The comparison of AOD-9604 vs HGH Fragment 176-191 comes down to a single atom. Both are 16-residue peptides corresponding to the C-terminal region of the 191-amino-acid human growth hormone molecule; AOD-9604 carries tyrosine where the native fragment carries phenylalanine, a change of one hydroxyl group and 16 Da. That small difference separates two very different evidence bases, and this guide sets out the structures, the verified identity data and what the published literature actually reports for each.
Key takeaways
- HGH Fragment 176-191 is FLRIVQCRSVEGSCGF, CAS 66004-57-7, C78H123N23O22S2, MW 1799.1 Da — the last 16 residues of somatropin.
- AOD-9604 is YLRIVQCRSVEGSCGF, CAS 221231-10-3, C78H123N23O23S2, MW 1815.1 Da — the same sequence with Phe→Tyr at position 1.
- The 16 Da mass difference is one oxygen atom: tyrosine is phenylalanine with a para-hydroxyl. It is also the easiest way to tell the two apart on a certificate of analysis.
- Ng and colleagues (Hormone Research, 2000) and Heffernan and colleagues (Endocrinology, 2001) published the core preclinical work on AOD9604, the latter reporting that effects on lipid metabolism depended on the β3-adrenergic receptor in knockout mice.
- AOD-9604 progressed to human trials; a 12-week study showed a modest separation from placebo but a later 24-week study did not, and development was discontinued in 2007.
- The native 176-191 fragment has a much thinner literature, and reported lipolytic properties are frequently extrapolated from AOD9604 data rather than measured directly.
- Neither peptide contains the growth hormone receptor-binding surface; published work reports AOD9604 does not stimulate IGF-1 in the way intact growth hormone does.
AOD-9604 vs HGH Fragment 176-191 at a glance
Both compounds are hexadecapeptides derived from the C-terminus of human growth hormone. HGH Fragment 176-191 is the native sequence taken verbatim from residues 176 to 191. AOD-9604 is the same 16-residue span with the N-terminal phenylalanine replaced by tyrosine, giving a molecule that is 16 Da heavier and carries one additional hydroxyl group.
That is the entire structural difference. Everything else — the two cysteines, the disulfide, the charge distribution, the length — is identical. The divergence between them is historical rather than chemical: AOD-9604 was developed as a pharmaceutical candidate and accumulated a trial record, while the unmodified fragment did not.
Where the fragments come from in the 191-residue hormone
Human growth hormone is a 191-amino-acid single-chain protein of roughly 22 kDa folded into a four-helix bundle and stabilised by two disulfide bridges. The receptor-binding determinants that drive somatogenic signalling — and hence hepatic IGF-1 induction — are distributed across surfaces contributed by several helices.
The last 16 residues sit at the C-terminal tail. Excising them removes the receptor-binding architecture entirely, which is why neither peptide behaves as a growth hormone receptor agonist. What the region does retain, in the published animal work, is an association with lipid-metabolism effects that can be observed without the somatogenic effects of the intact hormone.
The two cysteines in the fragment correspond to Cys182 and Cys189 of the parent hormone, which form one of its two disulfide bridges. That the disulfide is fully contained within the excised span is part of why the fragment folds into a defined structure rather than remaining a floppy linear chain.
The one-residue difference and why it exists
The short answer: AOD-9604 replaces the fragment’s N-terminal phenylalanine with tyrosine. Because tyrosine is phenylalanine plus a para-hydroxyl, the peptide gains a single oxygen atom — formula C78H123N23O22S2 becomes C78H123N23O23S2, and mass rises from 1799.1 Da to 1815.1 Da.
The modification is often described in the literature as adding an N-terminal tyrosine to hGH(177-191), which is an equivalent way of describing the same 16-residue molecule. Both descriptions appear in the source literature and refer to identical material.
A tyrosine at the N-terminus provides a defined radiolabelling handle and a chromophore that simplifies quantification, both practical advantages for a compound intended for pharmaceutical development.
What the published research reports
The evidence bases are asymmetric. AOD9604 has a documented preclinical and clinical development history; the unmodified fragment does not, and much of what is claimed for it is transferred from AOD9604 papers.
Preclinical work on AOD9604
Ng and colleagues published metabolic studies of the synthetic lipolytic domain in Hormone Research in 2000. Heffernan and colleagues followed in Endocrinology in 2001 with a chronic-treatment study in obese mice and β3-adrenergic receptor knockout mice, comparing human growth hormone with AOD9604. The knockout arm is the informative part of that design: it tested whether the observed effects on lipid metabolism required an intact β3-adrenergic pathway.
Human studies of AOD9604
AOD9604 entered clinical development as an obesity candidate. A 12-week study reported a modest separation from placebo in body-weight change; a subsequent 24-week study did not reproduce it, and the programme was discontinued in 2007. Stier, Vos and colleagues later published a safety and tolerability review of the hexadecapeptide in the Journal of Endocrinology and Metabolism (2013).
Published accounts also report that AOD9604 retained lipolytic properties without stimulating IGF-1 in the manner intact growth hormone does — consistent with a peptide that lacks the receptor-binding surface.
The 176-191 fragment
By contrast, the unmodified fragment has not been the subject of a comparable human programme. Reference sources note explicitly that its presentation as a lipolytic peptide rests substantially on extrapolation from AOD9604 data rather than on direct measurement of the native sequence.
| Property | AOD-9604 | HGH Fragment 176-191 |
|---|---|---|
| CAS number | 221231-10-3 | 66004-57-7 |
| Sequence | YLRIVQCRSVEGSCGF | FLRIVQCRSVEGSCGF |
| Molecular formula | C78H123N23O23S2 | C78H123N23O22S2 |
| Molecular weight | 1815.1 Da | 1799.1 Da |
| Residues | 16 | 16 |
| Position 1 | Tyrosine | Phenylalanine |
| Cysteines | 2 (Cys182/Cys189 equivalents) | 2 (Cys182/Cys189 equivalents) |
| Relationship to hGH | hGH(176-191) with Phe→Tyr, or Tyr-hGH(177-191) | Native hGH(176-191) |
| GH receptor binding surface | Absent | Absent |
| Human trial record | Phase 2 obesity programme, discontinued 2007 | No comparable programme published |
| Class label | GH-derived fragment | GH-derived fragment |
The proposed mechanism and its limits
The mechanism proposed in the AOD9604 literature centres on the β3-adrenergic receptor rather than the growth hormone receptor. The Heffernan knockout study is the key piece of evidence: removing β3-adrenergic signalling changed the metabolic response to the peptide in that model.
Two caveats limit how far this can be taken. First, β3-adrenergic receptor biology in adipose tissue differs substantially between rodents and humans, and a mechanism demonstrated in mice does not transfer automatically. Second, a defined high-affinity receptor for either peptide has not been established; the mechanism is described in terms of a downstream pathway rather than a binding site.
The failure of the 24-week clinical study is consistent with that uncertainty. A compound with a clean preclinical mechanism and a modest short-term human signal that does not persist is a common pattern when the animal mechanism does not map cleanly onto human physiology.
Specifications and verified identity data
Both peptides are short enough that standard analytical methods resolve them cleanly. The mass difference of 16 Da is comfortably within the resolution of routine electrospray mass spectrometry.
| Mass in vial | Diluent | Concentration | Per 0.1 mL | Molarity if AOD-9604 (1815.1) | Molarity if Fragment (1799.1) |
|---|---|---|---|---|---|
| 2 mg | 1 mL | 2 mg/mL | 200 mcg | 1.10 mM | 1.11 mM |
| 2 mg | 2 mL | 1 mg/mL | 100 mcg | 0.551 mM | 0.556 mM |
| 5 mg | 1 mL | 5 mg/mL | 500 mcg | 2.75 mM | 2.78 mM |
| 5 mg | 2 mL | 2.5 mg/mL | 250 mcg | 1.38 mM | 1.39 mM |
| 5 mg | 3 mL | 1.67 mg/mL | 167 mcg | 0.919 mM | 0.927 mM |
| 10 mg | 2 mL | 5 mg/mL | 500 mcg | 2.75 mM | 2.78 mM |
| 10 mg | 3 mL | 3.33 mg/mL | 333 mcg | 1.84 mM | 1.85 mM |
Telling the two apart on a COA
Three checks separate AOD-9604 from HGH Fragment 176-191 on a certificate of analysis, and any one of them is decisive.
- The observed mass. AOD-9604 should confirm at 1815.1 Da; the native fragment at 1799.1 Da. A 16 Da discrepancy is the whole identity question, not a rounding error.
- The CAS number. 221231-10-3 for AOD-9604, 66004-57-7 for the fragment. Salt forms are usually noted separately as trifluoroacetate.
- The stated sequence. Position 1 is Y for AOD-9604 and F for the fragment. Residues 2 to 16 are identical, so only the first letter differs.
- The peptide content figure. Both are commonly supplied as TFA salts, so net peptide content is below the gross vial mass. Read purity and net content as separate numbers.
Handling, reconstitution and stability
Both peptides contain two cysteines, and cysteine is the most oxidation-sensitive residue in common use. That single feature drives most of the handling requirements for this pair.
Free thiols will slowly form intermolecular disulfides in solution, producing dimers that show as a late-eluting shoulder on reversed-phase HPLC. If a re-analysed working stock shows a new peak of roughly twice the expected mass, oxidative dimerisation is the first thing to check.
Limitations of the evidence
The most important limitation is the one this guide has returned to throughout: two molecules, one evidence base. AOD9604 carries the preclinical and clinical record; the unmodified fragment carries almost none, and treating the two as interchangeable is not supported.
The second limitation is the clinical outcome itself. AOD9604’s development ended because a 24-week study did not reproduce a 12-week result. That is a negative finding, and it belongs in any honest account of the compound.
Third, no defined receptor has been established for either peptide. The β3-adrenergic dependence reported in knockout mice describes a pathway requirement, not a binding site, and the species differences in that receptor’s adipose biology are substantial.
AOD-9604 and HGH Fragment 176-191 in the GenoPept store
Frequently asked questions
What is the difference between AOD-9604 and HGH Fragment 176-191?
One amino acid. Both are 16-residue peptides from the C-terminus of human growth hormone. HGH Fragment 176-191 is the native sequence FLRIVQCRSVEGSCGF at 1799.1 Da; AOD-9604 is YLRIVQCRSVEGSCGF at 1815.1 Da, with tyrosine substituted for phenylalanine at position 1. The 16 Da gap is one added oxygen atom.
Why does AOD-9604 weigh 16 Da more?
Because tyrosine is phenylalanine with an added para-hydroxyl group. Substituting one for the other adds a single oxygen atom, changing the molecular formula from C78H123N23O22S2 to C78H123N23O23S2 and the molecular weight from 1799.1 Da to 1815.1 Da. Nothing else in the sequence changes.
Do either of these peptides bind the growth hormone receptor?
No. The receptor-binding surfaces of human growth hormone are formed by residues distributed across several helices of the 191-amino-acid molecule, and none of that architecture survives in a 16-residue C-terminal fragment. Published accounts report that AOD9604 does not stimulate IGF-1 in the way intact growth hormone does, which is consistent with the absent binding surface.
What happened to the AOD-9604 clinical programme?
It was discontinued in 2007. A 12-week study reported a modest separation from placebo in body-weight change, but a subsequent 24-week study did not reproduce that result and development stopped. Stier, Vos and colleagues later published a safety and tolerability review of the hexadecapeptide in the Journal of Endocrinology and Metabolism in 2013.
Has HGH Fragment 176-191 itself been studied in humans?
Not in a comparable programme. Reference sources note that the fragment’s reputation as a lipolytic peptide rests largely on extrapolation from AOD9604 studies rather than on direct investigation of the native sequence. When reading claims about the fragment, it is worth checking which molecule the cited study actually used.
What is the proposed mechanism for AOD-9604?
The published preclinical account centres on the beta-3 adrenergic receptor rather than the growth hormone receptor. Heffernan and colleagues compared human growth hormone and AOD9604 in obese mice and in beta-3 adrenergic receptor knockout mice, and the knockout arm indicated that the metabolic response depended on that pathway. No defined high-affinity receptor for the peptide has been established.
How should these peptides be stored?
Lyophilised vials are held at −20 °C, protected from light and moisture. Both peptides contain two cysteines, so oxidation is the main degradation route: avoid reducing agents, minimise air exposure in solution, and aliquot reconstituted stocks to prevent repeated freeze-thaw. Refrigerate working solutions at 2–8 °C.
Why might a stored solution show an extra HPLC peak?
Most often oxidative dimerisation. Free thiols on the cysteine residues can form intermolecular disulfides over time in solution, producing a species of roughly twice the monomer mass that elutes later on a reversed-phase column. Confirming the mass of the new peak is the quickest way to distinguish a dimer from an unrelated impurity.
References
- Ng FM, Sun J, et al. Metabolic studies of a synthetic lipolytic domain (AOD9604) of human growth hormone. Hormone Research. 2000;53(6):274–278. PubMed
- Heffernan MA, et al. Effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and β3-AR knock-out mice. Endocrinology. 2001;142(12):5182–5189. PubMed
- Stier H, Vos E, et al. Safety and tolerability of the hexadecapeptide AOD9604 in humans. Journal of Endocrinology and Metabolism. 2013;3(1–2):7–15. Journal of Endocrinology and Metabolism
- Steyn FJ, Tolle V, Chen C, Epelbaum J. Neuroendocrine regulation of growth hormone secretion. Comprehensive Physiology. 2016;6(2):687–735. Wiley
Research-grade AOD-9604 and HGH Fragment 176-191
GenoPept supplies AOD-9604 and HGH Fragment 176-191 as lyophilised vials with a per-batch third-party certificate of analysis covering HPLC purity and mass confirmation — the mass figure that distinguishes the two — dispatched from the UK strictly for laboratory research.