Ipamorelin

Ipamorelin is a synthetic pentapeptide — five amino acids — that acts on the ghrelin receptor, formally the growth hormone secretagogue receptor. That receptor sits on the same pituitary cells as the GHRH receptor but responds to an entirely different signal, one associated with hunger and the fed–fasted state rather than with the hypothalamic growth axis.

It was developed to answer a specific problem with the peptides that preceded it. Earlier ghrelin-receptor compounds prompted growth hormone release but also raised cortisol and prolactin, two hormones whose movement complicates any experiment and confounds any interpretation. Ipamorelin was designed and selected for a narrower response, and in published comparisons it produces markedly less of that off-target activity.

Selectivity is its entire research identity. When a study needs to ask what happens when this receptor is engaged, without a stress-hormone response overlaid on the result, this is the compound the literature reaches for. Its interest lies in what it does not do.

• A cleaner readout: Comparative work shows it raises growth hormone with little movement in cortisol or prolactin. In an experiment, a hormone that stays still is as valuable as one that moves — it removes an alternative explanation for the result.

• Two receptors, one cell: It engages the ghrelin receptor while GHRH analogues engage a different one on the same pituitary cell. Having a selective tool for each is what makes the interaction between them measurable.

• Appetite signalling separated: Ghrelin itself is central to hunger, and earlier compounds in this class carried a strong appetite effect. A selective peptide that largely separates the growth hormone response from that signal is useful for isolating one from the other.

• Minimal effective sequence: At five residues it is among the smallest peptides with meaningful activity at this receptor, which makes it a practical reference point in structure–activity work on the receptor family.

A receptor is a protein in a cell's surface that changes shape when the right molecule binds, starting a response inside. The growth hormone secretagogue receptor was named before anyone knew what naturally activated it; the answer, found in 1999, was ghrelin, a hormone released by the stomach that rises before meals. The receptor therefore links feeding state to the growth axis.

Pituitary somatotrophs carry both this receptor and the GHRH receptor, and the two drive release through different internal signalling routes. Engaging both produces a larger response than either alone — not because one amplifies the other in a simple sense, but because two separate mechanisms are pushing the same cell at once. This is the mechanistic reason the two classes are studied in combination.

Selectivity is the harder part to achieve. Receptors in a family share structure, and a molecule shaped to fit one usually fits its relatives; activating those relatives is what produced the cortisol and prolactin responses of earlier compounds. Ipamorelin's value is that its shape discriminates well enough to leave those pathways largely untouched, which is what makes the experiment interpretable.

Its receptor pharmacology and selectivity are well characterised and were established in peer-reviewed work in the 1990s. That it raises growth hormone with comparatively little cortisol or prolactin movement has been reproduced and is the reason it displaced earlier compounds in research use.

Clinical development was pursued and discontinued, so there is no approved indication and no long-term human outcome data, and it is supplied for laboratory research only. Almost all published work concerns the hormone response itself. That is what it is chosen for: it raises growth hormone with markedly less movement in cortisol and prolactin than earlier compounds in its class, which is what keeps an experiment interpretable.

CJC-1295 is the usual partner, and the two are supplied together as CJC-1295 + Ipamorelin. The pairing has an actual mechanistic argument behind it — two receptors on one cell, engaged simultaneously by separate routes — which is more than can be said for most combinations.

GHRP-2 and GHRP-6 act on the same ghrelin receptor with broader off-target activity, and are the comparators that make the case for selectivity. Reading the three together is the clearest way to see what "selective" means in practice.