What ENPP1 inhibitors are in clinical trials currently?

Introduction to ENPP1
ENPP1 is a type II transmembrane glycoprotein that plays a fundamental role in extracellular nucleotide metabolism. It primarily hydrolyzes substrates such as ATP and 2′,3′-cGAMP, thereby modulating purinergic signaling and the activation of innate immune pathways. By degrading 2′,3′-cGAMP—an endogenous activator of the stimulator of interferon genes (STING) pathway—ENPP1 acts as a checkpoint that can dampen immune responses. As our understanding of this enzyme has advanced, it has become clear that ENPP1’s activity can influence numerous physiological processes, including mineralization of soft tissues and regulation of inflammation, while alterations in its expression affect disease progression in diverse pathological contexts.

Biological Role and Function of ENPP1
ENPP1 functions at the cell surface and in the extracellular matrix where it regulates the homeostasis of extracellular nucleotides. One of its most notable biological roles is the hydrolysis of cyclic dinucleotides, especially 2′,3′-cGAMP. By modulating the levels of this potent immune-stimulatory molecule, ENPP1 serves as a negative regulator of the STING pathway. This regulation is critical because STING activation leads to the production of type I interferons and other cytokines that stimulate immune responses. In addition to its immunomodulatory functions, ENPP1 is involved in controlling the deposition of calcium and pyrophosphate, thereby directly influencing tissue mineralization processes. As such, ENPP1 is integral to maintaining the balance between calcification and inhibition of ectopic mineral deposition in soft tissues.

Disease Associations with ENPP1
Disruptions in ENPP1 activity are associated with a wide range of diseases. Excessive activity of this enzyme has been linked to the development and progression of various cancers by suppressing effective anti-tumor immune responses, thus contributing to an immunosuppressive tumor microenvironment. In particular, high levels of ENPP1 expression have been correlated with advanced metastatic disease, poor prognosis, and resistance to immunotherapy, making it a promising target for cancer treatment. Beyond oncology, ENPP1 has also been implicated in metabolic disorders such as type 2 diabetes due to its participation in insulin signaling pathways, as well as in mineralization disorders like autosomal-recessive hypophosphatemic rickets type 2 (ARHR2) and osteomalacia, where its role in generating extracellular pyrophosphate becomes clinically significant.

ENPP1 Inhibitors
ENPP1 inhibitors are designed to block the activity of the enzyme, thereby preventing the degradation of critical signaling molecules such as 2′,3′-cGAMP. The inhibitors can increase the local concentration of 2′,3′-cGAMP, which in turn can lead to a more robust activation of the STING pathway and enhance innate and adaptive anti-tumor immune responses. The development of small-molecule inhibitors for ENPP1 is a promising strategy because it not only has the potential to break the immunosuppressive shield of many cancers but can also be tailored to have improved pharmacokinetic and pharmacodynamic properties, making them suitable for clinical applications.

Mechanism of Action
The primary mechanism by which ENPP1 inhibitors operate is through the competitive or allosteric blockade of the enzyme’s active site. By occupying the binding domain, these inhibitors prevent the hydrolysis of 2′,3′-cGAMP and other nucleotide substrates. There are both nucleotide-based inhibitors and non-nucleotide inhibitors under development. Non-nucleotide inhibitors, often characterized by diverse chemical scaffolds, have been optimized for potency, selectivity, and acceptable drug-like properties. Some inhibitors are designed to be cell impermeable, ensuring that they predominantly affect extracellular ENPP1, which is essential given the enzyme’s localization. The structural studies and computational modeling have further elucidated the interactions between ENPP1 and its inhibitors, establishing critical binding interactions that facilitate potent inhibition while minimizing off-target effects.

Potential Therapeutic Applications
Given the central role of ENPP1 in immune regulation and tissue mineralization, its inhibitors have broad potential therapeutic applications. In oncology, ENPP1 inhibitors can revert the immunosuppressive tumor microenvironment by allowing 2′,3′-cGAMP to persist extracellularly for longer periods, thereby promoting STING-mediated anti-tumor immunity. This effect can enhance the efficacy of immune checkpoint inhibitors and other immunotherapies. In addition, some ENPP1 inhibitors are being explored for their potential in metastatic cancers, where robust immune activation is critical for tumor control. Outside oncology, there is emerging interest in using these inhibitors in metabolic and calcification disorders, although the majority of the current clinical focus remains in the field of cancer immunotherapy.

Current Clinical Trials of ENPP1 Inhibitors
The most exciting developments in ENPP1 inhibitor research have now advanced to the clinical trial phase. Multiple candidates have been designed to target ENPP1 in patients with advanced and metastatic solid tumors, with a strong emphasis on cancer immunotherapy. These clinical studies are enrolling patients with various tumor types, including colorectal cancer and other advanced solid tumors, in both monotherapy and combination therapy settings. The trials are spread across early-phase (Phase 1) studies to later-phase (Phase 2) studies, and even expanded access programs have been reported.

List of ENPP1 Inhibitors in Trials
Several ENPP1 inhibitors have emerged in the clinical landscape. Key candidates include:

• RBS2418: This compound is being studied in multiple clinical trials targeting subjects with advanced, metastatic solid tumors as well as patients with advanced, metastatic, and progressive colorectal cancer. There is also an expanded access program for RBS2418 in specific cases (for example, in pancreatic cancer) that highlights its potential broader application.

• TXN10128: TXN10128 is under investigation in subjects with solid tumors. In a multicenter, open-label, Phase 1 dose escalation and expansion study, TXN10128 is being evaluated for its safety, tolerability, and preliminary anti-tumor efficacy in patients with locally advanced (unresectable) or metastatic solid tumors.

• SR-8541A: SR-8541A is another promising ENPP1 inhibitor currently in clinical trials. It is being explored in advanced/metastatic solid tumors in a Phase 1, dose escalation study where its pharmacokinetic profile, safety, and tolerability are being assessed. Additionally, a Phase 2 study of SR-8541A in combination with immunomodulators—specifically Botensilimab and Balstilimab—is underway for subjects with refractory metastatic microsatellite stable colorectal cancer, indicating its use in combination strategies for resistant forms of cancer.

• ISM5939: ISM5939 is being evaluated in a first-in-human Phase 1a/b, open-label, multicenter study. This study focuses on dose escalation, optimization, and expansion in patients with advanced and/or metastatic solid tumors.

These four inhibitors—RBS2418, TXN10128, SR-8541A, and ISM5939—represent the current frontiers of ENPP1-targeted therapy in clinical trials, and they are being investigated in various solid tumor contexts with the aim of enhancing anti-tumor immunity through the modulation of the STING pathway. Their selection underscores a strategic focus on harnessing the body’s natural immune responses to counteract aggressive cancers.

Phase and Status of Trials
The clinical studies of these inhibitors demonstrate a clear time sequence and progression in clinical development:

• RBS2418:
 – A study titled “Evaluation of RBS2418 in Subjects With Advanced, Metastatic Solid Tumors” is identified as a first-in-human trial that includes a Phase 1a/b dose escalation and expansion study. This trial is assessing RBS2418 both as a monotherapy and in combination with Pembrolizumab, a well-known checkpoint inhibitor, in subjects with advanced unresectable, recurrent, or metastatic tumors.
 – In another study, “Evaluation of RBS2418 in Patients with Advanced, Metastatic, and Progressive Colorectal Cancer” (a Phase 2a trial), RBS2418 is being evaluated in a randomized, double-blind, placebo-controlled format with best supportive care. This trial highlights its application in a more specific patient population with advanced colorectal cancer.
 – There is also an Expanded Access program for RBS2418, designed to provide treatment options for individual patients (for instance, in pancreatic cancer) outside the standard clinical trial framework, thereby addressing urgent clinical needs in a compassionate use setting.

• TXN10128:
 – TXN10128 is currently in a Phase 1 multicenter, open-label, dose-escalation and expansion study where its primary objective is to establish the safety profile, tolerability, and preliminary efficacy data in subjects with solid tumors. The trial’s design emphasizes careful dose escalation to identify an optimal therapeutic window.

• SR-8541A:
 – The Phase 1 study of SR-8541A is focused on evaluating the safety, tolerability, and pharmacokinetics when administered orally as a monotherapy in subjects with advanced or metastatic solid tumors. The aim is to determine the maximum tolerated dose and potential early signals of efficacy.
 – Further, the Phase 2 clinical trial involving SR-8541A is designed in combination with other immunotherapeutic agents, namely Botensilimab and Balstilimab, for patients with refractory metastatic microsatellite stable colorectal cancer. This study highlights the evolving strategy of combining ENPP1 inhibition with immune checkpoint blockade to potentiate the anti-tumor immune response.

• ISM5939:
 – ISM5939 is being tested in a first-in-human Phase 1a/b, open-label, multicenter study. This trial is designed to assess dose escalation, safety, pharmacokinetics, and the preliminary therapeutic window in patients with advanced and/or metastatic solid tumors, providing valuable early insight into the potential clinical utility of this novel inhibitor.

Taken together, these clinical trials indicate that the current clinical development of ENPP1 inhibitors is largely in the early-phase stages (Phase 1 and Phase 2), where safety and dosage are being rigorously studied. The emphasis on both monotherapy and combination therapy suggests that researchers are exploring multiple avenues to harness the immunostimulatory potential of these agents, either as stand-alone treatments or in concert with other therapies to overcome resistance seen in refractory tumors.

Challenges and Future Directions
While the clinical trials of ENPP1 inhibitors are promising, several challenges remain in their development, and future research directions have been outlined by investigators to refine their clinical applications further.

Development Challenges
Despite the strong mechanistic rationale for targeting ENPP1, several development challenges persist:

• Selectivity and Off-Target Effects:
One major challenge is achieving high selectivity for ENPP1 without affecting other members of the ENPP family or unrelated phosphodiesterases. Off-target inhibition could lead to adverse effects given the diverse roles of these enzymes in various physiological processes. The complexity of the enzyme’s active site necessitates sophisticated medicinal chemistry strategies and extensive structure-activity relationship studies to ensure potency and selectivity.

• Pharmacokinetic and Pharmacodynamic Profiles:
Optimization of the pharmacokinetic properties of small molecules is critical. These inhibitors must have a favorable bioavailability and a suitable half-life so that they can maintain effective concentrations in the extracellular space without causing toxicity. Additionally, the dynamic interplay between ENPP1 inhibition and the activation of the STING pathway requires precise dosing strategies to avoid overactivation of innate immune responses that may lead to systemic inflammation.

• Formulation and Administration:
Many ENPP1 inhibitors are designed to be cell impermeable so that their effect is confined to the extracellular space. This property, while advantageous, presents challenges in formulation to ensure that the compounds remain stable, are delivered efficiently, and reach the tumor microenvironment at therapeutic concentrations. Moreover, achieving the right balance between systemic exposure and localized effect in solid tumors is a hurdle that researchers continue to address.

• Combination Therapy Complexity:
The clinical trials involving agents such as SR-8541A in combination with checkpoint inhibitors like Botensilimab and Balstilimab further underscore the challenge of combination therapies. Combining ENPP1 inhibitors with immunotherapies involves complex pharmacodynamic interactions that can either potentiate efficacy or compound toxicity. Determining the optimal scheduling, dose, and sequence of administration is therefore a critical challenge in ongoing trials.

Future Research and Development Prospects
Looking ahead, several areas of research and development are likely to drive the clinical translation of ENPP1 inhibitors:

• Refinement of Inhibitor Design:
Continued advances in structure-guided drug design and high-throughput screening methods will be key. Future research will likely leverage emerging computational modeling techniques and detailed structural insights gleaned from crystal structures to design next-generation inhibitors with improved selectivity and potency. The iterative optimization of chemical scaffolds, as seen with the evolution from TH-based inhibitors to more potent non-nucleotide inhibitors, exemplifies the dynamic nature of this field.

• Biomarker Identification and Patient Stratification:
For ENPP1 inhibitors, identifying biomarkers that predict response is critical. Future studies may focus on genomic, proteomic, and immunologic markers that correlate with high ENPP1 expression or STING pathway suppression in tumors. Improved patient stratification could enhance the efficacy of these inhibitors by targeting treatments to those who are most likely to benefit, potentially reducing the risk of side effects by avoiding unnecessary exposure in non-responsive patients.

• Combination Strategies with Established Therapies:
The combination of ENPP1 inhibitors with checkpoint inhibitors or with conventional treatments such as chemotherapy or radiation therapy is an area with significant promise. Combination strategies could address inherent resistance mechanisms in tumors and potentiate anti-tumor immunity. Preclinical models suggest that by disrupting the degradative “dam” that ENPP1 forms, these inhibitors could synergize with other immunotherapeutic agents to produce durable clinical responses.

• Expanding Therapeutic Indications:
Although current trials have predominantly focused on advanced solid tumors and colorectal cancer, future research may broaden the therapeutic indications for ENPP1 inhibitors. There is potential for these inhibitors to be used in other malignancies where ENPP1-driven immunosuppression plays a central role, as well as in rare calcification disorders where dysregulated mineralization is a key aspect of the disease process.

• Improved Clinical Trial Designs:
Future trials may incorporate innovative designs such as adaptive trial protocols, basket trials, or umbrella trials that allow for more flexible assessment of therapeutic efficacy across diverse patient populations and tumor types. These designs can help optimize dose regimens, accelerate the identification of effective combination therapies, and improve our understanding of the safety profile in various clinical settings.

Conclusion
In summary, the current clinical landscape for ENPP1 inhibitors primarily comprises four key agents: RBS2418, TXN10128, SR-8541A, and ISM5939. RBS2418 is being evaluated in multiple clinical studies, including Phase 1a/b trials in advanced, metastatic solid tumors—with an expansion into Phase 2a studies in patients with progressive colorectal cancer—and it is also available under an expanded access program for individual patients. TXN10128 is under investigation in a Phase 1 trial targeting locally advanced or metastatic solid tumors. Similarly, SR-8541A is being studied as a monotherapy in a Phase 1 trial in advanced/metastatic solid tumors and in a Phase 2 setting in combination with immunomodulators for refractory metastatic colorectal cancer. Lastly, ISM5939 is being tested in a first-in-human Phase 1a/b trial in patients with advanced and/or metastatic solid tumors, marking it as a novel candidate in the clinical development pipeline.

We began with an introduction to ENPP1 highlighting its critical role in extracellular nucleotide hydrolysis and immune regulation. Its function as a negative regulator of STING, combined with its involvement in tissue mineralization, links ENPP1 to a variety of diseases ranging from cancer to metabolic and mineralization disorders. ENPP1 inhibitors, acting by preserving the levels of immune-stimulatory molecules like 2′,3′-cGAMP, hold significant promise in overcoming tumor-induced immunosuppression. Their mechanism of action and potential applications make them attractive candidates for cancer immunotherapy and possibly beyond.

In the clinical trials section, we reviewed the current ENPP1 inhibitors in development. RBS2418 is a versatile inhibitor studied across various solid tumors and is evaluated both as a stand-alone treatment and in combination with established immunotherapies. TXN10128, SR-8541A, and ISM5939 complete the picture as they are being tested in early-phase trials designed to understand safety, tolerability, and initial efficacy. The diverse phases and trial designs, from first-in-human studies to combination strategies, highlight the dynamic and evolving nature of this field. Through careful dose-escalation studies and combination trials, researchers are working to determine the best strategy to utilize these inhibitors to achieve a robust anti-tumor immune response while minimizing adverse effects.

Looking ahead, although the current clinical data are encouraging, significant challenges remain. These include the need for improved selectivity to avoid off-target effects, optimizing pharmacokinetic profiles, developing robust biomarkers for better patient selection, and refining combination therapy protocols. Future research is expected to address these issues through advanced computational design, innovative clinical trial methodologies, and a deeper understanding of the underlying molecular processes. If these challenges can be overcome, ENPP1 inhibitors have the potential to become a cornerstone in the treatment of various cancers and, perhaps, other ENPP1-associated disorders.

In conclusion, the clinical development of ENPP1 inhibitors is a highly active and promising area that illustrates the translation of biochemical insights into potentially transformative clinical therapies. The current trials with RBS2418, TXN10128, SR-8541A, and ISM5939 mark critical steps toward making effective anti-cancer immunotherapies available to patients. These inhibitors, by blocking ENPP1’s enzyme activity, hold the promise of reinvigorating the immune system against tumors while offering avenues for combination therapy to overcome resistance mechanisms. With ongoing trials providing valuable safety and efficacy data, the future of ENPP1 inhibition looks set to broaden the therapeutic landscape, not only in oncology but potentially in other areas where ENPP1’s regulation is disrupted. Continued research and clinical validation are essential to fully realize the potential of these novel inhibitors, ultimately leading to improved patient outcomes and new therapeutic paradigms.