Macimorelin

Pharmaceutical

Macimorelin acetate (MACRILEN) is an orally active, synthetic ghrelin receptor (GHSR-1a) agonist that was granted FDA approval in 2017 as a diagnostic agent for adult growth hormone deficiency (AGHD). It is the first and only oral growth hormone stimulation test, replacing invasive provocative tests that require intravenous infusions of insulin, arginine, glucagon, or GHRH. In the diagnostic protocol, a single oral dose of macimorelin is administered, and serum GH levels are measured at baseline, 45 minutes, 60 minutes, and 90 minutes. A peak GH response below the validated cutoff (2.8 ng/mL) confirms GH deficiency with high sensitivity and specificity. Pharmacologically, macimorelin is a peptidomimetic — a small molecule designed to mimic the structure and activity of the natural peptide ligand ghrelin at the GHSR-1a receptor. It triggers the same Gq-coupled signaling cascade (phospholipase C activation, intracellular calcium release, GH vesicle exocytosis) as endogenous ghrelin and peptide secretagogues like Ipamorelin. The key difference from research-grade secretagogues is that macimorelin has undergone full regulatory evaluation, including Phase III clinical trials demonstrating diagnostic concordance with the insulin tolerance test (ITT), the gold standard for GH deficiency diagnosis. Macimorelin's clinical significance extends beyond diagnostics. Its development validated the oral ghrelin-receptor agonist approach in a regulatory setting and established safety data for acute GHSR-1a stimulation in a well-characterized patient population. While it is approved exclusively as a single-dose diagnostic, the pharmacology is shared with research-grade secretagogues used chronically for GH optimization — providing a bridge between regulated medicine and the research peptide space.