Merck & Co., Inc. (NYSE:MRK) and Daiichi Sankyo announced on September 25 that they voluntarily withdrew the U.S. application for ifinatamab deruxtecan in adults whose extensive-stage small-cell lung cancer progressed during or after platinum-based chemotherapy. Following FDA discussions, the partners said the submitted evidence, including Phase 2 results, did not satisfy accelerated-approval requirements.
The investment question is whether a longer development path can preserve the opportunity. Ifinatamab deruxtecan, or I-DXd, is an investigational antibody-drug conjugate designed to deliver a cancer-killing payload to cells carrying a target protein.
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Bull Case
Merck & Co., Inc. and Daiichi Sankyo have evidence of tumor activity. Results reported in September 2025 showed a confirmed objective response rate of 48.2%, measuring tumor shrinkage, among 137 patients receiving 12 mg/kg in IDeate-Lung01. Median response duration was 5.3 months. Those findings support further investigation, although the trial did not compare I-DXd against standard chemotherapy.
Merck & Co., Inc. continues development through the randomized Phase 3 IDeate-Lung02 trial. It compares I-DXd with physician-selected chemotherapy after one prior platinum-based treatment regimen. Daiichi Sankyo said enrollment was nearly complete in the September 25 update.
That comparison could establish whether the response signal translates into better outcomes with acceptable safety. Phase 3 studies in prostate and esophageal cancers also continue, preserving opportunities that require their own supporting evidence.
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Bear Case
Merck & Co., Inc. had previously announced an October 10, 2026 FDA target decision date. Withdrawal removes that pending approval opportunity, and no replacement filing timetable was announced. A longer development period would defer potential revenue and require further research spending.
For Merck & Co., Inc., safety remains central. In the Phase 2 dataset, severe or worse treatment-related adverse events, classified as Grade 3 or higher, occurred in 36.5% of patients. Independently adjudicated treatment-related interstitial lung disease or pneumonitis occurred in 12.4%, including two fatal cases.
The FDA placed IDeate-Lung02 on partial clinical hold in December 2025 after more fatal interstitial lung disease events than expected. That U.S. hold was lifted in January 2026 following added safeguards. The partners did not identify the earlier hold as the reason for withdrawing the application.
A delayed launch would also need to establish a competitive position among treatments available at that time. Continued development preserves an opportunity, while comparative benefit and manageable toxicity remain essential.
Hedge Fund Sentiment
The filings available so far reflect positions held before Merck & Co., Inc. reported the withdrawal of its ifinatamab deruxtecan application. Insider Monkey’s database showed 101 hedge funds holding Merck & Co., Inc. at the end of 2Q2026, up from 98 funds three months earlier.
Conclusion
Merck & Co., Inc. retains an active development program, but the near-term approval case has weakened. Phase 3 must establish how efficacy compares with chemotherapy and whether the benefit justifies lung toxicity. Stronger evidence and subsequent FDA feedback will determine whether the drug’s promise becomes a credible commercial opportunity.
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This article is originally published at Insider Monkey.
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