Non-ischaemic dilated cardiomyopathy: recognising the genetic links.

The landscape of genetically related cardiac disease continues to evolve. Heritable genetic variants can be a primary cause of familial or sporadic dilated cardiomyopathy (DCM). There is also increasing recognition that genetic variation is an important determinant of susceptibility to acquired causes of DCM. Genetic forms of DCM can show a wide variety of phenotypic manifestations. Identifying patients who are most likely to benefit from genetic testing is paramount. The objective of this review is to highlight the importance of recognising genetic DCM, key genotype-phenotype correlations and the value of genetic testing in clinical management for both the individual and their family. This is likely to become more relevant as management strategies continue to be refined with genotype-specific recommendations and disease-modifying therapies.

Polymorphic Ventricular Tachycardia Storm After Coronary Artery Bypass Graft Surgery: A Form of 'Angry Purkinje Syndrome'.

BACKGROUND: Polymorphic ventricular tachycardia (PMVT) is a highly lethal arrhythmia which is commonly caused by acute myocardial ischaemia. PMVT mediated by short-coupled ventricular ectopy patients with ischaemic heart disease but in the absence of acute ischaemia may relate to transient peri-infarct Purkinje fibre irritability and has been termed 'Angry Purkinje Syndrome'. METHODS: We present a case series of three patients with PMVT storm 3-5 days following coronary artery bypass graft surgery (CABG). In all three cases, recurrent episodes of PMVT were initiated by monomorphic ventricular ectopy with a short coupling interval. Acute coronary ischaemia was excluded in all three patients with a coronary angiogram and graft study. Two out of three of the patients commenced oral quinidine sulphate with subsequent rapid suppression of arrhythmia. Implantable cardiac defibrillators were implanted in all three patients and revealed no recurrence of PMVT following hospital discharge. CONCLUSION: The Angry Purkinje Syndrome is a rare but important cause of ventricular tachycardia storm after CABG surgery and is mediated by short-coupled ventricular ectopy in the absence of acute myocardial ischaemia. This arrhythmia may be highly responsive to quinidine.

Incidence of Premature Lead Failure in 2088 TendrilTM Pacing Leads: A Single Centre Experience.

BACKGROUND: Recent reports describe a high rate of premature lead failure in the St Jude/Abbott TendrilTM 2088 (St. Jude Medical Inc., St. Paul, MN, USA) pacing lead principally manifested by electrical noise. This finding awaits confirmation. METHODS: We performed a retrospective analysis of 2088 TendrilTM leads among 362 patients implanted from 2010 to 2018. Eligible leads were those with device interrogations beyond one month from lead implantation. Review of serial device interrogations was conducted for each lead, particularly focussing on electrical noise as a marker of premature lead dysfunction. RESULTS: Four hundred and eight (408) leads among 337 patients were included in this study, with an average patient age of 81±11 years at the time of lead implantation. Mean follow-up was 2.5±1.8 years. There were eight leads with electrical noise indicating premature lead failure. This reflects an overall 1.7% rate of lead dysfunction; the failure rate was 6.2% at 4 years. The majority of cases were detected during routine checks without adverse clinical consequences. Four (4) cases required device reprogramming to avoid interference or inhibition due to noise. CONCLUSION: The rate of Tendril TM 2088 premature lead failure appears to be similar to recent local and international studies. This study reports a significantly higher rate of lead dysfunction at 4 years (6.2%) than the published Abbott product performance reports.

hERG K(+) channels: structure, function, and clinical significance.

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K(+) channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Kinetics of drug interaction with the Kv11.1 potassium channel.

The Kv11.1 potassium channel is the molecular target for the majority of drugs implicated in acquired long QT syndrome, the most common cause of drug-induced sudden cardiac death, and a common reason for drug restriction or withdrawal from the market. While the IC50 for block of Kv11.1 is commonly used to estimate the risk of acquired long QT syndrome, this approach is crude, and it is widely accepted that the kinetics of drug interactions with the channel are a critical component in understanding their mechanism of action and risk profiles. In this study we report the first directly measured kinetics of block and unblock of Kv11.1 by a QT prolonging drug: the antipsychotic clozapine. Our data show that clozapine binding to Kv11.1 is complex. There are at least two kinetically distinct components to both block and unblock, while the kinetics of unblock are dependent on the dose or duration of drug application. Based on these observations, we have proposed a model incorporating kinetically distinct binding to the open and inactivated states of Kv11.1 that can describe the observed kinetic features of clozapine block and correctly predict the overall affinity and apparent nonstate-dependent interaction of clozapine with Kv11.1. Mechanistic insights into drug block of Kv11.1 gained though detailed kinetic analyses such as this have a potential role in development of drugs targeted to specific channel states to reduce unwanted side effects, as well as in the design of better high-throughput preclinical tests for assessing the proarrhythmic effects of QT prolonging drugs.

Myocardial injury secondary to ICD shocks: insights from patients with lead fracture.

BACKGROUND: Patients who receive appropriate implantable cardioverter defibrillator (ICD) shocks have a subsequent adverse prognosis. Most data suggest that patients with inappropriate ICD shocks also have a subsequent adverse prognosis, although this is more controversial. The shocks may be an epiphenomenon, that is, a marker of underlying disease progression; however, it cannot be excluded that shocks cause direct myocardial damage. This latter question is difficult to clarify as the arrhythmia provoking the shock can also cause troponin release. Inappropriate shocks secondary to lead fracture are an ideal situation to examine this question; any troponin release in an otherwise well and hemodynamically stable patient, is likely due directly to the shocks. METHODS: All patients with Fidelis lead fracture admitted to our institution with inappropriate shocks were included in this study. Troponin (I or T) was considered positive if the level was above the 99th percentile reference cutoff. RESULTS: Elevated troponin levels were recorded in 16 of 22 patients (73%). Patients with elevated troponin received a higher number of shocks (20.3 ± 30.1 vs 5.3 ± 4.8, P = 0.07) compared with patients with normal troponin. Very elevated troponin levels (>0.8 mcg/L) were seen in five of 22 (22%) patients. The mean peak troponin level for these five patients was 7.06 ± 8.56 mcg/L; two patients had troponin levels that would be expected from a medium-sized myocardial infarction or severe myocarditis. CONCLUSION: Troponin elevation occurred in the majority of our patients after inappropriate ICD discharges secondary to lead fracture. This indicates that ICD shocks can cause myocardial injury.

Multisite Validation of a Functional Assay to Adjudicate SCN5A Brugada Syndrome-Associated Variants.

BACKGROUND: Brugada syndrome is an inheritable arrhythmia condition that is associated with rare, loss-of-function variants in SCN5A. Interpreting the pathogenicity of SCN5A missense variants is challenging, and ≈79% of SCN5A missense variants in ClinVar are currently classified as variants of uncertain significance. Automated patch clamp technology enables high-throughput functional studies of ion channel variants and can provide evidence for variant reclassification. METHODS: An in vitro SCN5A-Brugada syndrome automated patch clamp assay was independently performed at Vanderbilt University Medical Center and Victor Chang Cardiac Research Institute. The assay was calibrated according to ClinGen Sequence Variant Interpretation recommendations using high-confidence variant controls (n=49). Normal and abnormal ranges of function were established based on the distribution of benign variant assay results. Odds of pathogenicity values were derived from the experimental results according to ClinGen Sequence Variant Interpretation recommendations. The calibrated assay was then used to study SCN5A variants of uncertain significance observed in 4 families with Brugada syndrome and other arrhythmia phenotypes associated with SCN5A loss-of-function. RESULTS: Variant channel parameters generated independently at the 2 research sites showed strong correlations, including peak INa density (R2=0.86). The assay accurately distinguished benign controls (24/25 concordant variants) from pathogenic controls (23/24 concordant variants). Odds of pathogenicity values were 0.042 for normal function and 24.0 for abnormal function, corresponding to strong evidence for both American College of Medical Genetics and Genomics/Association for Molecular Pathology benign and pathogenic functional criteria (BS3 and PS3, respectively). Application of the assay to 4 clinical SCN5A variants of uncertain significance revealed loss-of-function for 3/4 variants, enabling reclassification to likely pathogenic. CONCLUSIONS: This validated high-throughput assay provides clinical-grade functional evidence to aid the classification of current and future SCN5A-Brugada syndrome variants of uncertain significance.

Degenerating regenerating torsades de pointes.

Ventricular fibrillation (VF) commonly ends in death. Isolated case reports describe the uncommon occurrence of spontaneous termination of VF. Torsades de pointes (TdP), a peculiar form of polymorphic ventricular tachycardia associated with a prolonged QT interval on the surface electrocardiogram, most often spontaneously terminates and then returns to the underlying rhythm. Here, we present an unusual case of TdP degenerating into VF, reorganizing into TdP, and then spontaneously terminating. Our case suggests that the mechanisms underlying the maintenance of TdP and VF are not dissimilar. The precipitants to this event and the likely mechanisms operative are discussed.

Return to Sport and Performance After Thumb Metacarpophalangeal Joint Collateral Ligament Surgery in the National Basketball Association.

Introduction Basketball players are at increased risk of thumb collateral ligament injury (ulnar collateral ligament (UCL) and radial collateral ligament (RCL)). Methods The National Basketball Association (NBA) players with thumb collateral ligament surgery were identified using publicly available data. Performance statistics, ligament injuries (UCL or RCL), return to sport (RTS) time, laterality, and injury dates were recorded. Cases were matched 1:1 with controls based on age (±1 year), body mass index (BMI), NBA experience (±1 year), and performance statistics prior to the index date. RTS was defined as playing in one NBA game postoperatively. Career longevity was evaluated. Summary statistics were calculated, and Student's t-tests (ɑ = 0.001) were performed. Results All 47 players identified with thumb collateral ligament surgeries returned to sport. Thirty-three players (age: 26.9 ± 3.0) had one year of postoperative NBA experience for performance analysis. Career length (case: 9.6 ± 4.1, control: 9.4 ± 4.3, p > 0.001) was not significantly different from controls (p > 0.001). The same season time to RTS (n = 20) was 7.1 ± 2.4 weeks. Off-season or season-ending surgery (n = 13) RTS time was 28.4 ± 18.7 weeks. Neither thumb collateral ligament (UCL, n = 7; RCL, n = 10; unknown, n = 16) had an identifiable difference between the groups when evaluating career length. Career length, games/season, and performance were not different for players who underwent surgery on their dominant thumb (63.6%, 21/33) compared to controls (p > 0.001). Conclusion RTS rate is high in NBA athletes undergoing thumb collateral ligament surgery. Players do not experience decreased performance or career length due to thumb collateral ligament surgery, regardless of a dominant or non-dominant thumb injury.

Multi-site validation of a functional assay to adjudicate SCN5A Brugada Syndrome-associated variants.

Brugada Syndrome (BrS) is an inheritable arrhythmia condition that is associated with rare, loss-of-function variants in the cardiac sodium channel gene, SCN5A. Interpreting the pathogenicity of SCN5A missense variants is challenging and ~79% of SCN5A missense variants in ClinVar are currently classified as Variants of Uncertain Significance (VUS). An in vitro SCN5A-BrS automated patch clamp assay was generated for high-throughput functional studies of NaV1.5. The assay was independently studied at two separate research sites - Vanderbilt University Medical Center and Victor Chang Cardiac Research Institute - revealing strong correlations, including peak INa density (R2=0.86). The assay was calibrated according to ClinGen Sequence Variant Interpretation recommendations using high-confidence variant controls (n=49). Normal and abnormal ranges of function were established based on the distribution of benign variant assay results. The assay accurately distinguished benign controls (24/25) from pathogenic controls (23/24). Odds of Pathogenicity values derived from the experimental results yielded 0.042 for normal function (BS3 criterion) and 24.0 for abnormal function (PS3 criterion), resulting in up to strong evidence for both ACMG criteria. The calibrated assay was then used to study SCN5A VUS observed in four families with BrS and other arrhythmia phenotypes associated with SCN5A loss-of-function. The assay revealed loss-of-function for three of four variants, enabling reclassification to likely pathogenic. This validated APC assay provides clinical-grade functional evidence for the reclassification of current VUS and will aid future SCN5A-BrS variant classification.

Derivation and validation of a simple exercise-based algorithm for prediction of genetic testing in relatives of LQTS probands.

BACKGROUND: Genetic testing can diagnose long-QT syndrome (LQTS) in asymptomatic relatives of patients with an identified mutation; however, it is costly and subject to availability. The accuracy of a simple algorithm that incorporates resting and exercise ECG parameters for screening LQTS in asymptomatic relatives was evaluated, with genetic testing as the gold standard. METHODS AND RESULTS: Asymptomatic first-degree relatives of genetically characterized probands were recruited from 5 centers. QT intervals were measured at rest, during exercise, and during recovery. Receiver operating characteristics were used to establish optimal cutoffs. An algorithm for identifying LQTS carriers was developed in a derivation cohort and validated in an independent cohort. The derivation cohort consisted of 69 relatives (28 with LQT1, 20 with LQT2, and 21 noncarriers). Mean age was 35±18 years, and resting corrected QT interval (QTc) was 466±39 ms. Abnormal resting QTc (females ≥480 ms; males ≥470 ms) was 100% specific for gene carrier status, but was observed in only 48% of patients; however, mutations were observed in 68% and 42% of patients with a borderline or normal resting QTc, respectively. Among these patients, 4-minute recovery QTc ≥445 ms correctly restratified 22 of 25 patients as having LQTS and 19 of 21 patients as being noncarriers. The combination of resting and 4-minute recovery QTc in a screening algorithm yielded a sensitivity of 0.94 and specificity of 0.90 for detecting LQTS carriers. When applied to the validation cohort (n=152; 58 with LQT1, 61 with LQT2, and 33 noncarriers; QTc=443±47 ms), sensitivity was 0.92 and specificity was 0.82. CONCLUSIONS: A simple algorithm that incorporates resting and exercise-recovery QTc is useful in identifying LQTS in asymptomatic relatives.

Greater response to cardiac resynchronization therapy in patients with true complete left bundle branch block: a PREDICT substudy.

AIMS: Cardiac resynchronization therapy (CRT) benefits patients with heart failure and a wide QRS complex. Still, one-third derive no clinical benefit and a majority of patients demonstrate no objective improvement of left ventricular (LV) function. Left bundle branch block (LBBB) is a strong predictor of response to CRT. We evaluated whether absence of electrocardiogram (ECG) markers of residual left bundle (LB) conduction in guideline-defined LBBB predicted a greater response to CRT. METHODS AND RESULTS: An r wave ≥1 mm in lead V1 (r-V1) and/or a q wave ≥1 mm in lead aVL (q-aVL) was used to identify patients with residual LB conduction. Forty patients with a wide QRS were prospectively enrolled and subdivided into three groups: complete LBBB (cLBBB), LBBB without r-V1 or q-aVL (n = 12); LBBB with residual LB conduction (rLBBB), LBBB with r-V1 and/or q-aVL (n = 15); and non-specific intraventricular conduction delay (IVCD), (n = 13). Following CRT: mean change in left ventricular ejection fraction was 11.9 ± 11.9% in cLBBB, 3.8 ± 5.4% in rLBBB (P= 0.045), and 2.5 ± 4.4% in IVCD (P= 0.02 cLBBB vs. IVCD); mean reduction in left ventricular end-systolic volume was 26.4 ± 39.2% in cLBBB, 14.3 ± 22.9% in rLBBB (P= 0.35), and 5.6 ± 17.3% in IVCD (P= 0.11 cLBBB vs. IVCD); mean change in native QRS duration was -8.0 ± 11.0 ms in cLBBB, -0.8 ± 8.24 ms in rLBBB (P= 0.07), and 0.15 ± 8.0 ms in IVCD (P= 0.048 cLBBB vs. IVCD). CONCLUSION: In patients with guideline-defined LBBB, the absence of ECG markers of residual LB conduction was predictive of a greater improvement in LV function with CRT.

Anticoagulation bridging around device surgery: compliance with guidelines.

BACKGROUND: Current guidelines recommend bridging anticoagulation in patients undergoing cardiac rhythm device surgery with a "moderate to high risk" of thromboembolism. Patients at "low risk" are advised to stop oral anticoagulation without bridging to the procedure. This study examines real world adherence to accepted guidelines and the clinical sequelae of nonadherence. METHODS: We performed a review of all patients undergoing device surgery receiving chronic anticoagulation over a prespecified time period of 14 months. Patients were classified per American College of Chest Physician guidelines as "moderate/high risk" or "low risk" of thromboembolism. We then compared perioperative management of anticoagulation to guideline recommendations and assessed the rate of perioperative bleeding and thromboembolism. RESULTS: One hundred and twenty-nine patients were included in this study. Sixty-two (48%) were classified as "moderate/high risk" and 67 (52%) "low risk." In the "moderate/high risk" group 47/62 (76%) received perioperative anticoagulation but only 25/62 (40%) were bridged both pre- and postprocedure or maintained on uninterrupted warfarin. In the "low risk" group, 22/67 (33%) received bridging therapy. Device pocket hematoma or perioperative bleeding occurred in 10/129 (8%) with 4/10 receiving inappropriate bridging for a calculated low risk of thromboembolism. There were no perioperative thromboembolisms. CONCLUSIONS: Our study identified significant underutilization of bridging, particularly in the postoperative period, in patients at "moderate/high risk" of thromboembolism. Conversely, bridging was overused in "low risk" patients and associated with bleeding complications. Physicians should be urged to follow current expert guidelines in regard to bridging anticoagulation for cardiac rhythm device surgery. (PACE 2012;35:1480-1486).

Arrhythmic Phenotypes Are a Defining Feature of Dilated Cardiomyopathy-Associated SCN5A Variants: A Systematic Review.

BACKGROUND: Variants in the SCN5A gene, that encodes the cardiac sodium channel, Nav1.5, are associated with a highly arrhythmogenic form of dilated cardiomyopathy (DCM). Our aim was to review the phenotypes, natural history, functional effects, and treatment outcomes of DCM-associated rare SCN5A variants. METHODS: A systematic review of reported DCM-associated rare SCN5A variants was undertaken using PubMed and Embase. RESULTS: Eighteen SCN5A rare variants in 29 families with DCM (173 affected individuals) were identified. Eleven variants had undergone experimental evaluation, with 7 of these resulting in increased sustained current flow during the action potential (eg, increased window current) and at resting membrane potentials (eg, creation of a new gating pore current). These variants were located in transmembrane voltage-sensing domains and had a consistent phenotype characterized by frequent multifocal narrow and broad complex ventricular premature beats (VPB; 72% of affected relatives), ventricular arrhythmias (33%), atrial arrhythmias (32%), sudden cardiac death (13%), and DCM (56%). This VPB-predominant phenotype was not seen with 1 variant that increased late sodium current, or with variants that reduced peak current density or had mixed effects. In the latter groups, affected individuals mainly showed sinus node dysfunction, conduction defects, and atrial arrhythmias, with infrequent VPB and ventricular arrhythmias. DCM did not occur in the absence of arrhythmias for any variant. Twelve studies (23 total patients) reported treatment success in the VPB-predominant cardiomyopathy using sodium channel-blocking drug therapy. CONCLUSIONS: SCN5A variants can present with a diverse spectrum of primary arrhythmic features. A majority of DCM-associated variants cause a multifocal VPB-predominant cardiomyopathy that is reversible with sodium channel blocking drug therapy. Early recognition of the distinctive phenotype and prompt genetic testing to identify variant carriers are needed. Our findings have implications for interpretation and management of SCN5A variants found in DCM patients with and without arrhythmias.

Covalent and non-covalent albumin binding of Au(i) bis-NHCs via post-synthetic amide modification.

Recent decades have witnessed the emergence of Au(i) bis-N-heterocyclic carbenes (NHCs) as potential anticancer agents. However, these systems exhibit little interaction with serum proteins (e.g., human serum albumin), which presumably impacts their pharmacokinetic profile and tumor exposure. Anticancer drugs bound to human serum albumin (HSA) often benefit from significant advantages, including longer circulatory half-lives, tumor targeted delivery, and easier administration relative to the drug alone. In this work, we present Au(i) bis-NHCs complexes, 7 and 9, capable of binding to HSA. Complex 7 contains a reactive maleimide moiety for covalent protein conjugation, whereas its congener 9 contains a naphthalimide fluorophore for non-covalent binding. A similar drug motif was used in both cases. Complexes 7 and 9 were prepared from a carboxylic acid functionalized Au(i) bis-NHC (complex 2) using a newly developed post-synthetic amide functionalization protocol that allows coupling to both aliphatic and aromatic amines. Analytical, and in vitro techniques were used to confirm protein binding, as well as cellular uptake and antiproliferative activity in A549 human lung cancer cells. The present findings highlight a hitherto unexplored approach to modifying Au(i) bis-NHC drug candidates for protein ligation and serve to showcase the relative benefits of covalent and non-covalent HSA binding.

PARP inhibitor-induced torsades de pointes in long QT syndrome: a case report.

BACKGROUND: Poly ADP-ribose polymerase (PARP) inhibitors target pathogenic BRCA mutations in chemotherapy-resistant malignancies. PARP inhibitors cause modest dose-dependent QT prolongation in the setting of a normal baseline QT interval. CASE SUMMARY: We describe a case of PARP inhibitor-induced torsades de pointes (TdP) in an 86-year-old gentleman prescribed rucaparib due to chemotherapy-resistant, metastatic prostate cancer with pre-existing long QT, with an apparent dose-dependent increase in QT interval. The patient presented with syncope and recurrent TdP requiring direct cardioversion reversion (200 J biphasic) and an isoprenaline infusion (2 µg/min). There were no other QT prolonging agents and no electrolyte or metabolic disturbance to account for this arrhythmia. Improvement in QT interval was observed within 72 h of rucaparib cessation. DISCUSSION: PARP inhibitors cause a modest, dose-dependent increase in QT interval in patients with a normal baseline. The safety of PARP inhibitors in patients with pre-existing long QT has not been evaluated. This is the first reported case of rucaparib-associated TdP in a patient with pre-existing long QT, highlighting the amplified effect of this agent in individuals with pre-existing QT prolongation and the risk of fatal arrhythmias.

Toward multifunctional anticancer therapeutics: post-synthetic carbonate functionalisation of asymmetric Au(i) bis-N-heterocyclic carbenes.

A post-synthetic strategy is reported that allows for functionalisation of Au(i)-bis NHCs via carbonate formation. The scope of this methodology was explored using both aromatic and aliphatic alcohols. As a demonstration of potential utility, the fluorescent Au(i)-bis NHC conjugate 5 was prepared; it was found to have enhanced stability when formulated with bovine serum albumin, localise within the mitochondria of A549 cells and do so without compromising the high cytotoxicity seen for the parent Au(i)-bis NHC system.

Human ether-a-go-go related gene (hERG) K+ channels: function and dysfunction.

The human Ether-a-go-go Related Gene (hERG) potassium channel plays a central role in regulating cardiac excitability and maintenance of normal cardiac rhythm. Mutations in hERG cause a third of all cases of congenital long QT syndrome, a disorder of cardiac repolarisation characterised by prolongation of the QT interval on the surface electrocardiogram, abnormal T waves, and a risk of sudden cardiac death due to ventricular arrhythmias. Additionally, the hERG channel protein is the molecular target for almost all drugs that cause the acquired form of long QT syndrome. Advances in understanding the structural basis of hERG gating, its traffic to the cell surface, and the molecular architecture involved in drug-block of hERG, are providing the foundation for rational treatment and prevention of hERG associated long QT syndrome. This review summarises the current knowledge of hERG function and dysfunction, and the areas of ongoing research.

Drug binding to the inactivated state is necessary but not sufficient for high-affinity binding to human ether-à-go-go-related gene channels.

Drug block of the human ether-à-go-go-related gene K(+) channel (hERG) is the most common cause of acquired long QT syndrome, a disorder of cardiac repolarization that may result in ventricular tachycardia and sudden cardiac death. We investigated the open versus inactivated state dependence of drug block by using hERG mutants N588K and N588E, which shift the voltage dependence of inactivation compared with wild-type but in which the mutated residue is remote from the drug-binding pocket in the channel pore. Four high-affinity drugs (cisapride, dofetilide, terfenadine, and astemizole) demonstrated lower affinity for the inactivation-deficient N588K mutant hERG channel compared with N588E and wild-type hERG. Three of four low-affinity drugs (erythromycin, perhexiline, and quinidine) demonstrated no preference for N588E over N588K channels, whereas dl-sotalol was an example of a low-affinity state-dependent blocker. All five state-dependent blockers showed an even lower affinity for S620T mutant hERG (no inactivation) compared with N588K mutant hERG (greatly reduced inactivation). Computer modeling indicates that the reduced affinity for S620T compared with N588K and wild-type channels can be explained by the relative kinetics of drug block and unblock compared with the kinetics of inactivation and recovery from inactivation. We were also able to calculate, for the first time, the relative affinities for the inactivated versus the open state, which for the drugs tested here ranged from 4- to 70-fold. Our results show that preferential binding to the inactivated state is necessary but not sufficient for high-affinity binding to hERG channels.