Ipamorelin vs MK-677
Ipamorelin is a synthetic pentapeptide and MK-677 (ibutamoren) an orally active small molecule, yet both are agonists at the ghrelin receptor GHS-R1a. This page compares their chemistry, pharmacology, clinical research, regulatory and WADA status, and the different analytical methods each requires.
Ipamorelin and MK-677 illustrate two routes to the same target. Ipamorelin is a pentapeptide descended from earlier growth hormone releasing peptides such as GHRP-6. MK-677, also called ibutamoren, is a non-peptide small molecule developed by Merck in the 1990s as an orally bioavailable ghrelin receptor agonist. MK-677 is not a peptide, although it is often discussed alongside peptide secretagogues.
Both stimulate growth hormone release by activating GHS-R1a, the receptor for the hormone ghrelin. Their different chemistry affects how long they act, how they are absorbed and how their identity and purity are tested. Neither is approved by the FDA, and both are prohibited in sport.
Side-by-side
| Attribute | Ipamorelin | MK-677 |
|---|---|---|
| Chemical class | Peptide (5 residues) | Small molecule (spiro-piperidine derivative); not a peptide |
| Structure | Aib-His-D-2-Nal-D-Phe-Lys-NH<sub>2</sub> | Non-peptide; usually supplied as the mesylate salt |
| Receptor target | GHS-R1a (ghrelin receptor) | GHS-R1a (ghrelin receptor) |
| Oral bioavailability | Poor, as for most peptides | Orally active by design |
| Duration of action | Short-acting in published pharmacology studies | Long-acting; sustained GH and IGF-1 elevation reported in human studies |
| Clinical research | Phase 2 trials in postoperative ileus; no approval | Studies in healthy older adults, GH deficiency and other settings; no approval |
| Evidence level | Early-clinical | Early-clinical |
| FDA status | Not FDA-approved | Not FDA-approved |
| WADA | Prohibited under S2 (GH secretagogues) | Prohibited under S2 (GH secretagogues) |
| Analytical methods | MS against peptide sequence mass; peptide HPLC; counter-ion and net peptide content | Small-molecule identity by LC-MS and NMR; salt-form confirmation; no sequence or net peptide content |
Same receptor, different molecules
GHS-R1a is a G protein-coupled receptor in the pituitary and hypothalamus that binds ghrelin, an acylated 28-residue peptide. Growth hormone releasing peptides were discovered before ghrelin, and chemists then sought non-peptide compounds that could activate the same receptor and survive the digestive tract. MK-677 emerged from that effort. Ipamorelin, in contrast, was optimized as a peptide for selectivity; in animal studies it raised growth hormone with little effect on ACTH or cortisol, setting it apart from older GHRPs.
Clinical research and observed effects
Ipamorelin was studied in phase 2 trials for postoperative ileus, based on the gut-motility effects of ghrelin receptor agonists, but the program did not lead to an approved product. MK-677 was studied over longer periods in healthy older adults and other groups, where it raised growth hormone and IGF-1 levels; reported findings included increased appetite, fluid retention and changes in measures of glucose metabolism. These were investigational studies, and neither compound has an approved indication.
Analytical differences
Testing reflects chemistry. Ipamorelin is characterized with peptide methods: mass spectrometry against the expected mass of its sequence, reversed-phase HPLC for related peptides, and attention to counter-ion content and chiral purity of its D-amino acids. MK-677 is characterized as a small molecule: identity by LC-MS and NMR, purity by HPLC with small-molecule impurity profiling, and confirmation of the mesylate salt. The sequence analyzer and net peptide content concepts apply to ipamorelin only. See also ipamorelin vs CJC-1295.
Frequently asked questions
Is MK-677 a peptide?
No. MK-677, also called ibutamoren, is a non-peptide small molecule. It is often grouped with peptides because it activates the same ghrelin receptor as growth hormone releasing peptides such as ipamorelin, but its structure contains no amino acid chain, and it is analyzed with small-molecule methods rather than peptide sequencing or net peptide content.
What is the difference between ipamorelin and MK-677?
Both are ghrelin receptor agonists that stimulate growth hormone release. Ipamorelin is a short synthetic peptide with poor oral bioavailability and a short duration of action in published studies. MK-677 is an orally active small molecule with a much longer duration of action. Both reached human studies, and neither is approved by the FDA.
Are ipamorelin and MK-677 banned in sport?
Yes. Growth hormone secretagogues, including ghrelin receptor agonists such as ipamorelin and MK-677, are prohibited at all times under section S2 of the WADA Prohibited List. Neither compound is approved by the FDA for any use, so neither is available as a licensed medicine in the US.
Full entries
- Ipamorelin
Ipamorelin is a synthetic pentapeptide growth hormone secretagogue that acts as a selective agonist of the ghrelin receptor (GHS-R1a). It is investigational and not approved for any use.
- MK-677 (ibutamoren)
MK-677 (ibutamoren) is an orally active small-molecule agonist of the ghrelin receptor that raises growth hormone and IGF-1 levels. It is not a peptide, and it is not approved for any use.
Cite this page