AOD-9604 vs HGH Fragment 176-191: how they differ

These two molecules are close relatives, but not the simple one is the other plus one bolt-on that vendor pages often imply. In the peer-reviewed analytical record, AOD-9604 is defined as residues 177-191 of human growth hormone carrying an added N-terminal tyrosine, while HGH Fragment 176-191 is the unmodified residues 176-191 of the same hormone. Line their PubChem records up and they differ by exactly one oxygen atom in the molecular formula and 16 g/mol in mass - the signature not of a bare residue addition but of a residue swap at the N-terminus. Everything below describes what the published records and databases document about these two compounds at the chemistry level, grouped strictly by molecular lineage, not any human or animal use. Kovalabs supplies AOD-9604 and HGH Fragment 176-191 strictly as research reagents for laboratory and in-vitro research only.

Key facts

What is HGH Fragment 176-191?

HGH Fragment 176-191 is exactly what its name announces: a synthetic peptide corresponding to the C-terminal 176-191 region of human growth hormone. Where full-length human growth hormone is a 191-residue protein, this fragment reproduces only the short tail at residues 176 through 191 - the last sixteen positions of the parent sequence, beginning at residue 176 with a phenylalanine. PubChem CID 16131230 resolves it to the molecular formula C78H123N23O22S2 at 1799.1 g/mol, carrying the CAS number 66004-57-7. The two sulfur atoms in that formula are not incidental: the fragment contains one intramolecular cysteine-cysteine disulfide bond, a covalent loop that constrains the peptide's conformation and is part of why the molecule is treated as a defined chemical entity rather than a loose string of residues. The compound is described in full on the HGH Fragment 176-191 research monograph.

What is AOD-9604?

AOD-9604 is a structurally modified analogue of the same C-terminal region, not the bare fragment. Cox HD and colleagues, in the peer-reviewed analytical chemistry record (Drug Testing and Analysis, 2015; epub 2014; PMID 25208511), define AOD-9604 as a peptide consisting of residues 177-191 of human growth hormone with an additional tyrosine residue at the N-terminus. That makes it distinct from the unmodified 176-191 fragment on two counts: it begins at residue 177 rather than 176, and it carries an added N-terminal tyrosine the fragment does not. "AOD" is a historical developer code, and that is all this post will say about the label - the code is an identifier, not a description of anything the molecule does. PubChem CID 71300630 resolves AOD-9604 to the molecular formula C78H123N23O23S2 at 1815.1 g/mol, with CAS number 221231-10-3 and the FDA UNII 7UP768IP4M. AOD-9604 retains the same intramolecular cysteine-cysteine disulfide bond and the same two sulfur atoms as the fragment. The compound is documented separately on the AOD-9604 research monograph.

How do the two molecules compare on paper?

Laid side by side, the comparison is unusually clean - the formulae differ by a single oxygen atom and the masses by 16 g/mol, and everything else about the disulfide topology is shared. No head-to-head study pits one against the other; this is a molecular-lineage comparison, not a performance ranking, and no outcome of either compound is asserted.

The one structural difference: a residue swap at the N-terminus

The entire delta between these two records is one oxygen atom in the molecular formula, and the underlying edit is a swap at the N-terminus rather than a simple append. The fragment runs 176-191 and begins with a phenylalanine residue; AOD-9604 drops residue 176, begins the chain at residue 177, and caps it with an added N-terminal tyrosine. Phenylalanine and tyrosine are near-identical aromatic amino acids - tyrosine is phenylalanine carrying a single phenol hydroxyl group, which is exactly one extra oxygen. That is why the carbon, hydrogen, nitrogen and sulfur counts are identical across the two formulae and only the oxygen count moves, from O22 to O23, lifting the mass from 1799.1 to 1815.1 g/mol. A bare residue addition would change the carbon count too; the fact that it does not is the tell that the N-terminal phenylalanine has effectively been exchanged for a hydroxyl-bearing tyrosine. Both molecules keep the same cysteine-cysteine disulfide loop and the same two sulfur atoms, so the disulfide topology is shared and only the N-terminal residue differs.

A practical note for anyone cross-referencing databases: because the two share most of the same region and very similar masses, the safest discriminator is not the name or the approximate molecular weight but the exact molecular formula and CAS pairing. C78H123N23O22S2 with CAS 66004-57-7 is the unmodified fragment; C78H123N23O23S2 with CAS 221231-10-3 is the modified analogue. Mixing those pairings is the kind of clerical slip that quietly propagates through vendor listings, and a single oxygen atom in the formula is the cheapest way to catch it.

Solid on chemistry, thin on either molecule in people

The answer is different at each level, so take them one at a time.

Well-established: the chemistry. The molecular formulae, weights, CAS numbers, PubChem CIDs and the residue-swap relationship are documented and independently checkable. That AOD-9604 is the 177-191 region plus an N-terminal tyrosine, distinct from the 176-191 fragment, is a structural fact recorded in the peer-reviewed analytical literature, not an interpretation. This tier is solid.

The middle ground: in-vitro and animal work. Where these compounds have been studied, the literature sits largely in cell and animal models, and most of it concerns the AOD-9604 analogue rather than the unmodified fragment. A molecule characterised in a dish or a rodent is a molecule characterised in a dish or a rodent - informative about chemistry and binding, silent about what it would do in a person. Any reported study should be read for its design and the endpoint it named, never for an effect.

The weakest part: human evidence. Robust, replicated human-outcome data for either compound is thin to absent. Neither is a licensed medicine and neither has been shown to produce defined outcomes in humans.

How are these peptides handled in the laboratory?

Both are supplied as lyophilised (freeze-dried) powder for laboratory reconstitution. As disulfide-bonded peptides, reconstitution means directing an appropriate laboratory solvent into the vial gently, and this is bench handling of a reagent, not a preparation method for any use in humans or animals. For storage, lyophilised peptide is held desiccated and cold: longer-term at -20 C or colder for the freeze-dried solid, short-term at 2 to 8 C. None of this is a preparation method for any use in humans or animals.

Identity and purity should be verifiable rather than assumed, which matters doubly here because the two compounds differ by a single oxygen atom in the formula. Every Kovalabs batch ships with a certificate of analysis, so the mass of the material in hand can be checked against the documented identifiers - 1799.1 g/mol and C78H123N23O22S2 for the fragment, 1815.1 g/mol and C78H123N23O23S2 for the modified analogue - rather than taken on trust. Cross-check the full monographs for each compound at HGH Fragment 176-191 research and AOD-9604 research.

Research use only

AOD-9604 and HGH Fragment 176-191 are supplied by Kovalabs for laboratory and in-vitro research only. They are not medicines, not supplements, and not for human or veterinary use, and nothing on this page describes a dose, a route, a schedule or an outcome. They are research compounds that have not been evaluated by the MHRA or any comparable regulator for safety or efficacy in humans or animals. Every batch is third-party tested with a certificate of analysis. See the full research disclaimer for terms.

Frequently asked questions

What is the difference between AOD-9604 and HGH Fragment 176-191?

AOD-9604 is residues 177-191 of human growth hormone with an added N-terminal tyrosine, while HGH Fragment 176-191 is the unmodified residues 176-191. So the analogue starts one residue later and carries an extra N-terminal tyrosine the fragment does not - in net composition, an N-terminal residue swap rather than a simple addition. Both share the same intramolecular cysteine-cysteine disulfide bond. The edit shows up as a single oxygen atom in the molecular formula (C78H123N23O23S2 for AOD-9604 against C78H123N23O22S2 for the fragment) and a 16 g/mol shift in molecular weight (1815.1 vs 1799.1 g/mol).

Is AOD-9604 the same molecule as HGH Fragment 176-191?

No - they are related but distinct molecules. AOD-9604 is a modified analogue spanning residues 177-191 with an added N-terminal tyrosine, so it shares most of the same region but is not the unmodified fragment. They have separate CAS numbers (221231-10-3 for AOD-9604, 66004-57-7 for the fragment) and separate PubChem records (CID 71300630 and CID 16131230 respectively). Treating data on one as if it characterised the other is a misidentification.

What does the AOD in AOD-9604 stand for?

"AOD" is a historical developer code attached to the compound during its development. It functions as an identifier label only, and its expansion is not reproduced here because a code is not a description of what the molecule does. For identity purposes the reliable handles are the CAS number (221231-10-3), the PubChem CID (71300630) and the UNII (7UP768IP4M).

How can I tell the two compounds apart reliably?

By exact molecular formula and CAS, not by name or approximate mass. The unmodified fragment is C78H123N23O22S2 (CAS 66004-57-7, PubChem CID 16131230); the modified analogue is C78H123N23O23S2 (CAS 221231-10-3, PubChem CID 71300630). The two differ by a single oxygen atom in the formula and 16 g/mol in mass, so a certificate of analysis is the cleanest way to confirm which one is in the vial.

How should these peptides be stored in the laboratory?

As bench handling of research reagents only. Lyophilised peptide is kept desiccated and cold, with longer-term storage at -20 C or colder and short-term holding at 2 to 8 C. Reconstitution is a gentle dissolution step using an appropriate laboratory solvent. None of this is a preparation method for any use in humans or animals.

Are either of these compounds approved for any use?

No. Neither AOD-9604 nor HGH Fragment 176-191 has been approved as a medicine by the MHRA or any comparable regulator. Kovalabs supplies both strictly as research reagents for laboratory and in-vitro use, not for human or veterinary use.

Sources

1. PubChem CID 16131230 (HGH Fragment 176-191, C78H123N23O22S2, 1799.1 g/mol, CAS 66004-57-7)
2. PubChem CID 71300630 (AOD-9604, C78H123N23O23S2, 1815.1 g/mol, CAS 221231-10-3, UNII 7UP768IP4M)
3. Cox HD et al. Detection and in vitro metabolism of AOD9604. Drug Test Anal 2015 (PMID 25208511; defines AOD-9604 as hGH residues 177-191 plus an N-terminal tyrosine)
4. HGH Fragment 176-191 research monograph
5. AOD-9604 research monograph