A $22 Billion Bet Just Hit Its First Big Speed Bump
Daiichi Sankyo and Merck pulled their FDA application for an antibody-drug conjugate in small cell lung cancer before regulators could reject it. It's the first real stumble for a $22 billion ADC partnership that was supposed to be a sure thing.
Pull the Plug Before They Pull It for You
Two weeks before the FDA was supposed to make a decision, Daiichi Sankyo and Merck yanked their own application. On September 25, they voluntarily withdrew the U.S. filing for ifinatamab deruxtecan (I-DXd), an antibody-drug conjugate (ADC) designed to treat one of the deadliest cancers in oncology. The FDA's action date had been October 10. It never got that far.
The reason? After talking with regulators, the companies concluded their data package "did not satisfy the requirements for accelerated approval." In plain English: the FDA told them the evidence wasn't strong enough, and the companies decided to walk away rather than get formally rejected. Think of it as pulling your college application before the admissions office sends a rejection letter. The outcome is the same, but the optics are slightly less painful.
This isn't just another regulatory hiccup. It's a meaningful stumble for one of the biggest pharma partnerships on the planet.
A Partnership Worth More Than Some Countries' GDP
Back in October 2023, Merck and Daiichi Sankyo struck a massive collaboration covering three ADC programs. The deal's potential value? Roughly $22 billion across all three drugs, with $4 billion paid upfront and billions more tied to milestones. Ifinatamab deruxtecan was one of three ADCs in that agreement, alongside patritumab deruxtecan (targeting HER3) and raludotatug deruxtecan.
The structure gave the two companies joint global development and commercialization rights, splitting profits 50/50 outside Japan. Daiichi Sankyo kept exclusive Japanese rights and handles all manufacturing. It was, by any measure, a blockbuster deal built on the premise that ADCs are the future of cancer treatment.
I-DXd was supposed to be an early proof point. Instead, it became an early headache.
The Cancer That Refuses to Be Tamed
Extensive-stage small cell lung cancer (ES-SCLC) is the kind of diagnosis oncologists dread delivering. Most patients already have widespread disease when they're found. Treatment is palliative; the goal isn't to cure, it's to buy time.
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The current standard of care is a combination of platinum-based chemotherapy, etoposide, and a PD-L1 inhibitor like atezolizumab or durvalumab. When patients relapse (and most do), options get thin fast: lurbinectedin, topotecan, irinotecan, or tarlatamab, depending on the situation.
Survival gains from modern treatments remain modest. Long-term survivors are rare. The need for better drugs in this space is enormous, which is exactly why I-DXd's stumble stings so much.
Good Data, But Not Good Enough
The application was built on results from the IDeate-Lung01 phase 2 trial. And honestly, the numbers weren't terrible. In the 12 mg/kg dose group (137 patients), the confirmed response rate was 48.2%, with responses lasting a median of 5.3 months. For a cancer this aggressive, nearly half of patients responding is noteworthy.
Dig deeper, and the picture gets more interesting. Second-line patients responded at a rate of 56.3%, while those treated in third line or later still showed a 45.7% response rate.
So why did the FDA balk? Accelerated approval is a special pathway that lets drugs reach patients faster based on surrogate endpoints (like response rates) before full survival data is available. But the bar isn't "good results"; it's "results that reasonably predict clinical benefit." The agency apparently wasn't convinced that this data package cleared that bar. A phase 2 trial with 137 patients, however promising, may simply not have been robust enough to satisfy regulators who've been tightening standards across oncology.
Wall Street Shrugged (Mostly)
Analysts treated the withdrawal as a setback, not a catastrophe. Merck is a diversified giant; losing one near-term catalyst hurts sentiment but doesn't reshape the investment thesis overnight. The bigger concern is what this signals about the I-DXd program's timeline.
The good news: Daiichi Sankyo and Merck aren't abandoning the drug. A phase 3 trial called IDeate-Lung02 is still enrolling patients, and the companies have said they'll continue development. If that larger, more rigorous study delivers positive results, the filing can be resubmitted with a much stronger evidence package.
The bad news: that takes time. Years, potentially. And in oncology, time is the one thing patients don't have.
The ADC Arms Race Isn't Slowing Down
I-DXd targets a protein called B7-H3. But it's not the only ADC chasing small cell lung cancer. The competitive landscape is crowded and getting more intense by the quarter.
DLL3-targeting ADCs are arguably the hottest subcategory right now. Zocilurtatug pelitecan (ZL-1310) has shown clinical benefit in a phase 1 study, including in patients with brain metastases. SHR-4849 and IBI3009 (now under Roche's global rights) are also DLL3-directed programs moving through early development. AbbVie's ABBV-706 takes a different approach, targeting SEZ6. And sacituzumab govitecan (Trodelvy), a TROP2-directed ADC, has shown some activity signals in pretreated ES-SCLC, though its broader lung cancer track record has been mixed.
Across the field, one theme keeps emerging: topoisomerase I inhibitor payloads (the toxic "warhead" that ADCs deliver to cancer cells) appear to be the most consistently active class, regardless of which protein the antibody targets. The race isn't just about picking the right target on the cancer cell; it's about the entire package of antibody, linker chemistry, and payload.
What Comes Next
The I-DXd story isn't over. The phase 3 trial is still running, and a positive readout there would give the companies exactly the kind of evidence the FDA wants. The $22 billion partnership remains intact, and the other two ADCs in the Merck-Daiichi deal continue their own development paths.
But this withdrawal is a reminder that even the hottest technology platforms have speed limits. ADCs have been the darlings of oncology for the past few years, propelled by the spectacular success of Enhertu (another Daiichi Sankyo ADC). The assumption has been that the DXd platform can do no wrong.
Turns out, it can. And in a disease as brutal as small cell lung cancer, the FDA isn't going to lower the bar just because the science looks cool. The data has to be undeniable. For I-DXd, "promising" wasn't enough.
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