GHRP-6: Discovery and Regulatory History

Introduction

The history of GHRP-6 spans more than four decades of peptide pharmacology research and is inseparable from the broader scientific narrative that led to the discovery of the ghrelin hormonal system. What began as an unexpected observation in an enkephalin analogue screening program in the late 1970s produced the founding member of the peptidyl growth hormone secretagogue class, a research tool that helped unmask an entirely unrecognized gastric endocrine circuit, and a pharmacological scaffold that has remained scientifically productive into the 2020s. This article summarizes that history based on published peer-reviewed literature and publicly available regulatory documents.

Discovery Period: The Enkephalin Connection (1977–1981)

The origin of GHRP-6 is conventionally traced to 1977 and the laboratory of Cyril Y. Bowers at Tulane University Medical School [1]. Bowers and colleagues were conducting structure-activity relationship work on opioid peptides — principally enkephalin analogues — when they observed that specific chemical modifications to the enkephalin pentapeptide scaffold conferred unexpected growth hormone-releasing activity in rat anterior pituitary cell cultures [2]. This was an incidental finding relative to the opioid pharmacology work; enkephalins were not previously known to be growth hormone secretagogues.

The GH-releasing activity appeared specific — not attributable to general cellular stimulation or opioid receptor engagement — and was sensitive to structural modification in ways that could be probed systematically. Bowers' group recognized they had identified a new pharmacophore for pituitary GH secretion distinct from hypothalamic growth hormone-releasing hormone (GHRH), which had not yet been isolated at that time [1].

In collaboration with computational chemist Frank Momany, who applied conformational energy calculations to optimize the pharmacophore, Bowers and colleagues synthesized progressively shorter and more potent enkephalin-derived peptides through 1977–1981 [2]. That optimization converged on a synthetic hexapeptide with the sequence His-(D-Trp)-Ala-Trp-(D-Phe)-Lys-NH2, which displayed potent, selective, and cross-species GH-releasing activity both in vitro and in vivo.

Early Research and Formal Characterization (1984–1996)

The formal introduction of GHRP-6 into the peer-reviewed literature occurred in 1984, when Bowers, Momany, Reynolds, and Hong published in Endocrinology [3]. This paper provided the first complete characterization of what would come to be known as GHRP-6, demonstrating:

• Selective, dose-dependent GH release from rat pituitary cells in vitro without concomitant release of LH, FSH, TSH, or prolactin
• In vivo GH-releasing activity across rats, monkeys, sheep, calves, and chickens under appropriate experimental conditions
• Activity independent of the GHRH system, which had by that time been characterized following Vale's and Guillemin's respective isolations from pancreatic tumors in 1982

The 1984 publication established GHRP-6 as the prototype of a new pharmacological class — synthetic growth hormone secretagogues — distinct from both GHRH and somatostatin, and generated sustained research interest across multiple groups.

Over the following decade, researchers developed second-generation peptidyl GHSs including GHRP-1, GHRP-2, and hexarelin, varying the hexapeptide scaffold to modulate potency, selectivity, and metabolic stability [2]. Medicinal chemists at Merck Research Laboratories developed non-peptide GHS compounds — including MK-0677 (ibutamoren) — using GHRP-6's pharmacological profile as the starting point, confirming the GH-releasing activity resided at a specific receptor with a druggable orthosteric pocket [4]. A mechanistic advance in 1989–1992 established that GHRP-6 and related compounds activated the phospholipase C/PKC/calcium signaling pathway in pituitary somatotrophs — distinct from the cAMP pathway used by GHRH — confirming a pharmacologically independent receptor [5].

Receptor Cloning and the Orphan Receptor Era (1996–1999)

The receptor for GHRP-6 was identified and cloned in 1996 by Howard and colleagues at Merck Research Laboratories, published in Science [6]. Using a functional expression cloning strategy in Xenopus oocytes and COS-7 cells, the authors screened pituitary cDNA libraries with GHRP-6 and its non-peptide analogue MK-0677 as pharmacological probes. The cloned receptor — a seven-transmembrane G protein-coupled receptor of 364–366 amino acids — was designated GHS-R1a.

The study confirmed expression in the hypothalamus and anterior pituitary, demonstrated high sequence conservation across human, porcine, and rat homologues, and established that GHS-R1a showed no significant homology to the GHRH receptor — confirming it as a pharmacologically distinct entity. At this stage GHS-R1a was an orphan receptor, with known synthetic agonists but no identified endogenous ligand — a status that would be resolved three years later.

Discovery of Ghrelin: Validating the GHRP-6 Research Platform (1999)

The orphan receptor status of GHS-R1a was resolved in 1999 by Kojima and colleagues at the National Cardiovascular Center Research Institute in Osaka, who published the isolation and characterization of ghrelin in Nature [7]. Reasoning by analogy with the known gastrointestinal origin of motilin, the Kojima group extracted and fractionated rat stomach tissue to identify the endogenous GHS-R1a agonist.

Ghrelin was characterized as a 28-amino acid peptide with an n-octanoyl modification on serine at position 3 — an unusual fatty acid modification essential for receptor activation. The discovery established that GHRP-6 had, for more than 15 years, been a pharmacological probe of an entirely uncharacterized gastric endocrine axis — an unusually productive instance of synthetic pharmacology preceding endogenous biology. The retroactive framing of GHRP-6 as a ghrelin receptor agonist placed it within a broader context encompassing appetite regulation, energy homeostasis, and neuroendocrine signaling, domains that became major foci of GHS-R1a research in the following two decades.

Regulatory Milestones

GHRP-6's scientific contribution to the ghrelin receptor field has not been accompanied by a conventional pharmaceutical regulatory pathway. The compound has been investigated principally as a research tool rather than a development candidate. Information on synthesis, purity standards, and third-party verification for research-use material is covered in the GHRP-6 sourcing and quality article.

In the US compounding context, FDA assessed GHRP-6 and determined that it does not appear in the USP or NF and is not a component of any approved drug product; a nomination for the 503B bulks list was assessed as insufficiently supported [8]. These determinations reflect the compound's research-use status. GHRP-6 has been used as an investigational pharmacological tool in published animal model experiments and in a human pharmacokinetic study enrolling nine healthy male volunteers (Noa et al., 2013) [9]. Notably, MK-0677 (ibutamoren) — developed by Merck using GHRP-6 as its pharmacological prototype — advanced into clinical development and has been studied in published randomized controlled trials, illustrating the translational value of the GHRP-6 scaffold.

Current Research Landscape

Scientific interest in GHRP-6 has remained active into the 2020s, with the focus of published research evolving substantially. The compound's role as a pharmacological GH secretagogue has been joined by a distinct and productive research trajectory focused on cytoprotective effects in cardiac, hepatic, and dermal tissue models, attributed to GHRP-6's interactions with the CD36 scavenger receptor characterized in the 2010s [2].

Published studies in the 2010s and 2020s have included preclinical investigations of GHRP-6 in models of liver fibrosis, wound healing, ischemia-reperfusion injury, and chemotherapy-induced organ toxicity — with authors generally attributing observed findings to CD36-mediated mechanisms [2, 10]. A 2024 study examining GHRP-6 in doxorubicin cardiotoxicity models continued this investigational line.

The 2022 cryo-EM structural characterization of GHRP-6 bound to GHS-R1a [11] provided new atomic-level detail on how the synthetic hexapeptide engages its receptor, reviving structural pharmacology interest in the compound as a reference ligand for ghrelin receptor research. GHRP-6 thus occupies a distinctive position in the pharmacological literature: the founding member of a compound class that preceded and ultimately helped identify its own endogenous receptor system, and whose biological profile continues to yield published findings across multiple tissue contexts more than 40 years after its first characterization.

References

1. Bowers CY. Growth hormone-releasing peptide (GHRP). Cell Mol Life Sci. 1998;54(12):1316–29. PMID: 9893715.

2. Berlanga-Acosta J, Guillén-Nieto G, Rodríguez-Rodríguez N, Bringas-Vega ML. Synthetic Growth Hormone-Releasing Peptides (GHRPs): A Historical Appraisal of the Evidences Supporting Their Cytoprotective Effects. Mediators Inflamm. 2017;2017:9274040. PMC: PMC5392015. https://pmc.ncbi.nlm.nih.gov/articles/PMC5392015/

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(5):1537–45. PMID: 6714155. https://pubmed.ncbi.nlm.nih.gov/6714155/

4. Smith RG, Van der Ploeg LH, Howard AD, Feighner SD, Cheng K, Hickey GJ, et al. Peptidomimetic regulation of growth hormone secretion. Endocr Rev. 1997;18(5):621–45. PMID: 9331547.

5. Cella SG, Peroni M, Tagliaferri V, Lucini V, Locatelli V, Müller EE. Protein kinase C-dependent growth hormone releasing peptides stimulate cAMP production by human pituitary somatotropinomas. J Clin Endocrinol Metab. 1996;81(8):2777–82. PMID: 8721987. https://pubmed.ncbi.nlm.nih.gov/8721987/

6. Howard AD, Feighner SD, Cully DF, Arena JP, Liberator PA, Rosenblum CI, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974–7. PMID: 8688086.

7. 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):656–60. PMID: 10604470. https://pubmed.ncbi.nlm.nih.gov/10604470/

8. US Food and Drug Administration. Drug Products Containing Certain Bulk Drug Substances That May Not Be Compounded Under Section 503A of the Federal Food, Drug, and Cosmetic Act. Docket FDA-2015-N-0863. Available at: https://www.fda.gov/drugs/human-drug-compounding/drug-products-containing-bulk-drug-substances-nominated-503b-bulks-list

9. Noa M, Mas R, Mendoza S, Rodríguez E, González JR, Oyarzábal A. Pharmacokinetic study of Growth Hormone-Releasing Peptide 6 (GHRP-6) in nine male healthy volunteers. Regul Toxicol Pharmacol. 2013;65(1):5–11. PMID: 23099431. https://pubmed.ncbi.nlm.nih.gov/23099431/

10. García-Ojalvo A, Pavon-Fuentes N, Llópiz-Arzuaga A, Fernández-Mayola M, Pedraza-Rivero L, Castel-Ruiz R, et al. Growth Hormone-Releasing Peptide 6 Enhances the Healing Process and Improves the Esthetic Outcome of the Wounds. Adv Skin Wound Care. 2016;29(7):315–20. PMC: PMC4854984. https://pmc.ncbi.nlm.nih.gov/articles/PMC4854984/

11. Zhao LH, Yin Y, Yang D, Liu B, Gu L, Ren Y, et al. Molecular recognition of an acyl-peptide hormone and activation of ghrelin receptor. Nat Commun. 2022;13(1):4476. PMC: PMC8379176. https://pmc.ncbi.nlm.nih.gov/articles/PMC8379176/