Reviving PALA: A New Role in Cancer Immunotherapy

Podcast Transcript

Dale Shepard, MD, PhD: Cancer Advances, a Cleveland Clinic podcast for medical professionals, exploring the latest innovative research and clinical advances in the field of oncology.

Thank you for joining us for another episode of Cancer Advances. I'm your host, Dr. Dale Sheppard, a Medical Oncologist, Director of International Programs for the Cancer Institute and Co-Director of the Sarcoma Program at Cleveland Clinic.

Today I'm happy to be joined by doctors George Stark and Christine McDonald. Dr. Stark is a Distinguished Scientist in the Department of Cell Biology, and Dr. McDonald is an Associate Staff in the Department of Inflammation and Immunity, both in the Lerner Research Institute here at Cleveland Clinic. They're here today to discuss the development of PALA for use in oncology. So welcome.

George Stark, PhD: Thank you.

Christine McDonald, PhD: Thank you.

Dale Shepard, MD, PhD: So maybe to kick things off you can give us a little bit of an idea of what you do here at the clinic. So maybe we'll start with you, Christine.

Christine McDonald, PhD: Sure. This is Christine McDonald, and I am a basic science researcher here at the Lerner Research Institute. We obviously do some work on cancer, but I would say a lot of our work is focused on how your body senses bacteria and responds to it. So generating an innate immune response, and that kind of led us to some immuno-oncology work here with Dr. Stark.

Dale Shepard, MD, PhD: Excellent. So we're going to talk about multidisciplinary research. So, excellent. Dr. Stark, what do you do here?

George Stark, PhD: Well, I sometimes work.

Dale Shepard, MD, PhD: I think more than sometimes.

George Stark, PhD: I'm a basic scientist as well. I've been here quite a while. My lab works on mechanisms of how cells respond to external cues, signaling pathways, how they respond to interferons, especially in other cytokines. And a major part of my research now is looking for new therapies in cancer. The PALA project is a part of that, but we have a couple of other irons in the fire as well.

Dale Shepard, MD, PhD: So we're going to just jump right in this whole PALA story. So this is a drug that's been around for quite a while, and you've worked on it for quite a while. So maybe give us a little bit of a backdrop of the history of this and where we were up until we made these newer findings.

George Stark, PhD: Sure. Well, the history goes back about 55 years. I was a professor of biochemistry at Stanford University. We were working on how to inhibit aspartate transcarbamoylase, which is an early enzyme in pyrimidine nucleotide biosynthesis. So an essential pathway.

And we developed this drug, shorthand PALA, which binds very tightly and very specifically to that enzyme. And it turned out... These were all in vitro studies, with purified proteins. But it turns out that this compound penetrates cells, it works as an antitumor drug in mice, and it eventually went into extensive clinical trials in humans. There are about 100 publications documenting those trials in the early eighties.

But unfortunately it is toxic, of course, because it's blocking an essential pathway, and there wasn't enough of a therapeutic window for it to be useful in people in the end. So it was left on the shelf for quite a while until I ran into Christine.

Dale Shepard, MD, PhD: Very good. So we'll let you pick up, and kind of where does the story go from there?

Christine McDonald, PhD: Well, so I got into this story 'cause, as I mentioned, we are interested in how cells sense bacteria. And so there's a family of receptors that are inside your cells called the NOD-like receptor family that wasn't discovered until about 2000. And we were interested in a particular family member, NOD2, and how it was regulated.

And it turns out that we uncovered that NOD2 is regulated by the enzyme that PALA targets. And this enzyme called CAD basically dampens NOD2's signaling. So when the enzyme binds to the drug PALA, it kind of releases a brake on NOD2 and enhances innate immune responses. And in discussions with George we were like, "Well, how can we leverage this?" And turned to looking to see if it had an effect in situations with cancer.

The interesting thing, though, is that the amount of drug that you use is very different, in order to stimulate an immune response than to shut down the metabolic pathway that the enzyme is in charge of. So we can use it at much lower doses that are immune-stimulatory. And we're finding some very promising results in our preclinical models, using this at this new lower immunostimulatory dosage.

Dale Shepard, MD, PhD: Which is the important part, because the prior studies used higher doses and there was too much toxicity. Was, and again just sort of as a backdrop, was there evidence of some activity at the lower doses but people... I do early phase trials, so I realize that the concept is a little flawed at times, it seems, that more is better. And so is trying to get in as much drug, whether you need it or not.

George Stark, PhD: Well, it wasn't really explored, because all we knew at the time was that it was an anti-metabolite. And so it was the usual dose response for an anti-metabolite. How much can you give the patient without killing them, and kill the cancer? And it turned out that that was too close. But in the process we gained an awful lot of information about how the drug behaves in people. So all that's very useful now that we're going back at it.

Dale Shepard, MD, PhD: That's usually, when we talk about drugs and things in this particular setting on this podcast, it's about drugs that are just being approved or things that are sort of further along in clinical development. So it's kind of an interesting look at behind the scenes here to talk to you two, because I guess the question might come up, how do you... First off you have these NOD proteins. How do you really figure out what protein interactions are important? There's lots of protein interactions, how do you know which one's important?

Christine McDonald, PhD: That's a great question, especially since the enzyme CAD is a really large protein. So it has a lot of surface area to interact with many different things.

Our studies have focused on NOD2 'cause it's near and dear to my heart, but there are most likely either other NOD family members or other pathways that this enzyme may be co-regulating at the same time. And so we have studies that are ongoing right now to try to dissect what other players are involved and which ones are dominant for certain phenotypes. So we're using things like CRISPR technology to knock out the expression of the different candidates, and then using other stimuli to stimulate those pathways without PALA, to see if we can mimic those results a different way.

Dale Shepard, MD, PhD: And then when we think about pathways, looking at CAD, looking at NOD, are there particular cancers that have differential expression of those proteins? Or is there a way to generate some specificity? Like anti-metabolites, you're going to affect all your cells. Is there a way to get some differentiation there?

George Stark, PhD: Well we're not using it at levels in which it behaves as an antimetabolite. We're a logger, or even more lower than that. So there's no toxicity in any of our experiments with PALA at the most effective doses.

So we have a very strong immunostimulatory agent, which is not toxic, and it turns out that the pathways that it activates are effective in many different cancers. So in mouse models we've begun with skin cancer, but we have very compelling data in mouse models of non-small cell lung cancer, triple negative breast cancer, a little less but still good data in melanoma, and beginning encouraging data in pancreatic cancer. So those are all the ones that we've tested, and it seems as though we have a very general agent.

And part of the work is exploring further exactly how to use the drug in models of cancer, and expanding the number of cancers that we're checking. But the other part, as Christine has already alluded to is, how in the world does it work? Why is this pathway so effective? What are the cells... Getting back at your question. What cells are the ones that are activated? Is it just immune cells, or are we acting on the cancer cells as well? So there's a lot of mechanistic information that we're in the process of gathering that will eventually help us to use the drug more intelligently.

Dale Shepard, MD, PhD: It's good to see that there's such a wide range of cancers that are being positively impacted, because if you think about some of the ones you listed, things like pancreatic cancer have not historically been very effectively treated with immune therapies. So it might be... I mean, as we think about mechanisms, why this mechanism not others? Are there tumor types that you've looked at so far where it didn't work?

Christine McDonald, PhD: I think the only one that we've had some negative results on is glioblastoma, and those results were preliminary but were not as encouraging as our other models. But the brain is a special place.

George Stark, PhD: And PALA is a highly charged molecule, so very unlikely to get into the brain. It was a shot we-

Dale Shepard, MD, PhD: So maybe an access issue.

George Stark, PhD: Yeah, I think probably it is.

Dale Shepard, MD, PhD: Interesting. What about combinations? Are you looking at ways to contribute to combinations with chemo, targeted agents, other immunotherapies?

Christine McDonald, PhD: Yes. And we actually have some really exciting data that suggests that it may combine well with immune checkpoint inhibitors in immunologically cold tumors. And then we have a little bit of data in breast cancer with paclitaxel, seeing a combinatorial effect as well.

So I think there are a lot of areas that we would love to explore further to see if we can enhance some of these already approved therapies as well.

Dale Shepard, MD, PhD: I guess when you think about mechanistically, and I guess these are sorts of things you guys are trying to sort out, do we think that these would be drugs we'd use together? Or maybe sort of do something to prompt the response initially, like giving chemo first, damaging cells and then coming in with immunotherapy? Or is it kind of too early to know?

George Stark, PhD: I think there are many possibilities, and these remain to be explored. But I think a lot of this we could do in mouse models.

So for example, is it useful as a neoadjuvant therapy to reduce the initial tumor, and also help to prevent the spread of the tumor if it's going to be surgically removed? As in the case of breast cancer, for example.

Another consideration that bears on your question is that we found that PALA, in the lung cancer model especially, PALA is curative as a single agent for about 50% of the mice. Long-term cures. And when we re-challenge those mice with the same orthotopically implanted tumor in the lungs, Lewis lung carcinoma, they are resistant. The tumor doesn't take, even in the absence of added therapy. So clear that the mice have mounted an immune response, which is another aspect that we can consider using in combination with other drugs.

Dale Shepard, MD, PhD: Interesting. And so that could have some impact if you use it in a neoadjuvant setting, and then maybe even follow with an adjuvant setting sort of as a boost or something.

George Stark, PhD: Yep.

Dale Shepard, MD, PhD: Interesting.

Christine McDonald, PhD: Yeah. So we have lots of ideas.

Dale Shepard, MD, PhD: You guys have lots of ideas.

Christine McDonald, PhD: Lots of ideas.

Dale Shepard, MD, PhD: Now speaking of ideas, how far along are we? Like I say, a lot of times we talk about things where we're... Some new great drug, kilatumumab that just got approved for something. Where are we along the way in the process?

Christine McDonald, PhD: We're trying to get together a clinical protocol to get an IND approval for topical application of our drug to treat non-melanoma skin cancers. We're on the path, it's been slower than I'd like, but all of the regulatory steps take a certain amount of time to get through.

But we're very excited about the topical application, not only because it seems to have an antitumor effect, but in contrast to what is already out there for standards of care in terms of a field treatment effect, we're not seeing the same types of side effects. So current standards of care are very inflammatory and not very well tolerated by patients, and we're not seeing those inflammatory reactions with our topical agent. So...

Dale Shepard, MD, PhD: So comparing to things like 5-FU or things like that.

Christine McDonald, PhD: Mm-hmm, or imiquimod.

Dale Shepard, MD, PhD: Gotcha. Let's talk a little bit about that in terms of delivery. So you mentioned topical. What sort of delivery mechanisms are you guys looking into?

George Stark, PhD: Well, we've worked with a company that develops topical agents, and they've put together for us a five component cream that works with PALA, all the components of which have already been approved by the FDA. So we're basically ready to go with that. We've used that extensively in the mouse model, so we know that it's very well tolerated, et cetera.

Just getting back to the question that you asked Christine, the other piece of this, in order to do clinical trials of course you need money. And there are two possible sources of money. So we've formed a company in order to raise money, and we're actively doing that. And we've raised some, enough to do the clinical trial in skin cancer at this point, but we were very hopeful that we'd be able to get funds from the NExT program of the NCI. And we prepared two different applications, one in skin cancer, one in lung cancer. But unfortunately, at least for the moment, that program is not accepting any applications. And we are hopeful that at some time in the future it will be, but at the moment it's shut down.

So the main recourse we have right now is to raise the money ourselves as best we can, and to proceed that way with the funding of the trials.

Dale Shepard, MD, PhD: Yeah. This is something that seemingly... basic science translate at the cellular level to animal studies, then to people. It's always these big gaps that you have to sort of overcome and build a bridge to get there.

For people that might be listening in that might be figuring out how you get from place to place to place, here at the Cleveland Clinic we have the Innovations Group. So I guess just from a very practical standpoint, how does one decide whether to go through the institution's Innovations program or do things like develop a company? And what's the thought process for people who might be listening and saying, "I have an idea, but where do I go?"

Christine McDonald, PhD: The first place you go is the Cleveland Clinic Innovations. And they've been very supportive of this project and helped a lot with some of the very early stages, and actually in terms of supporting the formulation of our topical and walking through the development process with us. At a certain stage though, the Cleveland Clinic isn't going to be able to fund every great idea. And so we were encouraged to form a company to try to fundraise to continue the development process. But Innovations has been a partner of ours since the beginning.

Dale Shepard, MD, PhD: So it's kind of a step along the way.

Christine McDonald, PhD: Yeah.

George Stark, PhD: Yeah. And sort of relevant to that, we have two patents filed now... Or I should say the Cleveland Clinic, which owns the intellectual property, has two patents filed, one for the topical use and one for systemic use of PALA. So at least the intellectual property is protected, which of course is essential if you're going to raise any money.

Dale Shepard, MD, PhD: Where do we think this is going to go from a timeframe standpoint? 'Cause this is something that people... They see the late stage clinical part and they say, "Well, my phase three trial took forever to accrue and report." And we're a few steps back from that. Where do you think we are? And actually in fact, this particular drug having been around, having, as you said, lots of studies, having been in humans, do you get a jumpstart on some of that because you already have phase one information, toxicity information? What do you think the trajectory looks like here?

Christine McDonald, PhD: I would love to have this approved yesterday, so I'm a little impatient. But optimistically, I'm hoping we can get some trials up and running in the next year, assuming the regulatory process goes smoothly. But we will have a jumpstart over some other compounds that are in development, because we do have the information from the original trials in terms of toxicity, side effects, PKPD, all the pharmacology that we can leverage and start at a different point in the journey to trying this as a clinical solution.

George Stark, PhD: We've been working very collaboratively, not only with the cancer center in the Cleveland Clinic, but also with the Case Comprehensive Cancer Center, of which of course the clinic is a participant. And they're actively now working with us to facilitate getting the FDA approval that we'll need for the trial. We're basically ready to go, because for the initial trial, as we understand it so far, for simple toxicity assay on the skin, which has never been done in humans, we will be able to use non-clinical-grade drug, the same drug that we've been using in the mice. If we go any further, of course we're going to have to have clinical-grade drug and that will take a while. So we're embarking on the synthesis of the drug.

But right now we think that all we need to go ahead with the skin cancer is to have the trial designed and good thoughts about that already, and to interact with the FDA and get the necessary IND so that we can go ahead. And that might be as soon as four to six months, if everything goes perfectly.

Dale Shepard, MD, PhD: Looks like you're getting close.

George Stark, PhD: I think so. Very exciting.

Dale Shepard, MD, PhD: Yeah. So you have interactions, NOD, CAD, you've got a good drug, you've got this trajectory here, which looks like it's going well. Are we also taking a look back to say, are there other drugs that might be working through this pathway? And then have something else in the background working on similar ways, in case there's a problem with this and optimize this. What does that look like?

Christine McDonald, PhD: I would say, I don't know if we have another compound in the pipeline. But we are working actively to try to understand how the drug is acting and, as you say, what are all the players involved in case there's another way we can get a similar effect. But a lot of this is trying to understand not just the clinical result, but how we get there.

George Stark, PhD: I must say that when we designed PALA, we did such a good job that it's not possible to make a derivative that's any better.

Dale Shepard, MD, PhD: It is the best one.

George Stark, PhD: It is the best one.

Christine McDonald, PhD: Not that he's biased.

George Stark, PhD: It was intelligent design at the time. And so I really honestly don't think it's going to be possible to improve on that drug.

But we have a colleague at Roswell Park Cancer Institute who is working on a drug that inhibits further down in the same pathway, and it seems to have effects which he's studying, but may also be a part of the antitumor mechanism of PALA. So another drug that's actually well along and being studied, and they give us some important information about how our drug works.

Dale Shepard, MD, PhD: So you started this journey a really long time ago.

George Stark, PhD: Yeah.

Dale Shepard, MD, PhD: And so it must be tremendously satisfying to see the progress.

George Stark, PhD: Well I started when I was a teenager. No, I wish.

Dale Shepard, MD, PhD: So that was like eight or 10 years ago, right?

George Stark, PhD: No, it's been a very strange journey because we were so full of hope all those years ago and then it didn't pan out. But now Christine has revived PALA from the dead, and it looks like it's going to have a very successful afterlife.

Dale Shepard, MD, PhD: It's an interesting story, 'cause there's a lot of interest in repurposing drugs and going back and saying, "Well, this had promise. Did we miss something?" It's rare that someone gets to resurrect their own drug that they've worked on and then show progress. So well done, guys.

Christine McDonald, PhD: Well, it's been very exciting. And serendipity of... I came to the Cleveland Clinic and out of this screen found this enzyme, and George was in the next building over to give us reagents that led to this finding. Which is amazing.

Dale Shepard, MD, PhD: It's a good example of how we all work together well.

Christine McDonald, PhD: Oh, yeah.

George Stark, PhD: Well, it's also a case of chance favoring the prepared mind, you might say, because there's so much luck involved in getting us to this point. When we designed the drug, we had no idea it would penetrate cells, for example. And it does very, very well. And the serendipity of Christine discovering that CAD had this inhibitory function on the activation of NOD2, again, and just being in the same institution, is just beyond any reasonable probability. So yeah.

Dale Shepard, MD, PhD: You've been doing great work, and good luck with your continued success. And we'll look forward to hearing some more promising results.

Christine McDonald, PhD: Great.

George Stark, PhD: We'd be happy to report them.

Christine McDonald, PhD: Oh, yes. Thank you for having us.

Dale Shepard, MD, PhD: Absolutely.

George Stark, PhD: Thank you.

Dale Shepard, MD, PhD: To make a direct online referral to our Cancer Institute, complete our online cancer patient referral form by visiting clevelandclinic.org/cancerpatientreferrals. You will receive confirmation once the appointment is scheduled.

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