Antibody-drug conjugates have moved from late-line options to first-line anchors in breast cancer, so the competitive question is no longer whether they work. It is which ADC a patient receives first, and whether a second ADC still works after it. AstraZeneca, Daiichi Sankyo and Gilead are now competing to define that sequence, not just to add another approved option to an already crowded class.
Why is the ADC sequencing map getting more crowded?
Breast cancer has never been one treatment market. Decisions are already shaped by tumour subtype, biomarker status, disease setting, prior therapy and patient-specific factors, and antibody-drug conjugates add another layer on top of all of it. The practical path a clinician now has to reason through runs from subtype and biomarker, to line of therapy, to which ADC to use, to a combination strategy, to what comes next.
That layering matters because an ADC that moves into an earlier line of therapy does more than expand the population eligible to receive it. It can redefine what remains viable afterward, since prior ADC exposure changes how a tumour responds to a second one and narrows what a clinician can reach for once the first has been used.
The result is a landscape where individual assets can no longer be evaluated in isolation from each other. An ADC's value increasingly depends on where it sits in a treatment pathway that multiple companies are still actively trying to write, rather than on its standalone trial data.
- 01
Confirm the biomarker
Establish HER2 or TROP2 status and prior therapy before selecting the ADC.
- 02
Place the line of therapy
Decide whether the asset is positioned for first line or a later, post-ADC setting.
- 03
Plan the combination
Evaluate pairing with HER2-targeted therapy, endocrine therapy or immunotherapy.
- 04
Evidence the sequence after it
Build data on what still works once a patient has progressed on this ADC.
How did ADCs move from late-line option to treatment anchor?
In December 2025, the FDA approved Enhertu plus pertuzumab for first-line treatment of unresectable or metastatic HER2-positive breast cancer. AstraZeneca and Daiichi Sankyo reported that the combination reduced the risk of disease progression or death by 44 percent compared with the previous standard THP regimen in the pivotal study, a result the companies' oncology leadership described as having the potential to redefine the first-line standard of care.
The importance of that shift goes beyond one label expansion. Moving Enhertu into first line changes the post-progression conversation for every patient who would previously have received the ADC only after other options had been exhausted, since the sequence a clinician can offer afterward now starts from a different point.
AstraZeneca and Daiichi Sankyo are also seeking to move Enhertu earlier still, into HER2-positive early breast cancer. In March 2026, the companies announced US Priority Review for Enhertu in the post-neoadjuvant setting for patients with residual invasive disease, based on data showing a 53 percent reduction in the risk of invasive disease recurrence or death compared with T-DM1. The direction is consistent: an ADC's value is increasingly tied to whether it can become a treatment anchor rather than a treatment reserved for after progression.
Why is TROP2 becoming a sequencing test case?
The same sequencing question is emerging in triple-negative and HR-positive, HER2-negative breast cancer through the TROP2 class. Gilead's Trodelvy established an important position in metastatic triple-negative breast cancer, and Datroway, developed by AstraZeneca and Daiichi Sankyo, is now moving into adjacent and earlier settings.
In May 2026, the FDA approved Datroway for adults with unresectable or metastatic triple-negative breast cancer who are not candidates for PD-1 or PD-L1 inhibitor therapy, with the companies describing it as the first TROP2-directed antibody-drug conjugate approved for first-line use in metastatic triple-negative disease. The European Medicines Agency's Committee for Medicinal Products for Human Use issued a positive opinion for the same first-line setting the following month.
The immediate competitive question is not simply whether Datroway can take share from Trodelvy. It is whether the TROP2 class creates a new treatment sequence in which clinicians choose between ADCs based on disease setting, prior therapy and the ability to use another ADC afterward, an open question a multicentre study is now examining directly by testing sequencing of Enhertu and Datroway in both directions.
| United States | European Union | |
|---|---|---|
| Primary change mechanism | FDA approval and guideline evolution | Regulatory action followed by country-level practice |
| Speed to routine use | Can shift within months of an approval | Often lags behind the approval by a period of years |
| Key challenge for companies | Rapid medical education around new treatment order | Country-sensitive evidence communication |
What decides sequencing besides efficacy?
ADCs are often discussed through headline efficacy results, and those results matter, but they are unlikely to be the only thing that decides sequencing in practice. Safety and tolerability, including interstitial lung disease, neutropenia and ocular toxicity risk depending on the specific payload, shape which line of therapy a given ADC is comfortable to use in and what a clinician will accept before or after another one.
Combination potential matters just as much: whether an ADC can be paired with HER2-targeted therapy, endocrine therapy or immunotherapy affects whether it can move into earlier, combination-heavy settings rather than being confined to monotherapy in later lines. And post-progression options, whether a clinician believes another ADC will still work after the first, increasingly shape which asset gets used at which point, independent of either drug's standalone efficacy number.
This creates a more demanding environment for a company developing a novel ADC. A new target or payload is not enough on its own if the asset cannot answer a practical sequencing question: where does it belong in the pathway, and what does it leave available afterward for the patient who eventually progresses.
Does the US and EU reach the same sequencing conclusions?
The US and EU draw on much of the same clinical evidence, but the path from data to routine treatment is not identical. In the US, FDA approvals and guideline updates can shift the clinical conversation quickly, and Enhertu's first-line HER2-positive approval and Datroway's first-line triple-negative approval both show how a single new indication can change the treatment map in a matter of months.
In the EU, the regulatory pathway is followed by country-level decisions around adoption, clinical practice and implementation. Datroway's positive CHMP opinion in June 2026 is an important regulatory step, but each country's own clinical pathway and treatment practice determines how quickly the option actually reaches patients, which can lag well behind the US timeline even for an identical approval.
For a company planning a global sequencing strategy, that gap means the pathway a US oncologist follows and the pathway a European oncologist follows for the same tumour subtype can diverge for a period of years, even when both are working from the same underlying trial data.