Ipamorelin vs Sermorelin: Two Different Receptors on the Same Axis

Ipamorelin acts at the ghrelin receptor, sermorelin at the GHRH receptor. Different pharmacology, different histories, no head-to-head trial.

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Ipamorelin and sermorelin are routinely grouped together as "growth hormone peptides", and the grouping is accurate only at the level of the physiological axis they both touch. At the level of pharmacology they have nothing in common: different receptors, different receptor families in terms of how they were discovered, different molecular sizes by a factor of five, and entirely different regulatory histories.

They are also not interchangeable in the literature. One was once an approved pharmaceutical product in the United States and has decades of published clinical work behind it. The other reached phase 2 in a surgical indication and its results were never published in the peer-reviewed literature.

What they are

PropertyIpamorelinSermorelin
Development codeNNC 26-0161Not established
Compound typeSynthetic pentapeptideSynthetic peptide (hormone fragment)
Peptide familyGrowth hormone secretagogues (ghrelin receptor agonists)Glucagon / secretin peptide superfamily (growth hormone-releasing hormone)
Primary targetGrowth hormone secretagogue receptor 1a (GHS-R1a, the ghrelin receptor)Growth hormone-releasing hormone receptor (GHRHR)
Secondary targetsNot establishedNot established
Receptor familyClass A (rhodopsin-like) G protein-coupled receptorsClass B1 (secretin-like) G protein-coupled receptors
Agonist / antagonistAgonist at the growth hormone secretagogue receptorAgonist at the GHRH receptor
Highest research phasePhase 2 (completed; results not published in the peer-reviewed literature)Reached regulatory approval as a pharmaceutical product; that product is no longer marketed
Regulatory status (United States)Not approved. Ipamorelin has not been approved by the U.S. Food and Drug Administration for any indication.Formerly approved, no longer marketed. Sermorelin acetate was approved in the United States and sold as Geref; the product was subsequently withdrawn from commercial sale and there is no currently marketed sermorelin product in the United States. That approval attached to a specific pharmaceutical product and its labelled indications and says nothing about research-grade material.
Human trials publishedYesYes

Every value in this table is read from the two compounds’ own library entries when the site is built, so it cannot disagree with them. Nothing here ranks one compound against the other.

Sermorelin is the first 29 amino acids of human growth hormone-releasing hormone, synthesised as a C-terminal amide — the shortest fragment of that hormone with full biological activity. It carries CAS registry number 86168-78-7 and a molecular weight of 3,357.9 g/mol. It was formerly approved in the United States as a pharmaceutical product and sold as Geref; that product was withdrawn from commercial sale and there is no currently marketed sermorelin product in the United States.

Ipamorelin is a synthetic pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH₂, CAS registry number 170851-70-4, molecular weight 711.85 g/mol. It is an agonist at the growth hormone secretagogue receptor and was described in its discovery paper as the first selective growth hormone secretagogue. It is not approved by the U.S. Food and Drug Administration for any indication.

The size difference is not incidental. Sermorelin is a hormone fragment; ipamorelin is a small synthetic peptidomimetic that bears no sequence relationship to any human hormone at all.

Receptor and mechanistic differences

These two compounds act at two different receptors that were discovered decades apart and by opposite routes.

Sermorelin acts at the growth hormone-releasing hormone receptor. That receptor is a class B1 G protein-coupled receptor, cloned and expressed from human anterior pituitary in 1993, signalling through Gs and cyclic AMP [3]. Sermorelin is the unmodified 1-29 fragment of the natural ligand, so it is an agonist at the receptor its parent hormone evolved for. It carries no stabilising substitutions, which is why dipeptidyl peptidase-4 clears it much as it clears the native hormone — the enzyme was characterised acting on GHRH in plasma in 1989 [2].

Ipamorelin acts at the growth hormone secretagogue receptor. That receptor, GHS-R1a, was identified in 1996 in pituitary and hypothalamus by working backwards from synthetic peptides that released growth hormone through a route the GHRH receptor could not explain [7]. Its natural ligand was not known at the time. Ghrelin, an acylated peptide from the stomach, was identified as that ligand in 1999 [14]. So GHS-R1a is a receptor that was found because of drugs, and whose hormone was found afterwards — the reverse of the usual sequence, and the reverse of sermorelin's history.

The two routes converge on the same cell and are separable there. Both ultimately act on pituitary somatotrophs, but through distinct receptors, which is why pharmacological profiling with GHRP and GHRH antagonists could place ipamorelin's action at the GHRP-like receptor rather than the GHRH receptor [10]. It is also why the two classes have been studied together: in patients with pituitary lesions, GHRH and GHRP-6 administered together produced a response distinguishable from either alone [9].

Ipamorelin's distinguishing property within its own class is selectivity. In swine it released growth hormone without measurable release of ACTH or cortisol, which the earlier hexapeptide secretagogues did not manage [10]. That is a comparison within the secretagogue class, not with sermorelin, and the development of the class is reviewed in the secretagogue literature [16].

Receptor
Sermorelin — GHRH receptor, class B1, Gs-coupled, cloned 1993 [3]. Ipamorelin — growth hormone secretagogue receptor 1a, the ghrelin receptor, identified 1996 [7].
Natural ligand
Sermorelin is a fragment of the natural ligand. Ipamorelin has no sequence relationship to ghrelin, the natural ligand of its receptor [14].
Size
29 residues against 5.
Enzymatic stability
Sermorelin carries no stabilising substitutions and is cleared rapidly [2, 4]. Ipamorelin is built from non-standard residues that are not dipeptidyl peptidase-4 substrates.
Reported half-life
Sermorelin, rapidly eliminated after intravenous administration [4]. Ipamorelin, terminal half-life of about two hours in healthy male volunteers [13].
Regulatory position
Sermorelin was formerly approved in the United States; the product is no longer marketed. Ipamorelin has never been approved for anything.

What the research compares

No trial has compared ipamorelin with sermorelin. There is no head-to-head study, and the two literatures do not overlap in population, endpoint or era. Sermorelin's clinical work is concentrated in the late 1980s and 1990s in paediatric growth and in growth hormone deficiency; ipamorelin's is a small pharmacokinetic study in healthy volunteers and two phase 2 trials in postoperative gastrointestinal function whose results were not published in the peer-reviewed literature.

What the literature does support is a comparison of receptor pharmacology, which is the comparison this page makes. The two receptors were characterised separately, the ligand for one was found after the receptor, and the classes have been distinguished experimentally using selective antagonists [10, 7].

One further contrast is documentary rather than pharmacological. Sermorelin's regulatory history is a matter of record: a specific product, a specific approval, specific labelled indications, and a withdrawal from commercial sale. Ipamorelin's is the more common pattern in this part of the library — a compound with a published discovery paper, early human pharmacokinetics, registered trials that completed, and no published results from them.

What the human research shows for each

Human clinical research

Results from pharmaceutical clinical trials describe the investigational material and populations used in those studies and should not be interpreted as establishing the effects of research-grade materials offered for laboratory use.

Sermorelin — the published clinical record

Pharmacokinetics and secretory response. In healthy men, growth hormone concentrations remained elevated for approximately three hours after intravenous injection, despite rapid elimination of the peptide itself; nasal bioavailability was measured at 3 to 5 per cent [4]. The dissociation between how long the peptide persists and how long the response lasts is the central pharmacological observation for this compound.

Growth hormone deficiency. Treatment of growth hormone deficiency with growth hormone-releasing hormone was reported in 1987 [1]. Growth response to GHRH(1-29)NH₂ was later compared with growth hormone itself [5], and sustained increases in growth velocity were reported in children with idiopathic short stature [6]. Radiation-induced growth hormone deficiency was treated with GHRH in a separate study [8], and a review consolidated its use in diagnosis and treatment of children with idiopathic growth hormone deficiency [11].

Limitations. This is a literature of small studies, largely in children, conducted in the 1980s and 1990s, and it belongs to a specific approved pharmaceutical product that is no longer marketed. The former approval covered diagnostic use and treatment of idiopathic growth hormone deficiency in children with growth failure; it says nothing about any other population.

Ipamorelin — a short clinical record, mostly unpublished

Pharmacokinetics in healthy volunteers. In healthy male volunteers given 15-minute infusions, pharmacokinetics were proportional to the amount administered, with a terminal half-life of about two hours, clearance of 0.078 L/h/kg and a steady-state volume of distribution of 0.22 L/kg. Growth hormone release occurred as a single episode peaking at approximately 0.67 hours [13].

Phase 2 in postoperative gastrointestinal function. Two registered phase 2 trials examined ipamorelin for management of postoperative ileus and for return of gastrointestinal function after surgery [20, 21]. Neither has a peer-reviewed publication reporting its results.

Limitations. One published pharmacokinetic study in healthy men, and two completed phase 2 trials whose results are not in the peer-reviewed literature. A completed trial without a publication is not a result, and the absence should be read as an absence rather than as a finding in either direction.

What the animal research shows

Animal research

The animal literatures are, again, separate rather than comparative.

Ipamorelin's discovery paper characterised it in swine, where it released growth hormone without measurable release of ACTH or cortisol — the property that gave it the description "first selective growth hormone secretagogue" [10]. In rats it induced longitudinal bone growth [12], and it counteracted a glucocorticoid-induced decrease in bone formation in adult rats [15]. Separate rodent work examined it in models of postoperative gastrointestinal dysmotility, which is the preclinical basis for the phase 2 programme described above [17, 18]. More recent rodent work compared the GHS-R1a agonists anamorelin and ipamorelin in a cisplatin model [19].

Sermorelin's preclinical position is different because it is a natural hormone fragment rather than a designed molecule: the relevant animal and plasma work concerns how quickly the native sequence is degraded, principally by dipeptidyl peptidase-4 [2], which is the problem that every later GHRH analogue in this library was built to solve.

Findings described in this section were observed in animals. Nothing in them establishes anything about humans, and the two sets of experiments were not designed to be compared with each other.

What the in vitro and receptor research shows

In vitro research

The receptor-level work is where the distinction between these two compounds is actually established rather than asserted.

The human anterior pituitary GHRH receptor was cloned and expressed in 1993, which defined sermorelin's target as a class B1 G protein-coupled receptor [3]. Three years later a separate receptor in pituitary and hypothalamus was shown to function in growth hormone release and to be the target of the synthetic secretagogues — the receptor ipamorelin acts at [7]. Ghrelin was then identified as that receptor's endogenous acylated ligand in 1999 [14].

Ipamorelin's own characterisation used GHRP and GHRH antagonists to place its action at the secretagogue receptor rather than the GHRH receptor, which is the experiment that separates the two classes pharmacologically [10]. The development of the secretagogue class as a whole, including the structure-activity work behind it, is set out in review [16].

No in vitro study has characterised the two compounds against each other, and there is no shared potency axis on which to place them: their receptors are different proteins.

Research status of each

Sermorelin was approved in the United States as a specific pharmaceutical product and that product is no longer marketed. There is no currently marketed sermorelin product in the country. The former approval covered diagnostic use and treatment of idiopathic growth hormone deficiency in children with growth failure, attached to that product and its labelling.

Ipamorelin has never been approved for any indication. Its highest published research phase is phase 2, completed, with results not published in the peer-reviewed literature [20, 21].

Neither status transfers to research-grade material. A former approval belonged to a manufactured pharmaceutical product with a label, a monograph and a marketing authorisation, and none of that extends to material supplied for laboratory use.

Frequently Asked Questions

What is the difference between ipamorelin and sermorelin?
They act at different receptors. Sermorelin is the first 29 amino acids of human growth hormone-releasing hormone and acts at the GHRH receptor, a class B1 G protein-coupled receptor cloned from human anterior pituitary in 1993 [3]. Ipamorelin is a synthetic pentapeptide acting at the growth hormone secretagogue receptor, GHS-R1a, identified in 1996 [7]. They are not variants of one another; they are separate pharmacological classes.
Which receptor is the ghrelin receptor?
The growth hormone secretagogue receptor, GHS-R1a — ipamorelin's target. It was identified in 1996 by working backwards from synthetic peptides that released growth hormone through a route the GHRH receptor could not explain [7], and its endogenous ligand, ghrelin, was identified three years later as an acylated peptide from the stomach [14]. It is a receptor that was found because of drugs, with its hormone found afterwards.
Have the two been compared in the same trial?
No. There is no head-to-head study. Sermorelin's clinical literature is concentrated in the late 1980s and 1990s, largely in paediatric growth and growth hormone deficiency [1, 5, 6]. Ipamorelin's consists of one published pharmacokinetic study in healthy men [13] and two completed phase 2 trials whose results were not published [20, 21].
What made ipamorelin 'the first selective growth hormone secretagogue'?
Its discovery paper reported that in swine it released growth hormone without measurable release of ACTH or cortisol, which the earlier hexapeptide secretagogues did not manage [10]. That is a comparison within the secretagogue class — against compounds such as GHRP-6 and GHRP-2 — and not a comparison with sermorelin, which acts at a different receptor entirely.
Is either compound FDA approved?
Ipamorelin has never been approved for any indication. Sermorelin was formerly approved in the United States as a specific pharmaceutical product sold as Geref; that product was withdrawn from commercial sale and there is no currently marketed sermorelin product in the country. A former approval attached to a manufactured product with a label and a marketing authorisation, and none of it extends to research-grade material.
How long does each persist in plasma?
Sermorelin is eliminated rapidly after intravenous administration, yet growth hormone concentrations in healthy men remained elevated for approximately three hours — a dissociation between residence time and duration of response [4]. Ipamorelin's reported terminal half-life in healthy male volunteers was about two hours, with clearance of 0.078 L/h/kg and a steady-state volume of distribution of 0.22 L/kg [13].
Can the two classes be studied together?
They have been. Because they act at distinct receptors converging on the same pituitary cells, GHRH and a secretagogue administered together have been reported to produce a response distinguishable from either alone in patients with pituitary lesions [9]. That is a published observation about two pharmacological classes, not a statement about any material supplied for laboratory use.
Why is ipamorelin so much smaller than sermorelin?
Because it is not a hormone fragment. Sermorelin is 29 residues of a natural human hormone. Ipamorelin is a five-residue synthetic peptidomimetic built partly from non-standard residues, with no sequence relationship to ghrelin, the natural ligand of the receptor it acts at [14, 10]. Its design came from structure-activity work on the secretagogue class rather than from a natural sequence [16].

References

  1. Treatment of growth-hormone deficiency with growth-hormone-releasing hormone Lancet; 1987. PMID 2879138 doi:10.1016/s0140-6736(87)90699-4
  2. Dipeptidylpeptidase IV and trypsin-like enzymatic degradation of human growth hormone-releasing hormone in plasma The Journal of Clinical Investigation; 1989. PMID 2565342 doi:10.1172/JCI114049
  3. Molecular cloning and expression of a human anterior pituitary receptor for growth hormone-releasing hormone Molecular Endocrinology; 1993. PMID 7680413 doi:10.1210/mend.7.1.7680413
  4. Pharmacokinetics of growth hormone-releasing hormone(1-29)-NH2 and stimulation of growth hormone secretion in healthy subjects after intravenous or intranasal administration Acta Paediatrica Supplement; 1993. PMID 8329825 doi:10.1111/j.1651-2227.1993.tb12827.x
  5. Growth response to growth hormone-releasing hormone(1-29)-NH2 compared with growth hormone Acta Paediatrica Supplement; 1993. PMID 8329826 doi:10.1111/j.1651-2227.1993.tb12828.x
  6. Treatment with GHRH(1-29)NH2 in children with idiopathic short stature induces a sustained increase in growth velocity Clinical Endocrinology; 1994. PMID 7955460 doi:10.1111/j.1365-2265.1994.tb02580.x
  7. A receptor in pituitary and hypothalamus that functions in growth hormone release Science; 1996. PMID 8688086 doi:10.1126/science.273.5277.974
  8. Treatment of radiation-induced growth hormone deficiency with growth hormone-releasing hormone Clinical Endocrinology; 1997. PMID 9231053 doi:10.1046/j.1365-2265.1997.1790998.x
  9. Growth hormone secretion elicited by GHRH, GHRP-6 or GHRH plus GHRP-6 in patients with microprolactinoma and macroprolactinoma before and after bromocriptine therapy Clinical Endocrinology; 1998. PMID 9509075 doi:10.1046/j.1365-2265.1998.00360.x
  10. Ipamorelin, the first selective growth hormone secretagogue European Journal of Endocrinology; 1998. PMID 9849822 doi:10.1530/eje.0.1390552
  11. Sermorelin: a review of its use in the diagnosis and treatment of children with idiopathic growth hormone deficiency BioDrugs; 1999. PMID 18031173
  12. Ipamorelin, a new growth-hormone-releasing peptide, induces longitudinal bone growth in rats Growth Hormone & IGF Research; 1999. PMID 10373343 doi:10.1054/ghir.1999.9998
  13. Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers Pharmaceutical Research; 1999. PMID 10496658 doi:10.1023/a:1018955126402
  14. Ghrelin is a growth-hormone-releasing acylated peptide from stomach Nature; 1999. PMID 10604470 doi:10.1038/45230
  15. The growth hormone secretagogue ipamorelin counteracts glucocorticoid-induced decrease in bone formation of adult rats Growth Hormone & IGF Research; 2001. PMID 11735244 doi:10.1054/ghir.2001.0239
  16. Development of growth hormone secretagogues Endocrine Reviews; 2005. PMID 15814848 doi:10.1210/er.2004-0019
  17. Efficacy of ipamorelin, a novel ghrelin mimetic, in a rodent model of postoperative ileus The Journal of Pharmacology and Experimental Therapeutics; 2009. PMID 19289567 doi:10.1124/jpet.108.149211
  18. Efficacy of ipamorelin, a ghrelin mimetic, on gastric dysmotility in a rodent model of postoperative ileus Journal of Experimental Pharmacology; 2012. PMID 27186127 doi:10.2147/JEP.S35396
  19. The growth hormone secretagogue receptor 1a agonists, anamorelin and ipamorelin, inhibit cisplatin-induced weight loss in ferrets: Anamorelin also exhibits anti-emetic effects via a central mechanism Physiology & Behavior; 2024. PMID 39043357 doi:10.1016/j.physbeh.2024.114644
  20. Safety and Efficacy of Ipamorelin for Management of Post-Operative Ileus. NCT00672074
  21. Safety and Efficacy of Ipamorelin Compared to Placebo for the Recovery of Gastrointestinal Function. NCT01280344

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