Advances in Cell-Based Immunotherapies: CAR-T Cell Therapy vs TCR Therapy

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To another episode of Conversations in Drug Development. My name is Harriet Edwards and I'm your podcast host for today's episode. Today I'm delighted to be joined by not one but two guests on our podcast, both regulatory experts. Patrick you may remember from a previous podcast, he leads our advanced therapy medicinal product regulatory function here at Boyd's and Monica Pialella also our regulatory expert for all things advanced therapies.

Both are part of the regulatory team here at Boyd's and I'm delighted to welcome you both to the podcast. Hi both.

Hi, thanks. Nice to be here. Hello, Harriet. Great to be here indeed. So unsurprisingly, today's episode is going to be a regulatory affairs one and particularly focused on advanced therapies as we have our experts in the house.

But we're going to specifically be talking about cell-based immunotherapies. Now I know that's quite a broad scope, isn't it, Patrick? So maybe you could narrow it down a little bit for us and let everybody know what's in store for today's episode. Yeah, of course.

Yeah, it is a very broad scope as you say, but today really what we're going to be talking about are products consisting of cells within the human immune system that can be manipulated to have specific therapeutic properties, such as tumor killing being the best example. It is a very broad field, lots of different modalities, CAR-T cells, TCR-T cells, natural killer cells, Tregs, et cetera. But we're going to limit the scope today really by and large to CAR-T cells and TCR-T cells.

And the most common application for those therapeutics, which is oncology. Sounds good. Certainly an exciting episode with lots of interesting content to come.

But maybe before we dive in, we should start with some basic definitions and just to set the scene, particularly for those listeners that maybe aren't working in advanced therapies or specifically in cell-based immunotherapies at the moment, because it is quite a niche area. So Monica, before we get stuck into the detail, can you give us a brief definition of what TCR-Ts and then later CAR-Ts are as well?

Yes, so starting with some basic definitions, TCR-T cells stand for T-cell receptor T-cells and this receptor contains an extracellular portion consisting of four CD3 chains surrounding two main peptide chains, the TCR-α and TCR-β and are these peptide chains responsible of conferring the ability to the T-cells to recognize polypeptide fragments which are presented by a major histocompatibility complex, also known MHC molecules on the cell surface.

And also the TCR characteristic is that they are naturally occurring antigen sensors and therefore they result in a near to physiological signaling pathway. So just to pick up on one of the points that you mentioned there Monica, TCR T-cells are MHC-restricted, I should say. So that means that they can only interact with antigens when they are attached to an MHC molecule. Is that correct?

Yes, that's correct. So the MHC, which is also known as human leukocyte antigen, or HLA in humans, is composed of a group of glycoproteins that are able to process and present antigens, even intracellular antigens, on the cell surface for the T cells in order to discriminate whatever is self from non-self. So overall, they play a very important role in the body's immune response to pathogens.

So that's a really good introduction to TCRTs. We did mention, though, at the beginning of the podcast, Patrick said that we would be discussing two major types of cell-based immunotherapy. So TCRTs, which we've just defined, but also CAR T cells, which I know maybe the term at least is familiar to lots of people, but perhaps we should start there with a basic definition as well. What are CAR T cells? For sure. So CAR T cells stand for chimeric contigem receptor T cells.

And this CAR receptor comprises a T cell receptor activation intracellular domain, which is fused with an extracellular antibody variable domain, which again here confers the specificity to the CAR T cells to identify specific antigens and kill those cells bearing those specific antigens, which in the case of oncology is tumor cells. And in this antibody variable domain, instead of occurring naturally, it's actually designed.

So that's what differences it from the TCR. And also these CAR T cells are only able to recognize cell surface antigens and therefore cannot be considered MHC restricted as instead the TCR T cells are. So quite significant differences in there between CAR T cells and TCR T cells that we just mentioned. One point in particular that you mentioned I want to pick up on a little bit further was the fact that CAR-Ts are not naturally derived.

So they are obviously designed, they are synthetically produced and are not naturally occurring. Let's talk a little bit more about how and why they have evolved, because they have evolved, haven't they, since their introduction? Yes, that's correct. So the CAR design has gone through an evolutionary process by a number of CAR generations. And this optimization is aimed at addressing some of the limitations of these therapies.

And just to mention a few, these include limitations of trafficking to solid tumors or the limited cell persistence once administered in the body or the limited antigen recognition. So we talked about multiple generations of CARs. There's not just been one evolutionary change. There's been many to date and will, I'm sure, continue to be so. What has changed since the first generation?

Yeah, so what we have described earlier is actually the first generation of CAR T-cells, but this does not make them ideal because T-cells often become exhausted since they lack of costimulatory signals. So in the second generation of CAR T-cells, which is, by the way, the design of all commercially available CAR T-cell products currently approved.

This second generation of CAR-T, they've seen the introduction of a co-stimulatory domain in their intracellular portion of the receptor, and this has resulted in a significantly improved cell persistence, and we've seen today up to a decade following therapy.

And it is in the third generation CAR-T cells where a second co-stimulatory signal or domain had been introduced, but although this resulted in an improved cell persistence in the body, did not really bring additional superior antitumor activity. And of course, researchers tried to go through many other iterations of the design of this linear construct, and all these iterations fall under the bucket of next-generation cars.

But more recently, researchers have started thinking a bit more outside of the box with innovative alternatives, such as the lateral cars, aimed again at improving the surface systems and target specificity. So it's fair to say that there is an awful lot of efforts and innovation going on with making sure these types of cells are the best that they can be. And I guess the science will continue to evolve and we'll see more generations of these CAR-T products.

But you did mention, Monica, that the second generation cars, so a little bit further behind where we are today in the lab, are the ones that are commercially available. So let's switch a little bit from the research and development aspect of these CAR T cells and talk a little bit more about the regulatory landscape. So what's actually happening in terms of products that may be either in clinical trials or indeed on the market already?

Yeah, so to date, we have six CAR Ts currently approved, both in Europe and in US, with the first one being Kimbriah from Novartis in 2017. And one main characteristic is that they are all directed against two major targets. So tumor expressing either CD19 or BCMA, which stands for B-cell moderation antigen.

Yeah, it's probably also worth mentioning at this juncture that all of those CAR-Ts that have been approved that Monica just mentioned are for hematological malignancies, so not solid tumors. And unlike CAR-Ts is that there are actually no TCR-T products currently on the market. Okay, so that's two interesting points that you mentioned there, Patrick.

Maybe let's pick up on both of those. To start with, why is it that you mentioned CAR T products are only limited to, at the moment, to hematological malignancies? Why is that, Monica? Do we have a particular reason? So CAR T-cells usually have a very high target specificity, and this makes them ideal for the treatment of hematological tumors, because in these tumors, antigens are often specific and are not expressed in other normal tissues.

However, this does not make them handy when it comes to treating solid tumors, because most of these solid tumors antigens are also expressed in small amounts in normal tissues, such as heart, lung, or liver. And this results in on-target off-tumor toxicity. And another important aspect to consider, as we mentioned before, CAR T-cells are only able to recognize and detect surface antigens. So this gives them access to a more limited pool of targets compared to TCR T-cells.

And additionally, it's worth mentioning that we have had concerns over safety for these CAR T-cell therapies, but I'm pretty sure we're going to discuss later at this podcast. Yes, absolutely. Absolutely. And safety is always a concern, isn't it? But we should specifically touch on that in relation to CAR-T cell therapy. So we certainly will. But Patrick, just to go back on the second point that you made around there not being any TCR-T cell products on the market yet.

Why is it that success has been limited in these particular cell types to date? Is there a specific reason? Have we developed CAR-Ts earlier, for example, than TCR-Ts or is there something else?

Yeah it's an interesting one i mean tcrts have actually been around quite a long time probably around the same sort of time as cartes certainly since the 1990s in the clinic and unlike cartes tcrts can recognize both surface and intracellular antigens as monica's mentioned so that does give them a potentially very wide range of targets including solid tumor antigens that lack those specific surface tumor markers there's also literature to suggest

that they have better tumor penetrance and distribution within the tumor compared to cartes so in theory huge potential in solid tumors, but as we say, there are none currently on the market. There are some limitations with TCRTs because they can only recognize specific MHC molecules. This does limit the patient pool that's treatable with one individual TCR, which is actually a major contributor to the limited success, to be honest.

There are also other factors such as target tumor antigen heterogeneity and off-target toxicity caused by cross-reactivity with healthy tissues. Okay, so there is still a lot of work to do in order to get these products, at least one of these products onto the market.

So maybe let's talk about the challenges that exist within development for these types of cell based immunotherapies, because I'm sure there are numerous ones.

There are obviously generic challenges that apply across the board to advanced therapies as a whole but then when we think more specifically about cell-based therapies and Patrick you and I have spoken in a previous podcast about challenges in in manufacturing supply of autologous versus allogeneic cell-based therapy for example so obviously we won't repeat that challenge and please do listen into the particular podcast for more information on that but Monica perhaps you

could give us a little bit more information into some of maybe the specific CMC challenges that exist for CAR-Ts and TCR-Ts. Well, we could spend a whole podcast talking about CMC challenges for these therapies, but for the purpose of today's podcast, perhaps it's worth mentioning that the heterogeneity in starting materials and interpatient inter-donor variability.

Because this can significantly affect the manufacturing process consistency, both between batches but also across indications and this can affect significantly the drug product purity and cell composition which as a result may impact the product functionality and the overall product efficacy profile. And let's not forget that many of these if not all of these types of cell therapies are genetically modified as well so there's also that to consider.

That's correct so that adds an extra layer of complexity for vector manufacture and control. This is because developing a vector, which can either be viral or non-viral vector, this is almost like developing another investigational medicinal product, like another drug substance. And this is also valid for emerging genome editing tools, where developers will need to characterize the starting material, such as nucleases and guide RNAs, and treat them really as separate drug substances.

Also when it comes to authoring the CMC section of the application as you would basically do for a viral vector. Well that's a really good point and I am deliberately skipping over the mention of genome editing not on purpose but purely because it is a topic within itself and we do have again another podcast on that which I'd really encourage people to go and listen to if you are interested in genome editing but a huge extra complexity it's fair to say something that's already very complex.

But maybe let's think about any other CMC considerations, Monica, that may be worth mentioning in particular in relation to CAR Ts and TCR Ts. For sure. So what's worth mentioning is probably challenges for developing a suitable potency assay. So usually a potency assay should measure a product attribute that is relevant to the product's intended therapeutic effect. But developers and regulators do know and are aware of the challenges of developing such an assay.

So guidance have been issued both in Europe and in the US to encourage the use of orthogonal methods in order to obtain more meaningful full information on the product potency and the general recommendation is usually to start as soon as possible in development.

Good advice to follow and in terms of CMC considerations I know that was a somewhat of a whistle stop till but there was a lot of information there and a lot of challenges that exist and things for people to certainly think about so a good signpost to maybe delve into in a little bit more detail on another day certainly plenty to ponder from a manufacturing perspective anyway, it's fair to say.

But earlier you mentioned, or at least alluded to the fact that there were safety concerns around CAR-Ts and TCR-T products, as there are with pretty much any innovative medicine, but there are some specific safety concerns.

So Patrick, maybe if you could expand a little bit further on that, what are the major risks and what are the general approaches to safety when we think about these products in particular?

Yeah, of course. So there are a number of risks that you need to consider. One is sites Heideckine release syndrome, which people may be more familiar with that term, a serious inflammatory reaction, which can cause organ failure and death in serious cases in patients. Also neurotoxicity, on target but off tumor effects and off target effects, such as cross reactivity with unknown targets. But also other things can be factored in as risks, such as the conditioning regime.

So when you perform chemotherapeutic depletion of the patient's T cells and reduce the tumor burden prior to the CAR T or the TCRT therapy, this has actually been associated with toxicity as well. So for example, if you want to delve a bit further into this, you can look at the Juno rocket clinical case study as this encapsulates the complexity of linking toxicity to causality. It's also worth thinking about the risk of secondary malignancies, which have been in the news recently.

So if using integrating viral vectors such as retroviral vectors or lentiviral vectors the latter have been used to manufacture all of the commercially available CAR T cell products that's a consideration or for gene edited products you will have off-target cutting the genome potentially which may lead to genotoxicity and secondary tumor formation and it'd probably be remiss of me to mention that with CRS really is considered to be one of the major risks but also is a higher

risk in CAR T over other types of immunotherapy due to highly highly prolific nature of the cells and more upregulation of the inflammatory pathways when compared to TCRTs for example.

So again a lot to consider from a safety perspective and I totally agree with you the Juno rocket study is a worthwhile case study to go and have a look at for more detail but you did mention secondary malignancies there Patrick and I wanted to pick up on that a little bit more because it is a hot topic it seems to be in the news a lot at the the moment.

So Monica, maybe could you give us a little bit more insight into the specifics of the concerns around secondary malignancies for TCRTs and CAR-T therapies? Yes, as you rightly said, there has been a lot in the news about this serious risk and FDA investigations of reports from both clinical trials and post-marketing adverse events resulted in two major changes.

First of all, on the long-term follow-up requirements. So now patients and clinical trial participants receiving these treatments will need to be monitored lifelong for new malignancies. And also the second major change has been on labelling, requesting the addition of a boxed warning to the products prescribing information for all the approved CAR T cell therapies to date.

A similar data review is being conducted by the European Pharmacovigilance Risk Assessment Committee, also known PRAC, and they've reached a similar outcome to the FDA. And in addition, a direct healthcare professional communication. Also named as DHPC, will be required as a reminder of the lifelong monitoring of patients.

That sounds quite scary to anyone that's maybe not working in this field to have additional warnings and concern and potentially some really serious side effects but I do think it's fair to mention that the overall benefits of these products do still absolutely outweigh the potential risks for their approved indication.

So obviously lots of patients that are receiving these types of products are you know very very sick, there may be a last mass line treatment, they're exhausted all other potential options. And so there is still huge benefit in providing these types of products. But Patrick, aside from the safety concerns and manufacturing challenges, there are surely other development challenges that exist from a non-clinical and clinical perspective. I guess it's not plain sailing there either.

No, it's not. And in many ways, when you go back to non-clinical package, it's a case of like like ripping it up and starting again, comparing with a lot of other modalities. Generating safety and proof of concept efficacy data with a human product in animals is very challenging, as has been recognized by the regulatory guidance, which exists out there.

Because there is a lack of suitable in vivo models due to both species and target specificity, and it's very difficult to measure human immune responses in animals. So conventional toxicity studies are usually not applicable, and measuring safety risks such as CRS, which we've mentioned in neurotoxicity, is not feasible in animals, or certainly very easy in animals.

So quite often you'll see in vitro methods being used to assess safety, which are actually usually focused on toxicity related to off-target effects, such as cross-reactivity studies and screening for potential damage to healthy cells that express the same tumour antigen or in other words on target off tumour toxicity. Also assessment of the effective starting clinical dose is not straightforward. It's frequently now based on literature and clinical precedent.

You might have some in vivo work to support that and translation into the clinic often requires a sort of more cautious dose escalation approach especially if if there's novelty in the product or in the target. Patrick, I'm hoping we're not going to put anybody off wanting to develop these types of products, because we have mentioned an awful lot of risks, safety concerns and challenges that exist. But I think it's really fair to say it's not all negative.

There has actually been some really positive progression in this field over the last few years, hasn't there? There definitely has. And the approved CAR-T products that have traditionally been seen as the last line of defence in refractory relapsed patients that you referred to earlier, Harriet, where they've undergone multiple rounds of treatment, chemotherapy in different doses. Hematopoietic stem cell transplantation, all would be considered ahead of CAR T-cell therapy.

But times are changing. The amount of data that's been generated because they got into the clinic a bit faster has allowed us to understand more about enhancing the product safety profiles through design of the CAR-Ts and the products, and better understanding and management of CRS and other toxicities that are opening up possibilities of CAR-Ts actually being used to treat patients as second-line therapies instead of being in sixth, seventh, eighth, ninth-line therapies, for example.

And that isn't just a hypothesis, is it, Monica? There are already some good examples of approvals for second-line therapy? Yes, there are indeed. So examples are Yaskarta and Brianzi, which have both received FDA approval to be used as second-line therapies for the treatment of large B-cell lymphoma. And others are Karvitky and Apsima, which have also received positive FDA approvals in recent days for the treatment of multiple myeloma, either in second line or third line therapy.

So I think it's worth mentioning that the products we've previously just discussed, Monica, they were CAR T-cell therapies. However, we are talking about TCRTs as well.

So Patrick, is there any potential positive news on the horizon for TCRTs? Yeah, there certainly is. So there's one product in particular, it's called a Famicel. It's being developed by a company called Adaptamume. Famicel is an autologous TCR T-cell therapy engineered to express an affinity-enhanced TCR targeting the MAGE A4 cancer antigen. And this is for the treatment of advanced sylnovia sarcoma, so solid tumor indication. They submitted the BLA to FDA back in December 2023.

FDA then accepted the BLA in January 24 with a priority review as supported by positive data from cohort one of the pivotal trial Spearhead 1, which met its primary endpoint for efficacy. So if approved, and obviously we have our fingers crossed that it does get approved, if Amicel would be the first TCR T-cell product to be approved globally.

We're definitely looking forward to hopefully some positive results from that in the not too distant future, perhaps even by the time the podcast comes to air, actually, we may, as you said, with that shortened review time of eight months, we may actually have some information and some news to share. So fingers crossed, very exciting.

But in terms of the future for cell-based immunotherapies as a whole we've very much focused on CAR T's and TCR T's today but is there any other potential positive news for other types of cells that you mentioned right at the beginning of the podcast Patrick or are we really restricted in clinical development and beyond to CAR T's and TCR T's?

Yeah, I mean, it's more limited outside of CAR-Ts and TCR-Ts, but there certainly is a lot of activity. I'll probably mention natural killer cells in particular. So even in their native form, natural killer cells are a key component of the innate immune response and represent the first line of defense against tumors. If you take it a step further and genetically modify NK cells with CARs and TCRs, this also has the potential for greater tumor target coverage for both liquid and solid tumors.

So if you had CAR or TCRNKs, they could also offer a potentially reduced burden of CRS and neurotoxicity, especially when compared to CAR-T, as there's less association with the release of plurinflammatory factors and have a shorter half-lives in vivo. So more limited persistence, essentially. However, this limited persistence in vivo makes it likely that repeat dosing has to be required to achieve a lasting therapeutic effect.

So you might need to apply some caution early in development when applying it to initial dose escalation studies but of course this should be balanced with the potential for an enhanced safety and efficacy profile and in this term there is certainly a lot of promise even though it's very early days in the clinic.

Definitely something else to watch out for then and something maybe we should discuss in a future podcast to see what the progress is but just to touch on we've talked about different types of cell-based immunotherapies and potentially different cells that could be used.

But with the broadening out of the modalities and the different types of therapies that we maybe will see in the future, Monica, is there any opportunity to maybe also increase the diversity of the indication or the application? Or do you think realistically we're restricted to oncology here? Actually, there is opportunity to increase also the application of these therapies in other fields beyond oncology, although we need to admit this is still very much in its infancy.

But an example is Atara Biotherapeutics, which have seen their IND clearance from the FDA in February 2024 for their allogeneic anti-CD19 CAR-T cell used as monotherapy for the treatment of systemic lupus erythematosus with kidney involvement, also known known as lupus nephritis. So definitely not a good space to watch out for. Definitely. And I think that's actually a really nice, positive way to wrap up the podcast today.

We've covered an awful lot, I think it's fair to say, and it's been incredibly interesting, particularly for those that maybe don't work in this space. It's a real eye-opener.

As I said before, I think it's really a good opportunity in the next 12 months to revisit this and have you back on the podcast both of you to see maybe what's changed because I think it's got the potential for huge evolution over even the next 12 months but to summarize the episode today and maybe your key take-home messages Monica if I could come to you first and then maybe to Patrick what would you say was the the key takeaways for today's episode?

Yeah, so starting with CAR-T cell therapies, certainly we have seen a successful application for the treatment of hematologic malignancies. And from a TCR-T cell perspective, we have seen promising application in solid cancers with the first therapy approval being in the horizon. In development, instead, we know the challenges remain. From a CMC perspective, for instance, we have mentioned earlier about purity of the cell populations in the final product.

But from a non-clinical point of view also the lack of suitable animal models but certainly history has generated significant learning to evolve the field.

Yeah and just to sort of finish off really when we talked about the safety profile of these products it is a continuous learning particularly when you're in the clinic so early but powerful efficacy of these approved CAR T cell therapies as you said earlier how it means they retain their favorable risk benefit profile and the emergence of other engineered immune cell types such as NK cells is also very promising for

the treatment of solid tumours, but there's definitely still a lot of unmet need. And as Monica just mentioned, the applications outside oncology are being investigated and they're very exciting, but it's still very much very early days. Wonderful. Well, it has been a pleasure to have you both on the podcast today, and we very much look forward to welcoming you back again in the not too distant future. Music.