Intro: Welcome to Precision Pathology Podcast, featuring interviews with authors and global thought leaders who are at the forefront of technological advances in precision diagnostics and therapeutics. Your host is Dr. George Netto, the Simon Flexner Professor and Chair of Pathology and the Laboratory Medicine at the University of Pennsylvania's Perelman School of Medicine in Philadelphia. Here is Dr. Netto.

George Netto (Host): Welcome to a new episode of our podcast. Our guest today is my friend and colleague, Dr. Ben Stanger. Ben is a professor. He's actually the Hannah Wise Professor of Oncology in our Department of Medicine. He is an expert physician scientist in pancreatic cancer. And he's here today to discuss his very exciting new study with really exciting findings that just appeared in Science in the last month, in March of this year, looking at cancer interception, which is a relatively new concept, the way we're doing it here at Penn. But cancer interception using KRAS inhibitors to see if we can delay or avoid completely the development of pancreatic cancer. Importantly, I want to mention that Ben is the PI and corresponding author on this. But that's in partnership with our cancer center director, Bob Vonderheide, who incidentally was just elected the president of AACR.

So, the study included also some collaborator from my other favorite place, Hopkins. And with that, welcome, Ben. And thank you very much for accepting our invitation.

Benjamin Stanger, MD: Well, thank you, George. It's a pleasure to be here and I look forward to talking about our studies.

Host: Wonderful. So, we have a versatile audience. And with that in mind, we will benefit—all of us—from just a couple of words about what's the concept of interception that many institutions like ours are trying to benefit from. And couple of word about the RAS inhibitors too.

Benjamin Stanger, MD: Sure. So, interception, it's becoming a buzzword. So, It is worth spending a minute talking about what that means and where it fits in the cancer therapy spectrum. So, I think, for cancer, we all know and understand what treatment is. You have someone with cancer, you treat them with chemotherapy or a targeted agent or immunotherapy. And then, at the other end, we have prevention. Prevention being stopping the cancer process from even getting started in the first place. So, sunscreen, the HPV vaccine, smoking cessation. These would be examples of prevention.

Interception is a slight variation on that, which is to say that the cancer process has already begun. There's a dysplasia, there's a precursor lesion. An analogy would be an adenoma in the colon or a dysplastic nevus in the skin. So, it's on its way to becoming cancer. And one performs some type of intervention at that stage to make it go away or prevent it from progressing. So, that's what interception is.

Host: Awesome. You mentioned in your introduction examples of interceptions that we do mechanically, is a polypectomy, right? Or we go ahead and take the dysplastic nevus. But there, you can identify the lesion, which is for our topic today, is not possible to go and mechanically take out a PanIN, for example.

So, the approach here is more drug-related or chemical. So, a couple of words about RAS We know that's like one of the last frontiers in inhibition. So, it would be good to share with our audience.

Benjamin Stanger, MD: Yeah. Well, you're absolutely right. We do interception currently for something like colon cancer. I'm a gastroenterologist, so that's what I do clinically, is remove those polyps. And you're absolutely right that we can do that because we can perform a colonoscopy and see them.

So for the majority of cancers, we can't easily identify the precursors. And also, the precursors come in different forms. There are less advanced and more advanced precursors. So, maybe I'll take a second to just talk about pancreas cancer. And it's precursors in the context of RAS. So, pancreatic cancer is currently the third leading cause of cancer death in the United States poised to become the second leading cause, overtaking colon cancer in the next couple of years.

And what's a little bit unique about it from a molecular standpoint is the prevalence of RAS mutations. RAS mutations are present by the latest estimates in about 95% of pancreatic ductal adenocarcinomas. But not only is it that frequently mutated, it's also thought to be the first mutation. So, it is a mutation that happens in cells in the pancreas. And the result of that is these precursor lesions, which we call PanINs for pancreatic intraepithelial neoplasias. These are microscopic lesions, so they can't be detected on a CT scan or an MRI or really anything other than on a histopathological specimen. So, we have no way of really identifying them, but we do know that all of them carry KRAS mutations as well. KRAS is a G protein. It was probably the first human oncogene to be cloned something like 40 years ago, and is one of the most prevalent oncogenes to be mutated in human cancer. And yet, despite it being around for so long, for 30 years, people have tried to develop drugs for KRAS to inhibit the mutant oncoprotein. And billions of dollars in many years, and lots of postdocs and so on and so forth really didn't go anywhere until about 10 years ago when some really remarkable chemistry enabled what's now a treasure trove of small molecule inhibitors of KRAS that are in various stages of preclinical and clinical development.

Host: Wow. Very well put. And as a gastroenterologist, it was very impressive how you describe PanIN, a lesion that, during my tenure at Hopkins, Ralph Hruban, the current chair, everything has PanIN. "This is PanIN one. This is PanIN—" It's like, "Come on. This is not PanIN." But it turned out to be true. It's a universal precursor for adenocarcinoma.

So, let's focus a little bit on the experiment. This is clearly an animal model study. But the reason it's on the podcast because it has serious implication for us soon down the road, hopefully in a clinical trial. So, what the model that you used and little bit about how the intervention was conducted and what did we find?

Benjamin Stanger, MD: Perfect. Perfect. So, we have kind of a perfect set of conditions in principle for interception. We know that the precursors have a particular mutation for which there are drugs. We know that these precursors are actually quite common in the population. We have no way of screening for them, yet we know that they're very prevalent. And so in principle, we could think about instead of the mechanical type of interceptions that we use for other cancer types of pharmacological interception, we have a KRAS inhibitor. We know these precursors have KRAS mutations. Is it possible in principle to make those precursors go away?

And here, we're lucky because the field, the pancreas cancer field has had a model for 25 years or so, first actually developed here at Penn. And it's called the KPC model. And the KPC stands for KRAS, p53 and Cre. So, Cre is a recombinase, it's a technical mechanism for turning on or off genes.

And so, in this model, there are mutations in KRAS that through the activity of this recombinase are directed to the pancreas. And mutations in the p53 tumor suppressor, which are also very, very common in pancreas cancer, are also directed to the pancreas. So, you end up with a mouse where most of the cells in the pancreas from the time of birth have mutations in KRAS and p53.

So, what happens in this model is the mice are born. And for the first few months of age, the pancreas looks quite normal. It's actually kind of remarkable that the pancreas develops normally with a p53 mutation and a KRAS mutation. But over weeks to months, the animals start to develop PanIN. They start to develop these precursors. And this is in fact one of the ways we know that the PanIN precursors are the precursors of cancer, because those PanIN lesions then go on to develop invasive pancreatic ductal adenocarcinoma in the mice.

So, we have this model system where we have the progression from the precursors to cancer. We know the rough timing involved, which is to say at seven, eight, nine weeks of age, the mice will have dozens of PanIN lesions but no cancer. But then, at three to four months of age, they will be developing cancer with a hundred percent penetrance by five or six months of age. So, we can now ask the question, and we did ask the question, if you give mice an inhibitor of KRAS at this vulnerable stage, at the PanIN stage, what then happens to the PanIN lesions? And what happens to the resulting progression to cancer?

Host: Wow. I didn't know what KPC stands for. So now, I know. I was hoping the P is for Penn, but it turned out to be p53. I'll take that too. So the way, incidentally, you know, looking at the images, a colleague of mine in pathology, which she goes by Beth, is a pathologist on this study, and you can see from the section, the PanIN looks exactly very similar to what we see in human. So, it's identifiable on sections. And I know, and you may talk about that, putting all the sections, serial levels of the section in a 3D model. You guys did code, you can see how much PanIN you have in a cubic centimeter, which is also an amazing technology—Precision Pathology podcast. So, you did the intervention. So by 40 weeks or 38 weeks, almost all the mice should, if you don't give them any drug, have pancreatic cancer. And by seven weeks, just to summarize, they will have PanIN. So, the intervention became, you give two inhibitors. One is more selective to the G12D, KRAS. And one is less selective. It also inhibit other mutations. So, you did that at week nine, in my understanding, I think where you anticipate there are PanIN. So, tell us what you found.

Benjamin Stanger, MD: Yeah. So, we did a few time points. And we really wanted to be robust and make sure that it worked in multiple different settings. As you say, we had two different types of inhibitors. The KPC model uses a version of KRAS that has a glycine to aspartic acid mutation a G12D. So, one of the inhibitors is specific for that mutant form of RAS.

The other inhibitor, we could call it a pan-RAS inhibitor, and it inhibits RAS in the GTP bound state. And it has a very broad spectrum of action, meaning that it inhibits mutant RAS regardless of the mutation. It also inhibits wild-type RAS and not only wild-type KRAS, which is the mutation we're dealing with here, but also wild-type NRAS and HRAS, all members of the RAS family. So, we gave these drugs, as you say, at around seven to nine weeks of age. The first result was that the PanINs really dramatically disappeared. After even 10 days of therapy, they were at least 80% gone. And if we gave either inhibitor for a month, they were almost 90-95% gone. So, these PanIN lesions depend on RAS signaling for them to persist. And the reason they go away, we think, is they die. They stain for cleaved caspase-3, which is a marker of cell death.

Host: Apoptosis for the pathologist.

Benjamin Stanger, MD: Exactly, via apoptosis. So, that was the first result, is, okay, the PanINs can be forced to go away with this therapy. The second question is, if we do this, what happens then to the development of cancer? And it's important to distinguish between this mouse model and what happens in people. Because in people, these lesions, you know, there may be dozens or more than a hundred of them. But each of them are separate, have a different KRAS mutation, and the rest of the pancreatic cells don't have KRAS mutations. In our model, every cell is at-risk, so we know that if we stop this drug, if we stop the PanINs, new ones will form. So, it's going to be something that would, in the mouse model, require ongoing therapy.

In any event, we treat the mice for a month and then stop the drug. And we saw that the time to the development of cancer in those animals was delayed and delayed by more than a month. But again, eventually, all the animals went on to develop cancer. So, we then realized, well, we need to keep that pressure on over a longer timeframe. So here, we gave the mice therapy for a prolonged period of time. And we did so with one week on and one week off, a so-called metronomic dosing schedule.

And there, really, it dramatically delayed the onset of tumors. So whereas the tumors would develop normally in the mice with an average timing of around four months, the onset of tumors in the setting of this interception, ongoing interception, was a year, a year or more. So, these are the oldest KPC animals in the planet, because normally they would all die of their tumors before this.

Host: Awesome. Very well put. Now that you explained it, something I didn't pick up, the reason they go on and develop, I was thinking that could be a potential failure, is every cell in their pancreas is going to go ahead and develop PanINs. And so, the moment you stop the drug, not surprisingly, they will. That's an advantage we have human over your model, which brings us to really why you're spending years of your life and battling for grants and developed all these models here at Penn. Ultimately, we want to cure pancreas cancer in human, not in mice. But this is incredibly valuable step toward that. What do you see the next steps? Are we planning clinical trial or what's the next step?

Benjamin Stanger, MD: We'd love to cure pancreas cancer. But even better yet, as our founder said, an ounce of prevention is worth a pound of cure. And so, even better, we'd like to intercept it before it even comes about. So, the two drugs that I talked about that we've used in the mice, they are what we call tool compounds. They're chemically very, very similar to two drugs that are currently making their way in the clinic, in patients.

And one of the surprises, the one that's furthest along is the Pan-RAS inhibitor, the one that inhibits all manner of RAS family members. And the big surprise there from the clinical studies is that the drug is actually pretty well-tolerated. And it's a surprise because RAS molecules do things in the body. They're involved in lots of different organ systems. And the thought initially when these drugs were being developed is inhibiting all RAS would be toxic. But it seems we have a therapeutic window. There's a dose where the precursors and the cancer cells need a certain amount of RAS signaling for their survival.

But the normal cells can make do with that level of the drug and be perfectly fine. And that's of course an area we're exploring right now is, well, some individuals might have a lower threshold for toxicity, but these drugs seem to be well-tolerated in people. So where does that bring us? Well, from the treatment standpoint, It says we're likely to have a new treatment for pancreas cancer, the first one in more than a decade. And the most recent data that's been released is very, very promising and looking like there'll be likely an FDA approval, sometime later this year for that drug.

What does it mean for interception? Well, I think the first step is, as we did in the mice, showing that in human beings, we can make the PanIN lesions, the precursors go away with a short-term treatment of this drug. And we and other groups, colleagues at other institutions, are in this planning stages right now for the clinical trials that will establish that. And there are a couple of different variations on how this would look, but they mostly take the form of individuals who would be going to the operating room for pancreatic surgery. So, they have another reason to have a surgery like a cyst. And we would then give those patients the RAS inhibitor for some period of time before they go to surgery, and then have the surgical resection specimen available to us to ask compared to others who don't have the RAS inhibitor, have we made these PanIN lesions go away? And can we safely give this drug to people who don't have cancer? Because currently, right, we have a different threshold for toxicity in people who have cancer versus those who don't have cancer. So, that's step one.

And if, as we believe, we can make PanINs go away in people, step two then becomes trying to do that in individuals at higher risk for pancreatic cancer and intercepting those lesions and showing that we can prevent the cancer from forming.

Host: And those individuals will be people like inherited BRCA1, BRCA2 family, would probably be one and people at higher risk.

Benjamin Stanger, MD: Exactly right. So, BRCA carriers, hereditary pancreatitis, patients would be among that group. It turns out that a high-risk group is actually individuals with new-onset diabetes. So even though diabetes is very, very common, a subset of those who develop diabetes develop it in relation to pancreatic cancer.

So, that might be a group that, as these studies proceed over the next few years, we might consider also including. It's just that because early detection has proven so difficult, we don't really know who has these early cancers. We need to take a broader view of how we might interrupt the sequence of events.

Host: Wonderful. I tremendously enjoyed this. And I'm sure our audience will. I must say one of the best guests in term of how you lay the story. You really are passionate about what you're doing, and we are so lucky to have you here at Penn. I'm grateful that you took sometimes to share this exciting study with our audience. Thank you, Ben, for coming.

Benjamin Stanger, MD: Thank you for having me. It's been a lot of fun.

Outro: We would like to extend our sincere gratitude to today's guests for their valuable contributions and thoughtful perspectives. We also wish to thank our dedicated production team and the technical team at DoctorPodcasting. Special thanks to Jennifer Vepler for her dedication and skill in coordinating our episodes.

Please note the opinions expressed by our speakers are their own and do not necessarily represent the Perelman School of Medicine or the University of Pennsylvania. Thank you for listening, and we look forward to welcoming you back for future episodes.