What GHRP-6 and GHRP-2 are
GHRP-6 and GHRP-2 are synthetic hexapeptides developed as growth hormone secretagogues, both coming out of Cyril Bowers's research program at Tulane University. GHRP-6, with the sequence His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, was described first, in 1984 (Bowers et al., Endocrinology, 114(5):1537, 1984). GHRP-2, also known as KP-102, followed out of the same screening program, developed with the German firm Polygen and later marketed in Japan by Kaken Pharmaceutical.
Both compounds release growth hormone without a matching rise in most other pituitary hormones, the property that separated them from earlier candidates when they were first synthesized. Neither is approved for therapeutic use anywhere. GHRP-2 carries one narrow diagnostic approval in Japan, covered below; GHRP-6 has none.
Mechanism: the same receptor, competing for the same site
Both peptides work through the growth hormone secretagogue receptor, GHS-R1a, a G-protein coupled receptor on pituitary somatotrophs and in the hypothalamic arcuate nucleus. Neither compound's target had a name when it was synthesized. That changed in 1999, when Kojima and colleagues purified a 28-amino acid acylated peptide from rat stomach extracts, showed it activated GHS-R1a with high potency, and named it ghrelin (Kojima et al., Nature, 402:656, 1999). GHRP-6 and GHRP-2 are both described as synthetic ghrelin mimetics rather than ghrelin analogues: they share a binding site with ghrelin, not a structure.
Cheng and colleagues tested whether GHRP-6 and GHRP-2 act through the same mechanism directly, using rat primary pituitary cells (Cheng et al., Life Sciences, 60(16):1385, 1997). GHRP-2 released GH through the same receptor and signaling pathway as GHRP-6, despite the structural difference between the two peptides. When the researchers combined both compounds at maximal concentrations, GH release was no higher than either compound produced alone. The two peptides compete for the same binding site rather than acting through separate, additive routes.
Potency and efficacy: what the comparative data shows
Raun and colleagues at Novo Nordisk ran the most complete head-to-head comparison, testing GHRP-6, GHRP-2, and their own compound ipamorelin across rat pituitary cells, anesthetized rats, and conscious swine (Raun et al., European Journal of Endocrinology, 139(5):552, 1998). In the rat pituitary cell assay, GHRP-6 had an EC50 of 2.2 ± 0.3 nmol/L with a maximal response (Emax) of 100 percent, the reference point the other compounds were measured against. In the in vivo rat and swine models, the paper reported that GHRP-2 displayed higher potency but lower efficacy than GHRP-6: a smaller dose of GHRP-2 was needed to start raising GH, but the peak response GHRP-2 could reach topped out below what GHRP-6 achieved at its own maximal dose.
That potency-efficacy tradeoff matters for protocol design. A more potent compound with a lower ceiling is not automatically the stronger secretagogue; it depends on whether a study is measuring threshold sensitivity or peak GH output. Raun's swine data put GHRP-6's own ED50 at 3.9 ± 1.4 nmol/kg, giving a concrete reference point for the dose range these hexapeptides operate in in a large-animal model.
Cortisol, ACTH, and appetite: the side-effect profile
Neither compound is selective for GH alone: Raun et al. reported that GHRP-6 and GHRP-2 both raise ACTH and cortisol at GH-releasing doses, the property that later motivated Novo Nordisk's development of ipamorelin as a more selective alternative. Arvat and colleagues measured GHRP-2's corticotropic activity directly in humans, giving GHRP-2 and hexarelin to six adults aged 22 to 27 and six older adults aged 66 to 73 (Arvat et al., Peptides, 18(6):885, 1997). Both peptides raised ACTH and cortisol to a degree the paper described as similar to human corticotropin-releasing hormone, hCRH, a hormone whose specific job is triggering that axis. GH responses were comparable between the two peptides and larger than the response to GHRH alone, though blunted in the older group.
Because both compounds activate GHS-R1a, the receptor ghrelin also uses to signal hunger, both carry an appetite effect independent of their GH activity. Wren and colleagues showed central administration of ghrelin and GH secretagogues in rats increased food intake and activated neurons in hypothalamic appetite centers, including the arcuate nucleus (Wren et al., Endocrinology, 143(1):155, 2002).
Laferrere and colleagues quantified the effect for GHRP-2 directly in humans, infusing seven lean men with GHRP-2 at 1 microgram/kg per hour subcutaneously for 270 minutes before a buffet-style meal; the men ate 35.9 ± 10.9 percent more than they did after a saline infusion (Laferrere et al., J Clin Endocrinol Metab, 90(2):611, 2005). Normalized for body weight, intake rose from 101.3 ± 10.5 kJ/kg under saline to 136.0 ± 13.0 kJ/kg under GHRP-2, a difference the authors reported as significant (p = 0.008). A protocol using either peptide in an animal model with food access has to account for this confound, since a shift in body composition could reflect increased intake rather than the GH pulse itself.
GHRH-axis dependence and regulatory status
Both peptides depend on an intact GHRH axis for their full effect, established through different experimental designs. Pandya and colleagues gave nine healthy men aged 20 to 30 a GHRH-receptor antagonist before injecting GHRP-6, and found the peak GH response fell from 33.8 to 6.2 µg/L, with the area under the curve dropping from 1701 to 376 µg·min/L (Pandya et al., J Clin Endocrinol Metab, 83(4):1186, 1998). GHRP-6 does not bind the GHRH receptor directly, but the hypothalamic GHRH pathway carries most of its downstream effect.
A parallel result for GHRP-2 comes from a chronic dosing study in GHRH-knockout mice, animals with pituitary hypoplasia and dwarfism from birth. Six weeks of twice-daily subcutaneous GHRP-2 at 10 micrograms failed to restore somatotroph proliferation, GH secretion, or longitudinal growth in these mice (Alba et al., Am J Physiol Endocrinol Metab, 2005). GHRP-2 acts through a receptor distinct from GHRH, the same conclusion reached for GHRP-6, but a functioning GHRH axis is what the pituitary needs for either peptide to produce a full response.
Regulatory status is where the two compounds diverge most sharply. Japan's Pharmaceuticals and Medical Devices Agency approved GHRP-2 in October 2004 under the generic name pralmorelin, sold as GHRP Kaken, for a single narrow purpose: a one-dose diagnostic test for growth hormone deficiency (Pralmorelin, Drugs in R&D, 5(4):236, 2004). That remains the only regulatory clearance either compound has received anywhere, and it is diagnostic, not therapeutic. GHRP-6 has never received an approval of any kind, diagnostic or otherwise, and no completed Phase 3 program exists for either peptide as a treatment.
Practical considerations for research protocols
GHRP-6 and GHRP-2 both ship as lyophilized powder and follow the reconstitution and storage behavior common to short synthetic peptides: stable for extended periods frozen and unreconstituted, more fragile once dissolved, per the reconstitution guide. In Indonesian conditions, ambient heat and humidity accelerate degradation of any reconstituted vial left outside refrigeration faster than in temperate climates, a point covered in the lyophilized peptide storage guide. Researchers converting between mass, concentration, and injection volume for a given protocol can use the dosing calculator once a reconstitution concentration is set.
For protocols where cortisol or appetite confounds are unacceptable, neither GHRP-6 nor GHRP-2 is the selective option. The CJC-1295 and ipamorelin research overview covers the CJC-1295 and ipamorelin combination Novo Nordisk developed specifically to avoid the ACTH and cortisol rise both older hexapeptides carry. Full compound-level detail on each peptide individually is in the GHRP-6 research overview and the GHRP-2 research overview.
GHRP-6 vs GHRP-2: key differences at a glance
| Feature | GHRP-6 | GHRP-2 |
|---|---|---|
| Origin | Bowers et al., Tulane, 1984 | Tulane/Polygen, following GHRP-6 |
| Receptor | GHS-R1a | GHS-R1a (same site, competes with GHRP-6) |
| In vitro potency and efficacy | EC50 2.2 nmol/L, Emax 100% (reference) | Higher potency, lower efficacy (Raun et al., 1998) |
| ACTH/cortisol effect | Raises ACTH and cortisol at GH-releasing doses | Raises ACTH and cortisol; response size similar to hCRH (Arvat et al., 1997) |
| Appetite effect | Orexigenic in rat models (Wren et al., 2002) | 35.9% more food intake in men vs saline (Laferrere et al., 2005) |
| GHRH-axis dependence | GH response drops sharply with GHRH-receptor blockade (Pandya et al., 1998) | Chronic dosing fails to restore GH secretion without GHRH (Alba et al., 2005) |
| Regulatory status | Never approved for any use | Approved in Japan as a single-dose GH deficiency diagnostic (pralmorelin, 2004) |
Neither compound is a clean tool for isolating the GH axis from cortisol and appetite effects. The choice between them in a research design usually comes down to which secondary variable matters less for the question being asked, and whether the study needs GHRP-2's narrower dose-response window or GHRP-6's better-characterized decades-long track record.