1. Introduction and scope
Growth hormone (GH) secretagogues are a structurally heterogeneous group of compounds that, in published experimental and clinical research, increase circulating GH by acting on one of two principal upstream pathways. This review summarises peer-reviewed literature comparing these compound classes at the level of reported receptor pharmacology and observed physiological effects. It is a thematic literature summary intended for an in-vitro, research-use-only context. It does not provide dosing, administration guidance, or therapeutic recommendations, and it makes no health claims. Where studies described clinical populations, those findings are reported strictly as published outcomes of the cited investigations.
The compounds discussed fall into three groups as characterised in the literature: GHRH-receptor agonists (GHRH analogues such as sermorelin, CJC-1295 and tesamorelin); ghrelin-receptor agonists, comprising the growth-hormone-releasing peptides (GHRPs) and related synthetic agonists such as ipamorelin; and non-peptide secretagogues, exemplified in the literature by MK-677 (ibutamoren).
2. The GH axis: GHRH, ghrelin and the GHS receptor
Secretion of GH from the anterior pituitary is described in the literature as being governed by dual hypothalamic input: growth-hormone-releasing hormone (GHRH), which stimulates GH synthesis and release through the GHRH receptor, and somatostatin, which is inhibitory. A separate stimulatory pathway was uncovered through pharmacological studies of synthetic peptides. Bowers and colleagues reported that a synthetic hexapeptide acted directly on the pituitary to release GH through a mechanism distinct from that of GHRH (Bowers et al., 1984). This observation implied the existence of a then-unidentified receptor.
That receptor was subsequently cloned and characterised as a G-protein-coupled receptor expressed in the pituitary and hypothalamus, designated the growth hormone secretagogue receptor (GHS-R) (Howard et al., 1996). Reported signalling proceeded through the phospholipase C pathway, contrasting with the cyclic-AMP-coupled signalling described for the GHRH receptor. The identification of an orphan receptor activated by synthetic ligands raised the question of its endogenous agonist. Kojima and colleagues isolated this ligand from stomach tissue, an acylated 28-amino-acid peptide they named ghrelin, and reported that it potently released GH (Kojima et al., 1999). The literature thus establishes two parallel stimulatory inputs to pituitary somatotrophs: the GHRH receptor and the ghrelin receptor (GHS-R).
3. GHRH-receptor agonists (GHRH analogues)
GHRH analogues are peptides that reproduce the receptor-binding region of native GHRH. Sermorelin corresponds to the biologically active 1-29 amino-acid sequence of human GHRH, and review literature has summarised the rationale for studying truncated GHRH fragments as a means of engaging the native GHRH receptor (Walker, 2006). A recurring observation in the literature is that native GHRH and short analogues are subject to rapid enzymatic degradation, which limits their experimental duration of action.
Two strategies to extend duration are described. CJC-1295 is a GHRH analogue engineered for prolonged activity; Teichman and colleagues reported that its administration produced sustained elevations of GH and insulin-like growth factor I (IGF-I) over an extended period in healthy adults (Teichman et al., 2006). Tesamorelin is a stabilised GHRH analogue studied in a clinical population: Falutz and colleagues reported, in a randomised controlled trial in patients with HIV, a selective reduction in visceral adipose tissue alongside changes in the lipid profile (Falutz et al., 2007). These findings are reported here solely as published outcomes of the cited investigations.
4. Ghrelin-receptor agonists and the GHRPs
The GHRPs and related compounds act as agonists at the ghrelin receptor (GHS-R) rather than the GHRH receptor. The hexapeptide characterised by Bowers and colleagues was the prototype of this class (Bowers et al., 1984), and the identification of ghrelin as the endogenous ligand provided the physiological context for understanding how these synthetic agonists operate (Kojima et al., 1999).
A theme in the literature is the pursuit of selectivity. Earlier GHRPs were reported to elevate hormones beyond GH, including adrenocorticotropic hormone (ACTH), cortisol and prolactin. Raun and colleagues described ipamorelin as a pentapeptide that released GH with potency comparable to earlier GHRPs but without the concomitant elevations of ACTH, cortisol and other pituitary hormones, characterising it as the first selective GH secretagogue of its class (Raun et al., 1998). This reported selectivity is one of the principal points of differentiation drawn within the ghrelin-receptor-agonist group.
5. Non-peptide secretagogues
A distinct research strand pursued orally available, non-peptide molecules that engage the same ghrelin-receptor pathway. MK-677 (ibutamoren) is the most extensively studied compound of this type in the published literature. Nass and colleagues reported, in a randomised placebo-controlled trial in healthy older adults, that an oral ghrelin mimetic increased GH and IGF-I and was associated with changes in fat-free mass over the study period (Nass et al., 2008). The interest in this compound class within the literature stems from its reported oral activity and prolonged exposure profile, which contrast with the peptide secretagogues that were typically studied by injection in the cited investigations.
6. Comparative mechanism: two receptors, two release patterns
The clearest distinction drawn across these studies is the receptor engaged. GHRH analogues act at the GHRH receptor, coupled in the literature to cyclic-AMP signalling, whereas GHRPs, ipamorelin and MK-677 act at the ghrelin receptor (GHS-R), reported to signal through phospholipase C (Howard et al., 1996). Because these are independent receptors, the literature describes their stimulation of GH release as proceeding through separate, complementary mechanisms.
A second comparative theme is the temporal pattern of reported GH release. Studies of GHRH analogues and ghrelin-pathway agonists frequently describe GH output that preserves a pulsatile character, consistent with stimulation of the physiological release machinery rather than direct hormone replacement. Against this, compounds engineered for extended exposure, such as the long-acting GHRH analogue studied by Teichman and colleagues (Teichman et al., 2006) and the orally active non-peptide studied by Nass and colleagues (Nass et al., 2008), were reported to produce more sustained elevations of GH and IGF-I. The literature therefore frames a contrast between agents that emphasise pulsatile stimulation and those that emphasise sustained signalling, with the caveat that these characterisations derive from heterogeneous study designs and are not directly comparable across trials.
A third theme concerns endocrine selectivity within the ghrelin-receptor group. Whereas early GHRPs were reported to affect multiple pituitary axes, ipamorelin was characterised as releasing GH without those additional hormonal effects (Raun et al., 1998). Selectivity of this kind is presented in the literature as a property that distinguishes compounds within a single receptor class.
7. Research applications
Across the cited literature, GH secretagogues have served as research tools for investigating the regulation of the somatotropic axis. The synthetic peptides of Bowers and colleagues were instrumental in demonstrating the existence of a non-GHRH stimulatory pathway (Bowers et al., 1984), which in turn enabled the receptor cloning (Howard et al., 1996) and ligand-identification work (Kojima et al., 1999) that defined the ghrelin system. The comparative use of GHRH-receptor and ghrelin-receptor agonists has allowed investigators to dissect the two stimulatory inputs experimentally. Clinical investigations within the literature, including those of visceral adiposity (Falutz et al., 2007) and of body composition in ageing populations (Nass et al., 2008), have examined the physiological consequences of stimulating these pathways and are reported here only as the documented findings of those studies.
8. Limitations and research-use-only note
The studies summarised here span more than two decades and differ substantially in design, model system, compound, and endpoints, which limits direct comparison. Several were conducted in defined clinical populations and their findings should not be generalised. Reported patterns of GH release and selectivity reflect the specific conditions of each investigation. This document is a summary of published peer-reviewed research provided for informational and comparative purposes only. It does not constitute dosing guidance, administration instructions, medical advice, or any health claim. The compounds discussed are research chemicals intended for in-vitro laboratory research and research-use-only applications. They are not supplied or described for use in humans or animals, and nothing in this review should be interpreted as describing or recommending such use. Readers should consult the primary cited literature for full methodological detail and the limitations stated by the original authors.