• Histologically confirmed NSCLC stage IIIB/IIIC or IV not amenable for curative treatment, with PD-L1 ≥ 50% measured by a validated method, and eligible for pembrolizumab monotherapy in the first-line setting

• At least one lesion, not previously irradiated and not chosen for biopsy during the study screening period, that can be accurately measured at baseline according to RECIST 1.1

• Subjects who received previous neo-adjuvant or adjuvant systemic therapy (other than immunotherapies) will be eligible if neo-adjuvant or adjuvant therapy was completed at least 12 months prior to the development of metastatic disease. Last dose of neoadjuvant or adjuvant therapy must be more than 12 months prior to enrollment/randomization

• Available unstained archived tumour tissue sample in sufficient quantity to allow for analyses. At least fifteen unstained slides or a tumour block (preferred)

• Male and female age ≥ 18 years at time of signing the ICF

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-2

• Adequate organ function as defined below

• Haemoglobin ≥9.0 g/dL

• Absolute neutrophil count (ANC) 1.5 x (\> 1500 per mm3)

• Platelet count ≥100 x 109/L (\>75,000 per mm3)

• Serum bilirubin ≤1.5 x institutional upper limit of normal (ULN).

• AST (SGOT)/ALT (SGPT) ≤2.5 x institutional upper limit of normal unless liver metastases are present, in which case it must be ≤5x ULN

• Measured creatinine clearance (CL) \>40 mL/min or Calculated creatinine CL \>40 mL/min by the Cockcroft-Gault formula (Cockcroft and Gault 1976) or by 24-hour urine collection for determination of creatinine clearance:

⁃ Males:

⁃ Creatinine CL (mL/min) = Weight (kg) x (140 - Age) 72 x serum creatinine (mg/dL)

⁃ Females:

⁃ Creatinine CL (mL/min)

⁃ = Weight (kg) x (140 - Age) x 0.85 72 x serum creatinine (mg/dL)

• Written informed consent obtained prior to any study specific procedure