Inhibition of voltage-gated Na+ currents by eleclazine in rat atrial and ventricular myocytes.

BACKGROUND: Atrial-ventricular differences in voltage-gated Na+ currents might be exploited for atrial-selective antiarrhythmic drug action for the suppression of atrial fibrillation without risk of ventricular tachyarrhythmia. Eleclazine (GS-6615) is a putative antiarrhythmic drug with properties similar to the prototypical atrial-selective Na+ channel blocker ranolazine that has been shown to be safe and well tolerated in patients. OBJECTIVE: The present study investigated atrial-ventricular differences in the biophysical properties and inhibition by eleclazine of voltage-gated Na+ currents. METHODS: The fast and late components of whole-cell voltage-gated Na+ currents (respectively, I Na and I NaL) were recorded at room temperature (∼22°C) from rat isolated atrial and ventricular myocytes. RESULTS: Atrial I Na activated at command potentials ∼5.5 mV more negative and inactivated at conditioning potentials ∼7 mV more negative than ventricular I Na. There was no difference between atrial and ventricular myocytes in the eleclazine inhibition of I NaL activated by 3 nM ATX-II (IC50s ∼200 nM). Eleclazine (10 µM) inhibited I Na in atrial and ventricular myocytes in a use-dependent manner consistent with preferential activated state block. Eleclazine produced voltage-dependent instantaneous inhibition in atrial and ventricular myocytes; it caused a negative shift in voltage of half-maximal inactivation and slowed the recovery of I Na from inactivation in both cell types. CONCLUSIONS: Differences exist between rat atrial and ventricular myocytes in the biophysical properties of I Na. The more negative voltage dependence of I Na activation/inactivation in atrial myocytes underlies differences between the 2 cell types in the voltage dependence of instantaneous inhibition by eleclazine. Eleclazine warrants further investigation as an atrial-selective antiarrhythmic drug.

A Novel Mechanism for Calmodulin-Dependent Inactivation of Transient Receptor Potential Vanilloid 6.

The paralogues TRPV5 and TRPV6 belong to the vanilloid subfamily of the transient receptor potential (TRP) superfamily of ion channels, and both play an important role in overall Ca2+ homeostasis. The functioning of the channels centers on a tightly controlled Ca2+-dependent feedback mechanism in which the direct binding of the universal Ca2+-binding protein calmodulin (CaM) to the channel's C-terminal tail is required for channel inactivation. We have investigated this interaction at the atomic level and propose that under basal cellular Ca2+ concentrations CaM is constitutively bound to the channel's C-tail via CaM C-lobe only contacts. When the cytosolic Ca2+ concentration increases charging the apo CaM N-lobe with Ca2+, the CaM:TRPV6 complex rearranges and the TRPV6 C-tail further engages the CaM N-lobe via a crucial interaction involving L707. In a cellular context, mutation of L707 significantly increased the rate of channel inactivation. Finally, we present a model for TRPV6 CaM-dependent inactivation, which involves a novel so-called "two-tail" mechanism whereby CaM bridges two TRPV6 monomers resulting in closure of the channel pore.

Atrial-ventricular differences in rabbit cardiac voltage-gated Na+ currents: Basis for atrial-selective block by ranolazine.

BACKGROUND: Class 1 antiarrhythmic drugs are highly effective in restoring and maintaining sinus rhythm in atrial fibrillation patients but carry a risk of ventricular tachyarrhythmia. The antianginal agent ranolazine is a prototypic atrial-selective voltage-gated Na+ channel blocker but the mechanisms underlying its atrial-selective action remain unclear. OBJECTIVE: The present study examined the mechanisms underlying the atrial-selective action of ranolazine. METHODS: Whole-cell voltage-gated Na+ currents (INa) were recorded at room temperature (∼22°C) from rabbit isolated left atrial and right ventricular myocytes. RESULTS: INa conductance density was ∼1.8-fold greater in atrial than in ventricular cells. Atrial INa was activated at command potentials ∼7 mV more negative and inactivated at conditioning potentials ∼11 mV more negative than ventricular INa. The onset of inactivation of INa was faster in atrial cells than in ventricular myocytes. Ranolazine (30 µM) inhibited INa in atrial and ventricular myocytes in a use-dependent manner consistent with preferential activated/inactivated state block. Ranolazine caused a significantly greater negative shift in voltage of half-maximal inactivation in atrial cells than in ventricular cells, the recovery from inactivation of INa was slowed by ranolazine to a greater extent in atrial myocytes than in ventricular cells, and ranolazine produced an instantaneous block that showed marked voltage dependence in atrial cells. CONCLUSION: Differences exist between rabbit atrial and ventricular myocytes in the biophysical properties of INa. The more negative voltage dependence of INa activation and inactivation, together with trapping of the drug in the inactivated channel, underlies an atrial-selective action of ranolazine.

New pyrimido-indole compound CD-160130 preferentially inhibits the KV11.1B isoform and produces antileukemic effects without cardiotoxicity.

KV11.1 (hERG1) channels are often overexpressed in human cancers. In leukemias, KV11.1 regulates pro-survival signals that promote resistance to chemotherapy, raising the possibility that inhibitors of KV11.1 could be therapeutically beneficial. However, because of the role of KV11.1 in cardiac repolarization, blocking these channels may cause cardiac arrhythmias. We show that CD-160130, a novel pyrimido-indole compound, blocks KV11.1 channels with a higher efficacy for the KV11.1 isoform B, in which the IC50 (1.8 µM) was approximately 10-fold lower than observed in KV11.1 isoform A. At this concentration, CD-160130 also had minor effects on Kir2.1, KV 1.3, Kv1.5, and KCa3.1. In vitro, CD-160130 induced leukemia cell apoptosis, and could overcome bone marrow mesenchymal stromal cell (MSC)-induced chemoresistance. This effect was caused by interference with the survival signaling pathways triggered by MSCs. In vivo, CD-160130 produced an antileukemic activity, stronger than that caused by cytarabine. Consistent with its atypical target specificity, CD-160130 did not bind to the main binding site of the arrhythmogenic KV11.1 blockers (the Phe656 pore residue). Importantly, in guinea pigs CD-160130 produced neither alteration of the cardiac action potential shape in dissociated cardiomyocytes nor any lengthening of the QT interval in vivo. Moreover, CD-160130 had no myelotoxicity on human bone marrow-derived cells. Therefore, CD-160130 is a promising first-in-class compound to attempt oncologic therapy without cardiotoxicity, based on targeting KV11.1. Because leukemia and cardiac cells tend to express different ratios of the A and B KV11.1 isoforms, the pharmacological properties of CD-160130 may depend, at least in part, on isoform specificity.

Computational design and discovery of "minimally structured" hERG blockers.

Molecular knowledge of hERG blocking liability can offer the possibility of optimizing lead compounds in a way that eliminates potentially lethal side effects. In this study, we computationally designed, synthesized, and tested a small series of "minimally structured" molecules. Some of these compounds were remarkably potent against hERG (6, IC(50) = 2.4 nM), allowing us to identify the minimal structural requirements for hERG blocking liability.