HGH Fragment 176-191 Peptide

HGH Fragment 176-191, also known as AOD (anti-obesity drug precursor) sequence, corresponds to the C-terminal lipolytic domain of human growth hormone (hGH residues 176-191). Ng and colleagues initially demonstrated that this short C-terminal region retains the fat-reducing activity of intact hGH but is dissociated from its anabolic and diabetogenic effects, establishing the conceptual basis for developing a 'pure lipolytic' GH-derived peptide (Ng et al., 1990; PMID 2123181).

Mechanism of action: Fragment 176-191 stimulates lipolysis primarily through upregulation and sensitization of β3-adrenergic receptors (β3-AR) on white and brown adipocyte plasma membranes. β3-AR activation elevates intracellular cAMP via Gs-coupled adenylate cyclase, activating protein kinase A (PKA), which in turn phosphorylates hormone-sensitive lipase (HSL) and perilipin-1 on the lipid droplet surface. Phosphorylated perilipin permits HSL access to the triacylglycerol core, while concurrent activation of adipose triglyceride lipase (ATGL) accelerates the rate-limiting first step of triglyceride hydrolysis. The net result is enhanced free fatty acid (FFA) release and increased β-oxidation flux.

Preclinical findings: In genetically obese (ob/ob) and diet-induced obese mouse models, chronic administration of Fragment 176-191 produced substantial reductions in adipose mass (commonly reported in the 30-50% range over 2-3 weeks) without measurable changes in food intake, lean body mass, plasma glucose, or insulin sensitivity. Critically, IGF-1 levels remained unchanged, indicating absence of GH receptor/JAK2-STAT5 signaling — the pathway responsible for hGH's anabolic and pro-diabetogenic effects. This pharmacological dissociation is a defining feature distinguishing C-terminal fragments from full-length GH.

Adipocyte and mitochondrial effects: In vitro studies on isolated rodent and human adipocytes demonstrated dose-dependent glycerol release in response to Fragment 176-191, blocked by β3-AR antagonists (e.g., SR59230A) but not by β1/β2 antagonists, confirming receptor selectivity. Additional reports describe upregulation of uncoupling protein-1 (UCP-1) in brown adipose tissue, consistent with enhanced thermogenic fatty acid oxidation rather than simple FFA mobilization.

Metabolic context: Because lipolytic effects depend on downstream FFA oxidation, peripheral metabolic capacity (mitochondrial density, carnitine availability) modulates net adipose loss in animal models. Heffernan et al. (2001) showed that the lipolytic response to hGH C-terminal peptides was preserved in older and obese rodent cohorts, where GH responsiveness is otherwise blunted — suggesting the fragment bypasses canonical GHR desensitization (PMID 11713220).

Collectively, this body of preclinical work positions HGH Fragment 176-191 as a useful research tool for dissecting β3-AR-mediated lipolytic signaling independently of somatotropic GH activity. The fragment is supplied for in vitro and laboratory research use only.

AOD-9604 (Tyr-hGH177-191) is a tyrosine-modified analog of HGH Fragment 176-191, developed at Monash University and Metabolic Pharmaceuticals as a candidate anti-obesity compound. The two peptides share the same C-terminal hGH-derived core sequence (residues 177-191) and the same intramolecular disulfide bond (Cys182-Cys189), but AOD-9604 carries an additional N-terminal tyrosine residue not present in the native hGH sequence. This tyrosine was added to facilitate radioiodination for pharmacokinetic tracing and was retained because it conferred modest stability advantages.

Sequence and structural comparison:

Mechanistic equivalence: Both peptides act through the β3-adrenergic receptor axis on adipocytes, stimulating cAMP-PKA-HSL signaling to mobilize stored triglyceride. In vitro lipolysis assays using isolated rodent epididymal fat pads or 3T3-L1 adipocytes demonstrate broadly comparable EC50 values for glycerol release, and both fragments fail to activate the GH receptor — neither induces STAT5 phosphorylation, IGF-1 production, or insulin resistance in preclinical models.

Pharmacokinetic differences: The N-terminal tyrosine of AOD-9604 confers modest protection against aminopeptidase cleavage, slightly extending plasma half-life relative to the unmodified fragment in rodent models. This stability gain was a practical consideration when selecting AOD-9604 for clinical development, where oral bioavailability and dose-response reproducibility were essential endpoints.

Clinical research history: AOD-9604 progressed through Phase I and Phase II human trials between 2001 and 2007. Initial intravenous and subcutaneous dosing studies established the absence of cardiovascular and glycemic adverse effects characteristic of intact hGH. A 12-week Phase IIb oral trial in obese adults (Ng et al., 2007) tested doses up to 30 mg/day; while early data suggested modest reductions in body weight and favorable safety, the pivotal endpoint was not met at later analysis, and clinical development for obesity was eventually discontinued (PMID 17381369). AOD-9604 was subsequently evaluated for osteoarthritis and cartilage-related applications.

Research-tool perspective: For laboratory comparisons of β3-AR-mediated lipolytic signaling, the two peptides are largely interchangeable. Investigators should note that published EC50 and in vivo potency data are not always directly transferable between the two due to assay format, vehicle, and species differences. Both compounds remain valuable for in vitro adipocyte biology, receptor pharmacology, and structure-activity studies of GH C-terminal lipolytic determinants. Supplied for laboratory research use only; not for human or veterinary administration.

AOD-9604 (the tyrosinated version of Fragment 176-191) advanced to Phase 2 clinical trials in Australia for obesity. Key findings from the randomized, double-blind trial (Stier et al.):

• Daily oral AOD-9604 for 12 weeks produced dose-dependent weight loss
• The 1 mg/day group lost 2.6 kg more than placebo
• No effect on IGF-1 levels, glucose tolerance, or insulin sensitivity — confirming the fragment does not activate the GH receptor
• No serious adverse events; safety profile indistinguishable from placebo
• AOD-9604 was subsequently approved in Australia (TGA) as a food substance, confirming its safety profile