Atosiban
Also known as: RWJ-22164, Tractocile, oxytocin/vasopressin receptor antagonist, tocolytic peptide
Oxytocin/vasopressin receptor antagonist — synthetic nonapeptide tocolytic
What it is
Atosiban (brand name Tractocile) is an intravenous drug used in hospital obstetric units to pause or slow premature uterine contractions when a baby is at risk of being born too early. Doctors give it to pregnant women between 24 and 34 weeks of gestation to buy time for steroid treatment and safe transfer to a specialist center.
The scientific side
atosiban is a synthetic cyclic nonapeptide that acts as a competitive antagonist at both the oxytocin receptor (OTR) and the vasopressin V1a receptor in uterine myometrium, decidua, and fetal membranes. Its primary tocolytic mechanism involves blocking oxytocin-driven uterotonic signaling during threatened preterm labor. Oxytocin exerts its contractile effect through OTR-coupled Gq/11 proteins, activating phospholipase C, generating inositol 1,4,5-trisphosphate (IP3) and diacylglycerol, mobilizing intracellular calcium from the sarcoplasmic reticulum, and activating myosin light-chain kinase — the central biochemical step in smooth muscle contraction. By occupying OTR without activating it, atosiban prevents IP3-mediated calcium release and suppresses myometrial contractile force. A pharmacological study comparing atosiban with second-generation small-molecule OTR antagonists (retosiban, epelsiban) in human myometrial preparations confirmed that atosiban produces potent, rapid, and reversible competitive antagonism, inhibiting inositol phosphate accumulation and oxytocin-stimulated contractions in a concentration-dependent manner. The vasopressin V1a antagonist activity is clinically relevant because vasopressin can independently drive myometrial contractions via a closely related signaling axis; dual receptor blockade is pharmacologically advantageous over agents targeting OTR alone. Unlike beta-agonist tocolytics such as ritodrine or hexoprenaline — which suppress uterine contractility through beta-2 adrenoreceptor-mediated cAMP elevation but cause significant maternal cardiovascular adverse effects — atosiban acts in a tissue-selective manner on uterine and placental receptors, underpinning its favorable maternal tolerability observed in multiple RCTs (PMID 37678995, PMID 28813702). Atosiban is administered intravenously as a three-phase regimen: an initial bolus, a high-rate loading infusion, then a lower-rate maintenance infusion, achieving rapid receptor occupancy and sustained suppression of uterine activity. The standard 48-hour course is designed to allow antenatal corticosteroid administration and in utero transfer to a perinatal center — clinical benefits that depend on delayed delivery rather than any direct improvement in fetal organ maturation. The APOSTEL 8 RCT found atosiban did not reduce composite neonatal adverse outcomes versus placebo at 30–33+6 weeks (RR 0.90, 95% CI 0.58–1.40), challenging the assumption that delivery delay alone produces measurable neonatal benefit. Changes in OTR expression density across gestation may partly explain the inconsistency between delayed delivery and improved neonatal outcomes.
Class: Oxytocin/vasopressin receptor antagonist — synthetic nonapeptide tocolytic
Administration & storage
- Administration
- Intravenous bolus injection (6.75 mg over 1 minute)Intravenous infusion via controlled infusion pump (loading phase and maintenance phase)Central or peripheral venous access — peripheral IV is standard in obstetric practice
- Storage
- Store below 30°C (Tractocile vials). Protect from light. Once diluted, the infusion solution should be used within 24 hours. Do not freeze. Keep out of reach of children.
- Cautions
- Atosiban is contraindicated before 24 weeks or after 33+6 weeks gestation (licensed indication boundary),Contraindicated in premature rupture of membranes beyond 30 weeks gestation,Contraindicated where there is suspicion of intrauterine infection, antepartum hemorrhage requiring immediate delivery, eclampsia or severe pre-eclampsia, intrauterine growth restriction with abnormal fetal heart rate, or placenta praevia,Non-cardiogenic pulmonary edema has been reported as a rare but serious adverse event, particularly in multiple gestations (PMID: 42688486, PMID: 37305518, PMID: 30897311),Fetal heart rate should be monitored during infusion; atosiban crosses the placenta and fetal plasma concentrations are approximately 12% of maternal levels,No dose adjustment is established for renal or hepatic impairment; use with caution in these populations,The APOSTEL 8 trial found no superiority over placebo in neonatal outcomes at 30–33+6 weeks, raising questions about benefit-risk in this gestational window
Legal & regulatory status
Atosiban is NOT approved by the US FDA. A New Drug Application was submitted to the FDA but was not approved, primarily due to concerns about efficacy data and neonatal safety signals observed in early studies. Atosiban…
Atosiban is a synthetic peptide oxytocin/vasopressin receptor antagonist and does not appear on the current WADA Prohibited List as a performance-enhancing substance. Tocolytic agents targeting oxytocin receptors have…
Atosiban is not listed as an approved drug product by Health Canada for routine commercial marketing in Canada. Clinical use in Canadian hospitals occurs under individual hospital formulary decisions or via Health…
What it's studied for
- Acute tocolysis for threatened preterm labor (24–34 weeks gestation) — delay of delivery by ≥48 hours Phase III RCT / Systematic Review
- Comparison with nifedipine for threatened preterm birth — efficacy and perinatal outcomes Individual Participant Data Meta-Analysis
- Tocolysis during fetoscopic endotracheal occlusion (FETO) procedure for congenital diaphragmatic hernia Prospective Cohort
- Tocolysis in twin pregnancies with threatened preterm birth Network Meta-Analysis / RCT Subgroup
- Tocolysis during open fetal myelomeningocele (MMC) repair — comparison with magnesium sulfate Prospective Cohort
- Neurodevelopmental outcomes of children born after in utero atosiban exposure RCT Follow-up Study
- Assisted reproduction — adjunct to embryo transfer in IVF for recurrent implantation failure Exploratory / Pilot RCT
Safety signals
- Non-cardiogenic pulmonary edema
- Nausea and vomiting
- Headache and dizziness
- Hot flushes and tachycardia
- Injection site reactions (pain, erythema)
- Potential association with higher rate of neonatal intubation (vs nifedipine)
- Fetal exposure via placental transfer — vasopressin receptor antagonism in fetus
- Association with intraventricular hemorrhage (IVH) in preterm infants — class-level tocolytic signal
All studies (11)
Frequently asked
Why is atosiban not available in the United States?
Atosiban was submitted for FDA approval but was not approved, largely because early clinical data did not demonstrate a sufficiently clear benefit on neonatal outcomes compared with the risks. The more recent APOSTEL 8 RCT confirmed that atosiban does not improve neonatal outcomes compared with placebo in the 30–33+6 week window, which is consistent with the FDA's historical concerns. In the US, nifedipine and indomethacin are used off-label as tocolytics instead.
Does atosiban improve the baby's outcome, or just delay delivery?
Current evidence suggests atosiban primarily delays delivery — it does not appear to directly improve neonatal outcomes. The APOSTEL 8 RCT (n=752) found the rate of serious neonatal complications was 8% with atosiban vs 9% with placebo — not a significant difference. The clinical rationale for its use remains giving time for antenatal corticosteroids to mature fetal lungs and enabling transfer to a specialist center, benefits that depend on the delay itself rather than any direct fetal effect of atosiban.
Is atosiban safe for the baby?
Atosiban does cross the placenta, with fetal concentrations reaching about 12% of maternal levels. Long-term follow-up of children exposed in utero (APOSTEL III follow-up, PMID 32124520) found no significant differences in neurodevelopment, executive function, or behavior at age 2.5–5.5 years compared with nifedipine-exposed children. A separate study on tocolytic exposure and intraventricular hemorrhage noted a class-level signal, but this has not been definitively attributed to atosiban specifically.
Can atosiban be used for IVF embryo transfer?
Atosiban has been investigated in several pilot studies and clinical trials (e.g., NCT01501214, NCT04118959) as a way to quiet uterine contractions before embryo transfer and potentially improve implantation rates in women with repeated IVF failure. This is not an approved indication anywhere, and evidence is preliminary. The practice is reported in some European and East Asian fertility centers. No large confirmatory RCT has yet established this use, so it should be considered investigational.
How does atosiban differ from nifedipine as a tocolytic?
Both atosiban and nifedipine are used to delay threatened preterm birth, and head-to-head evidence does not clearly favor either agent for neonatal outcomes. Mechanistically they are very different: atosiban blocks oxytocin and vasopressin receptors in the uterus directly, while nifedipine blocks L-type calcium channels systemically to relax smooth muscle. Atosiban has a more uterus-specific action and fewer cardiovascular maternal side effects. Nifedipine may prolong pregnancy longer on average (18 vs 10 days median in one IPDMA) and costs substantially less. Neither is FDA-approved in the US.