Hexarelin, and the receptor nobody was looking for

A family built before its receptor was known

The growth-hormone-releasing peptides are unusual in that the chemistry came first. Bowers, Momany and colleagues described a synthetic hexapeptide in Endocrinology in 1984 — the compound later called GHRP-6, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 — that released growth hormone from rat pituitary cells in culture and in animals. It came out of a programme that started from modified enkephalin analogues rather than from any known growth-hormone regulator, and a companion paper by the same group set out the conformational modelling behind it. For more than a decade its target was simply unknown.

Hexarelin is a direct descendant. Deghenghi and colleagues in Milan reported it in Life Sciences in 1994 as the GHRP-6 scaffold with the tryptophan at position two replaced by 2-methyl-D-tryptophan, a substitution chosen for chemical stability. In infant and adult rats they measured growth-hormone release comparable to GHRP-6.

Later analogues pushed in a different direction. Raun and colleagues at Novo Nordisk described ipamorelin in the European Journal of Endocrinology in 1998 as the first selective secretagogue in the family, meaning that in their swine model it released growth hormone without the accompanying rise in ACTH and cortisol seen with GHRP-6 and others. Selectivity, rather than raw potency, was the design goal.

The receptor, and then the hormone

Howard and colleagues at Merck cloned the receptor in 1996, reporting in Science a G-protein-coupled receptor in pituitary and hypothalamus that mediated the action of these secretagogues. They named it GHS-R, and noted that a receptor this specific implied an endogenous ligand nobody had found.

Kojima and colleagues found it three years later. In Nature in 1999 they reported a 28-residue peptide purified from rat stomach whose third residue, a serine, carries an n-octanoyl group — a fatty-acid modification without which the peptide was inactive at the receptor. They named it ghrelin. The synthetic family had been mimicking a gut hormone all along.

That sequence of events matters for reading the literature. Papers on hexarelin from before 1996 describe effects without a receptor; papers after 1999 tend to assume every effect runs through GHS-R. The CD36 work is the main reason that assumption needs checking.

Photolabelling the heart

Hexarelin had reported cardiovascular activity that did not obviously fit a pituitary receptor, so Bodart and colleagues in Montreal went looking for its cardiac binding site. In Circulation Research in 2002 they described labelling rat cardiac membranes with a radioactive, photoactivatable hexarelin derivative, purifying what it bound, and sequencing the result. The protein, about 84 kDa, matched CD36 — a multifunctional glycoprotein better known as a scavenger receptor for oxidised lipoproteins, expressed in cardiomyocytes and microvascular endothelium.

The functional experiment used isolated perfused hearts. Hexarelin raised coronary perfusion pressure in a concentration-dependent way. In hearts from CD36-null mice, and from a spontaneously hypertensive rat strain genetically deficient in CD36, that response was absent, and across preparations it tracked CD36 expression and covalent hexarelin binding.

Demers and colleagues then mapped the site. In the Biochemical Journal in 2004 they localised the photolabel to an 8 kDa CD36 fragment spanning Asn132 to Glu177, with Met169 as the likely contact point, and noted that it overlaps the region CD36 uses to bind oxidised LDL.

What this does and does not show

The CD36 work establishes that hexarelin has a second, structurally defined binding partner, and that in rodent heart preparations a measurable vascular response depends on it. It is a clean example of the knockout logic: the effect is present, the receptor is removed, the effect is gone.

It does not establish what any of this means outside those preparations. The authors' own interpretation pointed toward coronary vasospasm in hypercholesterolaemia and atherosclerosis as a research question, not toward a use. And because GHS-R and CD36 are both engaged by the same molecule, any study that reports a hexarelin effect without separating the two receptors is reporting a mixture.

For the GHRP family as a whole, the practical lesson is about specificity claims. Ipamorelin's selectivity was defined against hormone release, not against CD36, and the published binding work for the two receptors does not cover every analogue in the family to the same depth.

The limits

Hexarelin is not an approved drug in the United States. Nothing in this article describes an effect in a person, and none of it is a protocol. The compound pages in the Endocrine world name the receptor work and the models used, and the growth-hormone-axis article covers the GHRH-analogue side of the same system.