PART 2 Part 2 is comprised of a MAD treatment group with three cohorts. Following conclusion of the complementary HV MAD cohort in Part 1, a set of adult subjects diagnosed with CF currently on stable ivacaftor/lumacaftor background therapy for a minimum of three months will participate in the Part 2 complementary CF MAD cohort. Subjects will be randomized to receive either PTI-801 or placebo QD for a total of 14 days.