This clinical trial is focused on studying a type of lung cancer known as Non-Small Cell Lung Cancer (NSCLC). The study is investigating the effectiveness of a combination of two treatments: Niraparib and Pembrolizumab. Niraparib is a medication that is taken orally in tablet form, while Pembrolizumab is given as an infusion, which means it is administered directly into the bloodstream through a vein. The trial will compare the effects of these medications against a placebo, which is a substance with no active drug, to see if the combination can help maintain the stability of the disease or improve the response after initial chemotherapy treatment.

The purpose of the study is to determine if the combination of Niraparib and Pembrolizumab can help patients live longer without the disease getting worse, and to see if it can improve overall survival compared to the placebo. Participants in the study will have already received a first-line treatment with platinum-based chemotherapy and Pembrolizumab, and their disease should have either remained stable or shown improvement. The study will involve regular monitoring and assessments to track the progress of the disease and the effects of the treatment.

Throughout the study, participants will receive either the combination of Niraparib and Pembrolizumab or a placebo with Pembrolizumab. The trial is designed to be double-blind, meaning neither the participants nor the researchers will know who is receiving the actual medication or the placebo. This helps ensure that the results are unbiased. The study will continue for a set period, during which participants will be closely monitored for any changes in their condition and any side effects they may experience. The ultimate goal is to find a more effective maintenance therapy for patients with advanced or metastatic NSCLC.