First real-world study of fetal therapy with CFTR modulators in cystic fibrosis: Report from the MODUL-CF study.

Background: We aimed to build a cohort of Maternal-Cystic Fibrosis (CF) fetal dyads treated in utero with Variant Specific Therapy (VST), to assess the efficacy on Meconium Ileus (MI) and potential adverse effects of treatment.

Methods: Dyads were included if the foetus had a genetic diagnosis of CF and carried at least one variant responsive to VST. Standardized assessment included pre-VST Magnetic Resonance Imaging (MRI), repeated ultrasound (US), and VST drug concentrations in cord blood, maternal and infant plasma.

Results: We enrolled 13 dyads. One withdrew from the study. VST therapies (Elexacaftor (ELX)/Tezacaftor (TEZ)/Ivacaftor(IVA) (ETI) n = 11, ivacaftor (IVA) n = 1) were administered to the pregnant women between 19 and 36 weeks' of gestation for a median[IQR] of 35[55] days, either as a curative indication of MI (n = 8) or as a tertiary prevention of fetal CF-related intestinal symptoms (n = 4). One foetus experienced increased bowel dilatation after ETI introduction. MRI revealed intestinal atresia. One dyad received only 2 doses. In the other 6 cases, resolution of MI was observed within 14[10] days of ETI. Fetal development and neonatal tolerance were excellent. Fecal elastase at birth was always below 200 ng/g even in the ETI breast-fed infant. Cord-to-maternal concentration yielded median ratios of 0.40 for ELX, 0.54 for IVA and 1.59 for TEZ.

Conclusion: ETI administration from the third trimester of pregnancy enables MI resolution. Trans-placental transfer is high. Fetal tolerance at ETI initiation needs to be monitored by a standardized assessment.