Delivering Ultracompact CRISPR Systems With AAV

Lucas Harrington, PhD

CRISPR-based gene editing holds great potential to treat a wide range of diseases by correcting mutated genes. Although, because of the size of many CRISPR-based editing systems, options for delivery targeted at specific cells are limited.

Mammoth Biosciences is currently developing an Ultracompact CRISPR System that is able to be delivered in an adeno-associated virus (AAV) capsid, thus increasing its ability to be targeted at a particular cell type of interest. Following the company's presentation of preclinical data regarding this technology at the American Society of Gene & Cell Therapy (ASGCT) 27th Annual Meeting, held May 7 to 10, 2024, in Baltimore, MD, CGTLive® sat down with Lucas Harrington, PhD, the cofounder and chief scientific officer of Mammoth Biosciences, to learn more.

CGTLive: Can you give some background about what you presented at ASGCT this year?

Lucas Harrington, PhD: What we're focused on at Mammoth is developing what we call Ultracompact CRISPR systems. It's no secret that the biggest challenge in the field is: how do you actually get these innovative gene editing therapies into the right tissues? Because of size that's quite challenging and why most of the therapies that we've seen so far are in either ex vivo cells or in the liver. What we have been focusing on is getting those systems to be much smaller and they're actually about a third of the size of the original CRISPR systems like SpCas9 and so they open up a lot more delivery avenues, in particular with viral delivery with AAV.

The presentation we had was giving an update on actually showing that those systems work and that they're robust and all the work that's gone into optimizing them. Then we also gave an update on our lead program, which is targeting a gene called APOC3 for patients with either familial chylomicronemia syndrome or severe hypertriglyceridemia, which are both diseases that are characterized by elevated triglycerides.

Can you give an overview of the key points that you presented?

I think 1 key thing is just that these systems are robust editors, right? They can really go toe-to-toe with these much larger systems, which I think originally was counterintuitive for some folks in the field—that you have something that's a third of the size, is it still a robust system? And I think we pretty conclusively showed that by showing data both in mouse models for our lead program as well as in nonhuman primates, and comparing and benchmarking it relative to the standard Cas9 systems. Then I think the other big aspect is showing that progress in that lead program. So as just an example of some of the efficacy data that we've generated in our mouse models, we've been able to see up to 95% reduction in our hypertriglyceridemic mouse, which is on par with, if not better than what you see with the other programs using antisense oligonucleotides (ASOs) to target this same gene. Importantly, with gene-editing, this gene, APOC3, has programs currently that are targeting it with ASOs and siRNAs, transient methods where you're getting repeat dosing of those therapies. What we're trying to do with gene editing is actually do that in a permanent, durable way so you don't have to worry about patient compliance and getting those injections on a monthly or quarterly basis.

How would you summarize the big picture implications that doctors and the healthcare community should take away from this?

I think gene-editing is continuing to develop and mature. Our program is still preclinical. But I think the key thing is, there’s a lot on the horizon. As companies like ourselves start to break out of the liver all the work that’s been done over the last 2 to 3 decades understanding the genetics of these diseases where we haven't really been able to actually go and correct them—once we're able to target tissues outside the liver, that is actually going to be the floodgates [opening]—being able to leverage that information. I think that's kind of the general, broad overview. I think for our program in particular, we're quite excited about the preclinical data that we've generated and are looking forward to eventually getting that into the clinic.

Have there been any major challenges in this research so far?

I think one challenge is with gene-editing, you have a lot of different options looking at different guide RNAs. I think that's one of the key differences between historic gene editing systems like zinc fingers, where you're optimizing for one sequence; whereas here, we have lots of different guide RNAs to choose from. So, making the right choices there, from both the safety and efficacy standpoint, is a challenge with any program. But, I think so far with this program, we're quite encouraged and haven't seen any major roadblocks at this point.

Is there anything else you would like to share with the audience?

We just signed a new partnership with Regeneron that we're really excited about—towards that goal of actually expanding into these nonliver tissues. We think that's going to be really critical towards broadening the pipeline for us and actually being able to leverage the work that they've been doing on the delivery front. They've been retargeting AAV capsids to be able to have better tropism and better potency for different tissues. We've been working on the cargo side. We're really excited to put those 2 pieces together and start to push these therapies forward for patients.

This transcript has been edited for clarity.

Click here to view more coverage of the 2024 ASGCT Annual Meeting.

REFERENCE
1. Harrington L, Chen J. Reduction in triglycerides through a novel Ultracompact CRISPR system: Efficacy in mouse models and NHP studies. Presented at: ASGCT Annual Meeting 2024, May 7-10; Baltimore, Maryland. Abstract #15