Harry J. Witchel

Psychotropic drugs, cardiac arrhythmia, and sudden death

A variety of drugs targeted towards the central nervous system are associated with cardiac side effects, some of which are linked with reports of arrhythmia and sudden death. Some psychotropic drugs, particularly tricyclic antidepressants (TCAs) and antipsychotic agents, are correlated with iatrogenic prolongation of the QT interval of the electrocardiogram (ECG). In turn, this is associated with the arrhythmia torsades de pointes (TdP). This review discusses the association between psychotropic agents, arrhythmia and sudden death and, focusing on TCAs and antipsychotics, considers their range of cellular actions on the heart; potentially pro-arrhythmic interactions between psychotropic and other medications are also considered. At the cellular level TCAs, such as imipramine and amitriptyline, and antipsychotics, such as thioridazine, are associated with inhibition of potassium channels encoded by HERG. In many cases this cellular action correlates with ECG changes and a risk of TdP. However, not all psychotropic agents that inhibit HERG at the cellular level are associated equally with QT prolongation in patients, and the potential for QT prolongation is not always equally correlated with TdP. Differences in risk between classes of psychotropic drugs, and between individual drugs within a class, may result from additional cellular effects of particular agents, which may influence the consequent effects of inhibition of repolarizing potassium current.

herg1 Gene and HERG1 Protein Are Overexpressed in Colorectal Cancers and Regulate Cell Invasion of Tumor Cells

The acquisition of the capacity to invade surrounding tissues confers a more malignant phenotype to tumor cells and is necessary for the establishment of metastases. The understanding of the molecular mechanisms underlying cell invasion in human solid tumors such as colorectal cancers could provide not only more sensitive prognostic analyses but also novel molecular targets for cancer therapy. We report in this article that K+ ion channels belonging to the HERG family are important determinants for the acquisition of an invasive phenotype in colorectal cancers. The herg1 gene and HERG1 protein are expressed in many colon cancer cell lines, and the activity of HERG channels modulates colon cancer cell invasiveness. Moreover, the amount of HERG1 protein expressed on the plasma membrane is directly related to the invasive phenotype of colon cancer cells. Finally, both the herg1 gene and HERG1 protein were expressed in a high percentage of primary human colorectal cancers, with the highest incidence occurring in metastatic cancers, whereas no expression could be detected either in normal colonic mucosa or in adenomas.

Blockade of HERG potassium currents by fluvoxamine: incomplete attenuation by S6 mutations at F656 or Y652

Pharmacological blockade of the Human ether‐a‐go‐go related gene (HERG) potassium channel is commonly linked with acquired long QT syndrome and associated proarrhythmia. The objectives of this study were (i) to identify and characterise any inhibitory action on HERG of the selective‐serotonin re‐uptake inhibitor fluvoxamine, (ii) to then determine whether fluvoxamine shared the consensus molecular determinants of HERG blockade of those drugs so far tested. Heterologous HERG potassium current (IHERG) was measured at 37°C, using the whole‐cell patch‐clamp technique, from a mammalian cell line (Human embryonic kidney 293) expressing HERG channels. IHERG tails, following repolarisation from +20 to −40 mV, were blocked by fluvoxamine with an IC50 of 3.8 μM. Blockade of wild‐type HERG was of extremely rapid onset (within 10 ms) and showed voltage dependence, with fluvoxamine also inducing a leftward shift in voltage‐dependent activation of IHERG. Characteristics of block were consistent with a component of closed channel (or extremely rapidly developing open channel) blockade and dependence on open and inactivated channel states. The attenuated‐inactivation mutation S631A partially reduced the blocking effect of fluvoxamine. The S6 mutations, Y652A and F656A, and the pore helix mutant S631A only partially attenuated blockade by fluvoxamine at concentrations causing profound blockade of wild‐type HERG. All HERG‐blocking pharmaceuticals studied to date have been shown to block F656 mutant channels with over 100‐fold reduced potency compared to their blockade of the wild‐type channel. Fluvoxamine is therefore quite distinct in this regard from previously studied agents.

Familial And Acquired Long QT Syndrome And The Cardiac Rapid Delayed Rectifier Potassium Current

1. Long QT syndrome (LQTS) is a cardiac disorder characterized by syncope, seizures and sudden death; it can be congenital, idiopathic, or iatrogenic.

Inhibition of the current of heterologously expressed HERG potassium channels by flecainide and comparison with quinidine, propafenone and lignocaine

The inhibition of the cardiac ‘rapid’ delayed rectifier current (IKr) and its cloned equivalent HERG mediate QT interval prolonging effects of a wide range of clinically used drugs. In this study, we investigated the effects of the Class Ic antiarrhythmic agent flecainide (FLEC) on ionic current (IHERG) mediated by cloned HERG channels at 37°C . We also compared the inhibitory potency of FLEC with other Class I agents: quinidine (QUIN, Class Ia); lignocaine (LIG, Class Ib) and propafenone (PROPAF, Class Ic). Whole cell voltage clamp recordings of IHERG were made from an HEK293 cell line stably expressing HERG. FLEC inhibited IHERG ‘tails’ following test pulses to +30 mV with an IC50 of 3.91±0.68 μM (mean±s.e.mean) and a Hill co‐efficient close to 1 (0.76±0.09). In experiments in which IHERG tails were monitored following voltage commands to a range of test potentials, IHERG inhibition by FLEC was observed to be voltage‐dependent and to be associated with a ∼−5 mV shift of the activation curve for the current. Voltage‐dependence of inhibition was greatest over the range of potentials corresponding to the steep portion of the IHERG activation curve. The time‐course of IHERG tail deactivation was not significantly altered by FLEC. In experiments in which 10 s depolarizing pulses were applied from −80 to 0 mV, the level of current inhibition by FLEC did not increase between 1 and 10 s. Some time‐dependence of inhibition was observed during the first 200 – 300 ms of depolarization. This observation and the voltage‐dependence of inhibition are collectively consistent with FLEC exerting a rapid open channel state inhibition of IHERG. Under similar recording conditions QUIN inhibited IHERG with an IC50 of 0.41±0.04 μM and PROPAF inhibited IHERG with an IC50 of 0.44±0.07 μM. Similar to FLEC, both QUIN and PROPAF showed voltage‐dependence of inhibition and blockade developed rapidly during a sustained depolarization. LIG showed little effect on IHERG at low micromolar concentrations, but could inhibit the current at higher concentrations; the observed IC50 was 262.90±22.40 μM. Our data are consistent with FLEC, PROPAF and QUIN exerting IHERG blockade at clinically relevant concentrations. The rank potency as HERG blockers of the Class I drugs tested in this study was QUIN=PROPAF>FLEC>>LIG.

Inhibition of the current of heterologously expressed HERG potassium channels by imipramine and amitriptyline.

Tricyclic antidepressants (TCAs) are associated with cardiovascular side effects including prolongation of the QT interval of the ECG. In this report we studied the effects of two TCAs (imipramine and amitriptyline) on ionic current mediated by cloned HERG potassium channels. Voltage clamp measurements of HERG currents were made from CHO cells transiently transfected with HERG cDNA. HERG‐encoded potassium channels were inhibited in a reversible manner by both imipramine and amitriptyline. HERG tail currents (IHERG) following test pulses to +20 mV were inhibited by imipramine with an IC50 of 3.4±0.4 μM (mean±s.e.mean) and a Hill coefficient of 1.17±0.03 (n=5). 3 μM amitriptyline inhibited IHERG by 34±6% (n=3). The inhibition showed only weak voltage dependence. Using an ‘envelope of tails’ comprised of pulses to +20 mV of varying durations, the τ of activation was found to be 155±30 ms for control and 132±26 ms for 3 μM imipramine (n=5). Once maximal channel activation was achieved after 320 ms (as demonstrated by maximal tail currents), further prolongation of depolarization did not increase imipramine‐mediated HERG channel inhibition. Taking current measurements every second during a 10 s depolarizing pulse from −80 mV to 0 mV, block was observed during the first pulse in the presence of imipramine and the level of IHERG block was similar throughout the pulse (n=5). A three pulse protocol (two depolarizing pulses to +20 mV separated by 20 ms at −80 mV) revealed that imipramine did not significantly alter the kinetics of IHERG inactivation. The τ of inactivation was 8±2 ms and 5.6±0.4 ms (n=5) in the absence and presence of 3 μM imipramine, respectively, and currents inactivated to a similar extent. Our data are consistent with TCAs causing components of block of the HERG channel in both the closed and open states. Any component of open channel block occurs rapidly upon depolarization. Inhibition of IHERG by the prototype TCAs imipramine and amitriptyline may suggest a mechanism for QT prolongation associated with risks of arrhythmia and sudden death that accompany high concentrations of TCAs following overdose.

Inhibitory actions of the selective serotonin re-uptake inhibitor citalopram on HERG and ventricular L-type calcium currents

Using whole‐cell patch clamp recording of heterologous HERG‐mediated currents in transfected mammalian cells, we observed that the selective serotonin re‐uptake inhibitor citalopram blocks HERG with an IC50 of 3.97 μM. This is slightly less potent than fluoxetine in our system (IC50 of 1.50 μM). In isolated guinea pig ventricular cardiomyocytes citalopram inhibited L‐type calcium current (I Ca,L). The voltage dependence of I Ca,L inactivation in the presence of 100 μM citalopram was shifted significantly leftward. As a result, the I Ca,L ‘window’ in citalopram was found to be (a) smaller and (b) leftward‐shifted compared to control. The effects of citalopram on both calcium current amplitude and the I Ca,L ‘window’ may help to explain citaloprams good cardiac safety profile, given its propensity to block HERG at excessive dosages.

Tryptophan depletion reverses the therapeutic effect of selective serotonin reuptake inhibitors in social anxiety disorder

BACKGROUND Tryptophan depletion studies have suggested that central serotonin (5-hydroxytryptamine, 5-HT) function mediates the therapeutic effect of selective serotonin reuptake inhibitors (SSRIs) in depression and panic disorder. The present study tested the hypothesis that temporary reduction in central 5-HT transmission, through acute tryptophan depletion, could reverse the therapeutic effect of the SSRIs in social anxiety disorder (SAD) patients. METHODS Fourteen patients with SAD who showed sustained clinical improvement with SSRI treatment underwent tryptophan depletion in a double-blind, placebo-controlled, crossover design, over 2 days 1 week apart. At the peak time of depletion, the participants also underwent three behavioral challenges: autobiographical script, verbal task, and neutral script. Psychological outcome was assessed with the Spielberger State Anxiety Inventory (STAI) Form Y-1 and visual analog scales (VAS) measuring anxiety, depression, and somatic symptoms. RESULTS Anxiety was significantly increased on the depletion day compared with the control day, both on the STAI Form Y-1 and composite VAS score. Furthermore, there was a significant depletion x time interaction, explained mainly by the anxiogenic effect of the autobiographical script. In contrast, the verbal and the neutral tasks failed to differentiate between depletion and placebo. CONCLUSIONS Tryptophan depletion induced significant increase of anxiety in treated SAD patients, which was more prominent during the recital of an autobiographical script. This finding supports the notion that SSRIs improve social anxiety by increasing 5-HT availability. The autobiographical script seems to be a more robust challenge test for SAD than the stressful verbal task.

Mechanism of hERG K+ channel blockade by the fluoroquinolone antibiotic moxifloxacin

The fluoroquinolone antibiotic moxifloxacin has been associated with the acquired long QT syndrome and is used as a positive control in the evaluation of the QT‐interval prolonging potential of new drugs. In common with other QT‐prolonging agents, moxifloxacin is known to inhibit the hERG potassium K+ channel, but at present there is little mechanistic information available on this action. This study was conducted in order to characterise the inhibition of hERG current (IhERG) by moxifloxacin, and to determine the role in drug binding of the S6 aromatic amino‐acid residues Tyr652 and Phe656. hERG currents were studied using whole‐cell patch clamp (at room temperature and at 35–37°C) in an HEK293 cell line stably expressing hERG channels. Moxifloxacin reversibly inhibited currents in a dose‐dependent manner. We investigated the effects of different voltage commands to elicit hERG currents on moxifloxacin potency. Using a ‘step‐ramp’ protocol, the IC50 was 65 μM at room temperature and 29 μM at 35°C. When a ventricular action potential waveform was used to elicit currents, the IC50 was 114 μM. Block of hERG by moxifloxacin was found to be voltage‐dependent, occurred rapidly and was independent of stimulation frequency. Mutagenesis of the S6 helix residue Phe656 to Ala failed to eliminate or reduce the moxifloxacin‐mediated block whereas mutation of Tyr652 to Ala reduced moxifloxacin block by ∼66%. Our data demonstrate that moxifloxacin blocks the hERG channel with a preference for the activated channel state. The Tyr652 but not Phe656 S6 residue is involved in moxifloxacin block of hERG, concordant with an interaction in the channel inner cavity.

Pharmacology of the short QT syndrome N588K-hERG K+ channel mutation: differential impact on selected class I and class III antiarrhythmic drugs

The short QT syndrome (SQTS) is associated with cardiac arrhythmias and sudden death. The SQT1 form of SQTS results from an inactivation‐attenuated, gain‐of‐function mutation (N588K) to the human ether‐à‐go‐go‐related gene (hERG) potassium channel. Pharmacological blockade of this mutated hERG channel may have therapeutic value. However, hERG‐blocking potencies of canonical inhibitors such as E‐4031 and D‐sotalol are significantly reduced for N588K‐hERG. Here, five hERG‐blocking drugs were compared to determine their relative potencies for inhibiting N588K channels, and two other inactivation‐attenuated mutant channels were tested to investigate the association between impaired inactivation and altered drug potency.