Novel K+ Channel Targets in Atrial Fibrillation Drug Development—Where Are We?:

Experimental drugs for supraventricular tachycardia: an analysis of early phase clinical trials

INTRODUCTION

Supraventricular tachycardias (SVT) are a diverse group of commonly encountered arrhythmias arising at or above the atrioventricular (AV) node. Conventional anti-arrhythmic medications are restricted by extensive side-effect profiles and limited efficacy. Catheter ablation has emerged as a first-line therapy for many arrhythmias but is not a suitable option for all patients. This has prompted the exploration of novel pharmacological approaches targeting specific molecular mechanisms of SVT.

AREAS COVERED

This review article aims to summarize recent advancements in pharmacological therapeutics for SVT and their clinical implications. The understanding of molecular mechanisms underlying these arrhythmias, particularly atrial fibrillation, has opened up new possibilities for targeted interventions. Beyond the manipulation of ion channels and membrane potentials, pharmacotherapy now focuses on upstream targets such as inflammation, oxidative stress, and structural remodeling. This review strives to provide a comprehensive overview of recent advancements in pharmacological therapeutics directed at the management of SVT. We begin by providing a brief summary of the mechanisms and management of commonly encountered SVT before delving into individual agents, which in turn are stratified based on their molecular treatment targets.

EXPERT OPINION

The evolving landscape of pharmacologic therapy offers hope for more personalized and tailored interventions in the management of SVT.

Open channel block of Kv1.5 channels by HMQ1611

Kv1.5 channels conduct the ultra-rapid delayed rectifier potassium current (I_Kur). Pharmacological blockade of human Kv1.5 (hKv1.5) has been regarded as an effective treatment of re-entrant based atrial fibrillation, because Kv1.5 is highly expressed in human cardiac atria but scarcely in ventricles. The Kv1.5 blockade is also expected to be used in cancer therapeutics since Kv1.5 is overexpressed in some types of human tumors. Here, we investigated the blockade of hKv1.5 channels by HMQ1611, a symmetrical biphenyl derivative. hKv1.5 channels were heterologously expressed in Chinese hamster ovary cells. The effects of HMQ1611 on wild-type and 13 hKv1.5 mutant channels were examined using the whole-cell patch-clamp method, and molecular docking simulation was conducted to predict the docking position of HMQ1611 within Kv1.5 channels. We showed that HMQ1611 reversibly inhibited the hKv1.5 current in a concentration-dependent manner (IC₅₀ = 2.07 μM). HMQ1611 blockade of hKv1.5 current developed with time during depolarizing voltage-clamp steps, and this blockade was also voltage-dependent with a steep increase over the voltage range for channel openings. HMQ1611 inhibition was significantly reduced in the T479A, T480A, V505A, I508A, L510A, V512A, and V516A hKv1.5 mutant channels. Molecular docking analysis predicted that V505, V512, and T480 were involved in the blocking action of HMQ1611 on hKv1.5 channels. These results suggest that HMQ1611 inhibits hKv1.5 currents as an open channel blocker. Amino acid residues located at the base of the selectivity filter (T479 and T480) and in the S6 segment (V505, I508, L510, V512, and V516) of hKv1.5 appear to constitute potential binding sites for HMQ1611.

Inhibition of Small-Conductance, Ca2+-Activated K+ Current by Ondansetron

Background: Small-conductance Ca²⁺-activated K⁺ channels (SK channels) have been proposed as antiarrhythmic targets for the treatment of atrial fibrillation. We previously demonstrated that the 5-HT₃ receptor antagonist ondansetron inhibits heterologously expressed, human SK2 (hSK2) currents as well as native cardiac SK currents in a physiological extra-/intracellular [K⁺] gradient at therapeutic (i.e., sub-micromolar) concentrations. A recent study, using symmetrical [K⁺] conditions, challenged this result. The goal of the present study was to revisit the inhibitory effect of ondansetron on hSK2-mediated currents in symmetrical [K⁺] conditions.

Experimental Approach: The whole-cell patch clamp technique was used to investigate the effects of ondansetron and apamin on hSK2-mediated currents expressed in HEK 293 cells. Currents were measured in symmetrical [K⁺] conditions in the presence of 100 nM [Ca²⁺]_o.

Results: Expression of hSK2 produced inwardly rectifying whole-cell currents in the presence of 400 nM free cytosolic Ca²⁺. Ondansetron inhibited whole-cell hSK2 currents with IC₅₀ values of 154 and 113 nM at −80 and 40 mV, respectively. Macroscopic current inhibited by ondansetron and current inhibited by apamin exhibited inwardly rectifying current-voltage relationships with similar reversal potentials (apamin, ∼5 mV and ondansetron, ∼2 mV). Ondansetron (1 μM) in the continuing presence of apamin (100 nM) had no effect on hSK2-mediated whole-cell currents. Wild-type HEK 293 cells did not express ondansetron- or apamin-sensitive currents.

Conclusion: Ondansetron in sub-micromolar concentrations inhibits hSK2 currents even under altered ionic conditions.

Anti-arrhythmic investigations in large animal models of atrial fibrillation

Atrial fibrillation (AF) constitutes an increasing health problem in the aging population. Animal models reflecting human phenotypes are needed to understand the mechanisms of AF, as well as to test new pharmacological interventions. In recent years, a number of large animal models, primarily pigs, goats, dog and horses have been used in AF research. These animals can to a certain extent recapitulate the human pathophysiological characteristics and serve as valuable tools in investigating new pharmacological interventions for treating AF. This review focuses on anti-arrhythmic investigations in large animals. Initially, spontaneous AF in small and large mammals is discussed. This is followed by a short presentation of frequently used methods for inducing short- and long-term AF. The major focus of the review is on anti-arrhythmic compounds either frequently used in the human clinic (ranolazine, flecainide, vernakalant and amiodarone) or being promising new AF medicine candidates (I_K,Ach , I_SK,Ca and I_K2P blockers).

Inhibition of Small-Conductance Calcium-Activated Potassium Current (I K,Ca) Leads to Differential Atrial Electrophysiological Effects in a Horse Model of Persistent Atrial Fibrillation

Background

Small-conductance Ca²⁺-activated K⁺ (K_Ca2) channels have been proposed as a possible atrial-selective target to pharmacologically terminate atrial fibrillation (AF) and to maintain sinus rhythm. However, it has been hypothesized that the importance of the K_Ca2 current—and thereby the efficacy of small-conductance Ca²⁺-activated K⁺ current (I_K,Ca) inhibition—might be negatively related to AF duration and the extent of AF-induced remodeling.

Experimental Approach and Methods

To address the hypothesis of the efficacy of I_K,Ca inhibition being dependent on AF duration, the anti-arrhythmic properties of the I_K,Ca inhibitor NS8593 (5 mg/kg) and its influence on atrial conduction were studied using epicardial high-density contact mapping in horses with persistent AF. Eleven Standardbred mares with tachypacing-induced persistent AF (42 ± 5 days of AF) were studied in an open-chest experiment. Unipolar AF electrograms were recorded and isochronal high-density maps analyzed to allow for the reconstruction of wave patterns and changes in electrophysiological parameters, such as atrial conduction velocity and AF cycle length. Atrial anti-arrhythmic properties and adverse effects of NS8593 on ventricular electrophysiology were evaluated by continuous surface ECG monitoring.

Results

I_K,Ca inhibition by NS8593 administered intravenously had divergent effects on right and left AF complexity and propagation properties in this equine model of persistent AF. Despite global prolongation of AF cycle length, a slowing of conduction in the right atrium led to increased anisotropy and electrical dissociation, thus increasing AF complexity. In contrast, there was no significant change in AF complexity in the LA, and cardioversion of AF was not achieved.

Conclusions

Intra-atrial heterogeneity in response to I_K,Ca inhibition by NS8593 was observed. The investigated dose of NS8593 increased the AF cycle length but was not sufficient to induce cardioversion. In terms of propagation properties during AF, I_K,Ca inhibition by NS8593 led to divergent effects in the right and left atrium. This divergent behavior may have impeded the cardioversion success.

A benzopyran with antiarrhythmic activity is an inhibitor of Kir3.1-containing potassium channels

Atrial fibrillation (AF) is the most commonly diagnosed cardiac arrhythmia and is associated with increased morbidity and mortality. Currently approved AF antiarrhythmic drugs have limited efficacy and/or carry the risk of ventricular proarrhythmia. The cardiac acetylcholine activated inwardly rectifying K+ current (IKACh), composed of Kir3.1/Kir3.4 heterotetrameric and Kir3.4 homotetrameric channel subunits, is one of the best validated atrial-specific ion channels. Previous research pointed to a series of benzopyran derivatives with potential for treatment of arrhythmias, but their mechanism of action was not defined. Here, we characterize one of these compounds termed Benzopyran-G1 (BP-G1) and report that it selectively inhibits the Kir3.1 (GIRK1 or G1) subunit of the KACh channel. Homology modeling, molecular docking, and molecular dynamics simulations predicted that BP-G1 inhibits the IKACh channel by blocking the central cavity pore. We identified the unique F137 residue of Kir3.1 as the critical determinant for the IKACh-selective response to BP-G1. The compound interacts with Kir3.1 residues E141 and D173 through hydrogen bonds that proved critical for its inhibitory activity. BP-G1 effectively blocked the IKACh channel response to carbachol in an in vivo rodent model and displayed good selectivity and pharmacokinetic properties. Thus, BP-G1 is a potent and selective small-molecule inhibitor targeting Kir3.1-containing channels and is a useful tool for investigating the role of Kir3.1 heteromeric channels in vivo. The mechanism reported here could provide the molecular basis for future discovery of novel, selective IKACh channel blockers to treat atrial fibrillation with minimal side effects.

Effect of selective IK,ACh inhibition by XAF-1407 in an equine model of tachypacing-induced persistent atrial fibrillation

BACKGROUND AND PURPOSE

Inhibition of the G-protein gated ACh-activated inward rectifier potassium current, I_K,ACh may be an effective atrial selective treatment strategy for atrial fibrillation (AF). Therefore, the anti-arrhythmic and electrophysiological properties of a novel putatively potent and highly specific I_K,ACh inhibitor, XAF-1407 (3-methyl-1-[5-phenyl-4-[4-(2-pyrrolidin-1-ylethoxymethyl)-1-piperidyl]thieno[2,3-d]pyrimidin-6-yl]azetidin-3-ol), were characterised for the first time in vitro and investigated in horses with persistent AF.

EXPERIMENTAL APPROACH

The pharmacological ion channel profile of XAF-1407 was investigated using cell lines expressing relevant ion channels. In addition, eleven horses were implanted with implantable cardioverter defibrillators enabling atrial tachypacing into self-sustained AF. The electrophysiological effects of XAF-1407 were investigated after serial cardioversions over a period of 1 month. Cardioversion success, drug-induced changes of atrial tissue refractoriness, and ventricular electrophysiology were assessed at baseline (day 0) and days 3, 5, 11, 17, and 29 after AF induction.

KEY RESULTS

XAF-1407 potently and selectively inhibited K_ir 3.1/3.4 and K_ir 3.4/3.4, underlying the I_K,ACh current. XAF-1407 treatment in horses prolonged atrial effective refractory period as well as decreased atrial fibrillatory rate significantly (~20%) and successfully cardioverted AF, although with a decreasing efficacy over time. XAF-1407 shortened atrioventricular-nodal refractoriness, without effect on QRS duration. QTc prolongation (4%) within 15 min of drug infusion was observed, however, without any evidence of ventricular arrhythmia.

CONCLUSION AND IMPLICATIONS

XAF-1407 efficiently cardioverted sustained tachypacing-induced AF of short duration in horses without notable side effects. This supports I_K,ACh inhibition as a potentially safe treatment of paroxysmal AF in horses, suggesting potential clinical value for other species including humans.

On the mechanism of GIRK2 channel gating by phosphatidylinositol bisphosphate, sodium, and the Gβγ dimer

G protein-gated inwardly rectifying K⁺ (GIRK) channels belong to the inward-rectifier K⁺ (Kir) family, are abundantly expressed in the heart and the brain, and require that phosphatidylinositol bisphosphate is present so that intracellular channel-gating regulators such as Gβγ and Na⁺ ions can maintain the channel-open state. However, despite high-resolution structures (GIRK2) and a large number of functional studies, we do not have a coherent picture of how Gβγ and Na⁺ ions control gating of GIRK2 channels. Here, we utilized computational modeling and all-atom microsecond-scale molecular dynamics simulations to determine which gates are controlled by Na⁺ and Gβγ and how each regulator uses the channel domain movements to control gate transitions. We found that Na⁺ ions control the cytosolic gate of the channel through an anti-clockwise rotation, whereas Gβγ stabilizes the transmembrane gate in the open state through a rocking movement of the cytosolic domain. Both effects alter the way in which the channel interacts with phosphatidylinositol bisphosphate and thereby stabilizes the open states of the respective gates. These studies of GIRK channel dynamics present for the first time a comprehensive structural model that is consistent with the great body of literature on GIRK channel function.

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

Antiarrhythmic drugs for atrial fibrillation: Imminent impulses are emerging

Rhythm and rate strategies are considered equivalent for the management of atrial fibrillation (AF). Moreover, both strategies are intended for improving symptoms and quality of life. Despite the clinical availability of several antiarrhythmic drugs (AAD) the alternatives for the patient with comorbidities are significantly fewer because of the concern regarding many adverse effects, including proarrhythmias. The impetuous development of AF ablation gave rise to a false impression that AAD are a second line therapy. All these statements reflect, in fact, the weakness of the classical paradigm and classification regarding AAD and the gap between the current knowledge of AF mechanism and determinants and the "classical" AAD non-discriminatory action. A new paradigm in development of effective and safe AAD is based on modern knowledge of vulnerable parameters involved in the genesis and perpetuation of AF. New AAD will target specific triggers of AF and ion currents which are expressed preferentially in fibrillatory atrium. Such targets will include repolarizing currents and channels, as ultrarapid potassium current, two pore potassium current, the acetylcholine-gated potassium current, small-conductance calcium-dependent potassium channels, but, also, molecular targets involved in intracellular calcium kinetics, as Ca2+-calmodulin-dependent protein kinase, ryanodine receptors and non-coding miRNA. New mechanistic discoveries link AF to inflammation and modern anti-cytokine drugs. There is still a long way to win between basic research and clinical practice, but, without any doubt, antiarrhythmic drug therapy will remain and develop as a cornerstone therapy for AF not in conflict, but complementary and alternative to interventional therapy.

Investigational antiarrhythmic agents: promising drugs in early clinical development

INTRODUCTION

Although there have been important technological advances for the treatment of cardiac arrhythmias (e.g., catheter ablation technology), antiarrhythmic drugs (AADs) remain the cornerstone therapy for the majority of patients with arrhythmias. Most of the currently available AADs were coincidental findings and did not result from a systematic development process based on known arrhythmogenic mechanisms and specific targets. During the last 20 years, our understanding of cardiac electrophysiology and fundamental arrhythmia mechanisms has increased significantly, resulting in the identification of new potential targets for mechanism-based antiarrhythmic therapy. Areas covered: Here, we review the state-of-the-art in arrhythmogenic mechanisms and AAD therapy. Thereafter, we focus on a number of antiarrhythmic targets that have received significant attention recently: atrial-specific K+-channels, the late Na+-current, the cardiac ryanodine-receptor channel type-2, and the small-conductance Ca2+-activated K+-channel. We highlight for each of these targets available antiarrhythmic agents and the evidence for their antiarrhythmic effect in animal models and early clinical development. Expert opinion: Targeting AADs to specific subgroups of well-phenotyped patients is likely necessary to detect improved outcomes that may be obscured in the population at large. In addition, specific combinations of selective AADs may have synergistic effects and may enable a mechanism-based tailored antiarrhythmic therapy.

Atrial-selective K+ channel blockers: potential antiarrhythmic drugs in atrial fibrillation?

In the wake of demographic change in Western countries, atrial fibrillation has reached an epidemiological scale, yet current strategies for drug treatment of the arrhythmia lack sufficient efficacy and safety. In search of novel medications, atrial-selective drugs that specifically target atrial over other cardiac functions have been developed. Here, I will address drugs acting on potassium (K⁺) channels that are either predominantly expressed in atria or possess electrophysiological properties distinct in atria from ventricles. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two-pore domain K⁺ (K2P) channels (tandem of P domains, weak inward-rectifying K⁺ channels (TWIK-1), TWIK-related acid-sensitive K⁺ channels (TASK-1 and TASK-3)) that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Direct drug effects on these channels are described and their putative value in treatment of atrial fibrillation is discussed. Although many potential drug targets have emerged in the process of unravelling details of the pathophysiological mechanisms responsible for atrial fibrillation, we do not know whether novel antiarrhythmic drugs will be more successful when modulating many targets or a single specific one. The answer to this riddle can only be solved in a clinical context.

Atrial fibrillation: Therapeutic potential of atrial K+ channel blockers

Despite the epidemiological scale of atrial fibrillation, current treatment strategies are of limited efficacy and safety. Ideally, novel drugs should specifically correct the pathophysiological mechanisms responsible for atrial fibrillation with no other cardiac or extracardiac actions. Atrial-selective drugs are directed toward cellular targets with sufficiently different characteristics in atria and ventricles to modify only atrial function. Several potassium (K⁺) channels with either predominant expression in atria or distinct electrophysiological properties in atria and ventricles can serve as atrial-selective drug targets. These channels include the ultra-rapidly activating, delayed outward-rectifying Kv1.5 channel conducting I_Kur, the acetylcholine-activated inward-rectifying Kir3.1/Kir3.4 channel conducting I_K,ACh, the Ca²⁺-activated K⁺ channels of small conductance (SK) conducting I_SK, and the two pore domain K⁺ (K2P) channels TWIK-1, TASK-1 and TASK-3 that are responsible for voltage-independent background currents I_TWIK-1, I_TASK-1, and I_TASK-3. Here, we briefly review the characteristics of these K⁺ channels and their roles in atrial fibrillation. The antiarrhythmic potential of drugs targeting the described channels is discussed as well as their putative value in treatment of atrial fibrillation.

Antiarrhythmic Drug Therapy for Rhythm Control in Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and affects over 33 million people worldwide. AF is associated with stroke and systemic thromboembolism, unpleasant symptoms and reduced quality of life, heart failure, and increased mortality, and treatment of AF and its complications are associated with significant cost. Antiarrhythmic drugs (AADs) can suppress AF, allowing long-term maintenance of sinus rhythm, and have the potential to relieve symptoms and reverse or prevent adverse effects associated with AF. However, large randomized controlled studies evaluating use of AADs have not demonstrated a clear benefit to maintaining sinus rhythm, and AADs often have significant limitations, including a modest rate of overall success at maintaining sinus rhythm, frequent side effects, and potentially life-threatening toxicities. Although some of the currently available AADs have been available for almost 100 years, better tolerated and more efficacious AADs have recently been developed both for long-term maintenance of sinus rhythm and for chemical cardioversion of AF to sinus rhythm. Advances in automated AF detection with cardiac implantable electronic devices have suggested that AADs might be useful for suppressing AF to allow safe discontinuation of anticoagulation in select patients who are in sinus rhythm for prolonged periods of time. AADs may also have synergistic effects with catheter ablation of AF. This review summarizes the pharmacology and clinical use of currently available AADs for treatment of AF and discusses novel AADs and future directions for rhythm control in AF.

Mechanism of Proarrhythmic Effects of Potassium Channel Blockers

Any disturbance of electrical impulse formation in the heart and of impulse conduction or action potential (AP) repolarization can lead to rhythm disorders. Potassium (K(+)) channels play a prominent role in the AP repolarization process. In this review we describe the causes and mechanisms of proarrhythmic effects that arise as a response to blockers of cardiac K(+) channels. The largest and chemically most diverse groups of compound targets are Kv11.1 (hERG) and Kv7.1 (KvLQT1) channels. Finally, the proarrhythmic propensity of atrial-selective K(+) blockers inhibiting Kv1.5, Kir3.1/3.4, SK, and K2P channels is discussed.

Synthesis of a highly water-soluble acacetin prodrug for treating experimental atrial fibrillation in beagle dogs

We previously reported that duodenal administration of the natural flavone acacetin can effectively prevent the induction of experimental atrial fibrillation (AF) in canines; however, it may not be used intravenously to terminate AF due to its poor water-solubility. The present study was to design a water-soluble prodrug of acacetin and investigate its anti-AF effect in beagle dogs. Acacetin prodrug was synthesized by a three-step procedure. Aqueous solubility, bioconversion and anti-AF efficacy of acacetin prodrug were determined with different methodologies. Our results demonstrated that the synthesized phosphate sodium salt of acacetin prodrug had a remarkable increase of aqueous solubility in H₂O and clinically acceptable solution (5% glucose or 0.9% NaCl). The acacetin prodrug was effectively converted into acacetin in ex vivo rat plasma and liver microsome, and in vivo beagle dogs. Intravenous infusion of acacetin prodrug (3, 6 and 12 mg/kg) terminated experimental AF without increasing ECG QTc interval in beagle dogs. The intravenous LD₅₀ of acacetin prodrug was 721 mg/kg in mice. Our preclinical study indicates that the synthesized acacetin prodrug is highly water-soluble and safe; it effectively terminates experimental AF in beagle dogs and therefore may be a promising drug candidate for clinical trial to treat patients with acute AF.

Atrial-Selective Potassium Channel Blockers

Atrial fibrillation (AF) is associated with increased morbidity and mortality. Atrial-selective potassium (K(+)) channel blockers may represent a novel therapeutic target. The best validated atrial-specific ion currents are the acetylcholine-activated inward-rectifier K(+) current IK,ACh and ultrarapidly activating delayed-rectifier K(+) current IKur. Two-pore domain and small-conductance Ca(2+)-activated K(+) channels and Kv1.1 channels may also contribute to the atrial repolarization. We review the molecular and electrophysiologic characteristics of atrial-selective K(+) channels and their potential pathophysiologic role in AF. We summarize currently available K(+) channel blockers focusing on the most important compounds.