Engineered Bacteria for Cancer Treatment

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 Shepherd, a Medical Oncologist and Co-Director of the Sarcoma Program at Cleveland Clinic. Today, I'm happy to be joined by Dr. Mohammad Dwidar, Assistant Staff in Lerner Research Institute, and Assistant Professor of Molecular Medicine at the Cleveland Clinic Lerner College of Medicine. Today he's here with us to discuss engineered bacteria for treatment of cancer. So, welcome.

Mohammed Dwidar, PhD: Thank you. Thank you for hosting he here.

Dale Shepard, MD, PhD: Sure. Give us a little bit of an idea. What do you do here at Cleveland Clinic?

Mohammed Dwidar, PhD: So basically, our lab is interested in engineering microbes as therapeutics and probiotics and diagnostics. So we try to actually use this microbes for good things. So some microbes are good and those are the ones that we try to make them better probiotics, for example. And we have bad bacteria that are pathogens like salmonella that we are going to talk about today. And those ones, we can also make use of them for treatment of bad diseases like cancer, for example.

Dale Shepard, MD, PhD: Give us a little bit of an idea. So there's certain viral therapies that we use for cancer now.

Mohammed Dwidar, PhD: Right.

Dale Shepard, MD, PhD: Using bacteria for cancers is kind of a newer concept, right? Give us a little bit of an idea of the history of that, and then we'll go from there, kind of what you're doing now.

Mohammed Dwidar, PhD: First of all, the use of the bacteria for treatment of cancer is not that new. Subibo actually said it started almost a hundred years ago with the first trials of Dr. William Cooley, who was an oncologist, and he used to inject his sarcoma patients with a mixture of bad bacteria, Serratia and streptococci, directly into the tumor. And surprisingly at that time, he saw that some patients responded and the tumor started to shrink in this patients. Of course, many patients died at that time because of the septicemia, which that was before the antibiotic era. So that's very expected.

But the basics was like settled at that time and people started to realize that, "Yes, we can use bacteria really as a kind of immunotherapy. They didn't know anything about immunotherapy at that time. But with this advance that we see now in immunotherapy for cancer, this is basically the era of the immunotherapy, right? And that's why we started to rethink again about the bacteria. So we can think about the bacteria-based cancer therapy as one of the oldest types of immunotherapies for cancer.

Dale Shepard, MD, PhD: So how are you trying to engineer bacteria currently to treat cancer?

Mohammed Dwidar, PhD: So basically, if the bacteria is good in targeting tumors, why we don't use them? And the answer is easy, because if you try to inject wild type bacteria in the blood, that's the definition of septicemia, right? Some bacteria have natural tendency to go to the tumors like salmonella, that's what we are using, some anerobic bacteria like Clostridia, for example. Although this bacteria can... If you inject them in the blood, they will go to the tumor, but they actually still can go to other organs and they will cause toxicity. And of course this will lead to death at the end.

And then the other thing is that when they go to the tumor, they can reduce the tumor growth, but they will not eradicate the tumor completely. So that's why we need to do two things. The first thing is that we need to enhance the specificity of the bacteria to the cancer so that we can really improve the safety of the bacteria-based cancer therapy. And at the same time, we need to make this bacteria more effective as therapeutics, mainly as tools to induce strong immune response against the tumor.

Dale Shepard, MD, PhD: So the concept is that the bacteria goes into the tumor cell, grows, and then causes lysis of the cancer that then elicits an immune response or mechanistically how-

Mohammed Dwidar, PhD: Yes.

Dale Shepard, MD, PhD: Is that how this is working?

Mohammed Dwidar, PhD: Yes. Not exactly, because the bacteria don't not necessarily go to that tumor cell. So some bacteria, they are intracellular pathogens like listeria. They still go to the tumor. Some are extracellular pathogens like Clostridia. They go to the tumor. And both of them, they have a strong antitumor effect. Salmonella, we can say it's kind of facultative. They invade the cells, but also they live extracellularly. But the presence of the bacteria in the tumor microenvironment, even if they are not inside the tumor cell, just in the tumor microenvironment itself, will elicit a strong immune response that will make the immune system better recognize that tumor and hopefully, ultimately we wanted this to lead to establishment of adaptive immunity against the tumor. That's the ultimate goal.

Dale Shepard, MD, PhD: In the research you're doing, what's being done to more specifically target the bacteria to tumor cells?

Mohammed Dwidar, PhD: Basically, when we started this work almost six years ago, we asked a simple question, "How can we make the bacteria more specific to the tumor?" That's the first hurdle, right, that really prevent the widespread application of the bacteria-based cancer cell.

And then we thought, "Okay, the way we can do it is let's try to look at the tumor metabolome and see if we can use some of the metabolites that are enriched in the tumor as kind of a hook to target the bacteria there."

And then we looked at the long list of metabolites. We know that the tumor microenvironment is unique and there are severe metabolites that are enriched and severe that are depleted. But there was one metabolite, it's called kynurenine. And this metabolite is overproducing almost all solid tumors. So we thought, "Okay, this can be a nice target for us, a good target if we can make our bacteria really go after this kynurenine."

And that was the main challenge that our recent study was focusing on. How can we make the bacteria like salmonella or E. coli really target this kynurenine? And if that can be enough to really enhance the specificity to the tumor.

Dale Shepard, MD, PhD: When you think about the tumor microenvironment, and you mentioned that most tumors will produce these metabolites that attract bacteria, are there some cancers that we think may be more or less likely to be responsive? So I guess what I'm thinking is that one of the reasons immunotherapy, traditional immunotherapy, even chemotherapies have been thought not to work for pancreatic cancer, for instance, is like a dense tumor microenvironment. Is there some thought that bacteria can more easily get into that area of the tumor?

Mohammed Dwidar, PhD: So that's actually a great question. So the beauty of the bacteria-based cancer therapy in general, compared to the traditional immunotherapy... So traditional immunotherapies like the CAR T-cells or the drug conjugated antibodies, for example, this kind of therapy is usually target specific antigen on the tumor, and that's why it's easy for the tumor to escape it once they downregulate this antigen, right? But the bacteria-based cancer therapy in general, it's really like a general treatment that they go naturally to any tumor. Any solid tumor that you have anywhere in the body, as long as the bacteria can access it, they will go, and they will accumulate there. That's natural. That's even without any engineering. And that's due to many reasons. Above all is the immunosuppressive environment of the tumor will protect the bacteria there so that they will survive better, while they will be cleared physically in other tissues. What we wanted to do with kynurenine is just to enhance this specificity, but even without kynurenine, salmonella will go, but we will just try to work above this.

Now, some tumors, of course, will not produce as much as kynurenine as other tumors. So when we did the experiments, we did it in ovarian cancer and breast cancer. And right now, we saw that our bacteria works also in colon cancer models or in melanoma cancer models as well. But of course, we do not expect it to work in every single cancer, but we try to basically find a metabolite that's widely produced, which is kynurenine. Interestingly, we found that even the tumors like in our experiments that produce very little kynurenine, our bacteria can still go there and they can colonize them well.

Dale Shepard, MD, PhD: Just logistically, how long does the bacteria stay in the body? So is this something where you have to do repeated infusions, or do we know yet about what treatment would look like?

Mohammed Dwidar, PhD: So we did experiment in mice.

Dale Shepard, MD, PhD: Yeah.

Mohammed Dwidar, PhD: I don't know about humans. And it will depend also on if the humans, if this patient basically got a recent salmonella infection or not, and what is the antibody titer? Again, is the salmonella in his blood? But for mice, at least we saw that when the salmonella will stay there for let's say a couple of weeks. You just inject once in the blood, they will go to the tumor, they will accumulate there. They will go to the liver or the spleen, the other organs as well. But they will be cleared from this organs within a couple of days. Then they will stay in the tumor with fair number, like really [inaudible 00:10:45] gram, which is really a high number. And then they will stay like this for, let's say on average, a couple of weeks or so. But eventually, the mice will have adaptive immunity against this salmonella and they will kick them out.

Our hope is that in the future, we can make this salmonella better that at this time window, that by the time basically the body develop adaptive immunity against salmonella and kick the salmonella out, we hope that adaptive immunity against the cancer is also established, but we are not yet there.

Dale Shepard, MD, PhD: So it would be because of the development of sort of the adaptive immunity wouldn't be a chronic treatment because you wouldn't be able to sustain that?

Mohammed Dwidar, PhD: Yeah, definitely. We can't keep injecting... We can't keep the salmonella in someone body forever.

Dale Shepard, MD, PhD: Oh, you never know. People probably originally said, "You can't just keep giving chemo."

Mohammed Dwidar, PhD: Oh, that's right. Yes, but the idea is that even those that you keep giving chemotherapy, but I think the life becomes not that good at that time.

Dale Shepard, MD, PhD: In the mice that you've studied, is there evidence that there is some sort of septic symptoms until the bacteria clears the bloodstream and gets into the tumor microenvironment?

Mohammed Dwidar, PhD: Yeah, so that depends on the dose as well. What we found is that... So there is one strain of salmonella that's being actually used now in clinical trials. It's called salmonella typhimurium VNP20009, which was engineered in a different way. And it's being used now in a clinical trial. So what we found is that our strain, which is basically kynurenine, so that strain is safer. It means when you give both bacteria at the same dose, our strain cleared faster from other organs, while the VNP strain, the benchmark strain, stayed there for some time.

And because of this, we demonstrated that our strain is safer, and they have less toxicity on the mice. And we can see this by less splenomegaly at the end, less production of the cytokines in the blood, and better survival of the mice. But of course, if you give very high dose of our strain, that will also kill the mice. Even if you give it dead bacteria, there will be like a shock still, and they will kill the mice. So you have to still control the dose, but it is safer than the strain that is being now used in clinical trials.

Dale Shepard, MD, PhD: You talked about the fact that number of solid tumors express the metabolite, the-

Mohammed Dwidar, PhD: Kynurenine, yes.

Dale Shepard, MD, PhD: ... kynurenine. And even the tumors that don't express a lot or secrete a lot still will attract the bacteria. Are there locations that you've noted are more or less likely? So something in like a retroperitoneum? Are there tumor locations where it seems to matter?

Mohammed Dwidar, PhD: The location will matter because I imagine that if the tumor is in the brain, then the brain will not allow much inflammation. Basically, it cannot tolerate it, right? And the kynurenine actually accumulate because of the inflammation that's caused by the tumor. So one main reason why kynurenine accumulate in the tumor is because of the induction by interferon-gamma. As the tumor attracts more immune cells, they start to produce enough interferon-gamma and this interferon-gamma is what actually caused the tumor cells to produce kynurenine in response to interferon-gamma. So I don't think it's the location. Maybe it's like... We don't know why. Even when we do it in mice, and there is really variations in the kynurenine concentration among the tumors. Of course, there are several factors, the size of the tumor, how hot is the tumor, the immune cells as well, and location could be another factor as well.

Dale Shepard, MD, PhD: From the studies so far, it looks promising. Are there sort of things that you've already identified that might need tweaked to make this most effective?

Mohammed Dwidar, PhD: So our first study was just to make the bacteria go after kynurenine. We were just using this kynurenine as a hook. But our goal was not to make this bacteria more effective as a cancer. We just wanted to make it more safe. That's it. But right now, we are actually doing a lot to try to make it more effective in stimulating the immune system to attack that tumor, and we are trying to equip this salmonella now with different subtherapeutic payloads.

Dale Shepard, MD, PhD: What's the next step?

Mohammed Dwidar, PhD: So the next step is to try to equip it with different subtherapeutic payloads, which we are doing now to make it more destructive physically and more stimulatory for the immune system with the hope of within the next few years, three or four years or something like this, we can take the first version of our strain to the clinic or at least to clinical trials.

Dale Shepard, MD, PhD: Fantastic. So you work with the salmonella strain. Are there other types of bacteria or other approaches you guys are working on in parallel?

Mohammed Dwidar, PhD: No, we are mainly focusing on salmonella now because of several reasons. There are other groups that are focusing on listeria, and some groups are focusing on Clostridium. And there are others that are focusing on E. coli as well. And so, there are different bacteria that are being studied as cancer therapeutics. Salmonella has several advantages. It's one of the most studied organisms. And compared to Clostridia, for example, they have much amenable genetic tools. We can modify them easier than the Clostridium, which is obligate anaerobic bacteria. And compared to E. coli, they have the advantage that they are professional pathogens. So they can go and invade and can stay in the cancer, because that's their job, right? But E. coli is not that good.

Dale Shepard, MD, PhD: Sounds great. Yeah. I participated in a trial with Clostridium. We had to inject it into tumors because of the anaerobic nature.

Mohammed Dwidar, PhD: Yes, yes, exactly. Clostridium is really hard to go and manipulate genetically, because it's obligate anaerobes. So everything that takes like one hour for salmonella is going to take like one day for Clostridium.

Dale Shepard, MD, PhD: Well, some fascinating insights on how we can develop some novel ways to treat cancer. Appreciate you joining me.

Mohammed Dwidar, PhD: Yeah, I hope. 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.

This concludes this episode of Cancer Advances. For more podcast episodes, visit our website, clevelandclinic.org/canceradvancespodcast. Subscribe on Apple Podcasts, Spotify, or wherever you listen to podcasts.

Thank you for listening. Please join us again soon.