Motilin
MLN, motilin (1-22), endogenous motilin
Motilin is a 22-amino-acid hormone released cyclically from M-cells of the duodenum during fasting. It initiates phase III of the migrating motor complex: the powerful sweeping contractions that clear residual food and bacteria from the stomach and small intestine between meals, sometimes called the housekeeper wave. It is not a licensed medicine anywhere, but the antibiotic erythromycin is a motilin receptor agonist, which is why it is used off-label as a prokinetic.
Mechanism
Motilin is released in a striking cyclical pattern roughly every 90 to 120 minutes during fasting, with plasma peaks coinciding with the onset of gastric phase III activity. Feeding abolishes both the motilin peaks and the migrating motor complex. It acts on the motilin receptor (MLNR, formerly the orphan receptor GPR38), a class A G-protein-coupled receptor structurally related to the growth hormone secretagogue receptor. Motilin and ghrelin are structurally and evolutionarily related, and the two systems together govern the fasting gut.
Motilin receptors are expressed on enteric neurons of the human stomach and duodenum, and to a lesser extent on smooth muscle. The dominant human mechanism is neural: motilin acts on cholinergic enteric neurons to release acetylcholine, which drives the strong propulsive contractions of gastric phase III. This neural rather than direct-muscular action is the main reason findings in rodents do not transfer. Rats and mice lack a functional motilin gene and receptor entirely, a fact that quietly wrecked much of the early translational work in this field. Physiologically, phase III contractions clear undigested residue and limit small intestinal bacterial overgrowth, and the gastric phase III burst has been linked in humans to the onset of hunger, giving motilin a role in meal initiation alongside ghrelin.
What the research shows
Motilin was isolated from porcine duodenum in the early 1970s, and its link to interdigestive motility was established by work showing that plasma motilin peaks coincide with gastric phase III of the migrating motor complex in humans and that motilin infusion reproduces phase III on demand. The receptor remained orphan until 1999, when Feighner and colleagues identified GPR38 as the motilin receptor in human gastrointestinal tissue, localised it to enteric neurons of the duodenum and colon, and showed that erythromycin acts as an agonist at it, explaining an empirical clinical observation that predated the mechanism by years. Seino and colleagues had established the human motilin precursor sequence in 1987.
The drug development record is discouraging and worth stating plainly. Macrolide-derived motilin agonists (motilides) including alemcinal (ABT-229) and mitemcinal (GM-611) went into clinical trials for gastroparesis and functional dyspepsia; alemcinal not only failed to help but was associated with worsening of symptoms in diabetic gastroparesis. The core problem is tachyphylaxis: motilin receptors desensitise rapidly with sustained agonist exposure, so accelerated gastric emptying fades within days. Camicinal (GSK962040), a non-macrolide small molecule designed to avoid the structural features implicated in desensitisation, is more potent at the human motilin receptor than erythromycin (pEC50 7.9 versus 7.3). In a randomised, blinded, placebo-controlled trial in 23 feed-intolerant critically ill patients, a single 50 mg enteral dose significantly increased glucose absorption and showed a trend toward accelerated gastric emptying, with effects clearest in patients in whom the drug was detectable in plasma. A separate randomised crossover manometry study by Deloose and colleagues showed that camicinal 150 mg induced gastric phase III significantly faster than placebo. Despite this, no motilin agonist has reached the market for gastroparesis. Erythromycin remains the only motilin agonist in routine clinical use, and its usefulness is largely confined to short courses and acute settings precisely because of tachyphylaxis.
Evidence assessment
Limited evidence
Motilin infusion in humans reliably induces phase III activity. That physiology is solid and reproducible. But motilin itself has never been developed as a therapeutic, there are no controlled efficacy trials of the peptide, and every attempt to build a motilin receptor agonist into a gastroparesis drug has failed to reach market. The randomised human data that exist concern the small-molecule agonist camicinal, in 23 critically ill patients for a single dose. That is genuinely limited.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Receptor for motilin identified in the human gastrointestinal system Preclinical only
Identified the orphan receptor GPR38 as the motilin receptor, localised it to enteric neurons of human duodenum and colon, and demonstrated that erythromycin acts as an agonist at it, providing the mechanism for a long-standing empirical clinical practice.
Sequence of an intestinal cDNA encoding human motilin precursor Preclinical only
Established the human motilin precursor sequence: a prepromotilin comprising a signal peptide, the 22-residue mature motilin and a C-terminal extension.
GSK962040: a small molecule, selective motilin receptor agonist, effective as a stimulant of human and rabbit gastrointestinal motility Preclinical only
Camicinal is more potent at the human motilin receptor than erythromycin (pEC50 7.9 versus 7.3) and stimulates human and rabbit gastrointestinal motility, with a non-macrolide structure intended to reduce tachyphylaxis.
The effect of camicinal (GSK962040), a motilin agonist, on gastric emptying and glucose absorption in feed-intolerant critically ill patients: a randomized, blinded, placebo-controlled, clinical trial Preclinical only
A single enteral dose significantly increased glucose absorption and showed a trend toward accelerated gastric emptying, with results varying by whether drug was detectable in plasma. The clearest randomised human evidence that selective motilin receptor agonism produces a prokinetic effect, but single-dose and in only 23 patients, so it says nothing about tachyphylaxis.
Manometric evaluation of the motilin receptor agonist camicinal (GSK962040) in humans Preclinical only
Camicinal 150 mg induced a gastric phase III significantly faster than placebo and increased daytime gastric phase III contractions, confirming that the motilin receptor mechanism operates in humans as predicted.
Safety
Motilin itself is administered only in short research infusions, where the reported effects are abdominal cramping and an urge to defecate consistent with vigorous propulsive contraction. The clinically relevant safety profile belongs to erythromycin used as a prokinetic: QT interval prolongation with risk of torsades de pointes, extensive CYP3A4-mediated drug interactions, ototoxicity at high doses, nausea and abdominal pain, and a well-documented association between neonatal erythromycin exposure and infantile hypertrophic pyloric stenosis. Prolonged use of an antibiotic as a motility agent also promotes antimicrobial resistance and disturbs the gut microbiome, a genuine population-level harm that is often understated when erythromycin is described casually as a prokinetic. Tachyphylaxis means benefit typically wanes within days to weeks, so extended courses combine diminishing efficacy with undiminished risk. Motilin peptide sold as a research chemical has no licensed status, no clinical evidence base, and a half-life of a few minutes that makes any consumer application implausible.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Motilin has no UK marketing authorisation. No motilin receptor agonist is licensed as a prokinetic in the UK. Erythromycin is licensed as an antibiotic and used off-label for gastroparesis and to promote gastric emptying before endoscopy in upper gastrointestinal bleeding. Motilin peptide sold to consumers is an unlicensed research chemical. |
| United States | Motilin is not an approved medicine and no motilin receptor agonist is licensed for gastroparesis in the United States. Erythromycin, an antibiotic that happens to be a motilin receptor agonist, is widely used off-label as a prokinetic; this is off-label use of a licensed antibiotic, not an approved indication. |
| WADA (sport) | Not listed on the WADA Prohibited List. Note the structural relationship to ghrelin: ghrelin and ghrelin mimetics acting at the growth hormone secretagogue receptor are prohibited under section S2, but motilin acts at a different receptor and does not release growth hormone. |
Questions
Erythromycin and related macrolides are agonists at the motilin receptor, quite separately from their antibacterial action. They trigger the same powerful gastric contractions that endogenous motilin produces. This was observed clinically before anyone knew why; the receptor was only identified in 1999.
Tachyphylaxis. The motilin receptor desensitises rapidly with sustained agonist exposure, so the accelerated gastric emptying seen with the first doses fades within days. This is the main reason erythromycin is useful for short courses and acute situations but disappointing as long-term therapy, and it sank several purpose-built motilide drug programmes.
No. Motilin is not licensed anywhere and no motilin receptor agonist has reached the market for gastroparesis. Alemcinal and mitemcinal failed in trials, and camicinal, despite positive randomised data in 23 critically ill patients and a positive manometry study, has not been approved. Erythromycin used off-label remains the only motilin agonist in clinical use.
Because rats and mice do not have a functional motilin gene or receptor. This is an unusual and consequential species gap: a great deal of early preclinical motility work in rodents could not, in principle, have modelled motilin biology, and the useful animal models are rabbit, dog and non-human primate.