Study Assessing the Safety, Tolerability, Pharmacokinetics, Food Effect, and Drug-Drug Interactions of PTI-801 in Healthy Volunteers, and Safety, Tolerability, and Pharmacokinetics of PTI-801 in Subjects With Cystic Fibrosis

Part 1 Inclusion Criteria: * Adults age 18 to 55 years old, inclusive, at the time of informed consent. * Body mass index (BMI) ≥18 to \<30 kg/m2. * Subject must be a nonsmoker and a nontobacco user for a minimum of 30 days prior to screening and for the duration of the study. Part 1 Exclusion Criteria: * History or current evidence of any clinically significant cardiac, endocrinologic,hematologic, hepatobiliary, immunologic, metabolic, urologic, pulmonary, neurologic,dermatologic, psychiatric, renal, or other major disease, as determined by the investigator. * Presence of prolonged QT/ Corrected QT Interval (QTc) interval with Fridericia's correction formula (QTcF) \>450 msec at screening. * Abnormal liver function as defined by aspartate aminotransferase (AST), alanine aminotransferase (ALT) or bilirubin \> upper limit of the normal range. * Abnormal renal function at screening defined as: Creatinine clearance \<80 mL/min using the Cockcroft-Gault equation. * Participation in another clinical trial or treatment with an investigational agent within 30 days or 5 half-lives, whichever is longer, prior to Study Day 1. * History of cancer within the past 5 years (excluding nonmelanoma skin cancer). * History or current evidence of alcohol or drug abuse or dependence within 12 months of screening as determined by the investigator. * Positive urine screen for prohibited drugs (cocaine, cannabinoids, nicotine \[urine cotinine is the detection mechanism for nicotine\], opiates, barbiturates, amphetamines, and benzodiazepines) or positive alcohol test at screening. * Positive blood screen for human immunodeficiency virus (HIV), hepatitis B surface antigen (HBsAg), or hepatitis C virus antibody (HCVAb). * Has donated blood within 3 months of screening or plans to donate blood within 3 months of study completion. Part 1 HV DDI Cohort Additional Exclusion Criteria: * Concomitant use of known strong or moderate inhibitors or inducers of CYP1A2, CYP2B6, and CYP3A4 within 14 days or 5 half-lives (whichever is longer) prior to Day 1 and through the last PK sampling point on Day 20 * Use of grapefruit- or Seville orange-containing products within 48 hours prior to Day 1 and through the last PK sampling point on Day 20 * Use of alcohol- or caffeine-containing products within 48 hours prior to Day 1 and through the last PK sampling point on Day 20 Part 2 Inclusion Criteria: * Confirmed diagnosis of CF with the F508del/F508del genotype on record, along with clinical findings consistent with CF such as chronic sinopulmonary disease or gastrointestinal/nutritional abnormalities * Forced expiratory volume in 1 second (FEV1) 40-90% predicted, inclusive * Non-smoker and non-tobacco user for a minimum of 30 days prior to screening Part 2 Cohorts 1-3 Additional Inclusion Criterion: * Stable on ivacaftor/lumacaftor dosing for both label indication and per label dosing for a minimum of 3 months at the time of dosing Part 2 Cohort 6 Additional Inclusion Criterion: * Stable on tezacaftor/ivacaftor dosing for both label indication and per label dosing for a minimum of 1 month at the time dosing Part 2 Exclusion Criteria: * Participation in another clinical trial or treatment with an investigational agent within 28 days or 5 half-lives, whichever is longer, prior to Study Day 1 * History of cancer within the past 5 years (excluding cervical cancer in situ with curative therapy for at least one year prior to screening and non-melanoma skin cancer) * History of organ transplantation * Hospitalization, sinopulmonary infection, CF exacerbation, or other clinically significant infection or illness (as determined by the investigator) requiring an increase or addition of medication, such as antibiotics or corticosteroids, within 14 days of Day 1 * Initiation of any new chronic therapy (e.g., ibuprofen, hypertonic saline, azithromycin, Pulmozyme®, Cayston®, TOBI®)) or any change in chronic therapy (excluding pancreatic enzyme replacement therapy) within 28 days prior to Day 1 * History or current evidence of alcohol or drug abuse or dependence within 12 months of screening as determined by the investigator * Pregnant or nursing women Part 2 Cohort's 4 and 5 Additional Exclusion Criterion: * Currently taking or has taken a CFTR modulator within 30 days prior to initial dose of study drugs