TSC recommended early termination due to lack of futility of the trial.

• To evaluate the change in M. abscessus cfu/g in induced sputum samples from baseline to the end of treatment with RESP301 in patients with cystic fibrosis who have treatment-naïve or treatment-refractory M. abscessus-pulmonary disease
• To assess the safety and tolerability of RESP301 during treatment (28 days) and follow up (84 days) in patients with cystic fibrosis who have treatment naïve or treatment refractory M. abscessus-pulmonary disease

Investigators will undertake an eighteen-week single centre, open label study in participants with cystic fibrosis infected with Mycobacterium abscessus (M. abscessus)-pulmonary disease (-PD).

The study will treat particpants with cystic fibrosis (CF) attending the Adult Cystic Fibrosis Centre at the Royal Papworth Hospital, Cambridge, United Kingdom. Participants will be consented and screened for the RESP301-003 study to enable approximately 12 participants to commence treatment with RESP301.

Participants will have M abscessus-PD as defined by the ATS/IDSA, specifically: (i) two or more positive sputum cultures for M. abscessus; (ii) radiological change consistent with NTM-PD; and (iii) symptoms consistent with NTM-PD, after exclusion of other causes.

Participants will be recruited who (1) have not commenced antibiotic treatment for M. abscessus-PD or (2) have treatment refractory M. abscessus-PD (defined as remaining sputum culture positive after 6 months or more of treatment). Treatment-refractory participants will be suitable for enrolment in the study if date of first dosing is at least 2 months since a change in M. abscessus treatment (or 4 months since change of Clofazimine).

Inclusion Criteria:

1. Adult patients of ≥18 years at time of informed consent
2. Patients with a clinical diagnosis of CF and confirmed by genetic testing
3. Diagnosis of treatment naïve or treatment refractory M. abscessus-PD
4. Signed informed consent documentation (indicating an understanding of the purpose and a willingness to meet the requirements for participation in the study)

Exclusion Criteria:

1. FEV1 <40% predicted
2. Methaemoglobin concentration > 2%
3. Use of nitric oxide donor medications such as prilocaine, sodium nitroprusside, and nitroglycerine within 30 days of proposed first treatment
4. Use of phosphodiesterase inhibitors (e.g., sildenafil) within 30 days of proposed first treatment
5. Evidence of pulmonary hypertension
6. History of frequent low volume or massive haemoptysis
7. Liver disease (i.e. liver cirrhosis, portal hypertension)
8. Subjects who have undergone organ transplantation
9. Pregnancy or lactation (female participants only)
10. Subjects who will not use appropriate forms of contraception for the duration of the study
11. Contraindication or unable to complete lung function testing
12. Contraindication or unable to tolerate nebulised hypertonic saline
13. Changes to previous NTM antibiotic regimen within two months of first dose of study treatment (or 4 months for clofazimine)
14. Subject has received investigational treatment as part of another interventional clinical trial within two months of the proposed first day of treatment
15. Required antibiotic treatment for a pulmonary exacerbation within 2 weeks of enrolment to the study.
16. Inability to undergo study related activities and / or commitments
17. Any subject who in the opinion of the investigator would not be best served by participating in this clinical trial.