Category: Comparisons

Sermorelin and ipamorelin are routinely shelved together as “growth hormone peptides,” but they are not variations on one molecule the way two GHRH analogs are. They act on two entirely different receptors, through two different signaling pathways, and they descend from two different parent molecules. Sermorelin is a synthetic fragment of growth hormone–releasing hormone that works at the GHRH receptor; ipamorelin is a synthetic ghrelin mimetic that works at the ghrelin receptor. This article compares them at the level of receptor target, peptide structure, and what the published literature actually measured for each.

Two receptors, two pathways

The cleanest way to separate these compounds is by the receptor each one binds. Both pathways converge on the same downstream output — signaling the pituitary — but they enter through different doors.

• Sermorelin acts at the GHRH receptor (GHRH-R), the same G-protein–coupled receptor on anterior-pituitary somatotrophs that native growth hormone–releasing hormone uses.
• Ipamorelin acts at the growth hormone secretagogue receptor (GHS-R1a) — the ghrelin receptor. Its endogenous ligand is ghrelin, the acylated stomach peptide identified as the natural agonist of this receptor (Kojima et al., Nature, 1999).

Because these are physically distinct receptors with distinct endogenous ligands, the two compounds are not interchangeable and are not redundant. A GHRH analog and a ghrelin mimetic are categorically different tools, and that distinction is the entire point of comparing them. The same GHS-R1a/ghrelin axis is the one we examine in our companion post on ipamorelin vs GHRP-2 vs GHRP-6, where all three compounds share ipamorelin’s receptor but differ in selectivity.

Sermorelin: a GHRH-receptor analog, GRF(1-29)

Sermorelin is the synthetic 1–29 fragment of human growth hormone–releasing hormone, written as GRF(1-29) or GHRH(1-29)-NH2. Native human GHRH is a 44–amino acid peptide, GRF(1-44); the receptor-binding activity is concentrated in the front of that chain, and the first 29 residues are the shortest segment that retains the parent hormone’s signaling activity. Sermorelin is that core fragment, carrying no stabilizing modification.

At the GHRH receptor, agonist binding activates the receptor’s G-protein–coupled signaling cascade in pituitary somatotrophs, which is the mechanism by which native GHRH and its fragments drive growth hormone release. The albumin-bioconjugate study that introduced CJC-1295 confirmed that the hGRF(1-29) fragment acts at the GRF (GHRH) receptor on the anterior pituitary, using that fragment as the activating moiety (Jetté et al., Endocrinology, 2005). Sermorelin is the unmodified version of that same fragment.

As an unprotected peptide, sermorelin is short-lived in circulation. In healthy adult subjects, GHRH(1-29)-NH2 given intravenously was measured to produce a release of growth hormone, with the response characterized over a short pharmacokinetic window consistent with rapid clearance (Wilton et al., Acta Paediatr Suppl, 1993). That is what the study measured in its research subjects — a GHRH-receptor–mediated GH response — reported here as such.

Ipamorelin: a ghrelin mimetic at GHS-R1a

Ipamorelin is a synthetic pentapeptide — five amino-acid residues — with the published sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2. It is not a fragment of GHRH at all. It belongs to the growth-hormone-secretagogue family and functions as a ghrelin mimetic, meaning it activates the same GHS-R1a receptor that the natural hormone ghrelin uses, despite sharing no structural relationship with ghrelin itself.

The defining feature reported for ipamorelin in its introductory pharmacology paper is selectivity. In rats, ipamorelin was measured to release growth hormone with potency comparable to the earlier secretagogue GHRP-6, but — unlike GHRP-6 — it did not produce a significant rise in ACTH or cortisol, and the authors described it as the first selective growth hormone secretagogue on that basis (Raun et al., Eur J Endocrinol, 1998). That selectivity profile — a GH signal at GHS-R1a without the cortisol and prolactin co-stimulation seen with several other secretagogues — is the property most often cited to distinguish it from its GHRP relatives.

Structurally, the pentapeptide was engineered for stability: the N-terminal aminoisobutyric acid (Aib) residue and the D-amino-acid substitutions in the chain are non-standard residues that resist the enzymatic cleavage that rapidly degrades ordinary peptides. The molecule is small, synthetic, and built specifically to act at the ghrelin receptor.

Putting the two side by side

The comparison resolves along three clean axes, and on each one the two compounds sit on opposite sides:

• Receptor target: Sermorelin acts at the GHRH receptor; ipamorelin acts at the ghrelin receptor, GHS-R1a. Different receptors, different endogenous ligands.
• Molecular class and structure: Sermorelin is a 29–residue GHRH fragment, GRF(1-29). Ipamorelin is a 5–residue synthetic ghrelin-mimetic pentapeptide (Aib-His-D-2-Nal-D-Phe-Lys-NH2) with no sequence relationship to GHRH.
• What the literature measured: For sermorelin, a GHRH-receptor–mediated GH release in healthy subjects (Wilton et al., 1993). For ipamorelin, a GHS-R1a–mediated GH release without significant ACTH/cortisol elevation in rats — the selectivity finding (Raun et al., 1998).

Because they enter through different receptors, the two are sometimes discussed as complementary research tools rather than substitutes — a GHRH-receptor analog and a ghrelin-receptor agonist represent two independent inputs to the same pituitary system. That distinction is the same logic that separates the GHRH-analog family covered in our tesamorelin vs CJC-1295 comparison — where every compound shares sermorelin’s GHRH-receptor pathway — from the ghrelin-mimetic family that ipamorelin belongs to. Sermorelin and ipamorelin straddle that divide: one on each side.

Frequently asked questions

Do sermorelin and ipamorelin work on the same receptor?

No. Sermorelin acts at the GHRH receptor, the same receptor native growth hormone–releasing hormone uses. Ipamorelin acts at the ghrelin receptor, GHS-R1a. They are different receptors with different endogenous ligands, which is the central distinction between the two compounds.

Is ipamorelin a GHRH analog?

No. Ipamorelin is a ghrelin mimetic, not a GHRH analog. It is a synthetic pentapeptide (Aib-His-D-2-Nal-D-Phe-Lys-NH2) that shares no sequence relationship with GHRH. Sermorelin, by contrast, is a direct GHRH fragment.

What is sermorelin, structurally?

Sermorelin is GRF(1-29), the synthetic 1–29 fragment of human GHRH. Native GHRH is the 44–residue GRF(1-44); the first 29 residues are the shortest segment that retains the parent hormone’s receptor-signaling activity, and sermorelin is that fragment with no stabilizing modification.

What does “selective” mean for ipamorelin?

In its introductory pharmacology paper, ipamorelin was measured to release growth hormone in rats without a significant rise in ACTH or cortisol, in contrast to GHRP-6, which raised those hormones. The authors described it as the first selective growth hormone secretagogue on that basis (Raun et al., 1998). It describes the published pharmacology, not an outcome in any individual.

Why are they compared if they are so different?

Both are studied as inputs to the pituitary growth-hormone system, so they are frequently shelved together. The comparison is useful precisely because it surfaces the difference: one is a GHRH-receptor analog and the other is a ghrelin-receptor mimetic, two separate pathways rather than two versions of one compound.

How does ipamorelin relate to the GHRP peptides?

Ipamorelin shares the GHS-R1a (ghrelin) receptor with GHRP-2 and GHRP-6, which is why it is most directly compared with them rather than with sermorelin. The published distinction is selectivity — the absence of significant ACTH/cortisol stimulation in the introductory study. Our companion post on ipamorelin vs GHRP-2 vs GHRP-6 covers that family in detail.

References

1. Wilton P, et al. Pharmacokinetics of growth hormone-releasing hormone(1-29)-NH2 and stimulation of growth hormone secretion in healthy subjects after intravenous or intranasal administration. Acta Paediatrica Supplement. 1993. PMID: 8329825.
2. Jetté L, et al. Human growth hormone-releasing factor (hGRF)1-29-albumin bioconjugates activate the GRF receptor on the anterior pituitary in rats: identification of CJC-1295 as a long-lasting GRF analog. Endocrinology. 2005. PMID: 15817669.
3. Raun K, et al. Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology. 1998. PMID: 9849822.
4. Kojima M, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999. PMID: 10604470.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.

AOD-9604 and HGH Fragment 176-191 are two of the most frequently confused research peptides, and the confusion is understandable: both derive from the same place — the C-terminal (carboxyl) end of human growth hormone (hGH). They are not, however, the same molecule. HGH Fragment 176-191 is the unmodified fragment itself; AOD-9604 is a deliberately modified analog of it. This article describes the structural relationship between the two, what the single modification changes, and what published laboratory research measured for each.

Where both molecules come from

Human growth hormone is a 191-amino-acid protein. Decades of structure-function work narrowed the part of the molecule associated with lipid-metabolism activity to a short stretch at the carboxyl terminus. HGH Fragment 176-191 is exactly that stretch — residues 176 through 191 of human growth hormone, isolated and synthesized as a standalone 16-amino-acid peptide. Because it reproduces only the C-terminal region and not the full protein, it lacks most of the sequence that the intact hormone uses to engage the growth-hormone receptor.

The research lineage is well documented. Early work characterized a synthetic lipolytic domain of hGH at the carboxyl terminus containing residues 177-191 (Ng et al., 2000), and that body of work is the direct ancestor of both the bare 176-191 fragment and the modified analog that followed.

What AOD-9604 adds: the tyrosine modification

AOD-9604 is the same C-terminal region with one structural change. It corresponds to the hGH 176-191 sequence in which the N-terminal residue is a tyrosine. In native human growth hormone, position 176 is phenylalanine; AOD-9604 carries a tyrosine at that N-terminal position instead. The compound is therefore often written as Tyr-hGH(177-191) — a tyrosine on the front of the 177-191 segment — which is the same molecule as “hGH 176-191 with a tyrosine at the N-terminus.” Both descriptions point to one 16-residue peptide.

The reported sequence of AOD-9604 is Tyr-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe, with the two cysteine residues forming a disulfide bridge, as in the parent region of the intact hormone. The added tyrosine was introduced to give the synthetic analog a more stable N-terminus than the bare fragment. In short:

• HGH Fragment 176-191 — the C-terminal fragment of hGH, unmodified.
• AOD-9604 — the same C-terminal region presented with an N-terminal tyrosine (Tyr-hGH(177-191)), a modification intended to stabilize the molecule.

You can explore AOD-9604 in our research catalog; HGH Fragment 176-191 is referenced here only as the structural baseline AOD-9604 is built from.

What published research measured for the fragment region

The lipid-metabolism research on this family is built around what the peptides did in cell, tissue, and animal models — not around any outcome in a person. The most-cited primary studies came out of the Monash University group (Ng and colleagues), and they describe biochemical and rodent endpoints.

In Zucker fatty rats, the synthetic C-terminal domain stimulated hormone-sensitive lipase and inhibited acetyl-CoA carboxylase in adipose tissue, and chronic administration reduced body-weight gain and adipocyte cell size without inducing the insulin resistance or glucose intolerance seen with intact growth hormone in that model (Ng et al., 2000). A separate study reported that oral administration of a synthetic hGH fragment increased lipolytic activity and reduced lipogenic activity in adipose tissue of treated rodents, and noted activity in human adipose tissue ex vivo (Heffernan et al., 2000).

These are descriptions of what the assays and animal models registered — enzyme activity, tissue measurements, and body-composition readouts in research subjects. They are not statements about any result in a human reader.

What the AOD-9604 modification studies measured

Research specific to AOD-9604 (the tyrosine-modified analog) extended the same line of investigation and probed how the effect was mediated. In obese mice, chronic treatment with either human growth hormone or AOD-9604 was associated with increased fat oxidation and reduced body-weight gain in the study models (Heffernan et al., 2001, Int J Obes). A companion study using beta-3-adrenergic-receptor knock-out mice was designed to test whether the analog’s measured effect on lipid metabolism depended on that receptor pathway, and reported that the lipolytic response observed in normal animals was altered in the receptor-knockout model (Heffernan et al., 2001, Endocrinology).

An important point for interpreting this literature: these endpoints were measured in cell systems and rodent models. The compound later advanced into human clinical testing, where the pivotal trial did not meet its primary efficacy endpoint, and clinical development was discontinued in 2007. Nothing in the published record should be read as a benefit a reader would obtain; it is a record of what specific studies measured under specific laboratory conditions.

How the two compare, at a glance

The comparison reduces to one structural difference and its rationale:

• Common origin — both trace to the C-terminal 176-191 region of human growth hormone, the segment historically associated with the molecule’s lipid-metabolism activity.
• The fragment — HGH Fragment 176-191 is that region with no added modification.
• The analog — AOD-9604 presents the same region with an N-terminal tyrosine (Tyr-hGH(177-191)), a change introduced to stabilize the peptide.
• The research — the primary lipid-metabolism literature was generated on this fragment family and on AOD-9604 specifically, using biochemical assays and rodent models, with knock-out work probing the beta-3-adrenergic pathway.

Frequently asked questions

Is AOD-9604 the same as HGH Fragment 176-191?

No. They share the same source region of human growth hormone, but AOD-9604 is a modified analog. AOD-9604 carries an N-terminal tyrosine on the 177-191 segment (often written Tyr-hGH(177-191)), whereas HGH Fragment 176-191 is the unmodified C-terminal fragment.

What does the tyrosine modification in AOD-9604 do?

The N-terminal tyrosine was introduced to stabilize the synthetic peptide. Structurally, it replaces the phenylalanine that occupies position 176 in native human growth hormone, giving the analog a tyrosine at the front of the 177-191 sequence.

Where in human growth hormone do both peptides come from?

Both derive from the carboxyl-terminal (C-terminal) region of the 191-amino-acid hGH molecule — specifically the 176-191 stretch, which structure-function research associated with the hormone’s lipid-metabolism activity.

What did published studies actually measure for AOD-9604?

The primary research measured biochemical and animal-model endpoints: enzyme activity in adipose tissue, fat oxidation, body-weight gain, adipocyte size, and dependence on the beta-3-adrenergic receptor pathway in knock-out mice (Ng et al., 2000; Heffernan et al., 2000, 2001). These are laboratory measurements, not outcomes in people.

Why is AOD-9604 sometimes called HGH Fragment 176-191?

Retailers and reviews often group them together because both originate from the same hGH region, and AOD-9604 is built on residues 176-191. The labels are related but not identical: AOD-9604 is the tyrosine-modified version of that fragment, not the bare fragment.

Did AOD-9604 ever reach human trials?

Yes. After the rodent and biochemical research, AOD-9604 progressed into human clinical testing; the pivotal trial did not meet its primary efficacy endpoint and clinical development was discontinued in 2007.

References

1. Ng FM, et al. Molecular and cellular actions of a structural domain of human growth hormone (AOD9401) on lipid metabolism in Zucker fatty rats. J Mol Endocrinol. 2000. PMID: 11116208.
2. Heffernan MA, et al. Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism. Am J Physiol Endocrinol Metab. 2000. PMID: 10950816.
3. Heffernan M, et al. The effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and beta(3)-AR knock-out mice. Endocrinology. 2001. PMID: 11713213.
4. Heffernan MA, et al. Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment. Int J Obes Relat Metab Disord. 2001. PMID: 11673763.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.

“Semax” and “N-acetyl Semax” are listed side by side across the research-peptide market, and the two-word difference in the name hides a real chemical distinction. Semax is a synthetic heptapeptide built on a fragment of a natural hormone. N-acetyl Semax is the same heptapeptide carrying a small chemical modification at one end — and the form we stock carries a second modification at the other end. This article walks through what Semax is at the sequence level, what N-terminal acetylation and C-terminal amidation change about the molecule, and what the published literature has actually measured.

What Semax is: a synthetic ACTH(4–7) analog with a Pro-Gly-Pro tail

Semax is a synthetic heptapeptide — a chain of seven amino acids with the sequence Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP). Its structure is best understood as two parts joined together. The first four residues, Met-Glu-His-Phe, correspond to ACTH(4–7) — positions 4 through 7 of adrenocorticotropic hormone, which is also the N-terminal segment of the well-studied ACTH(4–10) neuropeptide region. To that fragment Semax appends a Pro-Gly-Pro tail at the C-terminus.

That construction is reflected in the public chemistry record. In PubChem, the compound is catalogued under the descriptive name “ACTH (4-7), Pro-Gly-Pro-” with the molecular formula C₃₇H₅₁N₉O₁₀S (CAS 80714-61-0), which directly encodes the ACTH(4–7)-plus-Pro-Gly-Pro design. The Pro-Gly-Pro tail is not cosmetic: it is a deliberate stabilizing element, because proline-containing termini resist many of the exopeptidases that would otherwise trim a short peptide from its ends. So at baseline, Semax is already an engineered analog, not a raw hormone fragment.

What N-terminal acetylation changes at the molecular level

The defining structural difference between plain Semax and N-acetyl Semax is a single chemical group: an acetyl group (CH₃CO–) attached to the free N-terminus of the peptide — the α-amino group of the leading methionine residue. This is a small change in mass but a meaningful one in chemistry.

Acetylation matters because the N-terminus is a primary point of enzymatic attack. Aminopeptidases recognize a free α-amino group and cleave residues off the front of a peptide one at a time. Capping that amino group with an acetyl moiety removes the free amine these enzymes recognize — the standard rationale for N-terminal acetylation as a stability modification. In effect:

• Plain Semax — free N-terminus; the leading methionine is exposed to aminopeptidase recognition.
• N-acetyl Semax — the N-terminus is blocked by an acetyl cap, removing the free amine that front-end exopeptidases target.

That front-end vulnerability is grounded in degradation studies of Semax itself. In a serum-enzyme study, the breakdown of Semax and of ACTH/MSH(4–10) was dissected using selective peptidase inhibitors, and aminopeptidase activity was identified as a substantial contributor to degradation, with inhibitors such as bestatin and puromycin reducing a notable share of the degrading activity (Potaman et al., Peptides, 1993). Capping the residue those enzymes attack is the molecular logic behind acetylating Semax.

What amidation adds — and why our product is the amidate form

It is important to be precise about exactly which molecule we stock. Our product is N-Acetyl Semax Amidate — not plain Semax, and not only the N-acetylated form. It carries two end modifications relative to the parent heptapeptide:

• N-terminal acetylation — an acetyl cap on the leading methionine, as described above.
• C-terminal amidation — the free carboxylic acid (–COOH) at the tail end is converted to a carboxamide (–CONH₂).

Where acetylation blocks front-end (aminopeptidase) cleavage, C-terminal amidation removes the free carboxyl group that carboxypeptidases recognize at the other end. The result is a molecule that differs from plain Semax at both ends: an acetyl group at the head and an amide at the tail. This is why “Semax vs N-acetyl Semax” is not a comparison of one molecule against a slightly tweaked copy — for our amidate product it is the reference heptapeptide versus a doubly end-capped analog of it.

What published studies on Semax actually measured

The body of published research was generated largely on plain Semax, so it is the reference point, not a record of our amidate form. Two strands of that work are relevant here: degradation/stability and downstream molecular signaling.

On degradation, beyond the serum-enzyme inhibitor study above (Potaman et al., 1993), a later study characterized the binding of Semax to plasma membranes of the rat forebrain basal nuclei and tracked its biodegradation there (Dolotov et al., Russian Journal of Bioorganic Chemistry, 2004). Together these establish which enzymes act on the molecule and where — the mechanistic basis for why end-capping modifications like acetylation and amidation are used.

On molecular signaling, research has measured changes in neurotrophin systems in rodent models. One study reported that a single administration of Semax was associated with increased BDNF protein and trkB expression in the rat hippocampus (Dolotov et al., Brain Res, 2006). A later study used real-time PCR to compare the time course of NGF and BDNF gene expression across rat hippocampus, frontal cortex, and retina under Semax action, reporting region-specific changes in those transcripts (Shadrina et al., J Mol Neurosci, 2010).

Two cautions are worth stating plainly. First, these are measurements in specific laboratory animal models, not statements about any effect in a person. Second, this literature was conducted predominantly on plain Semax; it does not directly characterize the N-acetyl amidate form, whose end caps are expected on chemical grounds to alter the molecule’s susceptibility to the enzymes those degradation studies identified.

How it compares to a related ACTH/peptide-analog reference

Semax is often discussed alongside Selank, another synthetic Russian-developed peptide built by attaching a stabilizing tail to a parent sequence. Selank is an analog of the immunomodulatory peptide tuftsin extended with a C-terminal Pro-Gly-Pro tripeptide — the same Pro-Gly-Pro stabilizing strategy seen in Semax. The shared motif illustrates a principle in this class: short peptides are chemically fragile at their termini, and the recurring engineering answer is to modify the ends — whether by adding a proline-rich tail or, as with N-acetyl Semax amidate, by capping both the N- and C-termini outright.

Frequently asked questions

What is the difference between Semax and N-acetyl Semax?

Plain Semax is the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro with free termini. N-acetyl Semax carries an acetyl group capping its N-terminus. The form we stock, N-Acetyl Semax Amidate, adds a second modification — C-terminal amidation — so it is capped at both ends relative to plain Semax.

What is Semax derived from?

Semax is a synthetic analog of an ACTH fragment. Its first four residues (Met-Glu-His-Phe) correspond to ACTH(4–7), the N-terminal part of the studied ACTH(4–10) region, and a Pro-Gly-Pro tail is attached at the C-terminus. PubChem catalogues the compound under the descriptive name “ACTH (4-7), Pro-Gly-Pro-.”

Why is Semax acetylated?

The N-terminus of a peptide is a primary target for aminopeptidases, which cleave residues from the free amino end. Degradation studies of Semax identified aminopeptidase activity as a substantial contributor to its breakdown (Potaman et al., 1993). Capping the N-terminus with an acetyl group removes the free amine those enzymes recognize, which is the standard chemical rationale for N-terminal acetylation.

What does C-terminal amidation change?

Amidation converts the free carboxylic acid at the peptide’s tail into a carboxamide, removing the free carboxyl group that carboxypeptidases recognize. Where acetylation protects the front end, amidation protects the back end, so the doubly modified molecule is capped at both termini.

Is our product plain Semax?

No. Our listed product is N-Acetyl Semax Amidate — the N-terminally acetylated and C-terminally amidated form. Plain “Semax” is used in this article only as the structural and research reference point for the comparison.

What have studies on Semax measured?

Published research on plain Semax has characterized which serum and tissue enzymes degrade it (Potaman et al., 1993; Dolotov et al., 2004) and measured changes in neurotrophin systems — increased BDNF and trkB expression in the rat hippocampus (Dolotov et al., 2006) and region-specific NGF/BDNF gene-expression dynamics (Shadrina et al., 2010) — in rodent models. These measurements are in laboratory animal models and were conducted on plain Semax, not the amidate form.

References

1. Potaman VN, et al. Degradation of ACTH/MSH(4-10) and its synthetic analog semax by rat serum enzymes: an inhibitor study. Peptides. 1993. PMID: 8392718.
2. Dolotov OV, et al. The binding of Semax, ACTH 4-10 heptapeptide, to plasma membranes of the rat forebrain basal nuclei and its biodegradation. Russian Journal of Bioorganic Chemistry. 2004. PMID: 15344653.
3. Dolotov OV, et al. Semax, an analog of ACTH(4-10) with cognitive effects, regulates BDNF and trkB expression in the rat hippocampus. Brain Research. 2006. PMID: 16996037.
4. Shadrina M, et al. Comparison of the temporary dynamics of NGF and BDNF gene expression in rat hippocampus, frontal cortex, and retina under Semax action. Journal of Molecular Neuroscience. 2010. PMID: 19662538.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.

Melanotan 1 and Melanotan 2 are often listed side by side and treated as near-identical, but at the molecular level they are two distinctly engineered analogs of the same parent peptide — α-melanocyte-stimulating hormone (α-MSH). One is linear and full-length; the other is cyclic and truncated. That single architectural choice is what separates their behavior at the melanocortin receptors in the published literature. This comparison covers what each molecule is, how their structures differ, and what receptor-pharmacology studies have characterized for each.

The shared starting point: α-MSH

Both compounds are derived from α-MSH, a 13-residue endogenous peptide in the melanocortin family. Native α-MSH is a linear sequence (Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂) that binds, with varying affinity, to a family of melanocortin receptors — designated MC1R through MC5R. Like many small linear peptides, the natural hormone is short-lived: enzymes degrade it quickly, and its activity at the receptor is correspondingly brief.

Both Melanotan analogs exist because researchers set out to address that fragility through deliberate structural modification. They took different routes to do so, and the two routes are precisely what distinguish the molecules.

Melanotan 1: a linear analog with two substitutions

Melanotan 1 — also known by the research name afamelanotide, or [Nle4, D-Phe7]-α-MSH (NDP-MSH) — keeps the full-length, linear backbone of α-MSH. It is not truncated or cyclized. Instead it carries two targeted amino-acid substitutions within that intact chain:

• Position 4: the native methionine is replaced with norleucine (Nle), removing an oxidation-prone residue.
• Position 7: the L-phenylalanine is replaced with its mirror-image D-phenylalanine (D-Phe), a stereochemical change that resists the enzymes that would normally cleave the L-form.

These two substitutions leave the molecule’s overall shape and length essentially that of the parent hormone, while making it far more durable. In early melanophore research models, a single administration of [Nle4, D-Phe7]-α-melanotropin produced pigment-cell dispersion in frog and lizard skin that persisted for weeks — an effect the authors measured as dramatically more prolonged than that of the natural hormone, which lasted only days (Hadley et al., Science, 1981). Melanotan 1 is therefore best understood as α-MSH with a stabilized but otherwise preserved linear structure.

Melanotan 2: a cyclic, truncated analog

Melanotan 2 takes a fundamentally different structural approach. Rather than preserving the full chain, it is a shortened, ring-closed peptide. Its design, reported in the foundational synthesis work, started from the same superpotent Nle4/D-Phe7 lineage and then truncated the sequence and joined two side chains together to form a lactam bridge — a covalent amide link between an aspartate (position 5) and a lysine (position 10) residue (Al-Obeidi et al., J Med Chem, 1989).

The consequences of that cyclization are structural:

• Cyclic, not linear: the lactam bridge constrains the peptide into a fixed ring conformation rather than a freely flexible chain.
• Truncated: residues are removed from both ends, so Melanotan 2 is a compact heptapeptide rather than a 13-residue chain.
• Conformationally locked: the ring restricts the molecule to the shape thought to be active, which the design study associated with high potency in skin bioassays.

In short, where Melanotan 1 is a lightly edited copy of the natural hormone, Melanotan 2 is a rebuilt, cyclized fragment of it. That is the core structural distinction between the two.

Receptor pharmacology: how the structures behave at melanocortin receptors

The structural divergence maps onto how each analog has been characterized at the melanocortin receptor subtypes in binding studies. This is the part of the comparison most often summarized as “selectivity,” and the published data describe a meaningful contrast.

Melanotan 2 is characterized as broadly non-selective. When the cyclic lactam analog (MTII) was tested on cells expressing the human MC1, MC3, MC4, and MC5 receptors, it bound across the subtypes rather than confining itself to one — the study measured it as a high-affinity ligand spanning multiple melanocortin receptors, including MC4R (Schiöth et al., Peptides, 1997). The conformationally locked ring, in other words, is compatible with engagement across the receptor family.

Melanotan 1 / NDP-MSH has been studied largely through the lens of MC1R. A receptor-mutation study examined how the D-Phe7 stereoisomer engages the melanocortin-1 receptor and found that it relies on different binding contacts than native α-MSH: alanine mutations that sharply reduced binding of the natural L-isomer left binding of [Nle4, D-Phe7]-α-MSH essentially unchanged, indicating the two stereoisomers attach to the receptor at partly distinct points (Frändberg et al., Biochem Biophys Res Commun, 1994). This is the kind of receptor-level detail that explains why the single D-Phe7 substitution so strongly alters the molecule’s interaction with MC1R.

The practical framing for a structural comparison is this: the cyclic, truncated Melanotan 2 is the broader melanocortin-receptor agonist in published binding work, engaging multiple subtypes including MC4R, whereas the linear, full-length Melanotan 1 lineage has been characterized primarily through its distinctive, stereoselective interaction with MC1R. The difference is a consequence of architecture — ring-constrained heptapeptide versus stabilized full-length chain.

Side-by-side summary

• Parent peptide: both are analogs of α-MSH.
• Backbone — Melanotan 1: linear, full-length (13-residue), research name afamelanotide / [Nle4, D-Phe7]-α-MSH.
• Backbone — Melanotan 2: cyclic, truncated heptapeptide closed by an Asp5–Lys10 lactam bridge.
• Key substitutions: Melanotan 1 uses Nle4 and D-Phe7 within an intact chain; Melanotan 2 builds on that lineage but adds truncation and cyclization.
• Receptor profile in cited studies: Melanotan 2 binds broadly across MC1/MC3/MC4/MC5; the Melanotan 1 / NDP-MSH lineage is characterized chiefly through a stereoselective MC1R interaction.

Frequently asked questions

What is the structural difference between Melanotan 1 and Melanotan 2?

Melanotan 1 (afamelanotide, [Nle4, D-Phe7]-α-MSH) is a linear, full-length analog of α-MSH with two amino-acid substitutions. Melanotan 2 is a cyclic, truncated heptapeptide closed by a lactam bridge between its position-5 and position-10 residues. Linear versus cyclic is the core distinction.

Is Melanotan 1 the same as afamelanotide?

Yes. “Melanotan 1,” “afamelanotide,” and “[Nle4, D-Phe7]-α-MSH” (NDP-MSH) are names for the same linear α-MSH analog described in the research literature.

Which Melanotan analog is more selective for a single melanocortin receptor?

In published binding work, Melanotan 2 (the cyclic lactam analog) was measured as a broad, non-selective ligand across the MC1, MC3, MC4, and MC5 receptors (Schiöth et al., 1997). The Melanotan 1 / NDP-MSH lineage has instead been characterized chiefly through its distinctive interaction with the MC1 receptor (Frändberg et al., 1994).

Why does Melanotan 2 have a ring structure?

Its lactam bridge — a covalent link between an aspartate and a lysine side chain — was introduced during its design to lock the peptide into a fixed conformation. The synthesis study associated this cyclized, conformationally constrained ring with high potency in skin bioassays (Al-Obeidi et al., 1989).

What does the D-Phe7 substitution do at the receptor level?

Replacing the native L-phenylalanine at position 7 with its D-isomer changes how the peptide contacts the MC1 receptor. A mutation study found the D-Phe7 form attaches at partly different receptor points than native α-MSH, so contacts that mattered for the natural hormone did not reduce binding of the analog (Frändberg et al., 1994).

Are Melanotan 1 and Melanotan 2 interchangeable?

No. They are different molecules — linear full-length versus cyclic truncated — with different characterized receptor-binding profiles in the published research. They are related by a common parent peptide, not equivalent.

References

1. Hadley ME, et al. Calcium-dependent prolonged effects on melanophores of [4-norleucine, 7-D-phenylalanine]-alpha-melanotropin. Science. 1981. PMID: 6973820.
2. Al-Obeidi F, et al. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics. Journal of Medicinal Chemistry. 1989. PMID: 2555512.
3. Frändberg PA, et al. Evidence for alternate points of attachment for alpha-MSH and its stereoisomer [Nle4, D-Phe7]-alpha-MSH at the melanocortin-1 receptor. Biochemical and Biophysical Research Communications. 1994. PMID: 8060302.
4. Schiöth HB, et al. Selectivity of cyclic [D-Nal7] and [D-Phe7] substituted MSH analogues for the melanocortin receptor subtypes. Peptides. 1997. PMID: 9357059.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.

Tesamorelin and CJC-1295 are both synthetic analogs of growth hormone–releasing hormone (GHRH), the hypothalamic peptide that signals the pituitary to release growth hormone. They are frequently shelved next to each other, yet they are built on different fragments of GHRH and stabilized by entirely different chemical strategies. This article compares the two at the structural level — fragment length, the modification each one carries, and the persistence each shows in published pharmacology — using Sermorelin, the unmodified reference fragment, as the baseline.

The shared starting point: native GHRH

Endogenous human GHRH exists as a 44–amino acid peptide, GRF(1-44). The receptor-binding activity is concentrated in the front of the molecule: the first 29 residues, GRF(1-29), are the shortest segment that retains the parent hormone’s signaling activity. Both compounds in this comparison descend from that template, but they keep different amounts of it.

Native GHRH is short-lived. The principal reason is dipeptidyl peptidase-IV (DPP-IV), a plasma enzyme that cleaves the peptide between its second and third residues within minutes, inactivating it. Every stabilization strategy applied to a GHRH analog is, at its core, an attempt to defend against that cleavage. The two compounds solve the same problem in two different ways.

• Sermorelin is GRF(1-29) itself — the unmodified 29–residue fragment, with no protective modification, and correspondingly short-acting.
• Tesamorelin keeps the full 44–residue sequence and adds a chemical group to the N-terminus.
• CJC-1295 keeps the shorter 29–residue fragment and edits the sequence itself with several substitutions.

Tesamorelin: the full fragment with an N-terminal acyl group

Tesamorelin is a stabilized analog of the complete GRF(1-44) sequence — it retains all 44 amino acids of native GHRH rather than truncating to the 1-29 core. Its single defining modification sits at the front of the molecule.

That modification is a trans-3-hexenoic acid group — a short six-carbon acyl chain carrying a double bond — conjugated to the N-terminal tyrosine residue. In the FDA chemistry documentation and the published trial literature, the compound is described as the synthetic 44–amino acid GHRH sequence bearing a hexenoyl moiety attached at the amino terminus. Capping the N-terminus this way blocks the DPP-IV cleavage site, which is the structural feature responsible for the molecule resisting the enzymatic breakdown that rapidly clears unmodified GHRH.

The persistence this buys is modest in absolute terms. A population pharmacokinetic analysis pooling HIV-infected patients and healthy subjects characterized tesamorelin’s clearance and distribution, consistent with a short circulating half-life on the order of minutes rather than hours (González-Sales et al., Clin Pharmacokinet, 2015). The hexenoyl cap defends the front of the molecule but does not make it long-circulating. As for the GH axis: in a randomized, placebo-controlled trial in patients with HIV-associated abdominal fat accumulation, tesamorelin was measured to raise circulating insulin-like growth factor I (IGF-I) and to reduce visceral adipose tissue over 26 weeks relative to placebo (Falutz et al., N Engl J Med, 2007). That is what the trial measured in its research subjects; it is not a statement about any individual outcome.

CJC-1295: the short fragment with engineered substitutions

CJC-1295 takes the opposite approach. Rather than capping the full-length peptide, it works from the shorter GRF(1-29) core and rewrites part of the sequence. The backbone carries four amino acid substitutions, each chosen to blunt a specific route of enzymatic degradation — including a substitution at the position DPP-IV attacks. The result is a 29–residue analog that resists plasma degradation far better than the bare fragment.

This is where CJC-1295 splits into the two products commonly sold side by side:

• CJC-1295 No DAC is the stabilized GRF(1-29) backbone on its own — the four substitutions and nothing further. It is the same molecule the research literature often labels “Mod GRF 1-29.” It resists degradation but still clears on a timescale of minutes.
• CJC-1295 with DAC adds a second modification on top of that backbone: a Drug Affinity Complex (DAC), a maleimide-bearing group that bonds covalently to serum albumin in circulation. Tethered to that large, long-lived carrier protein, the peptide persists for days. We cover that mechanism in detail in our companion explainer on what DAC is.

The persistence difference between the two CJC-1295 forms is large and well characterized. In a clinical pharmacology study in healthy adults, a single administration of CJC-1295 with DAC was measured to elevate circulating growth hormone (GH) and IGF-I for several days, with an estimated half-life on the order of about a week (Teichman et al., J Clin Endocrinol Metab, 2006). A later study profiled the same GH/IGF-1 axis activation in healthy adult men one week after a single administration (Sackmann-Sala et al., Growth Horm IGF Res, 2009). Those are measurements in the studies’ research subjects, reported here as such.

Putting the two side by side

The comparison holds cleanly along three axes, with sermorelin anchoring the unmodified end:

• Fragment length: Tesamorelin retains the full GRF(1-44) sequence; CJC-1295 (both forms) and sermorelin are built on the shorter GRF(1-29) core.
• Modification type: Tesamorelin is capped with an N-terminal hexenoyl (trans-3-hexenoic acid) group. CJC-1295 instead edits the backbone with four amino acid substitutions, and the DAC version adds an albumin-binding maleimide group. Sermorelin carries no modification at all.
• Persistence (as published): Sermorelin, tesamorelin, and CJC-1295 No DAC all clear on a timescale of minutes; CJC-1295 with DAC persists for days because of the albumin tether (Teichman et al., 2006; González-Sales et al., 2015).

Read together, the family tells a tidy structural story: sermorelin is the unprotected fragment, tesamorelin protects the full-length peptide at one end, CJC-1295 No DAC protects the short fragment from the inside, and CJC-1295 with DAC layers a circulation-extending hook on top of that — siblings drawn from one hormone, separated by where and how each is reinforced.

Frequently asked questions

Are tesamorelin and CJC-1295 the same thing?

No. Both are synthetic GHRH analogs, but they are different molecules. Tesamorelin is the full 44–amino acid GRF(1-44) sequence with an N-terminal hexenoyl cap; CJC-1295 is built on the shorter 29–residue GRF(1-29) fragment with four amino acid substitutions, optionally plus a DAC group.

What is the main structural difference between them?

Fragment length and modification strategy. Tesamorelin keeps the whole GRF(1-44) chain and caps the N-terminus with a chemical group; CJC-1295 truncates to GRF(1-29) and substitutes amino acids within the sequence itself.

What is tesamorelin’s N-terminal modification?

It is a trans-3-hexenoic acid group — a short six-carbon acyl chain with a double bond — conjugated to the N-terminal tyrosine. This hexenoyl cap blocks the DPP-IV cleavage site, the structural reason the molecule resists the enzymatic breakdown that rapidly clears native GHRH.

How does CJC-1295 with DAC differ from CJC-1295 No DAC?

Both share the same stabilized GRF(1-29) backbone. The “with DAC” form carries an additional albumin-binding group, which published research associates with a half-life measured in days; the “No DAC” form lacks that group and clears within minutes. Our companion article on what DAC is walks through the mechanism.

Where does sermorelin fit in?

Sermorelin is the reference point: it is GRF(1-29) with no stabilizing modification at all. CJC-1295 is that same fragment reinforced, and tesamorelin is the longer fragment reinforced a different way. Comparing each analog back to sermorelin isolates exactly what its modification contributes.

Which one persists longer in published studies?

Among these compounds, CJC-1295 with DAC is the one published pharmacology associates with multi-day persistence, owing to its albumin tether (Teichman et al., 2006). Tesamorelin, CJC-1295 No DAC, and sermorelin all clear on a timescale of minutes (González-Sales et al., 2015).

References

1. Falutz J, et al. Metabolic effects of a growth hormone-releasing factor in patients with HIV. New England Journal of Medicine. 2007. PMID: 18057338.
2. González-Sales M, et al. Population pharmacokinetic analysis of tesamorelin in HIV-infected patients and healthy subjects. Clinical Pharmacokinetics. 2015. PMID: 25358450.
3. Teichman SL, et al. Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. Journal of Clinical Endocrinology & Metabolism. 2006. PMID: 16352683.
4. Sackmann-Sala L, et al. Activation of the GH/IGF-1 axis by CJC-1295, a long-acting GHRH analog, results in serum protein profile changes in normal adult subjects. Growth Hormone & IGF Research. 2009. PMID: 19386527.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.

The names “TB-500” and “thymosin beta-4” are used almost interchangeably across research-peptide listings, and that conflation hides a real structural distinction. Thymosin β-4 (TB-4) is a full, naturally occurring protein. TB-500 is a much shorter synthetic peptide that vendors describe as corresponding to one region of that protein. They are related, but they are not the same molecule. This article walks through what each compound is at the chemical level, the actin-binding mechanism they share, and what the published literature has actually measured.

What thymosin β-4 is: the full 43-amino-acid protein

Thymosin β-4 is a small, naturally occurring 43-amino-acid peptide — an acidic, heat-stable molecule of roughly 5 kDa that is widely distributed in cells and tissues. Its defining biochemical role is as the principal G-actin–sequestering peptide: it binds free actin monomers (G-actin) and holds them in reserve, keeping them from assembling into actin filaments (F-actin) until they are needed.

That sequestering function has been characterized in detail. Early work established that purified thymosin β-4 forms a 1:1 complex with an actin monomer and inhibits its polymerization (Safer et al., J Biol Chem, 1991). Cell-level work in human polymorphonuclear leukocytes then measured that thymosin β-4 was abundant enough to sequester the majority of available G-actin in resting cells (Cassimeris et al., J Cell Biol, 1992), and a structural study determined that it sequesters by effectively capping both ends of the actin monomer, preventing its incorporation into a growing filament (Irobi et al., EMBO J, 2004). The key takeaway: thymosin β-4 is a complete, defined protein with a well-mapped interaction with actin — not a fragment of anything.

What “TB-500” is: a synthetic fragment of the actin-binding region

“TB-500” is a synthetic peptide, and the name does not denote the full thymosin β-4 protein. In the analytical literature, the material sold as TB-500 was identified as the N-terminal acetylated 17–23 fragment of human thymosin β-4, with the sequence Ac-LKKTETQ (Esposito et al., Drug Test Anal, 2012). That places it squarely inside thymosin β-4’s actin-binding region: the conserved motif LKKTET begins at residue 17 of the 43-amino-acid sequence and is the segment most often referred to as the protein’s actin-binding motif.

So the relationship is one of part to whole:

• Thymosin β-4 — the entire 43-amino-acid protein.
• TB-500 — a short synthetic peptide (Ac-LKKTETQ) corresponding to the central actin-binding region (around residues 17–23), with an N-terminal acetyl group that the natural protein does not carry at that position.

So “TB-500 vs thymosin beta-4” is not a comparison of two unrelated compounds, but of a full protein and a fragment derived from one of its functional domains.

Why the names get conflated — and why that’s imprecise

Research-peptide vendors frequently label TB-500 simply as “thymosin beta-4,” or list the two as a single product. The conflation is understandable, since the fragment comes from the protein’s most-studied region, but it is chemically imprecise for three reasons:

• Size. One is a 43-amino-acid protein; the other is a seven-residue peptide. They are not the same molecule and do not share the same molecular weight.
• Structure. TB-500 carries an N-terminal acetyl modification (the “Ac-” in Ac-LKKTETQ); it is not simply “a piece of” the unmodified protein clipped out intact.
• Published evidence. The two have not been studied to the same extent. Most of the foundational actin-biology literature was generated using the full thymosin β-4 protein, not the seven-residue fragment.

The accurate framing is that TB-500 is a synthetic fragment/analog of thymosin β-4’s actin-binding region, used loosely in commerce to stand in for the parent protein. Our combined listing for TB-500 / Thymosin Beta-4 reflects that the two names travel together in the research-supply market even though they describe different molecules.

The shared mechanism: the actin-binding motif

What ties the fragment to the full protein is the LKKTET motif. Because this motif is the part of thymosin β-4 most directly involved in contacting actin, a fragment built around it retains the feature central to the parent protein’s actin interaction.

The functional weight of that motif has been measured directly. In one study, an isolated peptide containing the seven-amino-acid actin-binding motif was reported to reproduce the angiogenic activity seen with the full thymosin β-4 protein in the assays used, while fragments lacking parts of that motif were inactive in those same models (Philp et al., FASEB J, 2003). In the models tested, the motif — not the rest of the sequence — carried the measured activity, which is what makes the actin-binding region the logical basis for a derived peptide.

It is worth stating plainly what this does and does not establish. These studies characterize a molecular interaction with actin measured in specific laboratory models. They do not establish that the fragment and the full protein are interchangeable across every endpoint, and the deeper actin-sequestering characterization — the 1:1 stoichiometry, the monomer-capping structure — was done on the complete protein.

Frequently asked questions

Is TB-500 the same as thymosin beta-4?

No. Thymosin β-4 is the full 43-amino-acid protein. TB-500 is a synthetic peptide that corresponds to a short fragment of that protein — the acetylated 17–23 region (Ac-LKKTETQ) containing its actin-binding motif. They are related but chemically distinct, and vendors often use the two names loosely as if they were one product.

Why is TB-500 called a “fragment” of thymosin beta-4?

Because its sequence matches only a small section of the full protein. The material identified as TB-500 was characterized as the N-terminal acetylated 17–23 fragment of human thymosin β-4 (Esposito et al., 2012), whereas the parent protein is 43 amino acids long.

What is the LKKTET motif?

LKKTET is a short, conserved sequence beginning at residue 17 of thymosin β-4 that is commonly described as the protein’s actin-binding motif. It is the part of the protein most directly involved in contacting actin, which is why a derived fragment is built around it.

How does thymosin beta-4 interact with actin?

Published research characterized thymosin β-4 as the principal G-actin–sequestering peptide: it forms a 1:1 complex with an actin monomer (Safer et al., 1991) and, structurally, caps both ends of the monomer to keep it out of filaments (Irobi et al., 2004). In resting human leukocytes it was measured as abundant enough to sequester most available G-actin (Cassimeris et al., 1992).

Have TB-500 and thymosin beta-4 been studied the same amount?

No. Most of the foundational actin-biology literature used the full thymosin β-4 protein. One study reported that an isolated peptide containing the seven-amino-acid actin-binding motif reproduced the angiogenic activity of the full protein in the models tested (Philp et al., 2003), but the deeper mechanistic characterization was done on the complete protein.

Why do vendors list them together?

Because the fragment is derived from the most-studied region of the protein, the research-supply market uses the names interchangeably. The precise framing: TB-500 is a synthetic fragment or analog of thymosin β-4’s actin-binding region, not the full protein.

References

1. Esposito S, et al. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Testing and Analysis. 2012. PMID: 22962027.
2. Safer D, et al. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. Journal of Biological Chemistry. 1991. PMID: 1999398.
3. Cassimeris L, et al. Thymosin beta 4 sequesters the majority of G-actin in resting human polymorphonuclear leukocytes. Journal of Cell Biology. 1992. PMID: 1447300.
4. Irobi E, et al. Structural basis of actin sequestration by thymosin-beta4: implications for WH2 proteins. EMBO Journal. 2004. PMID: 15329672.
5. Philp D, et al. The actin binding site on thymosin beta4 promotes angiogenesis. FASEB Journal. 2003. PMID: 14500546.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.

Ipamorelin, GHRP-2, and GHRP-6 are all synthetic peptides that belong to the same pharmacological family: the growth hormone secretagogues that act as agonists at the ghrelin receptor, formally the growth hormone secretagogue receptor type 1a (GHS-R1a). Because they share a receptor, they are often treated as interchangeable. The published research tells a more interesting story — they differ markedly in selectivity, meaning how cleanly each one acts on the growth-hormone axis versus the extent to which it also moves other pituitary and adrenal hormones in study subjects. This comparison walks through what each compound is at the molecular level and what cited studies actually measured.

The shared mechanism: GHS-R1a agonism

The starting point for all three is the same receptor. A foundational study cloned a G–protein-coupled receptor in the pituitary and hypothalamus and showed it to be the molecular target of the synthetic growth hormone secretagogues, including GHRP-6 (Howard et al., Science, 1996). That receptor — GHS-R1a — was later shown to be the receptor for the stomach hormone ghrelin, which is why these compounds are described as ghrelin mimetics: they activate the same receptor ghrelin does, despite sharing no sequence similarity with it.

What separates the members of the family is not the receptor they bind but the breadth of the hormonal response that binding produces. GHS-R1a is expressed in tissues beyond the GH-releasing cells of the pituitary, and the less selective members engage that broader distribution. The selectivity question is therefore the central one when these compounds are compared:

• Does the compound stimulate growth hormone (GH) more or less in isolation?
• Or does it also raise cortisol and ACTH (the adrenal axis) and prolactin?
• Does it activate the appetite-regulating circuitry that ghrelin itself drives?

Ipamorelin: the selective end of the family

Ipamorelin is a synthetic pentapeptide — five amino acids — and it is the compound that defined the selective end of this class. The landmark characterization study described it as “the first selective growth hormone secretagogue” and tested its specificity directly (Raun et al., European Journal of Endocrinology, 1998). In that work, ipamorelin released GH in the research models studied, but, very notably, it did not raise ACTH or cortisol to levels significantly different from those seen with GHRH alone, and it showed no significant effect on prolactin — a separation that the authors reported remained intact across a wide range of exposures, far beyond the level needed to release GH.

That measured profile — GH-axis activity without a parallel rise in the adrenal-axis and prolactin hormones — is what “selective” means here, and it is the property that distinguishes ipamorelin from the older hexapeptides below. The comparison is purely about the hormonal fingerprint observed in research subjects; it is not a statement about any outcome in any individual.

GHRP-2 and GHRP-6: potent, but less selective

GHRP-2 (also known as pralmorelin) and GHRP-6 are both synthetic hexapeptides — six amino acids — and both are potent GH secretagogues at GHS-R1a. Where they diverge from ipamorelin is in the off-target hormonal activity that accompanies the GH response. A human comparison study measured GHRP-2 alongside hexarelin against GHRH, TRH, and human CRH, and reported that both peptides produced potent GH release while also raising ACTH and cortisol to a degree comparable to CRH, along with a measurable prolactin response (Arvat et al., Peptides, 1997). In other words, the published data placed GHRP-2 firmly in the less-selective group: strong on GH, but not isolated to it.

GHRP-6 was the earliest clinically studied member of the family and carries an additional dimension that distinguishes it within the group: a pronounced link to appetite circuitry. Because GHS-R1a is the ghrelin receptor, and ghrelin is an appetite-stimulating signal, the secretagogues can in principle engage feeding pathways — and GHRP-6 is the member where this was most clearly demonstrated. A study in rats found that central administration of GHRP-6 significantly stimulated food intake and activated brain appetite centers, including the hypothalamus and orexin-producing neurons (Lawrence et al., Endocrinology, 2002). That finding describes what the compound did in a research model; it is reported here as a selectivity characteristic, not as a benefit.

Where Hexarelin fits, and how to read the spectrum

Hexarelin (also called examorelin) is another synthetic hexapeptide in the same family, derived structurally from GHRP-6, and it is one of the most potent GH secretagogues of the group. In the same human comparison that examined GHRP-2, hexarelin was measured side by side and showed a similar pattern: potent GH release accompanied by ACTH, cortisol, and prolactin activity (Arvat et al., Peptides, 1997). It therefore sits with GHRP-2 and GHRP-6 on the potent-but-broad side of the spectrum rather than the selective side.

Laid out together, the family forms a clear gradient defined by selectivity rather than by raw GH potency:

• Ipamorelin — pentapeptide; reported in study models to stimulate GH with minimal measured effect on cortisol, ACTH, and prolactin.
• GHRP-2 — hexapeptide; potent GH secretagogue that also raised ACTH, cortisol, and prolactin in the cited human study.
• GHRP-6 — hexapeptide; potent GH secretagogue additionally associated with appetite-center activation in a research model.
• Hexarelin — hexapeptide; among the most potent of the group, with a measured off-target hormonal profile resembling GHRP-2’s.

Read across that list, the variable that separates these compounds is consistent: every one is a GHS-R1a agonist, but ipamorelin’s published profile shows the cleanest separation between GH-axis activity and the cortisol, prolactin, and appetite responses the hexapeptides carry.

Frequently asked questions

Are ipamorelin, GHRP-2, and GHRP-6 the same type of compound?

Yes, in the broad sense: all three are synthetic peptides that act as agonists at the ghrelin receptor, GHS-R1a, and are classed as growth hormone secretagogues. They differ in length — ipamorelin is a pentapeptide, GHRP-2 and GHRP-6 are hexapeptides — and, more importantly, in their measured selectivity.

What makes ipamorelin “selective” compared with GHRP-2 and GHRP-6?

In its characterization study, ipamorelin stimulated GH in the research models tested but did not significantly raise ACTH, cortisol, or prolactin relative to GHRH alone (Raun et al., 1998). GHRP-2 and GHRP-6, by contrast, were reported to raise the adrenal-axis hormones and prolactin alongside GH, which is what places them in the less-selective group.

What is the main difference between GHRP-2 and GHRP-6?

Both are potent hexapeptide secretagogues at the same receptor. In the published literature, GHRP-2 is generally described as the more potent GH releaser, while GHRP-6 is the member most clearly associated with appetite-center activation — central GHRP-6 stimulated food intake in a rat model (Lawrence et al., 2002).

Why are these called ghrelin mimetics?

GHS-R1a, the receptor all of these peptides bind, is the same receptor activated by the natural hormone ghrelin (Howard et al., 1996). The synthetic peptides reproduce ghrelin’s receptor activation despite having no sequence resemblance to ghrelin, which is why they are described as ghrelin mimetics.

Where does hexarelin sit relative to the others?

Hexarelin is a hexapeptide derived from GHRP-6 and is one of the most potent members of the family. In the same human study that measured GHRP-2, hexarelin showed a comparable off-target profile, with ACTH, cortisol, and prolactin activity accompanying GH release (Arvat et al., 1997), placing it on the potent-but-less-selective side of the spectrum.

Do all four bind the same receptor?

Yes. Ipamorelin, GHRP-2, GHRP-6, and hexarelin are all agonists at GHS-R1a, the growth hormone secretagogue receptor. The differences between them lie in the breadth of the hormonal response that receptor activation produced in the cited studies, not in the identity of the receptor itself.

References

1. Howard AD, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996. PMID: 8688086.
2. Raun K, et al. Ipamorelin, the first selective growth hormone secretagogue. European Journal of Endocrinology. 1998. PMID: 9849822.
3. Arvat E, et al. Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Comparison with the effects of GHRH, TRH and hCRH. Peptides. 1997. PMID: 9285939.
4. Lawrence CB, et al. Acute central ghrelin and GH secretagogues induce feeding and activate brain appetite centers. Endocrinology. 2002. PMID: 11751604.

For research use only. The products and materials discussed are intended for laboratory research purposes and are not for human or veterinary use, diagnosis, or treatment. This article describes the chemical structure and published pharmacological research of a compound and does not constitute a claim of any effect in any individual.