Materials and Methods

Patient selection

In this retrospective cohort study, we reviewed the cases of 167 patients from five centers in Bulgaria with metastatic NSCLC treated with pembrolizumab after progression upon first-line platinum-based chemotherapy between April 2017 and February 2020. The procedure was approved by the Scientific Research Ethics Committee at the Hospital “Nadezhda” in Sofia. The eligibility criteria were as follows: (1) age ≥18 years old, (2) histologically confirmed diagnosis of NSCLC in the metastatic stage, (3) wild-type epidermal growth factor receptor/anaplastic lymphoma kinase, (4) Eastern Cooperative Oncology Group-Performance status (ECOG-PS) < 2, (5) disease progression after receiving one prior platinum-based systemic therapy for metastatic disease with measurable disease lesions, (6) available blood cell count and blood samples, and (7) available computed tomography (CT) scans. Immunotherapy were administered after at least 3 weeks after previous treatment. Patients were excluded if they had brain metastases (since corticosteroid use may compromise therapy), autoimmune disease, symptomatic interstitial lung disease, systemic immunosuppression, or prior treatment with immune-stimulatory antitumor agents, including checkpoint inhibitors. Patients did not show any clinical or computed tomography signs of active infection. Tumor PD-L1 status was assessed. Pembrolizumab was initially administered via 2 mg/kg intravenous (i.v.) injection over 60 minutes every 3 weeks and later via 200 mg i.v. injection (flat dose) every 3 weeks.