Hexarelin

Hexarelin (examorelin) is a synthetic hexapeptide growth hormone secretagogue that activates both the GHS-R1a (ghrelin receptor) and the scavenger receptor CD36. Developed as an analog of GHRP-6, hexarelin contains a 2-methyl-tryptophan substitution that confers enhanced stability and potency. It is one of the most potent peptide GH secretagogues characterized, though it is also notable for its significant cardiac activity independent of GH release. The dual receptor profile — GHSR for GH secretion and CD36 for cardiac signaling — makes hexarelin pharmacologically unique among the GHRP family. Hexarelin has been investigated in clinical studies examining both its GH-releasing and cardioprotective properties. It demonstrates dose-dependent GH release but is subject to tachyphylaxis (reduced response) with repeated dosing, which limits its utility for sustained GH elevation.

Hexarelin has been investigated for its dual agonist activity at GHS-R1a and CD36 receptors. At the pituitary level, hexarelin activates GHS-R1a through Gq-coupled signaling, stimulating calcium-dependent GH release from somatotrophs. However, researchers observed that hexarelin’s cardiac effects are mediated through CD36, a scavenger receptor expressed on cardiomyocytes, macrophages, and endothelial cells. Studies suggest that CD36 activation by hexarelin in cardiac tissue is associated with PPARγ-dependent signaling, Akt phosphorylation, and changes in cardiac gene expression profiles. In isolated heart models, researchers observed that hexarelin was associated with functional changes in cardiac contractility and coronary perfusion parameters independent of GH or IGF-1 signaling. The peptide also demonstrates notable tachyphylaxis at the pituitary level — repeated dosing leads to progressively diminished GH responses, attributed to GHSR desensitization, while cardiac CD36-mediated effects appear more sustained.

Cardiac CD36 Signaling

Bhatt et al. (2006) investigated hexarelin’s cardiac effects and identified CD36 as its cardiac receptor, distinct from the GHS-R1a responsible for GH release. Researchers demonstrated that hexarelin’s cardiac activity persisted in GHS-R1a knockout models, confirming the independent CD36-mediated pathway [1].

Clinical GH Release

Ghigo et al. (1994) characterized hexarelin’s dose-response relationship for GH release in human subjects, establishing it as one of the most potent peptide GH secretagogues with both intravenous and subcutaneous activity [2].

Tachyphylaxis

Ong et al. (1998) investigated the phenomenon of hexarelin tachyphylaxis, demonstrating that repeated daily dosing led to progressively reduced GH responses. This study characterized the desensitization kinetics and recovery patterns of pituitary GHSR [3].