Emerging therapeutic targets in the short QT syndrome

Introduction: Short QT Syndrome (SQTS) is a rare but dangerous condition characterised by abbreviated repolarisation, atrial and ventricular arrhythmias and risk of sudden death. Implantable cardioverter defibrillators (ICDs) are a first line protection against sudden death, but adjunct pharmacology is beneficial and desirable.

Areas covered: The genetic basis for genotyped SQTS variants (SQT1-SQT8) and evidence for arrhythmia substrates from experimental and simulation studies are discussed. The main ion channel/transporter targets for antiarrhythmic pharmacology are considered in respect of potential genotype-specific and non-specific treatments for the syndrome.

Expert opinion: Potassium channel blockade is valuable for restoring repolarisation and QT interval, though genotype-specific limitations exist in the use of some K⁺ channel inhibitors. A combination of K⁺ current inhibition during the action potential plateau, with sodium channel inhibition that collectively result in delaying repolarisation and post-repolarisation refractoriness is likely to be valuable in prolonging effective refractory period and wavelength for re-entry. Genotype-specific K⁺ channel inhibition is limited by a lack of targeted inhibitors in clinical use, though experimentally available selective inhibitors now exist. The relatively low proportion of successfully genotyped cases justifies an exome or genome sequencing approach, to reveal new mediators and targets, as demonstrated recently for SLC4A3 in SQT8.     

Synthesis and Biological Evaluation of Pyrazolo[1,5-a]pyrimidine Compounds as Potent and Selective Pim-1 Inhibitors

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Genome sequencing in a genetically elusive multigenerational long QT syndrome pedigree identifies a novel LQT2-causative deeply intronic KCNH2 variant

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Investigation of Cardiovascular Effects of Tetrahydro-β-carboline sstr3 antagonists

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Early drug development: assessment of proarrhythmic risk and cardiovascular safety

Introduction: hERG assays and thorough ECG trials have been mandated since 2005 to evaluate the QT interval and potential proarrhythmic risk of new chemical entities. The high cost of these studies and the shortcomings inherent in these binary and limited approaches to drug evaluation have prompted regulators to search for more cost effective and mechanistic paradigms to assess drug liability as exemplified by the CiPA initiative and the exposure response ICH E14(R3) guidance document.

Areas covered: This review profiles the changing regulatory landscape as it pertains to early drug development and outlines the analyses that can be performed to characterize preclinical and early clinical cardiovascular risk.

Expert commentary: It is further acknowledged that the narrow focus on the QT interval needs to be expanded to include a more comprehensive evaluation of cardiovascular risk since unanticipated off target effects have led to the withdrawal of multiple drugs after they had been approved and marketed.     

A novel assessment of nefazodone-induced hERG inhibition by electrophysiological and stereochemical method

Nefazodone was used widely as an antidepressant until it was withdrawn from the U.S. market in 2004 due to hepatotoxicity. We have investigated methods to predict various toxic effects of drug candidates to reduce the failure rate of drug discovery. An electrophysiological method was used to assess the cardiotoxicity of drug candidates. Small molecules, including withdrawn drugs, were evaluated using a patch-clamp method to establish a database of hERG inhibition. Nefazodone inhibited hERG channel activity in our system. However, nefazodone-induced hERG inhibition indicated only a theoretical risk of cardiotoxicity. Nefazodone inhibited the hERG channel in a concentration-dependent manner with an IC₅₀ of 45.3 nM in HEK-293 cells. Nefazodone accelerated both the recovery from inactivation and its onset. Nefazodone also accelerated steady-state inactivation, although it did not modify the voltage-dependent character. Alanine mutants of hERG S6 and pore region residues were used to identify the nefazodone-binding site on hERG. The hERG S6 point mutants Y652A and F656A largely abolished the inhibition by nefazodone. The pore region mutant S624A mildly reduced the inhibition by nefazodone but T623A had little effect. A docking study showed that the aromatic rings of nefazodone interact with Y652 and F656 via π–π interactions, while an amine interacted with the S624 residue in the pore region. In conclusion, Y652 and F656 in the S6 domain play critical roles in nefazodone binding.     

Bufalin,a bufanolide steroid from the parotoid glands of the Chinese toad,suppresses hERG K+ currents expressed in HEK293 cells

In this study, we investigated the effect of bufalin on the human ether‐à‐go‐go‐related gene (hERG) K⁺ channels using the perforated patch recording technique. We measured a half‐maximal inhibitory concentration (IC₅₀) of 24.83 μM and maximal inhibitory effect of 39.45 ± 1.14% with bufalin. These findings suggest that bufalin is a potent hERG K⁺ channel blocker and may provide a new way for understanding Chan Su‐induced arrhythmia.     

Proarrhythmic risk assessment using conventional and new in vitro assays

Drug-induced QT prolongation is a major safety issue in the drug discovery process. This study was conducted to assess the electrophysiological responses of four substances using established preclinical assays usually used in regulatory studies (hERG channel or Purkinje fiber action potential) and a new assay (human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs)-field potential).After acute exposure, moxifloxacin and dofetilide concentration-dependently decreased I_Kr amplitude (IC₅₀ values: 102 μM and 40 nM, respectively) and lengthened action potential (100 μM moxifloxacin: +23% and 10 nM dofetilide: +18%) and field potential (300 μM moxifloxacin: +76% and 10 nM dofetilide: +38%) durations. Dofetilide starting from 30 nM induced arrhythmia in hiPSC-CMs. Overnight application of pentamidine (10 and 100 μM) and arsenic (1 and 10 μM) decreased I_Kr, whereas they were devoid of effects after acute application. Long-term pentamidine incubation showed a time- and concentration-dependent effect on field potential duration.In conclusion, our data suggest that hiPSC-CMs represent a fully functional cellular electrophysiology model which may significantly improve the predictive validity of in vitro safety studies. Thereafter, lead candidates may be further investigated in patch-clamp assays for mechanistic studies on individual ionic channels or in a multicellular Purkinje fiber preparation for confirmatory studies on cardiac conduction.     

不同心血管风险的临床药物对hERG钾通道的作用研究

摘要：目的在GLP实验室评估膜片钳检测临床用药物对hERG钾通道作用的差异性和重复性,研究9种致尖端扭转性室速（TdP）风险的临床用药物（高风险临床用药物：苄普地尔、奎尼丁、索他洛尔;中风险临床用药物：昂丹司琼、西沙比利、特非那定;低风险临床用药物：雷诺嗪、维拉帕米和美西律）对hERG钾通道的阻断作用。方法采用全细胞膜片钳技术记录不同浓度的苄普地尔、奎尼丁、索他洛尔、昂丹司琼、西沙比利、特非那定、雷诺嗪、维拉帕米和美西律作用于外源稳定转染表达hERG钾通道的HEK293细胞（hERG-HEK293稳态细胞）后hERG电流（IKr）的变化,研究上述临床用药物对IKr作用的浓度依赖性及半数抑制浓度（IC50）。结果9种临床用药物对hERG-HEK293细胞上IKr作用具有浓度依赖性,且高风险临床用药物苄普地尔和奎尼丁的IC50值分别约为98.32 nmol?L-1和1.95 μmol?L-1,索他洛尔的IC50值大于300 μmol?L-1;中风险临床用药物昂丹司琼、西沙比利和特非那定的IC50值分别约为0.94 μmol?L-1、39.10 nmol?L-1和128.58 nmol?L-1;低风险临床用药物雷诺嗪、维拉帕米和美西律的IC50值分别约为9.94 μmol?L-1、235.49 nmol?L-1和65.56 μmol?L-1。本实验所得临床用药物IC50值基本与文献基本相符。结论临床用药物致TdP风险与hERG通道的阻滞作用密切相关,但hERG通道阻断不等同于TdP风险,还与心脏上表达的多种离子通道有关,某些临床用药物可以通过阻断钠通道和钙通道而降低风险。本研究结果提示本方法所得数据可靠,为国内GLP实验室进行hERG钾通道评价研究提供了参考依据,可用于药物心脏毒性评价。