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GHRP-2 vs Ipamorelin: A Growth Hormone Secretagogue Comparison
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Drop GHRP-2 and ipamorelin onto rat pituitary cells and judge them by growth hormone alone, and you’ll have a hard time telling which is which. Both are synthetic agonists of GHS-R1a, the growth hormone secretagogue receptor, so the top-line number looks about the same. The split shows up in the extras. One of these drags the stress axis along for the ride. The other mostly doesn’t. Below we go through what the primary papers actually measured, with every one linked so you can check our reading against the abstract instead of taking our word for it.
Key findings
- GHRP-2 is a synthetic six-residue peptide. Ipamorelin is a pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH2. Both bind GHS-R1a [1].
- On rat pituitary cells, ipamorelin’s EC50 was 1.3 nmol/L against 2.2 for GHRP-6. Basically a tie. GHRP-2 was more potent but with a lower ceiling in swine [1].
- In swine, GHRP-2 and GHRP-6 both bumped up ACTH and cortisol. Ipamorelin didn’t, even way past its GH-active range [1].
- In six healthy young adults, GHRP-2 lifted prolactin, ACTH and cortisol, the last two about as much as hCRH did [2].
- GHRP-2 still raised GH in patients whose GHRH receptor was knocked out by mutation, roughly 4.5-fold. That points to a GHRH-independent action [5].
What these two molecules are
GHRP-2, also called pralmorelin, is a synthetic hexapeptide, sequence D-Ala-D-beta-Nal-Ala-Trp-D-Phe-Lys-NH2. Ipamorelin is the shorter of the pair. Novo Nordisk chemists got to it by stripping the central Ala-Trp dipeptide out of the GHRP-1 series, which is a nice reminder that these peptides were engineered, not found [1]. Both sit in the growth hormone secretagogue class, and both act at GHS-R1a, not at the separate GHRH receptor. That last bit matters more than it sounds. The two receptor systems aren’t interchangeable, so a result on one tells you almost nothing about the other.
The receptor has a weird backstory. GHRP-6 was out in the literature for years before anyone knew what it actually bound. GHS-R1a didn’t get cloned until 1996 [3], and ghrelin, the body’s own key for that lock, wasn’t pulled out of stomach tissue until 1999 [4]. So for over a decade, people published on these peptides with no named target in hand. Keep that in your back pocket when you read the oldest stuff.
Receptor pharmacology, measured
Raun’s 1998 paper is the cleanest head-to-head we’ve got. In vitro on primary rat pituitary cells, ipamorelin released GH with potency and efficacy close to GHRP-6, EC50 1.3 nmol/L versus 2.2. Move to anaesthetised rats, then conscious swine, and the two keep tracking each other. GHRP-2 came out more potent, but with a lower maximum in the swine assay [1]. Ferro’s later structure-activity work backed up the chemistry: the shared Ala-Trp-(D-Phe)-Lys core is the stretch these peptides need to grab GHS-R1a, and you can detect intact peptide plus active metabolites binding the receptor in excretion samples [7].
It’s worth putting the swine numbers down, because “more potent, lower efficacy” is easy to say and easy to misread. GHRP-2 switched on at a low concentration, ED50 near 0.6 nmol/kg, but its maximal GH response topped out around 56 ng/mL. Ipamorelin needed more peptide to get moving, ED50 near 2.3 nmol/kg, yet reached a higher ceiling, about 65 ng/mL. GHRP-6 sat right by it at 3.9 nmol/kg and 74 ng/mL [1]. So GHRP-2 turns on early but doesn’t climb as high. Potency and efficacy are different axes, and jamming them into one word throws away the part that usually decides how you read an experiment.
Where they split: the stress axis
Read this part twice. In those same swine experiments, GHRP-2 and GHRP-6 both pushed ACTH and cortisol up. Ipamorelin didn’t, and it held flat even at concentrations over 200 times its GH ED50 [1]. None of the secretagogues touched FSH, LH, prolactin or TSH in that model. The human data line up on the peptide side: in Arvat’s young adults, GHRP-2 raised prolactin, ACTH and cortisol, with the ACTH and cortisol jump about as big as hCRH produced [2]. So if a design needs a secretagogue that reads cleanly on GH without poking the adrenal axis, the published numbers pull the two apart for you.
GHRP-2 and the GH axis
A 2001 study handed GHRP-2 to people carrying a homozygous inactivating mutation in the GHRH receptor. They still responded, roughly a 4.5-fold GH rise over baseline, though nowhere near the 79-fold jump in controls. ACTH, cortisol and prolactin barely moved [5]. The authors read that as a sign an intact GHRH system isn’t strictly required for GHRP-2 to work on somatotrophs. More recently, a 2024 retrospective in adolescents reported median GH peaks near 89 to 90 ng/mL in idiopathic and short-stature groups after a GHRP-2 load, against 3.4 ng/mL in the organic or genetic GH-deficient group [6]. That kind of gap is exactly why GHRP-2 shows up in provocative testing.
Side by side
| Property | GHRP-2 | Ipamorelin |
|---|---|---|
| Length | 6 residues (hexapeptide) | 5 residues (pentapeptide) |
| Target | GHS-R1a [1] | GHS-R1a [1] |
| Swine GH profile | Higher potency, lower max [1] | Like GHRP-6 [1] |
| ACTH / cortisol | Raised, hCRH-like in man [2] | Flat even at 200x ED50 [1] |
Characterization, not just a name on a label
Both are short synthetic chains, and that cuts two ways. Easy to make, and easy to get subtly wrong. A truncated or racemized batch can pass a lazy glance and still misbehave at the receptor, which is the exact structure-activity sensitivity Ferro’s group was poking at [7]. For a clean comparison you want three separate facts about each lot. Identity from mass spectrometry. Purity from HPLC. Net peptide content, which is not the same thing as purity. Keep them separate. When a study throws out a surprising GH or cortisol result, the first thing to ask is whether both peptides were characterized to the same bar, because a difference in the material can wear the costume of a difference in pharmacology.
Reading the older literature carefully
A lot of the early GHRP-2 and ipamorelin write-ups predate the cloned receptor and the discovery of ghrelin, so the mechanism language in them is sometimes fuzzy or just wrong by today’s standards. When you line up results, check what each paper actually measured, the model it used (cell, rodent, swine, human), and the sampling times. A GH number off a receptor-binding assay and a GH number from a conscious animal are answering two different questions. For how material like this gets characterized before it ever reaches a study, see our notes on third-party tested peptides, and for the single-compound picture, our GHRP-2 overview.
FAQ
Do GHRP-2 and ipamorelin bind the same receptor? Yes. Both are agonists at GHS-R1a, which is a separate receptor from the GHRH receptor [1].
What’s the main measured difference? In swine, GHRP-2 raised ACTH and cortisol while ipamorelin didn’t, even far above its GH-active range [1].
When was the receptor actually identified? GHS-R1a was cloned in 1996 and its natural ligand ghrelin isolated in 1999, both well after the first GHRPs were described [3][4].
References
- Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998. https://pubmed.ncbi.nlm.nih.gov/9849822/
- Arvat E, di Vito L, Maccagno B, et al. Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Peptides. 1997. https://pubmed.ncbi.nlm.nih.gov/9285939/
- Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996. https://pubmed.ncbi.nlm.nih.gov/8688086/
- Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999. https://pubmed.ncbi.nlm.nih.gov/10604470/
- Gondo RG, Aguiar-Oliveira MH, Hayashida CY, et al. Growth hormone-releasing peptide-2 stimulates GH secretion in GH-deficient patients with mutated GH-releasing hormone receptor. J Clin Endocrinol Metab. 2001. https://pubmed.ncbi.nlm.nih.gov/11443201/
- Onuki T, Hiroaki T, Sawano K, et al. Robust growth hormone responses to GH-releasing peptide 2 in adolescents. J Pediatr Endocrinol Metab. 2024. https://pubmed.ncbi.nlm.nih.gov/38958228/
- Ferro P, Krotov G, Zvereva I, et al. Structure-activity relationship for peptidic growth hormone secretagogues. Drug Test Anal. 2017. https://pubmed.ncbi.nlm.nih.gov/26811125/
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