Ipamorelin: The Peptides Came First, Then the Receptor, Then the Hormone

Ipamorelin belongs to a family of peptides with an unusual biography. They were reported releasing growth hormone before anyone knew how. The receptor they act at was found later. The body's own hormone for that receptor was found later still.

That order is the whole story, so here it is in the order it happened.

1980 and 1984: they released growth hormone, and nobody knew how

Growth hormone-releasing peptides were first described by Bowers and colleagues in 1980, in the journal Endocrinology. In 1984 the same group reported, in the same journal, a new synthetic hexapeptide, a laboratory-made peptide six amino acids long, called GHRP-6. It could release growth hormone from the pituitary gland.

It did so, in the research page's words, "via an unidentified mechanism independent of GHRH." GHRH is growth hormone-releasing hormone, the body's known signal for that job. So here was a compound doing the job by some other route, and nobody could say what the route was. The research page describes the two pathways as distinct, which means growth hormone release had a second route that hadn't been mapped.

1996: a receptor, and no key

Twelve years after that 1984 report, Howard and colleagues, writing in Science, described a receptor called GHS-R1a. A receptor is the docking site on a cell that a signaling molecule fits into. This one sits in the pituitary and the hypothalamus and regulates growth hormone release, and it is the receptor these peptides act at.

There was still a gap. The receptor's endogenous ligand, the body's own molecule that fits it, had not yet been identified. The lab-made peptides fit a lock whose natural key nobody had found.

1998: a surprise in the chemistry

Raun and colleagues reported ipamorelin in 1998, in the European Journal of Endocrinology. It came out of a chemistry program screening compounds that lacked a two-residue unit, alanine and tryptophan, from the middle of an earlier peptide called GHRP-1. Screening means testing a set of related compounds to see what each one does. It is a pentapeptide, five amino acids to GHRP-6's six, and a fully synthetic construct that doesn't correspond to any natural peptide sequence.

Here is the part the developers didn't design for. Bowers and colleagues had reported growth hormone release without a rise in four other hormones, but they hadn't reported on two more, ACTH and cortisol. Later swine work showed that GHRP-6 raised both. In the same conscious swine study, ipamorelin's ACTH and cortisol levels were not significantly different from those following GHRH stimulation. None of the secretagogues tested, ipamorelin included, altered the four other hormones.

The research page describes that as an unexpected finding rather than a designed outcome, and the developers themselves called it very surprising. It wasn't explained by a different receptor either: they found ipamorelin releases growth hormone through the same receptor as GHRP-6.

One limit belongs right beside it. The developers' foundational study characterized growth hormone release in rat pituitary cells in a dish, in anesthetized rats and in conscious swine. The ACTH and cortisol measurements were carried out only in the swine. The research page states the limit plainly: the selectivity rests on a single set of swine experiments from the original developer, and it has not been independently replicated. The research page adds that much of the defining preclinical work comes from that same developer.

1999: the key turns up

In 1999, Kojima and colleagues, writing in Nature, discovered ghrelin, the body's own peptide for that receptor. The paper's own title describes it as a growth hormone-releasing peptide from the stomach. That is a different part of the body from the pituitary and hypothalamus, where the receptor had been found. GHS-R1a was then identified as the ghrelin receptor. In the research page's words, that classified the growth hormone-releasing peptides "as ghrelin agonists or analogs," an analog being a compound built to resemble another.

An agonist is a compound that switches a receptor on. Once the receptor had a known natural hormone, anything that switched it on could be described in relation to that hormone. Fifteen years after GHRP-6 was reported, the family had a new label, supplied by a hormone that hadn't been discovered when the family was.

Same molecules, different category

Nothing about the peptides changed in 1999. What changed was what they were understood to be.

The numbers are worth stating both ways. Counted from GHRP-6's report in 1984, the receptor arrived twelve years later and the hormone fifteen. Counted from the class's first description in 1980, the gaps are sixteen years and nineteen. Either way, the explanation trailed the compounds by well over a decade.

That's a small, odd fact about how classification works. Ipamorelin doesn't correspond to any natural peptide sequence, yet it is now described by its relationship to a natural hormone found after it was made. The research page calls it a synthetic ghrelin receptor agonist. That sentence couldn't have been written in 1984 or in 1996, because it needed something nobody had found yet.

This piece is about the order in which these peptides were understood. It makes no claim about what any of them do for anyone.

The peptides came first. The receptor followed. The hormone came last and reclassified the family. 1984, the peptides. 1996, the receptor. 1998, ipamorelin. 1999, the hormone. Our full research write-up sets out what the published literature does and doesn't show.