A Phase III Efficacy and Safety Study of Intravenous Retosiban Versus Placebo for Women in Spontaneous Preterm Labor

The study's primary objective is to demonstrate the superiority of retosiban to prolong pregnancy and improve neonatal outcomes compared with placebo. It is a Phase III, randomized, double-blind, parallel-group, multicenter study and will be conducted in approximately 900 females, aged 12 to 45 years, with an uncomplicated, singleton pregnancy and intact membranes in preterm labor between 24\^0/7 and 33\^6/7 weeks of gestation. Eligible maternal subjects will be randomly assigned in a 1:1 ratio to receive either retosiban IV infusion or placebo IV infusion over 48 hours. If not previously administered, antenatal corticosteroid treatment should be administered as either (1) two 12-mg doses of betamethasone given intramuscularly 24 hours apart or (2) four 6-mg doses of betamethasone administered intramuscularly every 12 hours. A single rescue course of antenatal corticosteroids is permitted if the antecedent treatment was at least 7 days prior to study enrolment. Investigators have discretion to use a standardized regimen of magnesium sulphate, as well as intrapartum antibiotic prophylaxis for perinatal group B streptococcal infection. Prior to randomization, each subject will be stratified by progesterone treatment and gestational age. The progesterone strata will consist of subjects on established progesterone therapy or subjects not on established progesterone therapy at Screening. The study will comprise 6 phases: Screening, Inpatient Randomized Treatment, Post Infusion Assessment, Delivery, Maternal Post-Delivery Assessment, and Neonatal Medical Review. The duration of any subject's (maternal or neonatal) participation in the study will be variable and dependent on gestational ages (GA) at study entry and the date of delivery.