Fulcrum Therapeutics, Inc. (FULC) Earnings Call Transcript & Summary

Good morning. Welcome to the Fulcrum Therapeutics FTX-6058 in sickle cell disease Phase 1b trial update. [Operator Instructions] I'd like to turn the call over to Stephanie [indiscernible] from Stern Investor Relations. Stephanie, please proceed.

Thank you, Norma. Good morning, and welcome to the Fulcrum Therapeutics conference call. Please be reminded that remarks made during this call may contain forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These may include expressed or implied statements about our future expectations and plans and clinical development time lines, among others. While these forward-looking statements represent our views as of today, they should not be relied upon as representing our views in the future. We may update these statements in the future, but we are not taking on an obligation to do so. Please refer to our most recent filings with the Securities and Exchange Commission for a discussion of certain risks and uncertainties associated with our business, that may affect these forward-looking statements. With me on today's call are Bryan Stuart, President and Chief Executive Officer; Dr. Julie Kanter from the University of Alabama School of Medicine; and Judith Dunn, President of Research and Development; Chris Morabito, our Chief Medical Officer; and Paul Bruno, our Senior Director of Corporate Development, will also be available for Q&A. Let me quickly run through this morning's agenda. Bryan will begin the call. Judy will begin -- will provide an overview of the FTX-6058 Phase Ib program. Dr. Kanter will present the Phase Ib cohort 1 data that was announced this morning in a press release and will be presented at EHA. Bryan will conclude the discussion and open the call for Q&A. With that, it's my pleasure to turn the call over to Bryan.

Thanks, Stephanie. Good morning, everyone, and thank you for joining us today. As many of you know, our mission at Fulcrum is to treat the root cause of rare genetic diseases, and we are very excited about the potential of FTX-6058 clinical development program. The results that we're sharing today demonstrate proof of concept of FTX-6058 as an oral HbF inducer and that we are on track to potentially deliver a transformative therapy for people living with sickle cell disease. The current treatment landscape for sickle cell disease is very limited. Available therapies only target select symptoms. HbF induction is known to be an effective strategy for broadly improving clinical outcomes and addressing multiple symptoms, including vaso-occlusive crisis, anemia, pain, infection, stroke, heart disease and others and has long been a key therapeutic goal of the field. I'm very excited today to share the proof-of-concept data from our Phase Ib trial of FTX-6058 in adults living with sickle cell disease in which we observed that FTX-6058 increased HbF in our initial subjects from our Phase Ib trial. Based on genetic data, literature and emerging gene editing data, clinicians and key opinion leaders believe that 5% to 10% increases in HbF from baseline levels could be transformative for people living with sickle cell disease. We are excited to announce today that our initial subjects in our first dose cohort achieved rapid and robust HbF increases. These initial subjects observed increases of up to 6.3%. And encouragingly, the data suggests that maximal increases for these subjects have not yet been achieved. On today's call, we'll be sharing data from 6 subjects in the first cohort. Two subjects were dosed for at least 56 days. In these subjects, we observed HbF increases of 5.2% and 6.3%, respectively, which is already at levels we believe could provide meaningful benefit for people living with SCD. All 6 subjects in the study tolerated the drug well and no serious treatment emergent adverse events were experienced by any subject in the study. We are also pleased to see that these clinical findings are consistent with the work that we've done to date preclinically, demonstrating an approximate 1:1 correlation between mRNA and HbF. These initial data from this first cohort have provided us valuable insights that inform the path forward for the program through our planned registrational trial in 2023. Judy will go into more detail on that shortly. Today's data gives us confidence that FTX-6058 a once-daily oral HbF inducer has the potential to disrupt the current development and treatment landscape for sickle cell disease. So from our presentation today, we hope to leave you with a few key takeaways: Firstly, that these proof-of-concept data validates FTX-6058 as an oral HbF inducer. Also, in these patients, FTX-6058 is already achieving levels of HbF induction associated with broad clinical benefit for people living with SCD. And based on these data, we plan to complete enrollment of the Phase Ib dose-ranging study in 2022 and aligned with regulators on the design for the registrational trial beginning in 2023. Before I pass it on to Judy and Dr. Kanter, I want to thank the study participants, their families, physicians, the sickle cell disease community and our clinical investigators for their commitment to advancing this work. We, at Fulcrum, are proud to be working alongside the sickle cell community to advance what we hope will be a transformational therapy for this disease. Judy?

Thank you, Bryan. Before we walk through the data, I'd like to review our 6058 program for sickle cell disease and other nonsickle hemoglobinopathies. Sickle cell disease is a genetic disorder of the red blood cells, caused by a mutation in the [ HPV ] gene. This mutation in hemoglobin causes red blood cells to take on a characteristic sickle shape. These red blood cells often die prematurely, causing anemia and often damage to the blood vessels, prompting vaso-occlusive crisis, or VOCs, episodes of extreme pain that may include other serious symptoms like stroke, acute chest syndrome and other forms of organ injury. Sickle cell disease can have a devastating impact on quality of life and complications significantly increase the risk of death from the disease. Further, it is the most common type of inherited hemoglobinopathy. And [ this action ] is estimated 100,000 people in the United States and millions more worldwide. There remains a significant unmet medical need in sickle cell disease. Currently available treatments ameliorate only select symptoms of the disease. None target it's broad symptomatology. Although hydroxyurea, the current standard of care, can address multiple symptoms, not all patients respond, the durability of the response is inconsistent and it has safety and tolerability challenges. HbF induction is known to be an effective therapeutic strategy for broadly improving outcomes in sickle cell disease, reducing hemolysis, anemia, the occurrence of VOCs and decreasing the frequency of recurring events. Gene therapy approaches, which, like 6058, aims to induce fetal hemoglobin may potentially provide a functional cure, but remain highly invasive treatments. At Fulcrum, we are focused on developing 6058 as a convenient oral HbF inducer that we believe could be truly transformational and become the new standard of care for sickle cell disease. As Bryan mentioned earlier, based on extensive conversations with clinicians and other KOLs, we have set a goal of 5% to 10% HbF induction from baseline for 6058. Empirical evidence from published noninterventional studies confirms that 5% to 10% increases in HbF over baseline are associated with broad clinical benefit. A systematic literature review of more than 50 published analyses indicate that any increase in HbF levels provides benefit. As the only oral HbF inducer in the clinic, we believe 6058 has the potential to be a transformative therapy for sickle cell disease and other hemoglobinopathies. 6058 originated through our FulcrumSeek discovery engine and is a potent and highly selective EED inhibitor with a clean off-target profile. It's being developed as a once-daily oral treatment. We have composition of [indiscernible] patent through 2040. And as we just talked about, we have strong preclinical and now clinical evidence in sickle cell subjects that 6058 induces HPG mRNA and HbF protein. Along with the encouraging data supporting the clinical value of HbF induction, we also have an extensive body of data supporting 6058's ability to induce HbF. First, in our numerous preclinical studies, including those in CD34 positive human cells and the Townes' model for sickle cell disease, we saw consistent two to threefold induction at HPG mRNA that translated into the same fold induction in HbF protein. We saw this level of induction independent of baseline HbF levels. In December, we announced the results of our Phase I trial of 6058 in healthy volunteers. As a reminder, that trial is designed to evaluate the safety, tolerability, pharmacokinetics and pharmacodynamics of ascending doses of 6058 in healthy volunteers over the course of 14 days. From that trial, we were enthused to report robust target engagement, dose-dependent HPG mRNA inductions that either met or exceeded the full changes predicted to be transformative and increases in F-reticulocytes, indicating that HbF protein induction had begun. 6058 was also well tolerated at higher doses with no severe or serious adverse events. Notably, in the 6-, 10-, 20- and 30-milligram dose cohorts, we saw consistent dose-dependent increases in HPG mRNA. These data provide robust proof of mechanism and proof of biology, reinforcing our confidence in the potential of 6058. Today, Dr. Kanter will be presenting initial data from the first subject in the ongoing Phase Ib trial of 6058 in adults with sickle cell disease. As a reminder, there are three cohorts in this study. The first cohort began at a daily dose of 6 milligrams. Each cohort can enroll up to 10 subjects, and each cohort comprises of a 28-day treatment period. At the end of this period, all subjects are offered the opportunity to participate in an 8-week treatment extension. The goal in this Phase Ib study is to, first and foremost, see if FTX-6058 induces HbF protein in people with sickle cell disease. At this early stage, we were looking for any HbF protein induction. As Bryan mentioned earlier, based on today's data, we are very pleased to be seeing not just protein induction, but a change from baseline within the range of 5% to 10% that could be transformative. I'll now turn it over to Dr. Kanter to discuss the initial results of this ongoing Phase Ib trial. Dr. Kanter is an associate professor of hematology/oncology, and the Director of the adult sickle cell program at the University of Alabama at Birmingham as well as a notable expert in advocate on sickle cell disease. We're thrilled to have her with us today to share these new data and answer any questions you may have at the end of the call. Dr. Kanter?

Thank you, Judy. I'm very pleased to be here today to share these exciting data with all of you. The data are being presented in conjunction with the European Hematology Association, or EHA, Hybrid Congress today in Vienna and include data as of the cutoff on May 25, 2022. So looking at our subject demographics. I want to provide an overview here of the initial 6 subjects enrolled. They have an average age of 31 years, and their mean baseline fetal hemoglobin level is 5.7%, which is consistent with other recent observational and treatment sickle cell disease clinical studies. I'll note that all of the subjects have hemoglobin SS genotype and none were on current background hydroxyurea therapy. When we review the day 1 pharmacokinetic parameters for FTX-6058 6-milligram dose, you can see that I'll highlight the Cmax was lower and Tmax earlier in our individuals with sickle cell disease compared to healthy volunteers. Whereas half-life was similar between healthy volunteers and sickle cell disease subjects. Together, these preliminary observations suggest there are differences in absorption between the healthy volunteers and sickle cell disease subjects as opposed to differences in metabolism. These are very small sample sizes, but the data indicates that there is a difference in exposure between healthy volunteers and sickle cell disease subjects. In sickle cell disease subjects, the exposure from the 6-milligram dose is more similar to the exposure from a 4-milligram dose in healthy volunteers. FTX-6058 achieved up to 6.3% absolute increase in hemoglobin F. This is very exciting. FTX-6058 was able to increase up to 6.3%, absolute increase from baseline. On the slide in your slide set, you can see the data from all 5 subjects included in the pharmacodynamic analysis. Now let's look at the subjects one by one. Subject 1 completed the full 84-day treatment period in the study. This subject on your slide is shown in blue and achieved a 5.2% absolute fetal hemoglobin increase above baseline for a final hemoglobin F level of over 14%, while still increasing as of the last time point. The subject was not dispensed drug after day 56. Subject 2, who is in orange, achieved a 6.3% absolute hemoglobin F increase above baseline for a final hemoglobin F level of approximately 10% at day 56, and dosing is ongoing. Subject 3 is noted in black. This subject achieved similar levels of fetal hemoglobin induction compared to subjects 1 and 2 during the first 28 days of treatment, but then voluntarily withdrew from the study. The subjects withdrawal importantly was not associated with the safety or tolerability issue. We do not know what the final hemoglobin F level would have been attained, but the kinetics are very similar to subjects 1 and 2. Subjects 4 and 5 were nonadherent as assessed by [indiscernible] and subject self reporting and no fetal hemoglobin induction was observed in these subjects. These initial data represent compelling and important proof-of-concept for FTX-6058. Encouragingly, the first dose cohort utilizing the 6-milligram dose was able to achieve 5% to 10% absolute increases in fetal hemoglobin, which is known to be associated with clinical benefit in individuals with sickle cell disease. We observed the measurable increases in hemoglobin F as early as 14 days after treatment initiation. Also important, the relationship between the hemoglobin mRNA increases the gamma hemoglobin mRNA increases and the increases in hemoglobin F protein was consistent with the existing preclinical data. Also important in this proof of concept was that FTX-6058 decreases hemolysis or red cell breakdown. The hemoglobin induction observed in subjects 1 to 3 and also resulted in decreased hemolysis as shown here on this slide. We are looking at changes in absolute reticulocyte count, total hemoglobin and total bilirubin, all that are consistent with decreased hemolysis. In the subjects that achieved hemoglobin F induction, we observed a 20% decrease in absolute reticulocyte count, about 0.5 gram per deciliter increase in total hemoglobin and a 55% decrease in total bilirubin. Together, these changes indicate less anemia and decreased hemolysis, again, important for proof of concept. These are small numbers, but are important measures of hemolysis that are moving in the right direction. FTX-6058 was also generally well tolerated. On this slide, you can see that there is a summary of all of the treatment-emergent adverse events. The chart includes all of the subjects and every adverse events, whether they were or not on drug. Importantly, all TEAEs are nonserious. They resolved and redeemed unrelated to study drug. No SAEs were reported while on study drug, and there are no discontinuations reported due to TEAEs. Unlike hydroxyurea, no myelosuppression was observed. There was one subject that did report a moderate vaso-occlusive crisis during the study, and this was in someone who was self-proclaimed nonadherent to the medication. With that, I'll now turn it back to Bryan for concluding remarks. Bryan?

Thank you, Dr. Kanter. Again, we're thrilled to share today's data. With this early snapshot from the Phase Ib, we have demonstrated compelling proof of concept. We now know that FTX-6058 is an oral small molecule HbF inducer that based on our initial subjects at the first dose cohort can achieve meaningful HbF increases in people with sickle cell disease. HbF induction provides a direct path to addressing multiple aspects of disease symptomatology and FTX-6058 is the only HbF inducer in the clinic that has the potential to noninvasively address a major unmet need in SCD. Additionally, we now have a better understanding of the relationship between mRNA and HbF protein. FTX-6058 also improved biomarkers of hemolysis demonstrating improvement in red blood cell morphology and other measures of cell health. FTX-6058 was also well tolerated in adults with sickle cell disease with up to 3 months of exposure, the longest time frame measured in this study. Lastly, we have made great steps to advance our knowledge of 6058 and the disease pathology of sickle cell disease. This new data will inform our next steps. As we complete the Phase Ib, we will take these steps to incorporate into our learnings, introducing new adherent approaches and focusing on enrollment of subjects on hydroxyurea in the first 6-milligram cohort. We will also initiate a second 2-milligram cohort to facilitate identification of a minimally efficacious dose and plan to explore a third dose cohort to further elucidate the kinetics and induced greater amounts of HbF after gaining more experience in combination with HU. We plan to complete enrollment in all 3 cohorts by the end of 2022. The data from the Phase Ib will then inform dose selection for a planned registrational trial, which we plan to discuss with regulatory authorities and expect to initiate in 2023. I'm very pleased by today's data and even more enthusiastic about what's to come with FTX-6058. This program has the potential to transform the treatment landscape for sickle cell disease and to reignite the conversation about the critical need for sickle cell therapies that are less invasive and more accessible for patients. We look forward to continued collaboration with clinicians, investigators and the sickle cell community and continue to advance this important work. With that, I'd like to open up the call for Q&A. Operator, you may now open the line for questions.

[Operator Instructions] Our first question comes from the line of Judah Frommer from Crédit Suisse.

I just wanted to start out with compliance within these first 6 subjects. Any additional detail you can provide on why there were dropouts here? Do you think that not being on background hydroxyurea or being on kind of a chronic treatment program may have made a difference here? And can you elaborate on what you'll change going forward? As this being an oral option for patients should hopefully solve that compliance problem. That's how evident in this patient population.

Yes, absolutely. It's a great question. And maybe I can turn it over to Judy, and we can speak to both what we observed in these initial patients and what we have initiated.

Yes, we did learn a lot in this cohort. We learned a lot about enrollment. We learned a lot about operationalizing the trial. And understanding adherence is one of those important things that we were able to observe and most importantly, react to. So we have already, based on this experience, put observed dosing into the trial and we get the results of that as data comes in. But historically, across the industry, when you put in these sorts of technologies that help to improve adherence, that is the case. So that's already ongoing, and we will take that into phase -- with the registration trial. And the second part of your question was -- could you remind me?

Yes. No. I think that kind of covers it on the compliance side. But just in terms of the patients that will now be on background hydroxyurea, do you see any difference in potential compliance there and kind of the next piece of that was going to be, do you expect an additive effect to that?

Yes. So our preclinical data has indicated that there may well be an additive effect between hydroxyurea and 6058. In terms of predicting compliance, those of the [indiscernible], the patients who came into this trial were not on background therapy. So we wouldn't expect compliance or adherence to be more or less impacted by having to take hydroxyurea, especially in the upcoming cohorts, when patients are coming in on hydroxyurea, they're already on a stable dose for several months and we'll be able to verify that with them. So we don't think that taking more medication is going to necessarily impact compliance or adherence. And most importantly, we do have technology in place to assist us with both verifying adherence as well as improving it.

Okay. And just on the safety side of things, I certainly don't want to put words in your mouth. So is it fair to say that you don't believe that any of the discontinuations were tied to safety or drug reaction? I think you did say the drug was well tolerated. I assume that means in everybody. And then as you explore incremental doses, is it more about establishing, I guess, minimal applications dose along with, hopefully, maximal protein induction as you get into that third quarter? Or is there -- what do you think you'll explore in safety for the second and third cohort?

Yes. So two important and related questions. So first and foremost, I want to be clear that the adverse event data that we showed today does include adverse events from all patients who were enrolled in the trial, not just the subset of patients, which were the focus of some of the PK/PD. And so you can see by those adverse event listings that there were no adverse events that were serious or severe. And there is no pattern between adherence and adverse events. And so we do not believe they are correct. We do not believe that there was a relationship between any kind of tolerability and adherence. And then in terms of dose selection, moving forward for your question, Bryan outlined this, I think, very well, number one, we're really excited about the ability and I think an important observation for the field to show that an oral medication has the potential to induce HbF to these levels. So moving forward, we want to be really deliberate about those elections and achieve a couple of things. Given the robust response that we saw at 6 milligrams, we certainly do have to start to explore minimally effective doses. And so as Bryan said, we will look at HU. And then we won't go higher until we have some additional experience in combination with HU.

Our next question comes from Joseph Schwartz with SVB Securities.

Congrats on all the progress. I was wondering if you could talk about whether you'll be doing any other data analyses on these patients in order to inform your dosing strategy forward -- going forward, like the biomarkers you've spoken about previously?

I'm sorry, go ahead, Bryan.

No, no, go ahead, Judy. Apologies.

Yes. No, I was going to say, Joe, that yes, this protocol, this program remains in study conduct. We'd like to get a little bit more data at 6 milligrams, and we do continue to look at data generated from these participants as well as future. And we will look at -- as you know, very closely look at exposure relationships at things like target engagement, like mRNA, and now, ultimately, like [ HbS ]. And I think all of those relationships, PK/PD exposures are going to be important for us to understand and inform our dose selection.

Okay. And then I was wondering if you could talk a little bit more about your decision to next go down in dose and by so much to 2 milligrams from 6 milligrams. If the patients with sickle cell disease turn out to have lower absorption and 6058 has been well tolerated so far, can you just help me understand your thought process there? And have you considered whether you could study that lower 2-milligram dose with HU while you advance a higher dose as a monotherapy? And then will you continue to study the 6-milligram dose in these patients? Will they continue? And will we get any more data from this cohort this year?

So Joe, maybe I could speak to at a high level, just kind of disclosure how we're thinking about moving forward here, and then I can turn it over to Judy in terms of both thinking about the second cohort, but also what will inform the dosing level for the third dose cohort. So I think just at a high level, our goal is to, obviously, in the course of this Ib dose ranging to look at three different doses to understand HbF induction and safety and tolerability independently and in combination with HU and we want to inform a single dose that we can then take into a registrational trial next year, again, with that goal, of being able to achieve what we believe and the field believes will be transformational. And that is that 5% to 10% increase -- 5% to 10% absolute increase in HbF. So as we go throughout the remainder of this year, it's both going to be establishing a minimally efficacious dose and then selecting the third dose after we have more experience with HU. We believe we're now on track with the additions we've made in terms of sites and patient identification to complete enrollment in 2022. So we haven't guided to exactly when we'll share that data, but we feel like we'd be able to have all of that data -- essentially have all that enrollment completed this year. Judy, anything you wanna add to that?

No, Bryan, I think you well covered it. I think to maybe address some of Joe's more specific questions in terms of, will we look at 6 milligrams in additional subjects? And certainly, that is the case, Joe. We'll continue to collect more data at 6 milligrams. We'll continue to get acute exposure. And then in terms of understanding that absorption, that's a really important thing for us to understand. We have a single dose. It's early days and very few patients. We want to make sure that we validate these observations and understand them. And we don't even know that, that will hold at 2 milligrams. So this is exactly why we want to do these experiments to really understand this exposure relationship and the exposure in patients.

And Joe, just to make sure we answer all of your questions. Let me turn it over to Chris, and maybe Chris could speak to just why we selected 2 milligrams. And Joe, it sounds like you might be asking why 2 versus maybe something that's slightly higher, but I'll turn it over to Chris, and we can speak to that.

Yes. Sure. Joe, so if you remember from the PK data that we shared from the healthy volunteer trial, there is some variability. There's not much in the dose -- in terms of [indiscernible]. So it's that there is some variability. And between the two doses, we actually cover a pretty complete range of exposures and concentrations. So going down to 2 seems like a big jump. But in fact, it's not when we look at the variability that we've seen in [ healthy ]. We do expect that there will be some variability in patients as well. As Judy said, we don't understand the absorption at 2 milligrams is something that we need to understand. So going on to 2 in patients should give us a pretty complete range of exposures and concentrations. I'll also remind you that at 2 milligrams in healthy, we saw a significant -- in fact, we think maximal increases in targeted engagements by 2 weeks, and we have every reason to believe that will continue to be the case in patients. And also to remind you that in just 2 weeks in healthy volunteers, about half of the subjects had increases in HbF mRNA, could be more longer duration of treatment. So there's, again, a reason to believe that a 2-milligram dose level in patients could have positive effects on RNA and potentially on HbF.

Our next question comes from Dae Gon Ha with Stifel.

A couple from me as well. So first, I wanted to dig a little deeper on the non-adherent compliant patients. Getting some questions from investors wondering if you can provide a little bit more color, like how many doses did they take? How many pills were actually administered? Do you actually have any PK data? Was that included in the PK table that you showed today? And I guess per the SAP that was discussed in the PR as to the reason for why they were excluded from the analysis, I guess are you able to disclose what exactly that stipulates in terms of who is included and who is not? And I've got a couple of follow-up.

Judy, do you want to take that?

Sure. As I said, we did learn a lot about adherence. And what we saw in terms of adherence with this patient population in this particular study, it's not way out of bounds with what we see across the industry and these types of early trials. And I think importantly, understanding that with regard to tolerability is important. And as I mentioned, we do not believe the adherence was at all related to tolerability. In terms of what we have in the [ SAP] , we know this data, we're being very transparent about this data because we kept a very close eye on this, and we did three things: One, we had patient diaries where they reported what they were or were not taking every day. We did a pill count, and then we had patient self-report in the office as well as physician opinion. So three of those were quantitative in the [ SAP ] and then we also relied on physicians. What we can speak to is that we know that the patients that we did not include in that analysis did not have compliance at the 70% level. However, what we have done, as I said, has put something in place to improve that, and we believe that, that will be effective.

Got it. Okay. On the HbF induction in the preclinical data commentary you mentioned how the effect sizes were fairly consistent, the two to threefold induction versus baseline irrespective of what that baseline really is, hence, the fold induction usage. I guess, is your current thought process that, that same effect, meaning irrespective of baseline HbF induction would also hold in sickle cell patients? And then last question for me on the dose selection for the upcoming cohorts 2 and 3. I mean 2-milligram has been announced, but can you speak to maybe amending the protocol for a fourth cohort? And I guess what I'm wondering is, once you gain some HU experience in cohort 2, given the disparity or changes between healthy volunteer and sickle cell patient exposure to the drug, like how confident are you that your cohort 3 selection will be the most optimal in terms of inducing fetal hemoglobin versus what we're seeing with the 6-milligram?

And maybe I could -- we'll split those questions off. So first, in terms of two to threefold. I'll turn it over to Paul. And then maybe I could turn it back to Judy and we could speak to a couple of things. One, the flexibility that we have, as you're asking about, either to potentially add a fourth dose cohort or to make a current cohort larger. And then additionally, we can also just speak to some of the modeling we do and how that allows us to take the data from essentially all the dose cohorts to inform what that optimal dose will be, but let me turn it over to Paul.

Yes. The first thing I'd note is just that HbF continues to increase in subjects 1 through 3 at data cutoff adjusting we're not yet at maximum. So the one thing I'll note is for subjects 1 and 2 at that steady state level after about day 56, we're seeing about 1.6 fold and 2.7-fold induction, respectively. And this was all at a dose that had exposure more similar to a 4-milligram dose. So as we go forward, we believe that the two to threefold induction is still achievable, but we'll need more data to answer that definitively.

And then in terms of dose selection, Dae Gon, as we've talked about, we're going about this very deliberately. What we're utilizing, as you know, is a quantitative pharmacology approach. So when you say, will the next dose be the optimal dose? I think the way to think about this is that every dose will inform the selection of the optimal dose, and we will utilize these relationships between exposure and mRNA in healthy volunteers as well as in sickle cell patients to best understand the relationships and move forward with a goal of selecting a single dose to move into a registration trial. So we believe that with the current study design, we'll add some more patients at 6 to get up to 10. We'll select two more doses. We believe that this may well be sufficient to inform that selection. However, we would never take off the table our optionality to either study fourth dose or to study even more than 10 patients per cohort, but that's certainly going to be going to be data-driven.

Our next question comes from Ted Tenthoff with Piper Sandler.

Congrats on proof-of-concept data. And also, I just want to really sincerely thank you for taking all the time to answer these questions because I know it's a lot coming at you. Two from me and two from the side client, did you measure pancellularity of induction by cytometry? And was the vaso-occlusive crisis that was observed on treatment I think you said maybe that it was off, but I just want to see whether or not that had anything to do with compliance or discontinuation. And then I have two additional ones.

And thanks for the question. We really will continue to measure these parameters of pancellularity. If you remember, in our early preclinical data, we showed pancellularity in this response. So that's something that we're going to continue to characterize. And then I'm sorry, I lost you, was your second -- could you just repeat your second question. Was that about adverse events on and off the drug?

I think you mentioned for the vaso-occlusive crisis, when did that occur? And did it contribute to either nonadherence or discontinuation?

An important thing to understand. So first and foremost, the VOC did occur in a subject who had reported not being compliant and for whom we could not confirm compliance to an appropriate level. And that really is all we know about that. Again, as we said, we don't see any pattern between tolerability and adherence at all. And again, very few subjects. But what I can tell you is the VOC was on a patient who we cannot confirm exposure at all.

Great. And then just two additional ones. I was impressed by the clean safety profile, are there any assumed additive talk when you combine [indiscernible] data from the preclinical work? And then lastly, how long do you anticipate registrational trial would be? And how important will the secondary endpoints be or ultimately for approval? .

So Ted, I'll start out and I'll talk to you about the registration path and then I'll turn it over to Chris Morabito to talk about the additive -- the potential additive effects of HU and 6058, both in terms of efficacy and tox. I think, as you know, we have not yet spoken to the regulators about a registration program. So we -- I can share with you our current thinking. Our current thinking is that given the preponderance of evidence that supports HbF as a surrogate. And that is it moves in the right direction and it's predictive of clinical benefit. We feel very confident that we will be approaching the agency with an accelerated approval plan based on HbF. But then as you know, secondary endpoints, I think, confirmatory, we might consider them. In the case of an accelerated approval, we could consider conducting that study a bit longer and then looking at some clinical secondary or confirmatory endpoints that would really demonstrate the clinical value that would be recognized by patients and physicians. . So again, early days, but our plan is that we would approach the regulators with an accelerated plan based on an HbF surrogate, but then certainly plan to continue exposure to really demonstrate how HbF consistent with all the literature that we have, genetic data, et cetera, can provide real clinical benefit. And we think that's important to be able to demonstrate as well. And I'll turn it over to Chris to talk a little bit about the combination of HU in terms of safety and efficacy.

So we don't have any esthetical experience yet with hydroxyurea. And that's very important for us to gain before we think about dosing higher than 6 milligrams. We certainly need to do so. We know from our preclinical data that there is at least an additive effect on [ HbF ] in people or on cell lines anyway that have been previously treated with hydroxyurea. So people who have -- we expect that people who are on hydroxyurea will have an FTX-6058 response. And that could be something that's substantial, certainly we expect it to be something that would be within that goal of 5% to 10% at least based on the data that we're seeing today. So in terms of safety, that's something that we're going to have to demonstrate clinically, as you know, we are currently in chronic toxicology studies that this are ongoing. We haven't provided details about tox. I don't have any data to share with you about 6058 plus hydroxyurea and cost models. But we will certainly look as we are getting to the clinic with hydroxyurea for any evidence of potential effects from the combination remembering, of course, as all the clinicians out there know, there is significant toxicity associated with hydroxyurea. So that's something that we'll certainly be watching for as we introduce these to those patients.

Our next question comes from Matt Biegler with Oppenheimer.

Thanks for hosting this event for the encouraging early data. A couple for me. First, just a clarification on whether you were actively excluding patients on hydroxyurea for this -- for the first cohort or if that was just kind of serendipitous that you didn't see any of those patients? And maybe if you could provide some color on how many or what percentage of patients are on a backbone hydroxyurea just to kind of put that number into context? And then there does seem to be quite a bit of controversy around noncompliance. And I want to ask that question maybe a different way. Like we know that hydroxyurea activity is pretty binary, right? So some patients either respond or they don't. Is it possible that the noncompliant patients are actually just not responding to the treatment, and that's why they're not being compliant? Like is this a chicken or the egg kind of situation? Or maybe to ask another -- ask the question another way, how can we rule that out? Like how do we know that the patients are being noncompliant?

This is Dr. Kanter. May I answer that question, at least in part?

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So I enroll a lot of individuals on clinical trials and have a pretty decent reputation in terms of our ability to enroll and keep patients adherent. The fact that these individuals were enrolled and then really didn't take any drug just, to me, suggest they weren't good choices by the PI. And so I think the biggest mitigating influences the inclusion of more sites, which will allow more individuals to enroll. I mean to counter you on the on or off effect of hydroxyurea, we actually see a lot of individuals who have a partial effect on hydroxyurea, but they either -- their disease perceives that effect, they have breakthrough or they become less adherent over time, which we see happen especially on teenagers. So we actually see quite a number of individuals who have some effect, but not full effect. And in fact, that's probably our best target group for here. I fully expect that these are going to be additive effects without additional toxicity because we're looking at different mechanisms of hemoglobin F induction. So I don't think there'll be any reason that we should suspect any toxicity from that, and we should absolutely expect that they will have significant improvement from the addition of both -- and finally, I think individuals are looking for a great new oral medication. So I think that this is a good option for them. I'm hoping that answered all your questions. Oh, you asked about baseline hemoglobin F -- I mean baseline hydroxyurea utilization. It depends on if you're talking about children or adults. If you have old takers with sickle cell anemia, so hemoglobin SS and S beta 0, it's about 60% of people taking hydroxyurea far more in children than in adults and very dependent on whether they're seeing a hematologist as an adult or if they're seeing a primary care doctor.

Yes. Thanks so much, Dr. Kanter. Maybe we could just add your final question. Would you just turn it over to Judy, and we can speak to with these 2 patients in patients 4 and 5, as you've seen how we know and what our criteria was for nonadherence versus a patient that wasn't getting a response.

Yes. Thanks, Bryan. When we run these trials, we do have a lot of things in place in the statistical analysis plan as well as just operationalizing the trial. And as I mentioned before, patient diaries reporting how many pills they took. Pill counts that are returned as well as patient self-report when there are questions in the office and not all the time to do all of those add up. So we have to go with the preponderance of data as well as physician input. I think what you are asking about is, where the adverse events that may have been experienced? Were they at the time of enrollment and do they perhaps contribute due to lack of compliance? And what we can say and what we know from all of these data sources is that the compliance pattern did not correlate in any consistent way with any tolerability issues. So we can't point to changes in tolerability that led to adherence at all. So our early read on this is no, not at all. These patients were patients who had been nonadherent from the very earliest days. And really, we're not able to show any benefit probably nor were -- we can't confirm that they had any tolerability issues either. And as you see, we are showing you the full adverse event data set. And if you look at it, it's really that VOC that sticks out and that certainly was on a patient that reported on compliance.

Our next question comes from Matthew Harrison with Morgan Stanley.

I guess a couple for me. So the first one is, given the fact that you basically lost three patients out of the 6 main cohort, why aren't you considering expanding that cohort to include more patients, especially since you want to get combination data with hydroxyurea? And then second question, it sounds like you don't yet have any animal data or toxicology data on the combination. Can you just talk about what data you may or may not have on the combination and why you feel like you need to get sufficient clinical data before going higher in dosing? And then third, just on the kinetics of the response here, I guess the question is, it appears that you get better response over a longer period of time. Is there any thought to amending the protocol to extend the amount of time on dosing, especially for some of these earlier patients?

Maybe sort of first and foremost, I can address the first question, which is, would we consider expanding the size of the cohort? And I think we can absolutely do that. One of the things, as we said, we want to get more data here on 6. We want to get data with HU. As you mentioned, there were these two nonadherent patients. We believe we certainly already have and will be addressing adherence moving forward. So we have that flexibility as we think about all three of these dose cohorts to go beyond 10 subjects if we think that will be valuable. So that is by no means off the table. At the same time, as we gather more data, both on and off HU, we want to look at the predictability that we're seeing here. In terms of what we've seen with HU preclinically, maybe I could turn it over to Paul and we can speak to that before I turn it over to Chris to talk about kinetics.

Yes. Thanks, Bryan. So I could say we tested a combination of hydroxyurea and FTX-6058 in CD34s. We see no tox issues. We've seen no [indiscernible] effects, but obviously, that is a cellular assay. And so here, that the most important thing will be generating that clinical data at these lower doses before contemplating other potential higher dose cohorts.

But Matthew, I would also just add, there's no obvious concern that we've identified going into that. So we anticipate, and I think as Dr. Kanter said, based on what we have seen preclinically that the combination will be additive and that patients will respond who are on a background of HU. And then let me turn it over to Chris to talk about that kinetics question and the potential for more time, obviously, the fact that we are still seeing increases and we're seeing robust initial HbF, I think, is very encouraging, but that's certainly something we think about.

Yes. As we've been talking about, we expected to see increases in HbF measured as early as the month and potentially close to maximum by 3 months. But always were doubtful that 3 months would be the absolute time point that we could see maximum. And we've always built that into our thinking about how to proceed with this program. Our eyes are clearly on the goal of getting this into a pivotal trial. That's the trial that will define ultimately a target profile for this. For us to get there, we have to go through the Phase Ib, which is a proof-of-concept dose-ranging study. We've now defined proof of concept. So we're going to go through the steps to define the dose, but we think we can define the dose with 3 cohorts up to 3 months of dosing. So at this point, while it is possible to extend the study, I don't think we're going to need to. We're going to be able to get enough information on these 3 doses from these 3 cohorts, potentially more if we need to add more before we started pivotal study, which ultimately is going to be the most informative.

[Operator Instructions] Our next question comes from Tazeen Ahmad with Bank of America.

I was wondering if you could give us a little bit more color on the steps that you've taken to make sure the compliance improved in the study going forward. And based on whatever changes you make to increase compliance, how do you kind of project that changes would affect use in a real-world setting? Because I think people's view is that people are very compliant in a controlled study setting, but become less so in a real-world setting when a drug does get approved, and people have to use it, but no 1 is monitoring if they're using it. So I just would appreciate some color on what changes specifically you're making there. And then to your point about the two- to threefold increase in induction of fetal hemoglobin over time. I know that you had previously guided that the 3-month time frame might be too short to see that type of improvement. But just based on what you know so far, how long do you think it would take in order to see that level of induction?

Yes. Thanks, Tazeen. Maybe I'll start with the first question, and then I'll turn it over to Judy and we can speak more just about adherence and some of the things that we've implemented, which have been successful in other clinical trials as well. I think as Paul spoke to a little bit earlier. Again, everything that we saw preclinically predicted that we would get to two- to threefold induction with 6058. I think we're very encouraged by the healthy volunteer data that we were seeing mRNA increases. And this is early data. This is our initial data on a small number of patients from our first dose cohort, obviously, but to be able to see the increases that we are in terms of fold, and I think Paul referenced that it's 1. -- approximately 1.6 fold and 2.7 fold for the two subjects that have been dosed for 56 days. That's very encouraging to us. It's a small data set, but it tells us that, yes, we have a lot of conviction that based on that data, the healthy volunteer data and the preclinical data that we can absolutely get to that two to threefold. So very encouraging, and we want to see more data to confirm that. But what we're seeing at day 56 is beyond, I think, from a kinetics perspective, what we were anticipating. Let me turn it over to Judy, and we could talk about adherence and some of the things that we've already put into place.

Thanks, Tazeen, for the question. Adherence is such an important piece of this program, and it's important for patients in the real-world treatment. So in this particular program, we were able to see this very early on in this cohort. And what we've put in place is observed dosing. So that is a video interaction between a physician or coordinator and subjects in the trial who dose in front of them. Now moving forward, in a large global trial, there are technologies that have been available for quite some time that have a very high degree of analytics it's a cell phone app that is able to watch subjects taking the pill, not only just are they taking it, but actually, are they keeping it or swallowing it. And then send a report back to a central database as well as if they don't hear from the subject at a particular time, send a message to the subject to, "hey, you were supposed to take your meds". So these have been shown through a lot of data collected to be incredibly impactful in terms of improving adherence. So clearly, already, we have something in place, but we have a very tangible and data-driven and data supportive plan moving forward into the registration trial. I think you asked a really important question about how do we predict real-world compliance or adherence based on what we've seen in this program. And I don't know that I can speculate in terms of the connection between the two of them, but what we do know is that patients on medications tend to be more compliant when they are recognizing clinical benefit. And what we showed in this trial very early on and again with a few patients, but these markers of hemolysis, these markers are decreasing in anemia, moving in the right direction I think patients may not think, "oh, well, my HbF is improving", but what they will understand is, "hey, I'm feeling better". And so we are hopeful that with these very robust early changes in HbF and these early indicators of decrease in hemolysis that patients will recognize the efficacy and benefit and the value of what 6058 can deliver, and that has always been an important predictor of continued adherence is really the ability of patients to recognize value.

All right. And maybe can I just squeeze a question for Dr. Kanter. What type of incremental benefit do you think you'd like to see in combination with hydroxyurea. Just practically speaking, if this is a branded drug that makes it to market. Is it something that you feel you need to see a minimal level of increased efficacy on or just any additional efficacy would be good for you?

I think that's a great question. The first thing I wanted to reply to part of the earlier question in terms of time to increase fetal hemoglobin. What we know in sickle cell disease is that red blood cells that contain fetal hemoglobin live longer. And so the effect becomes cumulative. So over time, we absolutely expect -- even if we didn't -- if the dose wasn't changed, then you continue to see a rise in fetal hemoglobin as more cells survive longer.n In terms of either on its own or combined with hydroxyurea, we know that fetal hemoglobin is something that significantly alleviates sickle cell disease-related complications. So I think we'll be able to make some better assessments in terms of what's the minimal that we would sort of want to see, but certainly continued additive increase in fetal hemoglobin and decrease in hemolysis are the absolutely first two things that we want to see to make sure that there's going to be efficacy. I'm not sure that I can tell you at specific levels. Other than them, we want to see all of the things combined, we want to not just see increase in fetal hemoglobin, but also the corresponding decrease in hemolysis. And then over time, of course, looking clinically, we hope to see individuals feeling better. We hope to see individuals with less vaso-occlusive crisis. I hope that answers the question.

Thank you. And this concludes the today's conference call. We are out of time. I'd like to thank everyone for participating in today's conference. You may now disconnect. Everyone, have a wonderful day.