GHRP-2 vs GHRP-6: Sequence, Receptor Affinity, and Research History

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.

GHRP-2 and GHRP-6 are two hexapeptides from the same Tulane University program, sharing four of six residues and the same receptor, and yet one of them is an approved diagnostic agent in Japan while the other has never been approved anywhere. This article compares them by sequence, mass, receptor affinity profile, the published head-to-head cell data, regulatory status, and the blends in which each appears.

The short answer: two residues apart, same receptor

GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂. GHRP-2 is D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂. The C-terminal four residues are identical; GHRP-2 replaces the N-terminal histidine with D-alanine and the D-tryptophan with D-2-naphthylalanine. Both bind the growth hormone secretagogue receptor GHS-R1a, the class A G-protein-coupled receptor cloned by Howard and colleagues in 1996, and a 1997 head-to-head study in rat pituitary cells concluded that they act through the same receptor and mechanism. GHRP-6 additionally binds the scavenger receptor CD36, a property that has driven a separate line of medicinal chemistry.

GHRP-2, under the international nonproprietary name pralmorelin, is approved in Japan as a diagnostic agent. GHRP-6 is not an approved drug anywhere, and neither compound is approved in the United States. The compound-level article on GHRP-2 and GHRP-6 covers the Bowers program in detail; this article is about what separates the two.

Where the hexapeptides came from

Both compounds descend from Cyril Bowers' work at Tulane in the late 1970s and early 1980s on synthetic Met-enkephalin derivatives that released growth hormone without opioid activity. Bowers' own 2012 historical review in Methods in Enzymology lays out the sequence: unnatural growth hormone-releasing peptides discovered in 1976, GHRH isolated in 1982, the hypothesis of an undiscovered natural GHRP distinct from GHRH, and the eventual cloning of the receptor in 1996 and isolation of ghrelin in 1999. A 1985 Endocrinology paper by Sartor, Bowers and colleagues characterized GHRP-6 in rats and found the growth hormone response depended strongly on age and sex, with animals under 15 days old responding far more than adults.

GHRP-2 was the second-generation compound. A 2004 profile in Drugs in R&D records that it was developed by Polygen in Germany with Tulane, carried the codes GPA-748 and KP-102, and was licensed in Japan to Kaken. A 1994 Regulatory Peptides paper by Sawada and colleagues characterized KP-102 in urethane-anesthetized rats, and a 2004 Arzneimittelforschung paper by Doi and colleagues reports its pharmacology in conscious rats and dogs, including the finding that its growth hormone-releasing activity was less suppressed by endogenous somatostatin than that of GHRH. The same paper gives the CAS number 158861-67-7 and the dihydrochloride salt form of the Japanese product.

Head-to-head in rat pituitary cells

The direct comparison is a 1997 Life Sciences paper by Cheng and colleagues at Merck. In rat primary pituitary cells, GHRP-2 and GHRP-6 each released growth hormone synergistically with GRF, co-application of both at maximal concentrations produced no more release than either alone, each peptide desensitized the cells to the other but not to GRF, and a GHS-R peptide antagonist blocked GHRP-2 while a GRF antagonist did not. The authors concluded that GHRP-2 acts via the same receptor and mechanism as GHRP-6 despite the structural difference. That paper is the reason the two can be regarded as interchangeable ligands at GHS-R1a in a cell assay, with potency as the only variable.

A 1998 Xenobiotica paper by Johansen and colleagues compared the two with ipamorelin in rats and found that GHRP-6 was excreted mainly in bile while GHRP-2 and ipamorelin behaved differently, and a 1998 European Journal of Endocrinology paper by Raun and colleagues reported that in swine both GHRP-2 and GHRP-6 raised ACTH and cortisol at growth hormone-releasing concentrations, whereas ipamorelin did not. That selectivity contrast is covered in the article on ipamorelin.

Side-by-side comparison

The table shows the two structural consequences of the GHRP-2 substitutions: the loss of histidine removes an ionizable imidazole and changes HPLC retention and net charge, and the naphthyl group adds hydrophobicity and a second aromatic chromophore. GHRP-6, with two tryptophans, is the more light- and oxidation-sensitive of the pair.

GHRP-6 and CD36: the affinity GHRP-2 does not share

A 2012 Journal of Medicinal Chemistry paper by Proulx, Lubell and colleagues states that GHRP-6 "exhibits dual affinity for the growth hormone secretagogue receptor 1a and the cluster of differentiation 36 receptor," and describes a series of azapeptide analogs in which replacing D-Trp2, Ala3 or Trp4 with an aza-residue cut GHS-R1a affinity by at least 100-fold while, in some cases, retaining CD36 affinity. The point for a comparison is that the CD36 interaction depends on the D-Trp2 and the lysine-rich binding domain that GHRP-6 analogs such as hexarelin share, and GHRP-2, which replaces D-Trp2 with D-2-Nal, is not the parent compound of that literature. Any claim about GHRP-6 that rests on CD36 does not transfer to GHRP-2, and any claim that rests on GHS-R1a transfers in principle to both.

Regulatory status and analytical detection

GHRP-2 is approved in Japan as pralmorelin, where it is used as a provocative test agent; a 2022 Journal of the Endocrine Society paper by Suzuki and colleagues and the 2004 Doi paper both describe that diagnostic use. It is not an approved drug in the United States or Europe. GHRP-6 has no approval anywhere. Both are listed in the growth hormone-releasing categories of the WADA Prohibited List, and a 2010 Rapid Communications in Mass Spectrometry paper by Okano and colleagues describes an LC-MS/MS method for GHRP-2 and its main metabolite in human urine built for that purpose. The article on the WADA Prohibited List and peptides explains which classes are named. A research vial of either compound is not the Japanese product and is supplied only for in vitro laboratory work.

Which blends they appear in, and what the certificate must show

GHRP-6 is the one that shows up in blends, most commonly paired with a GHRH-type peptide on the receptor-synergy rationale from the Cheng paper and from Bowers' earlier work. US Prime Peptides lists GHRP-2 10 mg and GHRP-6 10 mg as single-compound vials and CJC-1295/GHRP-6 5 mg + 5 mg as a blend, each with a lot-matched certificate posted before purchase. For the single compounds the identity line should show about 818 Da or about 873 Da, and the chromatogram should be read for tryptophan oxidation products at +16 and +32 Da. For the blend, the 3.4 kDa CJC-1295 and 873 Da GHRP-6 are easily resolved by both mass spectrometry and HPLC, so per-component purity is straightforward; the article on CJC-1295 vs ipamorelin covers the same two-receptor logic for the other common pairing. What no standard certificate confirms for either hexapeptide is the D-configuration at positions 1, 2 and 5 of GHRP-2 or 2 and 5 of GHRP-6, since enantiomers have identical mass.

Frequently Asked Questions

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

Two residues. GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂; GHRP-2 replaces His1 with D-Ala and D-Trp2 with D-2-naphthylalanine, giving D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH₂. Both bind GHS-R1a; GHRP-6 also binds CD36. Their masses are about 873 Da and 818 Da.

Do GHRP-2 and GHRP-6 work through the same receptor?

Yes, at GHS-R1a. A 1997 study in rat primary pituitary cells found that they cross-desensitize, do not add to each other at maximal concentrations, and are blocked by the same antagonist, and concluded they act via the same receptor and mechanism. GHRP-6 has an additional affinity for CD36 that GHRP-2 is not reported to share.

Is GHRP-2 an approved drug?

In Japan, yes, under the name pralmorelin, as a diagnostic agent. It is not an approved drug in the United States. GHRP-6 is not approved anywhere. A research vial of either is not the Japanese product.

Why does GHRP-6 appear in blends more often than GHRP-2?

Commercial convention more than science. The receptor-synergy rationale for pairing a secretagogue-receptor peptide with a GHRH-receptor peptide applies to both, since they act at the same receptor; GHRP-6 was simply the first-generation compound and became the default partner in catalog blends.

Citations

1. Sartor O, Bowers CY, Reynolds GA, Momany FA. Variables determining the growth hormone response of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 in the rat. Endocrinology. 1985;117(4):1441-1447. PMID 4029086
2. Bowers CY. History to the discovery of ghrelin. Methods Enzymol. 2012;514:3-32. PMID 22975043
3. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. PMID 8688086
4. Cheng J, Wu TJ, Butler B, Cheng K. Growth hormone releasing peptides: a comparison of the growth hormone releasing activities of GHRP-2 and GHRP-6 in rat primary pituitary cells. Life Sci. 1997;60(16):1385-1392. PMID 9096259
5. Sawada H, Sugihara H, Onose H, et al. Effect of D-Ala-D-beta Nal-Ala-Trp-D-Phe-Lys-NH2 (KP-102) on GH secretion in urethan-anesthetized rats. Regul Pept. 1994;53(3):195-201. PMID 7846295
6. Doi N, Hirotani C, Ukai K, et al. Pharmacological characteristics of KP-102 (GHRP-2), a potent growth hormone-releasing peptide. Arzneimittelforschung. 2004;54(12):857-867. PMID 15646370
7. Pralmorelin: GHRP 2, GPA 748, growth hormone-releasing peptide 2, KP-102 D, KP-102 LN, KP-102D, KP-102LN. Drugs R D. 2004;5(4):236-239. PMID 15230633
8. Johansen PB, Hansen KT, Andersen JV, et al. Pharmacokinetic evaluation of ipamorelin and other peptidyl growth hormone secretagogues with emphasis on nasal absorption. Xenobiotica. 1998;28(11):1083-1092. PMID 9879640
9. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PMID 9849822
10. Proulx C, Picard É, Boeglin D, et al. Azapeptide analogues of the growth hormone releasing peptide 6 as cluster of differentiation 36 receptor ligands with reduced affinity for the growth hormone secretagogue receptor 1a. J Med Chem. 2012;55(14):6502-6511. PMID 22712585
11. 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(8):bvac088. PMID 35795807
12. Okano M, Sato M, Ikekita A, Kageyama S. Determination of growth hormone secretagogue pralmorelin (GHRP-2) and its metabolite in human urine by liquid chromatography/electrospray ionization tandem mass spectrometry. Rapid Commun Mass Spectrom. 2010;24(14):2046-2056. PMID 20552695

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

Compliance Notice: US Prime Peptides products are supplied strictly for laboratory and academic research purposes only. They are not intended for human or veterinary consumption, diagnosis, treatment, or prevention of any disease. All content on this page is educational in nature and does not constitute medical advice or product claims. Researchers are responsible for handling these compounds in accordance with their institution’s safety protocols and applicable laws.