Merck’s $4 Billion Daiichi ADC Pact Takes Its Third FDA Setback in Three Years
- Daiichi Sankyo and Merck have voluntarily withdrawn their application for accelerated approval of ifinatamab deruxtecan (I-DXd), a B7-H3-directed antibody-drug conjugate, in extensive-stage small cell lung cancer that has progressed on or after platinum chemotherapy, after the FDA said the data fell short, Fierce Biotech reported on 28 September 2026.
- The filing rested on the Phase 2 IDeate-Lung01 study of 187 patients. Enrolment in the Phase 3 IDeate-Lung02 trial is nearing completion, and the partners may return with a larger dataset. Merck’s oncology development head, Marjorie Green, said the company was disappointed.
- Merck paid Daiichi $4bn upfront in 2023 for three ADCs. The FDA rejected patritumab deruxtecan in 2024 on manufacturing grounds, and the partners withdrew that filing in 2025 after a confirmatory trial missed overall survival.
- I-DXd’s Phase 3 trials in small cell lung cancer, prostate cancer and oesophageal squamous cell carcinoma have primary completion dates in 2028, as does the Phase 2/3 ovarian trial of the third asset, raludotatug deruxtecan.
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The deal’s first return keeps slipping toward 2028, the year Keytruda begins to lose protection.
- Twice now the partners have withdrawn rather than wait for a rejection. Once after a confirmatory trial missed survival, and now after the FDA said single-arm Phase 2 data were not enough. Across three years and two assets, the $4bn upfront has produced no US approval.
- Competitors gain time. Amgen’s Imdelltra, a DLL3-targeted T-cell engager, already treats the same post-platinum population, and GSK’s B7-H3 ADC risvutatug rezetecan recently impressed in a Phase 3 trial in China. The delay narrows I-DXd’s lead in B7-H3.
- Merck spent the same day buying early. It licensed SciBrunch’s preclinical KRAS G12D drug for a $400m charge. The pattern is a company adding options at the front of the pipeline while the late-stage bets meant to replace Keytruda take longer.