Research-chemical catalogues routinely list AOD-9604 and “HGH fragment 176–191” as if the names were interchangeable. They are adjacent molecules, not the same compound, and the human evidence sits almost entirely on one of them.
AOD-9604 is a synthetic hexadecapeptide corresponding to the C-terminal lipolytic domain of human growth hormone, with tyrosine substituted for the native N-terminal phenylalanine (Tyr-hGH177–191). Unmodified fragment 176–191 retains that phenylalanine. Both are cyclised by a native-style disulphide. Only AOD-9604 was taken through a formal obesity development programme. That programme generated a usable safety dataset and did not produce a published, positive primary efficacy result. Metabolic Pharmaceuticals terminated the anti-obesity programme; no UK marketing authorisation followed.
This explainer grades the two fragments against the published record, not against vendor copy. It does not discuss human use, reconstitution, or self-administration. Both compounds, when sold as laboratory reagents, are research-use-only.
1. What each molecule actually is
Human growth hormone is a 191-residue protein. Work in the 1990s mapped much of its lipolytic and antilipogenic activity to the C-terminus, while somatogenic (IGF-1-mediated) actions mapped elsewhere. The research programme at Monash University and Metabolic Pharmaceuticals isolated that C-terminal domain as a standalone peptide.
Unmodified HGH fragment 176–191 is the native hexadecapeptide Phe-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe, with a disulphide between the two cysteines. Early papers often number the same stretch as hGH 177–191; the difference is whether the N-terminal phenylalanine is counted as residue 176 or the fragment is described as residues 177–191 of the parent hormone. Wu and Ng (1993) reported antilipogenic activity of synthetic 177–191 in adipose assays; Natera, Jiang and Ng (1994) reported reduced cumulative weight gain and adipose mass in obese mice after chronic treatment with that peptide.
AOD-9604 (CAS 221231-10-3) is Tyr-hGH177–191: the same C-terminal stretch with tyrosine in place of phenylalanine at the N-terminus, prepared by solid-phase synthesis and cyclised by disulphide bonding between the two cysteines. Moré (2014) describes the three-dimensional structure as similar to the homologous region of intact hGH. Ogru and colleagues (2000) published a conformational analysis of the cyclic C-terminal peptide. The tyrosine substitution and cyclic constraint were the pharmaceutical design choices that took AOD-9604, rather than the unmodified fragment, into human trials.
The two peptides are therefore related analogues, not synonyms. A certificate of analysis that names one does not identify the other.
2. Preclinical signal: lipolysis without IGF-1
The animal and ex-vivo literature is the strongest part of the file, and it is almost entirely an AOD-9604 / AOD-9401 file.
Ng et al. (2000, Hormone Research) studied AOD9604 as a synthetic lipolytic domain of hGH in obese Zucker rats. Heffernan, Jiang, Thorburn and Ng (2000, American Journal of Physiology — Endocrinology and Metabolism) reported that oral administration of a closely related synthetic fragment (AOD-9401) increased lipolytic activity and decreased lipogenic activity in isolated adipose tissue from obese rodents and humans, with an acute rise in energy expenditure and fat oxidation in ob/ob mice and a reduction of weight gain in obese animals.
Heffernan et al. (2001, Endocrinology) then compared chronic intraperitoneal hGH and AOD9604 in obese mice and in β3-adrenergic-receptor knockout mice. Fourteen days of AOD9604 reduced body weight and body fat in obese wild-type animals and raised repressed β3-AR RNA in fat cells toward lean-mouse levels. The chronic body-weight and lipolysis changes were absent in β3-AR knockout mice, although an acute experiment still showed increased energy expenditure and fat oxidation in the knockouts. The authors concluded that the lipolytic actions of hGH and AOD9604 are not mediated directly through the β3-AR, even though both compounds increase β3-AR expression, which may subsequently contribute to enhanced lipolytic sensitivity.
Across these systems, AOD9604 did not stimulate IGF-1 production and did not induce the insulin resistance associated with intact hGH (Moré, 2014, summarising the preclinical set). That dissociation — C-terminal metabolic activity without somatogenic signalling — is the scientific rationale for the fragment class. It is not evidence that either peptide is an effective anti-obesity intervention in humans.
The unmodified 176–191 / 177–191 fragment contributes the earlier antilipogenic papers (Wu 1993; Natera 1994). It does not have a comparable modern in-vivo programme, and it was not the molecule randomised in the later human studies.
3. Human evidence: a safety file, not an efficacy file
AOD-9604 is unusual among research-chemical fragments because it did enter controlled human trials. Stier, Vos and Kenley (2013) compiled the safety and tolerability dataset. Moré (2014) restated the same programme from a metabolism-and-toxicology angle.
Six human clinical trials were performed, including two intravenous and two oral dosing pilot studies, plus two oral Phase IIb studies in adults with obesity (Moré, 2014). In all of those studies AOD9604 was reported as safe and well tolerated, with a safety profile comparable to placebo and, in the intravenous work, to the recombinant-hGH reference arm (Stier 2013).
The second long-term study, METAOD006, was a randomised, double-blind, placebo-controlled, multicentre, parallel-group trial at 16 Australian hospitals and medical centres. Of 534 enrolled, 502 clinically obese adults (BMI 30–45 kg/m²; median 36.3) were randomised to a daily oral dose of 0.25 mg, 0.5 mg or 1 mg AOD9604 or placebo for 24 weeks (Stier 2013). Serious adverse events occurred in 18 of 502 participants (3.6%) and were not clustered on active treatment in a pattern that the authors judged treatment-related. No anti-AOD9604 antibodies were detected in the assayed subset. Stier concluded that daily oral AOD9604 at the doses studied over 24 weeks was safe and well tolerated, indistinguishable from placebo on the safety endpoints they reported.
That is a safety conclusion. It is not an efficacy conclusion. Moré (2014) states that effects of AOD9604 on weight loss were seen in initial trials but were not seen in the last study, in which an intensive diet and exercise regime was incorporated, and flags the efficacy write-up as “manuscript in preparation.” A completed, peer-reviewed primary-endpoint paper for METAOD006 has not entered the indexed literature. Development of AOD9604 as an anti-obesity drug was terminated in 2007; no marketing authorisation followed in the UK or elsewhere.
The honest grade for AOD-9604 is therefore limited: a real, placebo-controlled human safety database of several hundred participants; a preclinical metabolic mechanism that is internally consistent; and no published demonstration that the fragment reduced body mass versus placebo by a margin that survived a diet-and-exercise-controlled Phase IIb trial.
The honest grade for unmodified HGH fragment 176–191 is limited, tending to anecdotal for any human metabolic claim. The molecule is a documented research peptide with 1990s adipose-assay and obese-mouse data. It was not the investigational product in METAOD006. No randomised human trial of the unmodified hexadecapeptide has been published.
4. What the catalogue confusion costs the reader
Three substitutions of language appear repeatedly in vendor material and should be read as errors, not as evidence.
First, naming. Listing “HGH fragment 176–191 (AOD-9604)” as a single item collapses a tyrosine substitution, a pharmaceutical development history, and a human trial programme onto a different sequence. Certificates of analysis, HPLC traces and bioassays are sequence-specific. A lot labelled 176–191 is not a lot of AOD-9604.
Second, trial inheritance. The Stier 2013 safety compilation and the METAOD006 randomisation apply to oral AOD9604. They do not transfer to the unmodified fragment, to a different salt, or to a different route. Citing “six human trials, ~900 participants” beside a 176–191 product page is a category mistake.
Third, efficacy inflation. A preclinical lipolytic assay and a terminated obesity programme are not the same as a positive Phase III result. Contemporary licensed incretin medicines were not the comparator in these studies; the relevant fact is simpler: the fragment programme did not meet the bar its own sponsor set for an anti-obesity drug, and the efficacy manuscript flagged in 2014 did not appear.
None of this makes the C-terminal-domain hypothesis uninteresting. It does mean the human evidence column is thin, and thinner still for the unmodified fragment.
5. UK regulatory position
Neither AOD-9604 nor HGH fragment 176–191 holds a UK marketing authorisation. Neither is a licensed medicine. Under the Human Medicines Regulations 2012, a product presented as having properties for treating or preventing disease in human beings, or that may be used in or administered to human beings with a view to restoring, correcting or modifying physiological functions by pharmacological action, is a medicinal product. Presentation — label, website copy, implied purpose — is the test. Supply of an unlicensed medicinal product except under the specified exemptions is unlawful. The Medicines Act 1968 remains the parent statute.
Research-use-only reagents, labelled and sold for in-vitro or preclinical laboratory work and not presented for human administration, sit outside that medicines pathway. Relabelling, or marketing copy that implies personal use, can pull the same vial back across the presentation line. Peptide Data does not treat research-chemical availability as a proxy for UK legality of human use, and this article does not discuss personal use.
Separately, the World Anti-Doping Agency 2026 Prohibited List names both molecules under S2.2.3 (growth hormone, its analogues and fragments), with the explicit examples “AOD-9604 and hGH 176-191.” That is an anti-doping classification for athletes subject to the World Anti-Doping Code, including UK Anti-Doping. It is not a Medicines and Healthcare products Regulatory Agency licensing decision and does not convert either peptide into a controlled drug under the Misuse of Drugs Act 1971.
MHRA Drug Safety Updates covering GLP-1 receptor agonists and dual GIP/GLP-1 agonists do not apply to these fragments. They are a different pharmacological class.
6. Evidence grade and what would change it
| Compound | Human randomised evidence | Preclinical metabolic signal | UK licence | Working grade |
|---|---|---|---|---|
| AOD-9604 (Tyr-hGH177–191) | Six trials compiled by Stier 2013; METAOD006 n=502, 24 weeks, safety indistinguishable from placebo; primary weight endpoint not published as a positive result | Consistent lipolytic / antilipogenic and β3-AR-linked findings in obese rodents (Ng 2000; Heffernan 2000, 2001) | None | Limited |
| HGH fragment 176–191 (unmodified) | None published | Antilipogenic and obese-mouse weight-gain data for 177–191 (Wu 1993; Natera 1994) | None | Limited / anecdotal |
A published, pre-specified primary-endpoint analysis of METAOD006 — or a new randomised trial of either peptide against placebo with a body-composition endpoint and a statistical analysis plan — would be required to move AOD-9604. A first-in-human randomised study would be required to move unmodified 176–191 off the anecdotal column for human claims. Until then, the C-terminal fragment class is a documented biochemical hypothesis with one analogue that was tested in people, found tolerable, and not shown in the public literature to meet its obesity endpoint.
Peptide Data grades compounds against published evidence. It does not infer human utility from vendor interchangeability, from WADA listing, or from the existence of a terminated drug programme.
This article is AI-researched and editorially reviewed. It is provided for research and educational purposes only and is not medical advice. Research peptides are not licensed for human consumption in the UK.