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 discontinuation. And that's really what we saw in the trial. The discontinuation rates were really low.

But overall, I mean, this was a positive trial. SERENA-6 showed that switching to camizestrant at the first sign of an ESR1 mutation on CDK4/6 inhibitor plus AI improved progression-free survival. That's all we can really say from it right now.

Dr. Allison Zibelli: So, let's move on to ASCENT-04, which was a bit more straightforward. Sacituzumab govitecan plus pembrolizumab versus chemotherapy plus pembrolizumab in PD-L1-positive, triple-negative breast cancer. Could you talk about that study?

Dr. Rebecca Shatsky: Yeah, so this was also presented by the lovely Sara Tolaney from Dana-Farber. And this study made me really excited. And maybe that's because I'm a triple-negative breast cancer person. I mean, not to say that I don't treat hundreds of patients with hormone- positive, but our unmet needs in triple negative are huge because this is a disease where you have got to throw your best available therapy at it as soon as you can to improve survival because survival is so poor in this disease. The average survival with metastatic triple-negative breast cancer in the United States is still 13-18 months, and that's terrible.

And so, for full disclosure, I did have this trial open at my site. I was one of the site PIs. I'm not the global PI of the study, obviously. So, what this study was was for patients who had had at least a progression-free survival of 6 months after their curative intent therapy or de novo metastatic disease. They were PD-L1 positive as assessed by the Dako 22C3 assay of greater than or equal to a CPS score of 10. So, that's what the KEYNOTE-355 trial was based on as well. So, standard definition of PD-L1 positive in breast cancer here.

And basically, these patients were randomized 1:1 to either their sacituzumab govitecan plus pembrolizumab, day 1 they got both therapies, and then day 8 just the saci, as is standard for sacituzumab. And then the other group got the KEYNOTE-355 regimen. So, that is pembrolizumab with – your options are carbogem there, paclitaxel or nab-paclitaxel. And it's up to investigator's decision which upon those they decided. They followed these patients for disease progression or unacceptable toxicity.

It was really an impressive trial in my opinion because we know already that this didn't just improve progression-free survival, because survival is so poor in this disease, of course, we know that it improved overall survival. It's trending towards that very much, and I think that's going to be shown immediately. And then the objective response rates were better, which is key in this disease because in the first-line setting, you've got a lot of people who, especially your relapsed TNBC that don't respond to anything. And you lose a ton of patients even in the first-line setting in this disease.

And so, this was 222 patients to chemotherapy and pembro and 221 to sacituzumab plus pembro. Median follow-up has only been 14 months, so it's still super early here. Hazard ratio so far of progression-free survival is 0.65, highly statistically significant, narrow confidence intervals. And so, the median duration of response here for the saci group was 16.5 months versus 9.2 months. So, you're getting a 7-month progression-free survival benefit here, which in triple negative is pretty fantastic. I mean, this reminds me of when we saw the ASCENT data originally come out for sacituzumab, and we were all just so happy that we had this tool now that doubled progression-free and overall survival and made such a difference in this really horrible disease where patients do poorly.

So, OS is technically immature here, but it's really trending very heavily towards improvement in overall survival. Importantly, the treatment-related adverse events in this, I mean, we know sacituzumab causes neutropenia, people who are experienced with this drug know how to manage it at this point. There wasn't any really unexpected treatment-related adverse events. You get some people with sacituzumab who have diarrhea. It's usually pretty manageable with some Imodium. So, it was cytopenias predominantly in this disease in this population that were highlighted as far as adverse events.

But I'm going to be honest, like I was surprised that this wasn't the plenary over the SERENA-6 data because this, in my mind, there we have a practice-changing trial. I will immediately be trying to use this in my PD-L1 population because, to be honest, as a triple-negative breast cancer clinical specialist, when I get a patient with metastatic triple-negative breast cancer who's PD-L1 positive, I think, "Oh, thank God," because we know that part of the disease just does better in general. But now I have something that really could give them a durable response for much longer than I ever thought possible when I started really heavily treating this disease. And so, this was immediately practice-changing for me.

Dr. Allison Zibelli: I think that it's pretty clear that this is at least an option, if not the option, for this group of patients.

Dr. Rebecca Shatsky: Yeah, the duration of responses here was – it's just really important because, I mean, I do think this will make people live longer.

Dr. Allison Zibelli: So, moving on to the final study that we're going to discuss today, neoCARHP (LBA500), which was neoadjuvant taxane plus trastuzumab, pertuzumab, plus or minus carbo(platin) in HER2-positive early breast cancer. I think this is a study a lot of us have been waiting for. What was the design and the results of this trial?

Dr. Rebecca Shatsky: I was really excited about this as well because I'm one of those people that was waiting for this. This is a Chinese trial, so that is something to take note of. It wasn't an international trial, but it was a de-escalation trial which had become really popular in HER2-positive therapy because we know that we're overtreating HER2-positive breast cancer in a lot of patients. A lot of patients we're throwing the kitchen sink at it when maybe that is not necessary, and we can really de-escalate and try to personalize therapy a little bit better because these patients tend to do well.

So, the standard of care, of course, in HER2-positive curative intent breast cancer with tumors that are greater than 2 cm is to give them the TCHP regimen, which is docetaxel, carboplatin, trastuzumab, and pertuzumab. And that was sort of established by several trials in the NeoSphere trial, and now it's been repeated in a lot of different studies as well. And so, that's really the standard of care that most people in the United States use for HER2-positive curative intent breast cancer.

This was a trial to de-escalate the carboplatin, which I was super excited about because many of us who treat this disease a lot think carbo is the least important part of the therapy you're giving there. We don't really know that it's necessary. We've just been doing it for a long time, and we know that it adds a significant amount of toxicity. It causes thrombocytopenia, it causes severe nausea, really bad cytopenias that can be difficult in the last few cycles of this to manage.

So, this trial was created. It randomized patients one to one with stage 2 and 3 HER2-positive breast cancer to either get THP, a taxane, pertuzumab, trastuzumab, similar to the what we do in first-line metastatic HER2-positive versus the whole TCHP with a carboplatin AUC of 6, which is what's pretty standard. And it was a non-inferiority trial, so important there. It wasn't to establish superiority of this regimen, which none of us, I think, were looking for it to. And it was a modified intent-to-treat population. And so, all patients got at least one cycle of this to be assessed as a standard for an intent-to-treat trial. And so, they assumed a pCR rate of about 62.8% for both groups. And, of course, it included both HER2-positive triple positives and ER negatives, which are, you know, a bit different diseases, to be honest, but we all kind of categorize them and treat them the same.

And so, this trial was powered appropriately to detect a non-inferiority difference. And so, we had about 380 patients treated on both arms, and there was an absolute difference of only 1.8% of those treated with carbo versus those without. Which was fantastic because you really realized that de-escalation here may be something we can really do.

And so, the patients who got, of course, the taxane regimen had fewer adverse events. They had way fewer grade 3 and 4 adverse events than the THP group. No treatment-associated deaths occur, which is pretty standard for- this is a pretty safe regimen, but it causes a lot of hospitalizations due to diarrhea, due to cytopenias, and neutropenic fever, of course.

And so, I thought that this was something that I could potentially enact, you know, and be practice-changing. It's hard to say that when it's a trial that was only done in China, so it's not necessarily the United States population always. But I think for patients moving forward, especially those with, say, a 2.5 cm tumor, you know, node negative, those, I'd feel pretty comfortable not giving them the carboplatin here.

Notes that I want to make about this population is that the majority were stage 2 and not stage 3. They weren't necessarily your inflammatory HER2-positive breast cancer patients. And that the taxane that was utilized in the trial is a little different than what we use in the United States. The patients were allowed to get nab-paclitaxel, which we don't have FDA approval for in the first-line curative intent setting for HER2-positive breast cancer in the United States. So, a lot of them got abraxane, and then they also got paclitaxel. We tend to use docetaxel every 3 weeks in the United States. So, just to point out that difference. We don't really know if that's important or not, but it's just a little bit different to the population we standardly treat.

Dr. Allison Zibelli: So, are there patients that you would still give TCHP to?

Dr. Rebecca Shatsky: Yeah, great question. I've been asked that a lot in the past like week since ASCO. I'd say in my inflammatory breast cancer patients, that's a group I do tend to sometimes throw the kitchen sink at. Now, I don't actually use AC in those because I know that that was the concern, but I think the TRAIN-2 trial really showed us you don't need to use Adriamycin in HER2-positive disease unless it's like refractory. So, I don't know that I would throw this on my stage 3C or inflammatory breast cancer patients yet because the majority of this were not stage 3. So, in your really highly lymph node positive patients, I'm a little bit hesitant to de-escalate them from the start. This is more of a like, if there's serious toxicity concerns, dropping carbo is absolutely fine here.

Dr. Allison Zibelli: All right, great.

Thank you, Dr. Shatsky, for sharing your valuable insights with us on the ASCO Daily News Podcast today.

Dr. Rebecca Shatsky: Thanks so much, Dr. Zibelli and ASCO Daily News. I really want to thank you for inviting me to talk about this today. It was really fun, and I hope you find my opinions on some of this valuable. And so, I just want to thank everybody and my listeners as well.

Dr. Allison Zibelli: And thank you to our listeners for joining us today. You'll find the links to all the abstracts discussed today in the transcript of this episode. Finally, if you like this podcast and you learn things from it, please take a moment to rate, review, and describe because it helps other people find us wherever you get your podcasts. Thank you again.

Disclaimer:

The purpose of this podcast is to educate and to inform. This is not a substitute for professional medical care and is not intended for use in the diagnosis or treatment of individual conditions. Guests on this podcast express their own opinions, experience, and conclusions. Guest statements on the podcast do not express the opinions of ASCO. The mention of any product, service, organization, activity, or therapy should not be construed as an ASCO endorsement.

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Disclosures:

Dr. Allison Zibelli:

No relationships to disclose

Dr. Rebecca Shatsky:

Consulting or Advisory Role: Stemline, Astra Zeneca, Endeavor BioMedicines, Lilly, Novartis, TEMPUS, Guardant Health, Daiichi Sankyo/Astra Zeneca, Pfizer

Research Funding (Inst.): OBI Pharma, Astra Zeneca, Greenwich LifeSciences, Briacell, Gilead, OnKure, QuantumLeap Health, Stemline Therapeutics, Regor Therapeutics, Greenwich LifeSciences, Alterome Therapeutics

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