Macimorelin

Origin and Structure

Macimorelin is a small-molecule, orally bioavailable ghrelin receptor agonist marketed as Macrilen. Chemically it is a modified tripeptide mimetic rather than a classical hexapeptide GHRP, engineered for oral absorption, plasma stability, and a clean pharmacodynamic profile suitable for diagnostic use. It was developed by Aeterna Zentaris as AEZS-130 and acquired by Strongbridge Biopharma, and is now marketed in the United States by Novo Nordisk (via the Xeris Biopharma partnership following a 2022 commercial reshuffle of the asset).

Macimorelin's distinguishing feature in this library is its regulatory status: it is one of very few compounds here with an active FDA approval. The FDA approved macimorelin on December 20, 2017 for the diagnosis of adult growth hormone deficiency (AGHD). The European Medicines Agency followed with approval in January 2019.

Mechanism of Action

Macimorelin is an agonist at the ghrelin receptor (GHSR-1a) on pituitary somatotrophs. Oral administration at 0.5 mg/kg produces a robust, time-defined GH pulse that peaks between 45 and 60 minutes post-dose in subjects with intact pituitary GH reserve. In patients with true adult GHD, the GH response is blunted or absent — this differential response is the diagnostic basis of the test.

The clinical validation is specific: a peak GH < 2.8 ng/mL after oral macimorelin has been validated against the insulin-tolerance test (ITT), historically the gold standard for AGHD diagnosis. The pivotal study (the "Phase 3 macimorelin diagnostic trial" published by Garcia and colleagues in 2018) reported sensitivity of 87% and specificity of 96% using the 2.8 ng/mL cutoff, with substantially better tolerability and safety than the ITT, which requires induction of hypoglycemia and carries real cardiovascular and neurologic risk.

Clinical Niche

Macimorelin is the first oral GH stimulation test. Before its approval, the diagnostic options for AGHD were:

• Insulin-tolerance test (ITT). Gold standard but requires controlled induction of hypoglycemia, which is contraindicated in cardiovascular disease, seizure disorders, and older patients.
• Glucagon stimulation test. Safer than ITT but 3-4 hours long and poorly tolerated (nausea, vomiting).
• GHRH-plus-arginine. Validated but requires compounded GHRH, which became unavailable in the United States after sermorelin's commercial discontinuation.

Macimorelin occupies a genuinely useful clinical position: one 90-minute outpatient visit, oral administration, no hypoglycemia, no IV. That practical advantage — not a pharmacological breakthrough — is what drove the approval.

Practical Considerations

Macimorelin is administered as a single 0.5 mg/kg oral dose after an overnight fast of at least 8 hours. Blood samples for GH are drawn at 30, 45, 60, and 90 minutes post-dose. The test is contraindicated in patients with conditions that prolong the QT interval and in those taking strong CYP3A4 inducers, both of which can interfere with interpretation. Patients are instructed to discontinue GH replacement at least a week before testing.

Unlike the other compounds in this library, macimorelin is not used as a therapeutic in any setting. It is a single-use diagnostic agent, prescribed by endocrinologists, dispensed as a reconstituted oral solution, and administered under clinical supervision. It is the most regulated and most clinically well-defined ghrelin-receptor agonist on the market.

Safety and Regulatory Status

The safety profile in the Phase 3 program and post-marketing surveillance is favorable. Most frequently reported adverse events are dysgeusia (metallic or bitter taste, roughly 5% of patients), mild transient headache, fatigue, dizziness, and nausea — all short-lived and consistent with a single-dose diagnostic agent. The QT interval prolongation signal identified during development is the most clinically significant safety item, and ECG-based screening is recommended in the label for patients at risk.

Macimorelin is FDA-approved (Macrilen, December 2017) and EMA-approved. It is on the WADA prohibited list (class S2) as a GH secretagogue, though the single-dose diagnostic use case is distinct from the chronic dosing that concerns anti-doping authorities. For context on the broader GH secretagogue category, see ipamorelin for the therapeutic injectable counterpart and MK-677 for the other orally bioavailable ghrelin agonist in widespread use — macimorelin is the only one in that group with an FDA approval.