HGH Fragment 176-191 (AOD-9604): Growth Hormone Fragment

Overview

HGH Fragment 176-191 is a short synthetic peptide derived from the C-terminal region of human growth hormone (hGH). The name refers to the stretch of the 191-residue hGH molecule that the fragment models. In the peer-reviewed literature the best-characterised version of this fragment is AOD-9604, a tyrosine-modified analogue (Tyr-hGH177-191), and the two names are used interchangeably in the research-chemical market even though that conflation is not strictly precise. This page summarises what the published record actually shows. It is a neutral reference for laboratory and educational context, not a treatment guide, and it contains no instructions for use in people.

The scientific rationale traces to a long-standing observation: full-length growth hormone drives lipolysis (the breakdown of stored fat) while also raising blood glucose and IGF-1 and carrying the metabolic liabilities of chronic GH exposure ^[1]. Researchers asked whether the fat-mobilising activity of GH could be separated from the rest of the hormone's effects, and localised that activity to the C-terminal end of the molecule. HGH Fragment 176-191 / AOD-9604 is the product of that separation attempt.

Origin and rationale

Work in the late 1970s showed that short synthetic peptides copied from the C-terminal sequence of human growth hormone reproduced part of the hormone's metabolic activity, including effects on blood glucose, independent of the intact protein ^[2]. Follow-up biochemical studies in the early 1980s probed how these C-terminal peptides act at the tissue level, pointing to mechanisms distinct from the growth-promoting, IGF-1-mediated arm of GH signalling ^[3]. This body of work established the idea of a discrete "lipolytic domain" near the C-terminus and set up the later engineering of AOD-9604 as a stabilised, tyrosine-tagged fragment intended to keep the fat-metabolism activity while dropping the growth and glucose effects.

Structure and physicochemistry

AOD-9604 is a 16-residue peptide with the sequence Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe (single-letter YLRIVQCRSVEGSCGF). It carries a single intramolecular disulfide bond between its two cysteine residues, which closes the small C-terminal loop thought to be important for activity. The molecular formula is C78H123N23O23S2 with an average molar mass near 1815 g/mol, and the compound is commonly catalogued under CAS (Chemical Abstracts Service number) 221231-10-3. The added N-terminal tyrosine, which is not present in the exact native 176-191 stretch, was originally introduced to aid synthesis and radiolabelling. As supplied for research, the material is typically a white lyophilised powder.

Mechanism of action

The defining claim for this fragment is functional selectivity: it is reported to reproduce the fat-mobilising signal of growth hormone without engaging the receptor pathways responsible for GH's growth-promoting and diabetogenic effects. In human safety studies AOD-9604 did not raise serum IGF-1, which is consistent with an action that does not run through the classic GH-receptor / IGF-1 axis ^[7]. In rodent tissue the fragment increased lipolytic activity in fat depots and, in obese animals, was associated with greater fat oxidation and reduced weight gain ^[4][6].

Mechanistic experiments in mice implicated the β3-adrenergic receptor (β3-AR), a receptor that governs lipolysis and thermogenesis in adipose tissue. Both hGH and AOD-9604 reduced body weight and fat in obese mice, and the response was altered in β3-AR knock-out animals, arguing that the β-adrenergic pathway contributes to the fragment's effect on fat metabolism ^[5]. Importantly, this is a preclinical mechanistic picture assembled from cell and rodent data, not a validated mechanism of clinical fat loss in humans.

Preclinical research

The preclinical record is the strongest part of the file. In obese Zucker rats, daily AOD-9604 reduced body-weight gain by more than half relative to controls and increased lipolysis in adipose tissue, and, unlike intact hGH, did not worsen insulin sensitivity on euglycemic-clamp testing ^[4]. In genetically obese (ob/ob) mice, chronic treatment with hGH or the modified C-terminal fragment increased fat oxidation and produced weight loss ^[6]. Studies combining obese mice with β3-AR knock-outs tied part of that activity to β3-adrenergic signalling ^[5]. These are consistent, repeated animal findings from the groups that developed the compound. They establish biological plausibility; they do not by themselves establish efficacy in people.

Human and clinical research

This is where honesty matters most. Between 2001 and 2006 AOD-9604 went through a clinical programme of six randomised, double-blind, placebo-controlled trials involving roughly 900 mostly obese adults, summarised in a later safety publication ^[7]. The headline result on safety was strongly positive: AOD-9604 was well tolerated, its adverse-event profile was essentially indistinguishable from placebo, it did not raise IGF-1, it did not impair glucose tolerance the way GH does, and no anti-drug antibodies were detected ^[7]. On efficacy the story is very different. Early, smaller studies suggested statistically significant fat loss, but the larger trial did not meet its primary weight-loss endpoint, and the obesity drug-development programme was discontinued around 2007. In plain terms: the human data support a clean safety and tolerability profile, but they do not demonstrate a clinically meaningful weight-loss benefit, and the compound has no approved therapeutic indication in any jurisdiction.

Beyond metabolism, small exploratory animal studies have examined whether the fragment influences cartilage and joint tissue, but that work is very preliminary and has not been confirmed in controlled human trials.

Pharmacokinetics, handling, storage

Detailed human pharmacokinetics for the fragment are not well described in the public literature. Toxicology and pharmacokinetic testing in rats and cynomolgus monkeys reported no genotoxic signal in standard assays (Ames, chromosomal aberration, micronucleus) and a generally acceptable safety margin after chronic oral dosing ^[8]. The peptide appears to be cleared rapidly, so any circulating exposure is short-lived. As a research material it is handled as a lyophilised solid, reconstituted in bacteriostatic or sterile water for in-vitro or animal work, kept cold, protected from repeated freeze-thaw cycles, and stored frozen for longer-term stability. These are general good-laboratory-practice handling notes, not directions for administration to humans.

Regulatory and anti-doping status

AOD-9604 was developed as an investigational drug (originally by Metabolic Pharmaceuticals) and later floated as a nutraceutical ingredient ^[8], but it has not been approved as a medicine for any indication. Growth-hormone fragments of this class fall under the World Anti-Doping Agency's prohibited-substance framework, so the compound is banned in sport. For all of these reasons it is sold and studied strictly as a research chemical.

Research-use disclaimer

This monograph is a neutral summary of published work for laboratory and educational reference. It is not medical advice, it does not recommend or describe use in humans, and it provides no dosing or administration guidance. HGH Fragment 176-191 / AOD-9604 is not an approved drug. Any handling should follow applicable laws and institutional safety rules.