Background: Non-small cell lung cancer (NSCLC) is the most common type of lung cancer. Anti–programmed cell death-1 (PD-1) therapies have improved outcomes in patients with NSCLC; however, most patients do not respond or only respond for a limited time. The combination of cemiplimab (anti–PD-1) and chemotherapy has been studied for the first-line treatment of advanced NSCLC (aNSCLC), regardless of programmed cell death-ligand 1 (PD-L1) status. While the standard of care continues to evolve for NSCLC, one approach to potentially improve outcomes is the combination of checkpoint inhibitors (IO) with chemotherapy. Fianlimab (anti–lymphocyte activation gene 3 [LAG-3]) and cemiplimab are both high-affinity, fully human IgG4 monoclonal antibody IO therapies that have shown promising clinical activity when combined in patients with aNSCLC in a Phase 1 study (NCT03005782). This ongoing study will further investigate the combination of fianlimab + cemiplimab + chemotherapy. Methods: This is a randomized, multicenter, Phase 2/3 study of fianlimab + cemiplimab + platinum-doublet chemotherapy (chemo) versus cemiplimab + chemo in patients with unresectable stage IIIB/IIIC or stage IV NSCLC irrespective of PD-L1 expression levels (NCT05800015). This study will be conducted globally at approximately 210 sites. Patient eligibility criteria: ≥18 years old; squamous or non-squamous histology with stage IIIB, IIIC, or IV disease; valid PD-L1 result; ≥1 radiographically measurable lesion by computed tomography/magnetic resonance imaging; Eastern Cooperative Oncology Group performance status ≤1; adequate organ and bone marrow function. The Phase 2 portion will be used to determine the fianlimab dose selected for the Phase 3 portion: patients will be randomized 1:1:1 to receive fianlimab high dose + cemiplimab 350 mg + chemo (Arm A), fianlimab low dose + cemiplimab 350 mg + chemo (Arm B), or cemiplimab 350 mg + chemo + placebo (Arm C), Q3W IV. In Phase 3, patients will be randomized 1:1 into one of two treatment arms (either Arm A or Arm B, and Arm C) based on findings from Phase 2. The primary endpoint of Phase 2 is objective response rate (ORR) per blinded independent central review. The primary endpoint of Phase 3 is overall survival. The secondary endpoints include efficacy (ORR, disease control rate, time to tumor response, duration of response, progression-free survival, and overall survival), safety and tolerability, immunogenicity (anti-drug antibodies and neutralizing antibodies against fianlimab or cemiplimab in serum), patient-reported outcomes, and pharmacokinetics. This study and a parallel study of fianlimab + cemiplimab in patients with aNSCLC with tumors expressing PD-L1 ≥50% (NCT05785767) are currently open for enrollment. Clinical trial information: NCT05800015.