Growth hormone secretagogues: GHRH analogs versus ghrelin mimetics

Two receptors, not one

Compounds described as growth hormone secretagogues fall into two mechanistically unrelated groups. GHRH analogs — sermorelin, tesamorelin, CJC-1295 — act at the GHRH receptor on somatotrophs and amplify the amplitude of endogenous secretory pulses. Ghrelin mimetics — ipamorelin, GHRP-2, GHRP-6, hexarelin — act at the growth hormone secretagogue receptor GHS-R1a and both stimulate release and suppress somatostatin tone.

Because the two act at different receptors on the same cell, their effects in preclinical models are more than additive, which is the entire rationale for the combination preparations that pair a GHRH analog with a ghrelin mimetic.

The DAC question

CJC-1295 exists in two distinct forms and the distinction is the most frequently confused point in this class. CJC-1295 with DAC carries a Drug Affinity Complex — a maleimidopropionic acid group that forms a covalent bond with serum albumin, extending half-life from minutes to roughly six to eight days.

CJC-1295 without DAC is not a variant of that molecule; it is Modified GRF (1-29), a 29-residue GHRH fragment with four stabilising substitutions and a half-life measured in minutes. The literature on the two diverges accordingly: the DAC form produces a sustained elevation in circulating GH, while the no-DAC form preserves the pulsatile secretion pattern that much of the physiological literature treats as mechanistically significant.

Selectivity within the ghrelin mimetics

Ipamorelin is the most selective compound in the group, producing GH release in preclinical models with minimal effect on cortisol or prolactin. GHRP-6 is markedly less selective and is characterised in the literature by strong appetite signalling through central ghrelin receptor activation. GHRP-2 sits between the two. Hexarelin is the most potent per unit mass but shows the most pronounced receptor desensitisation on repeated exposure.

Selectivity is therefore the deciding variable in study design. A protocol interrogating GH axis behaviour specifically will confound itself with GHRP-6; one investigating appetite signalling may specifically require it.

Downstream markers

Growth hormone itself is pulsatile and a single timepoint measurement is close to uninterpretable. IGF-1, produced hepatically in response to GH and with a far longer half-life, is the conventional integrated readout for this class and is what most published work reports.

Note that IGF-1 LR3 — an analog with reduced binding-protein affinity and an extended half-life — is a downstream effector rather than a secretagogue, and belongs to a different part of the axis entirely despite being catalogued alongside these compounds.

Two receptor families, one endpoint

The compounds grouped as growth hormone secretagogues divide cleanly into two pharmacological families that share an endpoint and nothing else. Releasing-factor analogs — sermorelin, tesamorelin, the CJC-1295 series — act at the GHRH receptor on somatotrophs, a class B receptor coupling to Gs and raising cAMP. Ghrelin-receptor agonists — ipamorelin, hexarelin, the GHRP series — act at GHS-R1a, a class A receptor coupling to Gq and mobilising intracellular calcium. Both routes converge on growth-hormone release, which is why they are catalogued together and why they are so often conflated.

The distinction is not academic, because the two families behave differently in every respect that matters to an assay. They desensitise on different timescales, they respond differently to somatostatin tone in the model system, and in combination studies the published question is specifically whether the two signals are additive — a question that only makes sense once the arms are recognised as mechanistically separate. A study that treats a GHRH analog and a ghrelin-receptor agonist as interchangeable has lost the variable it was trying to measure.

Within each family, the differences are largely pharmacokinetic rather than pharmacodynamic. Modifications that slow enzymatic clearance — the DAC linker on CJC-1295, the hexenoyl group on tesamorelin, the D-amino-acid substitutions in the GHRP series — extend exposure without changing the receptor interaction. Separating what a modification does to affinity from what it does to residence time is one of the most useful habits when reading comparative literature.

Selectivity, controls and reading the literature critically

Selectivity is the axis along which the ghrelin-receptor agonists differ most. The earlier peptides in the series engage pathways beyond GHS-R1a in published models, while ipamorelin is characterised as a comparatively clean agonist, and hexarelin is described with additional interactions including binding at CD36 in cardiac tissue. A study attributing an observation specifically to ghrelin-receptor engagement therefore needs the more selective compound as its control arm, or an antagonist to close the argument.

Assay artefacts account for a large share of disagreement between laboratories in this field. Peptide adsorption to untreated plastic at low concentrations, serum peptidase activity shortening effective exposure, and freeze-thaw degradation of the working stock all reduce apparent potency without touching the biology. Low-bind consumables, single-use aliquots and reduced-serum exposure windows remove most of it.

Finally, the literature is dominated by in vitro and preclinical work, and the compounds discussed here are supplied for that purpose only. Nothing in this article is a protocol, a dosing recommendation or a statement about use in humans or animals.