Growth hormone secretagogues: GHRH analogues vs ghrelin mimetics

A growth hormone secretagogue is any compound that prompts the pituitary to release its own growth hormone rather than supplying GH from outside. The category is routinely discussed as one thing. It is two. GHRH-receptor analogues — sermorelin, tesamorelin, CJC-1295 — bind the growth hormone-releasing hormone receptor and amplify the pituitary's existing pulse pattern. Ghrelin-receptor agonists — ipamorelin and the GHRP series — bind GHS-R1a, a receptor from a different GPCR family, with different downstream signaling, a different selectivity profile for prolactin and the adrenal axis, and a direct link to appetite. Two receptors, two pharmacologies.

Every growth hormone secretagogue acts at one of two receptors

The GHRH receptor is a class B G-protein-coupled receptor, pituitary-specific in its original characterisation and cloned in 1992 (Mayo, Mol Endocrinol 1992; PMID: 1333056). It signals through Gs and cyclic AMP, and it is the receptor the hypothalamus uses to generate the normal GH pulse. Antagonist work showed that directly: in six healthy young men, a competitive GHRH-receptor antagonist suppressed pulsatile GH secretion overnight by roughly three quarters — reported as the first direct evidence that endogenous GHRH drives spontaneous GH pulses (Jaffe et al., J Clin Invest 1993; PMID: 8349808).

The second receptor was found the other way round — the ligand came last. A pituitary and hypothalamic receptor responsive to synthetic GH-releasing peptides was cloned in 1996 and named GHS-R1a (Howard et al., Science 1996; PMID: 8688086). Its natural ligand came three years later, when ghrelin was isolated from stomach (Kojima et al., Nature 1999; PMID: 10604470). GHS-R1a is a class A GPCR signaling through Gq and the phospholipase C / calcium pathway, expressed in pituitary and hypothalamus but also in the vagus and gut — which is why this arm carries effects the other does not.

The natural experiment that separates the two pathways

The cleanest demonstration that these are distinct pathways is genetic, not pharmacological. Eleven patients homozygous for an inactivating GHRH-receptor mutation — whose GHRH arm is effectively switched off — were compared with eight controls after GHRP-2 administration. The patients still mounted a measurable GH rise, roughly 4.5-fold over baseline against roughly 79-fold in controls. The authors concluded that an intact GHRH signaling system is not an absolute requirement for GHRP-2 to act, and that it exerts a GHRH-independent effect on pituitary somatotrophs (Gondo et al., JCEM 2001; PMID: 11443201).

That is the load-bearing conclusion: GHS-R1a agonism is not a roundabout way of stimulating the GHRH receptor, because it reaches the somatotroph by its own route. The gap between groups is harder to read — a blunted response fits part of the ghrelin-receptor effect running through hypothalamic GHRH neurons, but fits reduced somatotroph capacity in long-standing GH deficiency equally well, and the study was not built to separate those. Calling the arms either interchangeable or fully independent overstates the evidence.

GHRH-receptor analogues: sermorelin, tesamorelin, CJC-1295

Native GHRH is 44 amino acids, but the first 29 carry essentially the full biological activity. That fragment, amidated at the C-terminus, is sermorelin — GHRH(1-29)NH2. Its limitation is metabolic rather than pharmacological: dipeptidyl peptidase-4 cleaves between residues 2 and 3, giving it a circulating half-life measured in minutes.

Most of the medicinal chemistry in this arm exists to defeat that cleavage. Substituting D-alanine at position 2 blocks the DPP-4 site: in normal men studied in 1985, the D-Ala2 analogue was reported as roughly twice as potent as the unmodified fragment, with neither altering prolactin, TSH, ACTH, cortisol or thyroid hormones (Barron, Coy and Millar, Peptides 1985; PMID: 2866496). Later structure-activity work mapped which residues tolerate substitution without losing receptor affinity (Cervini et al., J Med Chem 1998; PMID: 9513600). Those papers are the ancestors of every modified GHRH analogue in circulation.

Tesamorelin solves the same problem differently: the full GHRH(1-44) sequence with a trans-3-hexenoyl group on the N-terminal tyrosine, sterically hindering degradation. It is the only compound in this arm to have completed large randomized trials, in HIV-associated abdominal fat accumulation, reported in NEJM (Falutz et al., 2007; PMID: 18057338) and later as a pooled analysis of two phase 3 trials with safety extension data (Falutz et al., JCEM 2010; PMID: 20554713). The tesamorelin literature summary covers those trials.

The CJC-1295 naming problem

This is the most common source of confusion in the class. CJC-1295 with DAC is a tetrasubstituted GHRH(1-29) analogue (D-Ala2, Gln8, Ala15, Leu27) carrying a Drug Affinity Complex — a maleimide linker that binds covalently to circulating albumin. That linker, not the substitutions, produces the long duration of action. In the trial most often cited for it, run in healthy adults, single administrations produced dose-dependent rises in mean plasma GH lasting six days or more and in IGF-I lasting nine to eleven days, with an estimated half-life of roughly six to eight days (Teichman et al., JCEM 2006; PMID: 16352683).

CJC-1295 without DAC is a different molecule pharmacokinetically — the same peptide minus the albumin linker, also called modified GRF(1-29), with a duration measured in tens of minutes rather than days. Teichman's findings describe the DAC version and do not transfer to it. When a study design references "CJC-1295," the first question is which one is meant; a no-DAC preparation blended with ipamorelin behaves nothing like the DAC construct on the time axis.

Ghrelin-receptor agonists: ipamorelin and the GHRP class

The GHRP series — GHRP-2, GHRP-6, hexarelin — were synthesized before their receptor was known. They release GH efficiently but not cleanly: in healthy adults, GHRP-2 and hexarelin raised prolactin, ACTH and cortisol alongside GH, the ACTH and cortisol responses comparable to hCRH (Arvat et al., Peptides 1997; PMID: 9285939). Because GHS-R1a is also the ghrelin receptor, agonists here engage appetite signaling; intravenous ghrelin infusion increased food intake in healthy lean and obese subjects alike (Druce et al., Int J Obes 2005; PMID: 15917842).

Ipamorelin was designed against exactly that problem; its founding paper is titled for the claim — the first selective growth hormone secretagogue. In anaesthetised rats and conscious swine it released GH with potency and efficacy comparable to GHRP-6. The selectivity finding came from the swine work: GHRP-6 and GHRP-2 both raised ACTH and cortisol, while ipamorelin raised neither to levels significantly different from those seen after GHRH stimulation, holding at doses reported as more than 200-fold above its ED50 for GH release (Raun et al., Eur J Endocrinol 1998; PMID: 9849822). That result came from animal models, not clinical trials, and was benchmarked against GHRH rather than no stimulus — the reason ipamorelin is treated separately from the older GHRPs.

Side by side: what actually differs

Property	GHRH-receptor analogues	GHS-R1a agonists
Receptor	GHRHR (class B GPCR, Gs/cAMP)	GHS-R1a (class A GPCR, Gq/PLC/Ca2+)
Endogenous ligand	GHRH (hypothalamic)	Ghrelin (predominantly gastric)
Representative compounds	Sermorelin, tesamorelin, CJC-1295 (with or without DAC)	Ipamorelin, GHRP-2, GHRP-6, hexarelin
Effect on the GH pulse	Amplifies existing pulse amplitude	Initiates release; also proposed to oppose somatostatin tone
Prolactin / ACTH / cortisol	Not altered in the Barron study	Raised by GHRP-2 and hexarelin in healthy adults (Arvat); not raised by ipamorelin above the GHRH comparator in swine (Raun)
Appetite signaling	No direct link	Shares its receptor with ghrelin
Main degradation liability	DPP-4 cleavage at residues 2-3	Not the primary design constraint

Where IGF-1 LR3 sits — downstream, not a secretagogue

IGF-1 LR3 is regularly filed alongside these compounds and does not belong to the class. It is an analogue of insulin-like growth factor I — the hepatic mediator through which most GH signaling is expressed — carrying an arginine substitution at position 3 and a 13-residue N-terminal extension that sharply reduce its affinity for IGF-binding proteins. It acts at the IGF-1 receptor, downstream of both pathways here.

Both secretagogue arms sit upstream of the IGF-I feedback loop, so their output is self-limiting: rising IGF-I restrains further GH release. A direct IGF-1 receptor agonist sits outside that loop. Treating it as a stronger secretagogue conflates two levels of the axis; the IGF-1 LR3 research summary covers its receptor pharmacology separately.

Reading the class correctly

Three questions resolve most of the confusion. Which receptor does the compound bind? If it is a GHRH analogue, what modification defeats DPP-4, and does it carry an albumin linker? If it is a GHS-R1a agonist, what is the published selectivity profile against prolactin and the adrenal axis, and in what species was it established?

Those answers also make identity verification substantive rather than cosmetic. Sermorelin, modified GRF(1-29) and the DAC construct are closely related sequences with very different pharmacokinetics, and a label does not distinguish them — an analytical certificate does. Every Merit lot ships with its HPLC and mass-spectrometry certificate, each one published and searchable by lot number. Comparative work is only interpretable if the compound in the vial is the one the design assumed.

Notes

This is a research literature summary. It is not a recommendation for any specific research protocol or design. For research use only. Not for human or veterinary use.

Selected references

1. Mayo KE. Molecular cloning and expression of a pituitary-specific receptor for growth hormone-releasing hormone. Mol Endocrinol 1992;6:1734-1744. PMID: 1333056
2. Jaffe CA, Friberg RD, Barkan AL. Suppression of growth hormone (GH) secretion by a selective GH-releasing hormone (GHRH) antagonist. Direct evidence for involvement of endogenous GHRH in the generation of GH pulses. J Clin Invest 1993;92:695-701. PMID: 8349808
3. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science 1996;273:974-977. PMID: 8688086
4. Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature 1999;402:656-660. PMID: 10604470
5. Gondo RG, Aguiar-Oliveira MH, Hayashida CY, et al. Growth hormone-releasing peptide-2 stimulates GH secretion in GH-deficient patients with mutated GH-releasing hormone receptor. J Clin Endocrinol Metab 2001;86:3279-3283. PMID: 11443201
6. Barron JL, Coy DH, Millar RP. Growth hormone responses to growth hormone-releasing hormone (1-29)-NH2 and a D-Ala2 analog in normal men. Peptides 1985;6:575-577. PMID: 2866496
7. Cervini LA, Donaldson CJ, Koerber SC, et al. Human growth hormone-releasing hormone hGHRH(1-29)-NH2: systematic structure-activity relationship studies. J Med Chem 1998;41:717-727. PMID: 9513600
8. Teichman SL, Neale A, Lawrence B, et al. Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. J Clin Endocrinol Metab 2006;91:799-805. PMID: 16352683
9. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol 1998;139:552-561. PMID: 9849822
10. Arvat E, di Vito L, Maccagno B, et al. Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Comparison with the effects of GHRH, TRH and hCRH. Peptides 1997;18:885-891. PMID: 9285939
11. Druce MR, Wren AM, Park AJ, et al. Ghrelin increases food intake in obese as well as lean subjects. Int J Obes (Lond) 2005;29:1130-1136. PMID: 15917842
12. Falutz J, Allas S, Blot K, et al. Metabolic effects of a growth hormone-releasing factor in patients with HIV. N Engl J Med 2007;357:2359-2370. PMID: 18057338
13. Falutz J, Mamputu JC, Potvin D, et al. Effects of tesamorelin (TH9507), a growth hormone-releasing factor analog, in human immunodeficiency virus-infected patients with excess abdominal fat: a pooled analysis of two multicenter, double-blind placebo-controlled phase 3 trials with safety extension data. J Clin Endocrinol Metab 2010;95:4291-4304. PMID: 20554713