Greetings. Welcome to Korro Bio's Analyst Day, discussing KRRO-121, a potential first-in-class treatment for ammonia control. At this time, all participants are in listen-only mode. A question and answer session will follow the formal presentation. If anyone should require operator assistance during the conference, please press star zero on your telephone keypad. Please note this conference is being recorded. This Analyst Day presentation will include forward-looking statements. Actual results could differ materially from these forward-looking statements. Please see slide two of the accompanying presentation and our most recent annual and quarterly reports filed with the SEC for important risk factors that could cause actual results to differ materially from those expressed or implied in the forward-looking statements. We undertake no obligation to update or revise the information provided during the discussion or in the accompanying presentation as a result of new information or future results or developments. Our presentation will include preclinical data from Korro's development candidate, KRRO-121. These data may not be indicative of future data or results of the other ongoing or future preclinical studies or clinical trials. After our prepared remarks, we'll open up the call for Q&A. I will now turn the call over to Ram Aiyar, CEO of Korro Bio. Welcome, everybody. I'm Ram Aiyar, CEO and President of Korro Bio. We are a company that's publicly traded on Nasdaq, and today we're gonna focus on our product candidate we call KRRO-121, a compound with the potential to treat patients with high ammonia across multiple diseases. I would like to introduce you to a couple of my colleagues today: Loïc Vincent, Chief Scientific Officer, Todd Chappell, our Chief Operating Officer, Michelle Dinneen, among many things, a mother and a patient caregiver, and finally, Dr. Bruce Scharschmidt, a physician-scientist whose focus has been on hepatology. More relevant to today, he's one of the clinical developers of a compound that has become the standard of care for urea cycle disorders, a set of genetic diseases where ammonia is a bad actor. My colleagues will be able to tell you much more about the patient need, starting with an interview-style session between Todd and Michelle. The clinical perspective on the lack of ammonia control, with Dr. Scharschmidt giving a clinical background, his clinical experience. Loïc going through the scientific thesis to providing a better solution, using a combination of genetic evidence and RNA editing, our platform. And finally, Todd providing an overview on the market opportunity based on an immense amount of work that the team has done. In the end, we will then open it up for our Q&A. Just in the U.S., we have north of about 50,000 individuals that show up a year to the hospital with severe neurological conditions, despite having severe diet restrictions, with a road that leads to very severe outcomes and eventually to mortality, all due to uncontrolled ammonia. Let me just pause there for a second. We're talking about more than 50,000 individuals a year just in the U.S.. Today, we will talk about an approach to controlling this ammonia in the human body in a very meaningful way. Any protein, its function is defined by its structure. Through the lifetime of the protein, as it undergoes many modifications, the function of the protein changes. These modifications give them an ability to either last longer, degrade faster, and potentially alter its function based on context. Our technology platform, OPERA, among many things and with many features, is able to change the structure of a protein transiently for a certain period of time to modify its function. We do this by changing a single alphabet on RNA and using the body's machinery to create a version of the protein that is functionally more active. We can do this with high precision to last transiently for a time where the protein provides benefit and then take it off, all the while without touching DNA and without causing permanent changes. As we go through the scientific thesis today, it will become clear how our platform is applicable both in rare indications, such as urea cycle disorders, as well as in common diseases, and the ability to use the body's existing machinery to, quote, unquote, "fix itself." This is just the beginning, as there are many other target disease pairs we can go after using such an elegant approach, a small list of which is on our pipeline. Today, however, is only about KRRO-121. Let's get started, as we have a jam-packed agenda. I am very excited to start. With that, I will turn it over to Michelle and Todd to share what it is to have a patient or a child living with urea cycle disorders. Hi, everyone. I'm Todd Chappell. I'm the Chief Operating Officer here at Korro. We're here at the Korro office at, in Cambridge, Massachusetts, and I'm joined by our special guest, Michelle Dinneen, who is, a mother of a daughter who has, been impacted by UCD. So, I have a couple questions for you, and thank you very much for joining us today. Of course. Thanks for having me. Yeah. So maybe we can just start by kind of asking you what a little bit about your daughter, you know, background in terms of when you first noticed symptoms, how she was diagnosed. Let's start there. Absolutely. So my daughter, Sophia, was born in 2008. We had a normal pregnancy, a normal delivery, discharged from the hospital. Everything was typical, standard, what we would expect. The day after we were discharged, she was starting to get a little irritable. She was vomiting, but being a new mom, I didn't know the difference between normal vomiting, normal irritability, and not normal vomiting and not normal irritability. So I was talking to my mother-in-law, who's, has six children, and she was telling me, "This is completely normal. You're gonna learn how to walk and just rock her and get her to sleep and get her to calm down. This is all normal." In the midst of that, we received a call. It was Friday evening around 9:00 P.M. We received a call from our pediatrician's office, and it was the physician on call, and he called to ask if Sophia was acting okay, and went on to tell me that her newborn screening test had flagged a response for, or a result for, a possible metabolic disorder. It was something that he had only seen once in 40 years. He couldn't provide much more detail. Told us that it could be life-threatening, and that they would call us in the morning. We didn't get very much sleep that night, and they did call back in the morning. The state lab had rerun the test, and it was positive again. So they had us go into Mass General. We did some blood work, and our life changed pretty dramatically that day. Hmm. And so did she start treatment right away with standard of care? Yes. So they paged the metabolic physician at Mass General, who came in and consulted and got her set up with what she needed. But they had to do some testing first. They had to do an ammonia test to test her ammonia levels, and they continued to do more, just generalized blood work, see if anything else was off. Right. And then started care pretty, pretty quickly after that. What did she start in terms of care? She began on Ammonul. We were in the hospital. It's an intravenous form. It's the standard medications for urea cycle disorder, so she was started on the IV version of that. She had to have blood draws very, very regularly, and we had to stop feeding her standard formula, and start her on a prescription formula- Mm-hmm ... that was low protein- Mm-hmm ... with balanced amino acids. Right. And so how often would you have to go into the hospital to be tested and, you know, to visit the physicians? At the beginning, we were there for a solid two weeks. I think that she spent the first week in the ICU, and then we stepped down so that we could learn how to manage medications and formula at home once she was stabilized. And then we would go in probably about every month at the beginning, and I would say we had many more proactive or, yeah, proactive ER visits that first year- Hmm ... because we were told to watch for the symptoms of irritability, vomiting, sleeping too much. Again, as a new mom, those are all things- Yeah ... that any parent knows are difficult to assess whether or not it's abnormal or- Yeah, and, you know, I'm a father of two kids myself, and to, you know, imagining what that's like is, you know, it's very difficult, I'm sure. So, tell me a little bit about those emergency visits. What was... what would happen that would prompt you to go to the emergency room, and what would happen at the emergency room? The concern was always her ammonia level. Mm. The things that we learned eventually to watch for with increased or elevated ammonia levels were her eyes. Her eyes got very glassy and couldn't... As she got older, you know, past a couple months old, she couldn't really hold that connection, eye contact, and we would notice from that. Also vomiting, if she wasn't sick otherwise, if there wasn't already something going on, so unexplained vomiting, diarrhea, anything that made her reduce what she was consuming or her formula. Mm-hmm. So the diet was very regimented, and if she didn't hit her goals every day, then there's always a concern that her ammonia would increase if her body began to catabolize, if she wasn't consuming enough calories. Hmm, mm-hmm. Any of those instances could send us to the emergency room- Wow ... for an ammonia check. Right. And so we were talking earlier, before we started this, about how you'd have to weigh food and, you know, understand exactly how many calories she got and the protein. So can you talk a little bit about that? Absolutely. We had spiral-bound notebooks. We still have them at home somewhere, where we have an incredibly accurate food log. And anything that she had, at the beginning, it was easy because it was formula, but once she started eating solid food or baby food even, anything that she didn't eat, we needed to weigh so that we could figure out how much she ate from that original container of baby food, for example. Or the example that I had given you, a banana. If she has a banana and we know it weighs 60 g, and at the end it weighs 20 g, then we know she ate 40 g, and we calculate the protein and calories based on that. Yeah. So just overall, like, what was the impact to your family, to you and to your family? It created an atmosphere of hypervigilance. Yeah. I think that's probably the way that I would summarize it, for myself, for my husband, for anybody that was helping with the care of Sophia. It delayed us putting her into any kind of daycare or, you know, shared childcare- Mm-hmm ... situation. So we needed some accommodations, you know, through work and with our families. They had to learn how to track the food and protein. Our nanny had to learn how to track everything, and watch for the signs of hyperammonemia. Right. And so the daily medication that she would have to take was sodium phenylbutyrate, is that correct? And that's three times a day. Yeah, so she started on sodium benzoate. Mm. Then she went to Buphenyl, the previous form of Ravicti, and then she was part of the clinical trial for Ravicti, actually. Oh, really? Yes. And then moved to Ravicti, but yep, they were three times a day. And that was probably pretty difficult. I can't get my son to brush his teeth every day, so, you know, I imagine it's probably pretty difficult to, you know, make sure that she's on schedule to take three times a day. Yes, she takes medication twice a day now, and I don't know how we ever got her on schedule for three times a day- Right ... back at that. Yeah. And so, how is she doing now? She's doing amazing. Good. She is healthy. She's 17. She is everything you'd expect from a 17-year-old girl. Mm-hmm. She gives her parents a run for their money some days. And she, she is a junior in high school. She's athletic, she snowboards, she is social, she's doing really well physically. Yeah. And the reason for that is that, as I understand it, at five years old, you decided to have a liver transplant. Yes. That's right. Yeah. Can you just talk a little bit about that process? Absolutely. So for us, at the time, about 10% of kids with urea cycle disorders required liver transplants. The other people, because of actual liver damage, the other patients who had liver transplants were more... It was more of a decision to manage symptoms and the possible damage. For Sophia, it was both. Her liver became enlarged at a very young age, and we watched that really closely. In February of the year that she was transplanted, it was around that time that her liver started showing up on ultrasound as incredibly enlarged, pushing against her other organs. Mm. Her liver enzymes began to come back as much more elevated than they had in the past, and there was no real explanation. She wasn't sick, for those enzymes to be elevated. Mm. So we started talking about the possibility of doing a liver transplant before she became closer to liver failure or compromised health. Right. And that must have been a difficult decision, just weighing the pros and cons for that. And then I guess also just be interested to understand, after the liver transplant, you know, getting immunosuppressants and things like that. Can you just... Can Can you talk a little bit about that as well? Yeah, the decision-making process was incredibly difficult. At that point, not a lot of Urea Cycle Disorder patients went through liver transplant, and it wasn't as though she was in liver failure, which would have made the decision for us. Mm-hmm. We had to make the decision whether or not we were gonna do it at all, do it now, do it five years from now. And then once you go through that evaluation and are listed, then you're on call- Mm-hmm ... for, you know, however it is long until you get that call. Yeah. And so for us, as we think about, you know, our research into urea cycle disorders, you know, what, what do you wish you would have had at that time in terms of standard of care to, to treat Sophia? I think that in Sophia's case, the liver transplant was inevitable because of the liver damage that she had, but I think the improved ammonia control- Mm ... would have been the ultimate factor in giving us some peace of mind. Right. I think that the constant not knowing where her ammonia level was, having some type of in-home device to check ammonia levels would have been a miracle. That was always the piece that was the most unsettling. Right. Having a medication, I mean, we have always thought that having a medication also, that you're not having to take three times a day, especially for adolescents as well- Yes ... you know, would really make a huge difference. That's something I've seen a lot more since she's been a teenager- Mm ...that hesitation to want to comply with medication and requirements around that.... I think that there's a natural rebellion when you become a teenager that makes you not want to follow every rule. Right. And I think that taking medication three times a day would be very stressful for a family. Right. Yeah. Well, great. Thank you very much for your time. We really appreciate it. It's wonderful to hear firsthand, you know, from someone that's gone through this, and, yeah, really appreciate it. Of course. Yeah. Thanks for having me. Our next guest is Dr. Scharschmidt, who will be giving a physician's perspective on UCD and hepatic encephalopathy. Well, thank you, Todd, and good morning, everyone. I'd first like to thank Korro for including me and everybody for their interest. And I do hope you'll excuse my hoarseness and occasional cough. You know, as someone blessed to have his young grandkids all living close, the occasional downside is getting exposed to every virus that circulates among grade school kids here in San Francisco. And I might add that I'm delighted that this program includes the parent of a UCD patient. You know, the perspective of UCD patients and families is critical. I, of course, bring somewhat different perspective and look at urea cycle disorders through a somewhat different lens. Now, by way of introduction, I'm a liver specialist by training. In my academic past, I was professor of medicine and chief of GI at the University of California, San Francisco, where with my surgical colleagues, helped launch the UCSF Liver Transplant Program. During that time, I served as editor-in-chief of the Journal of Clinical Investigation and president of the parent organization. I was then recruited to add clinical development to Chiron, one of the first two Bay Area biotechs, and after its acquisition by Novartis, and a brief stay at Novartis, and most relevant today, served as Chief Medical and Development Officer at Hyperion, where we developed and launched glycerol phenylbutyrate, which I'll refer to as GPB or Ravicti, for urea cycle disorders. We also conducted a large, successful phase II study of that same ammonia-lowering drug for patients with decompensated cirrhosis and hepatic encephalopathy. Now, since Hyperion's 2015 acquisition, I've been doing other things, and these are my disclosures. Now, both UCDs and HE are all about ammonia. When most people think of ammonia, they think of household cleaners, not something that our body makes or that circulates in our bloodstream. So imagine, just imagine for a moment, that either you or your child is diagnosed with a UCD. So you or your child becomes severely and acutely ill. Your physician or pediatrician orders a blood ammonia test, refers you to a metabolic geneticist to confirm the diagnosis of a UCD, and advises the following: "A severely protein-restricted diet, perhaps with dietary supplements, a short-acting tablet or liquid taken up to several times per day, a warning that noncompliance with diet or drug can trigger a hyperammonemic crisis, which may require hospitalization and cause permanent brain damage or even death, and a caution that even if you do everything right, a crisis might still occur. And by the way, you're among the lucky ones. UCDs are rare. Many physicians have never seen a UCD patient and don't think to check blood ammonia. Yours did." So hence the title of this slide. UCDs are cruel. They can affect children starting early in life, and they never let up, and they're unforgiving. You know, if we adults miss blood pressure pills or stop adhering to a healthy diet, we may never suffer the consequences. Not so for UCD patients. Now, bear with me for the next two slides while we dive into disease biology. You know, the details are complex, but the concept's pretty straightforward. We humans evolved as hunter-gatherers. Our ancestors didn't know when they'd find their next meal, so our bodies learned to store food. We store fat in adipose tissue and carbohydrates as glycogen in our liver and muscles, but we can't store protein. Dietary protein, which isn't incorporated into body tissues such as muscle or bone, is broken down, mainly in our intestines, and ammonia is released as a byproduct. So this explains why we humans are constructed such that all the blood from our intestines flows first through our liver before it reaches the rest of the body. As you can see at left, the liver triages nutrients, includes ammonia and other gut-derived toxins. As you can see at the right, it detoxifies ammonia through a series of enzymatic steps called the urea cycle, which, when working normally, gets rid of ammonia in the form of urea that comes out in our urine and keeps systemic ammonia levels low and protects the brain. The two main causes of high blood ammonia, therefore, are liver disease and enzymatic defects in the urea cycle, or UCDs. Patients with UCDs in whom the urea cycle is not working normally are often treated with what are called ammonia scavengers or alternate pathway drugs, so named because they provide an alternate pathway for ridding the body of ammonia. The two approved drugs are sodium phenylbutyrate, or Buphenyl, and glycerol phenylbutyrate, aka GPB or Ravicti. They're both products of phenylacetic acid, which is converted to phenylacetylglutamine, the urea surrogate that comes out in the urine. So the urea cycle, then, is a series of enzymatic steps which convert ammonia to urea. All UCDs are autosomal recessive disorders, except for OTC deficiency, which is X-linked. They're genetically very heterogeneous. Not only are there many different disease-causing mutations, but due to the vagaries of random X chromosome inactivation as the female embryo develops, identical twin sisters conceived with the very same abnormal gene may exhibit dramatically different disease severity. Crises can occur with all UCDs, but tend to be most common with defective enzymes early in the cycle, referred to as proximal disorders. The more severe the defect, the earlier the onset, and severity is generally assessed clinically based on blood ammonia. Ureagenesis can be measured directly using stable isotopes, but that is rarely done in clinical practice and has so far been mainly a research tool. We don't have, for example, a readily measurable biomarker of disease severity or drug effect, such as factor VIII levels for hemophilia A. Most UCD patients are not detected by newborn screening, and the best available data suggested the incidence is about 1 in 35,000 live births, which if all patients were diagnosed and lived a normal lifespan, would mean a U.S. prevalence of around 10,000. All patients are likely not diagnosed, and some don't have a normal lifespan, so the true prevalence is uncertain. In Hyperion's GPB trials, as you can see at lower right, we enrolled an approximately equal number of children and adults, and the population was skewed towards females with OTC deficiency. Finally, UCD patients benefit from dedicated physician and patient advocacy groups with whom we worked closely during development. The NIH-sponsored UCD Consortium has an ongoing longitudinal study, a valuable source of information, which I'll come back to later. Now, I began development of the drug that's now Ravicti with a lot of humility. You know, yes, I understood the biology of UCDs and had occasionally made the diagnosis correctly. Typically, in young women referred to our UCSF Hepatic clinic for what was thought to be unexplained hyperammonemia. These patients turned out typically to have OTC deficiency. But I'm not a metabolic geneticist. That's why I began by reading treatment guidelines, and they seemed to make perfect sense. And I quote, "The goal of treatment is to maintain normal levels of plasma ammonia through the use of the low-protein diet and medication while allowing for normal growth." Now, the first surprise for me was that there was no consensus among our investigators regarding ammonia control. Some barely measured it. You know, they reasoned it's a finicky blood test which can yield spurious results if not handled properly, so they relied on their clinical judgments. Other investigators were quite meticulous in keeping ammonia within normal limits. And the second surprise, not only to us at Hyperion, but also to our investigators, came when we measured daily ammonia burden with round-the-clock levels, as agreed to with FDA. We found that ammonia varied dramatically throughout the day and often increased several-fold after a meal. This begs the question: what's meant by keeping ammonia normal? Does it mean part of the time or all of the time? More importantly, do all the challenges associated with tight control really benefit patients? To answer this question, we, working with our academic investigators, did a post-hoc interrogation of our unique clinical trial data set. The answer was an unambiguous yes. UCD patients do benefit from tight ammonia control. Based on the analysis of over 1,000 ammonia samples in over 100 patients with round-the-clock ammonia measurements, we found that the risk and frequency of hyperammonemic crises correlated directly with daily ammonia exposure. Now, round-the-clock ammonia measurements are, of course, impracticable in routine practice. We found that daily exposure correlated well with fasting morning levels, which are really doable. As you can see from the graphic, crisis-free survival decreased as fasting morning ammonia levels increased. Now, those findings were presented at a plenary session and reviewed as quite important. But achieving tight ammonia control is tough. You know, this is particularly true of school-age children and adolescents who are no longer under their parents' watch and want to fit in at school. They want to eat a lunch, which looks like the other kids, and not have to take drugs sometimes several times a day, let alone drugs which can cause body odor. And although GPB is better tolerated and has meaningful advantages over sodium phenylbutyrate, both drugs may need to be dosed several times a day. Both can decrease serum levels of branched-chain amino acids, requiring monitoring and occasional supplementation, and both have a narrow therapeutic index, as high levels of phenylacetic acid can be toxic. Given these challenges facing UCD patients and their families, we explored the role of compliance and other factors as contributors to crises in our trials. The table at left catalogs what our investigators believed to have triggered crises among UCD patients in the year prior to enrollment in the GPB trials. Among 49 pediatric patients, intercurrent illness or infection together accounted for about 40% of crises, a figure which agrees reasonably well with the 33% reported from the UCDC-sponsored longitudinal study. Non-compliance with diet and/or drug together contributed to about a fifth of cases. But this likely understates the importance of non-compliance for several reasons. It's likely underreported, as patients and their families may understandably find it difficult to acknowledge non-compliance. Moreover, with the same illnesses, which trigger crises, often cause nausea or vomiting, which makes it difficult for patients to take their drug. The frequency of crises dropped by over half when patients enrolled in the GPB trials. Now, while we felt GPB was a better drug and deserved some credit, it was also our job to ensure that patients complied with treatment while they were on trial. Both our data and the longitudinal study data indicate that most patients who experience crises, in fact, experience multiple. Finally, and importantly, over 60% of patients are not prescribed drugs. Our impression working with our study sites was that many of these patients, and understand these are patients who have been diagnosed, but were not taking drug and therefore not in our trials, also experienced crises. These are, of course, patients who also might benefit from treatment, and that impression was borne out by unpublished data from the longitudinal study provided us by the consortium that was included in Hyperion's. NDA. Now, these crises are the most dramatic manifestations of UCD, and they may cause permanent brain damage or death. But the manifestations of urea cycle disorders comprise a spectrum, and I'll close with emerging information about OTC deficiency. Recall that OTC deficiency is the most common UCD subtype and is X-linked. It used to be believed that heterozygous females with OTC deficiency are asymptomatic because they have one normal X chromosome. But some really elegant work done by Andrea Gropman, in particular, has shown that some of these patients exhibit subtle abnormalities in cognitive function and metabolic brain imaging. And a recent report, following up over 100 such patients initially categorized as asymptomatic, showed that over a third developed overt neuropsychiatric diagnoses, usually in their late teens, and 40% crises. So I'd like to think there's still a lot we can do for these patients to help achieve better ammonia control and outcomes. Well, I'd like to close with a few slides on what could be viewed as an additional important disease opportunity. That is hepatic encephalopathy, which I'll refer to as HE, in patients with decompensated liver disease. Now, when the liver is badly scarred, for example, by chronic viral hepatitis or MASH, it becomes scarred. We refer to the scarring as cirrhosis. A cirrhotic liver not only fails to work properly, but the scarring impedes blood flow through the liver, which causes the pressure in the portal vein to go up. We call this portal hypertension, which results in blood flowing around the liver through newly formed collaterals, such that it reaches the systemic circulation and the brain unfiltered. HE is a major manifestation of decompensated cirrhosis. The only cure is liver transplantation, but due to donor organ shortage, many patients may wait months or years for a transplant or never get one at all. HE manifestations range from confusion to coma or death. Regardless of the outcome, they're always frightening, and they're very expensive. The prevalence is probably greater than 200,000, and the pharmacoeconomics are strong. Now, we knew all this before embarking on Hyperion's phase II trial. The big question for us, and really the question for the field, was about ammonia. It had been known for over a century that ammonia is elevated in HE patients, but was it a correlate, or was high ammonia, in fact, a cause? The answer is that it's not just a correlate, but a cause. The result of a randomized, double-blind, placebo-controlled study in 178 patients taught us that ammonia lowering decreases the risk and frequency of HE events, as well as HE-related hospitalizations. Now, this was a pretty big deal in the world of hepatology, but we took it one step further. Could ammonia lowering and the apparently beneficial drug effect be true, true, but unrelated? Might the drug be doing something else? The answer turned out to be no. It's all about ammonia. Borrowing on what we learned from UCD patients, we undertook the same type of post-hoc analysis with remarkably similar results. As you can see it, right, lowering fasting ammonia decreased the risk and frequency of HE events, and the relationship between ammonia and HE events was the same in the two treatment arms. But there was still another question we needed to answer. You know, our phase II study had been conducted at the same time as the rifaximin launch. Fast forward a couple of years, rifaximin has fully penetrated the market. We were considering phase III and wondering if there was still an unmet need. So we did an observational study. We simply asked investigators to enroll patients who had experienced an HE event in the last month and follow them without any new intervention. Among 265 patients enrolled at 30 mostly U.S. centers, and followed for an average of a little over two months, 27% experienced at least one additional on-study HE event, and sometimes multiple. 85% of the events resulted in hospitalization. 82% of patients were on rifaximin at baseline, and as you can see it, right, HE free survival didn't differ between patients who were or were not taking rifaximin at baseline. Now, this, of course, doesn't mean that rifaximin doesn't work, but it does underscore a continuing unmet need. And HE is not only hard on our patients, but it is expensive. The cost of HE-related hospitalizations is almost certainly in the $10,000s, and the total cost to our healthcare system, likely in the $ billions. So some final thoughts: Why is HE still such a big problem? Well, in addition to compliance, we're getting better at treating other potentially lethal complications of cirrhosis, and because HE, per se, doesn't increase transplant priority, we see more of it. It's also possible that ammonia may not be the only cause and/or that current treatment doesn't lower ammonia enough, that we need something better, which I think is likely. And finally, there's increasing interest in more subtle manifestations of HE, called minimal or covert HE. This is conceptually analogous to the milder illness we talked about earlier in females with OTC deficiency. And like the OTC-deficient patients, patients with covert HE have subtle cognitive and brain imaging abnormalities. There is no professional consensus on whether or how to treat or test for it, and there's no approved treatment. There is a consensus, however, that it's a real problem and more common than overt. So as a hepatologist, I really do believe we can do more for these patients. Thanks very much for your attention. Thank you, Dr. Scharschmidt, for going through the medical needs in hyperammonemia diseases. Hello, everyone. My name is Loïc Vincent, and I am the Chief Scientific Officer at Korro Bio. Today, I'm excited to present KRRO-121, our lead program targeting glutamine synthetase stabilization for the treatment of urea cycle disorders and hepatic encephalopathy. This represents a potentially first-in-class approach to ammonia control that could transform the lives of patients living with these high unmet medical conditions. Let me start with the concepts underlying KRRO-121. Glutamine synthetase, or GS, is a critical ammonia-clearing mechanism in the liver. It complements the urea cycle and provides an alternative pathway for ammonia detoxification. The target validation comes from robust genetic evidence that uncovered a key amino acid modification that can augment GS protein stability, and this human genetic validation gave us confidence that stabilizing GS could be therapeutic. The ammonia-lowering benefits of enhanced GS activity may address substantial unmet need in patients with poor ammonia control, as described by Dr. Scharschmidt. This includes not only UCD patients, but also patients with HE. KRRO-121 is designed to capitalize on this biology. It's a GalNAc-conjugated oligonucleotide that edits GS messenger RNA to generate a stable de novo GS variant, specifically in the liver, that has increased stability and ammonia clearance capacity. Our preclinical data that I'm going to run through today demonstrate that KRRO-121 has potential to trigger robust ammonia clearance, supporting a pan-UCD approach that may enable dietary liberalization, as well as efficacy in other ammonia-driven diseases, such as hepatic encephalopathy. Finally, we are anticipating a regulatory submission for KRRO-121 in the second half of 2026. To understand our approach, it's important to recognize that the liver clears ammonia for two complementary pathways. The first is the urea cycle, which is expressed primarily in the liver. The second is glutamine synthetase, expressed in many tissues, including liver, brain, and muscles. The key insight is that GS clears ammonia and can bypass the urea cycle entirely. When the urea cycle is impaired, whether due to CPS1, OTC, or any other enzyme deficiency, we can leverage this alternative pathway to clear ammonia, and this is why targeting GS stabilization in the liver is such a compelling approach for UCD. Now, let's understand why GS needs to be stabilized. GS protein levels are regulated by a glutamine-dependent feedback loop that causes degradation of GS when glutamine rises. When GS clears ammonia and produces glutamine, as shown on the left, rising glutamine levels trigger GS degradation. This creates a pernicious cycle. When ammonia is high and you need more clearance capacity, GS starts to degrade, reducing ammonia clearance precisely when you need it most... The degradation mechanism involves acetylation of key and terminal lysine residues shown on the right. At low glutamine levels, these lysine residues are not acetylated, and GS remains stable. However, at high glutamine levels, these lysines become acetylated, which triggers an ubiquitination signal leading to protein degradation. This feedback mechanism makes biological sense in normal physiology. It prevents glutamine from accumulating to toxic levels. But in patients with hyperammonemia, this feedback loop becomes counterproductive. This is the vulnerability we are targeting with KRRO-121. The genetic evidence supporting our approach is compelling. We see validation from both sides, loss of function and gain of function. On the loss of function side, there's a published case report of two siblings with a lysine 14 to asparagine mutation. This mutation mimics acetyllysine and makes GS prone to degradation, resulting in GS deficiency and hyperammonemia. This shows that when you destabilize GS, you get elevated ammonia. Even more exciting is the gain of function evidence. Nine patients have been identified with start loss variants that stabilize GS due to the loss of the N-terminal lysine residues, which serve as the degradation signal. These patients have increased GS stability, stable GS activity, and importantly, lower ammonia levels. This human genetic data validates our hypothesis. If we can prevent GS degradation, we can enhance ammonia clearance. After understanding the degradation mechanism and seeing the genetic validation, we form our therapeutic hypothesis. Preventing GS degradation will stabilize the protein and enable increased ammonia clearance. The slide shows our conceptual framework. If we can achieve liver-specific GS modification that prevents degradation, we hypothesize this will increase ammonia clearance capacity. The diagram illustrates the mechanism. Glutamate and ammonia are substrates for GS, which produces glutamine. In our modified GS variant, shown here with arginine replacing one of the critical lysine residues, we prevent the degradation signal. The key is liver specificity. We want to enhance GS activity in the liver, where it can clear ammonia from the bloodstream without affecting GS in other tissues like the brain, where it has different physiological roles. This hypothesis sets up our therapeutic approach. Now, let's see exactly what KRRO-121 does. On the left, KRRO-121, shown in red here, is a GalNAc-conjugated oligonucleotide, which ensures liver-specific RNA editing. The GalNAc moiety binds to asialo glycoprotein receptors that are highly expressed on hepatocytes, providing exquisite liver selectivity. When KRRO-121 enters the hepatocyte, it binds to GS messenger RNA and recruits endogenous Adenosine Deaminase Acting on RNA, so-called ADAR enzymes, to make an A to I edit. Inosine is read as a guanosine during translation, effectively changing a lysine codon to an arginine codon. The result shown on the right is a de novo GS variant with arginine instead of a lysine at the critical N-terminal position. This variant lacks the acetylation site, so it cannot be tagged for degradation, and it can maintain consistent ammonia clearance capacity even when glutamine levels rise. Now, let's look at the data. These slides show results from OTC-deficient human iPS-derived hepatocytes, which have a urea cycle defect carrying the D175V mutation in OTC. In vehicle-treated cells without ammonia challenge, we see normal GS levels at 1.0-fold. But when we add ammonium chloride to simulate high ammonia environment, GS levels drop dramatically to 0.4-fold, which is a 60% reduction. This recapitulates the degradation mechanism we described earlier. However, when we treat with KRRO-121, GS levels remain stable at 0.9-fold, even in the presence of high ammonia. Notably, only 20%-25% RNA editing is required to maintain GS stability under ammonia stress. We have reached similar results in ASS1-deficient iPS-derived hepatocytes. This demonstrates that our approach may work across different UCD genotypes, supporting KRRO-121's pan-UCD potential. Moving to in vivo studies, these are results from OTC-deficient mice, which is a well-characterized UCD model. We dosed vehicle or most optimized oligonucleotides at 10 mg/kg subcutaneously daily on day zero through day four, then measured outcomes on day 14 after the mice were challenged with ammonia, simulating protein consumption. On the left panel, we see a dramatic reduction in ammonia levels. The human upper limit of normal is 75 mcg per deciliter, which converts to approximately 450 micromolar in mice, shown by the dashed line. So we are bringing these mice much closer to normal ranges. Even during this metabolic stress, we see a non-significant change in plasma glutamine levels, shown on the right panel, meaning glutamine level stays within manageable ranges. This slide demonstrate our mechanism of action using stable isotope tracers. We use 15N-labeled glutamate as a target engagement tracer to directly measure GS activation. This is a powerful pharmacodynamic biomarker that allows us to track metabolic flux through the GS pathway. We dosed with vehicle our most optimized oligonucleotide at 10 mg/kg subcutaneously, daily, on day zero through day four, then challenged mice with 100 mg/kg glutamine, plus 15N-glutamate on day 11. Three key findings validate our mechanism. On the left, we see increased plasma 15N-glutamine over time in treated animals, with significantly higher AUC under the curve. This directly demonstrates that GS is taking up 15N-glutamate and ammonia to produce 15N-glutamine. In the middle panel, we see that the treatment decreased plasma ammonia throughout the time course, with ammonia staying well below the 450 micromolar upper limit of normal, even under this protein load challenge. On the right, the treatment increased total liver GS concentration, approximately 1.44 at peak. This shows that the stabilized variant accumulates in the liver over time, providing more enzymatic capacity. What's particularly elegant here is that we have demonstrated GS target engagement in OTC deficient mice. But I should mention that we have observed similar results in wild-type mice, which potentially validates that the mechanism of action provides disease-agnostic ammonia control. This slide shows data from CPS1 deficient mice. The data reported here were generated by Dr. Brunetti-Pierri and Sonia Ait-Tayeb, which is a well-recognized academic research group in Italy. They confirmed our findings. On the left, we see a significant reduction in ammonia levels following ammonia challenge, and on the right, a non-significant change in plasma glutamine levels. This independent validation across a different UCD genotype in vivo strengthens our confidence in the pan-UCD applicability and diet liberalization potential of KRRO-121. On this slide, this is our most important preclinical dataset because it uses the PXB-mice, which have humanized livers. These mice retain zonal GS expression patterns, making them highly relevant to human disease physiology. We dosed with vehicle or KRRO-121 at 50 mg/kg subcutaneously on day zero and 14, which is an every two weeks dosing regimen. Mice were then challenged with 350 mg/kg ammonia on day 21. We have four key findings from left to right. First, we see stable de novo GS variant and normal total protein levels in liver. This repeated weekly dosing at 50 mg/kg dose produces an amount of the edited variant, approximately 10%-15% of total GS protein in the liver. Second, statistically significant reduction in basal ammonia from over 100 micromolar in vehicle controls, down to about 70 micromolar with KRRO-121 treatment. Third, enhanced ammonia clearance during the challenge. Vehicle-treated animals spiked to around 1,600 micromolar ammonia, well above the 500 micromolar upper limit of normal. But KRRO-121-treated animals stay around 500-900 micromolar, with the majority of animals remaining at or near normal limits. Finally, we have steady glutamine levels per challenge, with no significant difference between vehicle and treated animals. This is important because it shows we are not causing problematic glutamine accumulation. Now, here's the critical mechanism insight. The potential ammonia lowering requires only a minimal amount of de novo GS. We are seeing dramatic efficacy, with only 10%-15% of total GS being the edited, stabilized variant in the liver. This suggests that the de novo variant is substantially more active or stable than the wild-type GS, which makes sense given that it is protected from degradation. This humanized mouse model provides strong confidence that KRRO-121 enables stable level of GS, providing robust ammonia control in a system that closely mimics human hepatic physiology. This is the critical translational bridge between rodent models and human clinical trials. An important safety consideration for any ammonia-lowering therapy is the potential impact on the central nervous system. Glutamine synthetase is expressed in brain astrocytes, where it plays a critical role in neurotransmitter recycling and ammonia detoxification. We needed to definitively address whether KRRO-121 reaches the brain or affects astrocyte populations. We dosed mice with vehicle and KRRO-121 at 10 or 20 mg /kg daily for 5 days. These are supratherapeutic doses given the daily regimen. Here, you see immunohistochemistry staining for GFAP, which is a well-known and recognized astrocyte marker in brain section from OTC deficient mice. The top panel shows vehicle-treated brain, and the bottom shows KRRO-121-treated brain. These images look virtually identical. There is no increase in astrocyte staining relative to vehicle treatment, indicating no astrocyte activation or gliosis. This data definitively demonstrates that KRRO-121 does not cross the blood-brain barrier and does not have an effect on astrocytes in the CNS. This is critical for the safety profile because it means we are enhancing ammonia clearance in the liver without the risk of disrupting the important physiological functions of GS in the brain. Now let's discuss the safety and the biodistribution profile. The data comes from our NHP repeat dose toxicology studies, where we dose cynomolgus monkeys with KRRO-121 once weekly for three doses. Starting with biodistribution on the left, we see greater than 90% of KRRO-121 delivered to the liver, with only minimal distribution to kidneys, spleen, and the injection site, as expected. Critically, we see less than 0.05 delivery to the bone marrow, brain, heart, lymph nodes, and muscles. This exquisite liver selectivity is exactly what we want. It means we are targeting the therapeutic site while minimizing potential off-target effects. The middle panel shows immunofluorescence imaging, confirming liver localization of KRRO-121 with pericentral GS. You can see the purple signal, that Cy5 label KRRO-121, colocalizing with the teal signal, which is GS with Cy7. The blue is DAPI, standing for nuclei. This beautiful colocalization confirms that KRRO-121 is delivered to hepatocytes in the pericentral zone, where GS is predominantly expressed. On the right, we report clinical chemistry and hematology data at six hours post the third dose. As shown here, we see no changes in liver or kidney function. There is no impact on coagulation, complement, platelets, or cytokines. Altogether, the safety data demonstrate liver-restricted delivery with no CNS exposure, no evidence of off-target editing, and a clean toxicology profile at doses well above the anticipated therapeutic range. Let me summarize why we are excited about KRRO-121's potential. We have three key pieces of evidence. First, for preclinical efficacy, we have demonstrated a pan-UCD approach impacting multiple UCD subtypes. We have robust ammonia control in both OTC and CPS1 mice challenged with ammonia, and we have shown diet liberalization potential, so ammonia reduction during pro-protein challenge. Next, for preclinical safety, we have seen no adverse safety signals in repeat dose range-finding toxicology studies, no impact on coagulation, complement, platelets, or cytokines, and again, no increase in astrocyte staining in mouse brain tissue. This provides strong confidence in KRRO-121's safety profile as we advance toward the clinic. Finally, we have demonstrated translation producing a stable de novo GS variant, which increases ammonia clearance and maintains normal glutamine levels. This scales from mice to monkeys and shows GalNAc specific liver delivery. Together, this strong preclinical data package demonstrates KRRO-121's first-in-class potential for patients with hyperammonemia. Let me close my section with our development timeline. As you can see, we nominated KRRO-121 as our development candidate in the second half of 2025. Our next major milestone is regulatory filing for our first human trial, which we are expecting to submit in the second half of 2026. I want to emphasize the statement at the bottom. We have a compelling product profile for controlling ammonia that we expect to drive strong patient engagement and recruitment. Thank you. And now I'm passing the mic to Todd Chappell, who's going to cover the market opportunity for KRRO-121. Hello again. As a reminder, I'm Todd Chappell, the Chief Operating Officer here at Korro. So one of my many roles here at Korro has been to develop the product positioning for our programs. So I'm excited to share with you today some of our thoughts on the unmet need for patients with elevated ammonia and the overall market opportunity for KRRO-121. Starting with the end in mind, we believe UCD and HE have significant unmet need for novel ammonia-lowering therapies, which presents a substantial pipeline and a product opportunity. If you look at each individually, UCD represents approximately 9,000 addressable patients in the US and Europe combined, and with a $1.5 billion market opportunity. Additionally, hepatic encephalopathy represents over 200,000 addressable patients in the U.S. and Europe combined, and over a $2 billion market opportunity. So what I'd like to do now is walk you through each, each one of these market opportunities and why we believe we can address the associated unmet medical need. So I won't spend much time on this slide, as Dr. Scharschmidt already mentioned the importance of novel therapeutics addressing reduction of ammonia. However, briefly, hyperammonemia, or high blood ammonia, is caused by the liver's inability to process ammonia, often from liver diseases like cirrhosis, inherited metabolic disorders, such as urea cycle defects, organic acidemias, or certain medications like valproic acid. Severe infections, gut issues, or triggers like dehydration, trauma, and diet changes can also lead to ammonia buildup that's toxic to the brain. So what do ammonia levels that trigger these hyperammonemia episodes look like, specifically in genetic diseases such as UCD and cirrhotic situations such as hepatic encephalopathy? Our data science team evaluated electronic medical records from participants across the United States to help us understand what levels trigger these events. In a cohort of genetically confirmed urea cycle disorder patients, mean ammonia was approximately 77 micromolar, in the context of mean ammonia being 35 micromolar or below for healthy individuals. Approximately 95% of these individuals had a measurement greater than 1.5 times the upper limit of normal. As a result, ammonia control is highly challenging in UCD patients today, typically requiring IV nitrogen scavengers and a strict diet that can lead to malnutrition. How do we believe we can address the unmet need associated with UCD? While nitrogen scavengers have played a significant role in the lives of these patients, unmet need remains. Ravicti has to be taken three times a day, which can be difficult for chronic diseases. Ravicti is administered in the background of a strict diet and still leads to crisis due to elevated ammonia. As Michelle clearly articulated, new therapies to address these unmet needs are needed. We believe KRRO-121 may offer a differentiated ammonia-lowering approach to potentially address all UCD patients and provide a convenient subcutaneous delivery administered on a weekly or monthly basis versus on a daily basis. Additionally, we believe KRRO-121 has the potential to result in further reduction of crises due to elevated ammonia and liberalized patient diet. Lastly, for UCD, let's drill down into the epidemiology in the US and Europe. In the US alone, genetic frequency analysis leads us to believe that there are approximately 66,500 UCD patients, with approximately 4,200 having severe post-neonatal onset, which is defined as symptomatic patients expected to benefit from pharmacological therapy. Then there are an additional 5,000 patients in Europe with UCD. Now, switching over to the opportunity for KRRO-121 and its and hepatic encephalopathy. What we're showing in these graphs is that ammonia measurement in uncontrolled HE patients are frequently above normal, which correlates to a higher HE risk. Dr. Scharschmidt already reviewed with us what's shown on the left-hand side. First, the HE events correlate with ammonia levels. And second, in a study with Ravicti, those events decrease with ammonia levels that are lowered. Similar to UCD, in the bar graphs in the middle and on the right-hand side, we're showing the electronic medical record data our team generated for severe recurring hepatic encephalopathy. In the middle graph, you can see the median levels of ammonia are already elevated in these patients, well above one point five times the upper limit of normal. On the right-hand side, we see that severe recurring HE represents 40% of the patient population, and of those patients, we estimate that nearly 75% have elevated ammonia. Lastly, available therapies do a poor job reducing ammonia in these patients, offering minimal reduction in ammonia levels. These elevated ammonia levels in HE patients result in increased healthcare resource utilization. Through our EMR analysis, we've shown that among severe recurring HE patients, the subset with high ammonia and stable MELD scores have a greater than twofold increase in HE-related hospitalization compared to the broader severe recurring HE population. We also see a nearly twofold increase in all-cause hospitalization for this high ammonia subgroup. This is important because the cost of HE-related hospitalization can exceed $75,000 per visit, resulting in over $10 billion in patient charges for HE in the U.S. each year. This underscores a strong pharmacoeconomic case for additional therapies in this space. Similar to UCD, we believe 121 has a compelling product profile to address HE patients with substantial unmet need for offering a differentiated ammonia-lowering approach, one that can directly lower ammonia, thereby reducing the number of HE events. And with GalNAc, 121 will be administered subcutaneously with improved dosing frequency compared to standard of care, ultimately leading to an overall improved quality of life and potential survival benefit. As stated earlier, we believe that the addressable patient population for recurring HE in the U.S. is approximately 80,000 and over 150,000 in Europe, resulting in nearly 230,000 total patients just in the U.S. and Europe. We define the addressable patient population as those with severe recurring HE events, high ammonia, and with sufficient liver function. Finally, although it won't be our initial approach, we also see an opportunity to expand to preventing initial HE events in patients at risk, similar to how rifaximin is being positioned in their current clinical trial. So with that, I'll turn it over to Ram for closing remarks. Thank you, Todd, and thank you everybody today for being here with us. I want to end and leave you with four things. First, I hope you appreciate the need for controlling ammonia in humans, and the significant unmet need that still exists. You heard from a caregiver, you heard from a clinician, you heard from what the current standard of care is. In the case of urea cycle disorders, the current standard of care involves the need to have a high level of control of your diet, in addition to taking a drug three times a day. Despite that, the mortality in this patient population and the eventual malnutrition that exists is pretty severe. In the case of hepatic encephalopathy, in a much larger patient population compared to urea cycle disorders, the current standard of care is highly ineffective in terms of preventing hospitalizations for those patients where ammonia control is challenging. So the need here, as you heard today, is extremely high. Two, the scientific evidence that our team has provided in utilizing a transient approach to stabilize glutamine synthetase is elegant in and of itself. Having the ability to stabilize an intracellular protein using a mechanism the body understands, and leveraging it to activate glutamine synthetase, is novel, and yet has the potential to provide benefit, that has never been seen before, all inspired from genetics. Over the last two years, this team has accelerated the development of this therapy in a very short period of time, and is a testament to the improvements in the platform as it pertains to both potency and delivery that we have demonstrated today. Third, KRRO-121 has the potential to modify the standard of care, to going from a therapy that's three times a day oral, to something that's likely once in two weeks or more infrequent, depending on what the clinical studies are to show. That is just the beginning. The convenience of taking medication, even if it were as good as the current standard of care, to improve significantly compliance, is highly beneficial for patients. On top of that, KRRO-121 has the ability or the potential to let adolescents who suffer from urea cycle disorders have a semi-normal life in terms of protein consumption. The next time you have a cheeseburger, you will remember these kids and the potential benefit you're providing them. This drug, in addition to improving quality of life, has the potential to prevent hospitalization in urea cycle disorder patients, a need that's very high, and hopefully we can demonstrate that in the clinical studies that we intend to run. Let me just pause there for a second. The ability to have an impact on ammonia in both these patient populations is tremendous, both from a caregiver standpoint, as you heard today, as well as from a patient perspective. You will hear more about the next steps and the additional guidance we will provide later this year as to what are the clinical inflection points that you will see. As you heard from Loïc, we intend to have a regulatory clearance through a regulatory application in the second half of this year. Once we have guidance in terms of what those clinical studies will look like, based on alignment in the multiple regulatory authorities, we will provide more clarity as to what the next steps are. Our initial intent with the clinical studies is to show that we have a profound ammonia-lowering effect in either one or both of these patient populations. Finally, I'd like to end with a little insight into what the possibilities of our OPERA platform look like. Today, I hope it gives you a window of how we at Korro are approaching product-market fit with RNA editing therapies. We've been able to demonstrate this with KRRO-121, especially how it's different from DNA editing, how creating a transient protein variant is very different than going after a Mendelian disease with a pathogenic variant. I hope you can see how this opens up the possibility of us affecting biology in very meaningful ways, not just in rare genetic diseases, but also in much larger patient populations. We've shown that learning from genetics, we can affect a similar change in a transient fashion, where you probably don't want to edit a certain transcript or modify a certain protein in a permanent fashion.... In the case of KRRO-121, where ammonia and glutamine control is going to be very important in various physiological situations, a permanent DNA modification is not feasible. This is an exciting day for me and for the company, and we've been working on specifically KRRO-121 over the last few years. Hopefully, you share the same excitement that we have in terms of having a meaningful impact on these patients, where ammonia is a bad actor, and the potential for the ability to control it across multiple patient populations, to not just improve outcomes, but also the quality of life of these patients. With that, I'd like to thank our speakers, Dr. Scharschmidt, Michelle, Loïc, Todd, and we'll be happy to open it up for questions. Thank you. We'll now be conducting a question and answer session. If you'd like to be placed in the question queue, please press star one on your telephone keypad. A confirmation tone will indicate your line is in the question queue. You may press star two if you'd like to remove a question from the queue. For participants using speaker equipment, it may be necessary to pick up your handset before pressing star one. One moment, please, while we pull for questions. Our first question today is coming from Yasmeen Rahimi from Piper Sandler. Your line is now live. Good morning, team. Congrats on an excellent presentation connecting the dots of unmet need, market opportunity, mechanistic rationale, and the really strong preclinical data that you shared with us. I guess my question is, I know, Ram, you noted you're going to give us more color once the regulatory filing has taken place, but I would love to hear from our KLs, how do you think about how has the regulatory path evolved? Is ammonia still a good POC biomarker, or should we be looking at glutamate? So doesn't mean that you're focusing on those, but if we could just maybe help understand what are the key biomarkers to assess in a clinical study and how the regulatory path could have evolved in this space, that would be really grateful. And again, congrats on a very thoughtful and meticulous presentation to us this morning. Thanks, Yas. This is Todd Chappell. So I'm going to be directing traffic here for these questions. So I think for this first question, we'll have Dr. Scharschmidt respond. Yeah, that's a very good question. Thanks. I mean, I think nothing has changed with respect to ammonia and Urea Cycle Disorders. It's, it's still absolutely the driver. But your question about glutamine is, is a very good one, and I think the short answer is that with respect to hyperammonemic crises, ammonia is the horse and glutamine is the cart. So, so what do I mean by that? Well, glutamine is the body's most abundant amino acid. So in a situation where the body can't get rid of waste nitrogen, for example, in inadequately treated UCD patients, glutamine becomes elevated as a consequence. However, and we, we looked at this really quite closely and published our findings, elevated glutamine has no predictive value for crises once ammonia is taken into account. So unlike current therapy, and by that I mean phenylacetic acid prodrug, where an elevated glutamine could be a marker of inadequate control or patients at risk. Now, in the case of the Korro product, elevated glutamine, if it occurs, and you heard from Loïc, that so far it hasn't been observed, would suggest to me at least, that the drug is doing its thing. It's working, and the glutamine is replacing urea as a vehicle for waste nitrogen excretion. Hope that answers your question. Thank you. Thank you. Next question today is coming from Steve Seedhouse from Cantor. Your line is now live. Good morning. Thanks so much for hosting the event. I have three questions. I'll just take them one by one, if I could. First, I guess, is just how fast do you think you can get the proof of concept, pharmacodynamic data, namely, obviously clinically relevant impact on ammonia levels? And are you planning to start, you know, the first study at a therapeutic dose in patients? We'll have Ram respond to that. Hi, Steve. Thank you for joining. You know, as I mentioned, you know, we are gaining alignment from the different regulatory bodies. As you can imagine, for a rare disease, you want to go far and wide as quickly as possible. And so I would say stay tuned. I will come back to you, or we will come back to you, later this year in terms of when that is likely going to be. The intent, however, is exactly as you described it, which is the first goal is to show a reduction in ammonia. It's just unclear at this point in time, specifically how we're guiding to in which patient population. All right, thanks, Ram. Second question is just, I mean, a little preamble here. First of all, I think one of the keys to RNA editing right now is finding targets where less than complete editing will confer maximal clinical benefit. And you were talking about, I think you showed 20%-25% editing is all that's required to rescue the phenotype in hepatocytes and maybe even less than that in the mice. So do you think that's the sweet spot, or is that leaving efficacy on the table if you could get more editing than that? What would happen if you got more editing than that? Could it be harmful? And what's your conviction that you can get to that editing level in humans? For this one, we'll have Loïc respond. Yeah, thank you. It's a very thoughtful question. So, you're correct. Now, what we have demonstrated for this program is 20%-25% editing is sufficient to activate glutamine synthetase, and in particular, to increase the half-life of our targets. Whether we can increase the editing, we have some data where we can go up to 40% editing and glutamine synthetase, and this translate into the similar activity in terms of detoxification of ammonia in our preclinical models. Whether increase in editing could have detrimental impact or safety issue in human, no, this is fairly unknown at this point of time. Well, I mean, the key message here is that you're not correcting a mutation. You don't need to have, like, you know, 90% editing. With much lower editing, you can really correct a pathway, and you can restore a normal function. So whether the RNA editing field is heading into that direction, I truly believe that modulating targets is going to be a big path forward as we are going to advance several programs with this type of strategy in the near future. Okay, great. Last question for me, just, obviously, this is a GalNAc conjugate, so there, there's limited read-through from the, the AATD program, in terms of the mode of delivery and how much editing you'd therefore expect. I'm wondering, though, separately about the target itself. So Glutamine Synthetase, how does its expression level in the liver compare to something like SERPINA1 or other targets you've tested, and how would the expression level of the target itself affect editing efficiency that, that you'd expect in humans? Just thinking about how to handicap, you know, the variables that would, be predictive of editing efficiency in humans once we get to the clinical trial. We'll continue with Loïc on this one. Yeah, so I mean, if you compare SERPINA1 and glutamine synthetase, they are fundamentally different targets. For the former, you try to correct a missense mutation. For the latter, you try to improve the function by changing the restoring the biology and increasing the half-life. Now, how it's going to translate into the clinic, here we're using GalNAc conjugated delivery. We have exquisite delivery of our ASO to the hepatocyte. And what we intend to do is, now we are generating data safety, but also we will be doing allosteric modeling and using PKPD modeling to define the dose that is going to be therapeutic in the clinic. I think here from the, you know, the data we have generated in preclinical setting, we have a large body of evidence demonstrating that, at least for Glutamine Synthetase, the level of editing we are reaching and the correction of ammonia detoxification is very relevant, and we aim to be at those that are efficient when we get into patients. I'd like to add to what just Loïc said, Steve, to complement a couple of points. You know, first thing, earlier this year, we shared our data on a SERPINA1 editing that is in vivo, we're close to 100% almost, okay? So it shows you the amount of progress that we've made from a editing standpoint and where the platform has come through. Second, I think that the reason why the speed at which we have made this progress is because of the potency improvements we've seen on our GalNAc conjugate. And so just to put that in context, the current 121 program is couple of logs full more potent than what we had taken to the clinic earlier with GalNAc delivery. And so, builds a lot of confidence in terms of how we are evolving the platform and builds a lot of confidence in terms of the amount of editing that we, you know, need to see in humans at a dose, that's gonna be relatively safe. Terrific. Thanks so much. Thank you. Next question today is coming from Luca Issi from RBC Capital Markets. Your line is now live. Hi, team. Congrats on pre-commit on the upcoming regulatory progresses on 121, and we have a follow-up question for Dr. Vincent on the endpoints. Oh, this is Cassie on for Luca, by the way. Is there a glutamine level and ammonia threshold that you'll be targeting with your first human study? We're just thinking from the regulatory perspective, do you think this will be compared to the phenylbutyrate, just like how AATD uses augmentation therapy to set the bar? And down the road, is OTC crisis rate going to be required for an approval and maybe Dr. Scharschmidt? Sorry. Yeah, first, let's have Dr. Scharschmidt respond to that. A good and perceptive question. No, I think we would anticipate something similar to what was the case for Ravicti. Ammonia, the approval was based on favorable ammonia control. And of course, the regulators also wanted to see that hyperammonemia crises were at least directionally favorable. I think it's unlikely that, given the infrequency, that a statistically significant change in crises would be needed, but again, I would anticipate that approval would be based, given precedent on favorable benefit with respect to ammonia, with other things at least leaning in the right direction. Thank you. Next question. ... Oh, sorry. Thank you, Bruce. This is Loïc. Yeah, so, a very thoughtful question. I mean, from a preclinical perspective, what we have been able to demonstrate with our, GalNAc conjugated, oligonucleotide, is, a decrease of, ammonia with effective detoxification, bringing the levels, below the upper limit of normal or rounded. So I think, you know, when we think about the translation, and as mentioned by Bruce, we will be targeting the same impact. It also has, you know, a benefit to potentially, trigger diet liberalization. As we have seen in our studies, whenever we have done challenges, we have been able to control and detoxify, ammonia, and this could have, you know, life-changing, consequences for patients, we usually from a diet perspective. Thank you. Our next question is coming from Ryan Deschner from Raymond James. Your line is now live. Thanks for the question. First one for two. First one for Dr. Scharschmidt. Do you think preventing GS degradation could reasonably enable enough ammonia clearance to allow human patients to return to a completely normal diet? And then maybe for Loïc, give a rough expectation for how often you expect to dose UCD patients with 121. Dr. Scharschmidt? You know, you know, I'll defer to Loïc, sort of on the frequency of dosing. That's typically something that would be defined in early clinical trials, SAD, MAD. You know, I think the advantages of 121 should be substantial. I mean, just perhaps to reiterate or put some color about what I said earlier, and perhaps the most obvious advantage is compared with very short half-life PAA prodrugs, is durable around-the-clock coverage, which should benefit all patients, even those with good compliance, for example, where they sleep or if they're too sick to take their meds. There's another theoretical advantage, which I think could turn out to be important, and that is in the case of PAA prodrugs. Ammonia removal is really a byproduct of PAA detoxification and conjugation with glutamine to form phenylacetylglutamine. So, so the timing and the amount of ammonia removed is not driven by high ammonia per se, but rather driven by, as well as limited by, the timing and amount of PAA, which needs to be metabolized and to be safely delivered. By contrast, you know, the more robust glutamine synthetase created by KRRO-121 should operate based on conventional enzyme kinetics and be specifically responsive to high ammonia. So a treatment, if you will, which is there when the patients need it. I mean, given the preclinical findings, which Loïc described, I can only imagine sort of good things, if you will, that, yes, patients would be allowed to live a more normal life, perhaps decrease their drug requirements, have a diet which doesn't require dietary supplements or comes something closer to normal. So yeah, that, that's my quick take on things. Loic? From a treatment perspective and dosing regimen, so what we have done in the preclinical setting, and we're still generating data, you know, and, you know, being studied from a safety and PK perspectives, we would intend to use every other week or as frequent administration subcu to patients. Thank you. Thank you. Next question today is coming from Kostas Biliouris from Oppenheimer. Your line is now live. Good morning, everyone. This is Kostas on for Andreas. Congratulations on the progress and the great data here. A couple of questions from me. The first one is around diagnosis. It seems that these diseases are underdiagnosed. Do you expect the diagnosis rate to increase in the future, potential with newborn screening or other approaches? And the second question is on the translation from mice and non-human primate data. How well those data have been translated to humans for the approved drugs? Thank you, and congrats again on the progress. Thank you. I will have Dr. Scharschmidt respond to the diagnosis question. You know, again, it's a really good question, particularly for disorders such as urea cycle disorders, where only a minority are picked up by newborn screening. You know, historically, as you allude to, when there's an effective treatment available, the diagnosis rate tends to increase, and certainly that could be the case here. I would just also add that the patients who are severely, sufficiently severely affected to come to medical attention are the ones who tend to be diagnosed. So if anything, diagnosis rate might go up, but the patients in greatest need, for the most part, probably already identified. Thank you. Loïc is going to respond to the translation question. Yeah, from a translation from mouse to NHPs and to human, so I'm working with GalNAc conjugated oligonucleotide. Usually, the translation from small species to human is relatively straightforward. I think for glutamine synthetase in particular, we're going to start with the targets. The target expression in UCD patients is consistent, and there's no change, and there's no impairment in vascularization in the liver. So from a delivery perspective, we don't expect any issue. In our preclinical setting, we use very stringent ammonia challenge, where we use like, you know, very high concentration to really test the potency of KRRO-121 in generating de novo variants of glutamine synthetase, and we have been able to generate consistent data. I think the most critical data that in our dataset is the PXB humanized liver mouse studies, where here we're using a mouse model but with a human liver. And we have been able to demonstrate the generation of our de novo variants, and also we have been able to report a very significant decrease in ammonia after challenge. So, and we would expect now translation from preclinical to the clinic, assuming that we would be in a therapeutic range from a safety and efficacy perspective. Thank you. Next question today is coming from Keay Nakae from Chardan. Your line is now live. Yes, thank you. Question for Loïc. Just trying to get an understanding of two things. One, I guess, how fast the OPERA can go. Can you tell us what kind of editing efficiencies you're able to achieve here? Editing efficiencies, what we are reporting here is that 20%-25%. In different mouse model and preclinical studies, we are able to go up to 40%. So we are in a range where now editing is sufficient to generate the glutamine synthetase that have improved half-lives in our preclinical setting. What is the correlation with the editing efficiency, the amount of editing and the reduction in ammonia? Is that linear? What does that graph look like? Yeah, no, from a correlation perspective, and it's a very good question. It's clear, you know, when we reach at least 20%, this translate into the increase in the glutamine synthetase. So maybe to go into a bit deeper into the mechanism of action. We are editing at a lysine, and we are editing a lysine into an arginine. So we are removing one of the ubiquitination sites that is going to trigger degradation of the protein, so the proteasome. So by doing that, we increase the half-life very significantly. To give you some perspective, the native glutamine synthetase half-life in human is about one-two days, and in the presence of excess of glutamine, the half-life is decreasing to down to six-12 hours. With the editing we are doing, and again, we have, like, say, 20%-30% editing, the half-life of the glutamine synthetase variant that we are generating based on the preclinical studies is about 14 days, so two weeks. So the half-life of the end product, which is the glutamine synthetase variant, is significantly improved, and this is the sustained ammonia detoxification that is now kicking in with our KRRO-121 treatment. All right. Thanks. Thank you. Thank you. Next question is coming from Myles Minter from William Blair. Your line is now live. Hi. Thanks for taking the questions. First one is just if you've looked at any sort of downstream markers on glutamine metabolism. I assume in UCD, it's like these patients, most of them are gonna be OTC deficient. So, like, aren't you just shunting this more towards a problem of a buildup of, like, carbamoyl phosphate or orotic acid, and you're just pushing this disease down the chain because the underlying urea cycle disorder is still there? That's the first one. The second one, maybe for Dr. Scharschmidt. There's been a lot of talk about, you know, lack of enrollment cadence in UCD trials. Been a ton of sponsors out there doing some work and just taking, you know, exceptional long periods of time to get patients into trials. Just any sort of comment on expected enrollment cadence in this one for Korro's products? Thanks very much. Great. We'll have Loïc respond to the first question. Yeah, so just for clarification, so in the urea cycle, now, here we're using model system where we have enzymes that are deficient. What we have demonstrated is that multiple enzyme deficiencies, like CPS1, OTC, ASS1, the urea cycle is defective. Here, what we are activating is the glutamine synthetase pathway. It's a complementary parallel pathway, has nothing to do with the urea cycle by itself. It's a separate pathway that complements the ammonia detoxification. And Dr. Scharschmidt, maybe you can respond to the next question. Sure, happy to. I mean, enrollment is never easy with an ultra-orphan disease. But just to give you some context, at Hyperion, over the four years that I was there, we completed five clinical trials in patients with urea cycle disorders. In addition to the pivotal, there were three or four supporting studies in different age groups. These were enrolled exclusively in North America, so U.S. sites, plus SickKids in Toronto. And I think the one point I would make is really the importance, and it's difficult for me to overstate this, but the importance of working closely with the patient advocacy group, the National Urea Cycle Disorders Foundation, and also the NIH-sponsored physician group, the UCD Consortium. So it was really an effective partnership that enabled on-time enrollment to this rare patient population. Thanks for the questions. Thank you. Next question is coming from Mitchell Kapoor from H.C. Wainwright. Your line is now live. Hey, everyone. Thanks for taking the questions. Just wanted to ask a little bit more on ideal trial design that you would envision, with, you know, the obvious caveat that you have to follow up with the FDA. Just thinking about, like, what would be clinically meaningful here, on a duration of follow-up basis, what would be required to definitively show benefit in HE? Is it something that would be weeks, months, or longer? And then separately, would you seek to enroll, you know, ideally stable HE patients or those with recent HE events at a high risk of recurrence? And would you think about hospitalization rate or something else, in terms of, you know, what would convince physicians as well as patients, that this therapy would be meaningful to them? Thanks. We'll have Ram respond first and then follow up with Dr. Scharschmidt. Thanks, Mitch. I'm sure the same questions are on everybody's mind on this call in terms of what the next steps are. I think if I could just take it one level above. Our goal first is to show that mechanistically we can reduce ammonia. There are a couple of ways in which we can show that, and different populations we can show that. So that will be the first study in which we can show impact that the mechanism work, the target is engaged, and is active. What that study exactly looks like, we will come back to you shortly, but that's the goal first. The second, you know, once we've established that, would be to understand, you know, dose, dosing schedule, in each of the different patient populations. So it is likely that it's going to be different in UCD patients or urea cycle patients versus what it is in hepatic encephalopathy patients. And then finally, as you alluded to, you know, what are gonna be hard outcomes, how you think about you know, showing benefit, in UCD, whether it's diet liberalization, or it's reduction in the crises. I think that those are things that we still need to get buy-in from regulators, either pre or post-approval. So, I know these questions are relevant, and I understand the reason to understand them, but, you know, once we have more clarity on what the full development plan looks like and the alignment with regulators, I think we'd be in a much better position to respond to that. Based on our preclinical studies and based on what we are seeing in human cells, that shouldn't take very long. Now, the contingent component there is that a GalNAc-conjugated ASO will take time to get to the hepatocytes, edit and then have impact, and so we don't know what that PD really looks like, but it shouldn't be months, is our expectation. Dr. Scharschmidt, any follow-up details? Just happy to add a little color perhaps about what an HE trial hypothetically might look like. I think one of your questions pertain to the patient population. And yes, as your question suggested, I think we would do something rather similar to what Salix did and we did at Hyperion. We would enroll patients in between events who are clinically stable, who are qualified based on past HE events. In the Lambda study that I quoted, it was a recent HE event. In the phase II study, which had been accepted by FDA as supporting, we did a look back for six months. So I think that would be the patient population. Yes, hospitalizations, I think, would be important. Reading through all of this is very strong pharmacoeconomics. In the phase II study, Rockey et al, with glycerol phenylbutyrate, about 50% of the patients were hospitalized, and hospitalizations were significantly reduced with treatment. If I remember correctly, I think your third question pertained to what we'd be looking at in the study. I would anticipate that patient benefit would need to be in the form of a decrease in overt HE events, which would be defined as grade 2 or higher. I could spend a lot of time on this, but to just give you sort of a short version, we spent quite a bit of time with FDA preparing for what would have been a phase III trial that was never executed before we were acquired. Developed an HE grading instrument because the Conn scale used by Salix was criticized. I think there is also the potential for greater power and smaller sample sizes because it's very likely that patients experiencing more subtle HE events, for example, manifested by transient confusion, which occurs at home, are being missed. So I think it all has implications for what the next trial might look like and how we could use what we've done in the past to inform the next trial around. Great. Thank you very much. ... Thank you. As a reminder, that's star one to be placed in the question queue. Our next question is coming from Catherine Novack, from Jones Research. Your line is now live. Hi, good morning, and thanks for taking my question. Just as we're thinking about PD biomarkers for target engagement, since this would be a de novo edit that you're making, can you demonstrate in vivo editing capabilities in healthy volunteers? What are your thoughts on initial readouts? Well, thank you, Catherine. That's a very thoughtful question. So I mean, to your question about can we demonstrate target engagement or any endpoint in healthy volunteer, what we can comment today is the data we have generated in preclinical setting. As you have noticed in the PXB-mice, when we looked at ammonia level before challenge, we have seen a 20%-30% reduction in basal ammonia and in the study that we have done with the 15N-glutamate, which is a target engagement approach, we have seen in a wild type the like target engagement as well. So as we are creating a de novo variant, it's possible that in a healthy volunteer, we could see some effect. At this point of time, I cannot really say yes or no. This is something we will be monitoring, of course, but now from the preclinical setting, there's an impact. Great. Great. And then, just wanted to get your thoughts. Does this strategy potentially, you know, shift the burden of ammonia clearance from liver to other organs, such as kidneys? You know, what are the long-term risks of relying on GS to clear ammonia from the liver? So maybe I start, and maybe, Bruce can add on this. So as we discussed, now, we are activating this complementary pathway. It's like a synthetic rescue. You see, your urea cycle is defective, and then you are activating the glutamine synthetase that will restore normal ammonia detoxification. The impact of KRRO-121 is happening in the liver, through the GalNAc conjugation, it's exquisite delivery to the liver, and there's minimal ammonia detoxification in the muscle, for instance. So likely, the site of action for our drug is going to be in the liver. Again, we are not editing both lysine in the N-terminal domain of glutamine synthetase, so at some point, the de novo variant is going to be degraded by the proteasome. This is happening at a much later time form as compared to the native glutamine synthetase. And now, again, our therapy is transient. We aim to treat the patients, you know, every other week or less frequently. But now, if we stop the treatment, clearly, we're not generating any new de novo variants. So that would be a strategy where we can fine-tune the therapy in our patients to maintain safety and efficacy. Bruce, do you want to add on this? I guess just to comment, maybe to rephrase the question, is there some potential downside or, perhaps injurious consequence of- ... the approach that's been outlined? I think my take would be probably no. I might add that the biology of ammonia and nitrogen metabolism is quite complex, but the final pathway for getting rid of ammonia is excretion of urea in the urine. Now, in the case of phenylacetic acid prodrugs, urea is replaced by what's essentially a foreign compound, phenylacetylglutamine, but I'm really not aware of any renal consequences of PAA prodrugs. I mean, if anything, the KRRO-121 should be more patient-friendly, if you will, because the vehicle for excretion is being shifted from urea to glutamine, which is a normal body constituent and amino acid. So no, it's hard for me to envision any sort of renal downside, if you will, to the approach being outlined. Okay, great. Well, thank you very much for taking my questions. Thank you. Next question today is coming from Bill Mui from Clear Street. Your line is now live. Good morning, and thanks. I have sort of a multi-parter that has to do with therapeutic window. Given you're looking at sort of an editing efficiency that needs to be tuned rather than maximized, what can we expect in terms of intra-patient variability of editing? And in the case that something a patient does have editing above and beyond the intended range, what are the theoretical issues that could arise in terms of adverse events, and how serious versus transient might those be? Thank you. So thank you for the question. So when it comes to treating patients, one may think that we will see some variability across patients. That it comes down to the potency and the specificity of our drug. What we have done, now we have generated an optimized GalNAc-conjugated oligonucleotide that consistently induce relevant editing in all our preclinical settings, whether be in vitro and in vivo. What we can expect in patients without editing- ... We may have some viability in terms of editing, ranging from like 20%-40%. Likely, we have enough to generate the urea synthesis. Now, to think about, you know, the specificity of our drug, we have done all the off-target prediction in terms of, you know, in silico or looking in vitro, in primary human hepatocytes or in vivo, in, on the humanized liver from the PXB-mice. We have not seen any off-target whatsoever. So if you think about, if large editing is happening, then, this should translate into, meaningful production of the urea synthesis variants, but because we have not seen any off-target at the GS, messenger RNA transcript level, we don't expect, any downside of having high, editing in patients. Thank you. Thank you. We've reached the end of our question-and-answer session. I'd like to turn the floor back over to Ram for any further closing comments. Well, thank you, everybody, for listening in for this almost two-hour session. Really appreciate the time this morning. You know, as you've seen, we've been very thoughtful in terms of laying out the clinical need, the scientific basis of our potential therapy, the market opportunity here that is relatively large and underserved across the two patient population, and the speed with which we can generate a reasonable data set to show that our drug is working in the near term. I wanted to thank Dr. Scharschmidt for being here with us, for Michelle for spending time with us at Korro and talking through her story with Sophia. I'm really looking forward to the next update that we would give with 121, which will likely be during the time of our regulatory filing, which we have guided to as the second half of this year. Thank you, everybody. Thank you. That does conclude today's teleconference webcast. You may disconnect your line at this time and have a wonderful day. We thank you for your participation today.