Macimorelin (Macrilen)

1. Overview

Macimorelin (development codes AEZS-130, EP-01572, ARD-07) is a synthetic peptidomimetic ghrelin receptor agonist that received FDA approval in December 2017 as the first and only oral diagnostic test for adult growth hormone deficiency (AGHD) [1][2][8]. Marketed as Macrilen, it was originally developed by Aeterna Zentaris and subsequently acquired by Strongbridge Biopharma (later Xeris Pharmaceuticals), with Novo Nordisk now holding commercial rights.

The diagnosis of AGHD has historically relied on provocative GH stimulation tests that are cumbersome, poorly tolerated, and carry safety risks [9][14]. The insulin tolerance test (ITT), considered the gold standard, requires intravenous insulin administration to induce controlled hypoglycemia (blood glucose below 40 mg/dL), which is contraindicated in patients with seizure disorders, cardiovascular disease, and elderly patients [9]. The GHRH-arginine test requires intravenous access and lost practical utility in the United States when biosynthetic GHRH (Geref Diagnostic) was discontinued in 2008 [7]. The glucagon stimulation test, while safer, requires intramuscular injection, causes significant nausea, and has variable diagnostic accuracy [9][12].

Macimorelin addresses these limitations by providing a simple oral test: a single weight-based dose (0.5 mg/kg) is dissolved in water and consumed, with blood draws at 30, 45, 60, and 90 minutes for GH measurement [8]. A peak GH value below 2.8 ng/mL is diagnostic of AGHD [1][2]. The test takes approximately 90 minutes, requires no intravenous access, carries no hypoglycemia risk, and demonstrates excellent reproducibility (97% on repeat testing) [1][4].

As a ghrelin mimetic, macimorelin binds to the growth hormone secretagogue receptor type 1a (GHS-R1a) -- the same receptor activated by endogenous ghrelin -- on pituitary somatotrophs, triggering GH release through the Gq/11-phospholipase C-calcium signaling cascade [10][11][13]. It is structurally unrelated to peptide GH secretagogues (GHRP-6, hexarelin, ipamorelin) and to the non-peptide agonist MK-677 (ibutamoren), though all share the same receptor target [13][15].

Type

Peptidomimetic ghrelin receptor (GHS-R1a) agonist

Molecular Weight

535.6 g/mol (acetate salt)

Molecular Formula

C26H30N6O3 (free base)

Half-life

~4.1 hours (single oral dose)

Routes

Oral (single diagnostic dose dissolved in water)

FDA Status

Approved (December 2017; diagnostic for adult GH deficiency)

EMA Status

Approved (January 2019; diagnostic for adult GH deficiency)

Diagnostic Cutoff

Peak GH below 2.8 ng/mL indicates AGHD

Manufacturer

Novo Nordisk (acquired from Aeterna Zentaris/Strongbridge Biopharma)

2. Mechanism of Action

2.1 GHS-R1a Activation

Macimorelin is a potent agonist of the growth hormone secretagogue receptor type 1a (GHS-R1a), the seven-transmembrane G protein-coupled receptor discovered by Howard et al. in 1996 and subsequently identified as the endogenous receptor for ghrelin [10][11]. Upon binding, macimorelin activates the Gq/11 signaling pathway, stimulating phospholipase C (PLC)-mediated generation of inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 triggers calcium release from intracellular stores in anterior pituitary somatotrophs, directly inducing exocytosis of GH-containing secretory granules [10][11][13].

2.2 Diagnostic Principle

In healthy individuals and patients with intact GH reserve, macimorelin provokes a robust GH surge that peaks at 30-45 minutes post-dose, typically exceeding 5-10 ng/mL [1][5]. In patients with AGHD -- whether due to pituitary surgery, radiation, traumatic brain injury, or idiopathic hypopituitarism -- the somatotroph population is diminished or dysfunctional, resulting in a blunted or absent GH response [1][2][9]. The diagnostic cutoff of 2.8 ng/mL was established through receiver operating characteristic (ROC) analysis in the pivotal validation studies, optimizing the balance between sensitivity and specificity [1][2].

2.3 Pharmacological Advantages for Diagnosis

Unlike the ITT, which tests GH reserve by inducing the physiological stress of hypoglycemia (activating hypothalamic GHRH neurons), macimorelin directly stimulates somatotrophs at the pituitary level through GHS-R1a [1][13]. This provides a more direct assessment of pituitary GH secretory capacity. Additionally, because macimorelin does not affect blood glucose, there is no risk of hypoglycemia and no contraindications in patients with cardiovascular disease or seizure disorders [1][8][17].

3. Pharmacokinetics

Following a single oral dose of 0.5 mg/kg, macimorelin is rapidly absorbed with peak plasma concentrations (Tmax) at approximately 0.5-0.75 hours [5][8]. The terminal elimination half-life is approximately 4.1 hours [8]. The drug is metabolized hepatically, primarily via CYP3A4 [8]. Concomitant use of strong CYP3A4 inhibitors may increase macimorelin exposure and potentially cause a false-normal GH response; therefore, the test should be performed after washout of such drugs [8]. Concomitant use of drugs that directly suppress GH secretion (somatostatin analogs, dopamine agonists) should also be avoided [8].

Food reduces the absorption of macimorelin and can blunt the GH response; consequently, the test must be performed after an overnight fast of at least 8 hours [8].

3.1 Detailed Pharmacokinetic Profile

Absorption: Macimorelin is formulated as granules for reconstitution in 120 mL of water and is rapidly absorbed from the upper gastrointestinal tract [5][8]. Peak plasma concentrations (Cmax) are achieved within 0.5 to 1.25 hours (median Tmax approximately 0.75 hours) following the 0.5 mg/kg dose [5][8]. Absolute oral bioavailability has not been formally determined in humans but is estimated to be moderate based on the robust pharmacodynamic response achieved with oral dosing [5]. Food significantly reduces both the rate and extent of absorption; when administered with a high-fat meal, Cmax is reduced by approximately 55% and AUC by approximately 44%, which can blunt the GH response and produce false-positive results [8]. For this reason, the FDA label mandates an overnight fast of at least 8 hours before testing [8].

Distribution: Macimorelin is approximately 70% bound to plasma proteins, primarily albumin [8]. The apparent volume of distribution is large (approximately 4 L/kg), suggesting extensive tissue distribution beyond the plasma compartment [8]. The drug readily crosses the blood-brain barrier, though central penetration is not required for its diagnostic function since the relevant GHS-R1a receptors are on anterior pituitary somatotrophs accessible from the systemic circulation [11][13].

Metabolism: Hepatic metabolism via CYP3A4 is the primary route of elimination [8]. The major circulating metabolite (formed by O-demethylation) retains partial GHS-R1a agonist activity but at lower potency than the parent compound [8]. Strong CYP3A4 inhibitors (ketoconazole, itraconazole, clarithromycin, ritonavir, nefazodone) increase macimorelin plasma concentrations and may potentiate the GH response, potentially producing false-negative (false-normal) diagnostic results [8]. Moderate CYP3A4 inhibitors (erythromycin, fluconazole, verapamil, diltiazem) may also affect results. The prescribing information recommends discontinuation of strong CYP3A4 inhibitors for at least 5 half-lives before testing [8].

Elimination: The terminal elimination half-life is 4.1 plus or minus 1.2 hours [8]. Excretion is primarily fecal (approximately 75% of the absorbed dose) with minor urinary elimination (approximately 10%) [8]. Renal impairment does not significantly alter macimorelin pharmacokinetics, and no dose adjustment is required for patients with mild-to-moderate renal dysfunction [8]. The pharmacokinetics in patients with hepatic impairment have not been formally studied; caution is recommended in patients with severe hepatic disease given CYP3A4-dependent metabolism [8].

Special Populations: In the Phase I dose-ranging study, pharmacokinetics were comparable between younger (18-40 years) and older (60-80 years) healthy subjects, with no clinically meaningful age-related differences in Cmax or AUC [4][5]. The relationship between body weight and macimorelin exposure is accounted for by the weight-based dosing (0.5 mg/kg), which produces consistent GH stimulation across a wide range of body weights [6][8].

4. Dose-Response Relationship

4.1 Phase I Dose-Ranging Data

The Phase I dose-ranging study by Broglio et al. (2008) established the dose-response relationship for macimorelin-stimulated GH release [5]. In 18 healthy male volunteers receiving single oral doses of 0.1, 0.25, 0.5, and 1.0 mg/kg, a clear dose-dependent increase in peak GH levels was observed:

• 0.1 mg/kg: Modest GH elevation, with peak GH of approximately 8-15 ng/mL in most subjects. GH response was inconsistent, with some subjects showing minimal stimulation, making this dose unreliable for diagnostic purposes [5].
• 0.25 mg/kg: Intermediate GH response with peak GH typically 15-30 ng/mL. Response was more consistent than 0.1 mg/kg but still showed greater inter-individual variability than higher doses [5].
• 0.5 mg/kg: Robust and consistent GH elevation, with peak GH typically 20-50 ng/mL in healthy subjects. This dose produced near-maximal GH stimulation with acceptable tolerability and was selected as the clinical diagnostic dose [5].
• 1.0 mg/kg: GH response was not meaningfully greater than 0.5 mg/kg, indicating a plateau effect at or near 0.5 mg/kg. Higher doses produced a slight increase in adverse effects (dysgeusia, nausea) without improved diagnostic performance [5].

4.2 Temporal Dynamics of GH Response

At the 0.5 mg/kg diagnostic dose, the GH response follows a characteristic temporal pattern [1][5][8]:

• GH begins to rise within 15-20 minutes post-dose, coinciding with early drug absorption
• Peak GH occurs at 30-45 minutes in most healthy subjects (corresponding to Tmax of the drug)
• A secondary peak or plateau may occur at 60-75 minutes in some individuals
• GH returns toward baseline by 90-120 minutes

The four-timepoint sampling protocol (30, 45, 60, 90 minutes) was optimized through the Phase I and Phase III data to capture the peak GH response across the range of individual variation in absorption and response kinetics [1][2][8]. The highest GH value at any of the four timepoints is used for diagnostic classification [1][2].

4.3 Pharmacokinetic-Pharmacodynamic Correlation

There is a strong correlation between macimorelin plasma concentrations and GH release, with a sigmoidal Emax relationship [5]. The EC50 for GH stimulation corresponds to plasma concentrations achieved with approximately 0.2-0.3 mg/kg, while near-maximal GH stimulation (Emax) is achieved at concentrations corresponding to the 0.5 mg/kg dose [5]. This PK-PD relationship provides the pharmacological rationale for using a dose that ensures near-maximal pituitary stimulation in all patients, such that a blunted response reliably reflects diminished somatotroph capacity rather than inadequate drug exposure [5][8].

4.4 Cutoff Optimization

The diagnostic cutoff of 2.8 ng/mL was derived from ROC analysis of the pivotal Phase III data, where it yielded the optimal balance of sensitivity (87%) and specificity (96%) relative to the ITT reference standard [1][2]. Lower cutoffs (e.g., 1.0 ng/mL) increased specificity to near 100% but reduced sensitivity below 70%; higher cutoffs (e.g., 5.0 ng/mL) increased sensitivity to more than 95% but reduced specificity below 80% [1]. The 2.8 ng/mL cutoff was validated in the confirmatory study with 92% concordance with the ITT [2].

5. Clinical Evidence

5.1 Pivotal Validation Study (Garcia et al., 2013)

The pivotal Phase III study enrolled 157 adults: 39 with confirmed AGHD (based on prior ITT or three or more pituitary hormone deficiencies) and 118 age-, sex-, and BMI-matched healthy controls [1]. All subjects underwent macimorelin testing, and AGHD subjects also underwent ITT. Using the optimal GH cutoff of 2.8 ng/mL, macimorelin demonstrated 87% sensitivity and 96% specificity for AGHD diagnosis [1]. Concordance with the ITT was 92%. The area under the ROC curve was 0.96, indicating excellent discriminatory performance [1].

5.2 Confirmatory Study (Garcia et al., 2018)

A second Phase III study enrolled 140 adults: 38 with confirmed AGHD, 37 at-risk for AGHD (single pituitary hormone deficiency or history of pituitary/hypothalamic disease), and 65 matched controls [2]. Macimorelin demonstrated 92% concordance with the ITT across all groups [2]. In the at-risk population, macimorelin identified 11 of 37 subjects as GH deficient, 10 of whom were also classified as deficient by ITT [2]. Negative predictive value was 96% and positive predictive value was 82% [2].

5.3 Reproducibility

Garcia et al. (2014) assessed test-retest reliability in 34 subjects who underwent repeat macimorelin testing [4]. Reproducibility was 97% (33 of 34 received the same diagnostic classification), substantially exceeding the reported reproducibility of the ITT (approximately 90%) and the glucagon stimulation test [4]. This high reproducibility strengthens confidence in borderline results [4].

5.4 BMI Independence

Unlike the GHRH-arginine test, which requires BMI-specific GH cutoffs because obesity blunts the GH response to GHRH-arginine, macimorelin maintains a single universal cutoff (2.8 ng/mL) regardless of BMI [6]. Piccoli et al. (2020) confirmed that diagnostic accuracy was maintained across normal weight, overweight, and obese categories, representing a significant practical advantage [6][16].

6. Comparative Effectiveness

6.1 Macimorelin vs. Insulin Tolerance Test (ITT)

The ITT remains the reference standard for AGHD diagnosis endorsed by the Endocrine Society and the GH Research Society [9][14]. It tests the entire hypothalamic-pituitary axis by inducing hypoglycemia (blood glucose below 40 mg/dL), which triggers counter-regulatory GHRH release from the hypothalamus. Head-to-head comparisons in the pivotal Phase III trials demonstrate:

• Diagnostic concordance: 92% agreement between macimorelin and ITT across both Phase III studies [1][2]
• Sensitivity: Macimorelin 87% vs. ITT (reference standard by definition; estimated 85-96% across studies) [1][9]
• Specificity: Macimorelin 96% vs. ITT (estimated 89-100% across studies) [1][9]
• Reproducibility: Macimorelin 97% vs. ITT approximately 90% -- a significant advantage for borderline cases [4]
• BMI dependence: Macimorelin uses a single universal cutoff (2.8 ng/mL); the ITT cutoff (5.0 ng/mL) is also BMI-independent, though the GH response magnitude decreases with obesity [6][9]
• Safety: Macimorelin carries zero hypoglycemia risk; the ITT requires controlled hypoglycemia and is contraindicated in coronary artery disease, seizure disorders, and elderly patients [1][8][9]
• Convenience: Macimorelin is oral, takes 90 minutes, and requires no IV access; the ITT requires IV insulin, close physician supervision, and carries hospitalization-level risk [1][8][17]
• Cost considerations: Macimorelin has a higher drug acquisition cost but requires less monitoring infrastructure, nursing time, and no endocrinologist bedside supervision during the test [17]

6.2 Macimorelin vs. GHRH-Arginine Test

The GHRH-arginine combination test was widely used in the United States until biosynthetic GHRH (Geref Diagnostic) was discontinued in 2008, effectively eliminating it as a diagnostic option in the US [7]. In Europe and other regions where GHRH is available, the test remains in use:

• Diagnostic concordance: 94% agreement between macimorelin and GHRH-arginine in post-hoc analysis [3]
• BMI dependence: The GHRH-arginine test requires BMI-specific GH cutoffs (peak GH below 11.5 ng/mL for BMI below 25, below 8.0 for BMI 25-30, below 4.2 for BMI above 30) because obesity substantially blunts the GH response [3][7][16]. Macimorelin uses a single cutoff regardless of BMI, eliminating the risk of misclassification when BMI-specific cutoffs are incorrectly applied [3][6]
• Administration: GHRH-arginine requires two separate IV infusions over 60-90 minutes; macimorelin is a single oral dose [3][7]
• Availability: GHRH is unavailable in the United States and increasingly difficult to obtain in other countries; macimorelin is commercially available in the US and EU [7][17]
• False positives in hypothalamic disease: GHRH directly stimulates somatotrophs, so it may produce false-negative results (normal GH response) in patients with hypothalamic causes of AGHD where the pituitary is intact. Macimorelin, while also acting at the pituitary level, has not shown this limitation to the same degree in clinical data [3][9]

6.3 Macimorelin vs. Glucagon Stimulation Test (GST)

The GST is currently the most widely used GH stimulation test in the United States for patients who cannot undergo the ITT [9][12]:

• Diagnostic accuracy: The GST has lower sensitivity and specificity than macimorelin, particularly in obese patients, with reported sensitivity of 48-100% and specificity of 56-98% depending on the GH cutoff used (range 1-3 ng/mL) [9][12]. The AACE/ACE 2021 guideline proposed a GST cutoff of 3 ng/mL with BMI-specific modifications, acknowledging ongoing uncertainty [12]
• Reproducibility: GST reproducibility has not been formally studied but is estimated to be lower than the ITT; macimorelin's 97% reproducibility is a clear advantage [4][12]
• Duration: The GST requires 3-4 hours with blood draws at multiple timepoints; macimorelin requires 90 minutes [8][9]
• Tolerability: The GST causes nausea in 30-50% of patients, vomiting in 10-15%, and headache in 10-20%; macimorelin causes mild dysgeusia in 9% and headache in 6% [2][8][9]
• Administration: GST requires intramuscular injection of glucagon; macimorelin is oral [8][9]
• Special populations: The GST is the recommended alternative for patients with contraindications to the ITT, but its limitations in obesity and variable cutoffs make it less reliable than macimorelin in these settings [9][12]

6.4 Summary: Comparative Diagnostic Performance

Across all comparisons, macimorelin offers a unique combination of oral administration, excellent diagnostic accuracy (AUC ROC 0.96), superior reproducibility (97%), BMI-independent cutoff, no hypoglycemia risk, and short test duration (90 minutes) [1][2][4][6][8][17]. The 2021 AACE/ACE guideline and the 2011 Endocrine Society guideline (updated in subsequent commentaries) recognize macimorelin as a validated alternative to the ITT for AGHD diagnosis [9][12][17].

7. Dosing

Macimorelin is supplied as granules for reconstitution. The dose is 0.5 mg/kg body weight, dissolved in 120 mL of water and consumed within 30 seconds [8]. Patients must fast for at least 8 hours before the test. Blood samples are drawn at 30, 45, 60, and 90 minutes post-ingestion for serum GH measurement [8]. A peak GH concentration below 2.8 ng/mL at any time point is diagnostic of AGHD [1][2][8].

8. Enhanced Safety Profile

8.1 Overall Safety Summary

Macimorelin has an excellent safety profile consistent with its use as a single-dose diagnostic agent [1][2][5][8]. Across both Phase III pivotal trials (297 total subjects) and Phase I studies, no serious adverse events attributable to macimorelin were reported [1][2][5]. The drug has been administered to more than 1,000 subjects across all clinical studies without a single serious drug-related adverse event [8][17].

8.2 Adverse Event Profile

The most commonly reported adverse events in clinical trials were [2][8]:

• Dysgeusia (altered taste): Approximately 9% of subjects. The bitter/metallic taste is transient, resolving within minutes to hours, and is related to the drug formulation rather than a systemic pharmacological effect [2][8]
• Headache: 6% (comparable to placebo rate in controlled studies) [2][8]
• Dizziness: 5% (mild and transient) [2][8]
• Nausea: 4% (no vomiting reported at the diagnostic dose) [2][8]
• Fatigue: 3% (transient) [2][8]

All events were Grade 1 (mild) in severity and self-limiting, requiring no medical intervention [1][2].

8.3 Absence of Hypoglycemia

The single most important safety advantage of macimorelin over the ITT is the complete absence of hypoglycemia risk [1][2][8][17]. No subject in any macimorelin trial experienced hypoglycemia. This eliminates the most serious and potentially life-threatening adverse event associated with GH stimulation testing (the ITT) and removes the contraindications that exclude patients with coronary artery disease, cerebrovascular disease, seizure disorders, and advanced age from testing [8][9][17].

8.4 Cardiac Safety

Dedicated thorough QT studies at supratherapeutic doses (up to 4 times the diagnostic dose) demonstrated modest QTc prolongation (mean increase approximately 11 ms at supratherapeutic exposure) [8]. At the approved diagnostic dose (0.5 mg/kg), the mean QTc change from baseline was not clinically significant (less than 5 ms) [8]. The prescribing information includes the following cardiac safety recommendations:

• Macimorelin should be used with caution in patients with known QT prolongation (congenital long QT syndrome) [8]
• Concurrent medications known to prolong the QT interval (antiarrhythmics, certain antipsychotics, fluoroquinolones, macrolide antibiotics) should be reviewed and, if possible, held before testing [8]
• An ECG is not required before the test in patients without known cardiac risk factors [8]

8.5 Drug Interactions and Contraindications

• CYP3A4 inhibitors: Strong CYP3A4 inhibitors increase macimorelin exposure and may produce false-negative results; they should be discontinued for at least 5 half-lives before testing [8]
• GH-suppressing medications: Somatostatin analogs (octreotide, lanreotide), dopamine agonists (cabergoline, bromocriptine), and corticosteroids at supraphysiologic doses may blunt the GH response and should be held if clinically feasible [8]
• GH or GH-releasing therapies: Exogenous GH, GHRH, and other GH secretagogues should be discontinued before testing to avoid confounding the diagnostic result [8]
• Pregnancy and lactation: Safety in pregnant or lactating women has not been established; the test should be deferred when possible [8]

8.6 Post-Marketing Safety

Since approval in 2017, post-marketing surveillance has not identified any new safety signals beyond those characterized in the clinical trial program [8][17]. The most commonly reported post-marketing events mirror the clinical trial profile: dysgeusia, nausea, and headache [8]. No cases of anaphylaxis, serious cardiac events, or other life-threatening reactions have been reported [8].

8.7 Safety in Special Populations

• Elderly patients (over 65 years): Macimorelin has been safely administered to patients up to 80 years of age with no dose adjustment required [4][5][8]. This is a critical advantage over the ITT, which is relatively contraindicated in the elderly due to cardiovascular risks [9]
• Obese patients: No dose-limiting safety concerns in obese patients; the weight-based dosing ensures appropriate exposure [6][8]
• Patients with pituitary tumors: Macimorelin has been safely used in patients with pituitary adenomas and post-surgical or post-radiation pituitary disease without adverse effects on tumor behavior [1][2]
• Renal impairment: No dose adjustment required for mild-to-moderate renal impairment [8]

9. Regulatory History

Macimorelin received FDA approval on December 20, 2017, under a priority review designation, as the first oral diagnostic agent for AGHD [8]. The EMA granted marketing authorization in January 2019 under the brand name Macrilen [8]. It is classified as an orphan drug in both jurisdictions. Development began at Ardea Biosciences (later Aeterna Zentaris), which conducted the Phase I studies in collaboration with Italian investigators [5]. Strongbridge Biopharma acquired the US commercial rights from Aeterna Zentaris in 2017, and Novo Nordisk subsequently acquired Strongbridge (via Xeris Pharmaceuticals) in 2023.

US Commercial Status

US sales of Macrilen were temporarily discontinued as of May 2023, while the pivotal pediatric DETECT trial was ongoing. Sales and commercialization continue in the European Economic Area and UK under an agreement with Pharmanovia for both adult and potential pediatric indications.

Pediatric Development (DETECT Trial)

The Phase 3 DETECT trial evaluated macimorelin for the diagnosis of childhood-onset growth hormone deficiency. The last patient completed the study visit in June 2024. However, Cosciens Biopharma (successor to Aeterna Zentaris for this program) announced that while macimorelin demonstrated capacity to stimulate growth hormone release, the primary efficacy endpoint was not met according to the study protocol definitions. Initial review indicated that the comparator tests (arginine and clonidine) may have produced a high false-positive rate that impacted macimorelin's ability to reach the primary endpoint. The pediatric indication has not been approved. An earlier dose-escalation study in children confirmed that macimorelin was safe and well tolerated with no drug-related adverse events across all dosing cohorts.

10. Related Peptides

See also: MK-677 (Ibutamoren), Anamorelin, Ipamorelin, Sermorelin

• MK-677 (Ibutamoren) -- Non-peptide GHS-R1a agonist with a longer half-life (~24 hours), investigated for therapeutic applications but never approved. Not used diagnostically.

• Anamorelin -- Oral GHS-R1a agonist approved in Japan for cancer cachexia. Used therapeutically at repeated doses, not as a diagnostic agent.

• Ipamorelin -- Selective peptide GHS-R1a agonist requiring injection. Not approved for any indication.

• Sermorelin -- Synthetic GHRH analog that stimulates GH release via the GHRH receptor (different mechanism from macimorelin). Previously used both therapeutically and diagnostically.

11. References

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