This trial will consist of two parts: Part 1 and Part 2. Part 1 will enroll adult healthy volunteers (HV) into four treatment groups. The first group will enroll HV into a single ascending dose (SAD) treatment group consisting of three cohorts. The second group will enroll HV into a multiple ascending dose (MAD) treatment group consisting of three cohorts. The third group will enroll HV into a food effect (FE) treatment group consisting of one cohort. The fourth group will enroll HV into a drug-drug interactions (DDI) treatment group consisting of one cohort. Approximately 76 subjects will be enrolled in Part 1.

Part 2 Cohorts 1 through 3 will enroll adult subjects with cystic fibrosis (CF) currently on stable ivacaftor/lumacaftor background therapy for a minimum of three months. Part 2 Cohorts 4 and Cohort 5 will enroll adult subjects with CF not currently receiving cystic fibrosis conductance regulator (CFTR) modulator therapy within 30 days prior to Day 1. Part 2 Cohort 6 will enroll adult subjects with cystic fibrosis on stable tezacaftor/ivacaftor background therapy. Approximately 104 subjects will be enrolled in Part 2.

PART 1 The SAD treatment group is comprised of three cohorts where HV will be randomized to either PTI-801 or placebo. The MAD treatment group is comprised of three cohorts where subjects will be randomized to receive either PTI-801 or placebo once daily (QD) for a total of 7 days. HV will participate in a FE treatment group ,the FE treatment group is comprised of one cohort where subjects will be randomized to receive an initial single dose of PTI-801 either after an overnight fast of at least 10 hours (fasted group) or after an overnight fast of at least 10 hours followed by the consumption of a high fat high and high calorie meal (fed group). A set of HV will participate in a DDI treatment group. The DDI treatment group is comprised of one cohort where subjects will receive a 3-drug cocktail consisting of caffeine, bupropion, and midazolam on Day 1. On Day 4, subjects will be randomized to receive either PTI-801 or placebo QD for a total of 12 days. On Day 17, subjects will receive the 3-drug cocktail in combination with PTI-801 or placebo.

PART 2 Part 2 is comprised of a MAD treatment group with three cohorts, a co-administration group with two cohorts and a treatment group with one cohort.

Following the conclusion of the complementary HV MAD Cohort in Part 1, a set of adult subjects diagnosed with CF currently on a 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.

Following the conclusion of CF MAD Cohort 1 in Part 2, a set of adult subjects diagnosed with CF not currently receiving or have received background CFTR modulator therapy for a minimum of 30 days prior to Day 1 will participate in the Part 2 CF PTI-801 and PTI-808 co-administration cohort. Subjects will be randomized to receive either PTI-801 co-administered with PTI-808 or placebos QD for a total of 14 days.

Following the conclusion of the CF MAD Cohort 1 in Part 2, a set of adult subjects diagnosed with CF currently on a stable tezacaftor/ivacaftor background therapy for a minimum of one month will participate in the Part 2 complementary CF cohort. Subjects will be randomized to receive either PTI-801 or placebo QD for a total of 14 days.

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