Ipamorelin is a synthetic pentapeptide developed by Novo Nordisk in the late 1990s and is widely considered the first GHS-receptor agonist with selectivity for GH release comparable to endogenous GHRH. It is notable for producing robust pituitary GH pulses without significant co-stimulation of cortisol, prolactin, or ACTH — even at doses far exceeding its effective GH-releasing threshold.
Ipamorelin acts as a selective agonist of the growth hormone secretagogue receptor type 1a (GHS-R1a) — a G-protein coupled receptor expressed in the pituitary gland, hypothalamus, and multiple peripheral tissues. When ipamorelin binds GHS-R1a, it activates phospholipase C and triggers intracellular calcium mobilisation in somatotroph cells of the anterior pituitary, stimulating pulsatile release of growth hormone into circulation.
What distinguishes ipamorelin from earlier GH secretagogues is its receptor selectivity. In a landmark 1998 study by Raun et al., ipamorelin produced GH release comparable to GHRP-6 in conscious pigs, but at doses over 200-fold above the GH-releasing ED50, cortisol and ACTH remained statistically indistinguishable from GHRH controls. FSH, LH, prolactin, and TSH were unaffected at all doses tested.
Ipamorelin also suppresses somatostatin (the endogenous GH inhibitor) at the hypothalamic level, meaning it acts on both stimulatory and inhibitory arms of GH regulation. Its short terminal half-life (~2 hours IV) produces a discrete, time-limited GH pulse rather than sustained elevation.
GHRPs such as GHRP-6 and GHRP-2 stimulated GHS-R1a but also reliably elevated cortisol and ACTH, confounding research designs. Ipamorelin's compact Aib-His-D-2Nal-D-Phe-Lys-NH₂ structure — derived from a major Novo Nordisk chemistry programme — confers high receptor selectivity without adrenocortical activation.
This selectivity profile is of interest in research paradigms requiring isolated GH axis stimulation without adrenocortical confounders.
Ipamorelin's target receptor (GHS-R1a) is expressed throughout the GI tract, and ipamorelin has been evaluated as a potential prokinetic agent in postoperative ileus models. In the most significant human trial to date (2014), ipamorelin was well tolerated but did not significantly outperform placebo on the primary endpoint.
More recent experimental literature examines ipamorelin in nociception-related signalling and GI motility pathway research, expanding its utility beyond GH secretion studies.