hGH Fragment 176-191: The Lipolytic C-Terminal Domain of Growth Hormone

Research summary. hGH Fragment 176-191 is a synthetic 16-amino-acid peptide corresponding to the C-terminal domain of human growth hormone, specifically residues 176 through 191. The fragment contains the structural region of the parent hormone associated with its lipolytic (fat-mobilising) activity, while lacking the regions responsible for engaging the GH receptor and downstream IGF-1-axis signalling. AOD9604 (covered separately in this catalogue) is a derivative of this fragment with an additional N-terminal tyrosine for stability; the two molecules are closely related but distinct.

Molecular profile

• Sequence: Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (16 residues with Cys7–Cys15 disulfide bridge)
• Molecular formula: C₇₈H₁₂₅N₂₃O₂₃S₂
• Molecular weight: ~1817.1 g/mol
• CAS Number: 66004-57-7

The disulfide bridge between cysteines 7 and 15 (corresponding to positions 182 and 189 in the parent hGH) is critical to the fragment's structural integrity and biological activity. Without this bridge, the small fragment loses much of its activity.

Mechanism of action

The mechanistic story for hGH Fragment 176-191 has evolved over the decades of its investigation:

• Original hypothesis (β3-adrenergic receptor pathway). Early work proposed that the fragment acts on β3-adrenergic receptors (ADRB3) on adipocytes and skeletal muscle, stimulating lipolysis through cAMP-dependent activation of hormone-sensitive lipase, alongside thermogenic effects in skeletal muscle. Mice lacking ADRB3 receptors show attenuated lipolytic responses to the fragment, supporting a role for this pathway.
• Subsequent refinements. Follow-up work has demonstrated that ADRB3-knockout mice retain some lipolytic response to the fragment, indicating that additional receptor systems contribute. The full mechanistic picture remains incompletely characterised.
• Decoupling from IGF-1 axis. A consistent finding is that the fragment does not significantly elevate IGF-1, does not stimulate longitudinal bone growth, and does not produce the insulin-counterregulatory effects of intact hGH. This dissociation of the lipolytic function from the broader hGH endocrine profile is the distinguishing feature that has driven research interest.

Preclinical research highlights

Adipocyte lipolysis. In isolated rat adipocytes and in adipose-tissue preparations from genetically obese (ob/ob) mice, hGH Fragment 176-191 has been reported to stimulate fatty-acid release at concentrations and time-courses consistent with direct lipolytic activity.

Body composition in obese rodents. Studies in obese mouse models have reported reductions in fat mass, with reported reductions of approximately 50% in weight gain over multi-week dosing in some preparations, alongside increased energy expenditure and preserved food intake.

Lean tissue specificity. Lean (non-obese) animals administered the fragment have shown more limited body-composition changes than obese animals, suggesting feedback mechanisms that limit lipolysis near homeostatic adiposity.

Cartilage research. A separate line of investigation has examined the fragment in cartilage repair models. Direct intra-articular administration in rodent osteoarthritis preparations has been reported to reduce cartilage degeneration scores and inflammatory markers, with proposed mechanisms involving chondrocyte metabolism modulation distinct from the fragment's lipolytic activity.

Glucose handling. Studies of the fragment have reported modest blood-glucose-lowering effects through mechanisms that do not produce the insulin resistance characteristic of intact hGH chronic dosing.

Distinction from AOD9604

The fragment 176-191 sequence has been studied in several closely related forms, of which AOD9604 is the most commonly named:

• hGH Fragment 176-191: The native C-terminal sequence of hGH (residues 176-191)
• AOD9604: Fragment 176-191 with an added N-terminal tyrosine to extend the molecule by one residue, modify its stability profile, and (in the original development context) provide a distinct pharmaceutical-development entity

In practice, the activities of the two forms overlap substantially, and much of the published "AOD9604" literature is mechanistically informative about the parent fragment as well.

Regulatory and ethical considerations

Growth hormone fragments, including hGH Fragment 176-191 and AOD9604, are listed by the World Anti-Doping Agency as prohibited substances under category S2. The compound has not received broad regulatory approval as a therapeutic, despite Phase 2 clinical investigation in obesity contexts (under the AOD9604 designation) earlier in its development history.

Current research status

hGH Fragment 176-191 remains an investigational research peptide. It is widely available in research-peptide catalogues and is used as:

• A research tool for studying lipolysis-specific signalling decoupled from IGF-1 axis activity
• A reference compound for studying β3-adrenergic-independent fat-mobilising mechanisms
• An adjunct in cartilage-repair preclinical research

It is not an approved therapeutic.

Key takeaways for researchers

• hGH Fragment 176-191 is the synthetic C-terminal domain of human growth hormone, comprising 16 residues with a Cys7-Cys15 disulfide bridge.
• It retains the lipolytic activity of the parent hormone while lacking the IGF-1-axis-stimulating, growth-promoting, and insulin-counterregulatory effects.
• Mechanistic studies suggest β3-adrenergic and additional, incompletely characterised receptor pathways.
• It is closely related to AOD9604 (a one-residue extended derivative with similar pharmacology).
• It is on the WADA Prohibited List and is not an approved therapeutic.

References

1. Ng FM, Sun J, Sharma L, Libinaki R, Jiang WJ, Gianello R. Metabolic studies of a synthetic lipolytic domain (AOD9604) of human growth hormone. Hormone Research. 2000;53(6):274–278.

---

This article is provided for educational and research purposes only. hGH Fragment 176-191 is a research peptide. It is not an approved drug or therapeutic agent and is not intended for human consumption, diagnosis, treatment, cure, or prevention of any disease or condition. hGH Fragment 176-191 is listed by the World Anti-Doping Agency as a prohibited substance. All work involving this peptide should be conducted by qualified personnel within an appropriate research setting and in compliance with applicable institutional and regulatory requirements.