Progression Free Survival (PFS) Assessed by Investigator

Pembrolizumab Arm

Pembrolizumab is administrated at 200 mg fixed dose i.v. on day 1 of every 3 week cycle for a maximum or 2 years (expected maximum of 36 doses), or until progression of disease determined according to RECIST 1.1 criteria or lack of tolerability, or until the patient declines further treatment. Pembrolizumab: Pembrolizumab (MK-3475) is a potent and highly selective humanised monoclonal antibody (mAb) of the IgG4/kappa isotype designed to directly block the interaction between PD-1 and its ligands, PD-L1 and PD-L2. This blockade enhances functional activity of the target lymphocytes to facilitate tumour regression and ultimately immune rejection.

Standard Chemotherapy Arm

Gemcitabine (i.v. 1000 mg/m2) or vinorelbine (i.v. 30 mg/m2, or p.o 60/80 mg/m2) chemotherapy will be chosen on a per patient basis and delivered according to local standards. Chemotherapy will be administered on days 1 and 8 of every 3-week cycle. A maximum number of treatment cycles is not mandated. Patients randomised to the control arm will be allowed to cross over to receive pembrolizumab at progression, if cross-over criteria are met. Pembrolizumab administration will follow the same schedule as for patients in the experimental arm, i.e. 200 mg fixed dose i.v. on day 1 of every 3-week cycle for a maximum of 2 years or until trial termination. Gemcitabine: Gemcitabine replaces one of the building blocks of nucleic acids, in this case cytidine, during DNA replication. The process arrests tumour growth, as new nucleosides cannot be attached to the "faulty" nucleoside, resulting in apoptosis (cellular "suicide"). Vinorelbine: Vinorelbine is a vinca alkaloid cytotoxic chemotherapy that is available in intravenous and oral preparations with EMA licenses in lung cancer and breast cancer. Vinorelbine blocks mitosis in phase G2-M, causing cell death in interphase or at the following mitosis.

Analysis denominator · Participants