This is an open-label, multiple-ascending dose study of ARCT-032 in adults with CF who are not on CFTR modulator therapy. After successful screening, eligible participants enrolled in cohorts 1-3 will receive nebulized ARCT-032 daily for 4 weeks and then will be followed for safety for a total of 12 weeks. In cohort 4, eligible participants will receive nebulized ARCT-032 for 12 weeks and will then be followed for safety for an additional 12 weeks.