Dr. Monty Pal and Dr. Hope Rugo discuss advances in antibody-drug conjugates for various breast cancer types as well as treatment strategies in the new era of oral SERDs for HR-positive breast cancer.

TRANSCRIPT

Dr. Monty Pal: Hello, and welcome to the ASCO Daily News Podcast. I'm your host, Dr. Monty Pal. I'm a medical oncologist and vice chair of academic affairs here at the City of Hope Comprehensive Cancer Center, Los Angeles. Today, I'm thrilled to be joined by Dr. Hope Rugo, an internationally renowned breast medical oncologist and my colleague here at City of Hope, where she leads the Women's Cancers Program and serves as division chief of breast medical oncology.

Dr. Rugo is going to share with us exciting advances in antibody-drug conjugates (ADCs) that are expanding treatment options in various breast cancer types. She'll also address some of the complex questions arising in the new era of oral SERDs (selective estrogen receptor degraders) that are revolutionizing treatment in the hormone receptor-positive breast cancer space. Our full disclosures are available in the transcript of this episode.

Dr. Rugo, welcome, and thanks so much for being on the podcast today.

Dr. Hope Rugo: Thank you. Pleasure to be here.

Dr. Monty Pal: So, I'm going to switch to first names if you don't mind.

The first topic is actually a really exciting one, Hope, and this is antibody-drug conjugates. I don't know if I've ever shared this with you, but I actually started my training at UCLA, I was a med student and resident there, and it was in Dennis Slamon's lab. I worked very closely with Mark Pegram and a handful of others. This is right around the time I think a lot of HER2-directed therapies were really evolving initially in the clinics. Now we've got antibody-drug conjugates. Our audience is well-familiar with the mechanism there but tell us about how ADCs have really started to reshape therapy for HER2-positive breast cancer.

Dr. Hope Rugo: Yeah, I mean, this is a really great place to start. I mean, we have had such major advances in breast cancer just this year, I think really changing the paradigm of treating patients. But HER2-positive disease, we've been used to having sequenced success of new agents. And I think the two biggest areas where we've made advances in HER2-positive disease, which were remarkably advanced this year in 2025, have been in antibody-drug conjugates with trastuzumab deruxtecan and with new oral tyrosine kinase inhibitors (TKIs) that have less of a target on EGFR and more on HER2, so they have an overall more tolerable toxicity profile and therefore a potentially better efficacy in the clinic. At least that's what we're seeing with these new strategies that we couldn't really pursue in the past because of toxicities of the oral TKIs.

So, although our topic is ADCs, I'm going to include the TKI because it's so important in our thinking about treating HER2-positive disease. In the metastatic setting, we've seen these remarkable improvements in progression-free and overall survival in the second-line setting with T-DXd, or trastuzumab deruxtecan, compared to T-DM1. And then sequencing ADCs with giving T-DXd after T-DM1 was better than an oral tyrosine kinase or a trastuzumab combination with standard chemotherapy. That was DESTINY-Breast03 and DESTINY-Breast02.

So, then we've had other trials since then, and T-DXd has moved into the early-stage setting, which I'll talk about in just a moment. But the next big trial for T-DXd in HER2-positive disease was moving it to the first-line setting to supplant what has become an established treatment for now quite a long time: the so-called CLEOPATRA regimen, which used the combined antibodies trastuzumab, pertuzumab with a taxane as first-line therapy. And then we've proceeded on with maintenance with ongoing HP for patients with responding or stable disease.

And we'd seen long-term data showing, you know, at 8 years there was a group of patients whose cancers had never progressed and continued improved overall survival. So, T-DXd was studied in DESTINY-Breast09, either alone or in combination with pertuzumab compared to THP. The patient population had received a little bit more prior treatment, but interestingly, not a lot compared to CLEOPATRA.

And they designed the trial to be T-DXd continued until progression with or without pertuzumab versus THP, which would go for six cycles and then stop around six cycles, and then stop and continue HP. Patients who had hormone receptor-positive disease could use hormone therapy, and this is one of the issues with this dataset because, surprisingly in this dataset and one other I'll mention, very few patients took hormone therapy. And even in the maintenance trial, the HER2CLIMB-05, less than 50% took hormone therapy as maintenance. This is kind of shocking to me and highlights an area of really important education, that outcome is improved when you add endocrine therapy for hormone receptor-positive HER2-positive metastatic disease in the maintenance phase, and it's a really important part of treatment.

But suffice it to say, you know, you're kind of studying continued chemo versus stopping chemo in maintenance. And T-DXd, as we all expected, in combination with pertuzumab was superior to THP in terms of progression-free survival, really remarkably improved. And you could stop the chemo with toxicity, but most people continued it with T-DXd. Again, not a lot of people got hormone therapy, which is an issue, and you stop the chemo in the control arm.

So, this has brought up a lot of interest in trying to use T-DXd as an induction and then go to maintenance, much as we do with the CLEOPATRA regimen with hormone therapy. But it brings up another issue. So first, T-DXd is superior; it's a great treatment. Not everybody needs to have it because we don't know whether it's better to give T-DXd first or second with progression - that we need a little bit longer follow-up. But just earlier this week, interestingly, the third week of December, the U.S. FDA approved T-DXd in the DESTINY-Breast09 approach with pertuzumab. So as I mentioned earlier, there was a T-DXd-alone arm; that arm has not yet reported. So very interesting, we don't know if you need pertuzumab or not.

So what about the maintenance? That's the other area where we've made a huge advance here. So, we all want to stop chemo and we want to stop T-DXd. You don't want somebody being nauseated for two years while they're on treatment, and also there's a small number of patients with mostly de novo metastatic HER2-positive disease who are cured of their disease. We'd like to expand that, and I think these new drugs give us the opportunity to improve the number of patients who might be cured from metastatic disease.

So the first maintenance study we saw was adding palbociclib, the CDK4/6 inhibitor, to endocrine therapy and HP, essentially. There, we had a remarkable improvement in progression-free survival difference of 15.2 months: 29 to 44 months, really huge. At San Antonio this year, we saw data with this oral tyrosine kinase inhibitor tucatinib, already showed it was great in a triplet, but as maintenance in combination with HP, it showed also a remarkable improvement in progression-free survival. But the numbers were all shifted down. So in PATINA, the control arm was in the 24-month range; here it was the tucatinib-HP arm that was in the 25 months and 16 months for control. So there was a differential benefit in ER-negative and ER-positive disease. So I think we're all thinking that our ideal approach moving forward would be to give T-DXd to most patients, we see how they do, and treat to best response. And then, stop the T-DXd, start HP, trastuzumab, pertuzumab for ER-negative, with tucatinib for ER-positive with palbociclib.

We also have early data that suggests that both approaches may reduce the development of brain metastases, an issue in HER2-positive disease, and delay time to progression of brain metastases as seen in HER2CLIMB-05 in very early data - small numbers, but still quite intriguing that you might delay progression of brain metastases with tucatinib that clearly has efficacy in the brain.

So, I think that this is a hugely exciting advance for our patients, and these approaches are quickly moving into the early stage setting. T-DXd compared to standard chemo, essentially followed by THP, so a sequenced approach resulted in more pathologic complete responses than a standard THP-AC-type neoadjuvant therapy. T-DXd alone for eight cycles wasn't better, and that's interesting. We still need the sequenced non-cross-resistant chemo.

But I think even more importantly, the data from DESTINY-Breast05 looking at T-DXd versus T-DM1 in patients with residual disease after neoadjuvant HER2-targeted therapy showed a remarkable improvement in invasive disease-free survival with T-DXd versus T-DM1, and quite early. It was a high-risk population, higher risk than the T-DM1 trial with