Macimorelin (Macrilen / Ghryvelin)
FDA-approved oral ghrelin receptor agonist used as a diagnostic test for adult growth hormone deficiency.
At a glance
What it is: FDA-approved oral ghrelin receptor agonist used as a diagnostic test for adult growth hormone deficiency..
Primary research applications:
- Diagnosis of adult growth hormone deficiency (FDA-approved indication)
- Pediatric GH deficiency diagnostic research
Editorial summary: Macimorelin is the only orally-administered GH secretagogue with FDA approval as a clinical tool. It's used to diagnose adult GH deficiency by reliably stimulating a measurable GH peak after a single oral dose. Its therapeutic use as a chronic GH-raising agent is not approved and not the subject of major clinical programs.
- Class / structure
- Orally bioavailable ghrelin receptor agonist (small synthetic peptide)
- Half-life
- ~4 hours
- First described
- Approved 2017 (Aeterna Zentaris / Strongbridge)
- Regulatory status
- FDA-approved 2017 for adult GHD diagnostic testing; EMA-approved 2018
What is Macimorelin?
Macimorelin is a small synthetic peptide that activates the growth hormone secretagogue receptor (GHSR-1a, the ghrelin receptor) in the pituitary, triggering a measurable pulse of growth hormone release. Its FDA-approved use is purely diagnostic — to test whether a patient's pituitary can produce GH on demand, which is the operational definition of GH sufficiency.[1]
Discovery and development
Macimorelin was developed by Aeterna Zentaris (later marketed by Strongbridge and Novo Nordisk) as an oral alternative to the insulin tolerance test and arginine-GHRH testing — established but cumbersome ways of diagnosing adult growth hormone deficiency. The orally administered ghrelin receptor agonist offered a simpler, safer, and more reliable diagnostic protocol.
FDA approval came in 2017 with the brand name Macrilen, followed by EMA approval in 2018 (branded Ghryvelin in the EU). It is the only oral GH secretagogue with regulatory approval anywhere in the world.
Mechanism of action
Like other GH secretagogues (ipamorelin, hexarelin, MK-677), macimorelin binds GHSR-1a in the anterior pituitary and triggers GH release. Its key feature for diagnostic use is reliability: a healthy pituitary will produce a clear GH peak (typically >2.8 ng/mL) within 60 minutes; a GH-deficient pituitary will not.
Pharmacokinetics
Orally bioavailable with peak plasma concentration around 30–60 minutes after dosing. Half-life approximately 4 hours. The diagnostic protocol uses a single 0.5 mg/kg oral dose followed by serial GH measurements at 30, 45, 60, and 90 minutes.
What the research shows
The peer-reviewed literature on Macimorelin is summarized below across two tiers: human research (the highest standard), and preclinical / emerging research (animal models and early-stage human work).
Claims and the evidence behind them
This table summarizes commonly discussed claims and how the published evidence weighs in. The aim is clarity — supported claims, claims that look promising but need more data, and claims that outrun the science.
| Claim | What the evidence shows | Verdict |
|---|---|---|
| Reliably diagnoses adult GH deficiency by oral test | Phase 3 evidence | Established |
| Safer and simpler than insulin tolerance test | Comparative trial data | Supported |
| Useful as a chronic GH-raising therapy | Not the indication; no Phase 3 chronic-use data | Unsupported |
Reported user experiences
How the research describes administration
FDA-approved diagnostic protocol: single oral 0.5 mg/kg dose after overnight fast, followed by GH measurement at 30, 45, 60, and 90 minutes. Performed in clinical setting with cardiac monitoring.
Editorial note
Administration details above describe how the peptide is given in published studies. We summarize this for educational completeness — these descriptions are not protocols, dosing recommendations, or instructions for personal use. Decisions about treatment require an appropriately licensed clinician.
Safety considerations and open questions
The takeaway
Macimorelin is the answer to a specific clinical problem — diagnosing adult GH deficiency without the burden of insulin-tolerance testing. It is not a chronic therapy, not a body-composition tool, and not a research-peptide-class compound. For users encountering it in peptide discussions, the appropriate framing is: an FDA-approved oral ghrelin receptor agonist whose practical use is diagnostic, not therapeutic.
Frequently asked questions
Is macimorelin similar to MK-677?
Mechanistically yes — both are oral GH secretagogues acting on GHSR-1a. Practically very different: macimorelin is FDA-approved for single-dose diagnostic use; MK-677 (ibutamoren) is an unapproved compound used in research and grey-market wellness contexts for chronic GH-raising effects.
Can macimorelin be used to raise GH chronically?
It hasn't been developed for that indication and no Phase 3 chronic-use data supports it. The cost and short half-life also make daily use impractical compared to alternatives.
How does macimorelin compare to the insulin tolerance test?
Comparable diagnostic accuracy with substantially better practical profile — single oral dose vs. IV insulin infusion with hypoglycemia monitoring. Macimorelin has largely supplanted the ITT in adult GHD diagnosis where available.
References
- Strasburger CJ, et al. Oral macimorelin: a paradigm shift in growth hormone deficiency diagnosis. Endocrine. 2018;60(2):357-363. https://pubmed.ncbi.nlm.nih.gov/29460186/
- Garcia JM, Biller BMK, et al. Macimorelin as a diagnostic test for adult GH deficiency. J Clin Endocrinol Metab. 2018;103(8):3083-3093. https://pubmed.ncbi.nlm.nih.gov/29860474/
- FDA approval label, Macrilen (macimorelin) oral solution. December 2017. https://www.accessdata.fda.gov/drugsatfda_docs/label/2017/205598s000lbl.pdf