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### Hexarelin peptide: a systematic review

Hexarelin is a synthetic hexapeptide belonging to the growth hormone releasing peptide (GHRP) family. It has attracted much attention for its ability to effectively promote the secretion of growth hormone (GH). This article will systematically explain the multidimensional characteristics of Hexarelin from the aspects of structural characteristics, physicochemical properties, biological activity, application fields and safety.

#### 1. Structural and chemical characteristics of Hexarelin

1. **Amino acid sequence and modification**

The sequence of Hexarelin is **His-D-2-Methyl-Trp-Ala-Trp-D-Phe-Lys-NH₂**, and its unique design includes the following key modifications:

- **D-amino acids**: D-2-methyltryptophan (D-2-Methyl-Trp) and D-phenylalanine (D-Phe) enhance peptidase resistance and prolong half-life.

- **C-terminal amidation**: Amidation of Lys further stabilizes the structure and reduces degradation.

2. **Molecular weight and chemical formula**

- Molecular formula: C₄₆H₅₈N₁₂O₆

- Molecular weight: 907.04 Da

- CAS number: 140703-51-1

3. **Structure-function relationship**

- **His residue**: mediates binding to growth hormone secretagogue receptor (GHS-R1a).

- **Tryptophan double structure**: enhances receptor activation efficiency and improves GH release efficiency.

#### 2. Physical and chemical properties

1. **Appearance and color**

- The pure product is a **white to off-white crystalline powder**, which is loose and porous after freeze-drying.

- It is colorless and transparent in solution, and may be slightly opalescent at high concentrations.

2. **Solubility**

- **Water solubility**: easily soluble in pure water (1-5 mg/mL), with the best solubility at pH 5-7.

- **Organic solvents**: Soluble in methanol and DMSO, but precipitates in acetone.

- **pH sensitivity**: Acidic conditions (pH <4) may cause partial degradation, and alkaline environment (pH >8) reduces stability.

3. **Stability**

- **Thermal stability**: Stable when stored at 4℃, half-life of about 30 days at 25℃, avoid high temperature (>40℃).

- **Photosensitivity**: Need to be stored away from light, ultraviolet radiation can cause disulfide bond rupture.

- **Storage conditions**: Recommended to store in a dry state at -20℃, and use within 7 days at 4℃ after reconstitution.

#### 3. Biological activity and mechanism of action

1. **GH release mechanism**

- Stimulate GH secretion by activating GHS-R1a receptors in the hypothalamus and pituitary, and the effect is dose-dependent.

- Synergizes with endogenous growth hormone-releasing hormone (GHRH) to enhance GH pulse amplitude.

2. **Receptor selectivity**

- The affinity for GHS-R1a (EC₅₀≈0.8 nM) is significantly higher than that for GHRP-6, and does not activate the appetite regulation pathway of ghrelin.

3. **Pharmacokinetics**

- The half-life is about 2-3 hours (rodents), and the half-life in humans is not yet clear, but it is estimated to be 1-2 hours.

- Metabolic pathway: Mainly excreted through the kidneys, and liver cytochrome P450 participates in some metabolism.

#### 4. Application areas and research progress

1. **Muscle atrophy and trauma repair**

- Animal studies have shown that Hexarelin can reverse glucocorticoid-induced muscle atrophy and promote muscle fiber proliferation (IGF-1 mediated).

- In the myocardial ischemia model, it exhibits anti-apoptosis and pro-angiogenesis effects.

2. **Anti-aging and metabolic regulation**

- Enhance the activity of the GH/IGF-1 axis and improve the lean body mass ratio and bone density of elderly individuals.

- Regulate lipid metabolism: reduce low-density lipoprotein (LDL) and improve insulin sensitivity.

3. **Neuroprotective effect**

- Pass the blood-brain barrier, reduce β-amyloid protein deposition, and improve cognitive function in mice in Alzheimer's disease models.

4. **Potential for sports medicine**

- In non-athlete studies, increase muscle strength and endurance, but be wary of the risk of prohibition in competitive sports.

####5. Advantages and limitations

1. **Core advantages**

- **High efficiency**: GH release efficiency is 1.5 times that of GHRP-6.

- **Selectivity**: Less appetite stimulation (compared to GHRP-2/GHRP-6).

- **Stability**: D-amino acid design reduces enzymatic hydrolysis rate.

2. **Potential side effects**

- Transient increase in cortisol, long-term use may affect the balance of the HPA axis.

- Occasionally, erythema or itching at the injection site is seen.

3. **Research limitations**

- Clinical data are concentrated in animal models, and long-term safety in humans has not yet been determined.

- The regulatory mechanism of GH pulse frequency is still controversial.

#### 6. Future research directions

1. **Structural optimization**: Develop oral or sustained-release dosage forms to improve bioavailability.

2. **Combination therapy**: Combined with GHRH to simulate the physiological GH secretion pattern.

3. **Clinical transformation**: Promote phase II clinical trials to verify its efficacy in sarcopenia and metabolic syndrome.

#### Conclusion

Hexarelin has become a hot topic in peptide research due to its unique structural design and multi-effect biological activities. Although it has great potential in translational medicine, its clinical application still requires rigorous safety evaluation. In the future, through dosage form innovation and mechanism exploration, Hexarelin may play a more important role in the field of anti-aging and metabolic diseases.