Atosiban is a medication developed to delay preterm birth in pregnant women. This drug acts as a uterine relaxant, aiming to prolong gestation and allow for further fetal development. Its primary purpose is to provide additional time for interventions that improve newborn outcomes, such as administering corticosteroids to mature the baby’s lungs.
How Atosiban Works
Labor typically begins with uterine contractions, largely stimulated by the natural hormone oxytocin. Oxytocin binds to specific receptors on the muscle cells of the uterus, triggering a cascade of events that lead to contractions. Atosiban functions as an oxytocin receptor antagonist, binding to these receptors and blocking oxytocin from initiating contractions.
One can imagine oxytocin as a key designed to fit into a specific lock on the uterine muscle cell. When the oxytocin key turns the lock, it causes the muscle to contract. Atosiban, however, is like a key that fits into the same lock but cannot turn it. By occupying the lock, atosiban prevents the oxytocin key from entering, thus keeping the uterine muscle relaxed and helping to suppress contractions.
Clinical Application
Atosiban is prescribed under specific clinical circumstances to manage preterm labor. The criteria for its use include pregnant women experiencing regular uterine contractions, typically at a rate of at least four contractions every 30 minutes, each lasting for 30 seconds or longer. There must also be evidence of cervical changes, such as a dilation between 1 and 3 centimeters and an effacement of 50% or more. The medication is generally indicated for pregnancies between 24 and 33 completed weeks of gestation, provided the fetal heart rate is normal.
The administration of atosiban occurs intravenously within a hospital setting, following a three-stage protocol. Treatment begins with an initial bolus injection, where 6.75 mg of atosiban is delivered over approximately one minute, to quickly establish therapeutic levels. Immediately following the bolus, a high-dose “loading” infusion of 0.3 mg per minute is administered for three hours to effectively suppress contractions.
After the loading period, the infusion rate is reduced to a lower-dose “maintenance” infusion of 0.1 mg per minute, which can continue for up to 45 hours. The total duration of atosiban treatment should not exceed 48 hours, with a maximum cumulative dose of 330.75 mg.
Side Effects and Contraindications
Atosiban is generally considered to have a favorable safety profile for both the mother and the fetus compared to other tocolytic agents. Maternal side effects are typically mild and transient. Common reactions include headache, dizziness, nausea, and vomiting. Women may also experience hot flushes or reactions at the injection site.
While atosiban is considered safe for the baby, some studies have noted a slightly higher risk of neonatal apnea in infants whose mothers received the drug. However, serious fetal or neonatal complications are rare. Close monitoring of both the mother and the fetus is standard practice during atosiban administration to detect any potential concerns.
There are specific situations where atosiban must not be used; these are known as contraindications. These include:
Gestational age less than 24 weeks or more than 33 completed weeks.
Premature rupture of membranes if the gestational age is beyond 30 weeks.
Severe pre-eclampsia or eclampsia.
Uterine bleeding that requires immediate delivery.
Suspected intrauterine infection.
Placenta previa or abruptio placentae.
Abnormal fetal heart rate.
Efficacy and Regulatory Status
Atosiban has demonstrated comparable effectiveness to other medications used to delay preterm labor, such as beta-agonists or calcium channel blockers. A key advantage of atosiban is its association with fewer maternal side effects compared to these alternative tocolytic agents. This improved safety profile often makes it a preferred option for managing preterm contractions. Studies indicate that atosiban can effectively delay delivery for up to 48 hours, providing time for necessary interventions like corticosteroid administration for fetal lung maturity.
Globally, atosiban has achieved widespread regulatory approval in many countries. It was approved in the European Union in January 2000 and launched in April 2000, receiving approval in 67 countries by June 2007. However, atosiban is not approved by the Food and Drug Administration (FDA) for use in the United States. The FDA’s decision was partly due to concerns regarding a potential association between atosiban and infant deaths in some trials compared to placebo, and study designs that did not conclusively demonstrate improved neonatal outcomes.