Today’s forecast inside the tumor microenvironment: cloudy with a chance of immune evasion, scattered HER2 signals, and one very suspicious front moving in from the antibody-drug conjugate coast.
That is the biological weather report behind a new Nature Cancer study from the BEGONIA platform trial, where researchers tested durvalumab plus trastuzumab deruxtecan, mercifully shortened to T-DXd, as first-line treatment for women with locally advanced unresectable or metastatic hormone-receptor-negative, HER2-low breast cancer Schmid et al., 2026.
Translation: this is a tough breast cancer neighborhood. Hormone-receptor-negative means the tumor is not conveniently fueled by estrogen or progesterone, so hormone-blocking therapies are not the answer. HER2-low means the cancer has some HER2 protein on the surface, but not enough to qualify as classic HER2-positive disease. Historically, that was oncology’s version of saying, “Technically there’s a doorbell, but no one thought we could use it.”
T-DXd has changed that.
The Smart Bomb With a Guest Pass
T-DXd is an antibody-drug conjugate, which is basically a guided missile wearing a lab coat. The antibody part looks for HER2. The drug payload rides along like extremely toxic luggage. Once the drug reaches a cancer cell, it delivers chemotherapy more directly than old-school “everyone duck” chemotherapy.
Even better, T-DXd has a bystander effect. After it enters HER2-expressing cells, its payload can affect nearby tumor cells too, including ones with less HER2. In a mixed-up tumor, where some cells wear the HER2 nametag and others are pretending they forgot theirs at home, that matters.
Durvalumab plays a different role. It blocks PD-L1, one of the “nothing to see here” signals tumors use to calm down immune cells. Your immune system is basically running a 24/7 security service, and PD-L1 is one of the fake badges tumors flash at the front desk.
So the idea is elegant: T-DXd damages tumor cells and may make the area more immune-visible, while durvalumab tries to stop the tumor from shushing the T-cells. A tiny cellular buddy-cop movie, except everyone is wearing gloves and the soundtrack is peer review.
What Did BEGONIA Find?
This BEGONIA arm enrolled 58 women with untreated advanced or metastatic HR-negative, HER2-low breast cancer. They received durvalumab and T-DXd every three weeks. The study was open-label and phase 1b/2, meaning everyone knew what treatment was being given, and this was not yet the giant randomized trial that changes guidelines overnight while oncologists spill coffee on their keyboards.
The main result: 36 of 58 participants had a confirmed objective response, for an objective response rate of 62.1%. That is a lot of tumors shrinking. But the study had set a prespecified goal of 38 responses among 57 evaluable participants, or 66.6%, so technically it missed that bar.
This is where cancer research gets annoyingly nuanced, because of course it does. Missing the formal threshold does not mean “nothing happened.” It means the result should be treated with caution, especially because this was a single-arm study without a direct comparison group. Still, the signals were notable: median duration of response was 15.2 months, median progression-free survival was 12.6 months, and median overall survival was 30.3 months.
For metastatic HR-negative breast cancer, those numbers get attention. Not confetti-cannon attention. More like “everyone in the room sits up a little straighter” attention.
The PD-L1 Plot Twist
One especially interesting wrinkle: most participants had PD-L1-negative tumors. That matters because checkpoint inhibitors tend to work better in PD-L1-positive triple-negative breast cancer. In KEYNOTE-355, pembrolizumab plus chemotherapy improved overall survival mainly in patients whose tumors had higher PD-L1 expression Cortes et al., 2022.
Here, responses appeared in both PD-L1-positive and PD-L1-negative groups. Tiny sample sizes apply, so please keep your statistical seatbelt fastened. But if future trials confirm this, the combination might help some patients who do not neatly fit the usual immunotherapy-friendly category.
Cancer biology loves categories until Tuesday, then it starts eating the labels.
Why HER2-Low Became a Big Deal
HER2-low breast cancer became clinically important because T-DXd showed survival benefits where older HER2-targeted therapies had not. In DESTINY-Breast04, T-DXd improved progression-free and overall survival compared with physician’s choice chemotherapy in previously treated HER2-low metastatic breast cancer Modi et al., 2022. Longer follow-up supported T-DXd as a standard option after prior chemotherapy Modi et al., 2025.
DESTINY-Breast06 pushed the idea further in HR-positive disease, showing longer progression-free survival for HER2-low and HER2-ultralow metastatic breast cancer after endocrine therapy Bardia et al., 2024.
The BEGONIA study asks a slightly different question: can we move an antibody-drug conjugate plus immunotherapy earlier for HR-negative, HER2-low disease? If larger trials reproduce the result, this could mean more first-line options for a group that badly needs them.
The Catch, Because Biology Always Brings Snacks
Safety matters here. Nearly all participants had adverse events. Grade 3 or 4 events occurred in 48.3%. Nausea, fatigue, constipation, neutropenia, and vomiting were common. The lung toxicity signal needs special respect: adjudicated drug-related interstitial lung disease or pneumonitis occurred in 20.7%, mostly grade 1 or 2, but one participant had a grade 5 event.
That is the sober part. T-DXd can be powerful, but lung inflammation is not a footnote. It is a “call your care team early if breathing changes” situation.
So the takeaway is not “new miracle combo, please cue dramatic orchestra.” It is better than that: a plausible, biologically smart strategy showed meaningful activity in a hard-to-treat setting, including many PD-L1-negative tumors. Now it needs the grown-up test: larger randomized trials, careful safety monitoring, and better ways to identify who is most likely to benefit.
Cancer research rarely hands us clean answers. More often it gives us a promising clue, a pile of caveats, and a headache with a DOI. This one is worth watching.
References
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Schmid P, Im S-A, Nowecki Z, et al. First-line durvalumab in combination with trastuzumab deruxtecan in women with locally advanced unresectable or metastatic, hormone-receptor-negative, HER2-low breast cancer: multicenter, open-label, phase 1b/2 BEGONIA platform trial. Nature Cancer. 2026. DOI: 10.1038/s43018-026-01181-8
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Modi S, Jacot W, Yamashita T, et al. Trastuzumab deruxtecan in previously treated HER2-low advanced breast cancer. New England Journal of Medicine. 2022;387:9-20. DOI: 10.1056/NEJMoa2203690. PMCID: PMC10561652
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Cortes J, Rugo HS, Cescon DW, et al. Pembrolizumab plus chemotherapy in advanced triple-negative breast cancer. New England Journal of Medicine. 2022;387:217-226. DOI: 10.1056/NEJMoa2202809
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Bardia A, Hu X, Dent R, et al. Trastuzumab deruxtecan after endocrine therapy in metastatic breast cancer. New England Journal of Medicine. 2024;391:2110-2122. DOI: 10.1056/NEJMoa2407086
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Modi S, Jacot W, Yamashita T, et al. Trastuzumab deruxtecan in HER2-low metastatic breast cancer: long-term survival analysis of the randomized, phase 3 DESTINY-Breast04 trial. Nature Medicine. 2025. DOI: 10.1038/s41591-025-03981-4. PMCID: PMC12705423
Disclaimer: The image accompanying this article is for illustrative purposes only and does not depict actual experimental results, data, or biological mechanisms.