blocking new fat formation -- accounts for the net reduction in adipose tissue observed in animal studies.

Beta3-AR Dependence

The absolute requirement for beta3-AR signaling was demonstrated by Heffernan et al. (2001) using beta3-AR knockout mice. Chronic treatment with the GH C-terminal fragment produced no changes in body weight or lipolysis in knockout animals, while identical treatment significantly reduced body weight in wild-type obese mice [9]. Acute administration still enhanced energy expenditure in knockout animals, suggesting involvement of secondary pathways for immediate metabolic effects, but confirming that the sustained anti-obesity action depends entirely on beta3-AR [9].

Dissociation from Growth Hormone Actions

Multiple studies have confirmed that the C-terminal fragment does not [4][6][8]:

• Bind to the growth hormone receptor (GHR)
• Stimulate IGF-1 production
• Promote linear growth or organ growth
• Cause insulin resistance or hyperglycemia
• Stimulate cell proliferation

This clean dissociation between the lipolytic and growth-promoting domains of hGH was the central scientific rationale for developing the fragment as a potential anti-obesity therapeutic [1][4][10].

4. Researched Applications

4.1 Fat Loss and Lipolysis

Evidence level: Preclinical (for unmodified fragment); Phase II failed (for AOD-9604 form)

Animal studies with the hGH C-terminal fragment domain consistently demonstrated [1][2][4][5][8]:

• Reduced cumulative body weight gain (greater than 50% reduction vs. controls in obese Zucker rats over 19 days)
• Decreased adipose tissue mass and adipocyte cell size
• Increased fat oxidation
• No effect on food intake or lean body mass
• No diabetogenic effects or insulin resistance

However, when the modified form (AOD-9604) was advanced to human clinical trials for obesity treatment, results were disappointing. The pivotal Phase IIb OPTIONS Study (536 obese adults, 24 weeks) failed to demonstrate statistically significant weight loss versus placebo, and obesity development was discontinued in 2007 [11]. Whether the unmodified fragment would show different results in humans is unknown, as it has never been clinically tested.

The disconnect between robust animal efficacy and modest human results may reflect species differences in beta3-AR expression and density in adipose tissue (humans have substantially lower beta3-AR expression than rodents), oral bioavailability limitations, or insufficient dose optimization.

4.2 Cancer Research

Evidence level: In vitro only

Habibullah et al. (2022) reported that hGH fragment 176-191 enhanced the cytotoxicity of doxorubicin-loaded chitosan nanoparticles against MCF-7 breast cancer cells in vitro [13]. This is an early exploratory finding with no in vivo follow-up, and no clinical relevance has been established.

5. Clinical Evidence Summary

6. Dosing in Research

The following table summarizes doses used in published preclinical research. No human clinical trials have been conducted with the unmodified HGH Fragment 176-191. All human trial data pertains to the AOD-9604 (tyrosine-modified) derivative. These are not therapeutic recommendations.

7. Safety and Side Effects

7.1 Preclinical Safety

The hGH C-terminal fragment domain has demonstrated a favorable preclinical safety profile, primarily characterized by the absence of the adverse effects associated with intact growth hormone. In animal studies, the fragment did not produce insulin resistance, hyperglycemia, IGF-1 elevation, or growth-promoting effects [4][5][6][8]. Euglycemic clamp studies in obese Zucker rats showed no adverse effect on insulin sensitivity after chronic fragment treatment, in contrast to intact hGH which significantly impaired insulin action [4].

7.2 Human Safety Data (AOD-9604 Form Only)

All human safety data pertains to AOD-9604, not the unmodified fragment. Across six human clinical trials enrolling 893 participants, AOD-9604 demonstrated a safety profile "indistinguishable from placebo" [11]. No serious adverse events related to AOD-9604 were reported, and no effects on IGF-1, glucose, insulin, HbA1c, liver function, kidney function, or hematological parameters were observed [11].

7.3 Important Limitations

• The unmodified HGH Fragment 176-191 has never been tested in humans. Extrapolation of human safety data from AOD-9604 trials to the unmodified fragment, while structurally reasonable, is scientifically unvalidated.
• Long-term safety data beyond 24 weeks do not exist for any form of the fragment.
• Research peptide market products may contain impurities, degradation products, or inaccurate concentrations that introduce additional safety uncertainties.
• No data exist for subcutaneous injection of either form in humans (all clinical trials used oral administration).
• HGH Fragment 176-191 is rapidly degraded in serum, and the pharmacokinetics of subcutaneous administration have not been characterized.

7.4 Anti-Doping Detection

Both HGH Fragment 176-191 and AOD-9604 are prohibited at all times by WADA under S0 (Non-Approved Substances) [14]. Cox et al. (2015) developed a validated urine detection method with a limit of detection of 50 pg/mL and identified six metabolites, with the central loop fragment CRSVEGSCG being the most stable metabolic marker for anti-doping testing [12].

8. HGH Fragment 176-191 vs. AOD-9604

Both peptides derive from the same C-terminal region of human growth hormone and share the same core mechanism of action. The key distinctions are:

Structural difference: AOD-9604 has a tyrosine at the N-terminus where HGH Fragment 176-191 retains the native leucine. This single amino acid modification improves metabolic stability and potency [5][10].

Clinical data: All six human clinical trials (over 890 participants) were conducted with AOD-9604 exclusively. HGH Fragment 176-191 has zero human clinical data.

Stability: AOD-9604 demonstrates greater resistance to proteolytic degradation and higher oral bioavailability than the unmodified fragment [6][10].

Regulatory pathway: AOD-9604 has a self-affirmed GRAS determination as a food ingredient (2014) and has been reviewed by the FDA PCAC [17]. HGH Fragment 176-191 has no regulatory history.

Shared properties: Both stimulate lipolysis and inhibit lipogenesis through beta3-AR signaling. Neither activates the GH receptor, stimulates IGF-1, or produces growth-promoting effects [6][8][9].

9. Regulatory Status

HGH Fragment 176-191 has no regulatory approval for any indication in any jurisdiction. It is not FDA-approved, not included in any pharmacopeia, and has not undergone formal regulatory review. It is classified as a research compound only.

Under WADA rules, HGH Fragment 176-191 is prohibited at all times as a non-approved substance (S0 category). WADA clarified in 2013 that GH fragment peptides fall under this prohibition regardless of whether they have undergone formal drug development [14].

The unregulated research peptide market distributes HGH Fragment 176-191 widely, often marketed with fat loss claims extrapolated from preclinical data and conflated with the AOD-9604 clinical trial results. Consumers should be aware that these are structurally distinct molecules and that no human efficacy or safety data exist for the unmodified fragment.

10. Pharmacokinetics

The pharmacokinetics of HGH Fragment 176-191 are poorly characterized, as no formal human PK studies have been conducted. Available data are derived from preclinical studies and extrapolation from the structurally related AOD-9604 [4][6][12].

Absorption (subcutaneous). No formal SC pharmacokinetic data exist for the unmodified fragment. As a small peptide (approximately 1,817 Da) with an intramolecular disulfide bond, absorption from a subcutaneous depot would be expected to be rapid but incomplete, with significant peptidase degradation occurring at the injection site before systemic absorption. This is in contrast to AOD-9604, which was primarily studied via oral administration.

Serum stability. The unmodified fragment is rapidly degraded in serum by peptidases and proteases. While no formal half-life determination has been published, the peptide is described as "rapidly degraded" in biological fluids. Cox et al. (2015) identified six metabolites of AOD-9604 in vitro, with the central disulfide loop fragment CRSVEGSCG being the most stable metabolic marker [12]. The Cys182-Cys189 disulfide bond creates a constrained cyclic core that resists degradation somewhat longer than the linear N-terminal and C-terminal extensions.

Oral bioavailability (AOD-9604 form). Heffernan et al. (2000) demonstrated that the tyrosine-modified form (AOD-9604) achieved measurable oral bioavailability in ob/ob mice, producing body weight reduction and increased fat oxidation after oral administration [6]. The unmodified fragment would be expected to have lower oral bioavailability due to the absence of the N-terminal tyrosine modification that was specifically introduced to improve stability.

Metabolism. Degraded by serum and tissue peptidases through sequential cleavage from both termini. The disulfide-constrained cyclic core (residues Cys7-Cys14 in fragment numbering) is more resistant to degradation and likely represents the primary active pharmacophore. No hepatic or renal clearance studies exist [12].

Distribution. No distribution data available. As a small peptide, the fragment would be expected to distribute broadly in extracellular fluid with minimal plasma protein binding and rapid tissue penetration.

Practical PK limitations. The extremely short in vivo half-life of the unmodified fragment means that achieving sustained biological effects through subcutaneous injection would require multiple daily injections or sustained-release formulations. This pharmacokinetic limitation likely contributed to the development of AOD-9604 with its improved stability profile, and may partly explain the disconnect between robust acute in vitro lipolytic activity and the challenges of demonstrating in vivo efficacy.

11. Dose-Response Relationship

Preclinical dose-response (in vitro). Wu and Ng (1993) demonstrated that the C-terminal fragment inhibited lipogenesis in isolated rat adipocytes at equimolar concentrations to intact hGH, establishing that the fragment retained the full antilipogenic potency of the parent hormone in the in vitro setting [1].

In vivo fat loss dose-response (AOD-9604 form). Ng et al. (2000) showed that AOD-9604 at 500 mcg/kg/day IP for 19 days in obese Zucker rats produced greater than 50% reduction in body weight gain with no adverse effects on insulin sensitivity [4]. Heffernan et al. (2001) demonstrated dose-dependent increases in fat oxidation and weight loss in ob/ob mice with chronic IP treatment, with the fragment being less potent per unit dose than intact hGH but producing the fat loss effect without the diabetogenic effects [8].

Human dose-response (AOD-9604 only). The pivotal Phase IIb OPTIONS study tested multiple oral doses of AOD-9604 in 536 obese adults over 24 weeks. Despite the robust preclinical data, no dose level achieved statistically significant weight loss versus placebo, and obesity development was discontinued in 2007 [11]. This failure may reflect species differences in beta3-AR expression (humans have substantially lower beta3-AR density in adipose tissue than rodents), oral bioavailability limitations, or inadequate dose optimization.

Dose-response for beta3-AR upregulation. The lipolytic mechanism is entirely dependent on beta3-AR signaling (confirmed by knockout studies in beta3-AR null mice where chronic treatment produced no weight loss) [9]. The dose-response relationship is therefore constrained by beta3-AR expression levels, which are inherently lower in human white adipose tissue than in rodent brown and white adipose tissue.

No human dose-response data for unmodified fragment. As the unmodified HGH Fragment 176-191 has never been studied in humans, there are no dose-response data to guide usage. Community-reported doses in the research peptide market (typically 250-500 mcg subcutaneously 1-2x daily) have no scientific basis and are not validated by any clinical study.

12. Comparative Effectiveness

HGH Fragment 176-191 vs AOD-9604. AOD-9604 (tyrosine-modified fragment) has greater metabolic stability, higher oral bioavailability, and stronger lipolytic activity per unit dose than the unmodified fragment in preclinical comparisons [5][10]. All human clinical data (6 trials, 893 participants) were generated exclusively with AOD-9604. Despite this, AOD-9604 failed to demonstrate significant weight loss in the pivotal Phase IIb trial. The unmodified fragment, with inferior stability, would be expected to perform no better [11].

HGH Fragment 176-191 vs full-length hGH. Both the fragment and intact hGH stimulate lipolysis and reduce adipose tissue mass in animal models. However, intact hGH also activates the GH receptor (producing IGF-1 elevation, growth promotion, and insulin resistance), while the fragment does not [1][4][8]. This dissociation is the theoretical advantage of the fragment: fat loss without diabetogenic effects. However, intact hGH produces lipolysis through multiple mechanisms (direct HSL activation, beta-3 AR independent pathways), while the fragment relies solely on beta3-AR, potentially limiting its efficacy in humans with low beta3-AR expression.

HGH Fragment 176-191 vs GH secretagogues. GH secretagogues (CJC-1295, ipamorelin, MK-677) stimulate endogenous GH release, which then drives lipolysis through multiple pathways (both direct and indirect). They also produce GH receptor-mediated effects (IGF-1 elevation, growth promotion) absent with the fragment. For pure fat loss without anabolic or growth effects, the fragment is theoretically cleaner; for overall metabolic and body composition effects, GH secretagogues provide broader action.

HGH Fragment 176-191 vs established weight loss therapeutics. Approved GLP-1 RA-based therapies (semaglutide 2.4 mg, tirzepatide) produce 15-21% body weight loss in clinical trials -- dramatically exceeding any demonstrated effect of the GH C-terminal fragment domain. For evidence-based weight management, approved pharmacotherapies have incomparably stronger clinical support.

13. Enhanced Safety Profile

Favorable preclinical safety profile. The central safety advantage of the GH C-terminal fragment is its clean dissociation from GH receptor activation. Multiple studies confirm the fragment does not: bind the GH receptor, stimulate IGF-1 production, promote linear growth or organ growth, cause insulin resistance or hyperglycemia, or stimulate cell proliferation [1][4][6][8].

No diabetogenic effects. Euglycemic clamp studies in obese Zucker rats demonstrated that the fragment (AOD-9604 form) produced no adverse effects on insulin sensitivity after chronic treatment, in direct contrast to intact hGH which significantly impaired insulin action [4].

Human safety data (AOD-9604 form only). Across 6 clinical trials with 893 participants, AOD-9604 demonstrated a safety profile "indistinguishable from placebo." No serious adverse events, no effects on IGF-1, glucose, insulin, HbA1c, liver function, kidney function, or hematological parameters [11]. This safety data can only be cautiously extrapolated to the unmodified fragment.

No human safety data for unmodified fragment. The critical limitation is that HGH Fragment 176-191 itself has zero human safety data. All subcutaneous injection of this peptide in humans is unvalidated. Injection of research-grade peptides introduces risks of endotoxin contamination, incorrect concentration, degradation products, and immunogenic impurities.

Long-term safety unknown. Maximum human exposure is 24 weeks (AOD-9604 form only). Chronic effects on adipose tissue biology, endocrine function, and potential off-target effects of sustained beta3-AR agonism are uncharacterized.

Anti-doping detection. Both forms are prohibited by WADA under S0 (Non-Approved Substances). Cox et al. developed urine detection methods with a 50 pg/mL limit of detection and identified the CRSVEGSCG disulfide loop as the most stable metabolic marker [12][14].

Research peptide market risks. Products from unregulated sources may contain impurities, degradation products, or inaccurate concentrations. As a peptide with a disulfide bond, improper handling (heat, oxidation) can produce misfolded or inactive material. No quality control standards apply to research-grade peptides.

14. Related Peptides

See also: AOD-9604, Human Growth Hormone (hGH), IGF-1 LR3, IGF-1 DES, CJC-1295, Ipamorelin, Tesamorelin

15. References

1. [1] Wu Z, Ng FM. (1993). Antilipogenic action of synthetic C-terminal sequence 177-191 of human growth hormone. Biochem Mol Biol Int. PubMed
2. [2] Natera SH, Jiang WJ, Ng FM. (1994). Reduction of cumulative body weight gain and adipose tissue mass in obese mice: response to chronic treatment with synthetic hGH 177-191 peptide. Biochem Mol Biol Int. PubMed
3. [3] Wu Z, Ng FM. (1994). Effect of an antilipogenic fragment of human growth hormone on glucose transport in rat adipocytes. Biochem Mol Biol Int. PubMed
4. [4] Ng FM, Sun J, Sharma L, Libinaka R, Jiang WJ, Gianello R. (2000). Metabolic studies of a synthetic lipolytic domain (AOD9604) of human growth hormone. Hormone Research. DOI PubMed
5. [5] Ng FM, Jiang WJ, Gianello R, Pitt S, Rae T. (2000). Molecular and cellular actions of a structural domain of human growth hormone (AOD9401) on lipid metabolism in Zucker fatty rats. Journal of Molecular Endocrinology. PubMed
6. [6] Heffernan MA, Jiang WJ, Thorburn AW, Ng FM. (2000). Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism. American Journal of Physiology - Endocrinology and Metabolism. PubMed
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8. [8] Heffernan MA, Thorburn AW, Fam B, Summers R, Conway-Campbell B, Waters MJ, Ng FM. (2001). Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment. International Journal of Obesity. DOI PubMed
9. [9] Heffernan M, Summers RJ, Thorburn A, Ogru E, Gianello R, Jiang WJ, Ng FM. (2001). The effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and beta(3)-AR knock-out mice. Endocrinology. DOI PubMed
10. [10] Jiang WJ, Gianello R, Heffernan M, Ogru E, Libinaki R, Ng FM. (2001). Development of a Human Growth Hormone Peptide Analogue AOD9604 into an Anti-Obesity Drug. In: Bentham Science (eds) Growth Hormone and Related Proteins.
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13. [13] Habibullah MM, Mohan S, Syed NK, et al. (2022). Human Growth Hormone Fragment 176-191 Peptide Enhances the Toxicity of Doxorubicin-Loaded Chitosan Nanoparticles Against MCF-7 Breast Cancer Cells. Drug Design, Development and Therapy. DOI PubMed
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