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Macimorelin
Macimorelin (Macrilen) is a real, FDA-approved oral ghrelin-receptor agonist used as a one-time diagnostic test for adult growth hormone deficiency — not a growth-hormone treatment, and not the kind of repeat-dose "GH secretagogue" peptide sold for bodybuilding elsewhere in this Library.
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In brief
Should you care? Only relevant if you're being evaluated for adult growth hormone deficiency — a single-dose diagnostic test, not something taken repeatedly.The short version
- An oral ghrelin-receptor agonist that provokes a one-time GH response used to help diagnose deficiency — not a growth-hormone treatment itself.
- Approved December 2017 as the first oral alternative to the insulin tolerance test, the prior reference standard.
- Should not be confused with unapproved injectable "GH secretagogue" peptides like ipamorelin or the GHRPs, which target the same receptor but are sold repeatedly for bodybuilding rather than as a single diagnostic dose.
An oral ghrelin mimetic used as a diagnostic, not a treatment
Macimorelin is a synthetic, orally active peptidomimetic that binds and activates the ghrelin receptor (GHSR1a) — the same receptor targeted by injectable growth-hormone-releasing peptides such as ipamorelin and the GHRPs elsewhere in this Library. In a functioning pituitary, activating that receptor triggers a burst of growth hormone release. Macimorelin exploits that single burst purely as a diagnostic signal rather than as a repeated treatment: a patient drinks one dose, and blood is drawn at intervals afterward to see whether GH rises the way it should in someone without deficiency.
The approval and pivotal trial
The FDA approved macimorelin (as Macrilen) on 20 December 2017 to help diagnose adult growth hormone deficiency (AGHD). The pivotal trial (Garcia et al., J Clin Endocrinol Metab 2018;103(8):3083-3093, PMID 29860473) compared macimorelin against the pre-existing reference standard, the insulin tolerance test (ITT), in 157 subjects. Using a GH cutoff of 2.8 ng/mL for macimorelin versus 5.1 ng/mL for the ITT, negative agreement between the two tests was 95.4% and positive agreement was 74.3%; macimorelin showed 87% sensitivity and 96% specificity relative to the ITT, with 97% reproducibility on retesting.
Why an oral option matters here
The ITT remains a reference standard, but it works by deliberately inducing hypoglycemia (dangerously low blood sugar) to provoke a stress-driven GH response — a procedure that requires close medical supervision and is contraindicated in patients with seizure disorders or cardiovascular disease. Macimorelin reaches comparable diagnostic accuracy with a single oral dose and no deliberately induced physiological stress, at the cost of being a newer test with less universal availability and clinician familiarity than the decades-old ITT.
Current status
More complicated than most entries in this Library: Aeterna Zentaris's US/Canada commercialization partnership with Novo Nordisk ended in 2022, and the company discontinued US commercial sales of Macrilen effective 23 May 2023 while it seeks a new US partner — the underlying FDA approval has not been withdrawn, but a US patient cannot currently obtain it through normal commercial channels. It remains available and actively marketed in the UK and European Economic Area under the brand name Ghryvelin, through a separate licensing deal with Pharmanovia. Administered only in a clinical setting as a single supervised diagnostic dose — not self-administered, and not approved or trialed as a repeat-dose growth-hormone-boosting product.
References
- Garcia JM, Biller BMK, Korbonits M, et al., "Macimorelin as a Diagnostic Test for Adult GH Deficiency," J Clin Endocrinol Metab 2018;103(8):3083-3093, PMID 29860473, DOI: 10.1210/jc.2018-00665.
- FDA approval history for Macrilen (macimorelin), 20 December 2017; Aeterna Zentaris, discontinuation of US commercial sales of Macrilen effective 23 May 2023 following termination of its Novo Nordisk license; ongoing marketing in the UK/EEA as Ghryvelin under license to Pharmanovia.