Phase 1 Study of ATRA Plus a PD-1 Inhibitor in EGFR-Mutant NSCLC After Third-Generation EGFR-TKI Resistance

The goal of this Phase 1 clinical trial is to learn whether all-trans retinoic acid (ATRA) combined with tislelizumab is safe and tolerable in adults with EGFR-mutant non-small cell lung cancer (NSCLC). The study is for people whose cancer has worsened after treatment with a third-generation EGFR tyrosine kinase inhibitor and at least one standard systemic treatment, or who cannot tolerate or are not suitable for currently available standard treatments. The main questions this study aims to answer are: * What side effects and dose-limiting toxicities occur with the combination of ATRA and tislelizumab? * What is the highest dose of ATRA that can be given safely with tislelizumab, and what dose should be recommended for future studies? Researchers will also look for early signs that the combination can shrink or control the cancer and will assess how long participants live without their cancer getting worse and how long they live overall. Tumor tissue and blood samples will be studied to explore changes in the cancer and the immune system and to identify possible markers associated with treatment response or side effects. All participants will receive ATRA and tislelizumab; there is no comparison or placebo group. Participants will: * Take ATRA by mouth twice daily, beginning 7 days before the first dose of tislelizumab. The ATRA dose will be assigned according to the dose level being studied. * Receive tislelizumab by intravenous infusion once every 3 weeks. * Undergo regular safety assessments, including physical examinations, blood tests, vital-sign measurements, and monitoring for side effects. * Undergo imaging examinations approximately every 6 weeks during the first year to assess the cancer. * Provide tumor tissue and blood samples for biomarker research. * Attend an end-of-treatment visit, safety follow-up visits, and survival follow-up approximately every 3 months after treatment ends.