GHRP-2

GHSR1a Pharmacology and Receptor Research

GHRP-2 has been used as a reference ligand in GHSR1a (ghrelin receptor) pharmacology research, both in primary rat pituitary cell preparations and in heterologous expression systems. The Bowers laboratory's medicinal-chemistry arc from the 1977 enkephalin observation through the 1981 synthesis of GHRP-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2) and the late-1980s synthesis of GHRP-2 established the hexapeptide template that the entire GHRP class shares^[1].

The D-2Nal substitution at position 2 of the hexapeptide is the structural change from GHRP-6 that confers higher GH-releasing potency in conscious rats (Doi et al. 2004)^[2]. The same D-2Nal residue was retained in the Ipamorelin redesign at position 3 of a five-residue backbone, alongside Aib substitution at the N-terminus and deletion of the central Ala-Trp dipeptide. The two molecules are companion entries in the GHRP medicinal-chemistry record: GHRP-2 is the more potent hexapeptide; Ipamorelin is the pentapeptide redesigned in 1998 (Raun et al., PMID 9849822) for narrower receptor engagement in animal preparations^[7].

HPA Axis Co-Activation and CRF Pathway Research

GHRP-2's broader receptor profile has been studied in rat and human preparations as a model of CRF-mediated hypothalamic-pituitary-adrenal co-activation. Hirotani et al. 2005 demonstrated in rats that KP-102's ACTH-releasing activity runs through hypothalamic CRF release, with a secondary AVP component identified by additive testing^[3]. The mechanism is distinct from the somatotroph GHSR1a pathway and indicates non-somatotroph engagement at the hypothalamic level.

In humans, Arvat et al. 1997 reported ACTH and cortisol responses comparable to hCRH after intravenous bolus GHRP-2 in n=12 healthy adults, alongside modest prolactin elevation lower than TRH^[4]. Laferrère et al. 2006 documented dose-dependent cortisol elevation with subcutaneous infusion in n=19 lean and obese subjects, significant only at the higher infusion rate^[5]. The dataset on GHRP-2's human HPA profile is directly measured, in contrast to the animal-only inference base for the Ipamorelin selectivity literature. Suzuki et al. 2022 examined the diagnostic utility of the combined GH and ACTH or cortisol response in a pituitary-disorder cohort and reported sensitivity 71% and specificity 100% for secondary adrenal insufficiency screening in the n=36 population studied^[8].

Orexigenic Effect and Hypothalamic Ghrelin-Axis Research

GHRP-2 has been used as a synthetic ghrelin-mimetic to investigate hypothalamic GHSR1a-mediated food-intake research in humans. Laferrère et al. 2005 reported that subcutaneous infusion of one microgram per kilogram of body weight per hour of GHRP-2 over 270 minutes in n=7 lean healthy men was associated with a 35.9 plus or minus 10.9 percent increase in ad libitum food intake at a buffet meal versus saline infusion^[9]. GH levels rose during infusion; macronutrient composition of intake was unchanged.

Laferrère et al. 2006 extended this work in n=19 lean and obese subjects across two infusion rates (low and high)^[5]. Dose-dependent food intake increases (10.2 percent at the lower rate, 33.5 percent at the higher rate) were reported, with obese subjects responding comparably to lean subjects. The mechanism is hypothalamic GHSR1a activation in arcuate-nucleus NPY/AgRP neurons, mirroring the pre-meal release pattern of endogenous ghrelin (Kojima et al. 1999). The orexigenic pharmacology is the mechanistic basis for the later development of anamorelin (Adlumiz), the oral nonpeptide GHSR1a agonist that received Japan PMDA approval in January 2021 for cancer cachexia.

Comparative GHS Pharmacology with GHRH and Hexarelin

GHRP-2 has been used as a reference comparator in head-to-head studies of GHSR1a agonists and the GHRH receptor pathway. Arvat et al. 1997 ran the most-cited human comparison in n=12 (six young and six elderly subjects) at low intravenous bolus doses, measuring GHRP-2 and hexarelin against GHRH, TRH, and hCRH^[4]. GHRP-2 and hexarelin produced similar strong GH responses that exceeded GHRH alone; both produced prolactin elevations lower than TRH; both produced ACTH and cortisol responses similar to hCRH.

Tiulpakov et al. 1995 reported that combined intravenous GHRH and GHRP-2 produced synergistic GH release greater than either agent alone in n=8 young men, the canonical Bowers two-column synergy between the hypothalamic GHRH-R and the pituitary GHSR1a inputs to somatotroph activation^[10]. Veldhuis et al. 2009 extended the comparative work in n=24 young men with experimentally induced hypogonadism, reporting that GHRP-2 GH efficacy was preserved across hypogonadism^[11]. These human comparator data sit alongside the Doi et al. 2004 conscious-rat and conscious-dog characterization of KP-102, which documented species-specific differences in GHRH response that GHRP-2 did not display^[2].

Diagnostic Validation and Clinical Endocrinology Context

GHRP-2 (as pralmorelin hydrochloride 100 microgram vial, GHRP Kaken 100) was approved by the Japan PMDA in October 2004 as a single-dose intravenous diagnostic test for growth hormone deficiency in adults and children aged four years and older^[12]. The diagnostic protocol is a one-microgram-per-kilogram intravenous bolus with GH sampling at standardized time points; the diagnostic cutoff is a peak GH value of approximately 15 micrograms per liter, with values at or above the cutoff classified as normal and values below classified as GH-deficient. The pralmorelin cutoff corresponds to approximately 3 micrograms per liter on the insulin tolerance test.

Suzuki et al. 2022 reported a modern (n=36) diagnostic-validation cohort using the pralmorelin test in a Japanese pituitary-disorder population, including secondary adrenal insufficiency screening using the documented ACTH and cortisol response^[8]. The single test yields information on both GH axis and HPA axis simultaneously, a feature enabled by GHRP-2's broader receptor profile.

The FDA-approved diagnostic peer in the US is macimorelin (Macrilen), an oral nonpeptide GHSR1a agonist approved December 2017 for adult GH deficiency diagnosis based on the Garcia et al. 2018 head-to-head trial against the insulin tolerance test^[13]. Macimorelin has displaced pralmorelin in US clinical practice; pralmorelin is not US-approved.

Replication, Selectivity, and Clinical Trial Status

The selectivity literature on GHRPs has an asymmetric evidence base that is load-bearing for any comparative claim between GHRP-2 and Ipamorelin. Raun et al. 1998 reported that Ipamorelin did not measurably elevate ACTH, cortisol, prolactin, FSH, LH, or TSH at doses greater than 200 times the GH ED50 in rats and swine^[7]. No published human study has measured cortisol or prolactin response to Ipamorelin specifically. GHRP-2's broader profile, by contrast, has been measured directly in humans (Arvat 1997^[4]; Laferrère 2006^[5]) and characterized mechanistically in rats via the Hirotani et al. CRF pathway^[3]. The honest comparative read is that GHRP-2 has the methodologically richer receptor-profile dataset; Ipamorelin has an animal-only selectivity inference whose human profile has not been measured in any peer-reviewed published study.

The Japan PMDA approval is single-dose intravenous diagnostic only, not chronic therapeutic. The Kaken KP-102LN intranasal pediatric Phase II program in Japanese short-stature concluded without statistically significant linear growth signals, and the Wyeth US adult Phase II program was discontinued in the early 2000s without published data^[14]. No Phase 3 therapeutic trial of GHRP-2 has been completed in any indication. The GHRP-2 literature contrasts on this point with the peer GHSR1a class: macimorelin (FDA-approved 2017 diagnostic), capromorelin (FDA-approved veterinary), and anamorelin (Japan PMDA-approved 2021 for cancer cachexia) all reached approval as oral nonpeptide agonists. The injectable peptide GHRP segment has zero chronic-use therapeutic approvals.

GHRP-2 is banned at all times under WADA 2026 Prohibited List S2.2.4 (Growth Hormone Releasing Factors), with no Therapeutic Use Exemption pathway^[15]. Cox et al. 2010 documented GHRP-2 at approximately 50 micrograms in each tablet of over-the-counter nutritional supplements sold in Cyprus, mislabeled with incorrect chemical structure, creating inadvertent doping risk^[16]. The peptide is not on the FDA 503A Category 1 positive list; the prior interim Category 3 (insufficient data to evaluate) designation was retired by FDA on January 7, 2025. Framing GHRP-2 as the most potent GHRP is a chemistry observation that does not translate to clinical-utility ranking, given that the entire injectable peptide GHRP class is clinically underdeveloped relative to the oral nonpeptide GHSR1a agonists.