Study to Assess the Safety and PK of Oral and IV Xenleta in Adults With Cystic Fibrosis

Inclusion Criteria: 1. Signed informed consent. 2. Adult patients, ≥ 18 years of age. 3. Genetic confirmation of CF diagnosis by a report from a genetic test, such as "F508 deletion detected." 4. Weight \> 40 kgs. 5. Forced expiration volume (FEV)1 \> 40% predicted, as measured during the most recent evaluation. 6. Mentally and physically able to participate in the study as determined by the Investigator, ie, clinically stable with no significant changes in health status within 28 days prior to, and including, Day 1. 7. Vital signs within the following ranges: 1. Tympanic temperature, \< 38°C 2. Systolic blood pressure, 90 to 160 mmHg 3. Diastolic blood pressure, 50 to 90 mmHg 4. Heart rate \< 100 beats per minute at rest 5. Respiration rate 12 to 20 breaths per minute 6. Oxygen saturation to be documented. No selection criteria; supplemental oxygen use is acceptable. 8. Negative beta-human chorionic gonadotropin (β-hCG) urine or serum pregnancy test for females of childbearing potential. 9. Willing to commit to acceptable methods of contraception as defined in the protocol. Exclusion Criteria: 1. Known history of chronic liver or biliary disease, Gilbert's syndrome, or any of the following at Screening: aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \> 2 x upper limit of normal (ULN), total bilirubin \> 1.5 x ULN. 2. Prolonged baseline corrected QT interval corrected according to Fridericia (QTcF) defined as \> 440 ms (females) and \> 430 ms (males). 3. Family history or presence of prolonged QTc syndrome, Torsades de Pointes, or known conduction defects (eg, bundle branch block, atrioventricular block). 4. Use of Orkambi® (lumacaftor/ivacaftor) within 28 days prior to Day 1. 5. Use of cytochrome P450 (CYP)3A substrates that prolong the QT interval within 24 hours prior to Day 1. 6. Use of strong and moderate Cytochrome P450 (CYP3A) inducers or P-glycoprotein (P-gp) inducers within 28 days prior to Day 1. 7. Use of strong inhibitors of CYP3A4 within 24 hours prior to Day 1. 8. Serum potassium level below the normal reference range at Screening. 9. Known allergy to pleuromutilin class of antibiotic or any of the excipients of the lefamulin formulations. 10. Consumption of grapefruit, grapefruit juice, grapefruit products, pomelo, or Seville oranges within 24 hours before Day 1. 11. Use of vaporized nicotine or cannabidiol products, smoking (regularly or intermittently) more than 5 cigarettes (or equivalent) per day, or any use of tobacco other than in cigarettes or cigars within 28 days of Day 1. 12. Positive blood test for hepatitis C, human immunodeficiency virus (HIV), or hepatitis B antigen or core antibody (indicating active infection). 13. Positive test for drugs of abuse or alcohol at Screening or Day 1 that cannot be satisfactorily supported by medical history. 14. Use of an investigational product within the 30 days prior to Day 1 (3 months prior to Day 1 if the study drug was a new chemical entity). 15. Difficulty swallowing tablets. 16. Females who are pregnant or breastfeeding. 17. Does not have suitable venous access for multiple venipuncture or cannulation. 18. Any medical, psychological, cognitive, social, or legal conditions that, in the opinion of the Investigator, would interfere with the patient's ability to give an informed consent and/or participate fully in the study. 19. Any other reason, in the opinion of the Investigator, the patient is unsuitable to participate.