What Is Retosiban? Definition and What Research Reports

Definition

Retosiban (development code GSK221149A) is a synthetic, orally active small molecule that acts as a selective antagonist at the oxytocin receptor. It was investigated as a tocolytic agent — a drug intended to suppress uterine contractions — in the setting of spontaneous preterm labour. Chemically it belongs to the diketopiperazine class rather than the peptide class, which distinguishes it from atosiban, the peptide-based oxytocin/vasopressin receptor antagonist used in some countries as an intravenous tocolytic. Retosiban has not been approved for marketing in any jurisdiction; the compound exists in the literature as an investigational agent whose clinical programme was discontinued.

Where the Term Comes From and Why It Appears in Peptide Reference Lists

Retosiban is frequently encountered alongside peptide terminology because its molecular target, the oxytocin receptor, is a G protein-coupled receptor whose endogenous ligand — oxytocin — is a nine-amino-acid cyclic peptide. Research programmes aimed at blocking oxytocin signalling have historically produced both peptide antagonists (atosiban, barusiban) and non-peptide small molecules (retosiban, nolasiban). Retosiban sits in the second group. In practical terms, when a paper or database entry describes retosiban as an "oxytocin receptor antagonist," it is describing receptor pharmacology, not peptide chemistry.

A second reason the name recurs in receptor-pharmacology discussions is that retosiban has been used as a laboratory tool compound. Because it binds the human oxytocin receptor with high selectivity, investigators have used it in isolated human myometrial preparations to probe how mechanical and hormonal signals converge on uterine smooth muscle.

Quick Reference Table

What the Published Literature Reports

Receptor and Tissue-Level Work

Preclinical and ex vivo work characterised how retosiban interacts with the oxytocin receptor in human uterine tissue. A 2015 Endocrinology study examined the effect of the oxytocin receptor antagonist retosiban (GSK221149A) on the response of human myometrial explants to prolonged mechanical stretch, reporting on stretch-induced changes in the tissue (PMID 26207346). A 2018 paper in The Journal of Clinical Endocrinology and Metabolism reported that retosiban prevented stretch-induced human myometrial contractility and delayed labour in cynomolgus monkeys (PMID 29293998). A 2020 Endocrinology report described functionally selective inhibition of the oxytocin receptor by retosiban in human myometrial smooth muscle, indicating that the compound did not block all receptor-linked signalling pathways equally (PMID 31907536).

Early-Phase Human Pharmacology

Phase 1 work characterised how the compound behaved in healthy volunteers. A phase 1 randomised, placebo-controlled study assessed the pharmacokinetics, safety and tolerability of retosiban in healthy, non-pregnant Japanese subjects (PMID 28419732). Cardiac safety was addressed separately: researchers conducted a randomised, placebo- and positive-controlled crossover thorough QT/QTc study evaluating the effect of retosiban on cardiac repolarisation in healthy men and women (PMID 26138866). Thorough QT studies of this design are a standard regulatory step used to determine whether a drug meaningfully prolongs the QT interval.

Phase 2 Studies in Spontaneous Preterm Labour

Two phase 2 investigations in women with spontaneous preterm labour were published in the British Journal of Clinical Pharmacology. The 2015 report described a phase 2 proof-of-concept study of treatment of spontaneous preterm labour with retosiban (PMID 25819462). A 2017 report described a phase 2 pilot dose-ranging study of retosiban in the same population, examining more than one exposure level (PMID 28556962). Both were exploratory studies; the study designs were intended to generate signals about effect and exposure rather than to establish definitive clinical benefit.

Phase 3 Trials and Their Early Termination

The programme advanced to randomised trials comparing retosiban with placebo or with atosiban in spontaneous preterm labour, reported in American Journal of Perinatology in 2021 (PMID 32380566). These trials did not reach their planned enrolment, and the published account reflects a truncated dataset rather than a fully powered comparison. A separate follow-up publication, the ARIOS study, reported on safety and outcomes in infants born to mothers who had participated in retosiban treatment trials (PMID 34352924). Infant follow-up of this kind is a standard component of obstetric drug development, since exposure in utero raises questions that maternal endpoints alone cannot answer.

Systematic Review

The available clinical data were pooled in a 2021 systematic review and meta-analysis in Current Medical Research and Opinion, which examined novel oxytocin receptor antagonists for tocolysis and specifically synthesised the efficacy, safety and tolerability data for retosiban (PMID 34134590). Readers seeking a single consolidated overview of the retosiban clinical dataset generally start with this review, because it aggregates the phase 2 and terminated phase 3 material into one analysis.

Reported Tolerability: What Studies Report

Safety and tolerability were named endpoints across several retosiban publications rather than incidental observations. The phase 1 study in healthy non-pregnant Japanese subjects was explicitly framed around pharmacokinetics, safety and tolerability (PMID 28419732), and the thorough QT/QTc crossover study in healthy men and women was designed specifically to characterise effects on cardiac repolarisation (PMID 26138866). The 2021 systematic review evaluated safety and tolerability alongside efficacy across the available retosiban dataset (PMID 34134590), and the ARIOS follow-up reported safety and outcomes in infants born to mothers enrolled in retosiban trials (PMID 34352924). Because the phase 3 programme was stopped before full enrolment (PMID 32380566), the safety database for retosiban remains smaller than it would have been had the trials completed.

How the Term Is Used in Practice

• As a comparator class member: papers on tocolysis often list retosiban alongside atosiban, nolasiban and barusiban when describing oxytocin receptor antagonists.
• As a tool compound: myometrial physiology studies have used retosiban to block oxytocin receptor signalling experimentally (PMID 31907536).
• As a case study in drug development: the discontinued phase 3 programme is cited in discussions of the difficulty of running adequately powered preterm labour trials (PMID 32380566).

Common Points of Confusion

Retosiban is not a peptide. It is a small-molecule antagonist; the peptide connection is via its target's natural ligand, oxytocin. Retosiban is not atosiban. Atosiban is a peptide analogue used clinically in some countries; retosiban is a distinct oral small molecule that was compared against atosiban in randomised trials (PMID 32380566). Retosiban is not an approved drug. Its entire human dataset comes from clinical trials conducted under investigational protocols, and the programme did not proceed to registration.

This page is for educational purposes only and is not medical advice; consult a licensed physician or qualified healthcare professional regarding any medical condition, medication or treatment decision. Nothing here describes a protocol, and retosiban is an investigational compound that is not an approved medicine.

References

• Retosiban Prevents Stretch-Induced Human Myometrial Contractility and Delays Labor in Cynomolgus Monkeys (The Journal of Clinical Endocrinology and Metabolism, 2018)
• Randomized Trials of Retosiban Versus Placebo or Atosiban in Spontaneous Preterm Labor (American Journal of Perinatology, 2021)
• Novel oxytocin receptor antagonists for tocolysis: a systematic review and meta-analysis of the available data on the efficacy, safety, and tolerability of retosiban (Current Medical Research and Opinion, 2021)
• Treatment of spontaneous preterm labour with retosiban: a phase II pilot dose-ranging study (British Journal of Clinical Pharmacology, 2017)
• Effect of Retosiban on Cardiac Repolarization in a Randomized, Placebo- and Positive-controlled, Crossover Thorough QT/QTc Study in Healthy Men and Women (Clinical Therapeutics, 2015)
• Safety and Outcomes in Infants Born to Mothers Participating in Retosiban Treatment Trials: ARIOS Follow-up Study (American Journal of Perinatology, 2023)
• Functionally Selective Inhibition of the Oxytocin Receptor by Retosiban in Human Myometrial Smooth Muscle (Endocrinology, 2020)
• A Phase 1 Randomized, Placebo-Controlled Study Assessing the Pharmacokinetics, Safety, and Tolerability of Retosiban in Healthy, Nonpregnant Japanese Subjects (Clinical Pharmacology in Drug Development, 2018)
• Treatment of spontaneous preterm labour with retosiban: a phase 2 proof-of-concept study (British Journal of Clinical Pharmacology, 2015)
• The Effect of an Oxytocin Receptor Antagonist (Retosiban, GSK221149A) on the Response of Human Myometrial Explants to Prolonged Mechanical Stretch (Endocrinology, 2015)