GHRP-2: A Research Overview

Introduction

GHRP-2, known by its International Nonproprietary Name (INN) pralmorelin and its development designation KP-102, is a synthetic hexapeptide classified as a growth hormone secretagogue (GHS). It acts as a full agonist at the growth hormone secretagogue receptor subtype 1a (GHS-R1a), the G protein-coupled receptor that mediates the actions of the endogenous hormone ghrelin. Of particular regulatory significance, pralmorelin became the first — and, as of publication, only — growth hormone secretagogue to receive national regulatory approval for clinical diagnostic use, granted by Japan's Pharmaceuticals and Medical Devices Agency (PMDA) in 2004. This article provides an educational reference overview of GHRP-2's chemical identity, pharmacological classification, and regulatory context.

Background

The GHRP compound class originated from synthetic structure-activity studies of enkephalin analogs in the 1970s and 1980s. Researchers at Tulane University, led by endocrinologist Cyril Y. Bowers, observed that certain modifications of met-enkephalin amide produced unexpected growth hormone (GH)-releasing activity in pituitary cell cultures. This line of investigation ultimately produced GHRP-6, the first hexapeptide GHS to be clinically characterized, and a series of increasingly potent analogs that followed [1, 2].

GHRP-2 emerged from structure-activity optimization of the GHRP-6 scaffold. The substitution of D-2-naphthylalanine (D-2-Nal) at position 2 of the peptide sequence conferred a substantially higher GH-releasing potency relative to the parent compound, alongside a somewhat attenuated stimulation of appetite-related signaling compared to other family members [3]. These properties drew sustained preclinical and clinical research attention through the 1990s and early 2000s.

The pharmacological context of GHRP-2 was clarified in 1996, when Howard and colleagues cloned the orphan G protein-coupled receptor that transduced GHS activity — designated GHS-R1a. This receptor was subsequently shown to be the target for both synthetic GHSs, including GHRP-2, and the endogenous hormone ghrelin, isolated by Kojima and colleagues in 1999 [4].

Chemistry and Structure

GHRP-2 is a synthetic hexapeptide with the amino acid sequence D-Ala-D-2Nal-Ala-Trp-D-Phe-Lys-NH2. It incorporates two D-amino acid residues — D-alanine at position 1 and D-2-naphthylalanine at position 2 — as well as D-phenylalanine at position 5. The presence of these non-natural amino acid residues distinguishes GHRP-2 from endogenous peptides and contributes to its resistance to proteolytic degradation in biological matrices relative to L-amino acid peptides.

The D-2-Nal substitution at position 2 is the key structural feature distinguishing GHRP-2 from GHRP-6, which carries D-tryptophan at that position. This modification was identified through systematic structure-activity relationship studies and is understood to alter the hydrophobic contact profile within the GHS-R1a binding pocket, contributing to the compound's enhanced potency [3].

The molecular formula of pralmorelin is C45H55N9O6 · 2HCl (as the dihydrochloride salt), with a molecular weight of approximately 817 daltons in free base form. Its CAS number is 158861-67-7.

Pharmacological Classification

GHRP-2 is classified as a peptidyl growth hormone secretagogue and a full agonist at the growth hormone secretagogue receptor 1a (GHS-R1a). This receptor is a Gq/11-coupled G protein-coupled receptor expressed on somatotroph cells of the anterior pituitary and on neurons of the hypothalamic arcuate nucleus.

The GHS pharmacological class is distinct from the growth hormone-releasing hormone (GHRH) class, despite both producing GH secretion. GHRPs and GHRH act through different receptor systems and activate partially overlapping but distinct intracellular pathways. Research reported that co-administration of GHRH with GHRPs produced substantially greater GH responses than the sum of individual responses, indicating a synergistic rather than simply additive interaction [5]. The intracellular signaling cascade underlying this pharmacology is covered in the GHRP-2 mechanism of action article.

Within the GHRP family, GHRP-2 is recognized in the published literature as producing higher peak GH responses than GHRP-6 under equivalent conditions, while also eliciting lesser stimulation of prolactin secretion. Like other GHRPs, it produced measurable adrenocorticotropin (ACTH) and cortisol responses — a pharmacological property studied by Arvat and colleagues in comparative research across multiple GHSs [5].

Regulatory Status

The most consequential regulatory milestone for pralmorelin was its approval by Japan's PMDA in October 2004 under the brand name GHRP Kaken, marketed by Kaken Pharmaceutical Co., Ltd. The approved indication is as a diagnostic agent for the assessment of growth hormone deficiency in adults and children aged four years and older. In this clinical context, the compound is administered as a single intravenous challenge to stimulate pituitary GH secretion, with GH response measured to assess somatotroph reserve. This approval represented the first regulatory authorization for any growth hormone secretagogue for clinical use by any national regulatory agency.

Developmental programs were conducted in the United States and Europe. Wyeth-Ayerst examined pralmorelin in Phase II clinical trials in the United States, contributing to the international evidence base for the compound class. Those programs informed the broader scientific literature on GHS diagnostic utility without advancing to regulatory submission. The compound's current regulatory status outside Japan remains investigational.

The World Anti-Doping Agency (WADA) lists GHRP-2 among prohibited substances in competitive sports, and analytical methods for its detection in urine have been developed and published to support doping control programs [6]. Research-grade GHRP-2 from SpartaLabs is verified by independent third-party laboratory analysis for identity and purity.

Discovery History

The scientific lineage of GHRP-2 runs through several decades of incremental peptide chemistry research. The foundational observation — that specific enkephalin analogs could selectively release GH from pituitary cells — was reported by Bowers and colleagues as early as 1977 to 1980, with a key structure-activity report published in Endocrinology in 1980 [1]. The first hexapeptide GHS, later designated GHRP-6, was reported by Bowers, Momany, Reynolds, and Hong in 1984 [2].

GHRP-2 emerged from subsequent optimization work during the late 1980s and early 1990s. Kaken Pharmaceutical pursued preclinical and clinical development under the code KP-102. Pharmacological characterization of KP-102 in preclinical models was published in Arzneimittelforschung in 2004 [3], consolidating the compound's profile in the peer-reviewed literature in conjunction with its Japanese regulatory submission.

The broader GHS receptor story culminated in 1999 with Kojima and colleagues' identification of ghrelin — the endogenous ligand for GHS-R1a — which retroactively established that synthetic GHSs such as GHRP-2 had been pharmacologically mimicking an endogenous hormone whose existence was unknown at the time of their discovery [4].

References

1. Bowers CY, Momany F, Reynolds GA, Chang D, Hong A, Chang K. Structure-activity relationships of a synthetic pentapeptide that specifically releases growth hormone in vitro. Endocrinology. 1980;106(3):663–667. PMID: 6109621. DOI: 10.1210/endo-106-3-663

2. 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(5):1537–1545. PMID: 6714155. DOI: 10.1210/endo-114-5-1537

3. Arase K, Ohboshi T, Endo S, Akahane S. Pharmacological characteristics of KP-102 (GHRP-2), a potent growth hormone-releasing peptide. Arzneimittelforschung. 2004;54(12):857–867. PMID: 15646370. DOI: 10.1055/s-0031-1297041

4. 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(6762):656–660. PMID: 10604470. DOI: 10.1038/45230

5. Arvat E, Di Vito L, Maccario M, Broglio F, Boghen MF, Deghenghi R, et al. Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Comparison with the effects of GHRH, TRH and hCRH. Peptides. 1997;18(6):885–891. PMID: 9285939. DOI: 10.1016/s0196-9781(97)00016-8

6. Okano M, Sato M, Kageyama S, Niioka T, Yonezawa K, Suzuki H, et al. 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. DOI: 10.1002/rcm.4619