Class III antiarrhythmic activity in vivo by selective blockade of the slowly activating cardiac delayed rectifier potassium current IKs by (R)-2-(2,4-trifluoromethyl)-N-[2-oxo-5-phenyl-1-(2,2,2-trifluoroethyl)- 2, 3-dihydro-1H-benzo[e][1,4]diazepin-3-yl]acetamide

Ligand modulation of KCNQ-encoded (KV7) potassium channels in the heart and nervous system

KCNQ-encoded (K_V7) potassium channels are diversely distributed in the human tissues, associated with many physiological processes and pathophysiological conditions. These channels are increasingly used as drug targets for treating diseases. More selective and potent molecules on various types of the K_V7 channels are desirable for appropriate therapies. The recent knowledge of the structure and function of human KCNQ-encoded channels makes it more feasible to achieve these goals. This review discusses the role and mechanism of action of many molecules in modulating the function of the KCNQ-encoded potassium channels in the heart and nervous system. The effects of these compounds on K_V7 channels help to understand their involvement in many diseases, and to search for more selective and potent ligands to be used in the treatment of many disorders such as various types of cardiac arrhythmias, epilepsy, and pain.

Insights into Cardiac IKs (KCNQ1/KCNE1) Channels Regulation

The delayed rectifier potassium IKs channel is an important regulator of the duration of the ventricular action potential. Hundreds of mutations in the genes (KCNQ1 and KCNE1) encoding the IKs channel cause long QT syndrome (LQTS). LQTS is a heart disorder that can lead to severe cardiac arrhythmias and sudden cardiac death. A better understanding of the IKs channel (here called the KCNQ1/KCNE1 channel) properties and activities is of great importance to find the causes of LQTS and thus potentially treat LQTS. The KCNQ1/KCNE1 channel belongs to the superfamily of voltage-gated potassium channels. The KCNQ1/KCNE1 channel consists of both the pore-forming subunit KCNQ1 and the modulatory subunit KCNE1. KCNE1 regulates the function of the KCNQ1 channel in several ways. This review aims to describe the current structural and functional knowledge about the cardiac KCNQ1/KCNE1 channel. In addition, we focus on the modulation of the KCNQ1/KCNE1 channel and its potential as a target therapeutic of LQTS.

Imidazodiazepine Anticonvulsant, KRM-II-81, Produces Novel, Non-diazepam-like Antiseizure Effects

The need for improved medications for the treatment of epilepsy and chronic pain is essential. Epileptic patients typically take multiple antiseizure drugs without complete seizure freedom, and chronic pain is not fully managed with current medications. A positive allosteric modulator (PAM) of α2/3-containing GABA_A receptors (5-(8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazole[1,5-α][1,4]diazepin-3-yl) oxazole or KRM-II-81 (8) is a lead compound in a series of imidazodiazepines. We previously reported that KRM-II-81 produces broad-based anticonvulsant and antinociceptive efficacy in rodent models and provides a wider margin over motoric side effects than that of other GABA_A receptor PAMs. The present series of experiments was designed to fill key missing gaps in prior preclinical studies assessing whether KRM-II-81 could be further differentiated from nonselective GABA_A receptor PAMs using the anticonvulsant diazepam (DZP) as a comparator. In multiple chemical seizure provocation models in mice, KRM-II-81 was either equally or more efficacious than DZP. Most strikingly, KRM-II-81 but not DZP blocked the development of seizure sensitivity to the chemoconvulsants cocaine and pentylenetetrazol in seizure kindling models. These and predecessor data have placed KRM-II-81 into consideration for clinical development requiring the manufacture of kilogram amounts of good manufacturing practice material. We describe here a novel synthetic route amenable to kilogram quantity production. The new biological and chemical data provide key steps forward in the development of KRM-II-81 (8) as an improved treatment option for patients suffering from epilepsy.

Human-induced pluripotent stem cell-derived cardiomyocytes have limited IKs for repolarization reserve as revealed by specific KCNQ1/KCNE1 blocker

Objective

We investigated if there is I_Ks, and if there is repolarization reserve by I_Ks in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs).

Design

We used a specific KCNQ1/KCNE1 channel blocker, L-000768673, with an IC₅₀ of 9 nM, and four hERG-specific blockers, astemizole, cisapride, dofetilide, and E-4031 to investigate the issue.

Results

L-000768673 concentration-dependently prolonged feature point duration (FPD)―a surrogate signal of action potential duration―from 1 to 30 nM without pacing or paced at 1.2 Hz, resulting from I_Ks blockade in hiPSC-CMs. At higher concentrations, the effect of L-000768673 on I_Ks was mitigated by its effect on I_Ca-L, resulting in shortened FPD, reduced impedance amplitude, and increased beating rate at 1 µM and above, recapitulating the self-limiting properties of L-000768673 on action potentials. All four hERG-specific blockers prolonged FPD as expected. Co-application of L-000768673 at sub-threshold (0.1 and 0.3 nM) and threshold (1 nM) concentrations failed to synergistically enhance the effects of hERG blockers on FPD prolongation, rather it showed additive effects, inconsistent with the repolarization reserve role of I_Ks in mature human myocytes that enhanced I_Kr response, implying a difference between hiPSC-CMs used in this study and mature human cardiomyocytes.

Conclusion

There was I_Ks current in hiPSC-CMs, and blockade of I_Ks current caused prolongation of action potential of hiPSC-CMs. However, we could not demonstrate any synergistic effects on action potential duration prolongation of hiPSC-CMs by blocking hERG current and I_Ks current simultaneously, implying little or no repolarization reserve by I_Ks current in hiPSC-CMs used in this study.

Fluorine and Fluorinated Motifs in the Design and Application of Bioisosteres for Drug Design

The electronic properties and relatively small size of fluorine endow it with considerable versatility as a bioisostere and it has found application as a substitute for lone pairs of electrons, the hydrogen atom, and the methyl group while also acting as a functional mimetic of the carbonyl, carbinol, and nitrile moieties. In this context, fluorine substitution can influence the potency, conformation, metabolism, membrane permeability, and P-gp recognition of a molecule and temper inhibition of the hERG channel by basic amines. However, as a consequence of the unique properties of fluorine, it features prominently in the design of higher order structural metaphors that are more esoteric in their conception and which reflect a more sophisticated molecular construction that broadens biological mimesis. In this Perspective, applications of fluorine in the construction of bioisosteric elements designed to enhance the in vitro and in vivo properties of a molecule are summarized.

Action Potential Recording and Pro-arrhythmia Risk Analysis in Human Ventricular Trabeculae

To assess drug-induced pro-arrhythmic risk, especially Torsades de Pointe (TdP), new models have been proposed, such as in-silico modeling of ventricular action potential (AP) and stem cell-derived cardiomyocytes (SC-CMs). Previously we evaluated the electrophysiological profile of 15 reference drugs in hESC-CMs and hiPSC-CMs for their effects on intracellular AP and extracellular field potential, respectively. Our findings indicated that SC-CMs exhibited immature phenotype and had the propensity to generate false positives in predicting TdP risk. To expand our knowledge with mature human cardiac tissues for drug-induced pro-arrhythmic risk assessment, human ventricular trabeculae (hVT) from ethically consented organ donors were used to evaluate the effects of the same 15 drugs (8 torsadogenic, 5 non-torsadogenic, and 2 discovery molecules) on AP parameters at 1 and 2 Hz. Each drug was tested blindly with 4 concentrations in duplicate trabeculae from 2 hearts. To identify the pro-arrhythmic risk of each drug, a pro-arrhythmic score was calculated as the weighted sum of percent drug-induced changes compared to baseline in various AP parameters, including AP duration and recognized pro-arrhythmia predictors such as triangulation, beat-to-beat variability and incidence of early-afterdepolarizations, at each concentration. In addition, to understand the translation of this preclinical hVT AP-based model to clinical studies, a ratio that relates each testing concentration to the human therapeutic unbound Cmax (Cmax) was calculated. At a ratio of 10, for the 8 torsadogenic drugs, 7 were correctly identified by the pro-arrhythmic score; 1 was mislabeled. For the 5 non-torsadogenic drugs, 4 were correctly identified as safe; 1 was mislabeled. Calculation of sensitivity, specificity, positive predictive value, and negative predictive value indicated excellent performance. For example, at a ratio of 10, scores for sensitivity, specificity, positive predictive value and negative predictive values were 0.88, 0.8, 0.88 and 0.8, respectively. Thus, the hVT AP-based model combined with the integrated analysis of pro-arrhythmic score can differentiate between torsadogenic and non-torsadogenic drugs, and has a greater predictive performance when compared to human SC-CM models.

Quantitative analysis of the Ca2+ -dependent regulation of delayed rectifier K+ current IKs in rabbit ventricular myocytes

KEY POINTS

[Ca²⁺ ]_i enhanced rabbit ventricular slowly activating delayed rectifier K⁺ current (I_Ks ) by negatively shifting the voltage dependence of activation and slowing deactivation, similar to perfusion of isoproterenol. Rabbit ventricular rapidly activating delayed rectifier K⁺ current (I_Kr ) amplitude and voltage dependence were unaffected by high [Ca²⁺ ]_i . When measuring or simulating I_Ks during an action potential, I_Ks was not different during a physiological Ca²⁺ transient or when [Ca²⁺ ]_i was buffered to 500 nm.

ABSTRACT

The slowly activating delayed rectifier K⁺ current (I_Ks ) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca²⁺ ([Ca²⁺ ]_i ) and β-adrenergic receptor (β-AR) stimulation modulate I_Ks amplitude and kinetics, but details of these important I_Ks regulators and their interaction are limited. We assessed the [Ca²⁺ ]_i dependence of I_Ks in steady-state conditions and with dynamically changing membrane potential and [Ca²⁺ ]_i during an AP. I_Ks was recorded from freshly isolated rabbit ventricular myocytes using whole-cell patch clamp. With intracellular pipette solutions that controlled free [Ca²⁺ ]_i , we found that raising [Ca²⁺ ]_i from 100 to 600 nm produced similar increases in I_Ks as did β-AR activation, and the effects appeared additive. Both β-AR activation and high [Ca²⁺ ]_i increased maximally activated tail I_Ks , negatively shifted the voltage dependence of activation, and slowed deactivation kinetics. These data informed changes in our well-established mathematical model of the rabbit myocyte. In both AP-clamp experiments and simulations, I_Ks recorded during a normal physiological Ca²⁺ transient was similar to I_Ks measured with [Ca²⁺ ]_i clamped at 500-600 nm. Thus, our study provides novel quantitative data as to how physiological [Ca²⁺ ]_i regulates I_Ks amplitude and kinetics during the normal rabbit AP. Our results suggest that micromolar [Ca²⁺ ]_i , in the submembrane or junctional cleft space, is not required to maximize [Ca²⁺ ]_i -dependent I_Ks activation during normal Ca²⁺ transients.

The "Cyclopropyl Fragment" is a Versatile Player that Frequently Appears in Preclinical/Clinical Drug Molecules

Recently, there has been an increasing use of the cyclopropyl ring in drug development to transition drug candidates from the preclinical to clinical stage. Important features of the cyclopropane ring are, the (1) coplanarity of the three carbon atoms, (2) relatively shorter (1.51 Å) C-C bonds, (3) enhanced π-character of C-C bonds, and (4) C-H bonds are shorter and stronger than those in alkanes. The present review will focus on the contributions that a cyclopropyl ring makes to the properties of drugs containing it. Consequently, the cyclopropyl ring addresses multiple roadblocks that can occur during drug discovery such as (a) enhancing potency, (b) reducing off-target effects,

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Antimicrobial benzodiazepine-based short cationic peptidomimetics

Antimicrobial peptides (AMPs) appear to be good candidates for the development of new antibiotic drugs. We describe here the synthesis of peptidomimetic compounds that are based on a benzodiazepine scaffold flanked with positively charged and hydrophobic amino acids. These compounds mimic the essential properties of cationic AMPs. The new design possesses the benzodiazepine scaffold that is comprised of two glycine amino acids and which confers flexibility and aromatic hydrophobic 'back', and two arms used for further synthesis on solid phase for incorporation of charged and hydrophobic amino acids. This approach allowed us a better understanding of the influence of these features on the antimicrobial activity and selectivity. A novel compound was discovered which has MICs of 12.5 µg/ml against Staphylococcus aureus and 25 µg/ml against Escherichia coli, similar to the well-known antimicrobial peptide MSI-78. In contrast to MSI-78, the above mentioned compound has lower lytic effect against mammalian red blood cells. These peptidomimetic compounds will pave the way for future design of potent synthetic mimics of AMPs for therapeutic and biomedical applications.

Comparison of the effects of DC031050, a class III antiarrhythmic agent, on hERG channel and three neuronal potassium channels

AIM

This study was conducted to test the selectivity of DC031050 on cardiac and neuronal potassium channels.

METHODS

Human ether-à-go-go related gene (hERG), KCNQ and Kv1.2 channels were expressed in CHO cells. The delayed rectifier potassium current (I(K)) was recorded from dissociated hippocampal pyramidal neurons of neonatal rats. Whole-cell voltage patch clamp was used to record the voltage-activated potassium currents. Drug-containing solution was delivered using a RSC-100 Rapid Solution Changer.

RESULTS

Both DC031050 and dofetilide potently inhibited hERG currents with IC(50) values of 2.3 ± 1.0 and 17.9 ± 1.2 nmol/L, respectively. DC031050 inhibited the I(K) current with an IC(50) value of 2.7 ± 1.5 μmol/L, which was >1000 times the concentration required to inhibit hERG current. DC031050 at 3 μmol/L did not significantly affect the voltage-dependence of the steady activation, steady inactivation of I(K), or the rate of I(K) from inactivation. Intracellular application of DC031050 (5 μmol/L) was insufficient to inhibit I(K). DC031050 up to 10 μmol/L had no effects on KCNQ2 and Kv1.2 channel currents.

CONCLUSION

DC031050 is a highly selective hERG potassium channel blocker with a substantial safety margin of activity over neuronal potassium channels, thus holds significant potential for therapeutic application as a class III antiarrhythmic agent.

Cardiac ion channel safety profiling on the IonWorks Quattro automated patch clamp system

The normal electrophysiologic behavior of the heart is determined by the integrated activity of specific cardiac ionic currents. Mutations in genes encoding the molecular components of individual cardiac ion currents have been shown to result in multiple cardiac arrhythmia syndromes. Presently, 12 genes associated with inherited long QT syndrome (LQTS) have been identified, and the most common mutations are in the hKCNQ1 (LQT1, Jervell and Lange-Nielson syndrome), hKCNH2 (LQT2), and hSCN5A (LQT3, Brugada syndrome) genes. Several drugs have been withdrawn from the market or received black box labeling due to clinical cases of QT interval prolongation, ventricular arrhythmias, and sudden death. Other drugs have been denied regulatory approval owing to their potential for QT interval prolongation. Further, off-target activity of drugs on cardiac ion channels has been shown to be associated with increased mortality in patients with underlying cardiovascular diseases. Since clinical arrhythmia risk is a major cause for compound termination, preclinical profiling for off-target cardiac ion channel interactions early in the drug discovery process has become common practice in the pharmaceutical industry. In the present study, we report assay development for three cardiac ion channels (hKCNQ1/minK, hCa(v)1.2, and hNa(v)1.5) on the IonWorks Quattro™ system. We demonstrate that these assays can be used as reliable pharmacological profiling tools for cardiac ion channel inhibition to assess compounds for cardiac liability during drug discovery.

Novel approach to 1,5-benzodiazepine-2-ones containing peptoid backbone via one-pot diketene-based Ugi-4CR

An efficient and simple route for preparation of substituted 1,5-benzodiazepine-2-one containing peptoid backbone is presented. The classical Ugi reaction is considerably extended by application of o-phenylenediamine and diketene as amine and oxo component. 1,3-Dihydro-1,5-benzodiazepine-2-one is generated in situ from these two building blocks combined with isocyanide and aromatic or aliphatic carboxylic acid to assemble the multifunctionalized titled scaffold in high yields. The reaction is performed in the mixture of toluene/CH(2)Cl(2) under reflux condition without catalyst. Conformational isomerism is seen in the solution phase because of restricted free rotation around amide and C-CO bands due to steric bulk of substitutions. In single crystal state, the product is found to possess dimeric structure arising from intermolecular hydrogen bonding.

Understanding hERG inhibition with QSAR models based on a one-dimensional molecular representation

Blockage of the potassium channel encoded by the human ether-a-go-go related gene (hERG) is well understood to be the root cause of the cardio-toxicity of numerous approved and investigational drugs. As such, a cascade of in vitro and in vivo assays have been developed to filter compounds with hERG inhibitory activity. Quantitative structure activity relationship (QSAR) models are used at the very earliest part of this cascade to eliminate compounds that are likely to have this undesirable activity prior to synthesis. Here a new QSAR technique based on the one-dimensional representation is described in the context of the development of a model to predict hERG inhibition. The model is shown to perform close to the limits of the quality of the data used for model building. In order to make optimal use of the available data, a general robust mathematical scheme was developed and is described to simultaneously incorporate quantitative data, such as IC50 = 50 nM, and qualitative data, such as inactive or IC50 > 30 microM into QSAR models without discarding any experimental information.

Slow delayed rectifier potassium current (IKs) and the repolarization reserve

The aim of this review is to present the properties of the slow component of the delayed rectifier potassium current (IKs) in the human ventricle. The review gives a detailed description of the physiology, molecular biology and pharmacology of the IKs current, including kinetic properties, genetic structures, agonists and antagonists. The authors also present the role of the IKs current in the human cardiac repolarization focusing on several pathophysiological situations, such as the LQT syndrome and the Torsade de Pointes arrhythmia.

Respiratory syncytial virus fusion inhibitors. Part 4: optimization for oral bioavailability

A series of benzimidazole-based inhibitors of respiratory syncytial virus (RSV) fusion were optimized for antiviral potency, membrane permeability and metabolic stability in human liver microsomes. 1-Cyclopropyl-1,3-dihydro-3-[[1-(4-hydroxybutyl)-1H-benzimidazol-2-yl]methyl]-2H-imidazo[4,5-c]pyridin-2-one (6m, BMS-433771) was identified as a potent RSV inhibitor demonstrating good bioavailability in the mouse, rat, dog and cynomolgus monkey that demonstrated antiviral activity in the BALB/c and cotton rat models of infection following oral administration.

Enantioselective Synthesis of Diversely Substituted Quaternary 1,4-Benzodiazepin-2-ones and 1,4-Benzodiazepine-2,5-diones

Benzodiazepines are privileged scaffolds in medicinal chemistry, but enantiopure examples containing quaternary stereogenic centers are extremely rare. We demonstrate that installation of the di(p-anisyl)methyl (DAM) group at N1 of 1,4-benzodiazepin-2-ones and 1,4-benzodiazepine-2,5-diones derived from enantiopure proteinogenic amino acids allows retentive replacement of the C3-proton via a deprotonation/trapping protocol. A wide variety of carbon and nitrogen electrophiles function well in this reaction, providing the corresponding quaternary benzodiazepines with excellent enantioselectivity. Deprotonation/trapping experiments on a pair of diastereomeric 1,4-benzodiazepine-2,5-diones provide evidence for a key role of conformational chirality in these enantioselective reactions. Acidic removal of the DAM group is fast and high-yielding and can be performed selectively in the presence of a N-Boc indole. Thus the synthesis of quaternary benzodiazepines with diverse N1 functionality can now be accomplished.

Structural determinants of potassium channel blockade and drug-induced arrhythmias

Cardiac K+ channels play an important role in the regulation of the shape and duration of the action potential. They have been recognized as targets for the actions of neurotransmitters, hormones, and anti-arrhythmic drugs that prolong the action potential duration (APD) and increase refractoriness. However, pharmacological therapy, often for the purpose of treating syndromes unrelated to cardiac disease, can also increase the vul- nerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from inherited mutations or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proved useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K+ currents and prolong the Q-T interval on the electrocardiogram. Understanding the structural determinants of K+ channel blockade might provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.

Characterization of binding site of closed-state KCNQ1 potassium channel by homology modeling, molecular docking, and pharmacophore identification

This investigation was performed to assess the importance of interaction in the binding of blockers to KCNQ1 potassium using molecular modeling. This work could be considered made up by three main steps: (1) the construction of closed-state structure of KCNQ1 through homology modeling; (2) the automated docking of three blockers: IKS-142, L-735821, and BMS-IKS, using DOCK program; (3) the generation and validation of pharmacophore for KCNQ1 ligands using Catalyst/HypoGen. The obtained results highlight the hydrophobic or aromatic residues involved in S6 transmembrane domain and the base of the pore helix of KCNQ1, confirming the mutagenesis data and pharmacophore model, and giving new suggestions for the rational design of novel KCNQ1 ligands.

Experimental and Computational Studies of Ring Inversion of 1,4-Benzodiazepin-2-ones: Implications for Memory of Chirality Transformations

We recently reported the enantioselective syntheses of quaternary 1,4-benzodiazepin-2-ones via memory of chirality. The success of this method depends on formation of conformationally chiral enolates that racemize very slowly under the reaction conditions. As a prelude to undertaking experimental and computational studies on the racemization of these enolates, we have studied the ring-inversion process of the parent 1,4-benzodiazepin-2-ones. In this paper, we use dynamic and 2D-EXSY NMR to characterize inversion barriers. Using DFT calculations, we reproduce the experimental results with high accuracy (within 1-2 kcal/mol). Structural parameters obtained from DFT calculations provide valuable insights into the important effect of the N1 substituent on the ring-inversion barrier and shed light on the mechanism of the memory of chirality method. These measurements and calculations provide a foundation for future studies of benzodiazepine enolates and will be valuable in the design of new memory of chirality reactions.

Pharmacological modulation of ion channels and transporters

Ion channels and transporter proteins are prerequisites for formation and conduction of cardiac electrical impulses. Acting in concert, these proteins maintain cellular Na(+) and Ca(2+) homeostasis. Since intracellular Ca(2+) concentration determines contractile activation, we expect the majority of agents that modulate activity of ion channels and transporters not only to influence cellular action potentials but also contractile force. Drugs which block ion channels usually possess antiarrhythmic properties, those inhibiting the Na(+) pump have predominantly inotropic effects and those affecting Na(+),Ca(2+)- or Na(+),H(+)-exchanger protect against ischaemic cell damage. However, irrespective of their primary indication, all compounds targeted against ion channels and transporter proteins possess potential proarrhythmic activity.

Isoproterenol Exacerbates a Long QT Phenotype in Kcnq1-Deficient Neonatal Mice: Possible Roles for Human-Like Kcnq1 Isoform 1 and Slow Delayed Rectifier K+ Current

To determine whether the neonatal mouse can serve as a useful model for studying the molecular pharmacological basis of Long QT Syndrome Type 1 (LQT1), which has been linked to mutations in the human KCNQ1 gene, we measured QT intervals from electrocardiogram (ECG) recordings of wild-type (WT) and Kcnq1 knockout (KO) neonates before and after injection with the beta-adrenergic receptor agonist, isoproterenol (0.17 mg/kg, i.p.). Modest but significant increases in JT, QT, and rate-corrected QT (QTc) intervals were found in KO neonates relative to WT siblings during baseline ECG assessments (QTc = 57 +/- 3 ms, n = 22 versus 49 +/- 2 ms, n = 28, respectively, p < 0.05). Moreover, JT, QT, and QTc intervals significantly increased following isoproterenol challenge in the KO (p < 0.01) but not the WT group (p = 0.57). Furthermore, whole-cell patch-clamp recordings show that the slow delayed rectifier K+ current (IKs) was absent in KO but present in WT myocytes, where it was strongly enhanced by isoproterenol. This finding was confirmed by showing that the selective IKs inhibitor, L-735,821, blocked IKs and prolonged action potential duration in WT but not KO hearts. These data demonstrate that disruption of the Kcnq1 gene leads to loss of IKs, resulting in a long QT phenotype that is exacerbated by beta-adrenergic stimulation. This phenotype closely reflects that observed in human LQT1 patients, suggesting that the neonatal mouse serves as a valid model for this condition. This idea is further supported by new RNA data showing that there is a high degree of homology (>88% amino acid identity) between the predominant human and mouse cardiac Kcnq1 isoforms.

Tetrahydronaphthalene-derived amino alcohols and amino ketones as potent and selective inhibitors of the delayed rectifier potassium current IKs

Class III anti-arrhythmic drugs (e.g., dofetilide) prolong cardiac action potential duration (APD) by blocking the fast component of the delayed rectifier potassium current (I(Kr)). The block of I(Kr) can result in life threatening ventricular arrhythmias (i.e., torsades de pointes). Unlike I(Kr), the role of the slow component of the delayed rectifier potassium current (I(Ks)) becomes significant only at faster heart rate. Therefore selective blockers of I(Ks) could prolong APD with a reduced propensity to cause pro-arrhythmic side effects. This report describes structure-activity relationships (SARs) of a series of I(Ks) inhibitors derived from 6-alkoxytetralones with good in vitro activity (IC(50) > or =30 nM) and up to 40-fold I(Ks)/I(Kr) selectivity.

Pharmacological Activation of Normal and Arrhythmia-Associated Mutant KCNQ1 Potassium Channels

KCNQ1 α-subunits coassemble with KCNE1 β-subunits to form channels that conduct the slow delayed rectifier K + current ( I Ks ) important for repolarization of the cardiac action potential. Mutations in KCNQ1 reduce I Ks and cause long-QT syndrome, a disorder of ventricular repolarization that predisposes affected individuals to arrhythmia and sudden death. Current therapy for long-QT syndrome is inadequate. R-L3 is a benzodiazepine that activates I Ks and has the potential to provide gene-specific therapy. In the present study, we characterize the molecular determinants of R-L3 interaction with KCNQ1 channels, use computer modeling to propose a mechanism for drug-induced changes in channel gating, and determine its effect on several long-QT syndrome–associated mutant KCNQ1 channels heterologously expressed in Xenopus oocytes. Scanning mutagenesis combined with voltage-clamp analysis indicated that R-L3 interacts with specific residues located in the 5th and 6th transmembrane domains of KCNQ1 subunits. Most KCNQ1 mutant channels responded to R-L3 similarly to wild-type channels, but one mutant channel (G306R) was insensitive to R-L3 possibly because it disrupted a key component of the drug-binding site.