GHRP-2 and GHRP-6: The First-Generation Growth Hormone Releasing Peptides

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

This article explains what GHRP-2 and GHRP-6 are chemically, how a 1981 enkephalin-analog design program produced them, how the search for their receptor led to the cloning of GHS-R1a in 1996 and the discovery of ghrelin in 1999, what cell and animal studies have measured, and how the two hexapeptides differ from each other and from ipamorelin. It closes with regulatory status and what a research certificate should show.

GHRP-2 and GHRP-6 are synthetic hexapeptides with D-amino acids at alternating positions

GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂, a six-residue C-terminal amide with D-tryptophan at position 2 and D-phenylalanine at position 5. GHRP-2 is D-Ala-D-(2-naphthyl)Ala-Ala-Trp-D-Phe-Lys-NH₂; it keeps the Ala-Trp-D-Phe-Lys-NH₂ core but replaces the N-terminal His-D-Trp pair with D-alanine and the non-natural residue D-2-naphthylalanine. Neither sequence occurs in any organism. Both were designed, not isolated, which is why the older literature calls them "unnatural" growth hormone releasing peptides.

The D-residues matter for two reasons. They protect the backbone from common proteases, and they fix the turn geometry that the original design work identified as necessary for activity. The two aromatic side chains at positions 2 and 4 (indole or naphthyl, then indole) and the D-aromatic at position 5 create a compact hydrophobic face; the C-terminal lysine amide carries the positive charge. These features recur in later secretagogues, including ipamorelin, which is covered in its own article at Ipamorelin: The Selective Ghrelin Receptor Agonist in the Research Literature.

The hexapeptides came out of an enkephalin-analog design program, not from a natural hormone

GHRP-6 was designed by iterating on weakly active enkephalin analogs, years before any natural ligand for its receptor was known. A 1981 Endocrinology paper by Momany, Bowers and colleagues reported that [D-Trp²]- and [D-Phe²]-methionine enkephalin amide released small amounts of GH from rat pituitary tissue in vitro. Using conformational energy calculations to predict which shapes were favored, they synthesized and tested progressively more active sequences, arriving at Tyr-D-Trp-Ala-Trp-D-Phe-NH₂, which was active in the rat pituitary assay at roughly one-thousandth the concentration of the starting analogs.

Two companion papers in Endocrinology in 1984 extended the series. The Momany paper reported conformational analysis and in vitro activity for the pentapeptide His-D-Trp-Ala-Trp-D-Phe-NH₂ and the hexapeptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂, both active in rat pituitary incubations at 1 to 10 ng/mL of medium. The Bowers paper characterized the hexapeptide, naming it [His¹,Lys⁶]GHRP, and reported that it released GH in vitro without releasing LH, FSH, TSH or prolactin, that somatostatin-14 and somatostatin-28 blocked the response, and that the response was observed in rats, monkeys, lambs and calves. The authors noted that the GH response lasted longer than the response to the then-new GHRH fragments.

The key conceptual point in Bowers' own 2012 history in Methods in Enzymology is that between 1982 and 1984 the group concluded that GHRP was not simply a GHRH mimic. It acted on the pituitary in vitro without GHRH, yet its action in animals required GHRH. That mismatch led to the hypothesis of an undiscovered natural hormone acting through a distinct receptor.

The receptor hunt led to GHS-R1a in 1996 and ghrelin in 1999

GHRP-6 was the tool that revealed a receptor system nobody had predicted. In 1991, Sethumadhavan, Veeraragavan and Bowers reported in Biochemical and Biophysical Research Communications that a radioiodinated Tyr-Ala-GHRP derivative bound specifically, reversibly and saturably to membranes from rat anterior pituitary and hypothalamus, with competition analysis suggesting two classes of sites. Only [D-Lys³]GHRP, certain substance P antagonists and a GHRH analog competed for binding. The same year, Bowers, Sartor, Reynolds and Badger showed in Endocrinology that GHRP acted at both pituitary and hypothalamic sites in rats through receptors that were neither GHRH receptors nor opiate receptors.

In 1996, Howard and colleagues at Merck reported in Science the cloning of a G protein-coupled receptor from swine and human pituitary and hypothalamus that was the target of both peptide and non-peptide secretagogues. This became GHS-R1a, the growth hormone secretagogue receptor. Smith and colleagues' 1997 review in Endocrine Reviews framed the peptidomimetic program that followed. The cloned receptor was an orphan: its natural ligand was unknown.

That gap closed in 1999 when Kojima, Kangawa and colleagues reported in Nature the purification from rat stomach of a 28-residue peptide with an n-octanoyl group on serine 3 that activated GHS-R. They named it ghrelin. For background on this receptor class, see What Are GPCRs? The Receptor Family Most Research Peptides Bind.

What cell and animal studies have measured for GHRP-6 and GHRP-2

Most of the mechanistic literature uses GHRP-6 as a reference agonist in isolated pituitary cells and rodent models, measuring calcium signaling and hormone release rather than any outcome. Bresson-Bepoldin and Dufy-Barbe reported in the Journal of Neuroendocrinology in 1996 that GHRP-6 produced a biphasic rise in cytosolic calcium in identified rat somatotrophs: an initial release from intracellular stores, then sustained influx through voltage-sensitive channels. Blocking the store release with thapsigargin or the influx with nifedipine reduced the secretory response, and depleting protein kinase C abolished it.

A separate line of work found that some GHRP-family peptides bind a second, unrelated protein. Bodart and colleagues reported in Circulation Research in 2002 that a photoactivatable hexarelin derivative labeled an 84 kDa protein in rat cardiac membranes whose N-terminal sequence matched CD36, a scavenger receptor expressed in cardiomyocytes and microvascular endothelium. Hexarelin raised coronary perfusion pressure in perfused rat hearts in a concentration-dependent way, and the response was absent in hearts from CD36-null mice. Hexarelin is a 2-methyl-tryptophan variant of GHRP-6, so this finding is usually read as applying to the hexapeptide family rather than to GHRP-6 specifically.

For GHRP-2, the animal literature is narrower and often diagnostic in orientation. In rat pituitary preparations and in swine, Raun and colleagues (below) used GHRP-2 and GHRP-6 as comparators when characterizing ipamorelin. What is not established in any of this work is a human outcome: the model systems are cells, perfused organs and farm or laboratory animals, and the endpoints are hormone concentrations, calcium traces and perfusion pressures.

How GHRP-2 differs from GHRP-6, and how both differ from ipamorelin

GHRP-2 is more potent but less efficacious than GHRP-6 in the swine model, and both release ACTH and cortisol where ipamorelin does not. Those are the three comparative facts that come from Raun and colleagues' 1998 paper in the European Journal of Endocrinology, the primary characterization of ipamorelin. In primary rat pituitary cells, ipamorelin and GHRP-6 released GH with similar potency and efficacy (EC₅₀ values of 1.3 and 2.2 nmol/L). In conscious swine, GHRP-2 showed a lower ED₅₀ than GHRP-6 but a lower maximal plasma GH response. Both GHRP-2 and GHRP-6 raised plasma ACTH and cortisol in swine; ipamorelin did not, which is the basis for calling it the first selective secretagogue.

Structurally the three sit on a continuum. GHRP-6 is the parent. GHRP-2 swaps the N-terminal His-D-Trp for D-Ala-D-2-Nal, which increases receptor affinity. Ipamorelin (Aib-His-D-2-Nal-D-Phe-Lys-NH₂) deletes the central Ala-Trp pair entirely and adds the helix-favoring residue Aib at the N-terminus. A side-by-side comparison of the two hexapeptides appears in GHRP-2 vs GHRP-6: Sequence, Receptor Affinity, and Research History.

Regulatory status: a Japanese diagnostic agent, no US approval, and a named WADA class

Neither GHRP-2 nor GHRP-6 is an approved drug in the United States. GHRP-2 has a different history in Japan, where it is marketed under the INN pralmorelin as a pituitary stimulation test agent used to assess GH reserve. The clinical literature around that use is diagnostic, not interventional: Suzuki and colleagues' 2022 paper in the Journal of the Endocrine Society, for example, evaluated the GHRP-2 test in 36 adults with hypothalamic-pituitary disorders and recommended measuring ACTH alongside GH during the test. That Japanese diagnostic product is a regulated pharmaceutical and is not the same thing as a research vial.

GHRP-6 has never been approved anywhere. Both compounds, together with GHRP-1, GHRP-3 through GHRP-5, hexarelin and alexamorelin, are named explicitly in the growth hormone releasing peptides subsection of class S2 on the World Anti-Doping Agency Prohibited List, which matters for sports-science and anti-doping laboratories that handle reference material. A reference table of status across the catalog is maintained at FDA Status of Common Research Peptides: A Reference Table.

The 1990s volunteer studies in the historical literature used pharmaceutical-grade material under investigational protocols; they do not describe a research vial, which US Prime Peptides supplies strictly for in vitro work.

What a certificate of analysis should show for a D-amino-acid hexapeptide

For GHRP-2 and GHRP-6, the certificate lines that carry the most information are identity by mass spectrometry, purity by HPLC, net peptide content and the counter-ion. Mass spectrometry confirms the expected monoisotopic mass (872.4 for GHRP-6, 817.4 for GHRP-2) but cannot distinguish D-Trp from L-Trp, because enantiomers have identical mass. Stereochemical identity rests on the synthesis record and, where a laboratory wants to verify it, on chiral amino acid analysis after hydrolysis. See LC-MS Identity Confirmation: What the Mass Spectrum Proves About a Peptide for the limits of the identity line.

Two features of these sequences deserve attention on the HPLC trace. Tryptophan is oxidation-prone, so a small late-eluting or early-eluting shoulder may represent oxidized Trp rather than a synthetic impurity. Both peptides are typically supplied as acetate or trifluoroacetate salts; the salt form affects net peptide content and should be declared.

US Prime Peptides posts lot-matched certificates before purchase for GHRP-2 10 mg, GHRP-6 5 mg and GHRP-6 10 mg, each covering identity by mass spectrometry, HPLC purity, net peptide content, endotoxin, sterility and heavy metals, with an independent third-party report.

Frequently Asked Questions

What is the difference between GHRP-2 and GHRP-6?

Both are six-residue amides that share the Ala-Trp-D-Phe-Lys-NH₂ C-terminal core. GHRP-6 begins His-D-Trp; GHRP-2 begins D-Ala-D-2-naphthylalanine. In the 1998 swine comparison by Raun and colleagues, GHRP-2 was more potent but produced a lower maximal GH response than GHRP-6, and both raised ACTH and cortisol in that model.

Is GHRP-6 the same as ghrelin?

No. Ghrelin is a 28-residue natural peptide with an octanoyl group on serine 3, identified in rat stomach in 1999. GHRP-6 is a synthetic hexapeptide designed in the early 1980s. They act at the same receptor, GHS-R1a, but GHRP-6 was discovered fifteen years before ghrelin and shares no sequence with it.

Is GHRP-2 an approved drug anywhere?

GHRP-2 is marketed in Japan under the name pralmorelin as a diagnostic agent for pituitary stimulation testing. It is not an approved drug in the United States, and GHRP-6 is not approved in any jurisdiction. Both appear by name in class S2 of the WADA Prohibited List.

Why can't mass spectrometry confirm the D-amino acids in GHRP-6?

D- and L-amino acids are mirror images with identical mass, so a mass spectrum of GHRP-6 looks the same whether position 2 carries D-Trp or L-Trp. Identity by MS confirms the sequence mass and fragmentation pattern; stereochemistry depends on the synthesis record or on separate chiral amino acid analysis.

Citations

1. Momany FA, Bowers CY, Reynolds GA, Chang D, Hong A, Newlander K. Design, synthesis, and biological activity of peptides which release growth hormone in vitro. Endocrinology. 1981;108:31-39. PMID 6109621
2. Momany FA, Bowers CY, Reynolds GA, Hong A, Newlander K. Conformational energy studies and in vitro and in vivo activity data on growth hormone-releasing peptides. Endocrinology. 1984;114:1531-1536. PMID 6425039
3. Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114:1537-1545. PMID 6714155
4. Bowers CY, Sartor AO, Reynolds GA, Badger TM. On the actions of the growth hormone-releasing hexapeptide, GHRP. Endocrinology. 1991;128:2027-2035. PMID 2004615
5. Sethumadhavan K, Veeraragavan K, Bowers CY. Demonstration and characterization of the specific binding of growth hormone-releasing peptide to rat anterior pituitary and hypothalamic membranes. Biochem Biophys Res Commun. 1991;178:31-37. PMID 1712588
6. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273:974-977. PMID 8688086
7. Bresson-Bepoldin L, Dufy-Barbe L. GHRP6-stimulated hormone secretion in somatotrophs: involvement of intracellular and extracellular calcium sources. J Neuroendocrinol. 1996;8:309-314. PMID 8861287
8. Smith RG, Van der Ploeg LH, Howard AD, et al. Peptidomimetic regulation of growth hormone secretion. Endocr Rev. 1997;18:621-645. PMID 9331545
9. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139:552-561. PMID 9849822
10. Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402:656-660. PMID 10604470
11. Bodart V, Febbraio M, Demers A, et al. CD36 mediates the cardiovascular action of growth hormone-releasing peptides in the heart. Circ Res. 2002;90:844-849. PMID 11988484
12. Bowers CY. History to the discovery of ghrelin. Methods Enzymol. 2012;514:3-32. PMID 22975043
13. Suzuki S, Ruike Y, Ishiwata K, et al. Clinical usefulness of the growth hormone-releasing peptide-2 test for hypothalamic-pituitary disorder. J Endocr Soc. 2022;6:bvac088. PMID 35795807

This article is for laboratory research reference only. US Prime Peptides products are supplied strictly for in vitro research. Not for human consumption.

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