Growth-hormone-axis research peptides

The growth-hormone axis is a neuroendocrine cascade where hypothalamic GHRH and ghrelin drive pulsatile pituitary GH release, with IGF-1 as the principal downstream effector. Synthetic GHRH analogues (sermorelin, CJC-1295, tesamorelin) and GHRP-class ghrelin-receptor agonists (ipamorelin, hexarelin, GHRP-2/6) activate this axis through pharmacologically distinct receptors — the mechanistic basis of the canonical GHRH+GHRP dual-pathway combination.

The growth-hormone axis is the neuroendocrine cascade by which the hypothalamus regulates pulsatile release of growth hormone (GH) from anterior-pituitary somatotrophs, driving hepatic and peripheral synthesis of insulin-like growth factor 1 (IGF-1) and downstream effects on protein synthesis, lipolysis, glucose metabolism, and tissue growth. Two opposing hypothalamic peptides regulate GH secretion — GHRH stimulating and somatostatin inhibiting — with ghrelin (the gastric-derived 'hunger hormone') providing a third parallel amplification signal through the growth-hormone secretagogue receptor (GHSR-1a). This three-signal architecture produces the characteristic pulsatile GH release pattern with overnight peaks that drive the diurnal rhythm of IGF-1 production. Pharmacological engagement of the axis exploits the receptor-level separation between these signals. GHRH is a 44-amino-acid hypothalamic peptide whose N-terminal 1-29 fragment retains full biological activity — the pharmacological starting point for all synthetic GHRH analogues on the market. Sermorelin (GHRH 1-29) is the prototype, unmodified analogue with the strictest pulsatility preservation but shortest plasma half-life (~5-12 minutes) requiring multiple daily administrations. CJC-1295 no-DAC (Mod GRF 1-29) adds four protease-resistance amino-acid substitutions extending half-life to ~30 minutes while preserving pulsatile release. CJC-1295 DAC extends this dramatically to 6-8 days through covalent albumin binding via a maleimide-modified lysine 30 — producing sustained non-pulsatile receptor occupancy that is pharmacologically distinct from the shorter-acting variants. Tesamorelin is the only currently FDA-approved GHRH analogue (Egrifta, for HIV-associated lipodystrophy), using a single N-terminal trans-3-hexenoyl modification of the full GHRH 1-44 sequence to achieve ~26-38 minute half-life supporting once-daily dosing with preserved pulsatility. The GHRP class engages a distinct pituitary receptor. Ipamorelin, hexarelin, GHRP-2, and GHRP-6 are ghrelin-receptor (GHSR-1a) agonists activating Gαq/11-coupled phospholipase Cβ signalling with intracellular calcium release — a pathway completely separate from GHRH's Gαs/cAMP/PKA cascade. Combined GHRH+GHRP administration produces GH pulses substantially larger than either monotherapy through synergistic dual-receptor activation plus somatostatin-tone suppression from GHSR-1a signalling. Within the GHRP class, ipamorelin is the selectivity outlier — producing GH release without measurable cortisol, ACTH, prolactin, or aldosterone spillover (Raun et al., Eur J Endocrinol, 1998), making it the preferred component when clean GH-axis isolation is required. Hexarelin uniquely binds cardiac CD36 receptors producing cardioprotective effects distinct from the GH-axis pharmacology, though at the cost of substantial chronic-dosing desensitisation. Pre-clinical and clinical GH-axis assays include timed serum GH sampling after single-dose administration (typically every 15-30 minutes over 2-4 hours for pulse characterisation), 24-hour pulsatile GH profiling with frequent sampling (every 10-20 minutes) for pulse amplitude, frequency, and area-under-curve analysis, serum IGF-1 and IGFBP-3 as integrated pharmacodynamic readouts, downstream metabolic endpoints (fasting glucose, HOMA-IR, body composition by DEXA), and — for tesamorelin specifically — CT-measured visceral adipose tissue volume in HIV-lipodystrophy contexts. UK regulatory status varies within this class. Tesamorelin holds FDA approval as Egrifta but no MHRA authorisation. Sermorelin was FDA-approved as Geref from 1990-2008 but was discontinued commercially. All other compounds (CJC-1295 variants, ipamorelin, hexarelin) are unlicensed research chemicals in the UK. Every compound in this category is captured by WADA S2 (Peptide Hormones, Growth Factors, Related Substances and Mimetics) as GHRH analogue or GH secretagogue. WADA detection includes direct peptide mass-spec identification, GH-isoform-ratio testing, and IGF-1 elevation monitoring. Open research questions include the mechanistic separability of GIPR agonism-versus-antagonism contribution to tirzepatide-class metabolic benefits, whether pulsatile-preserving protocols achieve durable IGF-1 sensitivity superior to sustained-elevation DAC-variant approaches, and comparative long-term safety of chronic vs cyclical dual-pathway secretagogue protocols.

Peptides in this category