The aim of this study is to see how effective combination immunotherapy (fianlimab and cemiplimab) is in treating advanced non-small cell lung cancer, in comparison to cemiplimab by itselfA Randomized, Double-Blind Phase 2/3 Study of Fianlimab (Anti-LAG-3 Antibody) in Combination With Cemiplimab (Anti-PD-1 Antibody) Versus Cemiplimab Monotherapy in First-Line Treatment of Patients With Advanced Non-Small Cell Lung Cancer (NSCLC) With Tumors Expressing PD-L1 ≥50%

Exclusion

1. Patients who have never smoked, defined as smoking ≤100 cigarettes in a lifetime.
2. Active or untreated brain metastases or spinal cord compression. Patients are eligible if central nervous system (CNS) metastases are adequately treated, and patients have neurologically returned to baseline (except for residual signs or symptoms related to the CNS treatment) for at least 2 weeks prior to enrollment. Patients must be off (immunosuppressive doses of) corticosteroid therapy.
3. Patients with tumors tested positive for actionable epidermal growth factor receptor (EGFR) gene mutations, anaplastic lymphoma kinase (ALK) gene translocations, or c-ros oncogene 1 (ROS1) fusions, as described in the protocol.
4. Encephalitis, meningitis, or uncontrolled seizures in the year prior to enrollment.
5. History of interstitial lung disease (eg, idiopathic pulmonary fibrosis or organizing pneumonia), of active, noninfectious pneumonitis that required immune-suppressive doses of glucocorticoids to assist with management, or of pneumonitis within the last 5 years. A history of radiation pneumonitis in the radiation field is permitted as long as pneumonitis resolved ≥6 months prior to enrollment.
6. Known primary immunodeficiencies, either cellular (eg, DiGeorge syndrome, T-cell-negative severe combined immunodeficiency [SCID]) or combined T- and B-cell immunodeficiencies (eg, T- and B-cell negative SCID, Wiskott Aldrich syndrome, ataxia telangiectasia, common variable immunodeficiency).
7. Ongoing or recent (within 2 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments, which may suggest risk of immune-mediated treatment-emergent adverse events (imTEAEs). Patients with uncontrolled type 1 diabetes mellitus or with uncontrolled adrenal insufficiency are excluded. The following are not exclusionary: vitiligo, childhood asthma that has resolved, residual hypothyroidism that required only hormone replacement, or psoriasis that does not require systemic treatment.
8. Patients with a condition requiring corticosteroid therapy (>10 mg prednisone/day or equivalent) within 14 days of randomization. Physiologic replacement doses are allowed even if they are >10 mg of prednisone/day or equivalent, as long as they are not being administered for immunosuppressive intent. Patients with clinically relevant systemic immune suppression within the last 3 months before trial enrollment are excluded. Inhaled or topical steroids are permitted, provided that they are not for treatment of an autoimmune disorder.

9. Patients who have received prior systemic therapies are excluded with the exception of the following:

   1. Adjuvant or neoadjuvant platinum-based doublet chemotherapy (after surgery and/or radiation therapy) if recurrent or metastatic disease develops more than 6 months after completing therapy as long as toxicities have resolved to CTCAE grade ≤1 or baseline with the exception of alopecia and peripheral neuropathy.
   2. Anti-PD-(L) 1 with or without LAG-3 as an adjuvant or neoadjuvant therapy as long as the last dose is >12 months prior to enrollment.
   3. Prior exposure to other immunomodulatory or vaccine therapies as an adjuvant or neoadjuvant therapy, Cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4) antibodies as long as the last dose is >6 months prior to enrollment. Immune-mediated AEs must be resolved to CTCAE grade ≤1 or baseline by the time of enrollment. Endocrine immune-mediated AEs controlled with hormonal or other non-immunosuppressive therapies without resolution prior to enrollment are allowed.

Note: Other protocol-defined Inclusion/ Exclusion Criteria apply.