Ipamorelin: The First Selective Growth Hormone Secretagogue — Structure and Receptor Pharmacology
Development History
Ipamorelin was developed in the mid-1990s at Novo Nordisk by Raun, Hansen, and colleagues as part of a research program aimed at identifying growth hormone secretagogue receptor (GHS-R1a) agonists with improved selectivity profiles. Earlier GHS-R1a agonists, including GHRP-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂) and GHRP-2, demonstrated potent GH-releasing activity but also stimulated release of ACTH, cortisol, and prolactin—complicating their use as research tools for studying isolated GH-axis pharmacology.
Ipamorelin was the first compound in its class to demonstrate GH release selectivity in preclinical models, earning it the designation as "the first selective growth hormone secretagogue."
Chemical Structure
Ipamorelin is a synthetic pentapeptide with the sequence Aib-His-D-2Nal-D-Phe-Lys-NH₂ and molecular weight of approximately 712 Da. Notable structural features include:
- Aib (α-aminoisobutyric acid) at position 1 — constrains backbone torsion angles and promotes helical conformation
- D-2-naphthylalanine (D-2Nal) at position 3 — an unnatural amino acid providing enhanced hydrophobic interaction with the GHS-R1a binding pocket
- D-Phe at position 4 — D-configuration provides resistance to endopeptidases
- C-terminal amide — protects against carboxypeptidase degradation
The incorporation of multiple non-natural and D-configured amino acids gives ipamorelin significant stability against proteolytic degradation compared to natural peptide ligands.
Receptor Pharmacology
Ipamorelin binds to the growth hormone secretagogue receptor type 1a (GHS-R1a), a G-protein-coupled receptor that signals primarily through Gq/11-phospholipase C activation, leading to IP₃-mediated calcium mobilization from intracellular stores. The receptor was cloned in 1996 by Howard et al. and later identified as the receptor for the endogenous ligand ghrelin (discovered in 1999 by Kojima et al.).
Selectivity Data
In the landmark 1998 publication, Raun et al. demonstrated that ipamorelin produced dose-dependent GH release in pituitary cell cultures without significant stimulation of ACTH, cortisol, FSH, LH, prolactin, or TSH at concentrations up to 100-fold above the GH-releasing EC₅₀.
Key Published Studies
- Hansen BS et al. (1999) — Structure-activity relationships of ipamorelin analogs; characterized the pharmacophore requirements for GHS-R1a selectivity.
- Johansen PB et al. (2004) — Pharmacokinetic characterization of ipamorelin in preclinical models.
- Gobburu JV et al. (2004) — Population pharmacokinetic-pharmacodynamic modeling of ipamorelin.
Research Applications and Future Directions
Ipamorelin's selectivity profile makes it a valuable pharmacological tool for studying GHS-R1a-mediated signaling independent of other pituitary hormone axes. Current research directions include investigation of GHS-R1a receptor desensitization kinetics, comparison with other selective GHS-R1a agonists, and use as a reference compound in GH secretagogue screening assays.
Available at Crush Research: Ipamorelin 10mg and as part of the CJC-1295/Ipamorelin Blend. View Certificates of Analysis.
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