What are the therapeutic candidates targeting CTLA4?

Introduction to CTLA4
CTLA4, or cytotoxic T lymphocyte–associated antigen 4, is a critical inhibitory receptor expressed predominantly on T cells that plays a central role in the regulation of immune responses. It serves as an immune checkpoint by acting as a “brake” on T cell activation and proliferation, thereby maintaining immune homeostasis and preventing excessive immune-mediated tissue damage. Numerous studies have elucidated its structure, function, and significance in immune regulation; for example, early reports demonstrated that CTLA4 is upregulated on T cells after activation, where it competes with the co-stimulatory receptor CD28 for binding to shared ligands—B7-1 (CD80) and B7-2 (CD86)—on antigen‐presenting cells. This competitive binding, which is driven by the higher affinity of CTLA4 for these ligands, is central to dampening immune responses, thereby protecting the host from autoimmunity yet also, when dysregulated, hindering anti-tumor immunity.

Role of CTLA4 in Immune Regulation
CTLA4 is widely recognized for its unique role in immune regulation. Basally, it is expressed at low levels on naïve T cells but is rapidly upregulated upon T cell activation. In its activated state, CTLA4 exerts an inhibitory signal through multiple mechanisms. One major pathway involves its competition with CD28 for the B7 ligands that are present on dendritic cells and other antigen-presenting cells. Because CTLA4 binds these ligands with a higher affinity, it effectively sequesters them, reducing the availability of positive co-stimulation through CD28 and thereby turning down T cell activation. Additionally, CTLA4 is constitutively expressed on regulatory T cells (Tregs), where it is instrumental in maintaining peripheral tolerance. Through mechanisms that include the trans-endocytosis of CD80/CD86 molecules from antigen-presenting cells, CTLA4 helps Tregs suppress effector T cell responses and maintain immune equilibrium. The ability to manipulate these pathways makes CTLA4 a potent lever in modulating both autoimmunity and anti-tumor immunity.

CTLA4 as a Therapeutic Target
The dualistic nature of CTLA4—balancing immune activation in normal physiology while also contributing to tumor immune evasion—makes it an attractive therapeutic target for cancer immunotherapy. In the context of cancer, tumor cells as well as the tumor microenvironment may exploit the CTLA4 pathway to dampen the immune response, thus reducing cytotoxic T cell activity against malignant cells. Blockade of CTLA4 using monoclonal antibodies can reverse this suppression, leading to enhanced T cell proliferation, increased cytokine production, and ultimately a more robust anti-tumor immune response. This rationale provided the impetus for the development of drugs that target CTLA4, aiming to “release the brakes” on T cells and improve the immune system’s capacity to recognize and disrupt tumor growth.

Current Therapeutic Candidates
The therapeutic landscape targeting CTLA4 is broad and spans both approved treatments and emerging candidates that are in various stages of preclinical and clinical development. The candidates can be divided into two broad classes: approved therapies that have already demonstrated clinical benefit and emerging therapies that seek to improve upon existing approaches by enhancing efficacy and reducing toxicity.

Approved Therapies
The flagship therapy in this category is ipilimumab, marketed under the trade name Yervoy. Ipilimumab was the first CTLA4-blocking antibody to receive regulatory approval and has been extensively studied in patients with metastatic melanoma, where it demonstrated a durable improvement in overall survival. Ipilimumab is a human monoclonal antibody belonging to the IgG1 subclass and works by binding to CTLA4 on T cells. The binding action blocks the interaction between CTLA4 and its ligands on antigen-presenting cells, thereby removing key inhibitory signals during T cell activation. The clinical success of ipilimumab has not only led to its approval but has also paved the way for the development of additional agents targeting CTLA4. Its approval and clinical profile are supported by multiple trials that have highlighted both its efficacy and the challenges associated with immune-related adverse events (irAEs), which are a common class effect of CTLA4 blockade.

Emerging Therapies
Building on the success and limitations of ipilimumab, several next‐generation therapeutic candidates have emerged with the goal of achieving enhanced efficacy and improved safety profiles. These include:

• Tremelimumab – Initially evaluated in early clinical trials, tremelimumab is another anti‐CTLA4 monoclonal antibody which, although similar in mechanism to ipilimumab, belongs to the IgG2 subclass. Its different Fc characteristics may allow for distinct pharmacokinetics and toxicity profiles that could result in either enhanced efficacy in certain patient populations or a reduction in irAEs. Tremelimumab continues to be investigated in trials for various solid tumors, including lung cancer and hepatocellular carcinoma.

• ADG126 – Emerging as a next‐generation, masked anti‐CTLA4 antibody, ADG126 is designed to minimize dose-dependent toxicities while maintaining or even enhancing anti-tumor efficacy. By “masking” the antibody’s Fc region or modifying its binding kinetics, ADG126 aims to selectively engage CTLA4 in the tumor microenvironment, potentially reducing systemic autoimmunity and improving tolerability. Patents detail the design strategies for anti‐CTLA4 antibodies like ADG126, emphasizing safer profiles and enhanced tumor penetration.

• Gotistobart – Developed by OncoC4, gotistobart is positioned as a next-generation anti‐CTLA4 antibody that specifically targets regulatory T cells (Tregs) within the tumor microenvironment. By selectively depleting intratumoral Tregs, gotistobart is designed to spare normal tissues while boosting anti-tumor immune responses. Preclinical data and early clinical trial signals indicate that gotistobart might achieve significant immune modulation with a potentially lower incidence of irAEs compared to traditional agents.

• HL32-Fab-based candidates – Structural studies have characterized the interaction between CTLA4 and novel antibody fragments like HL32-Fab. These candidates are in early stages of development, where the aim is to optimize binding affinity and specificity while maintaining a safety profile that minimizes systemic T cell hyperactivation. Their design often leverages detailed crystallographic data to enhance efficacy while reducing off-target effects.

• Bispecific antibodies and combination formats – In addition to monotherapy antibodies, new technologies are emerging that combine CTLA4-targeting with other immune checkpoint inhibition strategies. For instance, bispecific antibodies that concurrently target CTLA4 along with PD-1, PD-L1, or other relevant receptors (such as Cadonilimab, which targets CTLA4 as well as PD-1) are under investigation. These bispecific constructs are intended to provide a synergistic boost to anti-tumor immunity while carefully balancing toxicity. Such candidates can potentially address resistance mechanisms that arise when a single checkpoint is targeted and might permit lower dosing to reduce adverse effects.

Mechanisms of Action
A thorough understanding of the mechanisms of action of CTLA4 inhibitors is essential for appreciating both their therapeutic potential and their limitations. These agents modulate the immune response via several convergent and divergent pathways, ultimately tipping the balance towards enhanced T cell activation and anti-tumor immunity.

Immune Modulation by CTLA4 Inhibitors
CTLA4 inhibitors work primarily by disrupting the inhibitory signals that prevent T cell activation. They do this in several ways:

• Competitive Inhibition – All CTLA4-blocking antibodies, such as ipilimumab and tremelimumab, bind to the CTLA4 receptor, thereby preventing its interaction with the B7 ligands (CD80 and CD86) on antigen-presenting cells. Without this inhibitory signal, CD28 (the co-stimulatory receptor) is free to bind to B7 ligands, leading to increased T cell activation, proliferation, and cytokine production.

• Regulatory T Cell Depletion – In addition to their direct effects on conventional T cells, CTLA4 inhibitors also modulate the function of Tregs, which constitutively express high levels of CTLA4. Certain antibody isotypes, such as the IgG1 class (as seen with ipilimumab), can mediate antibody-dependent cellular cytotoxicity (ADCC), leading to the selective depletion of Tregs in the tumor microenvironment. This selective depletion can further enhance the anti-tumor immune response by reducing the immunosuppressive milieu within the tumor.

• Modulation of Immune Synapse Dynamics – By interfering with the CTLA4-mediated “braking” of T cell signaling, these agents indirectly promote the formation of a more stable immune synapse between T cells and antigen-presenting cells. Such stable synapses are critical for sustained T cell activation and effective tumor cell killing. Structural investigations have shed light on how modifications in antibody structure (for example, in HL32-based candidates) can influence this process, thereby informing design strategies that enhance immune activation while minimizing adverse events.

• Improved Co-stimulation – The perturbation of CTLA4 interactions by blocking antibodies enhances the availability of B7 ligands for CD28 binding. This provision of an unopposed co-stimulatory signal is thought to be a key driver of enhanced anti-tumor immunity when CTLA4 inhibitors are administered.

Comparison of Different Candidates
While all CTLA4-targeting agents share these core mechanisms, differences in their molecular structure, isotype, epitope specificity, and Fc receptor engagement lead to variations in clinical performance:

• Isotype Differences – Ipilimumab is an IgG1 antibody and has demonstrated ADCC activity, which can lead to Treg depletion. Tremelimumab, on the other hand, is an IgG2 antibody, which typically has less ADCC activity; this difference may influence both efficacy and toxicity, as less Treg depletion might translate into fewer immune-related adverse events but could also lead to a difference in overall immune activation.

• Epitope and Affinity – Emerging candidates such as ADG126 and HL32-Fab-based molecules have been designed based on detailed structural data to target unique CTLA4 epitopes. These candidates may offer improved binding specificity and affinity, leading to more effective blockade within the tumor microenvironment while potentially reducing off-tumor effects.

• Format and Masking Technologies – Some novel candidates utilize masking strategies (for example, ADG126) to limit systemic exposure and focus the drug’s activity within the tumor. Such approaches can lower the risk of irAEs while preserving the anti-tumor efficacy.

• Bispecific Formats – Bispecific antibodies that simultaneously block CTLA4 along with another inhibitory receptor (commonly PD-1) aim to harness synergistic effects. These agents combine the benefits of two checkpoint inhibitors into a single therapeutic entity, which may allow lower doses of each active component and help overcome resistance mechanisms. However, their design is complex, and optimizing the balance of inhibition between the two targets remains challenging.

Clinical Trials and Efficacy
Clinical development of CTLA4 inhibitors has been robust, with many trials evaluating their efficacy, safety, and optimal dosing in various cancer types. The outcomes of these trials have provided a wealth of data that continues to inform future therapeutic strategies.

Overview of Clinical Trials
Ipilimumab’s pivotal trials in metastatic melanoma set the stage for CTLA4-targeted therapy. These trials showed that despite a relatively modest overall response rate, the durability of responses was impressive, and some patients experienced long-term survival benefits. Numerous phase II and phase III trials have since expanded the use of CTLA4 inhibitors to other malignancies, including non-small cell lung cancer, renal cell carcinoma, and hepatocellular carcinoma. Emerging agents like tremelimumab have likewise been evaluated in multiple phase II studies across different solid tumors. Clinical evaluations of next-generation antibodies such as ADG126 and gotistobart are ongoing, with preliminary data suggesting potentially enhanced safety profiles and comparable or superior efficacy in select patient populations. In some trials, combinations of CTLA4 inhibitors with other immunotherapies, particularly PD-1/PD-L1 inhibitors, are being rigorously tested in order to address the limitations observed when these agents are used as monotherapies.

Efficacy and Safety Profiles
The efficacy of CTLA4 inhibitors is largely characterized by their ability to produce durable responses in a subset of patients. Ipilimumab has been shown to extend overall survival in melanoma patients compared to conventional therapies, although it is associated with a high frequency of immune-related adverse events such as colitis, dermatitis, and endocrinopathies. Tremelimumab’s clinical data suggest various degrees of activity across different cancers, but variability in response rates and tolerability continues to be a subject of active investigation. Emerging candidates, which are engineered to optimize binding and minimize systemic exposure, have the potential to reduce toxicities while maintaining or enhancing anti-tumor effects. For example, early clinical data with candidates such as gotistobart have indicated promising efficacy when used in combination regimens; however, definitive clinical outcomes and long-term safety data are still pending. Comparative studies have also been conducted to understand the nuances between these agents—for instance, differences in the extent of ADCC-mediated Treg depletion between IgG1 and IgG2 antibodies have significant therapeutic implications. In general, while CTLA4-targeted therapies show potent immune-stimulatory effects, their safety profiles present a significant challenge. The management of irAEs remains a critical component of clinical use, and the design of next-generation agents is increasingly focusing on strategies to mitigate these adverse effects through dosing modifications, masking technologies, and combination therapy approaches.

Future Directions and Challenges
Despite considerable advances in CTLA4-targeted therapy, several important challenges remain, and ongoing research is required to fully harness the therapeutic potential of these agents while minimizing toxicity.

Challenges in Targeting CTLA4
The primary challenge when targeting CTLA4 lies in the fine balance between augmenting anti-tumor immunity and avoiding excessive immune activation that can lead to autoimmunity. Some of the challenges include:

• Immune-Related Adverse Events (irAEs) – CTLA4 inhibitors are notorious for their potential to cause a range of irAEs. These adverse events arise due to the fundamental role of CTLA4 in maintaining self-tolerance. The management of irAEs involves the use of immunosuppressive agents and treatment interruptions, which can compromise therapeutic efficacy.

• Dosing and Scheduling Optimization – Determining the optimal dosing strategy for CTLA4 inhibitors is complex. Overdosing may lead to excessive toxicity, while underdosing might result in insufficient immune activation. The development of novel agents like ADG126 is directly addressing this challenge by employing masking strategies to restrict activity to the tumor microenvironment, thereby potentially allowing safer dosing regimens.

• Biomarker Identification – Predictive biomarkers that can identify which patients are most likely to benefit from CTLA4-targeted therapy are urgently needed. Current biomarkers such as CTLA4 expression levels or the tumor immune microenvironment profile have not yet fully translated into reliable clinical tools.

• Resistance Mechanisms – As with other immunotherapies, primary or acquired resistance to CTLA4 inhibitors remains a critical problem. Understanding the underlying mechanisms of resistance—for example, how tumor-induced immunosuppression or compensatory upregulation of other checkpoint molecules may limit efficacy—is an ongoing area of research.

Future Research Directions
Looking ahead, several strategic avenues are being explored in the development of CTLA4-targeted therapies:

• Next-Generation Antibody Engineering – Innovations in antibody engineering, including the design of masked antibodies that are selectively activated in the tumor environment, are promising approaches to improve the therapeutic index of CTLA4 inhibitors. Detailed structural studies provide insights that are guiding next-generation designs to optimize binding specificity and reduce systemic toxicity.

• Bispecific and Multispecific Agents – Combining CTLA4 blockade with inhibition of other checkpoints (for instance, PD-1/PD-L1) in a single bispecific antibody molecule is an emerging strategy. These agents can potentially deliver synergistic immune activation while reducing the need for high doses of individual antibodies, thereby limiting toxicities.

• Combination Therapies – Future research is likely to focus on combining CTLA4 inhibitors with other therapeutic modalities, such as radiation therapy, chemotherapy, or targeted agents. Such combinations may overcome resistance mechanisms and enhance overall response rates, and several ongoing clinical trials are evaluating these strategies.

• Personalized Immunotherapy – Advances in genomic, transcriptomic, and proteomic technologies are expected to refine patient selection by identifying biomarkers that predict response to CTLA4-targeted therapy. The integration of these data into clinical decision-making would pave the way for more personalized immunotherapeutic regimens.

• Optimization of Dosing Regimens – Research into adjusting dosing schedules to balance efficacy and toxicity is critically important. With the advent of new agents that offer improved safety profiles, future clinical trials will likely investigate fractionated dosing, adjusted treatment intervals, or intermittent dosing strategies that aim to maximize benefit while minimizing irAEs.

• Understanding Mechanisms of Resistance – Ongoing studies are focused on dissecting the molecular and cellular pathways that contribute to resistance against CTLA4 inhibition. By understanding factors such as the dynamics of T cell activation, regulatory T cell infiltration, and compensatory immune checkpoint upregulation, future therapies can be tailored or combined with agents that target these resistance mechanisms.

Conclusion
In summary, therapeutic candidates targeting CTLA4 represent a critical and evolving class of immunotherapeutic agents that have fundamentally altered the treatment landscape for several forms of cancer. At the forefront is ipilimumab, the first approved CTLA4-blocking antibody, which set a precedent by demonstrating durable responses in metastatic melanoma despite considerable challenges related to immune-related adverse events. Building on this legacy, emerging therapies such as tremelimumab, ADG126, gotistobart, and novel candidates derived from advanced antibody engineering techniques (e.g. HL32-Fab-based molecules) are being developed to enhance efficacy and mitigate toxicity. These new agents not only leverage improved binding specificity and alternative isotype properties but also employ innovative masking strategies and bispecific formats to focus their activity within the tumor microenvironment.

Mechanistically, CTLA4 inhibitors function by disrupting the inhibitory signals that restrain T cell activation, thereby enhancing co-stimulation through CD28 engagement, promoting effector cell expansion, and in certain cases, depleting regulatory T cells within the tumor. However, the potent immune modulation that underpins these therapies also leads to the risk of irAEs, necessitating a delicate balance that is currently being addressed through optimized dosing regimens, innovative drug designs, and combination therapeutic strategies. Clinical trials over the past decade have not only validated the anti-tumor efficacy of CTLA4 blockade but have also underscored the need for a personalized approach to immunotherapy that considers patient-specific biomarkers and the complex dynamics of the tumor immune microenvironment.

Looking forward, future research directions in CTLA4-targeted therapy are expected to focus on next-generation antibody engineering, the development of bispecific and multispecific agents to overcome resistance, and the integration of precision medicine strategies to optimize patient selection and dosing. Continued efforts to understand and counteract the mechanisms of resistance as well as to reduce systemic toxicity through targeted delivery and combination approaches will be essential for the evolution of this therapeutic class. Ultimately, these advancements hold the promise of significantly improving clinical outcomes not only for patients with melanoma but also for a wide array of solid tumors that leverage the CTLA4 pathway for immune evasion.

Thus, while current approved therapies such as ipilimumab have transformed the cancer treatment paradigm, emerging candidates provide a glimpse of a future where the full therapeutic potential of CTLA4 blockade is realized with fewer adverse effects and greater efficacy through personalisation and innovative combination strategies. The path forward is an exciting interplay of scientific discovery, innovative drug design and precise clinical application, all focused on unleashing the full power of the immune system against cancer while meticulously managing the inherent risks associated with immune modulation.