The purpose of this study is to determine whether PUR118 is safe and tolerable in a population of subjects with Cystic Fibrosis.

Inclusion Criteria:

• Males or non-pregnant, non-lactating healthy female.
• Age 18 to 55 years of age (inclusive) on day of signing informed consent.
• Subject has a BMI of 17 or greater.
• Subject has a confirmed diagnosis of cystic fibrosis documented by a compatible clinical or radiographic presentation, and definitive accepted lab and clinical criteria (e.g., sweat chloride > 60 mEq/L or 2 disease causing CFTR mutations) OR is regularly attending a Consultant led clinic where the diagnosis of Cystic Fibrosis is not in doubt.
• Subject has an FEV1 ≥ 45% of predicted at screening.
• Subject has an oxygen saturation >92% on room air as determined by pulse oximetry at screening.
• Subject is a non-smoker or ex-smoker has not smoked for at least six months prior to screening.
• Subject is otherwise medically stable without clinically significant abnormal screening results that depart from their usual baseline level of health as a subject with CF.
• Must be willing and able to communicate in English and participate in the whole study.
• Must provide written informed consent.

Exclusion Criteria:

• Subject has unstable lung disease, as defined by a change in medical regimen during the preceding 2-weeks, or a significant new finding on chest radiograph (pneumothorax, lobar/segmental collapse), or in the opinion of the Investigator, has had a decline in pulmonary status within the last year not considered a part of the usual, chronic progression of CF lung disease and part of their baseline health condition as a subject with CF.
• Subject has had an exacerbation of respiratory symptoms within the past 30 days that required initiation of a new or altered respiratory therapy.
• Subject had either an upper or lower respiratory illness within the 30 days prior to dosing days, or has symptoms from such an illness that have not resolved.
• Subject has a history of lung transplantation.
• Females of child bearing potential who are pregnant, or lactating. Females who are sexually active and either not surgically sterile or not willing to use an acceptable form of contraception
• Clinically significant abnormal biochemistry, haematology or urinalysis not consistent with CF as judged by the Investigator
• Failure to satisfy the Investigator of fitness to participate for any other reason.