after joining the study, the participant is randomly assigned to receive either SAR445399 or a matched placebo. the assignment is double‑blinded, meaning neither the participant nor the study staff know which product is being given.

before any medication is given, a series of baseline measurements are performed:

chest high‑resolution computerized tomography (HRCT) – a detailed lung scan used to calculate the mucus plug score (MPS);

forced expiratory volume in 1 second (FEV1) – a lung function test performed both after and before using a bronchodilator; and

standard laboratory tests, vital‑sign measurements, and a 12‑lead electrocardiogram (ECG) to record the participant’s health status.

the participant receives the first subcutaneous injection (injection under the skin) of the assigned product.

the product is supplied as a powder for solution for injection; the dose listed in the protocol is 00 mg.

the injection is prepared and administered by qualified study personnel in accordance with the study protocol.

at each scheduled study visit, the participant receives another subcutaneous injection of the same product (either SAR445399 or placebo).

the timing of these visits follows the study schedule, which continues for up to 52 weeks from the first dose.

throughout the study, the participant undergoes periodic evaluations, including:

repeat HRCT scans to monitor changes in the mucus plug score;

repeated FEV1 measurements (both post‑ and pre‑bronchodilator) to assess lung function;

recording of any pulmonary exacerbations (PEx) – worsening of breathing symptoms;

monitoring for any adverse events (AEs), including serious events, events leading to discontinuation, and events of special interest;

laboratory tests, vital‑sign checks, and ECGs to detect any safety concerns.

at week 24, a comprehensive set of evaluations is performed:

HRCT to determine the change in mucus plug score from baseline;

FEV1 measurements to evaluate lung function changes;

calculation of the annualized rate of pulmonary exacerbations and severe exacerbations up to week 24;

assessment of whether the participant remained free of exacerbations during the 24‑week period;

collection of blood samples to measure the plasma concentration of SAR445399 and to test for anti‑drug antibodies.

at week 52, the final study visit includes:

repeat HRCT and FEV1 testing to evaluate long‑term changes;

calculation of pulmonary exacerbation rates and severe exacerbation rates from baseline to week 52;

determination of exacerbation‑free status for the entire 52‑week period;

final safety review, including all adverse events, laboratory abnormalities, and ECG findings;

final blood sampling for drug concentration and anti‑drug antibody analysis.

after the week 52 visit, the participant’s involvement in the trial ends.

all study data are recorded, and the participant may be offered standard follow‑up care as determined by the treating physician.