Ashish Kamat: Hello, and welcome to UroToday's Bladder Cancer Center of Excellence. I'm Ashish Kamat, Professor of Urologic Oncology at MD Anderson Cancer Center and it's a distinct pleasure to invite and welcome and have here today Professor Jeffrey Ross who's going to talk to us about micropapillary urothelial carcinoma. Dr. Ross really needs no introduction to most of us. He is professor of pathology, oncology and urology at SUNY University in New York and is the medical director of Foundation Medicine.

Dr. Ross, thank you so much for taking the time and spending it with us today and talking about a topic that really is very near and dear to my heart with a lot of the clinical work and in publications that have done with micropapillary cancer over the years, starting back in the late nineties. And it's great to see all this molecular work emerging from your group that addresses this deadly disease. So Jeff, with that, the stage is yours.

Jeffrey Ross: Thank you very much, Ashish. It's my pleasure to be the speaker for this UroToday session. And I hope we'll have a really interesting discussion about this rare, but critically important, subtype of urothelial bladder cancer that has such an aggressive clinical course. My first slide that I will show will be my disclosures. And here they are, for the audience to quickly look at. And let's jump right in about this interesting micropapillary.

The term micropapillary really refers to an aggregate or clustering of urothelial malignant cells into small balls and papillary formations that often have a surrounding free space. Sometimes they look like they're actually invading lymphovascular channels, which the lower panel on the right actually does show them doing, which they do readily. But often, these are just retraction artifacts around these micro papillae. They are not full papillae like we see in classic papillary urothelial carcinoma because they do not contain a central fibrovascular core.

The incidence of MPUC varies in the published literature, but in most clinically advanced data sets, not with a lot of non-invasive, superficial disease in the group, then the percentage of cases goes up significantly. So for example, at Foundation Medicine, where all the cases are clinically advanced and metastatic, it's 8.2% of urothelial bladder cancer. More common in men and on occasion, can also involve the upper tract. The lymphovascular invasion is such a significant finding and you can see it very, very superficially in radical cystectomy samples and in some TURBTs leads many urologic oncologists and practicing urologists to have a very aggressive approach of this disease with great concern that even in the absence of smooth muscle invasion on a TURBT sample, they should be considered relatively high stage and scheduled for either neoadjuvant, followed by radical cystectomy or some other type of very aggressive clinical treatment.

We did a regional study in 2013 as next-generation sequencing techniques became available for formal and fixed paraffin embedded materials to look at urothelial carcinomas. And in that study, we found a significant number back in 2013 that featured alterations in the HER2, or what we call ERBB2 gene, which was a surprise. A significant number of cases had alterations and a substantial number of those involved what we call the extracellular domain of the gene, the S310F and S310Y mutations. And when I looked at the histologies of these, I was struck by the high frequency of micro papillary architecture in these ERBB2 extracellular domain mutated cases led to a publication about this association where about 40% of the HER2 altered by gene sequence, not the amplifications, just the HER2 mutation cases being very frequently MPUC subtype.

In 2023, a 10-year interval, I decided to take another look and see if this really was holding up. Now, there were thousands of cases to review and 548 or 10% had an ERBB 2 either gene amplification or an extracellular domain mutation. And the 4% with the extracellular domain mutations were mostly 310F, but some S310Ys, about the ratio of 3:1. And again, we looked at the histology on these and found that although it was lower than originally described, the MPUC frequency was just under 30%. So clearly, an association with this extracellular domain mutation, which is associated with hyper-dimerization of the HER2 protein, with the HER3 protein could be actually causing a morphologic appearance. And this, just to digress for a second, is not new for HER2. We've known for many years that HER2 amplification can alter the histologic status of different types of cancers. And breast cancer was first to see this where the disease, Paget's disease, is 100% HER2 amplified because in culture we see HER2 alterations driving cell motility.

So I believe that the micropapillary architecture is not a coincidence in these HER2 altered tumors, but actually a result of the HER2 alteration, which may also be giving it this aggressive behavior of early spread beyond the bladder, even in patients who don't show significant smooth muscle invasion due to the high propensity for lymphovascular invasion. Other genes in the most recent study, like FGFR3, which we look for so carefully in clinically advanced bladder cancers for the potential of using anti-FGFR drugs were definitely higher in the non-altered cases, meaning that like we see in lung cancers and others, when a driver like ERBB2 amplification or ERBB2 extracellular domain mutation when they're present, other mutations that can be drivers like FGFR3 become far less frequent.

Recently, in a systemic review, of targeting HER2 and bladder cancer colleagues, including our dear friend Petro Grivas at the University of Washington and Seattle, looked at the numbers of clinical trials that have attempted to find an indication for anti-HER2 targeted therapy in patients with locally advanced and metastatic urothelial bladder cancer. We initially thought, way back after the approval of trastuzumab for the treatment of metastatic breast cancer, that bladder cancer would be one of the earliest non-breast cancers to gain FDA approval. But alas, that did not happen.

However, things are changing as we all know, and here's an example by the way of an ERBB2 S 310F extracellular domain patient who did receive trastuzumab and achieved a partial response. There's very little published information about the responsiveness of these extracellular domain mutations to the standard of care or the first generation anti HER2 targeted therapies with antibodies like trastuzumab pertuzumab and even trastuzumab T-DM1, as well as several different kinase inhibitors like Afatinib and Neratinib.

But now we come to the era of Trastuzumab Deruxtecan, also known as ENHERTU, which of course has gotten its first approval in metastatic advanced breast cancer. That was followed by approval in HER2 amplified gastroesophageal cancer. Then the recent approval of breast cancers with 1+ and 2+ IHC staining, achieving FDA approval for trastuzumab Deruxtecan. Then for the first time, a non amplification indication, lung cancer with exon 20 insertion mutation achieved an FDA approval for trastuzumab Deruxtecan for lung cancers with a sequence mutation, not a copy number increase, meaning that the IHC stain would be negative and the FISH assay would not show amplification. So a very significant breakthrough then.

While we've all been waiting to see about trastuzumab Deruxtecan's efficacy in urothelial bladder cancer and we got our first glimpse at that data back in February of this year at the ASCO GU meeting in San Francisco, where we first learned about some of the very promising results of treatment of patients with advanced disease with this combination of a HER2 guidance and irinotecan targeting molecule.

We're waiting for the next release of data which may well come at ASCO GU this coming February about the progress toward the clinical regulatory approval of trastuzumab Deruxtecan for urothelial carcinoma. We are all excited and very much anticipated. Regulatory approval is in the works for this very exciting drug.

To our knowledge, though, we don't have histologic data as to whether or not any of the cases that are in the trial for this drug are micropapillary subtypes. These are being registered into the trial solely based on IHC staining for HER2. And if the tumor is only a sequence alteration, these will likely be HER2 zero. But approval for trastuzumab Deruxtecan for HER2 overexpressed in bladder cancer would only enrich the chances that downstream, we would also get approval for those bladder cancers with normal copy number and no overexpression, but do have HER2 driving the tumor because they have a sequence mutation, particularly in the extracellular domain. And that's where the micropapillary variants will come in.

So extracellular domain, HER2 mutations, ERBB2 mutations as a driver of bladder cancer, is a unique form of the disease which enriches significantly the micropapillary urothelial carcinoma histology. We are excited about the possibility that beyond the traditional HER2 agents, the development of trastuzumab Deruxtecan has the potential not only to help patients with bladder cancer whose HER2 gene is amplified, but also eventually bladder cancer whose HER2 gene has a gene is a wild copy number, normal copy number, but has an activating mutation in the sequence causing it to be a driver mutation for the disease.

Thanks very much, everyone. Looking forward to my discussion with Ashish.

Ashish Kamat: Thanks, Jeff, so much for that succinct presentation on a fairly complicated topic. It's great how you distilled that into your talk in such a short time. A couple questions that I want to ask you because micropapillary bladder cancer, as you and I have talked about, has been near dear to my heart with my special interest in it. And back in the early 2000s, we published the largest series of micropapillary bladder cancer from MD Anderson.

And as you rightfully pointed out, most of these tumors tend to present at an advanced stage. And once they present at an advanced stage, of course, our patients require chemotherapy and a radical cystectomy. But the question often comes up because we do see a good number of these patients in the non-muscle invasive space and in the non-muscle invasive space, the standard best treatment for these patients is immunotherapy with BCG. Now there's also some evidence to suggest that the HER2 pathway is active in this anti-immune response. Do you have any insight into why that might be the case or how that would correlate with the poor response of micropapillary bladder cancer to BCG, which is what we see in the clinic?

Jeffrey Ross: Well, I think it's fascinating. We have seen some evidence that HER2 driven cancers are less responsive to immunotherapy in other settings. We've just started to look, in fact this morning we were working on using HER2 status to predict the pathologic complete response in the PURE-01 trial where pembrolizumab served as the single immunotherapy prior to radical cystectomy. Not enough cases probably to have statistical significance, but it does tend to look like when tumors have a driver, whether it's copy number increase or an extracellular mutation we just were talking about, they may be immunoresistant or resistant to checkpoint inhibitors. Further emphasis that it may not be in the future, so safe at least until the trastuzumab Deruxtecan is approved to do a topical or a intravesical therapy for a disease with this propensity for lymphovascular invasion. You may be able to control the superficial component of it, but it may be very secretly finding its way outside the bladder during that period of time. I've always felt that micropapillary urothelial carcinoma is muscle invasive, whether you see it or not, meaning treat it as if it were muscle invasive.

Ashish Kamat: And I'm with you on that because our investigations and our publications that sort of raise the awareness of micropapillary bladder cancer amongst the urologic oncology community because as you mentioned, prior to that the reported incidents in the literature was 1% or so. But when more pathologists recognized that it's clinically irrelevant, then the incidents started reporting more and more, with obviously, the caveats that you mentioned about the retraction artifacts and sometimes LVI being over called and made out to be micropapillary bladder cancer.

But I'm very interested in what you just mentioned about the preliminary work that you're doing with the PURE-01 cohort because in patients that have micropapillary bladder cancer, one of the thought processes was combine BCG with IO therapy and that might rescue them. But it sounds like some of the work you're doing suggests that IO therapy, and again that was a invasive cohort that you looked at, but across the board might not be the best strategy to be focused on with HER2 or expressing or micropapillary bladder cancer.

Jeffrey Ross: I think in this, if I might be bold enough to say, in this, we'll call it interval while we await the trastuzumab Deruxtecan approval in bladder carcinoma because I would think like in breast cancer, neoadjuvant use as was just shown in the San Antonio Breast Cancer Symposium last week, was a dramatic impact on these breast cancers. Chances of having pathologic complete response, the data was astounding. It was five times more potent at preventing, of yielding pathologic complete response than the standard trastuzumab regimen was. So that at least for the micropapillary cancers, maybe 40% of them that are HER2 driven, they would be receiving this drug in the neoadjuvant setting rather than immunotherapy, in the neoadjuvant setting. We had a case just this morning that I worked, on the trial that had just come through the cystectomy and the pathology report just came out, and it is a micropapillary subtype.

It does have a HER2 S 310Y mutation and it was a failed responder to the pembrolizumab. It's only one case. And of the 140 cases I looked at yesterday, we only had 21 that were HER2 driven. But of those 21, there are quite a number of sequence mutations. And it tends to be really an interest here, which is different than other tumor types in bladder. If the tumor has HER2 amplification, it won't have a sequence mutation. And if it has a sequence mutation, it won't have a copy number increase. A lot of other cancer types that have HER2 amplification will have sequence mutations as well, lung cancer for example. But not bladder. It's pretty much an either or, not always, but pretty much an either or.

Ashish Kamat: Yeah. I think the antibody drug conjugates, across the board, are moving into earlier and earlier stages of disease because as you mentioned, you have the target, you have the payload and you can sort of just give it where it's needed. With your years of work in this field, and again, from a clinical side, we've been frustrated with the whole story of the HER2, IHC and how do you actually do this. So for our folks that are listening that are in the community, for example, and may not have access to the high throughput sequencers, et cetera, what is your advice, from a pathology perspective, on how the IHC correlates with what you're seeing?

Jeffrey Ross: That's a wonderful question, Ashish. I was involved in this since, well, 1981, when I first began studying this just by luckily flying to Europe next to Robert Weinberg, and he told me about his discovery of the new gene. And I decided to, when I got back, to try to learn about it. And for so long we worked so hard to make it clear of what a positive case was and what a positive case wasn't, meaning a positive case had to have a copy number of HER2 gene of a significant number, probably six or more, and certainly five or more, or three plus continuous membranous staining on IHC. And Trastuzumab Deruxtecan has thrown all of this into great, great turmoil. I would say that if anyone thinks they can predict where HER2 testing is going now, I'd be impressed because my days of inventing the FISH test are long over because it's really fascinating how this drug works in HER2 1+ in breast cancer. And it's obviously got an approval and it was shown again in San Antonio to be similar in other settings beyond metastatic disease.

So for bladder, where we don't do HER2 testing routinely, I don't know of any lab that requires it in the management of all urothelial carcinomas. It's only done in a research setting. It is likely that that's going to change because there's going to get an FDA approval for this drug in bladder cancer, I would think in 2023 is possible, and certainly early 2024. And when that happens, labs will have to be able to test for HER2 status. Now, the question is you'll only get 70, 60, 70% of the positives if you only do immunohistochemistry or FISH, or both. The other positives, the ones that I was talking about today, don't have a positive stain or a positive copy number increase on FISH. They can only be detected by sequencing and really only accurately by next generation sequencing, and that may change.

Now, the approval for NHER2 and bladder is going to be copy number slash IHC. And no one's going to care about that other 30% of cases that are also HER2 driven, but you can't detect it that way. You only can detect it by sequencing. That will be what we call, I think, an expanded claim down the road. In lung cancer, I already know that patients who test negatively for the insertion exon 20 mutation, but have a copy number increase, are being put on Trastuzumab Deruxtecan. They're HER2 positive, there's no question there, HER2 positive. And for many, let's call it insurance carriers, HER2 positive is HER2 positive. They don't go deeper into understanding, well, what kind of positive? Is it a sequence mutation with an activated driver that way, or is it a copy number increase? They just say, "Oh, HER2 positive, this drug is on label." So there's a little confusion there, and this has been something I've been interested in for so long. I find it fascinating.

Ashish Kamat: Yeah, well that's why I thought I'd ask you the question, and thanks for summarizing it. I think sometimes the confusion, if it can be for our patients benefit is worth having. Right?

Jeffrey Ross: Right.

Ashish Kamat: So that's okay. Again, Jeff, I could chat with you forever. But in the interest of time, let me just take a moment to thank you for taking time out of your busy schedule for chatting with us and talking on this complex topic. I'm sure we'll have you again to maybe clarify some other issues with bladder cancer, molecular subtyping, et cetera, et cetera, down the road. But for now, thank you very much and hope to see you soon.

Jeffrey Ross: Thank you, Ashish. I hope I see you very soon and I will look forward to it.