What are the therapeutic candidates targeting P2X3?

Introduction to P2X3 Receptors

Structure and Function
P2X3 receptors are a member of the ionotropic purinergic receptor family. They are ATP‐gated non‐selective cation channels that exist predominantly as homotrimers or heterotrimers (e.g., in combination with P2X2 subunits to form P2X2/3 receptors). Structurally, each subunit possesses two transmembrane domains with a large extracellular loop that contains the ATP binding pocket. On activation by ATP, these receptors open to permit influx of cations (such as Na⁺ and Ca²⁺), which contributes to cell depolarization and subsequent signal transmission. The unique arrangement of the extracellular domains, including various loops (dorsal fin, left flipper, and lower body domains), creates sites for both ATP binding and potential allosteric modulation. Detailed structural studies facilitated by crystallography and molecular modeling have provided insights that now support rational design of antagonists by targeting both the orthosteric and allosteric sites of the channel.

Role in Human Physiology
In human physiology, P2X3 receptors are expressed predominantly on sensory neurons, especially on small-diameter C-fibers and Aδ fibers that innervate tissues in the skin, airways, and visceral organs. Their activation by extracellular ATP is a key mechanism in the initiation and modulation of pain, cough reflex, and other sensory responses. In addition to afferent signaling, P2X3 receptors are involved in the peripheral communication between cells following inflammatory events, where ATP is released from damaged or distressed cells. Because of this restricted and relatively selective pattern of expression, P2X3 receptors have drawn significant research interest for use as therapeutic targets in conditions where aberrant sensory signaling leads to chronic pain and cough.

Therapeutic Importance of P2X3 Receptors

Diseases Associated with P2X3
Dysregulation of P2X3 receptor signaling has been implicated in a variety of conditions. Primarily, two diseases have been in the spotlight:
• Chronic refractory cough: Elevated ATP levels in airway tissues, particularly during inflammation or irritation, can overactivate P2X3 receptors. This excessive activation leads to chronic cough—a condition that has historically been difficult to treat effectively with conventional antitussives.
• Pain conditions: P2X3 receptors are also key mediators of nociceptive transmission. In both inflammatory and neuropathic pain models, overexpression or hyperactivity of P2X3 receptors in dorsal root and trigeminal ganglia correlates with persistent pain states.
Other conditions where the P2X3 axis is emerging as a potential target include endometriosis-related pain, migraine (where ATP functions synergistically with other nociceptive molecules), and potentially even bladder dysfunction. The overall clinical relevance is underscored by the fact that modulation of these receptors can diminish the transmission of painful and irritating signals without affecting central processing in a major way, thereby reducing the possibility of central nervous system (CNS) side effects.

Mechanism of Action in Disease Modulation
P2X3 receptors mediate afferent sensitization by translating local extracellular ATP signals into nerve impulses. When these channels are overactivated, as frequently happens in response to tissue injury, inflammation, or chemical irritation, there is an exaggerated release of nociceptive signals to the central nervous system. Under normal circumstances, ATP is quickly degraded by ecto-nucleotidases, but under pathological conditions, its concentration rises, subsequently leading to sustained receptor activation. By antagonizing these receptors, many therapeutic candidates aim to interrupt this aberrant signal transduction, either by blocking ATP binding or by allosterically modulating the receptor’s conformation. This effectively dampens the hypersensitivity of sensory neurons, thereby reducing pain and cough reflexes. The innovative targeting of P2X3 is especially attractive given that its expression is relatively selective to peripheral afferents. This offers the promise of fewer adverse effects when compared to drugs that act centrally, thus providing an improved benefit-risk balance.

Current Therapeutic Candidates

Overview of Existing Candidates
Several therapeutic candidates have been developed to target P2X3 receptors, with many compounds advancing into clinical trials or even regulatory reviews. These candidates span different chemical scaffolds—from diaminopyrimidine derivatives to newer structural classes like pyrrolinones and benzimidazole-diones.

• Gefapixant (AF-219, MK-7264):
Gefapixant is arguably the most extensively studied and prominent P2X3 receptor antagonist. It was developed on the basis of diaminopyrimidine chemistry and has been evaluated for its efficacy in treating refractory chronic cough. Gefapixant functions by competitively inhibiting ATP binding at the receptor and has been shown to reduce aberrant afferent signaling. Although its clinical efficacy is well documented, one of the main issues with gefapixant is its associated taste disturbance (dysgeusia), which appears in a dose-dependent manner. Despite these adverse effects, gefapixant remains a benchmark candidate in this class, with multiple regulatory submissions (e.g., marketed as LYFNUA tablets 45 mg in some territories) and extensive clinical data available.

• Eliapixant:
Eliapixant is a next-generation P2X3 antagonist engineered to have higher in vitro potency and improved selectivity for the P2X3 receptor over P2X2/3 heteromers. Preclinical and early clinical data suggest that eliapixant has the potential to mitigate the taste disturbance seen with gefapixant, presenting an improved safety profile. Early studies indicate that eliapixant may be effective for both chronic cough and certain pain conditions, owing to its ability to block ATP-mediated sensitization at the peripheral terminals of sensory neurons.

• BLU-5937:
BLU-5937 is another promising candidate, being evaluated for its antitussive efficacy. Unlike gefapixant, BLU-5937 is designed to generate fewer taste-related side effects while still producing an analgesic and cough-suppressive effect. Clinical investigations have revealed that adverse events related to taste are minimal, and the compound shows a favorable tolerability profile. Although efficacy endpoints may vary between studies, BLU-5937 is often compared favorably with gefapixant in terms of its safety profile while maintaining clinical activity in reducing cough frequency.

• Sivopixant:
Sivopixant has been undergoing clinical evaluation—specifically in phase 2b trials—to determine its ability to reduce 24-hour cough frequency relative to placebo. Though initial results indicate that the primary endpoint might not have reached statistical significance across all doses, the greatest improvements were observed with higher doses, along with a tolerable side effect profile. This candidate represents the continuous effort to fine-tune dose-response profiles and maximize therapeutic outcomes without compromising safety.

• Camlipixant:
Camlipixant is being explored predominantly for its use in refractory chronic cough. Model‐based dose selection studies have suggested that doses of 25 mg and 50 mg twice daily are most appropriate to achieve sufficient receptor occupancy and therapeutic efficacy, without severe adverse effects. Camlipixant’s design stems from the drive to preserve efficacy while mitigating the dysgeusia frequently seen in first-generation compounds such as gefapixant.

• DT-0111 (Aspirex™):
Although often mentioned in the context of P2X2/3 receptor antagonism, DT-0111 is an example of a therapeutic candidate being developed as a water-soluble, inhaled agent. It is being investigated primarily for ATP-related pulmonary diseases, including chronic obstructive pulmonary disease (COPD) and chronic cough. Its formulation for inhalation suggests that it might achieve a high local concentration in the lungs while reducing systemic exposure—and attendant side effects.

• Additional Chemical Scaffolds and Patented Compounds:
Beyond these prominent candidates, several patents have disclosed novel diaminopyrimidine-based compounds that function as P2X3 and/or P2X2/3 receptor antagonists. These patented molecules claim to offer improved oral bioavailability, metabolic stability, and reduced adverse event profiles when compared with early-stage compounds. Furthermore, novel scaffolds like pyrrolinone derivatives and benzimidazole-4,7-dione analogues have been reported to show potent analgesic effects in preclinical models. Their development is being driven by the need to lower the incidence of taste disturbances and to provide candidates that are suitable for long-term therapy in chronic conditions.

Clinical Trial Status and Results
The clinical development of these P2X3 antagonists follows a timeline that reflects both early discovery as well as continual refinement:
• Gefapixant has been the most clinically advanced candidate. It has completed phase III clinical trials for refractory chronic cough in various regions and has garnered regulatory interest despite the high prevalence of dose-dependent taste disturbance. Its status has accelerated its potential approval for marketing in select jurisdictions, as indicated by submissions through agencies like the EMA and PMDA. For instance, gefapixant is available in some markets under trade names such as LYFNUA tablets 45 mg—with the approval process detailed in regulatory submissions.

• Eliapixant is in early phase clinical trials. Early-phase data have highlighted its potent and selective antagonism of the P2X3 receptor with clinical effectiveness coupled with a lower incidence of taste-related adverse effects. Although data are still emerging, preclinical evidence strongly suggests that its pharmacokinetic and pharmacodynamic profiles are favorable for addressing both chronic cough and pain, making it a candidate to progress quickly through the clinical pipeline.

• BLU-5937 has reached advanced phase clinical trials as well. Findings from phase 1/2 studies indicate that BLU-5937 can reduce cough frequency in refractory chronic cough patients while minimizing taste disturbances. However, given the variability in primary efficacy endpoints, further phase III studies are needed to establish its long-term efficacy and safety profile. The clinical trial outcomes for BLU-5937 are under active investigation, and the data so far position it as a promising candidate that may overcome the shortcomings of earlier molecules.

• Sivopixant has completed a phase 2b trial in which its ability to decrease the 24-hour cough frequency was evaluated. Although the overall significance compared to placebo has been challenged at lower doses, the highest dose demonstrated the most promising improvements in cough frequency. The safety and tolerability profiles observed in these trials suggest that further dose refinement and subsequent phase III trials may confirm its therapeutic potential.

• Camlipixant’s dosing strategy is currently being fine-tuned in preparation for phase III testing, following promising phase 2 dose-selection studies. Its model-based dose selection approach has identified 25 mg and 50 mg twice daily as the most effective regimens, balancing efficacy with minimal adverse effects—particularly in regard to taste disturbances. The candidate has garnered interest as a promising alternative to gefapixant for treating refractory chronic cough.

• DT-0111, while not directly a P2X3-specific antagonist in terms of its molecular target (as it also modulates P2X2/3 function), remains in preclinical development for direct pulmonary delivery. Its potential success hinges on the promise that inhalation formulations can deliver high concentrations locally with fewer systemic side effects, thereby broadening the treatment options for COPD and chronic cough.

In addition to the aforementioned compounds, the patent literature indicates a lively and ongoing effort to develop next-generation small molecules. Many of these compounds are still at the preclinical or early clinical stage. The advantages of these novel scaffolds include improved in vitro potency (with some candidates showing IC50 values in the low nanomolar range), enhanced selectivity (often achieving over 20-fold selectivity for P2X3 over P2X2/3 receptors), and promising pharmacokinetic profiles that are optimized for oral administration. Collectively, these data underscore the fact that the research and development pipeline for P2X3 antagonists is both robust and varied, with several compounds showing promise in reducing pain and cough by modulating aberrant ATP-mediated signaling.

Challenges and Future Directions

Developmental Challenges
Despite the promise of P2X3 antagonists, several developmental challenges remain. One of the primary limitations seen with early candidates like gefapixant is the high incidence of taste disturbances. P2X3 receptors are also expressed on taste sensory nerves; therefore, effective antagonism can inadvertently interfere with taste perception. This adverse event has driven developers to design molecules such as eliapixant, BLU-5937, and camlipixant that selectively target receptor subtypes or optimize receptor binding kinetics to reduce off-target effects.

Another challenge is the species-dependent pharmacology of these compounds. Early research highlighted significant differences in ligand binding between human and animal P2X3 receptors, demonstrating that candidate molecules must maintain high efficacy across species to ensure that preclinical efficacy reliably predicts clinical outcomes. Selectivity between P2X3 and P2X2/3 receptor subtypes also poses a significant hurdle; many clinical studies have sought to balance high receptor occupancy with minimal impact on heteromeric receptors, which can drive both efficacy and unwanted adverse events.

Additionally, the metabolic stability and oral bioavailability of these compounds need further optimization. Several candidates in the patent territory and early-phase development are designed to overcome issues related to first-pass metabolism, drug-drug interactions, and hepatobiliary toxicity—problems that can complicate chronic use in conditions like cough and pain. Such challenges have led to a refined focus on novel chemical scaffolds (like pyrrolinone and benzimidazole-dione derivatives) that are optimized through structure-activity relationship (SAR) studies and guided by the increased structural understanding of the receptor from crystallographic studies.

Finally, the balance between central and peripheral effects is crucial. Although peripheral sensory neurons are the target, complete systemic blockade may lead to unforeseen side effects in tissues with low-level receptor expression. Consequently, achieving the right therapeutic window remains a critical issue for clinical success.

Future Research and Potential Innovations
Looking forward, research on P2X3 receptor therapeutics is anticipated to combine the robust insights from structural biology with advances in medicinal chemistry. Future research directions include:

• Development of allosteric modulators:
The identification of allosteric binding sites on P2X3 receptors opens the possibility of designing modulators that fine-tune receptor activity rather than causing complete inhibition. Allosteric modulators may provide the advantage of preserving physiological signaling while reducing excessive receptor activation in pathological conditions. This approach can help minimize adverse effects like taste disturbances by avoiding full blockade of receptor activity.

• Improved structure-based drug design:
Recent crystallographic and cryo-EM studies have provided detailed snapshots of the receptor in its closed, open, and desensitized states, enabling the use of molecular modeling and in silico ligand docking. These techniques can help identify critical binding residues (for instance, in the dorsal fin and left flipper regions) and guide the optimization of candidate molecules with improved selectivity and potency. The iterative design process guided by these techniques is likely to reduce the trial-and-error approach traditionally associated with small-molecule drug discovery.

• Exploration of combination therapies and novel delivery methods:
Ongoing research may also explore the combination of P2X3 antagonists with agents that modulate related signaling pathways (such as inflammatory cytokines or neurotrophic factors) to achieve synergistic effects. Novel drug delivery systems—such as inhalable formulations (as in the case of DT-0111) or targeted nanoparticles—are also avenues designed to increase local drug concentration at the site of receptor expression while minimizing systemic exposure.

• Biological markers and personalized medicine approaches:
Given the growing understanding of the genetic polymorphisms that affect P2X3 receptor function, future clinical trials may incorporate biomarker studies to identify patient subgroups that are more likely to benefit from receptor blockade. This individualized approach could lead to more tailored treatment strategies and higher response rates.

• Optimization of dosing strategies:
Recent model-based dose selection studies, such as those guiding camlipixant’s dosing regimen, highlight the importance of establishing optimal dosing schedules that maintain therapeutic efficacy while reducing adverse events. Future studies will likely refine these dosing algorithms further by correlating pharmacokinetic parameters with functional outcomes in patients.

• New chemical modalities and patents:
The active patent landscape shows that there is significant intellectual property being generated around various P2X3 antagonists. These patents not only secure novel chemical backbones for future development but also highlight the innovation aimed at overcoming the limitations of earlier molecules. Continued activity in this space is expected to drive forward the next generation of therapeutic candidates that are even more potent, selective, and tolerable.

Conclusion
In summary, the therapeutic candidates targeting P2X3 receptors represent a diverse and rapidly evolving field in drug discovery. Starting from early-generation compounds like gefapixant—developed as diaminopyrimidines and which have undergone extensive clinical evaluation for refractory chronic cough—the focus has progressively shifted toward next-generation compounds that aim to mitigate adverse events such as taste disturbances while preserving therapeutic efficacy. The pipeline currently includes multiple chemical classes:

• Gefapixant, the most advanced candidate, has demonstrated robust clinical efficacy in reducing chronic cough but faces challenges related to dysgeusia.
• Eliapixant is positioned as a promising alternative with greater selectivity and fewer taste-related side effects, which could benefit both chronic cough and pain conditions.
• BLU-5937 and sivopixant are further examples of candidates that have progressed to later-phase clinical trials; both show efficacy in diminishing aberrant cough reflexes while striving to preserve a favorable tolerability profile.
• Camlipixant, guided by advanced model-based dose selection studies, shows potential to further refine dosing regimens in chronic cough treatment without severe adverse events.
• Additionally, DT-0111, which is designed for inhaled delivery, highlights the novel formulation approaches being explored to achieve high local drug exposure with reduced systemic risks.
• Patent literature reveals a range of novel small-molecule candidates—including diaminopyrimidine derivatives, pyrrolinone derivatives, and benzimidazole-dione analogues—that are engineered to overcome issues of selectivity, metabolic stability, and off-target effects.

From a general perspective, the evolution of P2X3 antagonists over the past decade has been driven by a deepening understanding of receptor structure and function. Specific insights gained from structural studies have enabled medicinal chemists to design compounds that more efficiently block the ATP binding site or modulate receptor activation allosterically. On the specific side, each candidate brings its own advantages and limitations. Gefapixant remains the gold standard in terms of clinical validation, whereas eliapixant and camlipixant may herald improved safety profiles that could make them preferable in long-term therapies. BLU-5937 and sivopixant are being adeptly positioned to address both efficacy and tolerability issues, while innovative delivery methods seen in DT-0111 promise to widen the spectrum of application, especially in pulmonary diseases. Taking a broader view, these candidates not only address the fundamental issue of overactive sensory signaling in chronic cough and pain but also open the door to personalized treatment strategies by leveraging emerging pharmacogenetic insights.

In conclusion, while current therapeutic candidates targeting P2X3 are already demonstrating significant promise in clinical settings, ongoing research – driven by new chemical modalities, improved structural insights, and innovative drug delivery approaches – is poised to expand and refine this therapeutic space further. The challenges of taste disturbance, receptor selectivity, and metabolic stability continue to inspire innovation. Future studies, integrating both clinical and preclinical data, will refine dosing strategies, improve safety profiles, and likely lead to the development of a new generation of P2X3 antagonists that offer robust therapeutic benefits for patients suffering from chronic cough, pain, and other P2X3‐related disorders. Ultimately, the rapid pace of research in this field, supported by a strong patent landscape and ongoing clinical trials, provides a solid foundation for the future success of P2X3-targeted therapeutics.