ASP-8731: An Investigational BACH1 Inhibitor for Sickle Cell Disease – Development and Discontinuation

I. Introduction to ASP-8731: An Investigational BACH1 Inhibitor

A. Overview of ASP-8731 and Therapeutic Rationale

ASP-8731, also identified by the code ML-0207, is an investigational small molecule drug that was developed as a novel inhibitor of BTB and CNC homolog 1 (BACH1).[1] The primary therapeutic indication for which ASP-8731 was being investigated was sickle cell disease (SCD), a severe and debilitating inherited hemoglobinopathy characterized by chronic hemolysis, vaso-occlusive crises, and progressive organ damage.[3] The fundamental rationale for its development in SCD stemmed from its potential to induce the expression of fetal hemoglobin (HbF). Increased levels of HbF are known to ameliorate the clinical manifestations of SCD by interfering with the polymerization of sickle hemoglobin (HbS).[5] Beyond HbF induction, the inhibition of BACH1 by ASP-8731 was also anticipated to modulate pathways related to oxidative stress and inflammation, which are key contributors to the pathophysiology of SCD.[3]

B. Origin and Development History

ASP-8731 was initially discovered and developed by Mitobridge, Inc..[3] In a significant strategic move, Mitobridge was acquired by Astellas Pharma in 2017 for an upfront payment of $225 million, with Astellas subsequently taking over the development of the compound.[3] Following the acquisition, Astellas primarily referred to the compound as ASP8731, while the earlier identifier ML-0207, used by Mitobridge, also appears in the research literature.[3] The existence of dual nomenclature (ASP-8731 and ML-0207) is a common occurrence in the pharmaceutical industry, particularly when a compound transitions from an originating company to an acquiring one, or when internal research codes differ from official development codes. Consistent linkage of these identifiers, as found in the documentation for this compound [3], is crucial for maintaining clarity and accuracy in scientific communication and tracking developmental progress. The substantial upfront investment by Astellas in the Mitobridge acquisition [3] underscores the high potential perceived in Mitobridge's assets, including the BACH1 inhibitor program, at that time. This initial high valuation and commitment make the eventual discontinuation of ASP-8731 following early clinical evaluation a particularly notable outcome, suggesting a significant and unexpected negative development.

II. Physicochemical Properties and Formulation of ASP-8731

A. Chemical Identification and Basic Properties

ASP-8731 is characterized as a small molecule drug.[3] Its unique Chemical Abstracts Service (CAS) number is 2488255-42-9 [1], and it is registered in the DrugBank database under the ID DB17696.[4] The compound is also known by its synonyms ASP8731, ASP 8731, and ML0207.[1]

The chemical formula for ASP-8731 is C20H21N5O4 [1], with an exact mass of 395.16 Daltons [1] and a molecular weight reported as 395.420 Daltons [1] or 395.41 Daltons [2]; such minor variations in molecular weight are common across different analytical sources or calculation methods. The International Union of Pure and Applied Chemistry (IUPAC) name for ASP-8731 is 2-(benzo[d]oxazol-2-ylamino)-N-(2-(2-hydroxyethoxy)ethyl)-1-methyl-1H-benzo[d]imidazole-5-carboxamide.[1] Its structure can be represented by the SMILES (Simplified Molecular Input Line Entry System) code: CN1C(NC2=NC3=C(O2)C=CC=C3)=NC4=C1C=CC(=C4)C(=O)NCCOCCO.[1] The InChIKey is LCQUGWVEZSJGJG-UHFFFAOYSA-N, and the InChI code is 1S/C20H21N5O4/c1-25-16-7-6-13(18(27)21-8-10-28-11-9-26)12-15(16)22-19(25)24-20-23-14-4-2-3-5-17(14)29-20/h2-7,12,26H,8-11H2,1H3,(H,21,27)(H,22,23,24).[1] Elemental analysis data provided are: C, 60.75%; H, 5.35%; N, 17.71%; O, 16.18%.[1]

The availability of such detailed chemical information, including structural identifiers and elemental composition, from research chemical suppliers like MedKoo [1] and MedChemExpress [2], even after the drug's development was halted, indicates that ASP-8731 was synthesized and characterized to a significant degree, as would be expected for a compound that progressed to clinical evaluation. The note from MedKoo that the product "is not in stock, which may be available by custom synthesis" [1] suggests that while not a readily available bulk chemical, its synthesis route is established.

B. Physical Characteristics and Formulation Information

ASP-8731 is described as a solid [2], with its appearance noted as off-white to light yellow.[2] An earlier source mentioned its appearance as "To be determined" [1], possibly reflecting initial research-grade material or batch variability. The purity of the compound is reported as >98%.[1]

Information regarding solubility was initially "To be determined".[1] However, MedChemExpress provides example solvent protocols demonstrating that ASP-8731 can achieve a solubility of ≥ 5 mg/mL (12.65 mM) using various solvent systems, including 10% DMSO, 40% PEG300, 5% Tween-80, and 45% Saline; or 10% DMSO and 90% (20% SBE-β-CD in Saline); or 10% DMSO and 90% Corn Oil.[2] These protocols, utilizing common pharmaceutical excipients such as Dimethyl Sulfoxide (DMSO), Polyethylene Glycol 300 (PEG300), Polysorbate 80 (Tween-80), and Sulfobutylether-β-cyclodextrin (SBE-β-CD), are indicative of early-stage formulation development work. Such efforts are crucial for preparing suitable concentrations for in vitro and in vivo preclinical studies and for guiding the development of an oral formulation intended for clinical trials.

For storage, ASP-8731 should be kept dry, dark, and at 0-4°C for short-term storage (days to weeks) or at -20°C for long-term storage (months to years).[1] MedChemExpress advises using the compound within one month if stored at -20°C.[2] It is typically shipped under ambient temperature as a non-hazardous chemical and is considered stable enough for a few weeks during ordinary shipping and customs procedures.[1] The intended route of administration for ASP-8731 in clinical settings was oral.[7]

C. Table 1: Key Drug Identification and Physicochemical Properties for ASP-8731

The following table summarizes the key identification and physicochemical properties of ASP-8731:

Parameter	Value	Source(s)
Name	ASP-8731	User Query
Alternative Names	ML-0207, ASP 8731, ASP8731	1
DrugBank ID	DB17696	User Query10
Type	Small Molecule	User Query7
CAS Number	2488255-42-9	User Query1
Chemical Formula	C20H21N5O4	1
Exact Mass	395.16 Da	1
Molecular Weight	395.420 Da / 395.41 Da	1
IUPAC Name	2-(benzo[d]oxazol-2-ylamino)-N-(2-(2-hydroxyethoxy)ethyl)-1-methyl-1H-benzo[d]imidazole-5-carboxamide	1
SMILES	CN1C(NC2=NC3=C(O2)C=CC=C3)=NC4=C1C=CC(=C4)C(=O)NCCOCCO	1
Appearance	Solid, Off-white to light yellow	2
Purity	>98%	1
Solubility (examples)	≥ 5 mg/mL (12.65 mM) in 10% DMSO/40% PEG300/5% Tween-80/45% Saline	2
Storage Conditions	Dry, dark, 0-4°C (short term) or -20°C (long term)	1
Intended Route of Admin.	Oral	7
Originator	Mitobridge, Inc.	3
Developer	Astellas Pharma, Mitobridge	7

III. Mechanism of Action and Pharmacological Rationale

A. BACH1 as a Therapeutic Target in Sickle Cell Disease

BTB and CNC homolog 1 (BACH1) is a transcription factor belonging to the Cap 'n' Collar and basic region Leucine Zipper (CNC-bZIP) protein family.[11] It is widely expressed in mammalian tissues and functions primarily as a transcriptional repressor.[3] BACH1 plays significant roles in regulating various cellular processes, including epigenetic modifications, heme homeostasis, oxidative stress responses, and immune system development.[11]

In the context of sickle cell disease, the role of BACH1 is particularly relevant due to the chronic state of oxidative stress and inflammation. Intravascular hemolysis, a hallmark of SCD, leads to the release of substantial amounts of free heme into circulation.[3] This free heme is a pro-oxidant and pro-inflammatory molecule, contributing significantly to endothelial dysfunction and vaso-occlusion, which are central to SCD pathology.[5]

Interestingly, heme itself can modulate BACH1 activity. Heme binds to BACH1, leading to the derepression of genes normally suppressed by BACH1.[3] One of the critical pathways regulated by BACH1 is mediated by Nuclear Factor Erythroid 2-related factor 2 (NRF2). BACH1 represses NRF2-mediated gene transcription; thus, when heme binds to BACH1 and alleviates this repression, NRF2 is free to bind to Antioxidant Response Elements (AREs) in the promoter regions of its target genes, thereby upregulating their expression.[5] These target genes include a suite of antioxidant and cytoprotective enzymes.

B. ASP-8731 as a BACH1 Inhibitor

ASP-8731 was developed as a selective, small molecule inhibitor of BACH1.[1] The therapeutic strategy was that by directly inhibiting BACH1, ASP-8731 would mimic the effect of heme binding to BACH1, thereby relieving its repressive function on gene transcription.[5] This inhibition was expected to lead to the activation of the NRF2 pathway, effectively upregulating the body's natural defense mechanisms against oxidative stress and promoting other beneficial gene expression changes relevant to SCD.[5]

C. Downstream Effects of BACH1 Inhibition by ASP-8731

The inhibition of BACH1 by ASP-8731 was hypothesized to confer therapeutic benefits in SCD through two primary downstream mechanisms:

1. Activation of NRF2 Pathway and Antioxidant/Anti-inflammatory Gene Expression: By preventing BACH1 from repressing NRF2, ASP-8731 was expected to increase the transcription of NRF2 target genes. These include critical antioxidant enzymes such as heme oxygenase-1 (HMOX1), ferritin heavy chain (FTH1), glutathione reductase (GR), the cystine/glutamate antiporter subunit SLC7A11 (involved in glutathione synthesis), glutamate-cysteine ligase modifier subunit (GCLM), and NAD(P)H quinone dehydrogenase 1 (NQO1).1 Upregulation of these genes would enhance cellular antioxidant capacity and anti-inflammatory responses. Indeed, pharmacological activation of the NRF2 pathway in SCD mouse models has been shown to protect against heme-induced vascular occlusion, reduce inflammation, and decrease hepatic necrosis.5
2. Induction of Fetal Hemoglobin (HbF): Crucially for SCD, NRF2 activation is also linked to the promotion of erythroid expression of the gamma-globin genes, HBG1 (A-gamma) and HBG2 (G-gamma).5 Gamma-globin chains combine with alpha-globin chains to form fetal hemoglobin (HbF, α2​γ2​). An increase in HbF levels in red blood cells is a well-established therapeutic strategy for SCD, as HbF interferes with the polymerization of sickle hemoglobin (HbS, α2​β2S​), thereby reducing red blood cell sickling, improving red cell survival, and mitigating many of the downstream pathological consequences of the disease.5 The induction of HbF was a primary therapeutic objective for ASP-8731 in SCD.3

This dual mechanism of action—inducing HbF to directly address the primary molecular defect in SCD and simultaneously upregulating antioxidant and anti-inflammatory pathways via NRF2 to combat secondary pathologies—represented a comprehensive and highly desirable therapeutic approach.[3] While leveraging the well-characterized NRF2 pathway through inhibition of its upstream repressor, BACH1, is a rational drug design strategy, NRF2 is known to have pleiotropic effects. Consequently, sustained and potent activation of NRF2, if not precisely controlled or if the therapeutic window is narrow, could potentially carry risks of unintended biological consequences, a factor that may have become relevant during human clinical trials. Furthermore, BACH1 itself is implicated in a wide array of biological processes and diseases, including heme homeostasis, immune regulation, cardiovascular diseases, and even cancer.[11] Although ASP-8731 was described as a "selective" BACH1 inhibitor [1], the broad physiological involvement of its target raises the possibility of effects in other systems if the drug's action or distribution is not highly specific, potentially contributing to unforeseen effects in humans not predicted by preclinical models.

IV. Preclinical Evidence for ASP-8731 in Sickle Cell Disease

The therapeutic potential of ASP-8731 for sickle cell disease was supported by a range of preclinical studies, encompassing in vitro experiments using various cell lines and primary cells, as well as in vivo studies in a relevant animal model of SCD.

A. In Vitro Efficacy Studies

• HepG2 Liver Cells: In this human liver carcinoma cell line, ASP-8731 demonstrated target engagement by increasing the mRNA levels of HMOX1 and FTH1.[1] This finding confirmed the drug's ability to activate NRF2-responsive antioxidant genes in a cellular context.
• Human Primary Pulmonary Arterial Endothelial Cells (PAEC): Endothelial cells play a critical role in the vasculopathy of SCD. In PAECs, ASP-8731 treatment led to a decrease in VCAM1 (Vascular Cell Adhesion Molecule 1) mRNA in response to TNF-α stimulation.[3] VCAM1 is an adhesion molecule upregulated during inflammation that mediates the adherence of leukocytes and sickled red blood cells to the endothelium, contributing to vaso-occlusion. Additionally, ASP-8731 prevented the hemin-induced decrease in glutathione (GTH), a key intracellular antioxidant.[3] These results pointed to anti-inflammatory and antioxidant protective effects in endothelial cells.
• Human Erythroid Differentiated CD34+ Cells: Studies using hematopoietic stem cells (CD34+) differentiated into erythroid cells provided direct evidence for HbF induction.
• In cells derived from SCD patients, ASP-8731 increased the mRNA levels of gamma-globin (HBG) and alpha-globin (HBA), but importantly, not beta-globin (HBB).[1] This specific pattern of globin gene modulation is desirable for increasing HbF production without increasing the problematic βS-globin.
• ASP-8731 treatment resulted in a 2-fold increase in the percentage of F-cells (erythrocytes expressing HbF), an effect comparable to that observed with hydroxyurea (HU), an established therapy for SCD.[3]
• When ASP-8731 and HU were administered together, they induced a greater number of HbF+ cells compared to either drug used alone, suggesting additive or synergistic effects.[3]
• Significantly, in CD34+ cells from one donor who was non-responsive to HU, ASP-8731 was still able to induce an approximately 2-fold increase in HbF+ cells.[3] This finding was particularly promising, suggesting potential utility in patients who do not benefit from HU.
• Further supporting its mechanism, ML-0207 (ASP-8731) was shown to induce the mRNA expression of NRF2 target genes, including HBG1, HBG2, HMOX1, SLC7A11, GCLM, and NQO1, in human bone marrow-derived CD34+ cells differentiated into erythrocytes.[5]

B. In Vivo Efficacy in Animal Models (Townes HbSS Mice)

The Townes HbSS mouse model, which expresses human sickle hemoglobin and recapitulates many features of human SCD, was used to evaluate ASP-8731 in vivo. Mice were treated by oral gavage once daily for 4 weeks with ASP-8731, HU, or vehicle.[3]

• Anti-inflammatory Effects: ASP-8731 treatment led to a marked decrease in hepatic ICAM-1 (Intercellular Adhesion Molecule 1) and NF-$\kappa$B phospho-p65 protein expression, both markers of inflammation.[3] It also resulted in decreased white blood cell (WBC) counts, which are often elevated in SCD and contribute to inflammation and vaso-occlusion.[3]
• Anti-vaso-occlusive Effects: ASP-8731 demonstrated an ability to inhibit heme-mediated microvascular stasis, a measure of vaso-occlusion, in these mice.[3] When ASP-8731 was used in combination with HU, it produced a significantly greater reduction in microvascular stasis compared to HU alone.[3]
• Induction of Antioxidant Pathways and HbF: Treatment with ASP-8731 markedly increased the expression of heme oxygenase-1 (HO-1) in the liver, consistent with NRF2 activation.[3] Furthermore, ASP-8731 increased gamma-globin expression and the percentage of F-cells in the blood of Townes-SS mice compared to vehicle-treated animals.[3] These increases in F-cells were accompanied by elevations in blood A-gamma globin protein and erythrocyte counts.[5]

The consistent upregulation of HMOX1 observed across different preclinical models (HepG2 cells, CD34+ cells, and Townes mouse livers) [1] served as a robust pharmacodynamic marker, confirming target engagement of BACH1 and subsequent activation of the NRF2 pathway by ASP-8731. While the preclinical data indicated an increase in HBA (alpha-globin) mRNA alongside HBG (gamma-globin) mRNA, but not HBB (beta-globin) mRNA [1], the balance of globin chain synthesis is a critical consideration. The primary therapeutic aim is to increase gamma-globin production to pair with existing alpha-globin chains to form HbF (α2​γ2​), thereby reducing the availability of alpha-globin to pair with βS-globin. However, a significant stoichiometric excess of alpha-globin chains relative to non-alpha chains (β, γ, δ) can be detrimental, leading to their precipitation and contributing to red cell damage, a phenomenon seen in beta-thalassemias. This aspect would necessitate careful monitoring of globin chain ratios and hematological parameters in human subjects.

C. Comparative and Combination Studies with Hydroxyurea (HU)

The preclinical development of ASP-8731 included important comparisons and combination studies with hydroxyurea, the long-standing standard-of-care therapy for SCD that also acts, in part, by inducing HbF.

• ASP-8731 demonstrated effects comparable to HU in its ability to increase F-cells in vitro.[3]
• The combination of ASP-8731 and HU resulted in a greater induction of HbF+ cells in vitro than either agent used alone.[3]
• In the Townes-SS mouse model, the combination of ASP-8731 and HU was more effective at reducing microvascular stasis than HU alone.[3]
• Crucially, ASP-8731 showed efficacy in inducing HbF in CD34+ cells derived from a donor who was non-responsive to HU treatment.[3]

Collectively, this preclinical package for ASP-8731 appeared robust and highly encouraging. The compound demonstrated efficacy across multiple key pathological features of SCD in both in vitro and in vivo settings. The positive results in comparison with, in combination with, and particularly in an HU-non-responsive context, built a strong scientific rationale for advancing ASP-8731 into clinical development for patients with SCD.

D. Table 2: Summary of Key Preclinical Findings for ASP-8731 in Sickle Cell Disease Models

Study Type/Model	Key Parameters Measured	Key Findings/Effects of ASP-8731	Comparison/Combination with HU	Source Snippet(s)
HepG2 liver cells	HMOX1 mRNA, FTH1 mRNA	Increased HMOX1 and FTH1 mRNA	N/A	1
Human Primary Pulmonary Arterial Endothelial Cells (PAEC)	VCAM1 mRNA (TNF-α stimulated), Glutathione (GTH) (hemin treated)	Decreased VCAM1 mRNA; Blocked hemin-induced GTH decrease	N/A	3
Human Erythroid Differentiated CD34+ cells (SCD/healthy)	HBG, HBA, HBB mRNA; %F-cells; NRF2 target genes (HMOX1, SLC7A11, etc.) mRNA	Increased HBG & HBA mRNA (not HBB); Increased %F-cells (2-fold); Increased NRF2 target gene mRNA	Similar F-cell induction to HU; Combination with HU induced more F-cells; Effective in HU-non-responsive donor cells	1
Townes HbSS Mice (oral gavage, 4 weeks)	Hepatic ICAM-1, NF-$\kappa$B phospho-p65, HO-1; WBC counts; Microvascular stasis; γ-globin, F-cells	Decreased hepatic ICAM-1 & NF-$\kappa$B; Increased hepatic HO-1; Decreased WBC counts; Inhibited heme-mediated microvascular stasis; Increased γ-globin & F-cells	Combination with HU significantly reduced microvascular stasis compared to HU alone	3

V. Clinical Development Program of ASP-8731

Following the promising preclinical data, ASP-8731 advanced into clinical development with a Phase 1 trial.

A. Phase 1 Clinical Trial (NCT05167526 / 8731-CL-0101)

A single Phase 1 clinical trial, registered under the identifier NCT05167526 (Astellas Study ID: 8731-CL-0101), was initiated to evaluate ASP-8731 in humans.[3]

• Title: The study was formally titled "A Phase 1 Combined Single and Multiple Ascending Oral Dose Study to Evaluate the Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of ASP8731 in Healthy Adult Participants, Including an Assessment of a Food Effect".[3] It was also described as "A Study to Find a Suitable Dose of ASP8731 and Check for Medical Problems at Each Dose in Healthy Adults".[10]
• Sponsor: The trial was sponsored by Astellas Pharma Global Development, Inc..[3]
• Phase and Design: This was a Phase 1, combined single ascending dose (SAD) and multiple ascending dose (MAD) study.[3] The trial was double-blinded, meaning both participants and investigators were unaware of the treatment assignments.[13]
• Participants: The study was conducted in healthy adult volunteers.[3] The planned enrollment was 36 participants.[13]
• Objectives and Route of Administration: The primary objectives were to assess the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of orally administered ASP8731.[3] The study also aimed to evaluate the effect of food on the pharmacokinetics of the drug.[3]
• Trial Dates: The trial commenced in March 2022 and its active period concluded around November 2022.[9]
• Primary Endpoints: The primary endpoints focused on safety and tolerability, encompassing the occurrence of adverse events and various patient laboratory abnormalities.[14] Specific details of these endpoints are not fully available in the provided information beyond this general description.

B. Termination of Clinical Trial NCT05167526

The Phase 1 clinical trial NCT05167526 was prematurely terminated.[3] Astellas Pharma officially terminated the trial on March 1, 2023.[7] Information from Astellas indicates the study "stopped early and will not start again".[13] While the primary completion date is listed as November 10, 2022 [9], this likely reflects the cessation of active data collection before the formal decision to terminate was finalized and publicly reported.

The reason cited for the termination was that "protocol defined stopping criteria were met".[3] It was also noted that the study was halted "well short of its enrollment target" [3], implying that the issues arose relatively early in the study's progression. The trial's purpose included checking for "medical problems at each dose" [10], suggesting that such problems were indeed encountered.

The termination of a Phase 1 trial in healthy volunteers due to "protocol defined stopping criteria" being met strongly suggests that ASP-8731 encountered significant safety or tolerability issues at the doses tested. In such first-in-human studies, particularly in healthy subjects where efficacy is not an endpoint, meeting stopping criteria is almost invariably linked to unacceptable adverse events, serious adverse events, or clinically significant laboratory abnormalities.[14] The fact that the trial was terminated "well short of its enrollment target" [3] further implies that these issues likely emerged early in the dose-escalation process, preventing the study from proceeding to higher dose levels or completing the planned cohorts.

A significant limitation in understanding the full context of the termination is the lack of specific details in the publicly available information regarding the exact nature of the "medical problems" or "adverse events" that triggered the stopping criteria.[10] Without knowledge of the specific toxicities observed (e.g., organ system affected, severity, dose-relationship), a definitive conclusion on the precise risk profile that led to the trial's halt cannot be drawn from the provided materials. This absence of detailed disclosure is not uncommon for early-phase trial discontinuations unless the company chooses to publish or present the findings.

The timeline of events—trial initiation in March 2022, apparent cessation of active study participation around November 2022, formal termination in March 2023, and the subsequent discontinuation of the drug's development program in April 2023 [7]—indicates a relatively rapid and decisive negative outcome once significant issues were identified. This timeframe suggests that the problems encountered were deemed serious and likely insurmountable for ASP-8731 in the context of its development for sickle cell disease.

C. Table 3: Details of Clinical Trial NCT05167526 for ASP-8731

Parameter	Value	Source Snippet(s)
ClinicalTrials.gov ID	NCT05167526	3
Astellas Study ID	8731-CL-0101	13
Phase	Phase 1	3
Title	A Phase 1 Combined Single and Multiple Ascending Oral Dose Study to Evaluate the Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of ASP8731 in Healthy Adult Participants, Including an Assessment of a Food Effect	3
Sponsor	Astellas Pharma Global Development, Inc.	3
Status	Terminated	3
Purpose	Evaluate safety, tolerability, PK, PD of single and multiple ascending oral doses of ASP8731 in healthy adults; assess food effect.	3
Condition	Healthy Volunteers	3
Intervention	ASP-8731, oral administration, single ascending dose (SAD) and multiple ascending dose (MAD)	3
Planned Enrollment	36	13
Actual Enrollment	Not specified, but "well short of its enrollment target"	3
Trial Dates	March 2022 - November 2022 (active period); Terminated March 1, 2023	7
Reason for Termination	"Protocol defined stopping criteria were met"	3

VI. Regulatory Status and Overall Development Outcome

A. Orphan Drug Designation

ASP-8731 (referred to as ASP 8731 in the designation) received Orphan Drug status from the U.S. Food and Drug Administration (FDA) for the treatment of Sickle Cell Anemia on September 1, 2022.[7] Orphan Drug Designation is a special status granted by regulatory authorities to drugs and biologics intended to treat rare diseases or conditions—defined in the U.S. as those affecting fewer than 200,000 people.[15] This designation aims to incentivize the development of such treatments by providing benefits to the sponsor, which can include tax credits for clinical testing, exemption from prescription drug user fees, and, most notably, a period of market exclusivity (typically seven years in the U.S.) upon approval.[15] To qualify, the drug must be intended for a life-threatening or chronically debilitating condition for which no satisfactory method of diagnosis, prevention, or treatment exists, or if such a method does exist, the new drug must demonstrate a potential significant benefit over available therapies.[15]

The granting of Orphan Drug status to ASP-8731 signified that, based on the preclinical evidence and the therapeutic rationale presented, the FDA recognized its potential to address an unmet medical need in the rare disease population of individuals with sickle cell anemia.[7] This designation was awarded while the Phase 1 clinical trial was underway (initiated March 2022, Orphan Drug status September 2022).[7] The subsequent failure of the drug in this early clinical phase, leading to its discontinuation, is therefore particularly poignant, as it represents a lost opportunity for a patient population with limited therapeutic options, despite the regulatory incentives designed to foster such developments. The emergence of negative clinical data after the orphan designation was secured ultimately overrode the encouragement and benefits offered by this status. No information regarding Orphan Drug status from the European Medicines Agency (EMA) for ASP-8731 was found in the provided materials.

B. Discontinuation of Development

The clinical development of ASP-8731/ML-0207 for sickle cell disease was officially discontinued by Astellas Pharma. An Astellas pipeline update from April 2023 confirmed the discontinuation of development for ASP8731/ML-0207 for sickle cell disease in Phase 1.[8] AdisInsight also reported this discontinuation for sickle cell anemia, effective April 30, 2023.[7] Consequently, "Anemia, Sickle Cell" is now listed as an "Inactive Indication" for ASP-8731, and the drug's highest development phase is recorded as "Discontinued Phase 1".[3]

The decision by a major pharmaceutical company like Astellas to discontinue the development of ASP-8731 [8], following the termination of a Phase 1 trial in healthy volunteers due to safety or tolerability issues [3], suggests that the problems encountered were considered significant and not readily resolvable through straightforward measures such as protocol amendments or formulation adjustments for this particular compound in the SCD indication. This implies a fundamental impediment to the further development of ASP-8731 for this purpose.

VII. Comprehensive Summary, Critical Analysis, and Future Perspectives

A. Recapitulation of ASP-8731's Profile and Trajectory

ASP-8731 (ML-0207) was an orally administered, selective small molecule inhibitor of BACH1, discovered by Mitobridge and advanced into early clinical development by Astellas Pharma following Mitobridge's acquisition. Its mechanism of action, centered on the inhibition of BACH1, offered a compelling dual therapeutic rationale for sickle cell disease: the induction of fetal hemoglobin (HbF) and the activation of the NRF2 pathway, leading to enhanced antioxidant and anti-inflammatory responses. This approach was supported by a robust body of preclinical evidence from in vitro and in vivo SCD models, which demonstrated efficacy in HbF induction (including in hydroxyurea-non-responsive cells), reduction of inflammation, and amelioration of vaso-occlusion.

Despite this promising preclinical profile and the attainment of Orphan Drug status in the USA, the clinical development of ASP-8731 was short-lived. The sole Phase 1 clinical trial (NCT05167526), designed to evaluate its safety, tolerability, pharmacokinetics, and pharmacodynamics in healthy adult volunteers, was prematurely terminated because "protocol defined stopping criteria were met".[3] Subsequently, Astellas Pharma discontinued the development of ASP-8731 for sickle cell disease in April 2023.[8]

B. Critical Analysis of Development and Discontinuation

The development trajectory of ASP-8731 is marked by a stark contrast between its extensive and highly encouraging preclinical data package and its ultimate failure in early human trials. The preclinical studies showcased multifaceted efficacy, including synergistic effects with hydroxyurea and activity in HU-non-responsive cellular models [3], painting a picture of a promising new therapeutic candidate for SCD.

However, the termination of the Phase 1 trial in healthy volunteers due to "protocol defined stopping criteria being met," further described as encountering "medical problems at each dose" and issues related to "patient laboratory abnormalities and adverse events," strongly points to unforeseen and significant safety or tolerability concerns in humans.[3] Such outcomes in healthy subjects, who are generally less vulnerable than patients with underlying diseases, are particularly concerning. These adverse findings were evidently not predicted by the preclinical toxicology studies that would have been prerequisite for initiating human trials. This discrepancy underscores the inherent limitations in the predictive power of preclinical models for human-specific responses, particularly concerning drug metabolism, distribution, and idiosyncratic toxicities. The human body's handling of ASP-8731 or its metabolites likely differed significantly from that in animal models, leading to the observed "medical problems."

The lack of specific, publicly available details regarding the nature and severity of the adverse events that triggered the trial's cessation prevents a complete toxicological assessment. However, the rapid decision by Astellas, a company that had made a substantial investment through the Mitobridge acquisition [3], to discontinue development [8] suggests that the observed issues were not minor or easily manageable. They were likely considered severe enough to render the risk-benefit profile unfavorable for further development, potentially involving critical organ systems or occurring at dose levels too low to be considered therapeutically viable for a chronic condition like SCD.

C. Implications for BACH1 Inhibition as a Therapeutic Strategy for SCD

The failure of ASP-8731 in Phase 1 does not necessarily invalidate BACH1 inhibition as a viable therapeutic strategy for sickle cell disease or other conditions. The adverse events leading to its discontinuation could be specific to the chemical properties of ASP-8731 itself (e.g., off-target activities, formation of toxic metabolites, unfavorable pharmacokinetic profile in humans leading to toxicity at exposures required for efficacy) rather than an inherent problem with inhibiting the BACH1 target as a class.

Indeed, interest in BACH1 as a therapeutic target persists. Other companies are reportedly pursuing the development of BACH1 inhibitors, with some, like IMMvention Therapeutix, explicitly claiming that the "druglike properties" of their candidates differentiate them from earlier BACH1 inhibitors.[3] This suggests an awareness within the field of potential liabilities associated with prior compounds and an effort to engineer molecules with improved safety and pharmacokinetic characteristics. These next-generation BACH1 inhibitors will likely be designed with careful consideration of the (unpublished) lessons from ASP-8731's development. The underlying therapeutic concept of simultaneously inducing HbF and bolstering antioxidant defenses via NRF2 activation through BACH1 inhibition remains an attractive and rational approach for the complex pathophysiology of SCD.

D. Concluding Remarks and Lessons Learned

The story of ASP-8731 serves as a salient reminder of the high attrition rates inherent in pharmaceutical research and development. Even compounds with a strong mechanistic rationale and compelling preclinical efficacy can encounter insurmountable hurdles in early clinical phases, most often due to unforeseen safety or tolerability issues in humans. This case highlights the critical challenges in translating findings from preclinical models to human clinical outcomes, emphasizing the importance of understanding human-specific pharmacology and toxicology as early as possible in the development process.

Future endeavors in the development of BACH1 inhibitors for sickle cell disease or other indications will undoubtedly benefit from a thorough (if non-public) understanding of the factors that led to ASP-8731's failure. A focus on optimizing molecular properties to enhance safety, improve pharmacokinetic profiles, and ensure target specificity will be paramount for the successful clinical translation of this therapeutic strategy. While ASP-8731 did not fulfill its initial promise, the pursuit of novel agents targeting BACH1 continues, reflecting the ongoing need for innovative treatments for sickle cell disease and the enduring appeal of its multifaceted mechanism of action.

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