GHRP-6

What Is GHRP-6?

GHRP-6 is a synthetic hexapeptide that releases growth hormone from the pituitary gland, both in living animals and humans and in isolated pituitary cells. It belongs to the growth hormone-releasing peptides (GHRPs), synthetic, non-natural peptides with potent stimulatory effects on growth hormone secretion.^[1][2]

GHRPs have no structural homology with growth hormone-releasing hormone (GHRH). They act through their own receptor, present at both the pituitary and the hypothalamus, which was cloned in the 1990s and shows no sequence homology with other known G-protein-coupled receptors.^[1] That receptor is now generally called the growth hormone secretagogue receptor, and its natural ligand is ghrelin.

GHRP-6 was the first hexapeptide in the class to be studied extensively in humans.^[1] Its research record spans endocrine testing in people with growth hormone deficiency, hypothyroidism, obesity and type 2 diabetes, and a growing set of animal studies on heart, gut and kidney injury.^[3][2][4] Most human data come from short-term endocrine testing rather than long-term clinical trials.

Chemical Structure and Molecular Identity

GHRP-6 is the hexapeptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, containing two D-configured aromatic residues and a C-terminal amide.^[5] PubChem lists it under CID 4345065 with the molecular formula C46H56N12O6 and a molecular weight of 873.0 g/mol.

The peptide carries three aromatic side chains from two tryptophans and one phenylalanine, plus a histidine imidazole and a lysine amine. The D-tryptophan at position 2 and D-phenylalanine at position 5 are features shared with later GHRPs such as hexarelin, which adds a methyl group to the D-tryptophan.

Key identifiers

• PubChem CID: 4345065
• InChIKey listed in PubChem: WZHKXNSOCOQYQX-UHFFFAOYSA-N (a record without defined stereocenters)
• Synonyms: growth hormone releasing hexapeptide, SKF-110679

Because the PubChem entry does not encode stereochemistry, the D-configuration at positions 2 and 5 is best confirmed from the residue-level sequence. GHRP-6 has also served as a template for conformational chemistry: a 2014 study synthesized eight GHRP-6 analogs containing N-aminoimidazolidin-2-one residues that rigidify the peptide backbone.^[6]

Research History and Development

GHRP-6 was among the earliest members of a new chemical class of growth hormone secretagogues that ranges from small synthetic peptides to non-peptide mimetics. A 1998 review described the class as releasing growth hormone through a unique dual and complementary action on the hypothalamus and pituitary.^[7]

Early mechanistic work, summarized in a 1993 review, examined GHRP-6 in experimental animal models and in obese subjects and described it as a new hexapeptide that specifically releases growth hormone.^[5]

Although GHRPs are of unnatural origin, investigators argued through the 1990s that their activity reflected the action of a natural hypothalamic hormone not yet identified, and that a natural GHRP-like ligand must exist.^[7][1] Interest persisted because of possible diagnostic and therapeutic value and because the peptides offered new insight into how growth hormone secretion is regulated.^[7]

After GHRP-6, a heptapeptide (GHRP-1) and two further hexapeptides, GHRP-2 and hexarelin, were synthesized for human studies, and non-peptide mimetics such as MK-0677 followed.^[1] More recent work has moved beyond endocrinology into cytoprotection, including cardiac, gastrointestinal and renal injury models.^[4][8][9]

Mechanisms Studied in Research

GHRP-6 releases growth hormone through its own receptor at the pituitary and hypothalamus, and studies point to functional opposition of somatostatin and cooperation with GHRH. The exact mechanism was still debated in the late 1990s.^[1]

Proposed endocrine mechanisms

• Somatostatin antagonism: evidence suggested GHRP-6 acts as a functional somatostatin antagonist at the pituitary level.^[2]
• Independence from hypothalamic factors: work in animal models and obese subjects found that GHRP-6 released growth hormone independently of GHRH and somatostatin.^[5]
• GHRH-linked action: reviews also proposed a partial GHRH-mediated mechanism, or an unidentified hypothalamic factor.^[1]
• Desensitization: responses to GHRPs partially desensitize, more with continuous infusion than with intermittent exposure, although prolonged administration still increased IGF-1 levels.^[1]

Cellular pathways in injury models

In a rat doxorubicin cardiotoxicity model, GHRP-6 sustained antioxidant defenses and upregulated the prosurvival gene Bcl-2.^[4] In a mouse acute kidney injury model, a self-assembling GHRP-6 hydrogel improved renal tubular epithelial cell survival through activation of the mTOR-P70 pathway and shifted metabolites including spermidine, L-glutamine and acetyl-CoA.^[9]

Key Findings From Published Studies

Human studies show that GHRP-6 is a strong growth hormone stimulus whose response depends on pituitary stalk integrity, thyroid status and blood glucose, while animal studies report protective effects in heart, gut and kidney injury.

Human endocrine studies

In short-statured children and adults with growth hormone deficiency, the growth hormone response to GHRP-6 was lower in deficient patients than in normal children, but individual results overlapped considerably. Responses to GHRP-6, alone or combined with GHRH, were almost completely blocked in patients with pituitary stalk transection.^[3]

In a study of 11 patients with primary hypothyroidism and 10 controls, each tested in random order on separate days, hypothyroid patients had markedly blunted responses to GHRH (mean peak 4.1 versus 24.9 µg/L) but higher responses to GHRP-6 than to GHRH (12.6 µg/L). Combining GHRP-6 with GHRH produced the largest responses in both groups.^[2] In 12 men with type 2 diabetes, peak growth hormone after combined GHRH and GHRP-6 was lower during a hyperglycemic clamp than a euglycemic clamp (112.45 versus 151.06 mU/L).^[10] GHRP-6 was also described as a powerful growth hormone releaser in obesity.^[5]

Animal and cell studies

• Heart: in healthy rats given doxorubicin, concurrent GHRP-6 prevented ventricular dilation, preserved left ventricular systolic function, attenuated damage to other organs and reduced morbidity and mortality.^[4]
• Gut and liver: GHRP-6 tripled migration of IEC-6 and HT29 intestinal cells without increasing proliferation in vitro, and in rat hepatic ischemia-reperfusion it reduced tissue damage markers by 50 to 85%, with added benefit alongside epidermal growth factor; lung and kidney injury were also reduced.^[8]
• Kidney: a GHRP-6 hydrogel enhanced recovery in a mouse acute kidney injury model.^[9]

Because PubChem's GHRP-6 record omits stereochemistry and a CAS number, researchers typically rely on HPLC purity and mass spectrometry identity data from certificates of analysis.

Regulatory and Research Status

GHRP-6 is not approved by the US Food and Drug Administration for any use and remains an investigational research compound. Growth hormone-releasing peptides as a group, including GHRP-6, are prohibited in sport under the World Anti-Doping Agency’s Prohibited List.

Clinical interest has focused on diagnostic testing of pituitary growth hormone reserve. Investigators suggested that GHRP-6 testing, alone or with GHRH, could be a cost-effective way to diagnose pituitary stalk transection and might help diagnose adult-onset growth hormone deficiency.^[3] Other authors cautioned that ambient glucose should be considered when interpreting combined GHRH and GHRP-6 tests in type 2 diabetes.^[10]

A 2020 review of growth hormone secretagogues in men with hypogonadism discussed GHRP-6 alongside sermorelin, GHRP-2, ibutamoren and ipamorelin, but concluded that a paucity of clinical data limits understanding of their role.^[11] The tissue-protection findings come from rodent and cell models and have not been confirmed in human trials. Analog chemistry also remains active, with GHRP-6 used as a scaffold for testing backbone constraints in peptide design.^[6]