Hexarelin: Dual GHS-R1a and CD36 Binding, Chemistry and Published Assay Data

Hexarelin is a synthetic hexapeptide first described under the development code EP-23905 and assigned the international nonproprietary name examorelin. Its six residues are L-histidine, D-2-methyltryptophan, L-alanine, L-tryptophan, D-phenylalanine and L-lysine, the C-terminus is an amide, and the resulting formula is C47H58N12O6 with a molecular weight of 887.0 g/mol.

Two of the six positions are D-configured and one carries a ring modification. Position 2 is D-2-methyltryptophan, a tryptophan bearing a methyl group at the 2-position of the indole, and position 5 is D-phenylalanine. The C-terminal lysine is amidated rather than left as a free acid. These three departures from ordinary L-amino-acid chemistry are what allow the peptide to persist in plasma long enough to be measurable.

The molecule carries an unusually heavy aromatic load for its size. Two indole rings and one benzene ring across six residues give it a computed XLogP of 2.3 and 65 heavy atoms, both higher than the amidated pentapeptides in the same family. Its topological polar surface area is 301 square angstroms and it presents 11 hydrogen-bond donors, figures consistent with a compact peptide that stays in aqueous buffer.

Hexarelin engages two structurally unrelated proteins. At the growth hormone secretagogue receptor 1a it acts as an agonist, the same class A G protein-coupled receptor that acyl-ghrelin activates. In rat cardiac membranes it also binds a photolabelled protein of relative molecular mass 84,000, identified by N-terminal sequencing as the scavenger receptor CD36.

The cardiac site was characterized before it was named. Bodart and colleagues perfused Langendorff rat hearts and saw coronary perfusion pressure rise in step with exposure level, then used a photoactivatable radioligand to show a single class of binding sites with a Kd of 14.5 nmol/L and a density of 91 fmol/mg protein. Nifedipine, chelerythrine and bisindolylmaleimide each partly blocked the vasoconstriction, implicating L-type calcium channels and protein kinase C; diclofenac and 1-(7-carboxyheptyl)imidazole did not, ruling out prostaglandins and thromboxanes.

Three years later the same group identified the protein. Deglycosylation and N-terminal sequencing matched rat CD36, a glycoprotein expressed in cardiomyocytes and microvascular endothelial cells. The functional test was genetic: the coronary pressure response was absent in hearts from CD36-null mice and from spontaneously hypertensive rats that are genetically deficient in CD36. That is the strongest form of evidence available for a receptor assignment, and it is why CD36 rather than GHS-R1a is treated as the mediator of the coronary response.

Demers and colleagues then mapped the contact region. Covalent photolabelling followed by enzymatic and chemical degradation yielded an 8 kDa fragment corresponding to CD36 residues Asn132 to Glu177. Cyanogen bromide cleavage of that fragment released the free ligand, placing Met169 at the point of contact. The region overlaps the oxidized low-density lipoprotein binding site, which spans Gln155 to Lys183, and the authors proposed that competition at this overlap contributes to the reduced atherosclerotic burden reported for growth hormone releasing peptides in apolipoprotein E-deficient mice.

Hexarelin differs from GHRP-6 by a single methyl group: both share the backbone His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, and hexarelin carries a 2-methyl substituent on the indole ring of the D-tryptophan at position 2. That one addition is the structural basis for the reported differences in receptor engagement between the two hexapeptides.

The sharper separation is not between hexarelin and its peptide siblings but between peptidyl and non-peptidyl secretagogues. In the rat cardiac binding assay, unlabelled hexarelin displaced the photoligand with an IC50 of 2.9 micromol/L, while MK-0677 and EP51389, both potent growth hormone secretagogues, failed to displace it at all. Both non-displacing compounds were also devoid of vasoconstrictive activity in the perfused heart. Potency at the pituitary receptor therefore does not predict activity at the cardiac site.

That dissociation is the reason hexarelin is used as a research tool rather than as one more entry in the secretagogue list. A compound with two independent targets makes it possible to ask which endpoints survive when the pituitary axis is bypassed, and the CD36-null and CD36-deficient animal work answers that question directly for the coronary response.

A 2026 review by Dominikowski and colleagues placed hexarelin within the wider set of peptides marketed outside regulatory approval that act on the growth hormone and IGF-1 axis. The authors stratified compounds by the strength of the evidence behind each, from randomized trial data down to classes with no controlled human record at all, and contrasted the peer-reviewed pharmacology against protocols circulating outside it. They recorded hormonal disturbances, fluid retention and musculoskeletal symptoms across the classes reviewed.

Hexarelin has been examined in rats, mice, hamsters and human cell lines across the 2023 to 2026 literature. The reported endpoints fall into four groups: renal tubular apoptosis after ischemia and reperfusion, oxidative cytotoxicity in a neuroblastoma line carrying a superoxide dismutase mutation, inflammatory cytokine output during viral infection, and retinal ganglion cell survival after optic nerve transection.

On the kidney, Guan and colleagues gave rats 100 µg/kg per day for 7 days before a 24-hour ischemia and reperfusion challenge. The exposed animals showed less tubular necrosis and dilatation, lower serum creatinine and reduced apoptosis, with Caspase-3, Bax and Bad downregulated and the anti-apoptotic protein Bcl-2 upregulated. The same direction of change appeared in HK-2 cells under hypoxia and reoxygenation. Molecular docking flagged MDM2 as a binding partner, and MDM2 and p53 expression were both suppressed in the animal and cell arms.

On oxidative cytotoxicity, Meanti and colleagues incubated SH-SY5Y cells expressing the SOD1-G93A mutant protein for 24 hours with 150 µM hydrogen peroxide, with or without 1 µM secretagogue. Hexarelin and the related compound JMV2894 both reduced the cytotoxicity, acting through molecules that regulate apoptosis and cell survival. Two compounds at a single concentration in one mutant line is a narrow result, and the authors framed it as a starting point rather than a mechanism.

On viral inflammation, Gauvin and colleagues worked in K18-hACE2 mice infected intranasally with SARS-CoV-2. Hexarelin at 10 µmol/kg, starting 30 minutes before infection and continuing daily for 9 days, raised survival relative to vehicle. At day 3 the measured cytokine and chemokine reductions were 44 percent for CCL2, 59 percent for CCL4, 43 percent for interferon alpha and 35 percent for interferon gamma. The framing throughout is CD36 modulation of the alveolar macrophage phenotype rather than any pituitary action.

On retinal neurons, Chow reported the most granular exposure-response curve in the recent set. Golden hamsters aged 8 to 9 weeks received 25, 50 or 100 µg/kg once daily for 5 days after optic nerve transection, and retinal ganglion cell survival at day 7, quantified by Tuj1 immunostaining on retinal whole mounts, rose from 51.2 percent in the saline group to 62.4, 68.5 and 74.6 percent. A twice-daily schedule at 100 and 150 µg/kg reached 91.4 and 109.2 percent against a 72.9 percent saline comparator.

Hexarelin has two published human studies with quantitative pituitary endpoints, both in small groups of healthy young men. Imbimbo and colleagues reported the acute growth hormone time course across three exposure levels in twelve volunteers in 1994, and Maccario and colleagues reported 24-hour secretory profiles under a repeated schedule in six volunteers in 2002.

The 1994 study was double-blind and placebo-controlled, with single boluses at 0.5, 1 and 2 µg/kg. Plasma growth hormone rose in step with exposure level, peaked at about 30 minutes and returned toward baseline with a half-life of roughly 55 minutes. Peak concentrations spanned 3.9 to 55.0 ng/mL across the range, and the response approached its ceiling at the highest level, meaning the useful measurement window sits below 2 µg/kg.

The 2002 study asked a different question: what happens under repetition. Six normal young men received hexarelin at 1.5 µg/kg either two or three times daily, with blood sampled every 20 minutes across 24 hours and multi-parameter deconvolution used to separate pulsatile secretion from clearance. Both schedules raised mean and integrated 24-hour growth hormone concentrations by the same amount, and the increase came from larger secretory pulse mass rather than more frequent pulses.

The selectivity finding from that study is the one worth recording precisely. Prolactin, adrenocorticotropin and cortisol secretion were unaltered across the sampling period, so the amplification was confined to the somatotroph and did not extend to lactotroph or corticotroph output. A 1 µg/kg challenge at the end of each 24-hour period was included specifically to monitor for down-regulation of the response.

Hexarelin is identified by its monoisotopic mass of 886.4602 Da, its InChIKey RVWNMGKSNGWLOL-GIIHNPQRSA-N and CAS number 140703-51-1. Laboratories separate it by reversed-phase liquid chromatography and confirm it by high-resolution tandem mass spectrometry, the same workflow applied across the growth hormone releasing peptide class.

Two structural features make the confirmation straightforward. The molecule carries two indole rings, giving strong ultraviolet absorbance near 280 nanometres and a chromophore that most amidated pentapeptides in this family lack. The C-terminal amide shifts the parent mass by one dalton relative to the free acid, so hydrolysis at that position is directly visible in the mass spectrum rather than inferred.

A 2025 result complicates attribution rather than detection. Pobee and colleagues incubated alexamorelin with pooled human hepatocytes from ten donors and analysed the products by liquid chromatography coupled to high-resolution tandem mass spectrometry. After 3 hours the alexamorelin signal had fallen roughly 150-fold, and the only metabolite detected was examorelin, which is hexarelin, released by carboxypeptidase cleavage of an alanine residue. Finding hexarelin in a sample therefore does not by itself establish which of the two compounds was present originally.

The structural characterization of both binding sites rests on photoaffinity labelling rather than on a solved receptor complex. No structure of hexarelin bound to GHS-R1a or to CD36 has been published; the cryo-electron microscopy record for the ghrelin receptor covers ghrelin, ibutamoren, macimorelin and anamorelin. For literature searching, note that part of the record appears under the name examorelin and part under the development code EP-23905.

Hexarelin is supplied as a lyophilized solid, usually as the acetate salt, and published handling figures come from supplier specifications rather than from peer-reviewed measurement. Vendor listings on PubChem include a grade specified at 90 percent or higher purity by HPLC, so purity varies by source and should be confirmed against the certificate accompanying the lot rather than assumed.

Salt form is the first thing to record. CAS number 140703-51-1 and PubChem CID 6918297 describe the free peptide, formula C47H58N12O6, molecular weight 887.0 g/mol. The acetate salt carries a variable acetate component and therefore a different mass per vial, and registry listings also show 208251-52-9 for a salt form. Peptide content by weight differs between the two, which matters whenever a stated milligram figure is converted to moles.

Three chemical features bear on storage. The C-terminal amide can hydrolyse to the free acid, a one-dalton shift that is the most useful degradation marker for this molecule by mass spectrometry. The two indole rings are the residues most exposed to oxidation and to light, so amber containers and cold storage matter more here than for peptides without tryptophan. The lysine side-chain amine is the reactive handle most likely to take part in unwanted acylation.

A computed XLogP of 2.3 sits at the upper end for a peptide of this size, driven by the aromatic load, and the topological polar surface area of 301 square angstroms indicates a molecule that stays in aqueous buffer rather than partitioning into membranes. The same aromatic residues assist analytical work: absorbance near 280 nanometres allows concentration to be checked by ultraviolet spectrophotometry without a separate assay.