Celltrion has applied to South Korea's Ministry of Food and Drug Safety for CT-P51, a biosimilar of Keytruda, the world's top-selling immuno-oncology drug. The filing follows Samsung Bioepis's submission of SB27 on the 10th and marks the start of a closely watched race for first-mover advantage in a market worth about 43.8 trillion won, or $31.7 billion, annually. CT-P51 showed pharmacokinetic equivalence to Keytruda in a 228-patient trial involving completely resected melanoma, with similar safety and immunogenicity results. The application covers major indications including non-small cell lung cancer, melanoma, gastric cancer and head and neck cancer. Celltrion plans sequential filings with the FDA, EMA and Health Canada and will rely on in-house manufacturing, direct sales and potential bundling with Vegzelma and future antibody-drug conjugates. Samsung Bioepis is pursuing large-scale evidence, having completed a global Phase 3 trial of SB27 in 555 non-small cell lung cancer patients across 14 countries, while using partners including Organon, Biogen and Sandoz for commercialization. Keytruda's substance patents expire in 2028 in South Korea, late 2029 in the United States and early 2031 in Europe. Competition also includes Formycon, Sandoz and Amgen, while MSD is promoting the subcutaneous Keytruda Qurex formulation before intravenous biosimilars arrive. CT-P51 would expand Celltrion's oncology portfolio from targeted therapies into immuno-oncology and could create cross-selling opportunities with Vegzelma.