The goal of this clinical trial is to learn if drug SPL84 is safe for adult patients with cystic fibrosis (CF). It will also learn if the drug works to treat works to treat CF with a specific mutation (3849 +10kb C-->T).

The purpose of this research study is to test the safety and effectiveness of multiple doses of the study drug, SPL84.

Researchers will compare drug SPL84 to a placebo (a look-alike substance that contains no drug) to see if drug SPL84 is safe and if it works to treat CF. In cohorts 1-3, SPL84 will be tested as a monotherapy, and in Cohort 4, SPL84 will be tested in participants who are already stable on CFTR modulator therapy.

Participants will take drug SPL84 or a placebo by inhalation every week for 9 weeks (cohorts 1-3) or 12 weeks (cohort 4) and visit the clinic approximately weekly for checkups and tests.

Cohort 1-3:

Inclusion Criteria:

• Diagnosis of CF and two CF causing mutations; 3849+10 Kb C->T mutation on one allele in the CF transmembrane conductance regulator (CFTR) gene (homozygote or compound heterozygote). Source documentation from a certified genetic laboratory is required.
• Body mass index (BMI) of ≥ 17 kg/m2.
• FEV1 40-90% predicted at screening.
• Non-smokers or vapers for at least 180 days (6 months) prior to screening, per participant report.

Exclusion Criteria:

• Use of Kalydeco, Orkambi, Symdeko/Symkevi or Trikafta/Kaftrio within 30 days of first dose with study intervention.
• Use of any investigational drug (other than SPL84) or device within 30 days of first dose with study intervention.
• Use of systemic steroids over 3 consecutive months in the last 6 months prior to screening, or use of systemic steroids in the last month prior to screening. Use of inhaled steroids above 1 mg.
• Use of CF medications, e.g. inhaled antibiotics, dornase alfa (Pulmozyme), hypertonic saline and physiotherapy should be on stable regimen for the period 28 days prior to screening; those participants taking inhaled antibiotics for prophylaxis must be on a stable regimen of these drugs for at least 90 days prior to first dose with study intervention.
• Any acute infection including acute upper respiratory or lower respiratory infections, pulmonary exacerbation, changes in therapy for pulmonary disease, or any non CF-related illness which results in the initiation of any new therapy within 14 days prior to first dose with study intervention.
• Hemoptysis of greater than 30 mL within 90 days prior to Day 1, or hospitalization for hemoptysis within 6 months of first dose with study intervention.
• Liver disease characterized by clinically significant cirrhosis and/or documented portal hypertension.
• History of any organ transplantation.
• Documented coronavirus disease (COVID-19) infection within 4 weeks prior to dosing.

Cohort 4:

Inclusion Criteria:

• Diagnosis of CF and two CF causing mutations; 3849+10 Kb C->T mutation on one allele in the CF transmembrane conductance regulator (CFTR) gene (homozygote or compound heterozygote). Source documentation from a certified genetic laboratory is required.
• Body mass index (BMI) of ≥ 17 kg/m2.
• FEV1 40-80% predicted at screening.
• Non-smokers or vapers for at least 180 days (6 months) prior to screening, per participant report.
• Stable adherence to standard use of Trikafta/Kaftio or Alyftrek for at least 3 months, or Alyftrek for 1 month after switching from Trikafta/Kaftio, according to prescribing information.

Exclusion Criteria:

• Previous participation in active arm of SPL84-002 study (Cohort 1-3)
• Use of any investigational drug (other than SPL84) or device within 30 days of first dose with study intervention.
• Use of systemic steroids over 3 consecutive months in the last 6 months prior to screening, or use of systemic steroids in the last month prior to screening. Use of inhaled steroids above 1 mg.
• Use of CF medications, e.g. inhaled antibiotics, dornase alfa (Pulmozyme), hypertonic saline and physiotherapy should be on stable regimen for the period 28 days prior to screening; those participants taking inhaled antibiotics for prophylaxis must be on a stable regimen of these drugs for at least 90 days prior to first dose with study intervention.
• Any acute infection including acute upper respiratory or lower respiratory infections, pulmonary exacerbation, changes in therapy for pulmonary disease, or any non CF-related illness which results in the initiation of any new therapy within 14 days prior to first dose with study intervention.
• Hemoptysis of greater than 30 mL within 90 days prior to Day 1, or hospitalization for hemoptysis within 6 months of first dose with study intervention.
• Liver disease characterized by clinically significant cirrhosis and/or documented portal hypertension.
• History of any organ transplantation.
• Documented coronavirus disease (COVID-19) infection within 4 weeks prior to dosing.