GHRP-6
growth hormone-releasing peptide 6, SKF-110679, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2
GHRP-6 is the original growth hormone-releasing peptide, described by Cyril Bowers and colleagues in 1984, more than a decade before anyone knew what receptor it acted on, and fifteen years before ghrelin, the natural hormone for that receptor, was discovered. It is a genuinely important compound in the history of endocrinology. It is also, after forty years, a compound with no clinical outcome trial and no approval anywhere.
Mechanism
GHRP-6 is a hexapeptide agonist at the growth hormone secretagogue receptor 1a. Its discovery inverted the usual order of drug development: Bowers and colleagues designed it from opioid peptide fragments and demonstrated in 1984 that it released growth hormone specifically, without releasing LH, FSH, TSH or prolactin, across rats, monkeys, lambs, calves and chicks. The receptor it acted on was not cloned until Howard and colleagues did so in 1996, and its endogenous ligand, ghrelin, was not identified until Kojima and colleagues found it in the stomach in 1999. GHRP-6 was therefore the pharmacological probe that revealed an entire hormonal system.
Mechanistically it works at both pituitary and hypothalamus. At the somatotroph, GHS-R1a couples through Gq to phospholipase C, mobilising intracellular calcium and triggering growth hormone exocytosis. In the hypothalamic arcuate nucleus it suppresses somatostatin tone and amplifies endogenous GHRH release, which is why its maximal effect requires an intact hypothalamic GHRH supply, and why combining GHRP-6 with GHRH produces a synergistic rather than additive response. Bowers demonstrated that synergy in normal men in 1990 and it has been reproduced since. GHRP-6 is not selective in the way ipamorelin is: it raises ACTH, cortisol and prolactin alongside growth hormone, and it is a potent appetite stimulant through arcuate NPY/AgRP neurons. A large body of preclinical work, much of it from Cuban groups, has also explored GHS-R1a-independent cytoprotective actions of GHRP-6 in heart, liver and other tissues, including effects mediated through the scavenger receptor CD36.
What the research shows
GHRP-6's human research record is deep on acute pharmacology and empty on outcomes. Bowers and colleagues established in 1990 that intravenous GHRP-6 releases growth hormone dose-dependently in normal men and acts synergistically with GHRH, and this acute effect has been examined in obese subjects, older adults, children, and patients with pituitary conditions. Pandya and colleagues later showed that GHRP-6 requires endogenous hypothalamic GHRH for maximal stimulation, confirming its dual hypothalamic and pituitary site of action. As a research tool, GHRP-6 has been enormously productive. It is the compound through which the ghrelin system was found.
None of that is a therapeutic evidence base. In forty years there has been no randomised controlled trial of GHRP-6 for any clinical outcome in humans. Not for growth, not for body composition, not for cachexia, not for cardiac protection, not for anything. The most substantial recent research programme is preclinical: Berlanga and colleagues showed that GHRP-6 prevents oxidant cytotoxicity and reduces myocardial necrosis in a model of acute myocardial infarction, with a body of follow-on animal work on infarct size, ventricular remodelling, fibrosis and doxorubicin cardiotoxicity. This is interesting, coherent, and entirely in cells and animals. It has not been tested in a human trial.
Anyone assessing GHRP-6 should hold two things at once. Its acute human pharmacology is among the best characterised in this class. We know with reasonable confidence what a single dose does to circulating hormones. And its clinical utility is entirely unknown, because the study that would establish it has never been run.
Evidence assessment
Limited evidence
Two verified human studies exist, but both measure only acute hormone release over minutes to hours; the remaining citations are preclinical, and no randomised controlled trial for any clinical outcome has been conducted in the four decades since the compound was described.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone Preclinical only
The hexapeptide elicited dose-related growth hormone release without concomitant release of LH, FSH, TSH or prolactin, and without species dependence.
Growth hormone (GH)-releasing peptide stimulates GH release in normal men and acts synergistically with GH-releasing hormone Mixed evidence
GHRP-6 released growth hormone dose-dependently and produced a synergistic rather than additive response when combined with GHRH.
Growth hormone (GH)-releasing peptide-6 requires endogenous hypothalamic GH-releasing hormone for maximal GH stimulation Mixed evidence
Maximal growth hormone stimulation by GHRP-6 depends on intact endogenous hypothalamic GHRH, confirming a dual hypothalamic and pituitary site of action.
Growth-hormone-releasing peptide 6 (GHRP6) prevents oxidant cytotoxicity and reduces myocardial necrosis in a model of acute myocardial infarction Preclinical only
GHRP-6 prevented oxidant-induced cytotoxicity and substantially reduced infarct mass while preserving ventricular wall thickness.
A receptor in pituitary and hypothalamus that functions in growth hormone release Preclinical only
Identified GHS-R1a as the receptor through which GHRP-6 and related secretagogues act, twelve years after GHRP-6 itself was described.
Safety
Acute administration is well tolerated in the human challenge studies conducted since 1990. Flushing, transient hunger and mild drowsiness are the usual reports after a single dose. Beyond that, the safety file is empty: no study has examined repeated GHRP-6 administration in humans over weeks or months, so there is no basis for statements about chronic safety in either direction. Two intrinsic properties warrant attention. GHRP-6 raises ACTH, cortisol and prolactin along with growth hormone, and the consequences of driving the adrenal axis repeatedly are unstudied. It is also one of the most potent appetite stimulants in the class, which is the point in cachexia research and a substantial problem in most other contexts. Whether tachyphylaxis develops with repeated use, as has been demonstrated for hexarelin, has not been formally established for GHRP-6. The general class hazards apply and are untested: glucose intolerance, fluid retention, arthralgia and the theoretical neoplasia concern from sustained IGF-1 elevation. Material bought outside a regulated supply chain carries the additional and entirely separate risks of unverified identity, purity, sterility and endotoxin content.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | No UK marketing authorisation. Supply for human use is unlawful under the Human Medicines Regulations 2012. Lawful only as a laboratory research chemical, and the MHRA treats accompanying dosing or administration instructions as marketing an unlicensed medicine. |
| United States | Not approved by the FDA for any indication and never submitted for approval. Not on Category 1 of the FDA's interim 503A bulk drug substances list, so it cannot lawfully be compounded for human use. Sold online as 'for research use only', a label that reflects no regulatory assessment of identity, purity or safety and functions largely to permit sale of an unapproved compound to consumers. |
| WADA (sport) | Prohibited at all times under section S2.2.4 of the WADA Prohibited List, which names GHRP-6 explicitly among prohibited growth hormone-releasing peptides. |
Questions
In research, it has been used for four decades as a pharmacological probe of the growth hormone axis and, historically, as the tool that led to the discovery of the ghrelin receptor and ghrelin itself. It has no approved clinical use anywhere in the world, and no randomised controlled trial has ever tested it for a clinical outcome.
Yes, pronouncedly. It is among the most potent appetite stimulants in this class, acting on NPY/AgRP neurons in the hypothalamic arcuate nucleus, the same mechanism by which ghrelin signals hunger. This is a therapeutic goal in cachexia research and an unwanted effect in most other settings.
All three are ghrelin receptor agonists. GHRP-6 is the original and the least potent per unit dose, with the strongest appetite effect. GHRP-2 is more potent as a growth hormone releaser and is the only one with a regulatory approval, in Japan for diagnostic use. Ipamorelin is the selective one, releasing growth hormone without raising cortisol, ACTH or prolactin.
There is a coherent body of animal evidence: reduced infarct size, less fibrosis, preserved mitochondrial function, protection against doxorubicin cardiotoxicity. All of it is preclinical. No human trial of GHRP-6 for any cardiac outcome has been conducted, so the finding remains a hypothesis rather than a result.