Macimorelin
GH Axis / DiagnosticAlso known as: Macrilen
Mechanism
An oral growth hormone stimulation test — the first and only FDA-approved oral test for diagnosing adult growth hormone deficiency. You drink a solution, then have blood drawn at specific intervals to measure your GH response. It works by mimicking ghrelin (the hunger hormone) to trigger GH release. Much simpler than older GH stimulation tests that required IV infusions.
Technical detail
Oral peptidomimetic ghrelin receptor (GHSR-1a) agonist. Molecular weight 535 Da (small molecule/peptidomimetic, not a full peptide). Binds GHSR-1a on pituitary somatotrophs, activating Gq/PLC/IP3/Ca2+ signaling cascade and triggering GH exocytosis. Diagnostic protocol: single oral dose (0.5 mg/kg dissolved in 120 mL water), fasting; blood draws at baseline, 30, 45, 60, and 90 minutes. GH peak <2.8 ng/mL confirms adult GH deficiency (AGHDx). Sensitivity 87%, specificity 96% vs insulin tolerance test (ITT). Oral bioavailability ~65%. Half-life ~4.1 hours. Metabolized by CYP3A4. Avoid strong CYP3A4 inhibitors (false positives). Phase III: GHLIQUID trial validated against ITT.
Effects
ENDOCRINE / GH AXIS [Tier 1 – Human Clinical / FDA-Approved]: Macimorelin is an oral ghrelin receptor agonist FDA-approved as a diagnostic test for adult growth hormone deficiency (AGHD). It is the ONLY oral GH stimulation test approved by the FDA, replacing the insulin tolerance test (ITT) and other cumbersome diagnostic protocols. Produces a reliable GH peak within 30-90 minutes of oral administration. Peak GH response used diagnostically — a cutoff of 2.8 ng/mL separates GH-deficient from GH-sufficient adults. DIAGNOSTIC APPLICATION [Tier 1 – FDA-Approved]: Single oral dose of 0.5 mg/kg produces dose-dependent GH release. Sensitivity 87%, specificity 96% for detecting AGHD compared to ITT (gold standard). Much safer than ITT (no hypoglycemia risk). Blood draws at 30, 45, 60, and 90 minutes post-dose. METABOLIC [Tier 1 – Human Clinical]: Transient effects only — designed as single-dose diagnostic, not chronic therapy. Single dose raises GH, ghrelin-like signaling. No clinically significant effects on glucose, insulin, or cortisol in single-dose diagnostic context. APPETITE [Tier 2 – Limited Human]: As a ghrelin mimetic, it does stimulate appetite transiently, but this is not clinically relevant in the single-dose diagnostic context. Chronic use would be expected to cause appetite stimulation similar to MK-677.
Practitioner Guide
CLINICAL POSITIONING: Macimorelin (Macrilen) is a DIAGNOSTIC TOOL, not a therapeutic peptide. Its value is in confirming or ruling out adult growth hormone deficiency before starting GH optimization protocols. Any practitioner running a peptide clinic should know about it as an alternative to the insulin tolerance test. DIAGNOSTIC PROTOCOL (FDA-APPROVED): • Patient fasts overnight (minimum 8 hours). • Administer 0.5 mg/kg body weight as oral solution (reconstituted powder). • Blood draws for serum GH at: baseline, 30 min, 45 min, 60 min, and 90 min post-dose. • Interpretation: Peak GH < 2.8 ng/mL = consistent with AGHD. Peak GH ≥ 2.8 ng/mL = GH deficiency unlikely. • Test takes ~2 hours total. No IV access needed (unlike ITT). No hypoglycemia risk (unlike ITT). WHY PRACTITIONERS SHOULD KNOW THIS: • Before prescribing GH peptides (Ipamorelin, CJC-1295, MK-677, etc.) for "anti-aging" or "GH optimization," responsible practice includes establishing that the patient actually has suboptimal GH levels. • Macimorelin test provides objective, FDA-validated evidence of GH status. • Insurance may cover the test when ordered for appropriate indications. • Protects the practitioner medically and legally — documented evidence of GH deficiency supports the clinical rationale for GH peptide therapy. DRUG INTERACTIONS TO KNOW: • Strong CYP3A4 inhibitors (ketoconazole, clarithromycin, itraconazole) may increase macimorelin exposure. • Drugs that affect GH release: recent use of GH, GH secretagogues, somatostatin analogs, or glucocorticoids may produce false results. Washout periods required. OFF-LABEL CONSIDERATIONS: Some practitioners have explored macimorelin as a therapeutic GH secretagogue (chronic oral dosing). This is off-label and NOT well-studied for chronic use. MK-677 is a better-characterized option for chronic oral GH stimulation. Macimorelin is substantially more expensive than MK-677.
Research Summary
TIER 1 (Human Clinical Trials / FDA-Approved): • Garcia et al. (2018, JCEM): Pivotal Phase III trial establishing macimorelin diagnostic accuracy. Sensitivity 87%, specificity 96% vs ITT for AGHD diagnosis. n=157. • FDA approval (2017) for diagnosis of AGHD in adults. First and only oral GH stimulation test. • Consistency and reproducibility superior to GHRH-arginine test and comparable to ITT, with dramatically better safety profile. TIER 2 (Limited Human / Strong Preclinical): • Pharmacokinetic studies showing reliable oral absorption, peak plasma levels at 0.5-1 hour, GH peak at 30-90 minutes. • Cross-validation studies against multiple GH stimulation tests confirming diagnostic reliability. • Drug interaction studies identifying CYP3A4 pathway as primary metabolic route. TIER 3 (Preclinical / Mechanistic): • Ghrelin receptor (GHS-R1a) binding characterization — oral peptidomimetic with strong receptor affinity. • Animal pharmacology confirming dose-dependent GH release via oral route. EVIDENCE GAPS: No chronic dosing studies in humans. Therapeutic (non-diagnostic) applications not investigated in clinical trials. Long-term safety of repeated dosing unknown. Cost-effectiveness vs ITT established, but cost-effectiveness vs clinical judgment alone debated.