GHRP-2

GHRP-2 has the most published Phase I human pharmacology of any peptide in the GHRP class. The foundational dose-response work for GHRP-6 was Bowers et al., JCEM 1990, 70:975-982 — an 18-volunteer study in healthy men that established dose-dependent intravenous GH release (peak serum GH 7.6, 16.5, and 68.7 μg/L at 0.1, 0.3, and 1.0 μg/kg respectively), demonstrated synergy with GHRH at submaximal doses, and proposed that GHRP signaling reflected "a new physiological system in need of further characterization." The GHRP-2 pharmacology followed in Bowers, Alster, Frentz, JCEM 1992, 74:292-298: intravenous peak GH of 17.8, 38.3, and 63.0 μg/L across the 0.25-1.0 μg/kg dose range, a terminal half-life of approximately 20 minutes, a distribution volume of 2.5 L, and oral bioavailability of approximately 0.3% — establishing early that oral delivery, while feasible, was not pharmacokinetically efficient. The intranasal route fared better: Johansen et al., Xenobiotica 1998, 28:1083-1092 reported intranasal bioavailability of approximately 50% for GHRP-2 in rats and somewhat lower for GHRP-6, which is the practical reason intranasal pralmorelin became the formulation Japanese researchers pursued for pediatric short-stature work.

The pediatric dataset is unusually rich for a research-only peptide. Pihoker, Kearns, French, Bowers, JCEM 1998, 83:1168-1172 gave a single 1 μg/kg IV dose to 10 prepubertal children (mean age 7.7 years) and built the PK-PD model. Mericq, Cassorla, Salazar, Avila, Iñiguez, Bowers, Merriam, JCEM 1998, 83:2355-2360 then dosed six prepubertal GH-deficient children with subcutaneous GHRP-2 at 0.3, 1.0, and 3.0 μg/kg/day in successive two-month periods over eight months. Growth velocity rose dose-dependently, but IGF-1 and IGFBP-3 did not, and the authors concluded that longer-acting formulations would be needed for therapeutic use. No regulator-grade pediatric short-stature indication closed.

The longest chronic-administration window in the GHRP class comes from Bowers, Granda, Mohan, Kuipers, Baylink, Veldhuis, JCEM 2004, 89:2290-2300. Seventeen older men and women received continuous subcutaneous GHRP-2 at 1 μg/kg/h for 30 days. Pulsatile GH secretion was elevated more than three-fold on day 1, IGF-1 reached a stable plateau by day 1 and held through day 30, and safety screening tests remained normal. The same β-arrestin GHSR-1a desensitization sequence that produces tachyphylaxis with Hexarelin (Rahim et al. 1998) applies in principle to GHRP-2 and GHRP-6 — receptor internalization on sustained agonist exposure is a class property — and the Bowers 2004 GH curves do show partial attenuation at days 14 and 30 relative to day 1, consistent with the established receptor pharmacology even when downstream IGF-1 holds steady at a new plateau.

The pharmacological signature that distinguishes GHRP-2 and GHRP-6 from Ipamorelin is the cortisol, ACTH, and prolactin spillover that the selective secretagogue does not produce. The Arvat 1997 head-to-head comparison established that GHRP-2 raises ACTH and cortisol at potency similar to hCRH itself; the same pattern holds for GHRP-6. The spillover is measurable rather than overwhelming, and acute-dose studies report mild or transient symptoms — but the contrast with the Raun 1998 paper for Ipamorelin (no meaningful elevation of ACTH, cortisol, or prolactin at doses 200-fold above the GH ED50) is the load-bearing reason modern practitioner stacks moved to the selective secretagogue. The comparative-pharmacology narrative parallels Hexarelin: the first-generation GHRPs have the deepest human dataset but the dirtiest hormonal signal.

The feature that uniquely distinguishes GHRP-6 is its appetite-stimulating effect, consistent with GHSR-1a activation of NPY/AgRP neurons in the arcuate nucleus and orexin neurons in the lateral hypothalamus — the same circuitry through which endogenous ghrelin drives hunger. Lawrence, Snape, Baudoin, Luckman, Endocrinology 2002, 143:155-162 showed that central administration of ghrelin or GHRP-6 in rats induces feeding, activates the arcuate, paraventricular, and dorsomedial nuclei plus lateral-hypothalamic orexin neurons, and that the feeding response is blocked by a Y1 NPY-receptor antagonist. The human parallel is Laferrère, Abraham, Russell, Bowers, JCEM 2005, 90:611-614: seven lean men received a 270-minute subcutaneous infusion of GHRP-2 versus saline, and food intake from a subsequent buffet meal rose approximately 36% on GHRP-2. GHRP-6 is reported in practitioner literature to produce more pronounced subjective hunger than GHRP-2 at equivalent GH-releasing doses, and is the cult-fitness choice when appetite stimulation is the goal — in bulking-phase use or in low-appetite older adults. The mechanism is robust; the published human appetite-effect literature is small (the Laferrère 2005 cohort of n=7 is the canonical reference) and a head-to-head appetite comparison of GHRP-2 versus GHRP-6 in humans has not appeared.

The class-level cytoprotective and cardiac preclinical literature is consolidated in Berlanga-Acosta et al., Clin Med Insights Cardiol 2017, Mao, Tokudome, Kishimoto, J Geriatr Cardiol 2014, and Mosa et al., Endocrine 2015, 49:307-323. The non-Hexarelin GHRPs have a thinner cardiac story because the CD36 channel that Hexarelin engages is structurally specific; GHRP-2 and GHRP-6 act predominantly through GHSR-1a.

The most consequential modern fact about GHRP-2 is its Japanese regulatory recognition. Pralmorelin hydrochloride (GHRP Kaken; KP-102D) was approved in 2004 by Japan's PMDA as a single-dose intravenous diagnostic agent for assessing adult growth-hormone deficiency in patients aged 4 years and older (Drugs in R&D 2004, 5:236-239), with a peak GH at or below 15.0 μg/L distinguishing GH-deficient patients from healthy controls. This remains the only modern regulator-grade approval of any GHRP in any jurisdiction, and the indication is diagnostic stimulation testing, not therapeutic GH replacement. Outside Japan, GHRP-2 has not been approved by the FDA, the EMA, or other major regulators; Wyeth's earlier US therapeutic-development program was discontinued. GHRP-6 has no comparable regulatory recognition and remains research-only globally. Both peptides appear on the WADA Prohibited List under S2 and are banned in-competition and out-of-competition.

The honest framing: GHRP-2's Phase I and chronic-dosing human dataset is the most published of any GHRP, the cortisol and prolactin spillover is a real and dose-dependent feature that pushed modern stacks toward the selective successor, the GHRP-6 hunger effect is mechanism-grounded but small-cohort in humans, and the Japanese pralmorelin approval applies to diagnostic stimulation testing, not to therapeutic body-composition or recovery use.