What CTNNB1 inhibitors are in clinical trials currently?

Introduction to CTNNB1
CTNNB1, which encodes the protein β‐catenin, plays a crucial role as both a cell adhesion molecule and a key mediator of the canonical Wnt signaling pathway. Its tightly regulated expression and localization are essential for many cellular functions including cell proliferation, differentiation, developmental processes, and tissue homeostasis. Dysregulation of CTNNB1 is associated with aberrant signaling that contributes to oncogenesis and cancer progression.

Role of CTNNB1 in Cellular Functions
Under physiological conditions, β‐catenin is localized at the plasma membrane where it interacts with cadherins to maintain cell–cell adhesion. In the cytoplasm, β‐catenin is controlled by a destruction complex that leads to its ubiquitin-mediated degradation when Wnt signaling is unactivated. However, upon Wnt ligand binding to its receptors, the destruction complex is inhibited; β‐catenin accumulates in the cytoplasm and translocates into the nucleus where it associates with transcriptional coactivators to drive the expression of target genes. These target genes are essential regulators of the cell cycle, fate determination, and stem cell renewal. Thus, CTNNB1 plays critical roles in cellular proliferation and differentiation, while also being instrumental for maintaining tissue architecture and regulating embryonic development.

Implications of CTNNB1 in Diseases
When CTNNB1 is mutated or overexpressed, the outcome can be the constitutive activation of Wnt/β‐catenin signaling, an event that has been implicated in multiple malignancies. Enhanced nuclear accumulation of β‐catenin and its interaction with transcriptional coactivators may result in overexpression of oncogenic drivers that promote tumor cell proliferation, invasion, metastasis, and resistance to apoptosis. Consequently, targeting CTNNB1 or its interactions has emerged as a promising strategy for therapeutics in cancers such as colorectal cancer, hepatocellular carcinoma, renal cell carcinoma, and other solid tumors in which aberrant Wnt/β‐catenin signaling is a dominant pathogenic driver.

Overview of CTNNB1 Inhibitors
Research developments over recent years have led to the identification of several therapeutic candidates designed to modulate CTNNB1 activity. Instead of directly “knocking out” CTNNB1 function entirely, most strategies focus on interfering with its accumulation, nuclear translocation, or its ability to generate a transcriptionally active complex with cofactors.

Mechanism of Action
The inhibitors currently in clinical investigation are designed to target different aspects of CTNNB1 function. For instance:

• Some small molecule inhibitors work by interfering with the binding of β‐catenin to its transcriptional coactivators (such as CBP), thereby reducing the transcription of oncogenic target genes. OP-724 is a prime example; it operates by disrupting the β‐catenin/CBP interaction to inhibit the transcriptional output and subsequent pathological signaling driven by β‐catenin.

• Other compounds, such as E7386, are oral small molecules that modulate β‐catenin signaling by affecting its regulatory dynamics. E7386 has been reported to modify the bioavailability of β‐catenin or alter its cellular distribution. This is achieved by indirectly promoting degradation or impairing its interaction with other components of the transcription complex.

• Another approach involves targeting CTNNB1 mRNA directly using siRNA-based or RNA interference–based platforms. ALN-BCAT, for example, employs an RNA interference mechanism to reduce CTNNB1 expression, thus lowering β‐catenin protein levels in tumor cells. Such agents represent a novel class of therapeutics that aim to “silence” the gene expression of CTNNB1 at the mRNA level.

Each modality exploits different vulnerabilities in the Wnt/β‐catenin pathway. While direct inhibition of CTNNB1 remains challenging owing to its role as a scaffolding protein with few enzymatic domains, these inhibitors instead focus on disrupting critical protein–protein interactions or reducing the intracellular abundance of β‐catenin, thereby blunting the oncogenic signal.

Development History and Milestones
Over the years, the clinical development of CTNNB1 inhibitors has reflected the evolving understanding of the Wnt/β‐catenin pathway. Early preclinical studies demonstrated that genetic knockdown of CTNNB1 in tumor models led to cell cycle arrest, reduced cellular proliferation, and increased apoptosis. These observations catalyzed the search for small molecules that could replicate these effects pharmacologically.

During the past decade, several candidates have advanced from preclinical models into early-phase clinical trials. For example, E7386 entered clinical evaluation following encouraging preclinical evidence of its ability to modulate β‐catenin signaling in tumor models. Its development milestones include multiple pharmacokinetic studies and phase 1 trials that confirmed its safety profile and appropriate bioavailability in human subjects. Similarly, OP-724, which originally demonstrated efficacy in preclinical models by disrupting the β‐catenin/CBP interaction, has been advanced into early clinical trials focusing on not only oncological indications but also on conditions where aberrant β‐catenin activity plays a role, such as liver cirrhosis. Lastly, ALN-BCAT represents the RNA interference–based approach and its progression into phase 1 studies marks a novel milestone as it utilises therapeutic gene silencing technology to target CTNNB1 directly.

Clinical Trials of CTNNB1 Inhibitors
Today, the clinical evaluation of CTNNB1 inhibitors is a multifaceted effort that includes several agents at various stages of development. The clinical trials currently underway are expanding our understanding of how best to target β‐catenin signaling in a safe and effective manner.

Current Clinical Trials
A detailed review of the synapse‐sourced references indicates that there are currently three major CTNNB1 inhibitors in clinical evaluation:

• E7386 – This small molecule modulator is consistently mentioned across several clinical trial references. E7386 is being investigated in multiple trial settings to evaluate its relative bioavailability, absolute bioavailability, and antitumor efficacy when used in combination with other anticancer agents, such as pembrolizumab. For instance, one trial is designed as an open-label, single-center, randomized pharmacoscintigraphic study to evaluate the relative bioavailability of E7386 following oral administration of a targeted release formulation compared to an immediate release tablet. In another study, E7386 is combined with pembrolizumab in a multicenter phase 1b/2 study targeting previously treated subjects with selected solid tumors, indicating an interest in its synergistic potential with immunotherapies. Additional trials have explored E7386 in combination with other anticancer drugs in subjects with solid tumors. These studies primarily focus on determining the optimal dosage, safety profile, and early efficacy signals in intractable and advanced solid malignancies.

• OP-724 – This agent functions as a selective modulator targeting the β‐catenin/CBP interaction. Clinical trials for OP-724 have been initiated in patient populations beyond conventional cancer indications. One trial focuses on evaluating the safety and tolerability of OP-724 in liver cirrhosis patients caused by HIV/HCV co-infection with hemophilia. Additionally, another open-label, dose-finding study investigates the use of OP-724 in patients with primary biliary cholangitis, a condition in which aberrant Wnt/β‐catenin signaling might also contribute to pathogenesis. Other trials, including mass balance studies using radiolabeled OP-724, are in early-phase clinical assessment to determine the compound’s pharmacokinetics in healthy volunteers, which is a necessary prerequisite prior to more extensive clinical applications. A further multicenter, placebo-controlled, randomized phase II trial (OP-724-002) is underway, which might assess its efficacy in conditions related to liver cirrhosis. Although these trials have been largely initiated in non-oncological indications, they substantially provide evidence on the compound’s modulation of CTNNB1-driven pathways. The outcomes of these studies will help to illuminate the potential for applying OP-724 to oncology indications where β‐catenin dysregulation is evident.

• ALN-BCAT – Representing a novel RNA interference (RNAi) therapeutic strategy, ALN-BCAT is an agent specifically formulated to reduce CTNNB1 mRNA and thereby decrease β‐catenin protein levels. It is currently being evaluated in a phase 1 study in patients with advanced or metastatic hepatocellular carcinoma as both monotherapy and in combination with pembrolizumab. The trial is designed to investigate the safety, tolerability, pharmacokinetics, and preliminary efficacy of ALN-BCAT. As this represents a relatively new modality for targeting CTNNB1, the data from this study will provide key insights into its clinical potential as well as into the broader applicability of RNAi technologies in oncology settings.

Phase and Status of Trials
The clinical development stages for these inhibitors range predominantly from phase 1 to early phase 2, reflecting an early stage of clinical evaluation and an ongoing assessment of safety, tolerability, pharmacokinetics, and indications of efficacy.

• For E7386, multiple phase 1 and phase 2 studies have been initiated. The open-label studies examining bioavailability (both relative and absolute) are classified as phase 1 studies aimed at understanding the pharmacological profile. The combination study with pembrolizumab in previously treated patients enters the phase 1b/2 stage, signaling a transitional phase where early safety is combined with a preliminary look at efficacy. The study evaluating E7386 in combination with other anticancer drugs is also in an early phase, largely focused on dose determination and therapeutic index establishment.

• OP-724 is being assessed in various phase 1 clinical studies. One study in liver cirrhosis patients caused by HIV/HCV co-infection is a phase 1 trial aiming at safety and tolerability assessment. Similarly, the dose-finding study in primary biliary cholangitis is a phase 1 trial specifically designed to establish the appropriate dosage for future efficacy studies. Additional pharmacokinetic studies, including mass balance evaluations with radiolabeled OP-724, are typically conducted as phase 1 assessments to ensure accurate characterization of the drug behavior in human subjects. The randomized phase II trial (OP-724-002) is a significant step forward if its data permit expansion of its use in related oncological indications.

• ALN-BCAT is currently in a phase 1 study format. The trial’s design includes both a monotherapy arm and an exploratory combination arm with pembrolizumab, which suggests that early investigations are concentrating on understanding the RNAi-mediated suppression of CTNNB1 in hepatocellular carcinoma and determining if there is a synergistic effect when combined with an immune checkpoint inhibitor.

Preliminary Results and Findings
At this point, the full results from many of these early-phase trials have not been published in detail in the provided synapse references. However, some preliminary findings and study designs do shed light on the progress of these agents:

• For E7386, the bioavailability studies have established that its novel formulation (targeted release tablet) is pharmacologically feasible, and early safety data appear to be acceptable over a range of doses. The combination trials with pembrolizumab have been designed in light of preclinical data suggesting that modulation of β‐catenin signaling can enhance anti-tumor immune responses. While detailed efficacy results are pending, the design of these trials suggests that there is a rationale for combining E7386 with immunotherapy in solid tumors.

• OP-724 has reached the stage of dose-finding in a non-oncologic population (liver cirrhosis and primary biliary cholangitis), focusing on its effect on β‐catenin signaling. Early-phase results, as inferred from study designs, indicate that the agent is well tolerated and that the pharmacokinetics support further evaluation. The mass balance study using a 14C labeled compound in healthy male subjects underscores the thorough pharmacological characterization undertaken prior to expanding the indication into oncology.

• For ALN-BCAT, the incorporation into a phase 1 trial in advanced hepatocellular carcinoma patients marks a novel approach. Although detailed preliminary efficacy data are not described in the references, the study design (including combination with pembrolizumab) is informed by both safety considerations and the growing evidence that silencing CTNNB1 can rejuvenate anti-tumor immune responses. This could potentially translate into improved progression-free survival and enhanced response rates among patients with CTNNB1-driven oncogenic profiles.

Future Directions and Challenges
While these CTNNB1 inhibitors represent meaningful advances in targeting a traditionally “undruggable” pathway, several challenges remain that must be addressed to maximize their clinical impact.

Challenges in Developing CTNNB1 Inhibitors
The challenges of developing CTNNB1 inhibitors are multifaceted and span the spectrum from scientific to clinical practice:

• **Selectivity and Specificity:**
CTNNB1 is highly conserved and functions in many normal cellular processes. Ensuring that inhibitors selectively target the pathological activation of β‐catenin without disrupting normal adhesion and homeostatic signaling is challenging. Off‐target effects could result in toxicity, notably affecting normal tissue integrity and function.

• **Drug–Protein Interaction Dynamics:**
Given that CTNNB1 tends to act as a scaffolding protein rather than an enzyme with a well-defined active site, designing small molecules that efficiently disrupt protein–protein interactions (such as β‐catenin/CBP) requires innovative medicinal chemistry strategies. Current candidates, like OP-724, illustrate these challenges but also highlight recent successes in targeting such interactions.

• **Biomarker Identification and Patient Stratification:**
Determining which patients are most likely to benefit from CTNNB1 inhibitors is critical. Biomarkers that reflect aberrant Wnt/β‐catenin signaling or direct measurements of nuclear β‐catenin could improve patient selection. However, standardized assays and thresholds for CTNNB1 pathway activation are still areas of active research.

• **Drug Formulation and Pharmacokinetics:**
Ensuring appropriate bioavailability, especially in oral formulations such as E7386, is essential for achieving therapeutic plasma concentrations with acceptable safety profiles. The development and refinement of formulations (for instance, targeted release tablets) play a crucial role in optimizing drug exposure and minimizing systemic side effects, as indicated by the ongoing bioavailability studies.

• **Resistance Mechanisms:**
As with many targeted therapies, there is always a risk of developing resistance through adaptive signaling pathways, compensatory mechanisms, or genetic alterations in the tumor cells. Detailed mechanistic studies are needed to anticipate and overcome potential resistance mechanisms to CTNNB1 inhibitors.

Potential Future Developments
Looking ahead, the future development of CTNNB1 inhibitors will likely be characterized by several promising trends:

• **Combination Therapies:**
Current clinical trials are already investigating the combination of CTNNB1 inhibitors with immunotherapies such as pembrolizumab. This approach is supported by preclinical evidence that Wnt/β‐catenin signaling can suppress anti-tumor immune responses. Combining agents like E7386 and ALN-BCAT with checkpoint inhibitors may provide synergistic benefits and address resistance.

• **Personalized Medicine Approaches:**
Advances in genomics and proteomics could lead to the identification of biomarkers that not only predict response to CTNNB1 inhibitors but also monitor treatment efficacy. Personalized treatment regimens may be developed by tailoring therapy based on the specific molecular aberrations in the Wnt/β‐catenin pathway present in a patient’s tumor.

• **Expansion of Indications:**
While many current trials have focused on advanced solid tumors and hepatocellular carcinoma, there remains potential for CTNNB1 inhibitors to be applied to a broader range of cancers including colorectal, renal, and even certain hematological malignancies. As more safety and efficacy data are accumulated, the approved indications for these agents may expand.

• **Novel Drug Delivery Systems:**
To overcome challenges related to bioavailability and off-target toxicity, future developments may integrate advanced drug delivery systems. Nanoformulations or targeted delivery vehicles could be designed to deliver CTNNB1 inhibitors more specifically to tumor tissues, thereby enhancing efficacy and reducing systemic toxicities.

• **Innovative Modalities:**
The success of ALN-BCAT as an RNA interference–based therapeutic opens the door for further development of nucleic acid–based approaches. Continued improvements in delivery methods and stability of RNAi molecules could allow for more robust suppression of CTNNB1 expression in tumor cells with potentially fewer off-target effects.

• **Improved Clinical Trial Designs:**
Adaptive trial designs that incorporate real-time biomarker analysis and individualized dose escalation may enhance the evaluation process for CTNNB1 inhibitors. Such approaches could accelerate the determination of safe and effective doses and facilitate the dynamic adjustment of treatment protocols based on early pharmacodynamic markers.

Conclusion
In summary, the current landscape of CTNNB1 inhibitors in clinical trials is marked by several innovative compounds that target different aspects of β‐catenin function. E7386 is a leading small molecule inhibitor that is being evaluated in multiple phase 1 and 2 studies – both as monotherapy through detailed bioavailability evaluations and in combination with immunotherapy agents like pembrolizumab for solid tumors. Concurrently, OP-724, which disrupts the β‐catenin/CBP interaction, is undergoing early-phase clinical trials in conditions such as liver cirrhosis and primary biliary cholangitis, with the potential to extend its application into oncology. Additionally, ALN-BCAT represents an RNA interference–based approach designed to reduce CTNNB1 expression, and is in phase 1 clinical studies in advanced hepatocellular carcinoma.

This general-specific-general overview illustrates how diverse inhibitors—spanning small molecules, protein–protein interaction disruptors, and RNAi agents—pursue the common goal of modulating aberrant CTNNB1 signaling in malignancies. However, challenges such as selectivity, appropriate patient selection, formulation issues, and the risk of resistance remain significant hurdles. Future directions are likely to include combination therapies with immunomodulators, personalized medicine approaches for better stratification, and advances in delivery technologies to improve efficacy and safety. Overall, while CTNNB1 inhibitors present a promising therapeutic avenue, continued research and innovative clinical trial designs will be essential to translate these early-phase findings into effective treatments for patients with CTNNB1-driven cancers.