Mechanisms of arsenic-induced prolongation of cardiac repolarization

Arsenic Inhibits CFTR-Mediated Chloride Secretion by Killifish (Fundulus heteroclitus) Opercular Membrane

Killifish are euryhaline teleosts that normally experience rapid changes in the salinity of the swim water. Acclimation to seawater is mediated by cortisol, which by activating glucocorticoid receptors, upregulates CFTR mediated Cl- secretion in the gill and operculum. Arsenic, a toxic metalloid that naturally occurs in the aquatic environment, has been shown to disrupt glucocorticoid hormone-mediated regulation of genes. Because little is known about the effects of environmentally relevant levels of arsenic on ion channels and salt homeostasis, studies were conducted to examine the effects of arsenic on the ability of killifish to acclimate to increased salinity. Arsenic in the swim water or administered by intraperitoneal injection prevented acclimation. To determine if arsenic blocked acclimation by inhibiting CFTR mediated Cl- secretion (Isc), opercular membranes were isolated and mounted in Ussing chambers and the effects of arsenic on Isc were measured. Arsenic (24 hr exposure) reduced Isc in opercular membranes isolated from salt water acclimated killifish. In addition, arsenic acutely (5-10 minutes) and reversibly inhibited Isc with an IC50 = 4.1 microM (305 ppb) when applied to the apical (seawater) side of the operculum, but not when added to the basolateral side of the operculum. Arsenic (4 microM for 60 minutes) also reduced mitochondrial respiration. Thus, environmentally relevant levels of arsenic block acclimation to seawater in killifish by reversibly inhibiting CFTR-mediated Cl- secretion by the opercular membrane, in part by inhibiting mitochondrial respiration.

Effects of phosphodiesterase (PDE) inhibitors on human ether-a-go-go related gene (hERG) channel activity

It is presumed that phosphodiesterase (PDE) inhibitors have two mechanisms for inhibition of hERG currents in the acute applications to cells: direct channel block, and downregulation of human ether-a-go-go related gene (hERG) activities by PKA-dependent pathway mediated phosphorylation through their inhibitory effects against PDE enzymes. However, it is unknown whether PDE inhibition contributes to the inhibitory effects of PDE inhibitors on hERG currents. This study examined the effects of various PDE inhibitors on hERG currents using both the whole-cell and perforated patch-clamp techniques in hERG transfected CHO-K1 cells. The study also investigated the contribution of the PKA-dependent pathway to the inhibitory effects of PDE inhibitors on hERG currents. Of the PDE inhibitors tested, vinpocetine, erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), vesnarinone, rolipram and dipyridamole decreased hERG currents in a concentration-dependent manner. Vinpocetine and vesnarinone markedly decreased the hERG current with an IC (50)of 0.13 and 20.6 microm, respectively, at comparatively low concentrations. Furthermore, vinpocetine caused a cumulative block of hERG currents. Milrinone, amrinone and zaprinast had no effect on the hERG current up to 100 microm. Of the PDE3 inhibitors (vesnarinone, amrinone and milrinone), only vesnarinone showed an hERG inhibitory effect. The inhibitory effects of vinpocetine and vesnarinone were not significantly affected by the co-application of protein kinase inhibitors. Furthermore, the protein kinase activators had no effect on hERG currents. It is concluded that vinpocetine and vesnarinone block the hERG channel directly, and that the inhibitory effect on intracellular PDE in the PKA-dependent pathway may not be involved in the inhibition of hERG currents in hERG transfected CHO-K1 cells.

Long QT syndrome: reduced repolarization reserve and the genetic link

Marked QT prolongation and torsades de pointes can occur not only in the congenital long QT syndromes (LQTSs) but also as a consequence of environmental stimuli, notably administration of certain drugs. A key feature of this 'acquired' form of the LQTS has been its unpredictable nature. That is, although risk factors have been identified in series of patients, they have not been terribly useful in addressing risk in an individual patient. Normal cardiac repolarization depends critically on the interplay of multiple ion currents, and these provide some redundancy, or 'reserve', to protect against excessive QT prolongation by drugs. We have proposed that lesions in these repolarizing mechanisms can remain subclinical but nevertheless increase risk on drug exposure, and have termed this situation 'reduced repolarization reserve'. The evidence in support of this concept is presented, and the known and potential contributions by genetic variants to risk is examined. Assessing variability in susceptibility to acquired LQTS provides a framework for analysis of other complex gene-environment interactions.

Are hERG channel inhibition and QT interval prolongation all there is in drug-induced torsadogenesis? A review of emerging trends

Contemporary preclinical in vitro and in vivo methods have been imperfect in predicting drug-induced Torsades de Pointes (TdP) in humans. A better understanding of additional relevant factors in the genesis of drug-induced TdP is necessary. New sophisticated in vitro techniques, such as arterially perfused ventricular wedge preparations or isolated perfused hearts, potentially offer a better understanding of torsadogenic mechanisms and a refinement of drug testing. Of particular interest are the dispersion of repolarization and the refractoriness of different cell types across the ventricular wall, triangulation of the action potential, reverse use dependence and instability of the action potential duration. In vivo models are currently refined by establishing parameters such as beat-to-beat variability and T-wave morphology as derived from the in vitro proarrhythmia indices. Animal models of proarrhythmia are to date not recommended for routine evaluation. A pharmacodynamic interaction with combinations of torsadogenic compounds is another area to be considered. Little is known about channel/receptor cross talk, although considerable evidence exists that cardiac G protein-coupled receptors can modulate hERG channel function. More investigations are necessary to further evaluate the role of altered gene expression, mutations, and polymorphisms in drug-induced TdP. A novel mechanism of drug-induced torsadogenesis is the reduced expression of hERG channel protein on the plasma membrane due to a trafficking defect. Pharmacokinetic and metabolism data are crucial for calculating the risk of a torsadogenic potential in man. Consideration of intracardiac accumulation can help in delineating pharmacokinetic-pharmacodyamic relationships. In silico virtual screening procedures with new chemical entities to predict hERG block may develop as a promising tool. The role of in silico modeling of TdP arrhythmia is likely to become increasingly important for organizing and integrating the vast amount of generated data. At present, however, in silico methods cannot replace existing preclinical in vitro and in vivo models.

Comparative Evaluation of HERG Currents and QT Intervals following Challenge with Suspected Torsadogenic and Nontorsadogenic Drugs

The purpose of the present study was to comparatively evaluate human HERG currents and QT intervals following challenge with suspected torsadogenic and nontorsadogenic drugs. Various concentrations of 14 different drugs were initially evaluated in terms of their relative potency to block I(HERG) in stably transfected human embryonic kidney cells. Four general categories of drugs were identified: high-potency blockers (IC50 < 0.1 microM) included lidoflazine, terfenadine, and haloperidol; moderate-potency blockers (0.1 microM < IC50 < 1 microM) included sertindole, thioridazine, and prenylamine; low-potency blockers (IC50 > 1 microM) included propafenone, loratadine, pyrilamine, lovastatin, and chlorpheniramine; and ineffective blockers (IC50 > 300 microM) included cimetidine, pentamidine, and arsenic trioxide. All measurements were performed using similar conditions and tested acute drug effects only (<30 min of drug exposure per measurement). Since two of the drugs that were ineffective I(HERG) blockers, arsenic trioxide and pentamidine, have been associated with cardiac repolarization delays (QT interval lengthening) and torsades de pointes ventricular arrhythmias in patients, we chose to evaluate them further using the isolated perfused rabbit heart model. Neither arsenic trioxide nor pentamidine had any significant effect on QT intervals in this model, even at relatively high (micromolar) concentrations. Similar results were obtained for loratadine in this model. When the hearts were challenged with a known torsadogenic drug such as cisapride, significant QT lengthening was rapidly induced. These results demonstrate that arsenic trioxide and pentamidine are essentially devoid of direct acute effects on cardiac repolarization or inhibition of I(HERG).

Refining detection of drug-induced proarrhythmia: QT interval and TRIaD

QT interval prolongation is so frequently associated with torsades de pointes (TdP) that it has come to be recognized as a surrogate marker of this unique tachyarrhythmia. However, not only does TdP not always follow QT interval prolongation, but TdP can occur even in the absence of a prolonged QT interval. Worse still, even shortening of the QT interval may be associated with serious arrhythmias (particularly ventricular tachycardia [VT] and ventricular fibrillation [VF]). It appears increasingly probable that the distinction between various ventricular tachyarrhythmias may be arbitrary, and drug-induced TdP, polymorphic VT, VT, catecholaminergic polymorphic VT, and VF may represent discrete entities within a spectrum of drug-induced proarrhythmia. Although they are differentiated by the coupling interval and the duration of QT interval, they appear to share a common substrate: a set of disturbances of repolarization characterized by Triangulation, Reverse use dependency, electrical Instability of the action potential, and Dispersion (TRIaD). It is becoming increasingly evident that augmentation of TRIaD, rather than changes in the duration of QT interval, provides the proarrhythmic substrate. In contrast, when not associated with an increase of TRIaD, QT interval prolongation can be an antiarrhythmic property. Electrophysiologically, augmentation of TRIaD can be explained by inhibition of hERG (human ether-a-go-go related gene) channel. Because drug-induced disturbances in repolarization commonly result from inhibition of hERG channels or I(Kr), hERG blockade and the resulting prolongation of QT interval are important properties of a drug to be studied. However, these need only be a concern if associated with TRIaD. More significantly, TRIaD so often precedes prolongation of action potential duration or QT interval and ventricular tachyarrhythmias that it should be considered a marker of proarrhythmia until proven otherwise, even in the absence of QT interval prolongation. Detecting drug-induced augmentation of TRIaD may offer an additional, more sensitive, and accurate indicator of the broader proarrhythmic potential of a drug than may QT interval prolongation alone.

HERG block, QT liability and sudden cardiac death

Non-cardiac drugs may prolong action potential duration (APD) and QT leading to Torsade de Pointes (TdP) and sudden cardiac death. TdP is rare and QT is used as a surrogate marker in the clinic. For non-cardiac drugs, APD/QT liability is always associated with a reduction in hERG current produced by either direct channel block or inhibition of trafficking. hERG and APD liabilities correlate better when APDs are measured in rabbit versus canine Purkinje fibres. hERG and APD/QT liabilities may be dissociated when hERG block is offset by block of calcium or sodium currents. hERG liability may be placed in context by calculating a safety margin (SM) from the IC50 for inhibition of hERG current measured by patch clamp divided by the effective therapeutic plasma concentration of the drug. The SM is uncertain because literature values for IC50 may vary by 50-fold and small differences in plasma protein binding have large effects. With quality control, the IC50 95% confidence limits vary less than twofold. Ideally, hERG liability should be determined during lead optimization. Patch damp has insufficient throughput for this purpose. A novel high-throughput screen has been developed to detect drugs that block hERG directly and/or inhibit hERG trafficking.

Kv1.5 surface expression is modulated by retrograde trafficking of newly endocytosed channels by the dynein motor

In this article we have investigated the mechanisms by which retrograde trafficking regulates the surface expression of the voltage-gated potassium channel, Kv1.5. Overexpression of p50/dynamitin, known to disrupt the dynein-dynactin complex responsible for carrying vesicle cargo, substantially increased outward K+ currents in HEK293 cells stably expressing Kv1.5 (0.57+/-0.07 nA/pF, n=12; to 1.18+/-0.2 nA/pF, n=12, P<0.01), as did treatment of the cells with a dynamin inhibitory peptide, which blocks endocytosis. Nocodazole pretreatment, which depolymerizes the microtubule cytoskeleton along which dynein tracks, also doubled Kv1.5 currents in HEK cells and sustained K+ currents in isolated rat atrial myocytes. These increased currents were blocked by 1 mmol/L 4-aminopyridine, and the specific Kv1.5 antagonist, DMM (100 nM). Confocal imaging of both HEK cells and myocytes, as well as experiments testing the sensitivity of the channel in living cells to external Proteinase K, showed that this increase of K+ current density was caused by a redistribution of channels toward the plasma membrane. Coimmunoprecipitation experiments demonstrated a direct interaction between Kv1.5 and the dynein motor complex in both heterologous cells and rat cardiac myocytes, supporting the role of this complex in Kv1.5 trafficking, which required an intact SH3-binding domain in the Kv1.5 N terminus to occur. These experiments highlight a pathway for Kv1.5 internalization from the cell surface involving early endosomes, followed by later trafficking by the dynein motor along microtubules. This work has significant implications for understanding the way Kv channel surface expression is regulated.

Protein trafficking abnormalities: a new mechanism in drug-induced long QT syndrome

Drug induced long QT syndrome (LQTS) can lead to cardiac arrhythmias and sudden death, and has emerged as a worldwide problem. Most drugs that cause this are thought to directly block a specific cardiac ion channel (KCNH2 or hERG) that carries the rapidly activating delayed rectifier potassium current, I(Kr). In this issue of the British Journal of Pharmacology, evidence is presented to support a new mechanism for causing drug induced LQTS. The drug pentamidine, at near therapeutic concentrations that do not cause direct KCNH2 channel block, disrupts normal KCNH2 channel protein processing and maturation to reduce its surface membrane expression. This indirect mechanism for reducing I(Kr) is novel, and whether other drugs may cause similar protein trafficking abnormalities is largely unknown.

Pentamidine reduces hERG expression to prolong the QT interval

Pentamidine, an antiprotozoal agent, has been traditionally known to cause QT prolongation and arrhythmias; however, its ionic mechanism has not been illustrated. In a stable HEK-293 cell line, we observed a concentration-dependent inhibition of the hERG current with an IC50 of 252 microM. In freshly isolated guinea-pig ventricular myocytes, pentamidine showed no effect on the L-type calcium current at concentrations up to 300 microM, with a slight prolongation of the action potential duration at this concentration. Since the effective concentrations of pentamidine on the hERG channel and APD were much higher than clinically relevant exposures (approximately 1 microM free or lower), we speculated that this drug might not prolong the QT interval through direct inhibition of I(Kr) channel. We therefore incubated hERG-HEK cells in 1 and 10 microM pentamidine-containing media (supplemented with 10% serum) for 48 h, and examined the hERG current densities in the vehicle control and pentamidine-treated cells. In all, 36 and 85% reductions of the current densities were caused by 1- and 10-microM pentamidine treatment (P<0.001 vs control), respectively. A similar level of reduction of the hERG polypeptides and a reduced intensity of the hERG protein on the surface membrane in treated cells were observed by Western blot analysis and laser-scanning confocal microscopy, respectively. Taken together, our data imply that chronic administration of pentamidine at clinically relevant exposure reduces the membrane expression of the hERG channel, which may most likely be the major mechanism of QT prolongation and torsade de pointes reported in man.

HERG-Lite®: A novel comprehensive high-throughput screen for drug-induced hERG risk

INTRODUCTION

Direct block of I(Kr) by non-antiarrhythmic drugs (NARDs) is a major cause of QT prolongation and torsades de pointes (TdP), and has made the hERG potassium channel a major target of drug safety programs in cardiotoxicity. Block of hERG currents is not the only way that drugs can adversely impact the repolarizing current I(Kr), however. We have shown recently that two drugs in clinical use do not block hERG but produce long QT syndrome (LQTS) and TdP by inhibiting trafficking of hERG to the cell surface. To address the need for an inexpensive, rapid, and comprehensive assay to predict both types of hERG risk early in the drug development process, we have developed a novel antibody-based chemiluminescent assay called HERG-Lite.

METHODS

HERG-Lite monitors the expression of hERG at the cell surface in two different stable mammalian cell lines. One cell line acts as a biosensor for drugs that inhibit hERG trafficking, while the other predicts hERG blockers based on their ability to act as pharmacological chaperones. In this study, we have validated the HERG-Lite assay using a panel of 100 drugs: 50 hERG blockers and 50 nonblockers.

RESULTS

HERG-Lite correctly predicted hERG risk for all 100 test compounds with no false positives or negatives. All 50 hERG blockers were detected as drugs with hERG risk in the HERG-Lite assay, and fell into two classes: B (for blocker) and C (for complex; block and trafficking inhibition).

DISCUSSION

HERG-Lite is the most comprehensive assay available for predicting drug-induced hERG risk. It accurately predicts both channel blockers and trafficking inhibitors in a rapid, cost-effective manner and is a valuable non-clinical assay for drug safety testing.

Acute promyelocytic leukemia: recent advances in therapy and molecular basis of response to arsenic therapies

PURPOSE OF REVIEW

While arsenic has long been known as a poison and environmental carcinogen, its dramatic effect in the treatment of acute promyelocytic leukemia (APL) has made its mechanism of action a topic of intense interest. This paper reviews recent findings that reveal why a traditional poison has become a magical potion for a major type of APL, which is characterized by a balanced chromosomal translocation t(15;17).

RECENT FINDINGS

Daily IV infusion of arsenic trioxide (As2O3; ATO) for 30 to 40 days can lead to complete remission in about 85% of patients with newly diagnosed or relapsed APL. Oral preparations of ATO and tetra-arsenic tetra-sulfide (As4S4) seem to be as effective as parenteral ATO, with similar toxicity profiles. The combination of all-trans retinoic acid and ATO in patients with newly diagnosed APL has yielded more durable remission than monotherapy. The mechanism of arsenic cytotoxicity is thought to involve posttranslational modification followed by degradation of the PML-retinoic acid receptor-alpha (PML-RARalpha) fusion protein; targeting of PML to nuclear bodies with restoration of its physiologic functions; and production of reactive oxygen species (ROS) by NADPH oxidase in leukemic cells or collapse of the mitochondrial transmembrane potential. The understanding of arsenic cytotoxicity has stimulated modifications that promise to improve efficacy, such as interfering with ROS scavenging or boosting of ROS production to enhance the cytotoxicity, and adding cAMP or interferons to ATO regimens.

SUMMARY

Recent advances in the clinical use of arsenic, the mechanism of arsenic-mediated cytotoxicity, and modulations of ATO to increase its efficacy and expand its clinical spectrum are reviewed.

Antimalarial drugs: QT prolongation and cardiac arrhythmias

Most available antimalarial drugs induce cardiac side effects. These side effects include various mild heart rate changes (amodiaquine) to excessive prolongation of the QT interval (halofantrine) which may lead to lethal arrhythmias such as Torsade de Pointes (TdP). The cellular mechanism of such events during antimalarial therapy is principally related to ion channel inhibition (e.g., human ether-a-go-go related gene channel) which may slow the repolarisation process and create a good substrate for arrhythmia (when dispersion of repolarisation is present). However, other antimalarial drugs do not show as potent cardiac side effects, like co-arthemeter and sulfadoxine-pyrimethamine. Considering that TdP are favoured by a complex combination of electrophysiological changes, a predictive cardiosafety strategy for new antimalarial drugs should comprise assays with an increasing level of information from ion channel level, cellular and organ level, to the whole organism. In this review, the actual knowledge on underlying mechanisms of QT prolongation and TdP is described, followed by the cardiac safety profiles of present antimalarial drugs.

ICH S7B draft guideline on the non-clinical strategy for testing delayed cardiac repolarisation risk of drugs: a critical analysis

The International Conference on Harmonization (ICH) stems from the initiative of three major world partners (Japan, USA, European Community) who composed a mutually accepted body of regulations concerning the safety, quality and efficacy requirements that new medicines have to meet in order to receive market approval. Documents on non-clinical safety pharmacology already composed by this organisation include two guidelines: the S7A adopted in 2000 and, its companion, the S7B guideline, in a draft form since 2001. The S7A guideline deals with general principles and recommendations on safety pharmacology studies designed to protect healthy volunteers and patients from potential drug-induced adverse reactions. The S7B recommends a general non-clinical testing strategy for determining the propensity of non-cardiovascular pharmaceuticals to delay ventricular repolarisation, an effect that at times progresses into life-threatening ventricular arrhythmia. In the most recent version of this document (June 2004), the strategy proposes experimental assays and a critical examination of other pertinent information for applying an 'evidence of risk' label to a compound. Regrettably, the guideline fails to deal satisfactorily with a number of crucial issues such as scoring the evidence of risk and the clinical consequences of such scoring. However, in the latter case, the S7B relies on the new ICH guideline E14 which is currently in preparation. E14 is the clinical counterpart of the S7B guideline which states that non-clinical data are a poor predictor of drug-induced repolarisation delay in humans. The present contribution summarises and assesses salient aspects of the S7A guideline as its founding principles are also applicable to the S7B guideline. The differences in strategies proposed by the various existing drafts of the latter document are critically examined together with some unresolved, crucial problems. The need for extending the objective of the S7B document to characterise the full electrophysiological profile of new pharmaceuticals is argued as this approach would more extensively assess the non-clinical cardiac safety of a drug. Finally, in order to overcome present difficulties in arriving at the definitive version of the S7B guideline, the Expert Working Group could reflect on the introduction of the S7B guideline recommendations in the S7A document, as originally intended, or on postponing the adoption of an harmonized text until the availability of novel scientific data allows solving presently contentious aspects of this and the E14 guidelines.

QT PRODACT: A Multi-site Study of In Vitro Action Potential Assays on 21 Compounds in Isolated Guinea-Pig Papillary Muscles

To construct a non-clinical database for drug-induced QT interval prolongation, the electrophysiological effects of 11 positive and 10 negative compounds on action potentials (AP) in guinea-pig papillary muscles were investigated in a multi-site study according to a standard protocol. Compounds with a selective inhibitory effect on the rapidly activated delayed rectifier potassium current (IKr) prolonged action potential duration at 90% repolarization (APD90) in a concentration-dependent manner, those showing Ca2+ current (ICa) inhibition shortened APD30, and those showing Na+ current (INa) inhibition decreased action potential amplitude (APA) and Vmax. Some of the mixed ion-channel blockers showed a bell-shaped concentration-response curve for APD90, probably due to their blockade of INa and/or ICa, sometimes leading to a false-negative result in the assay. In contrast, all positive compounds except for terfenadine and all negative compounds with IKr-blocking activity prolonged APD30-90 regardless of their INa- and/or ICa-blocking activities, suggesting that APD30-90 is a useful parameter for evaluating the IKr-blocking activity of test compounds. Furthermore, the assay is highly informative regarding the modulation of cardiac ion channels by test compounds. Therefore, when APD90 and APD30-90 are both measured, the action potential assay can be considered a useful method for assessing the risk of QT interval prolongation in humans in non-clinical safety pharmacology studies.

Drugs, QTc Interval Prolongation and Final ICH E14 Guideline

Regulatory concerns on the ability of an ever-increasing number of non-antiarrhythmic drugs to delay ventricular repolarisation, prolong the corrected QT (QTc) interval and induce potentially fatal ventricular tachyarrhythmias have culminated in the adoption of two, internationally harmonised, regulatory guidelines. On 12 May 2005, the International Conference on Harmonisation (ICH) reached an important milestone when it adopted the final texts for clinical (ICH topic E14) and non-clinical (ICH topic S7B) strategies by which drugs should be investigated for their potential to induce these effects during their development.ICH E14 provides recommendations to sponsors concerning the design, conduct, analysis and interpretation of clinical studies to assess the potential of a drug to delay cardiac repolarisation. Specifically, it calls for a clinical 'thorough QT/QTc study' (typically conducted in healthy volunteers), which is intended to determine whether a drug has a threshold pharmacological effect on cardiac repolarisation, as detected by QT/QTc interval prolongation. The E14 recommendations are generally applicable not only to new drugs that have systemic bioavailability but also to approved drugs when a new dose, route of administration or target population that may result in an increased risk is explored. The guideline provides for exceptions when this study may not be required.Recognising the fractious relationship between ICH E14 and ICH S7B, and the persistence of a number of issues that may require clarity and/or the emergence of other new scientific issues in the future, the ICH Steering Committee has formed an Implementation Working Group that is charged with providing clarity on aspects of the guideline that are ambiguous and responding to issues on which the sponsors are uncertain. This paper provides a commentary on some of the challenges that are likely to be faced by the sponsors of drugs during the next few years of application of these two guidelines. The adoption of these guidelines has left a number of questions unanswered and raised some new ones. When in doubt, the sponsor should seek formal regulatory clarity before making key decisions that may impact further development, assessment and approval of a new chemical entity. Although the goal of developing drugs with much lower torsadogenic potential and without inappropriate restriction in the use (or even rejection) of potentially beneficial drugs is within sight, it is questionable whether the risk of drug-induced pro-arrhythmia will be eliminated completely.

Drugs, QT Interval Prolongation and ICH E14

Regulatory concerns on the ability of an ever-increasing number of non-cardiovascular drugs to prolong the corrected QT (QTc) interval and induce potentially fatal ventricular tachyarrhythmias have culminated in initiatives to harmonise internationally the regulatory guidance on strategies by which to evaluate new drugs for this liability. The International Conference on Harmonisation (ICH) has released consensus texts for clinical (ICH topic E14) and non-clinical (ICH topic S7B) strategies as regulatory drafts for wider consultation. Draft ICH E14 calls for a clinical 'thorough QT/QTc study' (typically in healthy volunteers) for new drugs with systemic bioavailability, regardless of the non-clinical data. This indifference to non-clinical data has sparked off a major debate, even among the regulators. The 'thorough QT/QTc study' is intended to determine whether a drug has a threshold pharmacological effect on cardiac repolarisation, as detected by QT/QTc prolongation, and proposes the use of a positive control to validate the study. The guideline recommends exploration of the effect of concentrations that are higher than those achieved following the anticipated therapeutic doses and, consequently, a negative 'thorough QT/QTc study', even in the presence of non-clinical data of concern, will almost always allow standard collection of on-therapy ECGs. The proposed threshold of a 5 ms increase in mean placebo-corrected QTc interval for designating a study as positive for an effect, with all its implications for subsequent development of the drug and its regulatory assessment and labelling, has also generated a controversy. This paper provides an overview commentary on some contentious or ambiguous aspects of draft ICH E14 with a view to stimulating a debate and inviting scientifically supported comments from stakeholders in order to ensure that the application of the ICH E14 strategy, when finalised and adopted, does not result in either restriction in the use (or even rejection) of a potentially beneficial drug or approval of an otherwise hazardous drug without the restrictions required to promote its safe use.

Pentamidine-Induced Long QT Syndrome and Block of hERG Trafficking

The diamidine pentamidine is used to treat leishmaniasis, trypanosomiasis, and Pneumocystis carinii pneumonia. Treatment may be accompanied by prolongation of the QT interval of the electrocardiogram and torsades de pointes tachycardias. Up to now, it has been thought that therapeutic compounds causing QT prolongation are associated with direct block of the cardiac potassium channel human ether a-go-go-related gene (hERG), which encodes the alpha subunit of cardiac I(Kr) currents. We show that pentamidine has no acute effects on currents produced by hERG, KvLQT1/mink, Kv4.3, or SCNA5. Cardiac calcium currents and the guinea pig cardiac action potential were also not affected. After overnight exposure, however, pentamidine reduced hERG currents and inhibited trafficking and maturation of hERG with IC(50) values of 5 to 8 microM similar to therapeutic concentrations. Surface expression determined in a chemiluminescence assay was reduced on exposure to 10, 30, and 100 microM pentamidine by about 30, 40, and 70%, respectively. These effects were specific for hERG since expression of hKv1.5, KvLQT1/minK, and Kv4.3 was not altered. In isolated guinea pig ventricular myocytes, 10 microM pentamidine prolonged action potential duration APD(90) from 374.3 +/- 57.1 to 893.9 +/- 86.2 ms on overnight incubation. I(Kr) tail current density was reduced from 0.61 +/- 0.09 to 0.39 +/- 0.04 pA/pF. We conclude that pentamidine prolongs the cardiac action potential by block of hERG trafficking and reduction of the number of functional hERG channels at the cell surface. We propose that pentamidine, like arsenic trioxide, produces QT prolongation and torsades de pointes in patients by inhibition of hERG trafficking.