Growth, Body Composition, and Strength of Children With Cystic Fibrosis Treated With Elexacaftor/Tezacaftor/Ivacaftor (ETI).

Objective: We prospectively monitored rates of change for growth, body mass and composition, muscle strength, and FEV1 in 6-11-year-olds initiating ETI therapy, comparing them to those of US reference children. We assessed factors potentially contributing to rate of change and report ranges of individual variation.

Methods: Body composition was assessed using bioelectrical impedance analysis (BIA), and rates of change were analyzed using linear mixed effects regression models.

Results: At enrollment, median BMI-Z was 0.6 (IQR: -0.2, 1.1), percent body fat (PBF) was 22.7 (14.0, 31.5), and percent predicted(pp) FEV1 was 100 (90, 106). During ETI treatment, mean Z scores for annualized change rates of BMI (0.02 ± 0.07; p = 0.74), FMI (0.02 ± 0.08; p = 0.76), and FFMI (-0.03 ± 0.07; p = 0.68) were not different from zero. The most rapid weight gain occurred in girls (p = 0.01), 10-11-year-olds (p < 0.001), and those previously treated with a modulator (p = 0.005). Individual rates of change varied widely; PBF increased for 15 children (range: 0.7 to 10.0) and decreased for 12 (range: -0.7 to -9.5). Changes in body mass and composition were not significantly associated with changes in ppFEV1; regression coefficients were positive for FFMI (0.83) and SMMI (1.07) and negative for FMI (-0.29).

Conclusion: Healthy, well-nourished children with CF, as a group, experienced growth and body composition changes similar to those of US children, added muscle mass, and often added more FM than FFM during ETI therapy. Individual variation underscores the need for body composition monitoring and interventions that promote healthy physical maturation for all during ETI therapy.