What Is Ipamorelin? Pentapeptide Structure, GHS-R1a Pharmacology and Research Profile

Ipamorelin, sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2, is a synthetic five-residue peptide designed as a selective agonist at GHS-R1a, the growth hormone secretagogue receptor that the body's own hormone ghrelin also activates. At around 712 g/mol it is remarkably small for a secretagogue, and nearly all of its building blocks are non-canonical: an unnatural 2-aminoisobutyric acid at the N-terminus, a pair of D-amino acids in the centre and an amidated C-terminus. None of this is cosmetic. Every change is there either to block proteolytic attack or to fix the conformation the receptor recognises, and collectively they turn a five-residue chain into a compact, degradation-resistant scaffold with drug-like character.

When ipamorelin was first reported in the late 1990s, its selectivity set it apart. Previous GHRP-series secretagogues were described as shifting cortisol, prolactin and appetite-related signalling as well as growth hormone, whereas ipamorelin produced a relatively isolated GH response in animal studies — the reason it became a standard tool compound. We supply ipamorelin for laboratory research.

Discovery and structure: four modifications in five residues

The secretagogue story started with peptides developed from met-enkephalin analogues, which triggered GH release via a receptor that GHRH antagonists failed to block. That pointed to a separate receptor, cloned in 1996 as GHS-R1a; its natural ligand, ghrelin, was discovered in 1999. Ipamorelin emerged from the parallel medicinal chemistry programme, whose stated goal was better selectivity than the earlier compounds offered.

Each residue shows the thinking behind that programme. Position 1, Aib (2-aminoisobutyric acid), is alanine with an extra methyl on the alpha carbon; it strongly promotes helix and turn conformations and cannot be cleaved by aminopeptidases, so it stabilises both the fold and the N-terminus. Position 3, D-2-naphthylalanine, contributes a bulky bicyclic aromatic side chain in D form — size for hydrophobic receptor contact, and a stereochemistry normal proteases ignore. Position 4, D-phenylalanine, uses the same strategy with a smaller aromatic ring. Finally, amidating the C-terminal lysine eliminates the terminal negative charge, which in this receptor family correlates with stronger binding.

What results is formally a pentapeptide but behaves more like a small molecule: compact, protease-resistant and conformationally restricted. At 711.85 g/mol it is the lightest peptide in this catalogue's growth hormone section, around one-fifth the molecular weight of sermorelin — a fact with direct implications for molar comparisons.

Mechanism of action

Ipamorelin activates GHS-R1a, a class A GPCR found on pituitary somatotrophs and in the hypothalamus. Whereas the GHRH receptor signals via Gs and cAMP, GHS-R1a signals mainly through Gq: phospholipase C is activated, inositol trisphosphate and diacylglycerol are generated, and intracellular calcium rises to drive release of secretory granules. The receptor is also highly active even without a ligand, which has made it a popular model in inverse-agonist pharmacology.

As the two receptors rely on different second messengers within the same cell, GHRH receptor agonists and GHS-R1a agonists are very often studied together, and several models report a more-than-additive effect when they are combined. A second reported mechanism adds to this: activating the secretagogue receptor is linked to reduced somatostatin tone, lifting an inhibitory brake at the same moment the stimulatory signal arrives.

Ipamorelin's selectivity is its most-quoted attribute, and it should be stated carefully. In published animal studies it released GH without the ACTH, cortisol and prolactin increases produced by GHRP-6 and hexarelin at equivalent GH-releasing potency. That is a comparative observation from particular models, not an assurance of a clean profile in every system — and it is why ipamorelin is chosen to separate GHS-R1a signalling from the broader neuroendocrine background.

Research applications

Receptor pharmacology

Ipamorelin serves as a reference agonist in GHS-R1a binding, calcium flux and inositol phosphate accumulation assays, and in studies of the receptor's constitutive activity and biased signalling. This is probably its main use.

Comparative secretagogue studies

It is the benchmark for selectivity against which older GHRPs are judged and features in nearly every study ranking the class. For head-to-head detail, see ipamorelin versus GHRP-2 and ipamorelin versus GHRP-6.

Gastrointestinal motility models

Ghrelin receptor agonists have been tested in animal models of gastric emptying and postoperative ileus. This work is separate from the endocrine literature, and ipamorelin reached early clinical investigation in this area without obtaining approval.

Combination designs

Combining it with a GHRH receptor agonist is the most frequent experimental set-up; pre-blended research material is available as CJC-1295 with ipamorelin. Such studies should report the molar concentration of each peptide rather than a total mass, since their molecular weights differ by almost fivefold.

Available formats and vial sizes

We stock ipamorelin as lyophilized powder in sealed vials of 2 mg (€40), 5 mg (€50) and 10 mg (€75). Molar arithmetic works strongly in its favour: at 711.85 g/mol, 5 mg equals about 7.02 micromoles, close to five times the molar content of 5 mg of a GHRH analogue. Comparing a secretagogue and a GHRH analogue milligram for milligram is therefore not a controlled comparison. The compound is listed under GHRPs and secretagogues.

Reconstitution and storage at the bench

Worked example: 2 mL of bacteriostatic water added to a 5 mg vial gives 2.5 mg/mL (2,500 mcg/mL), and 0.1 mL — 10 units on a U-100 syringe scale — holds 250 mcg. Expressed as molarity the stock is about 3.51 mM, high for a peptide solution, so reaching the nanomolar concentrations used in receptor assays takes a lengthy serial dilution.

Ipamorelin is one of the easier peptides to work with. It has no cysteine, no methionine and no asparagine-glycine sequence, so the disulfide scrambling, oxidation and deamidation issues that dominate stability discussions for other peptides are irrelevant. Its two large aromatic side chains make it a little less water-loving than its size implies, so let the cake dissolve completely and check the solution is clear rather than assuming it dissolves instantly. Store lyophilized vials at −20 °C away from light and moisture; refrigerate and aliquot the reconstituted solution. The peptide storage guide gives general advice.

Purity and reading the certificate of analysis

Every lot is purified by reversed-phase HPLC to 99% or higher, with identity confirmed by mass and a certificate matched to the lot. Two checks are specific to ipamorelin. First, compare the measured mass with 711.85 g/mol for the C-terminally amidated pentapeptide; a reading about 1 Da higher indicates the free acid, which is pharmacologically a different compound. Second, recognise the certificate's limits: the D-configuration at positions 3 and 4 and the Aib at position 1 cannot be seen by mass spectrometry, because enantiomers share the same mass. A D-to-L epimer would give the correct mass and might elute close to the target peak. Confidence in stereochemistry depends on synthesis records, on chromatography sharp enough to show a single symmetrical peak and, where necessary, on dedicated chiral analysis. Our COA reading guide explains what each analytical section can and cannot demonstrate.

Regulatory position

Ipamorelin is not authorised as a medicine in the United States, the EU or elsewhere; early clinical work on gastrointestinal motility did not result in approval. It is not a food supplement ingredient and not a routinely available compounded medicine. Material supplied here is for laboratory research only and not for human consumption.

Related compounds and further reading

Among secretagogues, GHRP-2 and GHRP-6 are the older hexapeptides that served as the yardstick for ipamorelin's selectivity, while hexarelin is a more potent but less selective relative. On the other arm of the axis, sermorelin, Modified GRF 1-29 and tesamorelin act on the GHRH receptor through a completely different second-messenger pathway. The full picture is set out in the GH secretagogue landscape explained.