Safety and Outcomes in Infants Born to Mothers Participating in Retosiban Treatment Trials: ARIOS Follow-up Study

Objective: Retosiban, an oxytocin receptor antagonist, was developed for treating spontaneous preterm labor (sPTL) in women with intact membranes. This ARIOS follow-up study aimed to characterize clinical safety, morbidity, and mortality of infants exposed to retosiban or comparator over 2 years. Study Design: ARIOS prospectively assessed outcomes in infants whose mothers received at least one dose of retosiban or comparator (placebo/atosiban) in two Phase 3 sPTL trials. Both trials were terminated prematurely owing to poor enrolment. Infants could be enrolled into ARIOS from 28 days after estimated due date until hospital discharge or up to 9 months (corrected age). An internally developed questionnaire detailing medical conditions, mortality and resource use (Child Health Inventory; CHI), Ages and Stages Questionnaire-3 (ASQ-3), Modified Checklist for Autism in Toddlers-Revised with Follow-Up, and Child Behavior Checklist for Ages 1.5 to 5 were completed remotely by parents or legal guardians at prespecified intervals. Serious adverse events (SAEs) were primarily captured via CHI. No comparative statistical analysis was conducted between treatment arms. Results: A total of 49 (86%) infants who had received retosiban and 49 (78%) infants who had received a comparator were enrolled in ARIOS. No deaths occurred during the study. Nine infants experienced SAEs: 6/49 (12.2%) infants in the comparators group and 3/49 (6.1%) in the retosiban group. Of the nine SAEs, seven were due to infections, three, and four in the retosiban and comparators groups, respectively. Based on ASQ-3 score, the incidence of neurodevelopmental delay at 18 and 24 months were 0/18 (0%) and 2/25 (8%) with retosiban and 7/22 (31.8%) and 3/21 (14.3%) with comparator, respectively. Conclusion: The current study showed no unexpected adverse outcome or impairment with retosiban based on safety monitoring and neurodevelopment assessments. No further follow-up is intended owing to the discontinuation of clinical development of retosiban.