GHRP-2 (Growth Hormone Releasing Peptide-2, Pralmorelin)

1. Overview

GHRP-2 (Growth Hormone Releasing Peptide-2), also known by its INN pralmorelin and development codes KP-102 and GPA-748, is a synthetic hexapeptide growth hormone secretagogue with the amino acid sequence D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2 and a molecular weight of 817.97 g/mol [9][10]. It was developed through structure-activity relationship optimization of the original GHRP-6 scaffold by Cyril Bowers and colleagues, with the substitution of D-2-naphthylalanine (D-2-Nal) at position 2 producing substantially enhanced GH-releasing potency compared to the parent compound [1][2][3].

GHRP-2 holds the distinction of being the first -- and currently only -- growth hormone secretagogue to receive regulatory approval for clinical use. In October 2004, pralmorelin was approved in Japan by the Pharmaceuticals and Medical Devices Agency (PMDA) as a diagnostic agent for the assessment of growth hormone deficiency in both adults and children, marketed by Kaken Pharmaceutical Co., Ltd. under the trade name GHRP Kaken [9][16]. The approval was based on multicenter clinical trials across 84 Japanese facilities demonstrating reliable discrimination between GH-deficient patients and healthy controls [9].

While GHRP-2 produces the strongest GH release of the classical hexapeptide GHRPs per unit dose, it also stimulates ACTH, cortisol, and prolactin secretion, placing it between GHRP-6 and hexarelin in terms of selectivity [3][8]. Compared to GHRP-6, GHRP-2 produces notably less appetite stimulation while maintaining superior GH-releasing potency [7]. It was evaluated in Phase II clinical trials for the treatment of short stature in children, but further therapeutic development was discontinued when intranasal GHRP-2 failed to produce clinically meaningful growth promotion despite increasing endogenous GH secretion [12].

Molecular Weight

817.97 g/mol

Molecular Formula

C45H55N9O6

Sequence

D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2

CAS Number

158861-67-7

Half-life

Approximately 25-30 minutes (estimated from GH peak kinetics)

Routes Studied

Intravenous, subcutaneous, intranasal

Japan Approval

Approved 2004 (GH deficiency diagnostic; Kaken Pharmaceutical)

FDA Status

Not approved

WADA Status

Prohibited under S2 (peptide hormones, growth factors)

2. Mechanism of Action

GHRP-2 exerts its pharmacological effects primarily through activation of the growth hormone secretagogue receptor type 1a (GHS-R1a), the same G-protein-coupled receptor that serves as the target for the endogenous ligand ghrelin [4][5].

GHS-R1a Activation and GH Release

GHRP-2 binds to GHS-R1a on somatotroph cells of the anterior pituitary gland, activating Gq/11 protein coupling that stimulates phospholipase C (PLC). PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate to generate inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 triggers intracellular calcium release from the endoplasmic reticulum, producing GH exocytosis from secretory granules [4][10]. This PLC/calcium signaling pathway is distinct from the cAMP/protein kinase A pathway used by GHRH, which accounts for the synergistic GH release observed when GHRP-2 and GHRH are co-administered [3][15].

Hypothalamic Actions

At the hypothalamic level, GHRP-2 activates GHS-R1a on neurons in the arcuate nucleus, stimulating endogenous GHRH release and potentially suppressing somatostatin tone [10]. This hypothalamic component is essential for the full GH response, as demonstrated by studies showing that intact hypothalamic-pituitary connections are required for maximal GHRP-mediated GH release [1].

ACTH and Cortisol Stimulation

GHRP-2 stimulates the hypothalamic-pituitary-adrenal (HPA) axis, producing dose-dependent increases in ACTH and cortisol [8][19]. Mechanistic studies in rats demonstrated that this effect is predominantly mediated through hypothalamic corticotropin-releasing factor (CRF) release rather than direct pituitary corticotroph activation [15 from studies]. In patients with Cushing's disease, exaggerated ACTH responses to GHRP-2 have been observed, suggesting additional direct pituitary actions on corticotroph adenoma cells [8]. This HPA axis activation has led to investigation of GHRP-2 as a diagnostic tool for ACTH reserve assessment [19].

Appetite Stimulation

Like ghrelin, GHRP-2 activates NPY/AgRP neurons in the arcuate nucleus through GHS-R1a, producing orexigenic effects [7]. However, the appetite-stimulating effect of GHRP-2 is less pronounced than that of GHRP-6, possibly due to differences in receptor binding kinetics or pharmacokinetic properties at the hypothalamic level [7].

Structural Basis for Enhanced Potency

The substitution of D-2-naphthylalanine (D-2-Nal) at position 2 of the hexapeptide sequence -- replacing the D-Trp found in GHRP-6 -- confers enhanced binding affinity and GH-releasing potency [2][10]. The bulky aromatic naphthyl side chain is thought to improve hydrophobic contacts within the GHS-R1a binding pocket, resulting in more efficient receptor activation per molecule of peptide administered.

3. Researched Applications

Diagnosis of Growth Hormone Deficiency (Approved in Japan)

Evidence level: Strong (multicenter clinical trials, regulatory approval)

GHRP-2 (pralmorelin) is approved in Japan as a diagnostic agent for GH deficiency assessment in adults and children over 4 years of age [9]. In the pivotal Japanese multicenter trial, 126 children across 84 facilities were tested with a single 100 mcg IV bolus of pralmorelin. Peak GH levels above 16 ng/mL in children and above 9 ng/mL in adults were established as cutoff values distinguishing GH-sufficient from GH-deficient individuals [9][14]. The test offers advantages over the insulin tolerance test (ITT), including absence of hypoglycemia risk, simpler administration, and comparable diagnostic accuracy [14][15][19].

ACTH Reserve Assessment

Evidence level: Moderate (human clinical studies)

The ACTH-releasing property of GHRP-2, initially considered a side effect, has been investigated as a diagnostic tool for evaluating hypothalamic-pituitary-adrenal (HPA) axis function [19]. Takeno et al. (2004) demonstrated that the GHRP-2 stimulation test can assess ACTH reserve in patients with hypothalamopituitary disorders, offering a simpler alternative to insulin tolerance testing and CRH stimulation for evaluating adrenal axis integrity [19].

GH Secretion in Obesity

Evidence level: Moderate (human clinical studies)

GHRP-2 testing has been used to characterize GH secretory capacity in obese individuals, where standard provocative tests are often unreliable due to obesity-related GH suppression [17]. Laferrere et al. (2005) demonstrated that GHRP-2 could distinguish functional obesity-related GH suppression from true organic GH deficiency, providing useful clinical information in populations where diagnostic interpretation is otherwise challenging [17].

Growth Promotion (Discontinued)

Evidence level: Negative (Phase II discontinued)

GHRP-2 was evaluated in Phase II clinical trials for growth promotion in GH-deficient children via intranasal spray administration. Despite increasing endogenous GH secretion, the treatment failed to translate into clinically significant height velocity improvements over 24 weeks, and the therapeutic development program was discontinued [12]. This outcome highlighted the distinction between acutely stimulating GH release and achieving the sustained GH/IGF-1 elevation needed for growth promotion.

4. Clinical Evidence Summary

5. Dosing in Research

The following table summarizes doses used in published research studies. Except for the approved diagnostic use in Japan (100 mcg IV single bolus), these are not therapeutic recommendations.

Key dosing observations from the literature:

• The approved diagnostic dose in Japan is 100 mcg administered as a single IV bolus, with blood sampling for GH measurement at 15, 30, 45, and 60 minutes post-injection [9].
• The GH-releasing dose-response saturates at approximately 1 mcg/kg body weight IV; higher doses produce proportionally greater ACTH, cortisol, and prolactin elevation without additional GH benefit [3].
• GHRP-2 produces peak GH levels approximately 15-30 minutes after IV or subcutaneous injection, with GH returning to baseline within 60-90 minutes.
• Synergistic GH release occurs when GHRP-2 is combined with GHRH, producing responses that substantially exceed the sum of individual responses [3][18].

6. Safety and Side Effects

Observed Effects in Human Studies

GHRP-2 has been studied in numerous human clinical trials spanning over two decades. Its safety profile as a single-dose diagnostic agent in Japan is well-characterized [9][10][11]:

Cortisol and ACTH elevation: GHRP-2 produces moderate, dose-dependent ACTH and cortisol increases. The cortisol elevation is greater than that seen with ipamorelin (which produces essentially none) but less than that observed with hexarelin [3][8][13]. At the diagnostic dose of 100 mcg IV, the ACTH/cortisol response is transient and clinically insignificant in healthy subjects.

Prolactin elevation: Modest, transient prolactin increases have been documented, following a dose-dependent pattern [3]. This effect is less pronounced than with hexarelin or GHRP-6.

Appetite stimulation: GHRP-2 increases food intake and hunger ratings in healthy men, though the orexigenic effect is less than that produced by GHRP-6 at equivalent GH-releasing doses [7]. This moderate appetite stimulation reflects the compound's intermediate position between the strong appetite effects of GHRP-6 and the minimal appetite effects of ipamorelin.

Glucose metabolism: As with all GH secretagogues, potential insulin resistance effects exist from elevated GH. However, single-dose diagnostic use does not produce clinically meaningful glucose changes [10][11].

Injection site reactions: Local reactions are generally mild and transient with parenteral administration.

Comparison with Other GH Secretagogues

| Compound | GH Release | Appetite Increase | Cortisol Elevation | Prolactin Elevation | |---|---|---|---|---| | GHRP-2 | Strongest (hexapeptide class) | Moderate | Moderate | Moderate | | GHRP-6 | Strong | Strong | Dose-dependent | Dose-dependent | | Hexarelin | Strongest (overall) | Moderate | Significant | Significant | | Ipamorelin | Comparable | Minimal | Minimal | Minimal |

Safety in Diagnostic Use

In the Japanese approval studies, GHRP-2 administered as a single 100 mcg IV injection demonstrated a favorable safety profile with no serious adverse events reported across the multicenter trial population [9][10][11]. The general pharmacological safety assessment showed no significant cardiovascular, respiratory, or central nervous system effects at GH-releasing doses in preclinical models [11].

Theoretical Long-Term Concerns

Long-term safety data from controlled human studies are absent. As with other GH secretagogues, theoretical risks of chronic use include insulin resistance, fluid retention, and potential effects on neoplastic cell growth. Receptor desensitization (tachyphylaxis) may occur with sustained use, as documented with the related peptide hexarelin [16].

7. Regulatory Status

Japan (PMDA): Pralmorelin (GHRP Kaken) is approved as a diagnostic agent for GH deficiency assessment in adults and children, marketed by Kaken Pharmaceutical Co., Ltd. since 2004 [9].

FDA (United States): GHRP-2 is not approved by the FDA for any indication. It has not undergone the FDA drug approval process.

WADA: GHRP-2 is prohibited by the World Anti-Doping Agency under category S2 (Peptide Hormones, Growth Factors, Related Substances, and Mimetics) at all times, both in-competition and out-of-competition [20].

8. Pharmacokinetics

GHRP-2 pharmacokinetics reflect its nature as a small synthetic hexapeptide administered by injection or intranasal spray. Formal human pharmacokinetic studies are limited, with most parameters inferred from pharmacodynamic (GH response) kinetics and the Japanese regulatory approval data [9][10][11].

Intravenous administration. Following a single 100 mcg IV bolus (the approved diagnostic dose in Japan), GHRP-2 produces peak GH levels within 15-30 minutes, with GH returning to baseline by 60-90 minutes [9]. The estimated plasma elimination half-life of GHRP-2 itself is approximately 25 minutes based on the kinetics of the GH response curve [9][10]. The rapid clearance is consistent with peptidase-mediated degradation and renal elimination typical of small peptides.

Subcutaneous administration. When administered subcutaneously (100-300 mcg), GHRP-2 produces a somewhat delayed GH peak (approximately 30-60 minutes post-injection) compared to IV dosing, reflecting the absorption phase from the subcutaneous depot. The overall GH area under the curve (AUC) is comparable between SC and IV routes, though peak GH concentrations are approximately 20-30% lower with SC administration. The effective duration of action for a single SC dose extends to approximately 90-120 minutes [3].

Intranasal administration. In the Phase II pediatric study by Tanaka et al. (2014), intranasal GHRP-2 spray was administered twice daily [12]. While formal bioavailability data for the intranasal route were not published, the measurable GH response confirmed systemic absorption, though the magnitude of GH release was lower than that achieved with parenteral administration, consistent with limited transmucosal absorption of peptides.

Distribution and metabolism. As a small hexapeptide (817.97 Da), GHRP-2 distributes broadly to highly vascularized tissues including the pituitary and hypothalamus, where its pharmacological targets (GHS-R1a) are located. Metabolism occurs through peptidase-mediated cleavage in plasma and tissues. The peptide does not undergo hepatic CYP450-mediated metabolism, and no clinically relevant drug-drug interactions have been reported.

Dose-proportionality. GH release exhibits dose-proportional increases up to approximately 1 mcg/kg IV, at which point the GH response plateaus while ACTH, cortisol, and prolactin responses continue to increase in a dose-dependent manner [3]. This ceiling effect for GH but not other hormones has implications for dose selection in both diagnostic and off-label settings.

9. Dose-Response Relationships

GHRP-2 demonstrates well-characterized dose-response relationships for GH, ACTH, cortisol, and prolactin that inform both its diagnostic use and its pharmacological profile relative to other GH secretagogues [3][8][13].

GH release dose-response. Arvat et al. (1997) established the definitive dose-response in healthy adults receiving 0.1, 0.3, and 1.0 mcg/kg IV [3]. GH secretion increased dose-dependently from 0.1 to 1.0 mcg/kg, with the 1.0 mcg/kg dose producing peak GH levels of approximately 40-80 ng/mL in healthy subjects. The GH dose-response curve saturates at approximately 1 mcg/kg, with higher doses providing no additional GH benefit while increasing off-target hormonal effects [3].

ACTH and cortisol dose-response. Unlike GH, the ACTH and cortisol responses to GHRP-2 do not plateau at 1 mcg/kg and continue to increase at higher doses. At the diagnostic dose (approximately 1.4 mcg/kg for a 70 kg adult receiving 100 mcg), cortisol elevation is transient and clinically insignificant. At supraphysiological doses, the ACTH/cortisol response becomes proportionally greater, a factor that limits dose escalation beyond the GH saturation point [3][8].

Prolactin dose-response. Prolactin elevation follows a dose-dependent pattern similar to ACTH/cortisol but with lower absolute magnitude. At diagnostic doses, prolactin increases are modest and transient [3].

Synergy with GHRH. The combined administration of GHRP-2 with GHRH produces GH release that substantially exceeds the mathematical sum of individual responses [3][18]. Mericq et al. (2003) confirmed this synergy in prepubertal children, demonstrating that the complementary PLC/calcium (GHRP-2) and cAMP/PKA (GHRH) signaling pathways converge to amplify somatotroph GH output [18]. This synergistic dose-response is clinically relevant for diagnostic testing (GHRP-2 plus GHRH test) and informs the pharmacological basis of combining GHS-R1a agonists with GHRH analogs [15].

Diagnostic cutoff dose-response. At the approved Japanese diagnostic dose of 100 mcg IV, GH-sufficient subjects achieve peak GH levels well above the diagnostic cutoff (16 ng/mL for children, 9 ng/mL for adults), while GH-deficient patients fail to reach these thresholds [9][14]. This clear pharmacodynamic separation underlies the diagnostic utility of the GHRP-2 stimulation test.

10. Comparative Effectiveness

GHRP-2 vs. GHRP-6

GHRP-2 and GHRP-6 share the same GHS-R1a target but differ meaningfully in potency, selectivity, and side effect profile [1][2][7]. GHRP-2 produces approximately 25-50% greater peak GH release per unit dose than GHRP-6, attributable to the D-2-naphthylalanine substitution at position 2 that enhances receptor binding affinity [2][10]. GHRP-6 stimulates appetite substantially more than GHRP-2 at equivalent GH-releasing doses, reflecting differences in hypothalamic signaling kinetics [7]. For diagnostic applications, GHRP-2 is preferred due to its stronger GH response and more predictable dose-response relationship.

GHRP-2 vs. Ipamorelin

Ipamorelin is a third-generation pentapeptide GHS-R1a agonist distinguished by its high selectivity for GH release without elevation of ACTH, cortisol, or prolactin at GH-releasing doses [13]. In direct pharmacodynamic comparisons, GHRP-2 produces slightly greater maximal GH release than ipamorelin but at the cost of moderate cortisol and prolactin elevation. For repeated-dose protocols where hormonal specificity is desired, ipamorelin offers a cleaner pharmacological profile. For diagnostic GH stimulation testing, where a single dose is used and off-target effects are transient, GHRP-2's greater GH-releasing potency is advantageous.

GHRP-2 vs. Hexarelin

Hexarelin is often cited as the most potent GHRP for acute GH release, but it produces the greatest off-target hormonal effects (significant cortisol, ACTH, and prolactin elevation) and is subject to pronounced receptor desensitization (tachyphylaxis) with repeated dosing [16]. GHRP-2 occupies an intermediate position: stronger GH release than GHRP-6, comparable to hexarelin on a per-dose basis, but with less cortisol/prolactin elevation and possibly less tachyphylaxis [3][8].

GHRP-2 vs. MK-677 (Ibutamoren)

MK-677 is a non-peptide, orally active GHS-R1a agonist with a much longer half-life (approximately 24 hours) producing sustained 24-hour IGF-1 elevation [16]. GHRP-2, by contrast, produces transient GH pulses with rapid return to baseline, more closely mimicking physiological GH pulsatility. MK-677's oral bioavailability and once-daily dosing offer practical advantages, but its sustained GH axis activation may carry greater long-term metabolic risks (insulin resistance, fluid retention). GHRP-2 has the distinction of being the only GH secretagogue with regulatory approval for any clinical use (diagnostic in Japan) [9].

| Feature | GHRP-2 | GHRP-6 | Ipamorelin | Hexarelin | MK-677 | |---|---|---|---|---|---| | GH potency | Very high | High | Moderate-high | Highest (acute) | Moderate | | Cortisol elevation | Moderate | Dose-dependent | Minimal | Significant | Minimal | | Prolactin elevation | Moderate | Dose-dependent | Minimal | Significant | Minimal | | Appetite stimulation | Moderate | Strong | Minimal | Moderate | Moderate | | Route | IV, SC, intranasal | IV, SC | SC | IV, SC | Oral | | Tachyphylaxis | Possible | Possible | Unlikely | Pronounced | Unlikely | | Regulatory approval | Japan (diagnostic) | None | None | None | None |

11. Enhanced Safety Profile

The safety profile of GHRP-2 is the most extensively characterized among peptide GH secretagogues due to its regulatory approval in Japan and decades of clinical study [9][10][11].

Diagnostic use safety (definitive data). In the Japanese multicenter approval study involving 126 children across 84 facilities, a single 100 mcg IV bolus produced no serious adverse events [9]. The general pharmacological safety assessment by Doi et al. (2004) demonstrated no significant cardiovascular, respiratory, or central nervous system effects at GH-releasing doses in rats, dogs, and mice [11]. These data provide the strongest safety evidence in the GHRP class.

Endocrine effects: quantitative data. At the diagnostic dose, cortisol elevation is transient (peaking at 15-30 minutes, resolving within 60-90 minutes) and clinically insignificant in healthy subjects [3]. In patients with ACTH-dependent Cushing's syndrome, GHRP-2 produces exaggerated ACTH/cortisol responses, which forms the basis of its diagnostic utility in this condition but constitutes a contraindication for routine use [8]. Prolactin elevation at diagnostic doses is modest (typically less than 2-fold above baseline) and resolves within 60-90 minutes [3].

Glucose metabolism. As with all GH secretagogues, transient hyperglycemia and insulin resistance are potential consequences of elevated GH levels. However, single-dose diagnostic use does not produce clinically meaningful glucose perturbations [10][11]. The long-term metabolic effects of chronic off-label GHRP-2 use have not been evaluated in controlled human studies.

HPA axis safety. The GHRP-2-induced ACTH/cortisol response is predominantly mediated through hypothalamic CRF release rather than direct pituitary activation, suggesting that the HPA axis stimulation is physiologically regulated rather than autonomous [15 from studies]. Repeat dosing in diagnostic protocols (multiple tests) has not been associated with sustained HPA axis perturbation.

Cardiovascular safety. Preclinical safety pharmacology studies showed no significant effects on heart rate, blood pressure, electrocardiographic intervals, or cardiac contractility at doses producing maximal GH release [11]. No cardiovascular adverse events have been attributed to GHRP-2 in any published human study.

Comparison with insulin tolerance test. The GHRP-2 stimulation test offers a significant safety advantage over the insulin tolerance test (ITT), the historical gold standard for GH deficiency diagnosis. The ITT requires inducing symptomatic hypoglycemia (blood glucose below 40 mg/dL), which carries risks of seizures, loss of consciousness, and cardiovascular events in susceptible patients. The GHRP-2 test achieves comparable diagnostic accuracy without hypoglycemia risk, making it particularly suitable for children, elderly patients, and individuals with cardiovascular disease or seizure disorders [14][15][19].

Theoretical long-term concerns. In the absence of controlled long-term human data, the following theoretical risks of chronic GHRP-2 use should be noted: insulin resistance from sustained GH elevation, fluid retention, potential tachyphylaxis with chronic receptor stimulation, and uncharacterized effects on neoplastic cell growth from chronic GH/IGF-1 axis activation [16]. These concerns are shared across the entire GH secretagogue class and are not specific to GHRP-2.

12. Related Peptides

See also: GHRP-6, Ipamorelin, Hexarelin, MK-677 (Ibutamoren), CJC-1295

• GHRP-6 -- The parent compound from which GHRP-2 was derived. Acts on the same GHS-R1a receptor with slightly lower GH-releasing potency but stronger appetite stimulation. Extensively studied for cardioprotective and cytoprotective effects.

• Ipamorelin -- A third-generation GH secretagogue with far greater selectivity than GHRP-2: does not elevate cortisol, ACTH, or prolactin at GH-releasing doses, and produces minimal appetite stimulation [13].

• Hexarelin -- The most potent GHRP in terms of acute GH release, but with the greatest side effect burden including significant cortisol/prolactin elevation and documented receptor desensitization with chronic use.

• MK-677 (Ibutamoren) -- A non-peptide, orally active GHS-R1a agonist with prolonged duration of action (24-hour IGF-1 elevation) and different pharmacokinetics than injectable GHRPs.

• CJC-1295 -- A GHRH analog that acts through the GHRH receptor, complementary to GHRP-2. The combination produces synergistic GH release through converging PLC/calcium and cAMP/PKA signaling.

13. References

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