Show notes
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 quality of life on T-DXd indefinitely in the first-line setting is a big deal because, again, that CLEOPATRA regimen allows patients to drop their chemotherapy component about five to six months in. And with this, you're on a drug that feels very chemo-heavy indefinitely. And so, I think there's a lot more to investigate as far as what we're going to do with this data in the United States because it's a lot to commit a patient in the first-line metastatic setting. These de novo metastatic patients, some of them may be cured, honestly, on the HER2-targeting regimen. That's something we see these days. Dr. Allison Zibelli: So, very interesting trial. I'm sure we'll be talking about this for a long time. So, let's move on to SERENA-6, which was, I thought, a very interesting trial. This trial took patients with ER positive, advanced breast cancer after six months on an AI (aromatase inhibitor) and a CDK4/6 inhibitor. They did ctDNA every two to three months, and when they saw an ESR1 mutation emerge, they changed half of the patients to camizestrant plus CDK4/6 and kept the other half on the AI plus CDK4/6. Can you talk about that trial a little bit, please? Dr. Rebecca Shatsky: Yeah, so this was a big trial at ASCO25. This was presented as a Plenary Session. So, this was camizestrant plus a CDK4/6 inhibitor, and it could have been any of the three, so palbo, ribo, or abemaciclib in the first-line metastatic hormone-positive population, and patients were on an AI with that. They were, interestingly, tested by ctDNA at baseline to see if they had an ESR1 mutation. So, that was an interesting feature of this trial. But patients had to have already been on their CDK4/6 inhibitor plus AI for at least 6 months to enroll. And then, as you mentioned, they got ctDNA testing every 2 to 3 months. This was also a phase 3, double-blind, international trial. And I do want to highlight again, international here, because that's important when we're considering some of this data in the U.S. because it influences some of the results. So, this was presented by Dr. Nick Turner of the Royal Marsden in the UK. So, just a little bit of background for our listeners on ESR1 mutations and why they're important. This is the most common, basically, acquired resistance mutation to patients being treated with aromatase inhibitors. We know that treatment with aromatase inhibitors can induce this. It makes a conformational change in the estrogen receptor that makes the estrogen receptor constitutively active, which allows the cell to signal despite the influence of the aromatase inhibitor to decrease the estrogen production so that the ligand binding doesn't matter as much as far as the cell signaling and transcription is concerned. And camizestrant, you know, as an oral SERD, just to explain that a little bit too; these are estrogen receptor degraders. The first-in-class of a selective estrogen receptor degrader to make it to market was fulvestrant. And that's really been our standard-of-care estrogen degrader for the past 25 years, almost 25 years. And so, a lot of us are just looking for some of these oral SERDs to replace that. But regardless, they do tend to work in the ESR1-mutated population. And we know that patients on aromatase inhibitors, the estimates of patients developing an ESR1 mutation, depending on which study you look at, somewhere between 30% to 50% overall, patients will develop this mutation with hormone-positive metastatic breast cancer. There is a small percentage of patients that have these at baseline without even treatment of an aromatase inhibitor. The estimates of that are somewhere between 0.5 and up to 5%, depending on the trial you look at and the population. But regardless, there is a chance someone on their CDK4/6 inhibitor plus AI at 6 months' time course could have had an ESR1 mutation at that time. But anyway, so they got this ctDNA every 2 to 3 months, and once they were found to develop an ESR1 mutation, the patients were then switched to the oral SERD. AstraZeneca's version of the oral SERD is camizestrant, 75 mg daily. And then their type of CDK4/6 inhibitor was maintained, so they didn't switch the brand of their CDK4/6 inhibitor, importantly. And that was looked at then for progression-free survival, but these were patients with measurable disease by RECIST version 1.1. And the data cut off here was November of 2024. This was a big trial, you know, and I think that that's influential here because this was 3,256 patients, and that's a lot of patients. So, they were all eligible. And then 315 patients ended up being randomized to switch to camizestrant upon presence of that ESR1 mutation. So, that was 157 patients. And then the other half, so they were randomized 1:1, they continued on their AI without switching to an oral SERD. That was 158 patients. They were matched pretty well. And so, their baseline characteristics, you know, the two subgroups was good. But this was highly statistically significant data. I'm not going to diminish that in any way. Your hazard ratio was 0.44. Highly statistically significant confidence intervals. And you had a median progression-free survival in those that switched to camizestrant of 16 months, and then the non-switchers was 9.2 months. So, the progression-free survival benefit there was also consistent across the subgroups. And so, you had at 12 months, the PFS rate was 60.7% for the non-treatment group and 33.4% in the treatment group. What's interesting, though, is we don't have overall survival data. This is really immature, only 12% mature as far as overall survival. And again, because this was an international trial and patients in other countries right now do not have the access to oral SERDs that the United States does, the crossover rate, they were not allowed to crossover, and so, a very few patients, when we look at progression-free survival 2 and ultimately overall survival, were able to access an oral SERD in the off-trial here and in the non-treatment group. And so, that's really important as far as we look at these results. Adverse events were pretty minimal. These are very safe drugs, camizestrant and all the other oral SERDs. They have some mild toxicities. Camizestrant is known for something weird, which is called photopsia, which is some flashing lights in the periphery of the eye, but it doesn't seem to have any serious clinical significance that we know of. It has a little bit of bradycardia, but it's otherwise really well tolerated. You know, I hate to say that because that's very subjective, right? I'm not the one taking the drug. But it doesn't have any serious adverse events that would cause discontinua…
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