1 The inhibition of the cardiac`rapid' delayed recti®er current (I Kr ) and its cloned equivalent HERG mediate QT interval prolonging e ects of a wide range of clinically used drugs. In this study, we investigated the e ects of the Class Ic antiarrhythmic agent¯ecainide (FLEC) on ionic current (I HERG ) mediated by cloned HERG channels at 378C . 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). 2 Whole cell voltage clamp recordings of I HERG were made from an HEK293 cell line stably expressing HERG. FLEC inhibited I HERG`t ails' following test pulses to +30 mV with an IC 50 of 3.91+0.68 mM (mean+s.e.mean) and a Hill co-e cient close to 1 (0.76+0.09).3 In experiments in which I HERG tails were monitored following voltage commands to a range of test potentials, I HERG inhibition by FLEC was observed to be voltage-dependent and to be associated with a *75 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 I HERG activation curve. The time-course of I HERG tail deactivation was not signi®cantly altered by FLEC. 4 In experiments in which 10 s depolarizing pulses were applied from 780 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 ®rst 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 I HERG . 5 Under similar recording conditions QUIN inhibited I HERG with an IC 50 of 0.41+0.04 mM and PROPAF inhibited I HERG with an IC 50 of 0.44+0.07 mM. Similar to FLEC, both QUIN and PROPAF showed voltage-dependence of inhibition and blockade developed rapidly during a sustained depolarization. 6 LIG showed little e ect on I HERG at low micromolar concentrations, but could inhibit the current at higher concentrations; the observed IC 50 was 262.90+22.40 mM. 7 Our data are consistent with FLEC, PROPAF and QUIN exerting I HERG blockade at clinically relevant concentrations. The rank potency as HERG blockers of the Class I drugs tested in this study was QUIN=PROPAF4FLEC44LIG. British Journal of Pharmacology (2002) 136, 717 ± 729 Keywords: Antiarrhythmic; Class Ia; Class Ib; Class Ic;¯ecainide; HERG; I Kr ; QT interval; QT prolongation; quinidine Abbreviations: ANOVA, analysis of variance; d, fractional distance in the transmembrane ®eld sensed at drug receptor site; DISO, disopyramide; FLEC,¯ecainide; HERG, Human ether-a-go-go related gene; IC 50 , half maximal inhibitory drug concentration; I Ca,L , L-type calcium current; I HERG , current mediated by the HERG channel; I K , delayed recti®er potassium current; I Kr`r apid' delayed recti®er potassium current; I Ks`s low' delayed recti®er potassium current; I to , transient outward potassium current; k, slope factor describing voltage de...

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 IC 50 of 3.97 W WM. This is slightly less potent than fluoxetine in our system (IC 50 of 1.50 W WM). In isolated guinea pig ventricular cardiomyocytes citalopram inhibited L-type calcium current (I CaYL ). The voltage dependence of I CaYL inactivation in the presence of 100 W WM citalopram was shifted significantly leftward. As a result, the I CaYL`w indow' 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 CaYL window' may help to explain citalopram's good cardiac safety profile, given its propensity to block HERG at excessive dosages. ß 2002 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.

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The methanesulphonanilide agent dofetilide (UK-68,798) exerts Class III antiarrhythmic effects by inhibiting the cardiac rapid delayed rectifier potassium current (I(Kr)) encoded by HERG. The aim of the present study was to determine whether dofetilide also exhibits Class IV (L-type calcium-channel blocking) effects. L-type calcium current (I(Ca,L)) was measured from rabbit isolated ventricular myocytes, using the whole-cell patch-clamp technique under selective recording conditions. Positive control experiments demonstrated inhibition of I(Ca,L) elicited by pulses to + 10 mV by both nifedipine and externally applied Ni2+ ions. Three concentrations of dofetilide were tested: 100 nM, 1 microM and 10 microM. I(Ca,L) magnitude was not significantly reduced by any of the concentrations tested (P > 0.05; n = minimum of seven cells per drug concentration). The inactivation time-course of I(Ca,L) was also unaffected by 10 microM dofetilide. Heterologously expressed HERG current (I(HERG)) recorded from Chinese Hamster Ovary cells was extensively inhibited by 100 nm and 1 microM dofetilide, with inhibition at 1 microM not significantly different from 100% (P > 0.1). It is concluded that dofetilide produced no I(Ca,L) blocking effects at concentrations up to and exceeding that required for maximal I(HERG) inhibition. The findings support the notion that dofetilide is a highly selective Class III antiarrhythmic agent, devoid of Class IV antiarrhythmic activity.