American Society for Cell and Gene Therapy (ASCGT) conference recap by Mike Boland with Charlene Son Rigby

Here is the transcript of the podcast interview between Charlene Son Rigby and Mike Boland on the recent American Society for Cell and Gene Therapy (ASCGT) conference.

Welcome to the Science Plus Love Equals CURE Podcast. This is the STXBP1 10 Minute Monthly Update. This is podcast number 14.

I'm Charlene Son Rigby, president and co-founder of the STXBP1 Foundation, and I'm also mom to STXer Juno and her big brother, Luca. Today's topic is updates from the American Society for Cell and Gene Therapy. And I'm really delighted to have Mike Boland here.

So Mike is a dad to an STXer, Lucas, and he's also the strategic director of the ENDD Center at the University of Pennsylvania and CHOP. He was at the conference in person and is here to help us to understand the significance of what was presented and the research context. So I'm gonna give you an upfront warning, which is that this is gonna be a sciency podcast.

But I think that you're really gonna appreciate some of the detail and the depth that Mike is gonna be able to share with us about these important advances. So Mike, let's start with the first question. So there were actually five groups this year that were presenting.

“Can you tell us a little bit about this conference? Like who attends, what's the focus?

Sure, thanks for having me Charlene. This was the annual meeting of the American Society for Gene and Cell Therapy. The entire meeting is dedicated to various gene targeted therapies such as ASOs, CRISPR related therapies, AAV mediated therapies, and also some cell therapies such as cell replacement therapy for Parkinson's disease, for example.

Much of the meeting focuses on therapeutic approaches for rare diseases of the central nervous system, heart and liver, but there are also aspects of cancer therapy, CAR-T for example, which you hear a lot about in the news. It's attended primarily by basic and translational researchers, but there are also clinicians and governmental regulatory officers from the NIH and the FDA that attend.”

I have a basic question before we get into more of the content because we've been spending a lot of time talking about and thinking about clinical trials and the opportunity for our kids to get access to a new medicine. But a lot actually happens before we get to a clinical trial. And you mentioned those words about basic and translational researchers.

Can you lay out the main phases of therapy development and where the therapies are that are typically being presented at ASGCT?

Sure, there's a lot of hoops to jump through when you are trying to develop a therapy to a drug or to get into a clinical trial. There's usually a discovery phase in translational research where molecules or therapeutic approaches are screened or tested for their ability to appropriately modulate the protein of interest, whether to increase its expression or decrease its expression. And this is generally done in human cell lines or cells such as neurons derived from human pluripotent stem cells.

For example, in the case of hafluense efficiency, which is the primary disease mechanism for STXBP1, this occurs when a pathological mutation reduces expression of the protein in question by 50%. So we wanna test whether a given approach can restore protein levels back to normal. Once we have achieved that, we then move to preclinical testing of that therapy.

And this involves testing whether or not the therapy can correct disease-relevant deficits in cellular or animal disease models. From there, if that's successful, one would move the therapy to IND-enabling studies, usually in animal models like rodents and non-human primates, to further assess pharmacological profiles and any toxicity-related issues. If the therapy passes all of these hurdles, and then it's time to file for IND approval and the FDA and begin discussions about clinical trials.

Got it, got it. And I'm gonna ask you about IND approval in a minute, because I think that that's gonna be important, especially once we get to the discussion on capsida. But you also talked a lot about models, rodents, non-human primates.

Why do we work with models?

I mean, that's a great question. I mean, disease models come in a variety of shapes and sizes, cellular models that aren't related to neurons. Neurons derived from human-induced pluripotent stem cells, genetic mouse models, and even non-human primates.

But whether they are cellular or animal models, they enable us to better understand the genetic and molecular and functional causes or etiology of a disease. And they also provide a crucial means to test therapies.

Got it. And specifically in terms of like mice versus non-human primates, since it seems like a lot of the data that was presented at ASGCT was around those two models, like why do we need to use both of them and how are they different?

Well, we share about 90% of our DNA with mice and roughly 98% of our DNA with non-human primates like rhesus or macaque monkeys. Most of the genes and proteins we're interested in like STXBP1 are highly conserved between species, which makes other species fit for modeling human disease. Most preclinical research is performed in mouse or rat models because their genetics are more easily manipulated and their gestation and time enables for more frequent experiments.

In the case of STXBP1, we got relatively lucky that many of the symptoms we see in our children are replicated to some extent in mouse models of STXBP1 haploin sufficiency. The symptoms are also consistent across multiple mouse genetic backgrounds and in many different labs. Research using non-human primates is extremely expensive and carries the caveat of a much longer gestation time.

Therefore, their use is usually restricted to target engagement and toxicity assessments when a therapy has already shown efficacy in a rodent model.”

Yeah, and I mean, just to put a point on that, because I was at a meeting this week and they were talking about gestation time. So, you know, gestation times and also the time it takes to be an adult in terms of a mouse. It was, and I can't remember the amount of time, but it was in the terms of weeks to become an adult versus years, obviously, for humans.

Yeah, that's a great point. So we can test whether or not a given therapy has efficacy or is effective in newborn mice, or we can wait merely four to six to eight weeks and test whether or not it's effective in an adult mouse that has already developed symptoms of the disease.

Well, let's change gears a little bit and talk about the research that was presented. And so there were five research programs that were presented around STXBP1. And I should have pointed out earlier that last year, I think there was only one group that was presenting around STXBP1.

And so I always kind of try to measure things in terms of how we're how we're changing and expanding year to year. So it seems like it was pretty exciting.

Why don't we start with CAPSIDA?

So they presented they had a number of presentations, including on their STXBP1 work, as well as their CAPSID technology. And so maybe we could start out with, you know, CAPSID. What is that?

And, you know, what how does that relate to gene therapy?

Yeah, that's a really good point. So at this point, I think of gene therapy as a broad term that refers to a wide variety of therapies that target genes. And so these therapies range from several different types of ASOs or antisense oligonucleotides to many different types of AAV mediated therapies.

AAV, which is short for adeno associated virus, is commonly used to deliver a gene targeted therapy to the brain or to other organs. This is achieved by engineering the viral capsid, which is the outer shell of the virus that contains many proteins. You engineer this for specificity to the target organ or certain cell types within that organ.

For the last several years, ASOs have garnered much of the therapeutic focus for central nervous system disorders because the types of AAV currently available are terrific at expressing in mouse brains, but they're not very efficient in non-human primates or humans. The overwhelming theme of this year's ASGCT meeting...

It's like the hardest, the hardest acronym.

I know, I mess that one up all the time. I mean, I was surprised at how much this meeting highlighted recent advances in capsid evolution from academic groups such as End's own Bev Davidson, who actually just started a company around the technology that her group has developed, as well as other various companies including Capsida and Biogen. So these new capsids provide better expression in the CNS with enhanced liver detargeting at much lower doses when delivered by intravenous injection than the capsids currently in the clinic.

So what we're looking at is a wave of new therapies that are more efficient at lower doses, which is a really good thing for therapies that will be delivered by AAB.

Got it. And in terms of why the liver is important, that's because there's concern that the liver might get damaged with too much of the drug or too much of the capsid.

That's correct. So if you perform intravenous injection of AAV, you usually have to use much higher doses than you would via other methods. And those higher doses, the AAV tend to accumulate in the liver because everything is filtered through the liver in the bloodstream.

And so being able to detarget expression in the liver is a primary focus for a number of these different IV methodologies.

Right, right. Yeah, that makes sense since the liver cleans out our system.

Yeah, that's one of the biggest sources of toxicity with AAV mediated therapies is liver toxicity. So being able to detarget the liver is a big priority. If you're going to deliver by intravenous injection, which has its pros and cons.

Tell us what did CAPSIDA present.

OK, so they presented a couple of different things. Charles Chen from Mingxuan Xu's lab presented their work on a gene replacement strategy for STXBP1. This is in partnership with CAPSIDA.

They were able to demonstrate therapeutic efficacy in the form of seizure and learning and memory correction at moderate levels of AAV, which is very promising, using one of CAPSIDA's new evolved capsids. CAPSIDA also presented data from non-human primates on their best performing capsid that they plan to take to the clinic. This capsid was able to express in 40% of cells in the cortex and 67% of cells in the thalamus, and with five times less expression in the liver when given by IV injection.

Wow, that's great. So I know that there's still a lot of work to do, but it was really exciting to hear that Capsida was using the terminology IND Enabling Studies, and they said that their goal was clinical trials starting in the first half of 2025. And so what does that mean?

What are IND Enabling Studies?

So IND stands for Investigational New Drug, which is FDA terminology. IND Enabling Studies is another way to say preclinical research and anamodels, except here the focus is more on safety through pharmacology and toxicity assessments. Successful demonstration of target engagement, therapeutic efficacy, and favorable pharmacology and toxicity profiles leads to IND filing with the FDA, which is one of the last steps before a clinical trial can proceed.

Well, that is really exciting. I know that there's a lot that still needs to happen in this process, but it really does create a lot of hope for our community.

Yeah, it's really exciting.

Well, so let's talk about IV delivery because IV, there's a lot of ways that gene therapies can be delivered, but IV seems pretty attractive because it's so non-invasive. So can you talk a little bit about that?

Yeah, sure. You're absolutely right. There are a number of different ways to administer these virally-based therapies.

And it's important to note that all of these methods have drawbacks. So the least invasive method is IV, which is short for intravenous. This is just an injection of AAV into the bloodstream.

As I mentioned earlier, the primary drawback with this route of administration is AAV in the liver, which can cause health concerns if the levels are too high. This is why many of the new capsids are being evolved for D targeting of the liver. Another method is intrathecal injection, also called a lumbar puncture, which is injection into the cerebral spinal fluid through the spinal canal.

IV injection and lumbar punctures both carry the risk of dorsal root ganglion pathology, which is a type of neurodegeneration. So that is something to keep in mind with these two types of therapies. The lumbar puncture is also the primary method for administering ASOs, which need to be given as often as monthly or at least three to four times a year depending on the ASO.

The third method is called stereotactic intracranial injection, which is injection directly into the brain at a specific region or into the ventricles where the AAB can then spread throughout the brain. This method requires far lower concentration of virus to have a therapeutic effect, but is highly more sensitive type of surgery, essentially.

Right, right. Yeah, so I definitely see what you mean about real tradeoffs and pros and cons for each of the different methods. Well, let's switch gears and talk a little bit about ENCODED, one of our other biopharma partners that have been developing an STXBP1 program.

What data did ENCODED present?

ENCODED presented their platform for using cell type-specific regulatory elements to express STXBP1 in excitatory and inhibitory neurons, both of which require STXBP1 to function properly. So they're trying to use specific ways to express STXBP1 in different cell types.

Got it, got it. And you were talking about the evolved capsids. Is this an evolved capsid or is it a standard capsid?

All of their work on STXBP1 is in the standard capsid, which is in the clinic now. This is called AAV9, and it's the gold standard for targeting the central nervous system. Like I said, it's only one of the few capsids that is already in the clinic, but its major drawbacks are that it expresses well in mouse brains, but not effective in human primates, non-human primates, or in humans.

So that's why the interest in all of these new evolved capsids has arisen in the last few years.

Got it. So I did have a chance to look at their poster, and there was some wording that I didn't fully understand around that they used, and I'm just going to use the term and ask you to explain it. So they said, candidate sequence variance containing DT-A.

So this seemed to be a critical finding, I think, in the poster. And I just wanted to understand what is DT-A and what is its role?

Well, when I first saw the poster, I thought, DT-A stands for diphtheria toxin. I thought, why in the world are they incorporating diphtheria toxin into their AAV? But upon closer examination, it was clear that they're using the acronym to mean something different.

What they're trying to do is do DRG detargeting. So the dorsal root ganglion is one of these areas where you can get significant toxicity from AAV mediated therapies. And what they did was they screened thousands of different gene regulatory elements and identified some sequences that show low expression in the dorsal root ganglion, but not in the rest of the brain.

And they incorporated these into their gene replacement vector to potentially avoid the DRG pathology that we discussed earlier.

This neurodegenerative issue. Okay, that's an interesting strategy. Thank you.

Now let's talk a little bit about Biogen. What data did they present?

Biogen for the first time presented their STXBP1 gene replacement strategy that can also correct aberrant activity phenotypes and IPSL-derived neurons, and it can also correct seizures and learning memory deficits in mice, which is very exciting. This work was done using AAV9, but on a separate presentation, they demonstrated or talked about AAV capsids that they have involved in both humanized mice and also some involved in nonhuman primates. Presumably, they will use the evolved capsids for their gene complementation strategy in the future, and it's my understanding through discussions with them that they're currently performing toxicology studies on these evolved capsids in nonhuman primates.

While we're speaking about gene replacement therapy, it would be remiss if we didn't also mention the gene replacement strategies being developed at the ENDD Center in Bev Davidson's group. They're also working on such a strategy for STXBP1 using capsids evolved in their lab that have shown to express very well in a high percentage of nonhuman primate neurons as well as in human neurons, and at doses that are extremely low, on the order of 1 to 3 log fold lower than what is typically used for a therapeutic dose. So there's a really great potential for a dynamic range for therapeutic dosing with these capsids.

Well, that's really promising on the gene therapy front, and really thankful for the work from our biopharma as well as our academic partners, Capsida, Baylor, EnCoded, and Biogen. Let's switch gears and talk a little bit about antisense oligonucleotides. Alex Felix presented at ASGCT, and I think he was recognized with an award for his presentation, which is so cool.

So what are antisense oligonucleotides?

Yeah, anytime Alex gives a talk, he gets an award for it. He's really, really terrific. So antisense oligonucleotides, or ASOs for short, are short pieces of synthetic DNA that can bind to specific molecules of RNA.

They can block the ability of the RNA to make a protein, or they can alter the RNA's properties to boost protein expression.

Got it. And so Alex is using this specific approach called asteric-blocking oligo. So now we're really getting into the technical terminology.

Yeah, so asteric-blocking oligo is a type of ASO. They bind to specific regions of a given messenger RNA and interfere with proteins or other RNA molecules that can affect protein expression. So in other words, they block the repressors that would normally reduce protein expression and in so doing so are able to elevate expression.

Because our kids aren't making enough.

Exactly. So we want to elevate expression. So essentially we're blocking the repressors from inhibiting expression of STXBP1.

Therefore, we can elevate or increase the levels of STXBP1.

Got it. And he talked in his... the data he presented showed an increase in RNA.

And so can you remind me what RNA does and how that relates to the protein or the protein expression?

Yeah, Alex's ASL can increase the levels of STXBP1 messenger RNA, which is the molecule that codes for the STXBP1 protein. So Alex's ASOs have shown target engagement and efficacy in several cellular models, including human neurons as well as in mouse models. So by boosting the messenger RNA, you can increase levels of STXBP1.

He mentioned at the end of his talk that testing is now moving into a humanized mouse model, which is, I guess, a different type of model. Can you talk about what that actually is and why is it useful here?

So many of the gene targeted therapies being developed at end target the human DNA sequence. So if we try them in a normal mouse model, they're not effective because we need the human DNA sequence to test them.

Oh, and this is because of the 90%?

Exactly. So some of the therapies absolutely require human either gene coding sequences or regulatory sequences that regulate protein expression. And so because we want to elevate these therapies or evaluate these therapies in an animal model, we felt it was necessary to replace the mouse STXBP1 gene with the human equivalent.

This is what's called humanization. So we've essentially humanized the STXBP1 locus in the mouse so that now we can test human specific therapies in a living animal.

Very cool. Very, very cool.

Thank you so much for this recap of ASGCT.

Did I say it right?

So appreciate your time today and we'll look forward to seeing you at the summit.

Thanks, Charlene. It was fun. See you in a couple months.

Take care.

From STXBP1 Science + Love = CURE 10 Minute Update: May 2024, May 28, 2024

https://podcasts.apple.com/us/podcast/may-2024/id1713392388?i=1000657065605

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