GHRP-6 (Growth Hormone Releasing Peptide-6)

1. Overview

GHRP-6 (Growth Hormone Releasing Peptide-6) is a synthetic hexapeptide with the sequence His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 and a molecular weight of approximately 873 g/mol. It is the founding member of the growth hormone-releasing peptide (GHRP) family, from which all subsequent GHRPs -- including GHRP-2, hexarelin, and ipamorelin -- were derived [1][20].

The discovery of GHRP-6 traces back to the work of American endocrinologist Cyril Y. Bowers, who in the late 1970s and early 1980s observed that certain chemical modifications of met-enkephalin produced unexpected growth hormone-releasing activity in pituitary cell cultures. Through systematic structure-activity relationship studies, Bowers and colleagues developed GHRP-6, which was characterized in 1984 as the first synthetic peptide to specifically and dose-dependently release GH both in vitro and in vivo through a mechanism entirely distinct from GHRH [1]. This discovery ultimately led to the identification of the growth hormone secretagogue receptor (GHS-R1a) in 1996 by Howard et al. [9], and subsequently to the discovery of ghrelin, the endogenous ligand for this receptor, by Kojima et al. in 1999 [10].

GHRP-6 has been extensively studied in human clinical research, particularly as a GH stimulation test and diagnostic tool for GH deficiency [11][12]. Beyond its neuroendocrine effects, research from the Center for Genetic Engineering and Biotechnology (CIGB) in Havana, Cuba, has established GHRP-6 as a broad cytoprotective agent with documented protective effects in cardiac, hepatic, gastrointestinal, and neuronal tissues in preclinical models [16][17][20]. GHRP-6 is not approved for any therapeutic indication by the FDA or other regulatory agencies and remains an investigational compound.

Molecular Weight

873.01 g/mol

Sequence

His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂

Half-life

Distribution: ~7.6 min; Elimination: ~2.5 hours (IV)

Routes Studied

Intravenous, subcutaneous, intranasal, oral

FDA Status

Not approved for any therapeutic use

WADA Status

Prohibited under S2 (peptide hormones, growth factors)

2. Mechanism of Action

GHRP-6 exerts its pharmacological effects through activation of two distinct receptors: the growth hormone secretagogue receptor type 1a (GHS-R1a) and the scavenger receptor CD36. This dual receptor engagement accounts for both its neuroendocrine and cytoprotective properties [20].

GHS-R1a-Mediated GH Release

The primary neuroendocrine mechanism of GHRP-6 involves binding to GHS-R1a, a G-protein-coupled receptor expressed on somatotroph cells of the anterior pituitary gland and on hypothalamic neurons in the arcuate nucleus [9][10].

Pituitary actions: At the pituitary level, GHRP-6 binding to GHS-R1a activates Gq/11 protein coupling, which stimulates phospholipase C (PLC) activity, generating inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 triggers intracellular calcium release from the endoplasmic reticulum, leading to GH exocytosis from secretory granules. This signaling pathway is fundamentally distinct from the cAMP/protein kinase A pathway activated by GHRH, which explains the synergistic interaction between GHRP-6 and GHRH when co-administered [6][7]. In vitro studies using human pituitary somatotroph cells have directly confirmed that GHRP-6 stimulates phosphatidylinositol turnover [6 from studies list].

Hypothalamic actions: GHRP-6 also acts at the hypothalamic level, stimulating the release of endogenous GHRH from arcuate nucleus neurons and potentially suppressing somatostatin tone [3]. Evidence from patients with hypothalamopituitary disconnection, in whom GHRP-6-induced GH release was completely abolished, indicates that the hypothalamic component is the predominant site of action and is essential for the full GH response [3]. This was further confirmed by studies showing that GHRH antagonist pretreatment substantially reduced the GH response to GHRP-6 [study from PMID 9543138].

Synergy with GHRH: The combination of GHRP-6 and GHRH produces a synergistic GH response that substantially exceeds the arithmetic sum of the individual responses [2][4]. This synergy occurs because the two peptides act through complementary signaling pathways (PLC/calcium for GHRP-6 and cAMP/PKA for GHRH) that converge on GH secretion, and because GHRP-6 amplifies GHRH availability through its hypothalamic actions [2][6].

Appetite Stimulation

GHRP-6 is among the strongest appetite stimulants in the GHRP class. Activation of GHS-R1a on hypothalamic neurons in the arcuate nucleus stimulates neuropeptide Y (NPY) and agouti-related peptide (AgRP) expression, which are potent orexigenic signals [15]. This appetite stimulation occurs within 20-30 minutes of injection and is neurologically mediated through ghrelin receptor pathways rather than through hypoglycemia [15].

CD36-Mediated Cytoprotection

Beyond the ghrelin receptor, GHRP-6 binds to CD36, a class B scavenger receptor expressed on macrophages, endothelial cells, cardiomyocytes, and hepatocytes [20]. CD36 engagement activates prosurvival signaling cascades, most notably the PI3K/AKT pathway, which inhibits apoptosis and promotes cell survival under conditions of oxidative stress, ischemia, and inflammation [20][27]. Additionally, GHRP-6 through CD36 activation reduces reactive oxygen species (ROS) generation, enhances endogenous antioxidant defenses, and attenuates inflammatory cytokine production [16][17][20].

Secondary Neuroendocrine Effects

Unlike more selective GHS-R1a agonists such as ipamorelin, GHRP-6 also stimulates the release of ACTH, cortisol, and prolactin, particularly at doses above 100 mcg [7][13]. This broader neuroendocrine activation is dose-dependent and represents off-target receptor engagement that distinguishes GHRP-6 from newer, more selective GH secretagogues [23].

3. Researched Applications

All applications listed below are investigational. GHRP-6 is not approved for clinical use in any jurisdiction.

Diagnosis of Growth Hormone Deficiency

Evidence level: Strong (multicenter human studies)

The combined GHRH plus GHRP-6 test has been validated as a diagnostic tool for adult GH deficiency. In a landmark multicenter study by Popovic et al. (2000), 125 adults with organic pituitary disease and 125 healthy controls were tested with GHRH (1 mcg/kg) plus GHRP-6 (1 mcg/kg) administered intravenously [11]. The combined test reliably discriminated GH-deficient patients from controls under receiver operating characteristic (ROC) analysis and was proposed as a safe, convenient alternative to the insulin tolerance test -- the traditional gold standard -- which carries risks of hypoglycemia [11]. Subsequent studies confirmed the test's accuracy even in challenging populations such as elderly and severely obese men [12].

Cardioprotection

Evidence level: Moderate (preclinical animal studies)

GHRP-6 has shown significant cardioprotective effects across multiple preclinical models. In a porcine acute myocardial infarction model, Berlanga et al. (2006) demonstrated that GHRP-6 reduced infarct mass by 78% and preserved wall thickness by 50% compared with saline, while decreasing markers of oxidative stress and preserving antioxidant defense systems [16]. In rats with pressure-overload heart failure, chronic GHRP treatment improved left ventricular ejection fraction, reduced LV end-diastolic pressure, alleviated cardiac cachexia, suppressed circulating catecholamines, renin, angiotensin II, and aldosterone, and reduced cardiomyocyte apoptosis [study from PMID 15951341]. More recently, GHRP-6 was shown to prevent doxorubicin-induced cardiotoxicity through activation of PI3K/AKT prosurvival mechanisms [27].

Gastroprotection and Multi-Organ Protection

Evidence level: Moderate (preclinical animal studies)

GHRP-6 has demonstrated cytoprotective effects in the gastrointestinal tract and across multiple organ systems. A single prophylactic injection of GHRP-6 prevented the onset of Curling-like stress ulcers and luminal bleeding in rats, acting through a dual mechanism of direct epithelial cytoprotection and vagal efferent modulation [17]. In ischemia-reperfusion models, GHRP-6 pretreatment reduced organ injury by 50-85% across liver, kidney, and gut tissues [17]. In vitro, GHRP-6 caused a 3-fold increase in the migration rate of intestinal epithelial cells (IEC-6 and HT29), suggesting direct pro-reparative effects on gut mucosa [20].

Anti-Fibrotic Effects and Liver Protection

Evidence level: Moderate (preclinical animal studies)

In rats with CCl4-induced chronic liver fibrosis, GHRP-6 reduced fibrotic tissue accumulation by more than 75% and decreased the number of cirrhotic nodules by up to 60% [19]. At the molecular level, GHRP-6 treatment reduced transcriptional expression of pro-fibrotic genes TGF-beta1 and CTGF while inducing expression of PPARgamma (a transcription factor that opposes fibrogenesis) and MMP-13 (a matrix metalloproteinase involved in collagen degradation) [19]. This anti-fibrotic work, led by researchers at Cuba's CIGB, received the Annual Prize of the Cuban Academy of Sciences in 2016.

Wound Healing

Evidence level: Moderate (preclinical animal studies)

GHRP-6 accelerated wound closure in rat models, with measurable differences in wound area reduction appearing within the first 24 hours post-injury [18]. In full-thickness skin wounds, GHRP-6 enhanced extracellular matrix synthesis, collagen organization, and angiogenesis through CD36 receptor interactions [18]. In hypertrophic scar models, GHRP-6 dramatically reduced the onset of exuberant scarring by activating PPARgamma and reducing pro-inflammatory and pro-fibrogenic cytokine expression [18].

Neuroprotection

Evidence level: Preliminary (preclinical and early clinical)

GHRP-6, in combination with epidermal growth factor (EGF), has been investigated for neuroprotection in ischemic stroke. Preclinical studies demonstrated that co-administration of EGF and GHRP-6 within 4 hours of ischemic insult significantly improved survival, neurological outcome, and reduced infarct volume [26]. A Phase I/II clinical trial in 2024 evaluated this combination in acute ischemic stroke patients and reported improved outcomes compared with standard treatment [26]. The neuroprotective mechanisms are attributed to activation of pleiotropic survival pathways and increased IGF-1 expression in brain tissue, particularly the hippocampus [20].

Pneumoprotection

Evidence level: Preclinical (2026)

A 2026 study demonstrated for the first time that GHRP-6 has pneumoprotective effects. In mice with LPS-induced acute lung injury, a single GHRP-6 administration attenuated pulmonary damage at 24 hours. In a chronic model, seven daily GHRP-6 administrations following LPS instillation reduced the progression to interstitial fibrosis at 28 days. This extends the documented cytoprotective repertoire of GHRP-6 to pulmonary tissue, complementing its established cardioprotective, hepatoprotective, and gastroprotective effects.

Age-Related GH Decline

Evidence level: Moderate (human studies)

Studies in elderly subjects (ages 65-90) demonstrated that GH responses to GHRP-6 alone and combined GHRP-6/GHRH were preserved compared with younger adults, suggesting that age-related somatopause represents a functional and potentially reversible state rather than irreversible pituitary failure [4]. GH responses to GHRP-6 were found to be substantially greater than responses to GHRH alone in late adulthood [4].

4. Clinical Evidence Summary

5. Pharmacokinetics

Intravenous Administration

Garcia-Fernandez et al. (2012) conducted the definitive pharmacokinetic study in 9 healthy male volunteers at IV doses of 100, 200, and 400 mcg/kg [14]:

• Pharmacokinetic model: Bi-exponential (two-compartment) disposition
• Distribution half-life (t1/2 alpha): 7.6 minutes, reflecting rapid distribution from central to peripheral compartments
• Elimination half-life (t1/2 beta): 2.5 hours, reflecting slower elimination from the peripheral compartment
• Volume of distribution (Vd): Approximately 0.4-0.6 L/kg at steady state
• Protein binding: Moderate; not extensively characterized but consistent with extracellular distribution
• Clearance: Primarily through enzymatic proteolysis by tissue and circulating peptidases; the D-Trp at position 2 and D-Phe at position 5 provide partial resistance to aminopeptidases
• Dose proportionality: Plasma levels increase proportionally across the 100-400 mcg/kg range, though GH release saturates at approximately 1 mcg/kg [14]
• Renal elimination: Minimal excretion of intact peptide; degradation products eliminated renally
• Onset of GH release: Within 5-10 minutes of IV bolus
• Peak GH response: 15-30 minutes post-IV injection
• Duration of GH pulse: 60-90 minutes before return to baseline, despite the short distribution half-life, because somatotroph activation outlasts circulating peptide levels

Subcutaneous Administration

• Bioavailability: Approximately 40-60% relative to IV based on GH release comparisons
• Tmax for GH peak: 30-60 minutes after SC injection
• Absorption: Rapid from the SC depot; complete absorption within 30-45 minutes
• Duration of GH pulse: Similar to IV (60-90 minutes) once peak is reached

Intranasal Administration

• Bioavailability: Approximately 15-25% of IV GH-releasing activity
• Effective dose: 30 mcg/kg intranasal (Frieboes et al. 1999) [13]
• Tmax for GH peak: 30-45 minutes
• Sleep effects: Intranasal dosing (30 mcg/kg) increased nocturnal GH secretion and modulated sleep architecture; oral dosing (300 mcg/kg) did not significantly alter nocturnal GH, ACTH, or cortisol [13]

Oral Administration

• Bioavailability: Very low (less than 5% of IV activity), though detectable GH release has been achieved at oral doses of 300 mcg/kg in children [5]
• Limitation: Extensive first-pass metabolism and gastric acid degradation render oral GHRP-6 impractical for routine use

6. Dose-Response Relationships

GH Release Dose-Response

GHRP-6 GH dose-response has been characterized across multiple human studies:

• 0.1-0.3 mcg/kg IV: Threshold GH release; minimal cortisol/prolactin effects
• 1.0 mcg/kg IV: Standard diagnostic dose; produces robust GH peak of 15-50 mcg/L in normal subjects; this is the saturation dose for GH release [2][7][11]
• 2.0 mcg/kg IV: No further GH increase above 1.0 mcg/kg; disproportionate increase in cortisol and prolactin
• 100 mcg SC (flat dose): Approximate equivalent of 1 mcg/kg for a 100 kg individual; commonly used as a practical fixed dose in research protocols
• Dose above saturation: Additional GHRP-6 beyond the saturation dose increases ACTH, cortisol, and prolactin without additional GH benefit, narrowing the therapeutic index [7]

GH, Cortisol, and Prolactin Dose-Response Comparison

At escalating GHRP-6 doses, the hormonal profile shifts unfavorably:

• At 1 mcg/kg IV (standard dose):
  • GH peak: 15-50 mcg/L (robust, near-maximal)
  • Cortisol: 10-25% increase from baseline (minimal clinical significance)
  • Prolactin: 15-30% increase from baseline (clinically insignificant)
  • ACTH: Mild increase
• At 2 mcg/kg IV (supramaximal):
  • GH peak: Similar to 1 mcg/kg (saturated)
  • Cortisol: 30-50% increase
  • Prolactin: 40-60% increase
  • ACTH: Moderate increase
• Selective ratio: GHRP-6 is more selective than hexarelin but substantially less selective than ipamorelin, which produces no cortisol or prolactin elevation even at doses 200-fold above GH ED50 [23]

Synergy with GHRH

The combination of GHRP-6 plus GHRH produces synergistic (supra-additive) GH release:

• GHRP-6 alone (1 mcg/kg IV): GH peak approximately 20-40 mcg/L
• GHRH alone (1 mcg/kg IV): GH peak approximately 15-30 mcg/L
• GHRP-6 + GHRH combined: GH peak approximately 80-150 mcg/L, exceeding the arithmetic sum by 2-3 fold [2][4]
• Triple combination (GHRP-6 + GHRH + pyridostigmine): Produces "massive" GH release (greater than 100 mcg/L) even in obese subjects who are typically resistant to GH stimulation [study from PMID 7783658]

Appetite Stimulation Dose-Response

• Threshold: Appetite stimulation occurs at GH-releasing doses (approximately 1 mcg/kg SC or IV)
• Onset: 20-30 minutes post-injection
• Duration: 30-60 minutes
• Mechanism: Hypothalamic NPY/AgRP neuron activation via GHS-R1a; independent of hypoglycemia [15]
• Comparison: GHRP-6 produces the strongest appetite stimulation of any GHRP tested, exceeding GHRP-2 and hexarelin; ipamorelin produces minimal appetite effect [15][23]

7. Comparative Effectiveness

GHRP-6 vs. Ipamorelin (Selectivity Comparison)

This is the most important clinical comparison, as ipamorelin was specifically designed to overcome GHRP-6's selectivity limitations:

| Parameter | GHRP-6 | Ipamorelin | |---|---|---| | GH release potency | Strong | Comparable (dose-adjusted) | | Cortisol elevation | Dose-dependent (+10-50%) | None (even at 200x ED50) | | Prolactin elevation | Dose-dependent (+15-60%) | None | | ACTH elevation | Dose-dependent | None | | Appetite stimulation | Strong (strongest GHRP) | Minimal | | Desensitization | Moderate with chronic use | Mild | | GHS-R1a selectivity | Moderate | High | | Clinical development | Diagnostic tool (GHRH/GHRP-6 test) | Phase II (post-operative GI) | | Unique advantages | Extensive human PK data; cytoprotective/CD36 effects | Hormonal selectivity; chronic use suitability |

Ipamorelin is preferred when chronic GH augmentation is the goal and hormonal side effects must be minimized. GHRP-6 is preferred for acute diagnostic testing (where the GHRH/GHRP-6 combined test is validated) and in research contexts where its cytoprotective properties are relevant [23].

GHRP-6 vs. Hexarelin (Potency Comparison)

• GH potency: Hexarelin produces approximately 1.5-2 fold greater peak GH than GHRP-6 at equivalent doses due to its D-2-methyltryptophan modification
• Cortisol/prolactin: Hexarelin produces greater hormonal co-secretion than GHRP-6 at GH-equivalent doses
• Appetite: GHRP-6 produces stronger appetite stimulation than hexarelin
• Desensitization: Hexarelin shows more pronounced tachyphylaxis (significant by week 4 vs. moderate for GHRP-6)
• Cardiac CD36: Both bind CD36, but hexarelin has more extensive cardiac data in humans
• Metabolic stability: Hexarelin has longer half-life (~70 min vs. 7.6 min distribution/2.5 h elimination for GHRP-6) due to its methyltryptophan substitution

GHRP-6 vs. Insulin Tolerance Test (GH Deficiency Diagnosis)

The GHRH/GHRP-6 combined test has been validated against the gold-standard ITT:

• Diagnostic accuracy: GHRH/GHRP-6 test has comparable sensitivity and specificity to ITT for adult GH deficiency diagnosis [11][12]
• Safety advantage: No risk of hypoglycemia (ITT can cause seizures, is contraindicated in elderly and those with seizure disorders or coronary artery disease)
• Convenience: Single IV bolus vs. insulin-induced hypoglycemia requiring glucose monitoring and medical supervision
• Limitation: GHRH/GHRP-6 may over-diagnose GHD in very obese patients (BMI greater than 40), though GHRP-6 performs better than GHRH alone in obesity [12]

GHRP-6 Appetite Mechanism

GHRP-6's appetite-stimulating effect is mediated through a specific neurological pathway distinct from hunger driven by hypoglycemia:

• Receptor: GHS-R1a on arcuate nucleus neurons in the hypothalamus
• Downstream signaling: Activation of NPY (neuropeptide Y) and AgRP (agouti-related peptide) neurons, the primary orexigenic pathway in the brain [15]
• Timing: Appetite surge begins 20-30 minutes post-injection and lasts 30-60 minutes
• Characteristics: Described as a compelling urge to eat, independent of blood glucose level; does not require or cause hypoglycemia
• Comparison with ghrelin: Mimics the pre-meal ghrelin surge that signals hunger, as both act on the same receptor
• Clinical significance: This appetite effect can be therapeutically useful in cachexia and wasting conditions (preclinical data shows alleviation of cardiac cachexia), but is an unwanted side effect for GH augmentation purposes

8. Dosing in Research

The following table summarizes doses used in published research studies. These are not therapeutic recommendations. GHRP-6 is not approved for human use, and optimal dosing in humans has not been established through Phase III trials.

Key dosing observations from the literature:

• The saturation dose for GH release is approximately 100 mcg per subcutaneous injection (or approximately 1 mcg/kg body weight); doses above this threshold do not proportionally increase GH release but do increase cortisol and prolactin co-secretion [7][14].
• GH pulse onset occurs within 15-30 minutes of injection and returns to baseline within 60-90 minutes due to the short half-life (distribution half-life approximately 7.6 minutes, elimination half-life approximately 2.5 hours) [14].
• Oral bioavailability is lower than parenteral routes; oral doses of 300 mcg/kg were needed to achieve significant GH release in children [5].
• Intranasal administration (30 mcg/kg) produced significant nocturnal GH increases while oral and sublingual routes at comparable doses were less effective [13].

9. Safety and Side Effects

Observed Effects in Human Studies (Quantitative Rates)

GHRP-6 has been used in human research for over three decades, providing a substantial safety database from short-term administration studies. Quantitative adverse event data from published studies:

Cortisol and ACTH elevation:

• At 1 mcg/kg IV (standard diagnostic dose): Cortisol increase of 10-25% from baseline, ACTH increase of 15-30% (generally clinically insignificant) [7]
• At 2 mcg/kg IV and above: Cortisol increase of 30-50%, ACTH increase of 40-60% [7]
• Incidence of clinically significant cortisol elevation (more than 2x baseline): Less than 5% at standard doses, approximately 15-20% at supramaximal doses
• This effect distinguishes GHRP-6 from ipamorelin, which does not significantly alter ACTH or cortisol even at doses exceeding 200-fold the GH-releasing ED50 [23]

Prolactin elevation:

• At 1 mcg/kg IV: 15-30% increase from baseline (clinically insignificant) [7]
• At higher doses: 40-60% increase; no galactorrhea reported in any study
• Incidence of prolactin elevation above normal range: Less than 10% at standard doses

Appetite stimulation:

• Incidence: Approximately 30-50% of subjects report noticeable appetite increase at GH-releasing doses
• Onset: 20-30 minutes post-injection
• Duration: 30-60 minutes
• Mediated through hypothalamic GHS-R1a activation of NPY/AgRP pathways; not related to hypoglycemia [15][20]
• Strongest appetite effect among all GHRPs studied

Glucose metabolism:

• Fasting glucose increase: 5-15 mg/dL with repeated dosing (modest)
• Insulin sensitivity: May decrease by approximately 10-20% with chronic GH elevation, consistent with known GH diabetogenic effects [24]
• No cases of new-onset diabetes attributed to GHRP-6 in published studies

Local reactions: Transient flushing (10-20%), dizziness (less than 5%), and injection site reactions (less than 5%) have been reported with parenteral administration [14].

Water retention: Mild fluid retention in less than 10% of subjects, related to GH-mediated sodium retention [24].

Serious adverse events: Zero serious adverse events attributed to GHRP-6 across all published human studies spanning more than 30 years and hundreds of subjects [14][20][24].

Comparison of Side Effect Profiles Across GH Secretagogues

| Compound | GH Release | Appetite Increase | Cortisol Elevation | Prolactin Elevation | |---|---|---|---|---| | GHRP-6 | Strong | Strong | Dose-dependent | Dose-dependent | | GHRP-2 | Stronger | Moderate | Moderate | Moderate | | Hexarelin | Strongest | Moderate | Significant | Significant | | Ipamorelin | Comparable | Minimal | Minimal | Minimal |

Theoretical Long-Term Concerns

As with any agent stimulating the GH/IGF-1 axis, theoretical risks of chronic use include insulin resistance, fluid retention, joint pain, carpal tunnel syndrome, and potential effects on neoplastic cell growth. Long-term safety data from controlled human studies is absent. Hexarelin, a closely related GHRP, has shown receptor desensitization (tachyphylaxis) with chronic use, and similar effects may apply to GHRP-6 with sustained administration, though this has not been systematically studied [24].

Product Quality Concerns

As an unapproved compound available through unregulated channels, commercially sourced GHRP-6 carries risks of contamination, incorrect dosing, degradation products, and mislabeling. No pharmaceutical-grade GHRP-6 product exists for clinical use.

10. Regulatory Status

FDA: GHRP-6 is not approved by the U.S. Food and Drug Administration for any therapeutic indication. It has not undergone the formal drug approval process, though it has been used extensively in investigational settings for GH stimulation testing.

WADA: GHRP-6 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.

Cuba (CIGB): The most advanced clinical development of GHRP-6 has taken place in Cuba, where it has been investigated in combination therapies for stroke and wound healing at the Center for Genetic Engineering and Biotechnology (CIGB), including Phase I/II clinical trials [26].

11. Related Peptides

See also: Ipamorelin, CJC-1295

• Ipamorelin — A third-generation GH secretagogue derived from the GHRP scaffold. Acts on the same GHS-R1a receptor but with far greater selectivity: does not elevate cortisol, ACTH, or prolactin at GH-releasing doses, and produces minimal appetite stimulation [23].

• CJC-1295 — A modified GHRH analog that acts through the GHRH receptor (distinct from GHS-R1a). Acts synergistically with GHRPs when co-administered, reflecting the complementary PLC/calcium and cAMP/PKA signaling pathways for GH release.

• GHRP-2 — A more potent hexapeptide analog of GHRP-6 with stronger GH release per unit dose but still exhibiting cortisol and prolactin co-secretion. Produces less appetite stimulation than GHRP-6.

• Hexarelin — The most potent GHRP in terms of raw GH release, but with greater side effects including more pronounced cortisol/prolactin elevation and documented receptor desensitization (tachyphylaxis) requiring cycling.

• MK-677 (Ibutamoren) — A non-peptide, orally active GHS-R1a agonist with a prolonged duration of action. Raises IGF-1 levels for up to 24 hours but with different pharmacokinetic properties than injectable GHRPs.

12. References

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