Dr. Allison Zibelli and Dr. Rebecca Shatsky discuss advances in breast cancer research that were presented at the 2025 ASCO Annual Meeting, including a potential new standard of care for HER2+ breast cancer, the future of ER+ breast cancer management, and innovations in triple negative breast cancer therapy.

Transcript

Dr. Allison Zibelli: Hello and welcome to the ASCO Daily News Podcast. I'm Dr. Allison Zibelli, your guest host of the podcast today. I'm an associate professor of medicine and a breast medical oncologist at the Sidney Kimmel Comprehensive Cancer Center at Jefferson Health.

There was a substantial amount of exciting breast cancer data presented at the 2025 ASCO Annual Meeting, and I'm delighted to be joined by Dr. Rebecca Shatsky today to discuss some of these key advancements. Dr. Shatsky is an associate professor of medicine at UC San Diego and the head of breast medical oncology at the UC San Diego Health Moores Cancer Center, where she also serves as the director of the Breast Cancer Clinical Trials Program and the Inflammatory and Triple-Negative Breast Cancer Program.

Our full disclosures are available in the transcript of this episode.

Dr. Shatsky, it's great to have you on the podcast today.

Dr. Rebecca Shatsky: Thanks, Dr. Zibelli. It's wonderful to be here.

Dr. Allison Zibelli: So, we’re starting with DESTINY-Breast09, which was trastuzumab deruxtecan and pertuzumab versus our more standard regimen of taxane, trastuzumab pertuzumab for first-line treatment of metastatic HER2-positive breast cancer. Could you tell us a little bit about the study?

Dr. Rebecca Shatsky: Yeah, absolutely. So, this was a long-awaited study. When T-DXd, or trastuzumab deruxtecan, really hit the market, a lot of these DESTINY-Breast trials were started around the same time. Now, this was a global, randomized, phase 3 study presented by Dr. Sara Tolaney from the Dana-Farber Cancer Institute of Harvard in Boston. It was assessing essentially T-DXd in the first-line setting for metastatic HER2-positive breast cancer in addition to pertuzumab. And that was randomized against our standard-of-care regimen, which was established over a decade ago by the CLEOPATRA trial, and we've all been using that internationally for at least the past 10 years.

So, this was a large trial, and it was one-to-one-to-one of patients getting T-DXd plus pertuzumab, T-DXd alone, or THP, which mostly is used as docetaxel and trastuzumab and pertuzumab every three weeks for six cycles. And this was in over 1,000 patients; it was 1,159 patients with metastatic HER2-positive breast cancer. This was a very interesting trial. It was looking at the use of trastuzumab deruxtecan, but patients were started on this treatment for their first-line metastatic HER2-positive breast cancer with no end date to their T-DXd. So, it was, you know, you were started on T-DXd every 3 weeks until progression.

Now, CLEOPATRA is a little bit different than that, though, as we know. So, CLEOPATRA has a taxane plus trastuzumab and pertuzumab. But generally, patients drop the taxane after about six to seven cycles because, as we know, you can't be really on a taxane indefinitely. You get pretty substantial neuropathy as well as cytopenias, other things that end up happening. And so, in general, that regimen has sort of a limited time course for its chemotherapy portion, and the patients maintained after the taxane is dropped on their trastuzumab and their pertuzumab, plus or minus endocrine therapy if the investigator so desires.

And the primary endpoint of the trial was progression-free survival by blinded, independent central review (BICR) in the intent-to-treat population. And then it had its other endpoints as overall survival, investigator-assessed progression-free survival, objective response rates, and duration of response, and of course, safety.

As far as the results of this trial, so, I think that most of us key opinion leaders in breast oncology were expecting that this was going to be a positive trial. And it surely was. I mean, this is a really, really active drug, especially in HER2-positive disease, of course. So, the DESTINY-Breast03 data really established that, that this is a very effective treatment in HER2-positive metastatic breast cancer. And this trial really, again, showed that. So, there were 383 patients that ended up on the trastuzumab plus deruxtecan plus pertuzumab arm, and 387 got THP, the CLEOPATRA regimen.

What was really interesting also to note of this before I go on to the results was that 52% of patients on this trial had de novo metastatic disease. And that's pretty unusual for any kind of metastatic breast cancer trial. It kind of shows you, though, just how aggressive this disease is, that a lot of patients, they present with de novo metastatic disease. It's also reflecting the global nature of this trial where maybe the screening efforts are a little bit less than maybe in the United States, and more patients are presenting as later stage because to have a metastatic breast cancer trial in the United States with 52% de novo metastatic disease doesn't usually happen.

But regardless, the disease characteristics were pretty well matched between the two groups. 54% of the patients were triple positive, or you could say hormone-positive because whether they were PR positive or ER positive and PR negative doesn't really matter in this disease. And so, the interim data cutoff was February of this year, of 2025. So, the follow-up so far has been about 29 months, so the data is still really immature, only 38% mature for progression-free survival interim analysis.

But what we saw is that T-DXd plus pertuzumab, it really improved progression-free survival. It had a hazard ratio that was pretty phenomenal at 0.56 with a confidence interval that was pretty narrow of 0.44 to 0.71. So, very highly statistically significant data here. The progression-free survival was consistent across all subgroups. Overall survival, very much immature at this time, but of course, the trend is towards an overall survival benefit for the T-DXd group. The median durable response with T-DXd plus pertuzumab exceeded 3 years.

Now, importantly, though, I want to stress this, is grade 3 or above treatment-emergent adverse events occurred in both subgroups pretty equally. But there were 2 deaths in the T-DXd group due to interstitial lung disease. And there was a 12.1% adjudicated drug-induced interstitial lung disease/pneumonitis event rate in the T-DXd group and only 1%, and it was grade 1-2, in the THP group. So, that's really the caveat of this therapy, is we know that a percentage of patients are going to get interstitial lung disease, and that some may have very serious adverse events from it. So, that's always something I keep in the back of my mind when I treat patients with T-DXd.

And so, overall, the conclusions of the trial were pretty much a slam dunk. T-DXd plus pertuzumab, it had a highly statistically significant and clinically meaningful improvement in progression-free survival versus the CLEOPATRA regimen. And that was across all subgroups for first-line metastatic HER2-positive breast cancer here. And so, yeah, the data was pretty impressive. Just to go into the overall response rate, because that's always super important as well, you had 85.1% of patients having a confirmed overall RECIST response rate in the T-DXd plus pertuzumab group and a 78.6 in the CLEOPATRA group. The complete CR rate, complete response was 15.1% in the T-DXd group and 8.5 in the CLEOPATRA regimen. And it was really an effective regimen in this group, of course.

Dr. Allison Zibelli: So, the investigators say at the end of their abstract that this is the new standard of care. Would you agree with that statement?

Dr. Rebecca Shatsky: Yeah, that was a bold statement to make because I would say in the United States, not necessarily at the moment because the quality of life here, you have to think really hard about. Because one thing that's really important about the DESTINY-Breast09 data is that this was very much an international trial, and in many of the countries where patients enrolled on this, they were not able to access T-DXd off trial. And so, for them, this means T-DXd now or potentially never. And so, that is a really big difference whereas internationally, that may mean standard of care.

However, in the US, patients have no issues accessing T-DXd in the second- or third-line settings. And right now, it's the standard of care in the second line in the United States, with all patients basically getting this second-line therapy except for some unique patients where they may be doing a PATINA trial regimen, which we saw at San Antonio Breast Cancer in 2024 of the triple-positive patients getting hormonal therapy plus palbociclib, which had a really great durable response. That was super impressive as well. Or there is the patient that the investigator can pick KADCYLA because the patient really wants to preserve their hair or maybe it's more indolent disease.

But the quali...