HGH Frag 176-191: What Researchers Need to Know

HGH fragment 176-191 — commonly referred to in research literature as HGH frag or AOD-9604 precursor — is a 16-amino acid synthetic peptide derived from the C-terminal region of the full 191-amino acid human growth hormone (hGH) molecule, spanning positions 176 through 191 of the native sequence. Unlike full-length growth hormone, the 176-191 fragment does not activate the growth hormone receptor and therefore does not produce the insulin-desensitising, IGF-1-elevating, or bone-lengthening effects associated with exogenous hGH. What makes HGH frag a distinct research subject is a structural feature at its C-terminal end: a disulphide bridge formed by the residues cysteine-182 and cysteine-189, which confers a degree of receptor selectivity not present in the intact GH molecule.

This article covers the molecular structure of HGH frag 176-191 (CAS: 221231-10-3), its receptor binding profile distinct from full hGH, the published in vitro and preclinical evidence base, and its relationship to the related synthetic analogue AOD-9604. For research purposes only. Not for human consumption.

What Is HGH Fragment 176-191?

Human growth hormone (hGH) is a 191-amino acid single-chain polypeptide synthesised and secreted by somatotroph cells in the anterior pituitary gland. The full molecule has a complex three-dimensional structure that engages the growth hormone receptor (GHR), triggering downstream signalling cascades involving JAK2/STAT5, IGF-1 production in the liver, and a wide array of metabolic and anabolic effects.

HGH fragment 176-191 is a synthetic truncation of this molecule: a linear peptide comprising only the 16 C-terminal residues (positions 176–191). The sequence is: Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (with the disulphide bond between Cys-182 and Cys-189 cyclising the chain into a constrained loop conformation). Its CAS registry number is 221231-10-3 and its molecular formula is C₇₈H₁₂₅N₂₃O₂₃S₂, with a molecular weight of 1817.1 Da.

The key structural distinction from full-length hGH is the absence of the receptor-binding domain. hGH binds GHR primarily through two distinct site interactions on the protein surface — Site I and Site II — neither of which is encoded within positions 176–191. As a result, the fragment has no measurable affinity for the GHR in in vitro binding assays, and preclinical models confirm the absence of IGF-1 elevation, bone growth stimulation, and insulin desensitisation effects that characterise exogenous full-length hGH administration.

Molecular Structure: The Disulphide Bridge

The defining structural feature of HGH frag 176-191 is the intramolecular disulphide bond formed between the two cysteine residues at positions 182 and 189. This bond cyclises the peptide chain into a constrained loop — the loop spans residues 182–189, with a six-residue tail (176–181) preceding it and a single phenylalanine (position 191) following it.

This loop structure is hypothesised to be the pharmacophore responsible for the selective adipocyte-targeted activity observed in preclinical models. The related compound AOD-9604 (CAS: 221231-10-3; formally "AOD 9604" or "Advanced Obesity Drug 9604") is a modified version that adds a tyrosine residue at the N-terminus and uses an acetylated form of the fragment, a structural modification that alters its metabolic processing and half-life. AOD-9604 received Australian TGA investigational approval for obesity research trials in the 2000s, making it one of the few fragment-derived peptides with a documented Phase IIb clinical development history.

HGH Frag 176-191 vs AOD-9604: Structural Comparison

Receptor Binding Profile vs Full-Length hGH

Understanding the receptor profile of HGH frag 176-191 requires a clear mechanistic separation from full-length hGH. The complete 191-residue hGH molecule activates GHR through a hormone:receptor:receptor 2:1:1 complex — a well-characterised dimerisation event that triggers intracellular JAK2 transphosphorylation. The key binding sites on hGH for this interaction (Site I and Site II) are encoded in portions of the molecule distant from the 176–191 region.

Consequently, in receptor binding assays, HGH fragment 176-191 does not competitively displace full-length GH from the GHR. It does not stimulate hepatic IGF-1 production in cell-based assays. In rodent models comparing fragment administration to saline controls, no statistically significant changes in circulating IGF-1 were observed. This mechanistic separation is the foundation of the scientific interest in the fragment as a research tool: the ability to study specific aspects of GH peptide biology in isolation from the anabolic, pro-growth, and insulin-antagonising effects of the full hormone.

The preclinical lipolytic activity attributed to the fragment in adipocyte studies has been investigated using 3T3-L1 cell models, with observations suggesting differential β-adrenergic receptor interactions and adenylyl cyclase activation patterns compared to both full hGH and control conditions. The mechanistic pathway hypothesised involves cyclic AMP (cAMP) upregulation in adipocytes independent of GHR, though the precise receptor engagement underpinning this observation remains an active area of investigation.

Preclinical Evidence Base

The published research on HGH fragment 176-191 and its AOD-9604 derivative spans three main experimental contexts: in vitro adipocyte models, in vivo rodent studies, and the clinical development history of AOD-9604 (the stabilised analogue). This evidence base provides mechanistic context for the fragment's research profile without extrapolating to human clinical outcomes, in keeping with the compound's research-only status.

Research Context: Fragment Peptides in Combined Protocols

In preclinical research settings, growth hormone fragment peptides are sometimes studied alongside other research compounds as part of multi-peptide investigation protocols. The rationale for such combined research approaches varies by investigator, but frequently includes examining pathway independence (whether co-administration of mechanistically distinct compounds alters individual readouts) and building mechanistic maps of adipocyte and growth hormone receptor signalling.

Within the growth hormone peptide research area, HGH frag 176-191 is frequently studied in the context of the broader growth hormone secretagogue landscape — which includes GHRH analogues such as CJC-1295 and growth hormone-releasing peptides such as ipamorelin. These compounds operate via entirely different receptor pathways: CJC-1295 targets the GHRH receptor, ipamorelin targets the ghrelin/GHS receptor, and HGH frag operates via a GHR-independent mechanism. Research designs using these compounds in combination test the mechanistic independence of each pathway rather than seeking an additive effect from compounds acting at the same receptor. For research purposes only. Not for human consumption.

Laboratory Handling: Solubility and Stability

HGH fragment 176-191 (MW 1817.1 Da) is a lyophilised white powder in its supplied form. For in vitro research use, the standard reconstitution solvent is bacteriostatic water or 0.9% sodium chloride for aqueous working stocks. The peptide is soluble in aqueous solutions at physiological pH, with solubility typically >5 mg/mL in the above solvents. Researchers using the compound in adipocyte assays often prepare stock solutions at higher concentrations and dilute into cell culture medium to achieve working concentrations in the nanomolar to micromolar range appropriate to the assay readout.

A critical handling consideration is the preservation of the Cys-182–Cys-189 disulphide bridge. Reducing agents — including dithiothreitol (DTT), β-mercaptoethanol, and tris(2-carboxyethyl)phosphine (TCEP) — will cleave the disulphide bond and convert the cyclic fragment into a linear form with a substantially different conformational profile. All research protocols using HGH frag should be conducted in reducing agent-free buffers to ensure the conformationally active cyclised form is maintained throughout the assay.

Lyophilised peptide is stable at −20°C for 24 months under dry conditions. Reconstituted working solutions should be used within 7–14 days when stored at 4°C; for longer working life, aliquot and freeze at −20°C and avoid repeated freeze-thaw cycles that can promote aggregation and partial reduction of the disulphide bridge through oxidative cycling.

Regulatory and Compliance Position (UK)

HGH fragment 176-191 is not a listed controlled substance under the Misuse of Drugs Act 1971 and is not a licensed prescription medicine under the Human Medicines Regulations 2012. However, it occupies a regulatory grey area: full-length human growth hormone (somatropin) is a Prescription-Only Medicine (POM) in the UK, and derivative fragments share structural features with the parent compound. Suppliers operating in the UK research chemical market should be aware that regulatory frameworks applicable to growth hormone analogues are subject to periodic MHRA review.

Pure Grade Labs supplies HGH fragment 176-191 strictly as a research chemical, for laboratory use only, not for human consumption, and with no medical or clinical claims. Purchasers are responsible for ensuring compliance with applicable laws in their jurisdiction. This article is educational in nature and does not constitute medical advice, prescribing guidance, or an endorsement of human use.

Frequently Asked Questions

Cited Research