Hexarelin (His-D-2-Methyl-Trp-Ala-Trp-D-Phe-Lys-NH2, MW 887.05 g/mol, CAS 140703-51-1) is a synthetic hexapeptide GH secretagogue receptor agonist among the most potent synthetic GHRPs studied in cell-based receptor pharmacology systems. It has additionally been characterised as a CD36 ligand in cardiomyocyte cell model research.

Molecular Properties

| Property | Value |

|----------|--------|

| Molecular Formula | C₄₇H₅₈N₁₂O₆ |

| Molecular Weight | 887.05 g/mol |

| CAS Number | 140703-51-1 |

GHSR-1a Receptor Pharmacology

Hexarelin demonstrates high-affinity binding to the growth hormone secretagogue receptor 1a (GHSR-1a), a G-protein coupled receptor primarily coupled to Gq/11 signalling pathways. In vitro binding assays using HEK293 cells transfected with human GHSR-1a demonstrate binding affinity values in the nanomolar range, with reported Ki values ranging from 0.7-2.1 nM across multiple cell-based assay systems.

Receptor Binding Kinetics

Competitive radioligand binding studies utilising [¹²⁵I-Tyr⁴]-ghrelin or [³H]-MK-0677 as radioligands reveal Hexarelin exhibits rapid association kinetics with GHSR-1a. The compound demonstrates competitive inhibition patterns characteristic of orthosteric receptor binding, with Hill coefficients approximating unity in most cell model systems, indicating single-site binding behaviour.

Binding kinetic parameters in CHO-K1 cells expressing recombinant human GHSR-1a show association rate constants (kon) of approximately 1.2 × 10⁷ M⁻¹s⁻¹ and dissociation rate constants (koff) of 0.014 s⁻¹, yielding kinetically-derived KD values consistent with equilibrium binding measurements.

Intracellular Signalling Cascades

Upon GHSR-1a activation, Hexarelin initiates intracellular calcium mobilisation through phospholipase C (PLC) activation and inositol trisphosphate (IP₃) formation. Calcium imaging studies in HEK293-GHSR-1a cells demonstrate rapid calcium influx with EC₅₀ values typically ranging from 0.9-3.2 nM, correlating closely with binding affinity measurements.

The compound additionally stimulates protein kinase C (PKC) activation and downstream MAPK/ERK signalling pathways. In vitro phosphorylation assays reveal significant ERK1/2 activation within 5-15 minutes of Hexarelin exposure in GHSR-1a-expressing cell lines, with maximal phosphorylation occurring at concentrations corresponding to receptor saturation.

CD36 Receptor Interactions

Beyond GHSR-1a pharmacology, Hexarelin exhibits binding affinity for the CD36 scavenger receptor, particularly in cardiac cell model systems. CD36 is a class B scavenger receptor involved in fatty acid uptake and cellular metabolism regulation.

CD36 Binding Characteristics

In vitro binding assays using isolated cardiac myocyte preparations demonstrate Hexarelin-CD36 interactions with micromolar binding affinities, approximately 1000-fold lower than GHSR-1a binding. Competition binding studies using established CD36 ligands such as thrombospondin-1 peptides confirm specific receptor binding, though the physiological relevance of this interaction requires further characterisation.

Cardiac Cell Model Research

Studies utilising H9c2 cardiomyoblast cell lines and primary neonatal rat cardiomyocytes indicate Hexarelin may modulate cellular signalling pathways independent of GHSR-1a activation. These effects appear mediated through CD36 receptor binding, involving distinct intracellular signalling cascades including AMPK activation and altered cellular metabolic flux patterns.

Structure-Activity Relationships

The hexapeptide structure of Hexarelin incorporates several modifications optimising receptor binding and metabolic stability. The D-amino acid substitutions at positions 2 and 5 (D-2-methyl-tryptophan and D-phenylalanine) enhance proteolytic resistance while maintaining high receptor affinity.

Systematic structure-activity relationship studies demonstrate that the His¹ residue is crucial for GHSR-1a binding, while the D-Trp² modification significantly enhances binding affinity compared to natural L-tryptophan analogues. The C-terminal amidation prevents carboxypeptidase degradation, extending peptide stability in cell culture systems.

Enzyme Kinetic Studies

Metabolic stability assays using hepatic microsomal preparations reveal Hexarelin exhibits enhanced stability compared to endogenous ghrelin, with half-life values exceeding 2 hours in human liver microsome incubations. This improved stability profile makes it particularly suitable for extended in vitro receptor pharmacology studies.

Peptidase susceptibility screening demonstrates resistance to common aminopeptidases and endopeptidases, attributable to the D-amino acid modifications and N-terminal histidine residue.

Research Summary

Hexarelin represents a valuable pharmacological tool for GHSR-1a receptor research, demonstrating high-affinity binding, potent receptor activation, and excellent metabolic stability in cell-based assay systems. Its dual interaction with CD36 receptors provides additional research applications in cardiac cell model studies. The compound's well-characterised binding kinetics and intracellular signalling profiles make it suitable for investigating growth hormone secretagogue receptor pharmacology and associated downstream signalling pathways in various in vitro experimental paradigms.

All content is intended for in vitro laboratory research purposes only. Not for human or animal consumption. Not intended to diagnose, treat, cure, or prevent any condition.