Introduction

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Drugs that block the histamine receptor subtype H₁ are widely used to relieve the symptoms of allergic reactions (Babe and Serafin, 1996). During the past 20 years, second-generation H₁ receptor blockers have been developed to overcome the marked antimuscarinic and sedative properties displayed by first-generation antihistamines like diphenhydramine, promethazine, hydroxyzine, and pyrilamine (Sorkin and Heel, 1985). Because of their novel pharmacological profile, second-generation antihistamines such as terfenadine, astemizole, loratadine, cetirizine, and ebastine have been progressively replacing the older molecules on the market, thus becoming one of the most prescribed drug families in Western countries (Woosley, 1996).

Despite the enormous success of second-generation antihistamines, in the mid-1980s, ~10 years after their introduction into the market, several reports appeared in the literature indicating the rare occurrence of a form of polymorphic ventricular arrhythmia, the so-called torsade de pointes, after the administration of astemizole or terfenadine (Jackman et al., 1988). This ventricular arrhythmia, which occurs in the setting of a marked prolongation of the QT interval on the surface electrocardiogram, has been described either in patients taking intentional overdoses of these second-generation antihistamines (Craft, 1986; Davies et al., 1989) or in subjects with one or more predisposing factors to the development of cardiac arrhythmias (Monahan et al., 1990). These latter conditions included a reduced drug-metabolizing capacity of the patient (liver diseases, simultaneous administration of drugs known to inhibit hepatic metabolism such as macrolide antibiotics and ketoconazole), congenital prolongation of the QT interval, ischemic heart disease, congestive heart failure, and electrolyte imbalance, such as hypokalemia or hypomagnesemia (Woosley, 1996).

It has been suggested recently that the QT prolongation and ventricular arrhythmia caused by terfenadine and astemizole might be secondary to their ability to interfere with cardiac potassium channels involved in action potential repolarization (Berul and Morad, 1995) and in particular with the I_Kr component of the cardiac repolarizing current (Salata et al. 1995). The human ether-a-go-go related gene (HERG) (Warmke and Ganetzky, 1994) seems to represent the molecular basis of I_Kr because the K⁺ currents encoded by HERG display biophysical and pharmacological characteristics similar to those of I_Kr (Trudeau et al., 1995; Spector et al., 1996). In addition, evidence has been provided to show that HERG mutations are responsible for one form of long QT syndrome (LQTS-2), a chromosome 7-linked form of human arrhythmia (Curran et al., 1995). Altogether, these observations suggest that HERG is a primary target for both congenital (mutation-induced) and acquired (drug-induced) action potential prolongation, delayed repolarization, and cardiac arrhythmias.

Because of the occurrence of torsade de pointes in some of the patients taking terfenadine and astemizole, some authors have speculated that other nonsedating antihistamines might induce similar cardiotoxic effects (Good et al., 1994; Woosley, 1996). Among second-generation antihistamines, the piperazine H₁ receptor blocker cetirizine, which has been available for several years in Europe and was recently approved by the Food and Drug Administration for use in the United States, seems to lack arrhythmogenic potential both in humans (Sale et al., 1994) and in experimental animals (Hey et al., 1996). The aim of the current study was to (1) investigate the potential interaction of cetirizine with HERG K⁺ channels heterologously expressed in Xenopus laevis oocytes and in HEK 293 cells (Zhou et al., 1998), or endogenously present in SH-SY5Y human neuroblastoma cells (Arcangeli et al., 1995; Bianchi et al., 1998), and (2) compare the actions of the piperazine molecule with those of other second-generation antihistamines reported to be associated with (e.g., terfenadine, astemizole, and ebastine) (Roy et al., 1996; Suessbrich et al., 1996; Ko et al., 1997) or lacking in (e.g., loratadine) (Ko et al., 1997) HERG K⁺ channel-blocking activity.

	Materials and Methods

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X. laevis Oocyte Isolation

Ovarian lobes were surgically removed from adult female X. laevis frogs (Rettili di Schneider, Varese, Italy) and placed into 100-mm Petri dishes containing a Ca²⁺-free solution composed of 82.5 mM NaCl, 2 mM KCl, 1 mM MgCl₂, 5 mM HEPES, 2.5 mM pyruvic acid, 100 units/ml penicillin, and 100 µg/ml streptomycin, pH 7.5 with NaOH. After four extensive washes, the oocytes (stage V-VI) were dissociated at room temperature by collagenase treatment (type IA, 45-80 min at a concentration of 2 mg/ml). At the end of the collagenase treatment, the oocytes were placed in a Ca²⁺-containing solution composed of 100 mM NaCl, 2 mM KCl, 1.8 mM CaCl₂, 1 mM MgCl₂, 5 mM HEPES, 2.5 mM pyruvic acid, 100 units/ml penicillin, and 100 µg/ml streptomycin, pH 7.5 with NaOH. Dissociated oocytes then were placed in a 19° incubator and microinjected on the next day.

Molecular Biology and Oocyte Injection

The cloning of HERG has been described previously (Warmke and Ganetzky, 1994). The engineering of HEK 293 cells stably transfected with HERG cDNA was described by Zhou et al. (1998). HERG cDNA was linearized with the restriction enzyme EcoRI, and RNA was transcribed in vitro from linearized cDNAs by means of commercially available kits (mCAP; Stratagene, La Jolla, CA) using the SP6 RNA polymerase. cRNA was stored in a stock solution (250 ng/µl) at 20° in 0.1 M KCl. One day after isolation, X. laevis oocytes were microinjected with 76 nl of cRNA stock solution or appropriate dilutions. At 2-10 days after the cRNA microinjection, HERG K⁺ currents expressed in X. laevis oocytes were measured by the two-microelectrode voltage-clamp technique.

Cell Culture

Human neuroblastoma SH-SY5Y cells were cultured in Dulbecco's modified Eagle's medium, containing glucose (4.5 g/liter) and 5% fetal calf serum, and incubated at 37° in a humidified atmosphere with 10% CO₂ in 100-mm plastic Petri dishes. HEK 293 cells were cultured in minimal essential medium, supplemented with Earle's salts, nonessential amino acids (0.1 mM), penicillin (50 units/ml), streptomycin (50 µg/ml), G418 (0.4 mg/ml), and 10% fetal calf serum and incubated at 37° in a humidified atmosphere with 5% CO₂ in 100-mm plastic Petri dishes. For electrophysiological experiments, the cells were seeded onto glass coverslips (Fisher) coated with poly-L-lysine (30 µg/ml). All the experiments were performed 1-4 days after seeding at room temperature (22-23°).

Electrophysiology

Voltage-clamp with two microelectrodes. The oocytes were voltage-clamped with a commercially available amplifier (Warner OC-725A; Warner Instruments, Hamden, CT). Current and voltage electrodes were filled with 3 M KCl and 10 mM HEPES (pH 7.4; 1 M resistance). The bath solution contained 88 mM NaCl, 10 mM KCl, 2.6 mM MgCl₂, 0.18 mM CaCl₂, and 5 mM HEPES, pH 7.5. This solution was perfused in the recording chamber at a rate of ~0.2 ml/min. Data were stored on the hard disk of a 486 IBM compatible computer for off-line analysis. The pCLAMP software (version 6.0.2: Axon Instruments, Burlingame, CA) was used for data acquisition and analysis. Currents were recorded at room temperature.

Patch-clamp. Currents from the human neuroblastoma SH-SY5Y and the HERG-transfected HEK 293 cells were recorded at room temperature using a commercially available amplifier (Axopatch 200A: Axon Instruments). The whole-cell configuration of the patch-clamp technique (Hamill et al., 1981) was adopted using glass micropipettes of 3-7 M resistance. No compensation was performed for pipette resistance and cell capacitance. The cells were perfused with an extracellular solution containing 100 mM KCl, 10 mM EGTA, and 10 mM HEPES, pH 7.3 with KOH (for the SH-SY5Y cells), or 150 mM NaCl, 10 mM KCl, 3 mM CaCl₂, 1 mM MgCl₂, and 10 mM HEPES, pH 7.4 with NaOH (for the HERG-transfected HEK 293 cells). The pipettes were filled with 110 mM CsCl, 10 mM tetraethylammonium-Cl, 2 mM MgCl₂, 10 mM EGTA, 8 mM glucose, 2 mM Mg-ATP, 0.25 mM cAMP, and 10 mM HEPES, pH 7.3 with NaCl KOH (for the SH-SY5Y cells), or 130 mM K-Aspartate, 10 mM NaCl, 4 mM CaCl₂, 2 mM MgCl₂, 10 mM EGTA, 2 mM Mg-ATP, 0.25 mM cAMP, and 10 mM HEPES, pH 7.4 with NaOH (for the HERG-transfected HEK 293 cells).

Drugs and Statistics

All the reagents were purchased from Sigma Chemical (Milan, Italy). Astemizole was kindly provided by Janssen-Cilag (Rome, Italy). Loratadine was kindly obtained from Schering-Plough (Milan, Italy). Cetirizine was generously donated by UCB Pharma (Torino, Italy). The H₁ receptor antagonists were dissolved in dimethylsulfoxide at concentrations between 5 and 50 mM, and stock solutions were kept at 20°. Appropriate drug dilutions were prepared daily. The maximal dimethylsulfoxide concentration (0.6%) did not affect HERG K⁺ channels recorded in X. laevis oocytes, HEK 293 cells, or SH-SY5Y human neuroblastoma cells. Statistical significance between the data was obtained with the Student's t test or an analysis of variance followed by Tukey's test. When appropriate, data are expressed as the mean ± standard error.

	Results

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Differential effect of cetirizine on the K⁺ currents carried by HERG channels expressed in X. laevis oocytes compared with astemizole, terfenadine, and loratadine. On microinjection with HERG cRNA, X. laevis oocytes expressed a K⁺ current with biophysical properties that resembled those of I_Kr (Trudeau et al., 1995; Spector et al., 1996). This K⁺ current is activated by depolarization but displays a pronounced inward rectification of the current-voltage relationship at positive potentials (>0 mV), displays rather slow kinetics of activation, and exhibits a large inward component on repolarization to 100 mV, a value of membrane potential below the equilibrium potential for K⁺ ions (Fig. 1).

View larger version (41K): [in this window] [in a new window]   Fig. 1.   Effect of cetirizine on HERG K+ currents expressed in X. laevis oocytes showing a comparison of astemizole with loratadine. Representative current traces and current-to-voltage relationships were recorded from two different HERG-expressing oocytes under control conditions and after a 5-min perfusion with cetirizine (A, 1 and 10 µM) or astemizole (B, 1 and 10 µM). C, Representative current traces from a single HERG-expressing oocyte recorded under control conditions, after a 10-min perfusion with 30 µM loratadine, after a 20-min washout, and after a 10-min exposure to 30 µM extracellular cetirizine. Holding potential, 90 mV; test potentials, from 80 mV to +40 mV in 20-mV steps; return potential, 100 mV. Dashed lines, zero-current level.

View larger version (22K): [in this window] [in a new window]   Fig. 2.   Lack of effect of cetirizine in inhibiting HERG K+ channels in X. laevis oocytes and SH-SY5Y human neuroblastoma cells. A, Dose-response curve for HERG K+ channels block by four second-generation antihistamines. The inward HERG K+ tail currents recorded in X. laevis oocytes on repolarization to 100 mV after depolarizing pulses of 2 sec to 0 mV were normalized to the control value and expressed as a function of drug concentration. Solid lines, fits of the experimental data to the binding isotherm y = max/(1 + X/IC50)n, where X is the drug concentration, and n is the Hill coefficient. Fitted values for n were between 0.72 and 0.96. Each point is the mean ± standard error of three to six determinations. B, Cumulative block of HERG K+ currents expressed in X. laevis oocytes by terfenadine and lack of effect of cetirizine. The same experimental protocol (holding potential, 90 mV, 15 pulses to 0 mV for 300 msec followed by a 100-msec return to 120 mV; 0.5-Hz pulsing frequency) was repeated in the same HERG-expressing oocytes under control conditions, after a 6-min perfusion with 3 µM cetirizine, and after a 6-min exposure to 3 µM extracellular terfenadine. The peak inward K+ currents carried by HERG channels at 120 mV were normalized to the current value obtained in the first pulse of the control condition (no drug perfusion) for each cell and expressed as a function of time. C, Effect of the four different H1 receptor antagonists astemizole, terfenadine, loratadine, and cetirizine on IHERG constitutively expressed in SH-SY5Y human neuroblastoma cells. The same experimental protocol (holding potential, 60 mV; test potential, 0 mV for 10 sec; return potentials, from 0 to 140/180 mV in 20-mV steps for 100 msec) was performed inseveral SH-SY5Y cells under control conditions and after a 5-min perfusion with each H1 receptor antagonist (3 µM). Drug effects are reported as percent of inhibition of the inward IHERG current at 140 mV, without leak subtraction. Values, mean ± standard error of four determinations for each drug.

Comparison of the effects on HERG K⁺ channels constitutively expressed in SH-SY5Y human neuroblastoma cells (I_HERG) between cetirizine and other second-generation antihistamines. Beside the fundamental role of the I_Kr current in regulating action potential repolarization in cardiac cells, recent evidence suggests that HERG K⁺ channels (I_HERG) are also expressed in other excitable tissues such as the brain (Wymore et al., 1997), in several neuroblastoma cell lines (Arcangeli et al., 1995), and in other tumor cell lines such as TE671 human rhabdomyosarcoma, the human mammary gland adenocarcinoma SK-BR-3, the monoblastic leukemia line FLG29.1, the pituitary cell lines GH₃, GH₄, and MMQ, and others (Bianchi et al., 1998). For this reason, the SH-SY5Y clone of human neuroblastoma cells was used to compare the effects of the four different H₁ receptor blockers on mammalian cells that constitutively express HERG K⁺ channels.

Differential effect of the intracellular and extracellular applications of astemizole and cetirizine on I_HERG heterologously expressed after stable transfection of HEK 293 cells with HERG cDNA. The hypothesis that the lack of inhibitory action of cetirizine on I_HERG in both X. laevis oocytes and SH-SY5Y human neuroblastoma cells was caused by the relatively poor access of the drug to its putative intracellular receptor site on the channel has been investigated in a subsequent series of studies. To this aim, HEK 293 cells stably transfected with HERG cDNA (Zhou et al., 1998) were used. In fact, in these cells the ~50 times higher density of I_HERG compared with the SH-SY5Y human neuroblastoma cells (Taglialatela M, unpublished observations) allows the adequate recording of I_HERG with lower concentrations of extracellular K⁺ (10 mM). Using this extracellular K⁺ concentration, it is possible to obtain stable recording conditions lasting several minutes (up to 1 hr) and to resolve outward K⁺ currents carried by HERG channels.

View larger version (17K): [in this window] [in a new window]   Fig. 3.   Comparative effect of cetirizine and astemizole on IHERG heterologously expressed in HEK 293 cells stably transfected with HERG cDNA. A, Effect of extracellular perfusion with cetirizine. Representative current traces were recorded in the whole-cell configuration of the patch- clamp technique from a single HERG-transfected HEK 293 cell. Records were obtained under control conditions and after a 5-min perfusion with 3 µM cetirizine. Holding potential, 85 mV; test potentials, from 60 mV to +60 mV in 20-mV steps; return potential, 100 mV. B, Effect of intracellular exposure to cetirizine or astemizole: currents at 0 and 20 min. Representative current traces were recorded at time 0 min and after 20 min in three different HERG-transfected HEK 293 cells. Top, control cell (K-aspartate in the pipette). Middle, K-aspartate plus 3 µM cetirizine. Bottom, K-aspartate plus 3 µM astemizole. Holding potential, 85 mV; test potentials, from 60 mV to +60 mV in 20-mV steps; return potential, 100 mV. C, Effect of intracellular exposure to cetirizine or astemizole: time course of IHERG during the 20 min after breaking into the cell (time 0 min) under three different experimental situations: , control K-aspartate solution inside the pipette; , K-aspartate plus 3 µM cetirizine; and , K-aspartate plus 3 µM astemizole. The tail current values obtained in each experimental group were normalized to the current recorded at time 0 min (100%) (see Results).

	Discussion

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The results of the current study suggest that the four second-generation H₁ receptor antagonists terfenadine, astemizole, loratadine, and cetirizine display considerable heterogeneity in blocking constitutively and heterologously expressed HERG K⁺ channels. In fact, whereas cetirizine was completely devoid of any inhibitory action on these K⁺ channels, astemizole and terfenadine both inhibited HERG K⁺ channels with nanomolar affinities, whereas loratadine interfered with HERG K⁺ channels only at the highest concentrations used.

The observation that different H₁ receptor antagonists display marked differences in their ability to inhibit HERG K⁺ channels is of crucial clinical relevance considering that on one hand, these drugs are among the most frequently prescribed drugs in Western countries (Woosley, 1996), and on the other hand, these K⁺ channels have a crucial role in controlling the duration of the cardiac action potential (Curran et al., 1995; Trudeau et al., 1995; Spector et al., 1996). In fact, the induction of cardiac arrhythmias by terfenadine and astemizole has been documented extensively (Craft, 1986; Davies et al., 1989; Monahan et al., 1990). The cardiotoxic effects exerted by these two molecules has been mostly related to their ability to prolong cardiac repolarization and therefore to induce early afterdepolarizations, which are thought to be one of the mechanisms for the genesis of torsade de pointes (Singh, 1993). More recently, after the discovery that the K⁺ channels encoded by HERG represent the molecular basis of I_Kr (Sanguinetti et al., 1995), terfenadine- and astemizole-induced cardiotoxicities have been tightly associated with their ability to block HERG K⁺ channels (Roy et al., 1996; Suessbrich et al., 1996), although blockade of other cloned K⁺ channels has also been reported for terfenadine (Rampe et al., 1993; Crumb et al., 1995). The existence of a tight correlation between the cardiotoxic effects of H₁ receptor antagonists and HERG K⁺ channel blockade also is suggested by the observation that the IC₅₀ values for HERG K⁺ channels blockade by terfenadine and astemizole are close to the plasma concentration range (30-300 nM) measured in humans when ventricular arrhythmias occur (Hoppu et al., 1991; Yun et al., 1993; Woosley, 1996). Furthermore, it should be noted that the adverse cardiovascular effects of terfenadine, astemizole, and ebastine occur at plasma concentrations similar to those required to block peripheral H₁ receptors in guinea pigs (Hey et al., 1996).

The cardiac side effects of astemizole and terfenadine have led to the suggestion that other second-generation H₁ receptor antagonists also might display similar untoward cardiac effects (Good et al., 1994; Woosley, 1996). However, the observation in the current study that cetirizine was completely devoid of any interference with endogenously or heterologously expressed HERG K⁺ channels seems to suggest that torsade de pointes is not likely to occur during conventional therapy with this drug. This conclusion seems to be confirmed by the observation that cetirizine did not display significant prolongation of the QT interval in experimental animals (Hey et al., 1996) or humans (Sale et al., 1994) and that no study has yet appeared in the literature reporting cardiac arrhythmias or QT prolongation associated with its use (Woosley, 1996). Furthermore, in a recent pharmacosurveillance study in which the risk profile for heart rhythm disorders and cardiac deaths was determined for some of the most common nonsedating antihistamines, cetirizine displayed the lowest adverse drug reaction report rate per million defined daily doses (Lindquist and Edwards, 1997).

The lack of inhibitory effect of cetirizine on HERG K⁺ channels seems not to be the consequence of the poor permeability of the X. laevis oocyte membrane where HERG channels have been expressed because it was also observed in SH-SY5Y human neuroblastoma cells constitutively expressing I_HERG (Arcangeli et al., 1995; Bianchi et al., 1998), as well as in HERG-transfected HEK 293 cells (Zhou et al., 1998). Furthermore, the results showing that cetirizine did not block HERG K⁺ channels even at relatively high frequencies of stimulation (0.5 Hz) seems to rule out the possibility that cetirizine caused use-dependent blockade. In addition, it should be noted that the concentration of cetirizine (3 µM) used in the current study to evaluate the possible inhibition of I_HERG in SH-SY5Y and HERG-transfected HEK 293 cells was comparable to the levels of the drug observed in the plasma of normal subjects (1-5 µM) after the administration of doses two to six times higher than the commonly recommended daily therapeutical dose (Sale et al., 1994).

Loratadine inhibited HERG K⁺ channels only at the highest concentrations tested (30 µM in X. laevis oocytes and 3 µM in the SH-SY5Y human neuroblastoma cells). The fact that loratadine was more effective in the SH-SY5Y human neuroblastoma cells compared with oocytes can be accounted for by (1) the lower membrane permeability of the frog oocyte membrane with respect to that of mammalian cells, (2) the expression in mammalian cells of HERG splice variants (London et al., 1997; Bianchi et al., 1998) not present in the oocyte expression system, which might slightly influence loratadine binding, or (3) the different contents of monovalent and divalent cations in the extracellular solutions used in the two cell preparations. Our data confirm those of a recent study in which loratadine (up to 10 µM) failed to affect HERG K⁺ channels expressed in X. laevis oocytes but inhibited I_HERG stably expressed in a mammalian cell line with an IC₅₀ value of 3 µM (Lacerda et al., 1997). However, it should be mentioned that 3 µM loratadine did not inhibit I_Kr in guinea pig ventricular myocytes (Ko et al., 1997), whereas in our study, loratadine was found to effectively block I_HERG in human neuroblastoma cells (3 µM) and in X. laevis oocytes (30 µM). Although the discrepancy between the two studies remains unresolved at the moment, possible explanations might be found in the expression of different HERG splice variants or in differences in recording conditions. In conclusion, the current results clearly suggest that loratadine is at least 300 times less potent than astemizole or terfenadine in inhibiting HERG K⁺ channels. This observation might explain the lack of cardiac side effects associated with its use in humans (Woosley and Darrow, 1994; Brannan et al., 1995) and experimental animals (Hey et al., 1995), especially if one considers that after a single 40-mg dose, the C_max value of loratadine did not exceed 0.1 µM (Haria et al., 1994), a concentration at least 30 times lower than those used in the current study.

A direct comparison of HERG-blocking properties by the four H₁ receptor blockers also gives further insight into the structure-activity relationships for these molecules (Fig. 4). In fact, it has been suggested that the HERG K⁺ channel-blocking properties of terfenadine and its structural analogue ebastine are at least in part related to the substituting groups attached to the tertiary amine of the molecule rather than to the presence of the piperidine ring (Salata et al., 1995; Ko et al., 1997). This view seems to be confirmed by the observation that loratadine, which also exhibits a piperidine ring in its structure, was at least 300 times less potent than terfenadine in inhibiting HERG K⁺ channels. Furthermore, the aromatic ring structures common to most second-generation H₁ receptor antagonists seem not to be relevant for HERG K⁺ channels blockade. In fact, this region of the molecule confers H₁ receptor-blocking activity (Babe and Serafin, 1996); however, no correlation has been recently found between the ability to prolong the cardiac action potential duration, an effect possibly related to HERG K⁺ channels blockade, and the H₁ antagonistic activity by several antihistamines (Zhang, 1997).

View larger version (14K): [in this window] [in a new window]   Fig. 4.   Chemical structures of five different second-generation H1 receptor blockers.

Lipophilicity and bulkiness seem to be the two crucial parameters in the substituting groups attached to the tertiary amine conferring HERG K⁺ channel-blocking capacity to the antihistaminic molecule (Zhang, 1997). In fact, both cetirizine and loratadine have polar and smaller substitutions at the nitrogen atom (amido and carboxyl groups, respectively), whereas terfenadine, astemizole, and ebastine, the H₁ receptor antagonists most effective in inhibiting HERG K⁺ channels, have less polar and bulkier phenyl rings in the substituting side chains. This hypothesis is supported further by the observation that the more polar metabolites of terfenadine and astemizole, terfenadine carboxylate and norastemizole, respectively, do not display cardiotoxic potential (Hey et al., 1996). Furthermore, terfenadine carboxylate has been shown to be devoid of HERG K⁺ channel-blocking ability (Roy et al., 1996).

Because it has been demonstrated that HERG K⁺ channel blockade by terfenadine (Roy et al., 1996), as well as by the antiarrhythmic dofetilide (Kiehn et al., 1996), occurs at a site located on the cytoplasmic side of the channel, it seems possible to hypothesize that the lack of effect of cetirizine and the low potency of loratadine in inhibiting HERG K⁺ channels might be due to (1) a lower membrane permeability caused by their higher polarity or (2) their inability to interact with the terfenadine/astemizole receptor site on the channel molecule. The observation that internally applied cetirizine failed to inhibit I_HERG in HERG-transfected HEK 293 cells seems to suggest that the intracellular side of the channel molecule is insensitive to the drug, at least at the cytosolic concentrations reached in the current experiments. Therefore, it seems plausible to conclude that cetirizine lacks the ability to optimally interact with the terfenadine/astemizole receptor site on the intracellular side of the HERG K⁺ channel molecule.

In conclusion, the results of the current study suggest that second-generation H₁ receptor antagonists display marked heterogeneity in their blocking ability of HERG K⁺ channels. In particular, loratadine and cetirizine, which lack HERG-blocking ability, do not seem to induce ventricular arrhythmias such as torsade de pointes, whereas terfenadine and astemizole are potent blockers of HERG K⁺ channels and display significant arrhythmogenic potential. This conclusion might be of therapeutical significance for patients at risk of developing cardiac arrhythmias who require therapy with H₁ receptor blockers.

Finally, the observation that antihistamines greatly differ in their ability to interfere with HERG K⁺ channels and, consequently, to determine cardiotoxic effects emphasizes the importance of an evaluation of the possible blockade of HERG K⁺ channels, either constitutively present or heterologously expressed, during the early developmental phases of novel compounds belonging to this therapeutical class.