AOD-9604

Compendial Identification

AOD-9604 is a synthetic peptide corresponding to amino acid residues 176–191 of the human growth hormone polypeptide with the addition of an N-terminal tyrosyl residue to improve stability. The sequence Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe contains a disulfide bridge between cysteines 7 and 14 that constrains the molecule into a cyclic conformation. The peptide is registered under CAS number 221231-10-3 and exhibits a monoisotopic mass of 1815.83 Da.

Reference Standard Considerations

Identity confirmation of AOD-9604 reference material includes mass spectrometric verification of the intact peptide and verification of disulfide bond integrity by reduction/alkylation followed by mass shift analysis. The presence of two cysteine residues introduces the analytically important consideration of free-thiol contamination from incomplete oxidative folding during synthesis; reference grade material is specified at ≤ 1.0% free-thiol species relative to the cyclized parent. Original characterization studies describe the relationship between the cyclic conformation and the lipolytic pharmacological activity that distinguishes this fragment from full-length growth hormone.

Because AOD-9604 is not currently the subject of a USP or Ph. Eur. monograph, source qualification for reference material falls outside the formal compendial framework and must be evaluated against the documentary expectations that would apply if a monograph existed. The Delta Peptides Scientific Affairs compendial compliance audit records the six-criterion framework applied to research peptide reference-material sources, including the disulfide-integrity-relevant identification testing methodology required of any qualifying source.

Pharmacological Mechanism

AOD-9604 was developed to isolate the lipolytic activity of the human growth hormone molecule from its metabolic and somatogenic effects. The peptide stimulates lipolysis and inhibits lipogenesis in adipocytes through a mechanism that appears independent of the growth hormone receptor and that does not produce sustained elevations in serum insulin-like growth factor-1 (IGF-1). The proposed signalling cascade involves activation of beta-3 adrenergic receptor pathways or related G-protein-coupled receptors, with downstream activation of hormone-sensitive lipase and inhibition of acetyl-CoA carboxylase.

Adipocyte Selectivity

In cultured adipocytes and animal models of obesity, AOD-9604 administration produces measurable reductions in fat mass without the carbohydrate intolerance, hepatic gluconeogenic effects, or organ growth stimulation associated with full-length growth hormone administration. The pharmacological dissociation between lipolytic activity and somatogenic activity is the central scientific rationale for ongoing investigation of the fragment as a research tool for studying adipose biology.

Comparison with Other Metabolic Peptides

AOD-9604 differs mechanistically from growth hormone secretagogues such as GHRP-6, hexarelin, and ipamorelin, which act centrally to stimulate endogenous growth hormone release with consequent rises in IGF-1, lipolysis, and protein synthesis. Where secretagogues amplify the entire somatotropic axis, AOD-9604 isolates a single downstream effect (lipolysis) while leaving central endocrine architecture undisturbed. This distinction has implications for the design of comparative metabolic research protocols.

Investigational Research Context

AOD-9604 has been investigated in clinical studies for obesity management and, more recently, in osteoarthritis research where local administration may influence cartilage metabolism. The peptide has not been approved as a pharmaceutical product, although it has been classified as a generally recognized as safe (GRAS) ingredient by the U.S. FDA in specific food and supplement contexts. Reference-grade material is supplied exclusively for analytical and in vitro research applications.

Stability and Reconstitution

Lyophilized AOD-9604 is stable for 24–36 months when stored at −20 °C under desiccated, light-protected conditions. The principal degradation pathway is reduction of the Cys7–Cys14 disulfide bond, particularly in reconstituted aqueous solutions exposed to trace reductants or oxygen. Bacteriostatic water reconstitutions retain analytical purity (≥ 95.0%) for 14–21 days at 2–8 °C; longer-term storage is not recommended due to disulfide instability. The storage monograph details validated container-closure conditions for reference material.

Selected References

• Ng FM, Sun J, Sharma L, et al. Metabolic studies of a synthetic lipolytic domain (AOD9401) of human growth hormone. Horm Res. 2000;53(6):274-278. PMID 11146367
• Heffernan MA, Thorburn AW, Fam B, et al. Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment. Int J Obes. 2001;25(10):1442-1449. PMID 11673763
• Stier H, Vos E, Kenley D. Safety and tolerability of the hexadecapeptide AOD9604 in humans. J Endocrinol Metab. 2013;3(1-2):7-15. Journal of Endocrinology and Metabolism

Detailed Analytical Methods Reference

The reference analytical methods used to characterize AOD-9604 reference material are summarized below in the format used by major pharmacopoeias for related research peptides. These methods provide the basis for routine lot release testing and for the resolution of investigational questions concerning identity, purity, and impurity profile.

Reversed-Phase HPLC for Purity Determination

Routine purity assessment is performed using reversed-phase HPLC on a C18 column (250 × 4.6 mm, 5 μm particle size) at 40 °C. A gradient elution of 0.1% trifluoroacetic acid in water (mobile phase A) and 0.1% trifluoroacetic acid in acetonitrile (mobile phase B) provides baseline resolution of the parent peptide from common synthesis-related impurities. UV detection at 220 nm captures the peptide bond absorbance, with secondary detection at 280 nm providing identification of tyrosine and tryptophan-containing impurities. System suitability requires resolution > 2.0 between the parent peptide and the most closely eluting related substance, with peak tailing factor ≤ 1.5 for the parent peak.

Mass Spectrometric Identity Confirmation

Identity confirmation is performed by LC-ESI-MS with the system tuned for positive ion detection. The doubly charged ion [M+2H]²⁺ at m/z 909.4 is the most abundant species under standard conditions, with the triply charged [M+3H]³⁺ at m/z 606.6 providing confirmatory mass measurement. Tandem mass spectrometry following collision-induced dissociation generates a characteristic b-ion and y-ion series consistent with the published sequence; complete sequence coverage is achieved using a combination of trypsin and Asp-N digestions.

Disulfide Bond Integrity

The intramolecular disulfide bond between Cys7 and Cys14 is a critical structural feature of AOD-9604. Integrity is verified by Ellman's reagent assay for free thiol content (specification: < 1.0% molar free-thiol relative to parent), supplemented by mass spectrometric analysis following reduction with tris(2-carboxyethyl)phosphine and alkylation with iodoacetamide. The resulting mass shift of 116 Da per cysteine confirms quantitative reduction and provides a rigorous test of disulfide bond integrity.

Compendial Impurity Profile