This is a randomized, double-blind, cross-over, vehicle-controlled study to determine whether GS-5737 in 2.8% saline accelerates mucociliary clearance (MCC) in healthy subjects, compared to vehicle of 2.8% saline alone.

Inclusion Criteria:

• Body mass index (BMI) from ≥ 19 to ≤ 30 kg/m2
• Normal ECG, with no history or evidence of myocardial infarction, QT prolongation, or ventricular arrhythmias
• Normal (or abnormal but not clinically significant) blood pressure (BP) and heart rate(HR)
• History of lifetime smoking < 5 pack-years (ie, 1 pack per day x 1 year = 1 pack-year)and non-smokers of at least 60 days duration prior to Screening
• Negative screening tests for drugs of abuse (including alcohol)
• Hepatitis B (HBV) surface antigen negative
• Hepatitis C (HCV) antibody negative
• HIV-1, 2 antibody negative
• Male subjects who are sexually active must be willing to use highly effective contraception from the Baseline Visit through completion of the study and continuing for at least 90 days from the date of the last dose of study drug
• Male subjects must refrain from sperm donation from the Baseline Visit through completion of the study and continuing for at least 90 days from the date of the last dose of study drug
• Females of childbearing potential must have a negative serum pregnancy test
• Females of childbearing potential must agree to utilize protocol recommended highly effective contraception methods from Screening throughout the duration of study treatment and for 30 days following the last dose of study drug

Exclusion Criteria:

• Female subjects who are pregnant or lactating
• History of anaphylactic reaction to any drug
• Have previously participated in an investigational trial involving administration of any investigational compound within 30 days prior to the study drug dosing
• Presence or history of extrinsic allergy requiring treatment; hay fever is allowed unless it is active or has required treatment within the previous 2 months
• Have donated blood within 56 days of study dosing
• Presence or history of any pulmonary diseases (ie, asthma, emphysema, chronic bronchitis, chronic obstructive pulmonary disease (COPD), cystic fibrosis (CF), bronchiectasis, interstitial lung disease)
• Major surgery within 6 months of Screening
• Experienced symptoms of recent acute upper or lower respiratory tract infection requiring treatment within the 2 weeks prior to Screening
• Have had any radiation in the past year that would cause the subject exceed recommended limits by participating in this study protocol