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Atosiban
A synthetic peptide oxytocin receptor antagonist used to delay preterm labor by inhibiting uterine contractions, approved in Europe but not in the United States.
Overview
Atosiban is a synthetic nonapeptide that acts as a competitive antagonist at both oxytocin receptors and vasopressin V1a receptors on uterine smooth muscle cells. Developed as a tocolytic agent (a drug that suppresses premature labor), atosiban directly blocks the binding of oxytocin to its receptors on myometrial cells, preventing the intracellular calcium increase that triggers uterine contractions. This targeted mechanism of action provides effective tocolysis with a more favorable side effect profile compared to traditional tocolytics such as beta-agonists (ritodrine, terbutaline) and calcium channel blockers (nifedipine).
Atosiban was approved in the European Union in 2000 (marketed as Tractocile) for the delay of imminent preterm birth in adults with regular uterine contractions between 24 and 33 weeks of gestation. The drug is administered as an initial IV bolus followed by a high-dose infusion for 3 hours, then a lower-dose infusion for up to 45 hours, providing up to 48 hours of tocolysis — sufficient time for corticosteroid administration to promote fetal lung maturity and for maternal transfer to a facility with neonatal intensive care.
The pivotal clinical trials compared atosiban to beta-agonists (primarily ritodrine and salbutamol) and demonstrated comparable tocolytic efficacy with significantly fewer maternal side effects. In particular, atosiban caused dramatically less cardiovascular morbidity (tachycardia, palpitations, hypotension) and metabolic disturbance (hyperglycemia, hypokalemia) compared to beta-agonists, resulting in a much lower treatment discontinuation rate.
Atosiban was reviewed but not approved by the FDA due to concerns about fetal/neonatal outcomes in one trial that showed a numerical (but not statistically significant) increase in infant deaths in the atosiban group compared to placebo. This finding has been debated extensively, as it may have been related to baseline imbalances in gestational age between groups rather than a drug effect. Atosiban remains widely used in Europe and many other countries worldwide.