HGH Fragment 176-191: Molecular Structure

Molecular Structure and Properties#

HGH Fragment 176-191 is a peptide whose molecular structure and properties have been characterized through analytical chemistry and structural biology studies.

Amino Acid Sequence#

We describe HGH Fragment 176–191 (the C-terminal 16-residue segment of the mature 191-aa human growth hormone) in terms of sequence, structure, and physicochemical features, using primary literature for the residue mapping and fragment context.

Molecular structure and sequence

• Sequence and location: Residues 176–191 of mature hGH correspond to the one-letter sequence PLRIVQCRSVEGSCGP, derived from a residue-by-residue mapping of hGH and situated at the extreme C terminus. The segment contains Cys182 and Cys189, which form a disulfide bond in the intact hormone, establishing a loop within the C-terminal region (sequence and disulfide reported in Li 2004). Independent biological studies commonly use the closely related hGH(177–191) peptide (AOD-9401), which establishes the functional context of this C-terminal region in metabolic assays (Heffernan 2000).

Physicochemical properties

• Length and composition: The fragment is 16 amino acids long (PLRIVQCRSVEGSCGP) and comprises 2 basic residues (Arg2 total across the segment), 1 acidic residue (Glu), 3 polar uncharged residues (Gln, Ser, Ser), 2 cysteines (positions corresponding to 182 and 189 of hGH), and the remainder hydrophobic residues (including Pro and Gly).
• Isoelectric point (pI) and charge distribution: Using standard residue/terminus pKa values and Henderson–Hasselbalch calculations with free termini, the fragment’s net charge is mildly positive at acidic pH and approaches neutral to slightly negative near physiological pH in the reduced state (free thiols). In the oxidized state (cystines), cysteines do not contribute to deprotonation, shifting net charge to be more positive at higher pH. Estimates are provided in the embedded artifact; the sequence context and fragment definition are grounded in Li 2004 and Heffernan 2000.

Structural features

• Context in full hGH: Human GH is a class I cytokine with a four-helix bundle core; residues 176–191 constitute the distal C-terminal tail. The Cys182–Cys189 disulfide defines a looped element within this tail in the intact protein, and enzymatic cleavage studies place 176–191 within the COOH-terminal region used in fragment bioactivity assays (Li 2004; Heffernan 2000).
• Secondary-structure propensity: The presence of Pro (2) and Gly (2) alongside polar and small residues suggests that the isolated peptide is conformationally flexible, favoring loop/turn conformations over stable helices in solution; within the intact protein, the disulfide constraints the local loop.

Key data summary

Notes and assumptions for computed properties: The isoelectric points and net charges were estimated using typical pKa values (N-terminus ~8.0, C-terminus ~3.1, Arg 12.5, Glu 4.1, Cys 8.5) and Henderson–Hasselbalch equations, assuming free N- and C-termini and either reduced thiols or intramolecular disulfide. These values are estimates for the isolated peptide; actual values can be modulated by sequence context, local microenvironment, and ionic strength.

Stability and Formulation#

We synthesized what is known about the stability of HGH Fragment 176-191 (AOD9604) from experimental studies and analytical method papers. A summary table is embedded.

pH stability

• Direct pH–dependent stability data on AOD9604 are scarce. However, an hGH C‑terminal fragment analogue (AOD‑9401) was stable in Krebs–Ringer bicarbonate buffer at pH 7.4 with less than 5% loss during 16 h at 37 °C, suggesting neutral pH is not intrinsically destabilizing for closely related sequences. AOD9604 solutions used in studies did not measurably alter formulation pH or osmolarity, though precipitation occurred at higher concentrations, indicating solubility limits rather than pH instability.

Temperature sensitivity

• Biological matrices: AOD9604 is temperature‑labile in urine and plasma. In urine, intact peptide stability is poor at room temperature (<1 day) and only marginal at 2–8 °C (~1 day), whereas storage at −20 °C preserves signal for >20 days. In vitro in rat plasma at room temperature, the peptide shows an ~4 min half‑life with intact peptide undetectable by ~56 min, underscoring the need for rapid cooling/processing.

Degradation pathways

• Plasma/metabolism: The dominant pathway is rapid, sequential N‑terminal truncation in vivo and in vitro. After IV dosing in pigs, the principal circulating degradants lack two or three N‑terminal residues; truncations of up to 1–6 residues were observed, with no evidence of C‑terminal cleavage. The intact IV half‑life was ~3 min, far shorter than hGH, consistent with proteolysis. These findings imply that quantification strategies should consider both intact peptide and characteristic truncants.
• Gastrointestinal enzymes (proxy): For a related hGH fragment (AOD‑9401), pepsin and trypsin half‑lives were ~50 and ~170 minutes, respectively, suggesting moderate resistance to major GI proteases depending on sequence modifications; this contextualizes oral stability expectations but is not a direct measurement for AOD9604.

Formulation considerations

• Solution handling: In toxicology/PK studies, AOD9604 formulations were prepared fresh weekly, stored at 4 °C, and sometimes used PEG‑400 as vehicle; precipitation occurred at higher concentrations, indicating the need to verify solubility at intended dose strength and select appropriate excipients. Operationally, solutions were kept refrigerated between uses, consistent with mitigating temperature‑driven degradation and solubility issues.
• Sample preservation for analytics: Because of rapid proteolysis, urine and plasma samples should be processed promptly and frozen (≤ −20 °C) to limit loss of intact peptide; inclusion of protease‑inhibitor cocktails and rapid protein precipitation can further suppress ex vivo degradation.

Evidence gaps and practical guidance

• The literature provides strong evidence for temperature sensitivity in matrices and for N‑terminal truncation as the primary metabolic degradation route. However, direct AOD9604 pH‑stability profiling, quantitative temperature‑dependence across buffers, and chemical degradation routes such as deamidation or oxidation were not reported in the available sources. Until such data are available, practical formulation approaches include: using lyophilized drug substance for long‑term storage; preparing solutions shortly before use; maintaining refrigerated storage; selecting buffers near neutral pH with adequate ionic strength and adding solubilizers (e.g., PEGs) if needed; and minimizing hold times at ambient temperature during manufacturing and handling.

Pharmacokinetics#

HGH Fragment 176–191 (AOD9604) is a C‑terminal peptide of human growth hormone developed for metabolic indications. Pharmacokinetic evidence is predominantly preclinical, with no registered human PK trials identified.

Absorption and bioavailability

• Oral absorption is demonstrable in rats and pigs. In rats given 14C‑AOD9604 by oral gavage, tissue radioactivity peaked at ~30 min and plasma radioactivity was detectable by ~15 min; oral availability was estimated at ~40% based on radiolabel distribution. In pigs, oral gavage (2 mg/kg) produced measurable systemic exposure with Tmax ~60 min and authors concluded AOD9604 is orally bioavailable; oral AUCs exceeded IV AUCs in some experiments, which the authors attributed to prolonged uptake or experimental factors and therefore interpreted cautiously.

Distribution

• Whole‑body autoradiography after IV or oral 14C‑AOD9604 in rats showed rapid systemic distribution with prominent early localization to pancreas, pineal body, thyroid, liver, and kidney cortex; CNS distribution was low in early time points. Similar organ localization was seen after either route.

Metabolism

• AOD9604 undergoes rapid enzymatic degradation with sequential N‑terminal truncation in vitro and in vivo. In rat plasma in vitro, intact peptide declined with a half‑life of ~4 min and was largely absent by ~56 min; mass spectrometry in pigs identified −2 and −3 amino-acid N‑terminal truncation products as predominant in vivo metabolites, which retain reduced activity.

Elimination and half‑life

• In pigs, IV dosing (400 µg/kg) showed very rapid plasma clearance with a terminal half‑life of ~3 min and near‑complete plasma disappearance by ~12 min. In vitro rat plasma also showed rapid loss of intact peptide (t1/2 ~4 min), consistent with fast clearance observed in vivo. Specific excretion routes/percent recoveries were not reported in the cited excerpts.

Human data

• No human pharmacokinetic trials were identified in a focused ClinicalTrials.gov search for AOD9604/HGH Fragment 176–191 with PK/basic-science filters. The reviewed safety/metabolism article did not provide quantitative human PK parameters.

Key quantitative parameters (preclinical)

• Pig IV: t1/2 ~3 min; Tmax ~2 min; near‑complete plasma clearance by ~12 min.
• Pig oral (2 mg/kg): Tmax ~60 min; measurable exposure; higher AUC than IV in some experiments (interpret with caution).
• Rat oral (14C): tissue Tmax ~30 min; plasma detectable by ~15 min; oral availability estimated ~40%.
• Rat plasma in vitro: t1/2 of intact peptide ~4 min with rapid N‑terminal truncation.

Limitations

• Quantitative human PK parameters (absolute oral bioavailability, volume of distribution, clearance, excretion routes) were not found in registered trials or the reviewed publication excerpts. Preclinical AUC comparisons suggesting high oral exposure in pigs may reflect prolonged uptake or methodological factors and should be interpreted with caution.

Summary

• AOD9604 exhibits rapid absorption after oral dosing in rodents and pigs (Tmax ~30–60 min), rapid distribution to peripheral metabolic organs, and very fast enzymatic degradation with short systemic half‑lives after IV administration (minutes). Oral bioavailability is supported by radiolabel and exposure data in animals (rat estimate ~40%; pigs with demonstrable systemic exposure), but definitive human PK/bioavailability data were not identified.

Artifact summary table follows.

Related Reading#

• HGH Fragment 176-191 overview and research guide
• Peptides similar to HGH Fragment 176-191
• HGH Fragment 176-191 research evidence