Mechanism of Action
Myometrial Oxytocin Receptor Antagonism
Atosiban competitively antagonizes oxytocin receptors (OTR) on uterine myometrial cells. OTR activation normally triggers Gq-mediated phospholipase C activation, IP3-mediated calcium release from the sarcoplasmic reticulum, and protein kinase C activation, leading to myosin light chain phosphorylation and uterine contraction. Atosiban blocks all these downstream effects, reducing contraction frequency and amplitude.
Vasopressin V1a Antagonism
Atosiban also antagonizes V1a receptors, which contributes to tocolytic efficacy as vasopressin can stimulate myometrial contractions. The combined OT/V1a antagonism provides more complete blockade of the uterotonic signaling pathways active during labor initiation than OT-selective antagonism alone.
Research Summary
Preterm Labor Tocolysis
Approved (EU)Phase 3 trials: atosiban achieved 48-hour delivery delay in 79.7% of patients vs 61.8% placebo. Seven-day delivery delay achieved in 65.4% vs 39.6%. No significant improvement in perinatal outcomes (neonatal mortality, respiratory distress syndrome) -- consistent with the general tocolytic literature where delaying delivery 48 hours for steroids is the primary goal. Superiority over beta-agonists in tolerability with equivalent tocolytic efficacy.
IVF Implantation (off-label)
Active ResearchAtosiban administered around embryo transfer suppresses OT-mediated uterine contractility that may impair implantation. Small RCTs show mixed results; a Cochrane review found insufficient evidence. Some fertility centers use it for recurrent implantation failure as an adjuvant strategy while awaiting larger trials.
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Research Protocols
| Goal | Dose | Frequency | Route |
|---|---|---|---|
| Acute tocolysis (preterm labor) | 6.75 mg IV bolus over 1 min, then 18 mg/h (300 mcg/min) x3h, then 6 mg/h (100 mcg/min) up to 45h | Continuous IV infusion | Intravenous |
| IVF embryo transfer (research protocol) | 6.75 mg IV bolus 30-60 min before embryo transfer | Single dose per transfer | Intravenous |
Approved indication is gestational weeks 24-33+6 with cervical dilation <3 cm, intact membranes, and confirmed PTL. Up to 3 treatment cycles allowed.
Interactions
Safety Profile
Most common maternal adverse effects: nausea (13%), vomiting (7%), headache (9%), dizziness (6%), tachycardia (5%), injection site reactions. No significant cardiovascular effects -- major advantage over beta-agonist tocolytics (ritodrine, terbutaline) which cause maternal tachycardia, pulmonary edema, and hyperglycemia. Fetal/neonatal outcomes similar to placebo in trials; no adverse fetal effects from OT receptor blockade noted. Contraindicated before 24 weeks gestation (fetal safety data insufficient), with premature rupture of membranes, chorioamnionitis, antepartum hemorrhage, or fetal distress.
References
- [1]Romero R, et al. An oxytocin receptor antagonist (atosiban) in the treatment of preterm labor. Am J Obstet Gynecol. 2000;182(5):1173-1183.
- [2]Worldwide Atosiban versus Beta-agonists Study Group. Effectiveness and safety of the oxytocin antagonist atosiban versus beta-adrenergic agonists in the treatment of preterm labour. BJOG. 2001;108(2):133-142.