Ipamorelin, a synthetic pentapeptide growth hormone secretagogue, is a partial ghrelin analog and mimetic. Ipamorelin was discovered in the 1990s and is the first selective growth hormone-releasing peptide (GHRP). Like other GHRPs ipamorelin exerts its action through suppression of somatostatin and direct stimulation of GH release from anterior pituitary somatotrope cells, and is synergistic with GHRH analogs such as modified GRF (1-29) and CJC-1295. Unlike other GHRPs, ipamorelin is non-lipogenic, does not induce hunger, and does not induce release of ACTH (adrenocorticotropic hormone) or cortisol. Ipamorelin has been selected for development by Novo Nordisk.
Although less potent than GHRP-2, its selectiveness makes ipamorelin among the GHRP peptides. Raun et al discuss the selectiveness of ipamorelin for GH release shown in a study(1998):
Ipamorelin ... displays high GH releasing potency and efficacy in vitro and in vivo. ...In vitro, ipamorelin released GH from primary rat pituitary cells with a potency and efficacy similar to GHRP-6 ….A pharmacological profiling using GHRP and growth hormone-releasing hormone (GHRH) antagonists clearly demonstrated that ipamorelin, like GHRP-6, stimulates GH release via a GHRP-like receptor. In pentobarbital anaesthetised rats, ipamorelin released GH with a potency and efficacy comparable to GHRP-6 .... In conscious swine, ipamorelin released GH with an ED50 ... very similar to GHRP-6 None of the GH secretagogues tested affected FSH, LH, PRL or TSH plasma levels. Administration of both GHRP-6 and GHRP-2 resulted in increased plasma levels of ACTH and cortisol. Very surprisingly, ipamorelin did not release ACTH or cortisol in levels significantly different from those observed following GHRH stimulation. This lack of effect on ACTH and cortisol plasma levels was evident even at doses more than 200-fold higher than the ED50 for GH release. ipamorelin is the first GHRP-receptor agonist with a selectivity for GH release similar to that displayed by GHRH. The specificity of ipamorelin makes this compound a very interesting candidate for future clinical development.
Ipamorelin was originally discovered through an experimental program analyzing structures lacking the central dipeptide (Ala-Trp) common to other GHRPs.
Ipamorelin may be developed as a candidate for alternative, less-efficient administration systems that require a higher dose. In contrast to other GHRPs, ipamorelin can be applied at a 200-fold dose without any release of ACTH or cortisol or anything other than GH, meaning it also has a much higher margin of error and can be dosed in sufficient quantities to not require administration of a GHRH analog.
Ipamorelin releases a proportionate amount of GH even at very high doses.
Raun K, Hansen BS, Johansen NL, Thøgersen H, Madsen K, Ankersen M, Andersen PH. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998 Nov;139(5):552-61.
 Bowers CY, Momany F, Reynolds GA. In vitro and in vivo activity of a small synthetic peptide with potent GH releasing activity. 64th Annual Meeting of the Endocrine Society, San Francisco, 1982, p. 205.
Bowers CY, Momany F, Reynolds GA, Sartor O. Multiple receptors mediate GH release. 7th International Congress of Endocrinology, Quebec, Canada, 1984, p. 464.
 Brosnan-Cook, M. et al. (1998) Iontophoretic delivery of ipamorelin, a growth hormone secretagogue. Proceedings of 80th Annual Meeting Endocrine Society, New Orleans, USA. Abstract Pp1-186.
 Jogarao V S Gobburu; Henrik Agerso; William J Jusko . Pharmacokinetic-Pharmacodynamic Modeling of Ipamorelin, a Growth Hormone Releasing Peptide in Human Volunteers. Lars Ynddal Pharmaceutical Research: Sep 1999; 16, 9; ProQuest Nursing & Allied Health Source p. 1412.