Growth Hormone-Releasing Peptide (GHRP)

Growth hormone-releasing peptides are short synthetic sequences, typically six or seven residues, that stimulate growth hormone release through the growth hormone secretagogue receptor GHS-R1a. They were derived in the laboratory of Cyril Bowers from enkephalin fragments that released growth hormone without opioid activity, long before anyone knew what they were binding. The receptor was cloned before its natural ligand, and the search for it ended in 1999 with the isolation of ghrelin from the stomach.

The family includes GHRP-6, GHRP-2, hexarelin and the later, more selective ipamorelin. They signal through Gq and phospholipase C rather than the cyclic AMP arm used by GHRH, which is why giving both together produces a burst larger than the sum of either alone. GHRP-2, as pralmorelin, is used in Japan as a diagnostic agent, and the orally active non-peptide secretagogue macimorelin was approved by the FDA in 2017 for diagnosing adult growth hormone deficiency. No GHRP is an approved therapy anywhere.

The pharmacological interest is that GHRPs amplify endogenous pulses rather than replacing them, so IGF-1 feedback and somatostatin tone still shape the output. That preserved regulation is real but it is not safety, and the ceiling it imposes means achievable peaks are bounded by what the pituitary holds. In short human studies the effect on growth hormone is unambiguous; durable effects on body composition or function are not established.

Where this bites is selectivity. GHS-R1a agonism is not confined to growth hormone: GHRP-6 and hexarelin raise cortisol and prolactin and stimulate appetite, and the receptor is expressed well beyond the pituitary. Vendor material describing these peptides as targeted growth hormone releasers, or transferring ipamorelin's cleaner profile onto hexarelin, claims a class property the class does not have.