P059 Therapeutic drug monitoring of elexacaftor/tezacaftor/ivacaftor over 1 year in adult patients with cystic fibrosis

Objectives: In 2022, our first analysis of cmax values of elexacaftor, tezacaftor and ivacaftor showed that a significant portion of patients had cmax values outside the recommended range. With this abstract we want to give a follow up of cmax values over 1 year in our patient cohort. Methods: In this retrospective analysis, we evaluated the cmax values of elexacaftor, tezacaftor and ivacaftor with isotope dilution LC- MS/MS method (Habler K et al. 2021). Blood was taken during routine outpatients visits. Results were available after 7–14 days. Results: We collected blood samples from 167 patient with cystic fibrosis and intake of elexacafor, tezacaftor and ivacaftor. In 43 patients (approx. 30%) the dosis of ETI was reduced. For the poster, we will present the latest statistical analysis which is actually ongoing. Conclusion: A significant proportion of patients treated with ETI have elevated cmax values for elexacaftor. The reduction of the recommended dose did not cause a decrease in FEV1. With therapeutic drug monitoring, we were able to reduce the dose in one third of patients. This led also to a reduction in costs and side effects.