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Inhibition of Cardiac Ca2+ Release Channels (RyR2) Determines
Efficacy of Class I Antiarrhythmic Drugs in Catecholaminergic
Polymorphic Ventricular Tachycardia
Hyun Seok Hwang, PhD, Can Hasdemir, MD, Derek Laver, PhD, Divya Mehra, BPharm,
Kutsal Turhan, MD, Michela Faggioni, MD, Huiyong Yin, PhD, and Björn C. Knollmann, MD,PhDOates Institute for Experimental Therapeutics (H.S.H., M.F., B.C.K.), Vanderbilt University School
of Medicine, Division of Clinical Pharmacology, Nashville, TN; the Department of Cardiology
(C.H.) and Department of Thoracic Surgery (K.T.), Ege University School of Medicine, Izmir,
Turkey; the School of Biomedical Sciences and Pharmacy (D.L., D.M.), University of Newcastle
and HMRI, Callaghan, Australia; and the Department of Pharmacology and Chemistry (H.Y.),
Vanderbilt University School of Medicine, Nashville, TN
Abstract
Background—Catecholaminergic polymorphic ventricular tachycardia (CPVT) is caused by
mutations in the cardiac ryanodine receptor (RyR2) or calsequestrin (Casq2) and can be difficult
to treat. The class Ic antiarrhythmic drug flecainide blocks RyR2 channels and prevents CPVT in
mice and humans. It is not known whether other class I antiarrhythmic drugs also block RyR2
channels and to what extent RyR2 channel inhibition contributes to antiarrhythmic efficacy in
CPVT.
Methods and Results—We first measured the effect of all class I antiarrhythmic drugs
marketed in the United States (quinidine, procainamide, disopyramide, lidocaine, mexiletine,
flecainide, and propafenone) on single RyR2 channels incorporated into lipid bilayers. Only
flecainide and propafenone inhibited RyR2 channels, with the S-enantiomer of propafenone
having a significantly lower potency than R-propafenone or flecainide. In Casq2−/− myocytes, the
propafenone enantiomers and flecainide significantly reduced arrhythmogenic Ca2+ waves at
clinically relevant concentrations, whereas Na+ channel inhibitors without RyR2 blocking
properties did not. In Casq2−/− mice, 5 mg/kg R-propafenone or 20 mg/kg S-propafenone
prevented exercise-induced CPVT, whereas procainamide (20 mg/kg) or lidocaine (20 mg/kg)
were ineffective (n=5 to 9 mice, P<0.05). QRS duration was not significantly different, indicating
a similar degree of Na+ channel inhibition. Clinically, propafenone (900 mg/d) prevented ICD
shocks in a 22-year-old CPVT patient who had been refractory to maximal standard drug therapy
and bilateral stellate ganglionectomy.
Copyright © 2011 American Heart Association. All rights reserved.
Correspondence to Björn C. Knollmann, MD, PhD, Oates Institute for Experimental Therapeutics, Vanderbilt University School ofMedicine, Medical Research Building IV, Room 1265, 2215B Garland Ave, Nashville, TN [email protected].
Disclosures
None.
Drs Hwang, Hasdemir, and Laver contributed equally to this work.
The online-only Data Supplement is available at http://circep.ahajournals.org/cgi/content/full/CIRCEP.110.959916/DC1.
NIH Public AccessAuthor ManuscriptCirc Arrhythm Electrophysiol. Author manuscript; available in PMC 2013 May 29.
Published in final edited form as:
Circ Arrhythm Electrophysiol. 2011 April ; 4(2): 128–135. doi:10.1161/CIRCEP.110.959916.
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Conclusions—RyR2 cardiac Ca2+ release channel inhibition appears to determine efficacy of
class I drugs for the prevention of CPVT in Casq2−/− mice. Propafenone may be an alternative to
flecainide for CPVT patients symptomatic on β-blockers.
Keywords
class I antiarrhythmic drugs; propafenone; RyR2; catecholaminergic polymorphic ventricular
tachycardia; flecainide; ranolazine; tetrodotoxin; quinidine; procainamide; disopyramide;
lidocaine; mexiletine
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmia
syndrome characterized by physical or emotional stress-induced bidirectional or
polymorphic ventricular tachycardia.1 The more common autosomal-dominant form has
been linked to mutations in the gene encoding the cardiac Ca2+ release channel (RYR2).2 A
less common but more severe autosomal-recessive form is caused by mutations in the gene
encoding cardiac calsequestrin (CASQ2),3,4 the major Ca2+-binding protein in the
sarcoplasmic reticulum (SR).5 Ventricular myocytes isolated from mouse models of both
forms of CPVT exhibit catecholamine-induced premature SR Ca2+ release and spontaneous
Ca2+ waves that trigger delayed afterdepolarizations (DADs) and premature beats.6,7 Thus,
the spontaneous opening of SR Ca2+ release channels elicited by catecholaminergic stress
are the likely culprit for triggering ventricular arrhythmias in CPVT.8
Although significantly reduced, cardiac events remain unacceptably high in CPVT patients
treated with β-blockers.9 Even implantable cardioverter-defibrillators (ICDs) are not
necessarily effective, because defibrillation shocks can cause catecholamine release and
electrical storm, and deaths have been reported in CPVT patients with ICDs.10-12 Thus,
there is a need for better drug therapy in CPVT. We recently found that the class Ic
antiarrhythmic drug flecainide directly targets the molecular defect in CPVT by open-state
block of RyR2 channels13 and prevents CPVT in mice and humans.14 However, it is not
known whether other class I antiarrhythmic drugs also block RyR2 channels and to what
extent RyR2 channel inhibition contributes to antiarrhythmic efficacy in CPVT. Thus, we
tested the effect of all class I antiarrhythmic drugs currently marketed in the United States
on single RyR2 channels incorporated into lipid bilayers. We found that only propafenone
and flecainide inhibit RyR2 channels. The potency of RyR2 block determined
antiarrhythmic activity of Na+ channels blockers in vitro and in vivo in Casq2−/− mice, a
mouse model of CPVT. Clinically, propafenone prevented exercise-induced VT and
recurrent ICD shocks in a highly-symptomatic CPVT patient refractory to standard therapy.
Thus, RyR2 channel inhibition appears to be important for antiarrhythmic drug efficacy in
CPVT. Propafenone may be a promising therapeutic alternative to flecainide for CPVT
patients.
Methods
Animal Model and Experimental Procedures
All experiments were approved by the institutional animal care and use committees at
Animal Care and Use Committees of Vanderbilt University in the United States and
University of Newcastle in Australia and performed in accordance with National Institutes
of Health guidelines. Adult Casq2−/− mice (12 to 16 weeks old) were used for all
experiments. Casq2−/− mice consistently develop ventricular tachycardia during exercise or
after catecholamine challenge.6,14 All data were analyzed in blinded fashion regarding the
treatment groups. Quinidine hydrochloride monohydrate, procainamide hydrochloride,
disopyramide phosphate salt, lidocaine hydrochloride, mexiletine hydrochloride, flecainide
acetate, propafenone hydrochloride, ranolazine dihydrochloride, and tetrodotoxin were
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obtained from Sigma (St Louis, MO). (+)-S-propafenone and (-)-R-propafenone were
separated on a ChiralPak AD column (25×0.46 cm, Chiral Technologies, Exton, PA) using
hexane and 2-propanol containing 0.4% diethylamine. The flow rate was 1 mL and UV
absorption was monitored at 247 nm. The purity of these 2 enantiomers was >99%.15
Single RyR2 Channel Measurements
SR vesicles containing RyR2 channels were obtained from sheep hearts and were
reconstituted into artificial lipid bilayers.16 During SR-vesicle incorporation, the cis
(cytoplasmic) bath contained (in mmol) 250 Cs+ (230 CsCH3O3S, 20 CsCl), 1.0 CaCl2, and
500 mannitol; the trans (luminal) solution contained 50 Cs+ (30 CsCH3O3S, 20 CsCl2) and
1.0 CaCl2. After detection of channels in the bilayer the [Cs+] of the trans solutions was
increased to 250 mmol/L by means of aliquot addition of 4 mol/L CsCH3O3S. The
cytoplasmic solution was exchanged to one containing 2 mmol/L ATP and 0.1 μmol/L free
Ca2+ (1 mmol/L CaCl2+4.5 mmol/L BAPTA) via bath perfusion. The perfusion system
allowed exposure of a single channel to multiple drugs and concentrations that could be
applied in any sequence. Solutions were pH-buffered with 10 mmol/L N-tris
[Hydroxymethyl]methyl-2-aminoethanesulfonic acid (TES, ICN Biomedicals), and solutions
were titrated to pH 7.4 using CsOH (optical grade, ICN Biomedicals) and were redox-
buffered with 5 mmol/L glutathione (ICN Biomedicals).
Electric potentials are expressed using standard physiological convention (ie, cytoplasmic
side relative to the luminal side at virtual ground). Single-channel recordings were obtained
using bilayer potential difference of +40 mV. The current signal was digitized at 10 kHz and
low-pass filtered at 1 or 2 kHz with a gaussian digital filter. Open probability (Po) as well as
open and closed durations were measured by the 50% threshold detection method (Channel2
software by P.W. Gage and M. Smith, Australian National University, Canberra).
Cell Isolations and Ca2+ Fluorescence Recordings
Ventricular myocytes were isolated by a modified collagenase/protease method as
described.6 All the experiments were conducted in Tyrode solution containing (in mmol:
CaCl2 2, NaCl 134, KCl 5.4, MgCl2 1, glucose 10, and HEPES 10, pH 7.4. Final
concentration of Ca2+ was 2 mmol/L. After isolation, myocytes were loaded with Fura-2
acetoxymethyl ester (Fura-2AM).6,17 Briefly, myocytes were incubated with Fura-2AM (2
μmol/L) for 6 minutes at room temperature to load the indicator in the cytosol. Myocytes
were washed twice for 10 minutes with Tyrode solution. A minimum of 30 minutes was
allowed for deesterification before imaging the cells. Ca2+ fluorescence ratios (Fratio) were
recorded and normalized relative to the mean of vehicle group. The ratiometric fluorescent
records were analyzed using commercially available data analysis software (IonWizard,
IonOptix, Milton, MA). Spontaneous Ca2+ waves were measured using the following
protocol: Fura-2AM-loaded myocytes were field stimulated at 1 Hz for 20 seconds until they
reach a steady Ca2+ transient height. Then, stimulation was switched off and myocytes were
monitored for 40 seconds for the occurrence of spontaneous Ca2+ waves, followed by
application of caffeine (10 mmol/L) for 5 seconds using a rapid concentration clamp system.
Amplitudes of caffeine-induced Ca2+ transients were used as estimates of SR Ca2+ content.
Analysis was carried out as recently described by us.14 A spontaneous SR Ca2+ wave was
defined as any spontaneous increase of 0.07 ratiometric units or more from the diastolic
Fratio.
ECG Recordings of Exercise-Induced and Isoproterenol-Induced Ventricular Tachycardiain Mice
Treadmill Exercise Test—Exercise testing in conscious Casq2−/− mice was carried out
as previously described.6 Briefly, mice were initially anesthetized (pentobarbital, 70 mg/g),
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and an ECG transmitter (Data Sciences International, St Paul, MN) was implanted into the
abdominal cavity with subcutaneous electrodes in lead II configuration. Animals were
allowed to recover for at least 6 days after surgery before participating in the treadmill
exercise studies. Study drug or vehicle (DMSO) was injected intraperitoneally 30 minutes
before exercise. We previously established that a dose of 20 mg/kg flecainide results in a
serum flecainide serum concentration of 2.5 μmol/L 1 hour after intraperitoneal injection,
causes a 25% increase in QRS duration, and effectively suppressed exercise-induced CPVT
in mice.14 Because measurement of the QRS duration provides a rapid, noninvasive and
accurate biomarker of Na+ channel block18 and because the objective of our study was to
compare the efficacy of antiarrhythmic drugs at a similar degree of Na+ channel block, study
drugs were administered at doses that produce a 25% increase in QRS duration (online-only
Data Supplement Figure 1). Mice were placed individually into a special chamber of the
motorized rodent treadmill (Exer-6M, Columbus Instruments, Columbus, OH) and exercised
until they exhibited signs of exhaustion.6 For each mouse, treadmill testing was performed 4
times at an interval of >72 hours in randomized sequence. High-quality ECGs were recorded
and analyzed from 15 minutes before exercise until 12 hours after as previously reported.14
Isoproterenol Challenge—The β-adrenergic receptor agonist isoproterenol was used to
induce ventricular arrhythmia in anesthetized Casq2−/− mice as previously described.6
Briefly, mice were anesthetized with isoflurane vapor titrated to maintain the lightest
anesthesia possible. On average, 1.0% vol/vol isoflurane vapor was sufficient to maintain
adequate anesthesia. Loss of toe-pinch reflex and respiration rate was used to monitor levels
of anesthesia. Baseline ECG was recorded for 5 minutes, followed by an additional 10
minutes after administration of isoproterenol (3 mg/kg i.p.). R-propafenone (5 mg/kg) or
vehicle (DMSO) was injected intraperitoneally 30 minutes before administration of
isoproterenol. Isoproterenol challenge was performed 2 times in the same mouse at an
interval of >72 hours, with half the animals receiving first placebo, then R-propafenone, and
the other half receiving first R-propafenone, then placebo.
Human Studies
Propafenone is an approved antiarrhythmic drug and was administered as part of routine
clinical care. The patient’s legal guardian provided consent for retrospective data analysis,
left and subsequently right stellate ganglionectomy, and exercise stress testing. Patient data
collection was approved by the institutional ethics committee. ICD interrogations and
analysis were performed every 2 months on a regular basis or immediately if the patient
received ≥2 ICD shocks.
Statistical Analysis
Differences between the groups of conscious mice were assessed using a 1-way ANOVA
with repeated-measure analysis conducted with SPSS, version 17.0 (SPSS Inc, Chicago, IL),
followed by the Tukey post hoc test when appropriate. The incidence and rate of
spontaneous Ca2+ waves in myocytes and ventricular ectopy in mice were compared by
Fisher exact test and nonparametric Mann-Whitney test. Drug effects on single RyR2
channels were compared using 1-way ANOVA followed by the Tukey post hoc test when
appropriate. The single-channel concentration response curves for S- and R-propafenone
were compared by applying a t test to the differences between measured values and a
common Hill curve over the concentration range of 10 to 50 μmol/L. Results were
considered statistically significant if the probability value was <0.05.
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Results
Propafenone Inhibits RyR2 Ca2+ Release Channels by Open State Block
We first tested the effect of all class I antiarrhythmic drugs currently marketed in the United
States (quinidine, procainamide, disopyramide, lidocaine, mexiletine, flecainide, and
propafenone) on single RyR2 channels incorporated into artificial lipid bilayers. These
experiments were carried out using cytoplasmic and luminal [Ca2+] that mimic cellular
[Ca2+] during diastole (control condition: 2 mmol/L ATP, [Ca2+]cyt=0.1 μmol/L, and
[Ca2+]lum=1 mmol/L). Although class Ia and Ib drugs had no significant effect on RyR2
channels, both class Ic drugs flecainide and propafenone significantly inhibited RyR2
channels at a concentration of 20 μmol/L (Figure 1A).
Propafenone is used clinically as a racemic mixture of S- and R-enantiomeres.19 Both
enantiomers are equipotent Na+ channel inhibitors.20 Only S-propafenone has moderate β-
adrenergic receptor-blocking activity.20 Thus, we next examined the effects of R- and S-
propafenone on RyR2 channels. A representative example of RyR2 channel opening under
these control conditions is shown in Figure 1B (top trace). RyR2 open probability (Po) was
≈0.1. Addition of S-propafenone, R-propafenone, and flecainide to the cytoplasmic bath
caused similar effects on RyR2 channel gating (Figure 1B). The reduction in RyR2 Po was
primarily the consequence of reduced channel To, because all 3 drugs induced brief channel
closures to a subconductance state with ≈30% of the open state conductance (Figure 1B and
1C). The concentration dependencies of R-propafenone, S-propafenone, and flecainide on
To are shown in Figure 1C. Note that none of the 3 drugs significantly changed channel
closed times (Tc, Figure 1C), which is in contrast to the RyR2 channel inhibitor tetracaine13
that is not effective in CPVT myocytes.14 R-propafenone and flecainide reduced RyR2 Po
with similar potency, whereas S-propafenone was significantly less potent (Figure 1D).
Thus, RyR2 channel block by propafenone is stereoselective. The mechanism of
propafenone action is consistent with an open state block and similar to flecainide.
Only Na+ Channel Inhibitors With RyR2 Channel Blocking Properties ReduceIsoproterenol-Induced Ca2+ Waves in Casq2−/− Myocytes
We next tested the efficacy of class I antiarrhythmic drugs and other Na+ channel inhibitors
in ventricular myocytes isolated from a mouse model of CPVT, Casq2−/− mice. All study
drugs were applied for 30 minutes before measurements were obtained. As previously
reported,6 β-adrenergic stimulation with isoprotorenol causes spontaneous Ca2+ waves
(SCW) and triggered beats in Casq2−/− myocytes (Figure 2A). Compared with treatment
with vehicle, the rate of isoproterenol-stimulated Ca2+ waves was drastically reduced by
application of either R-propafenone or flecainide (Figure 2A and 2B), with an IC50 of
approximately 1.1±0.5 μmol/L and 2.0±0.2 μmol/L, respectively. Consistent with its lower
potency in single RyR2 channels (Figure 1C), S-propafenone was significantly less effective
compared to flecainide and R-propafenone in intact myocytes (S-propafenone, 6 μmol/L
0.41±0.12; P<0.05; Figure 2B). Class I agents that did not reduce RyR2 channel open
probability (Figure 1A) had also no significant effect on isoproterenol-induced SCW in
Casq2−/− myocytes (Figure 2A and 2B). Moreover, the selective Na+ channel blocker
tetrodotoxin and the late Na+ current inhibitor ranolazine also had no significant effect on
SCW rate (Figure 2B). At the same time, none of the drugs tested significantly changed
amplitude and decay rate of field-stimulated Ca2+ transients (data not shown) or SR Ca2+
content (Figure 2C). The latter is important because SR Ca2+ content is a determinant of the
incidence of spontaneous Ca2+ waves.21 Taken together, these results demonstrated that
RyR2 channel block is required to suppress arrhythmogenic Ca2+ waves, whereas Na+
channel block by itself has no significant effect.
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RyR2 Channel Block Is Required for Antiarrhythmic Efficacy of Class I Drugs in CPVT Mice
To test the therapeutic efficacy of R-propafenone in vivo, we first studied anesthetized
Casq2−/− mice challenged with isoproterenol (3 mg/kg). Casq2−/− mice exhibit
catecholamine-induced and exercise-induced polymorphic or bidirectional ventricular
tachycardia.6,13,14 Analogous to flecainide,14 pretreatment with R-propafenone (5 mg/kg
i.p.) prevented isoproterenol induced-ventricular ectopy (Figure 3). Ventricular extrasystoles
occurred in 9 of 14 Casq2−/− mice pretreated with vehicle (ventricular extrasystoles per
minute, 4.6±1.6), but only in 1 of 14 Casq2−/− mice pretreated with R-propafenone
(ventricular extrasystoles per minute, 0.3±0.3, Mann-Whitney test, P=0.01). Interestingly,
R-propafenone treatment significantly reduced the isoproterenol-induced heart rate response
(peak heart rate [bpm]: Veh, 558±11 versus R-prop, 476±10; P<0.05).
Next, we examined to what extent RyR2 and Na+ channel block contribute to antiarrhythmic
efficacy in vivo. To address this question, we compared the effect of 2 groups of drugs on
exercised-induced CPVT of conscious Casq2−/− mice: (1) class I antiarrhythmic drugs
without RyR2-blocking properties (procainamide and lidocaine, Figure 1A); and (2), class I
drugs with RyR2-blocking properties (S- and R-propafenone). Both propafenone
enantiomers are equipotent as Na+ channel blockers,20 but S-propafenone has lower potency
on RyR2 channels (Figure 1C) and correspondingly lower efficacy in suppressing SCW in
myocytes (Figure 2A). All drugs were dosed to produce a similar increase in QRS duration
(online-only Data Supplement Figure 1), an in vivo marker of Na+ channel inhibition.18
Pretreatment with a single injection of R-propafenone (5 mg/kg) completely prevented VT
during and for up to 4 hours after treadmill exercise, whereas procainamide (20 mg/kg) or
lidocaine (20 mg/kg) had no significant effect on exercise-induced VT (Figure 4). The time
course of R-propafenone reduction of VT is shown in online-only Data Supplement Figure
2. Consistent with its lower potency on RyR2 channels in vitro (Figure 1D), a 4-fold higher
dose of S-propafenone (20 mg/kg) than R-propafenone was required to suppress VT in vivo
(Figure 4). Collectively, these results indicated that RyR2 inhibition is required for
antiarrhythmic efficacy of class I agents in CPVT mice. Furthermore, the stereoselective
action of R-propafenone as RyR2 inhibitor translates into a higher potency of preventing
exercise or catecholamine-induced polymorphic ventricular tachycardia in vivo.
Propafenone Suppressed ICD Discharges and Exercise-Induced Ventricular Tachycardia ina CPVT Patient
We next tried propafenone in a 22-year-old CPVT patient (RyR2 missense mutation
p.L4105F)22 who was refractory to therapy. His medical history was remarkable for cerebral
palsy caused by birth hypoxia. Within 24 hours of admission, the patient had polymorphic
ventricular tachycardia associated with loss of consciousness requiring 4 direct-current
cardioversions. Holter monitoring showed frequent polymorphic and bidirectional VT. The
patient underwent placement of a single-chamber ICD in June 2007. Ventricular arrhythmias
were initially controlled with the combination of metoprolol (200 mg/d) and verapamil (120
mg/d).22 Starting in the fall of 2008, the patient had frequent appropriate ICD discharges (91
shocks during the 6-month period from November 2008 until April 2009) despite maximal
standard drug therapy and bilateral cardiac sympathetic denervation performed by stellate
ganglionectomy (Figure 5). Because flecainide is not available for clinical use in Turkey,
where the patient lives, propafenone (initially 300 mg/d, subsequently 600 mg/d, and finally
900 mg/d) was started in April 2009 and resulted in a dramatic reduction of ICD discharges
(Figure 5). Only 2 episodes of ICD discharges occurred during 12 months of follow-up, after
the propafenone dose was increased to 600 to 900 mg/d. Propafenone therapy also
completely prevented ventricular arrhythmias during treadmill exercise testing (online-only
Data Supplement Figure 3).
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Discussion
Only Class Ic Antiarrhythmic Drugs Propafenone and Flecainide Directly Target MolecularDefects in CPVT Patients
Over the last decade, the disease mechanism responsible for the CPVT has been well
established.8 Mutations in RyR2 or Casq2 cause a sensitization of the RyR2 Ca2+ release
channel complex to SR luminal Ca2+.23,24 As a result, RyR2 channels open prematurely
under conditions of high SR Ca2+ load, which typically occurs in conditions of β-adrenergic
stimulation and/or fast heart rates.25 The premature Ca2+ release from dyadic RyR2 channel
clusters triggers a propagated Ca2+ wave, which in turn activates the electrogenic NaCa
exchanger located on the cell membrane.26 The ensuing NaCa exchange produces a net
inward Na+ current, which depolarizes the cell membrane and generates a DAD.27 DADs of
sufficient amplitude activate voltage-gated Na+ channels and trigger full action potentials.
Given its unique pharmacological properties, propafenone has multiple modes of action in
CPVT: The RyR2 open channel block discovered here will reduce the likelihood of RyR2
opening triggering an arrhythmogenic Ca2+ wave.13 The class Ic-type tonic Na+ channel
inhibition during diastole will decrease the likelihood of any remaining DADs in triggering
premature ventricular extrasystoles.28 Although less potent as a RyR2 inhibitor, the β-
blocking activity of the S-enantiomere will also contribute to clinical efficacy of
propafenone racemate in CPVT. These multiple modes of propafenone action may explain
why propafenone was effective in a patient refractory to all currently known
pharmacological and surgical therapy in CPVT.
RyR2 Open Channel Block by Class Ic Antiarrhythmic Agents
It is interesting to note that local anesthetics and other drugs that inhibit Na+ channels often
also modulate RyR2 channels, albeit with a large variety of effects. In the present study, we
show that flecainide and propafenone effectively inhibit RyR2 by reducing channel mean
open durations via an open-state blocking mechanism. The RyR2 inhibitory action was not
shared by any of the clinically available class Ia or Ib antiarrhythmic agents. Other
anesthetics such as tetracaine, procaine, and QX314 also inhibit RyR2 channels29 but have a
significantly lower potency and a very different mode of action compared with that of
propafenone and flecainide.13 Tetracaine, unlike flecainide and propafenone, causes long
closures by binding to channels in their closed state, leading to substantially increased
closed-channel intervals.13 The prototypical class Ib agent lidocaine has no effects on RyR2
channels (Figure 1A) and activates RyR2 channels at higher concentrations.30 The tricyclic
antidepressant amitriptyline, which is a potent Na+ channel inhibitor, activates RyR2
channels at clinically relevant concentrations.16 Amitriptyline binds to RyR2 in their open
state and induces prolonged channel openings with ≈80% of the normal channel
conductance.16
The open-state RyR2 channel block probably contributes to the effectiveness of flecainide
and propafenone in suppressing DADs and triggered arrhythmias in experimental models.28
RyR2 channel inhibition may also explain the negative inotropic effects of propafenone and
flecainide, which exceed that expected from Na+ channel block alone31 and the significant
risk of aggravation of heart failure observed clinically with propafenone and flecainide.32
Implications for Drug Therapy in CPVT
Currently, β-blockers are the first line of therapy in CPVT.8 They are the only class of drugs
that has been shown to be effective in improving survival in CPVT.9 Because the incidence
of cardiac events remains considerable even on β-blockers9,33 and ICDs are not always
effective,10-12 there is a need for alternative therapies. Small case series suggest that Ca2+
channels blockers34 or stellate ganglionectomy35 may provide additional benefit. However,
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as illustrated by CPVT patients reported here and previously,14 neither therapy appears to be
completely effective in CPVT. Class Ia and Ib antiarrhythmic agents are not effective as
shown here and previously reported by others in CPVT mouse models36 and humans.33
These results are not surprising because lidocaine can activate RyR2 channels30 and would
be predicted to worsen the underlying molecular defect. We recently reported that
flecainide, a class Ic agent, appeared effective in a mouse model CPVT and in 2 patients
with drug-refractory CPVT, possibly because of its dual mode of action. Although a recent
study failed to find a significant effect of flecainide during an in vivo screen of
antiarrhythmic drugs in CPVT mice,36 this study was neither designed nor statistically
powered to examine flecainide action.37 Another group recently confirmed flecainide
efficacy in Purkinje cells.38 Our results with propafenone further support the hypothesis that
open-state block of RyR2 channels13 coupled with Na+ channel inhibition during diastole
presents a promising therapeutic approach in CPVT. Our finding that S-propafenone was
less effective than R-propafenone in reducing exercise-induced ventricular tachycardia in
CPVT mice suggests that the RyR2 blocking action may be more important than Na+
channel blocking activity for antiarrhythmic drug efficacy in CPVT. Because the
propafenone racemate used clinically has also β-blocking properties, it may be a promising
therapeutic option for CPVT patients who require life-long drug therapy.
Study Limitations
Our data do not exclude the possibility that in addition to the RyR2 inhibition discovered in
the present study, block of Na+ channels, and/or other ion channel also contribute to the
antiarrhythmic efficacy of propafenone in CPVT mice. The exact contribution of RyR2
channel block to propafenone action will have to be addressed by comparison with a
selective open channel RyR2 inhibitor, an agent that is currently not available. Although our
in vitro studies suggest that propafenone is a mechanism-based drug therapy in CPVT, our
clinical experience is limited to a single patient. Given the proarrhythmic effects of class Ic
agents in patients with ischemic heart disease or heart failure,39 the diagnosis of CPVT
should be clearly established and structural heart disease ruled out. As with any new
therapeutic regimen, future clinical studies have to be done to define the risks and benefits
of propafenone as treatment for CPVT.
Conclusion
In this study, we have shown that among clinically available class I antiarrhythmic drugs,
only flecainide and propafenone inhibit RyR2 channels, suppress arrhythmogenic Ca2+
waves, and prevent CPVT in Casq2−/− mice. Thus, the potency of RyR2 channel inhibition
rather than Na+ channel block appears to determine efficacy of class I agents for the
prevention of CPVT. Clinically, propafenone was effective in suppressing appropriate ICD
shocks in a CPVT patient refractory to standard therapy. Thus, propafenone and flecainide
both are promising mechanism-based drug therapies for CPVT patients. However, the
stereoselective action of propafenone discovered in the present study has to be considered
for its clinical use in CPVT patients, given the stereoselective metabolism of propafenone
racemate in humans.40,41
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
We thank Paul Johnson and Meegan Jones at the University of Newcastle for assistance with the single-channel
recording and Melissa Ryan at Vanderbilt University for myocyte isolations.
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Sources of Funding
This work was supported by National Institutes of Health grants HL88635 and HL71670 (to Dr Knollmann),
American Heart Association postdoctoral fellowship grant 09POST2240022 (to Dr Hwang), and an NSW Health
infrastructure grant and a project grant from the University of Newcastle (to Dr. Laver).
References
1. Leenhardt A, Lucet V, Denjoy I, Grau F, Ngoc DD, Coumel P. Cate-cholaminergic polymorphic
ventricular tachycardia in children: a 7-year follow-up of 21 patients. Circulation. 1995; 91:1512–
1519. [PubMed: 7867192]
2. Priori SG, Napolitano C, Memmi M, Colombi B, Drago F, Gasparini M, DeSimone L, Coltorti F,
Bloise R, Keegan R, Cruz Filho FE, Vignati G, Benatar A, DeLogu A. Clinical and molecular
characterization of patients with catecholaminergic polymorphic ventricular tachycardia.
Circulation. 2002; 106:69–74. [PubMed: 12093772]
3. Postma AV, Denjoy I, Hoorntje TM, Lupoglazoff JM, Da Costa A, Sebillon P, Mannens MM,
Wilde AA, Guicheney P. Absence of calse-questrin 2 causes severe forms of catecholaminergic
polymorphic ventricular tachycardia. Circ Res. 2002; 91:e21–e26. [PubMed: 12386154]
4. Lahat H, Pras E, Olender T, Avidan N, Ben-Asher E, Man O, Levy-Nissenbaum E, Khoury A,
Lorber A, Goldman B, Lancet D, Eldar M. A missense mutation in a highly conserved region of
CASQ2 is associated with autosomal recessive catecholamine-induced polymorphic ventricular
tachycardia in Bedouin families from Israel. Am J Hum Genet. 2001; 69:1378–1384. [PubMed:
11704930]
5. Campbell KP, MacLennan DH, Jorgensen AO, Mintzer MC. Purification and characterization of
calsequestrin from canine cardiac sarcoplasmic reticulum and identification of the 53,000 Dalton
glycoprotein. J Biol Chem. 1983; 258:1197–1204. [PubMed: 6337133]
6. Knollmann BC, Chopra N, Hlaing T, Akin B, Yang T, Ettensohn K, Knollmann BE, Horton KD,
Weissman NJ, Holinstat I, Zhang W, Roden DM, Jones LR, Franzini-Armstrong C, Pfeifer K.
Casq2 deletion causes sarcoplasmic reticulum volume increase, premature Ca2+ release, and
catecholaminergic polymorphic ventricular tachycardia. J Clin Invest. 2006; 116:2510–2520.
[PubMed: 16932808]
7. Liu N, Colombi B, Memmi M, Zissimopoulos S, Rizzi N, Negri S, Imbriani M, Napolitano C, Lai
FA, Priori SG. Arrhythmogenesis in catecholaminergic polymorphic ventricular tachycardia:
insights from a RyR2 R4496C knock-in mouse model. Circ Res. 2006; 99:292–298. [PubMed:
16825580]
8. Liu N, Rizzi N, Boveri L, Priori SG. Ryanodine receptor and calse-questrin in arrhythmogenesis:
what we have learnt from genetic diseases and transgenic mice. J Mol Cell Cardiol. 2009; 46:149–
159. [PubMed: 19027025]
9. Hayashi M, Denjoy I, Extramiana F, Maltret A, Buisson NR, Lupoglazoff JM, Klug D, Takatsuki S,
Villain E, Kamblock J, Messali A, Guicheney P, Lunardi J, Leenhardt A. Incidence and risk factors
of arrhythmic events in catecholaminergic polymorphic ventricular tachycardia. Circulation. 2009;
119:2426–2434. [PubMed: 19398665]
10. Postma AV, Denjoy I, Kamblock J, Alders M, Lupoglazoff JM, Vaksmann G, Dubosq-Bidot L,
Sebillon P, Mannens MM, Guicheney P, Wilde AA. Catecholaminergic polymorphic ventricular
tachycardia: RYR2 mutations, bradycardia, and follow up of the patients. J Med Genet. 2005;
42:863–870. [PubMed: 16272262]
11. Mohamed U, Gollob MH, Gow RM, Krahn AD. Sudden cardiac death despite an implantable
cardioverter-defibrillator in a young female with catecholaminergic ventricular tachycardia. Heart
Rhythm. 2006; 3:1486–1489. [PubMed: 17161793]
12. Pizzale S, Gollob MH, Gow R, Birnie DH. Sudden death in a young man with catecholaminergic
polymorphic ventricular tachycardia and paroxysmal atrial fibrillation. J Cardiovasc
Electrophysiol. 2008; 19:1319–1321. [PubMed: 18554199]
13. Hilliard FA, Steele DS, Laver D, Yang Z, Le Marchand SJ, Chopra N, Piston DW, Huke S,
Knollmann BC. Flecainide inhibits arrhythmogenic Ca(2+) waves by open state block of
ryanodine receptor Ca(2+) release channels and reduction of Ca(2+) spark mass. J Mol Cell
Cardiol. 2010; 48:293–301. [PubMed: 19835880]
Hwang et al. Page 9
Circ Arrhythm Electrophysiol. Author manuscript; available in PMC 2013 May 29.
NIH
-PA
Auth
or M
anuscrip
tN
IH-P
A A
uth
or M
anuscrip
tN
IH-P
A A
uth
or M
anuscrip
t
14. Watanabe H, Chopra N, Laver D, Hwang HS, Davies SS, Roach DE, Duff HJ, Roden DM, Wilde
AA, Knollmann BC. Flecainide prevents catechol-aminergic polymorphic ventricular tachycardia
in mice and humans. Nat Med. 2009; 15:380–383. [PubMed: 19330009]
15. Aboul-Enein HY, Bakr SA. Direct enantiomeric high performance liquid chromatographic
separation of propafenone and its major metabolite inserum on a cellulose tris-3,5-dimethylphenyl
carbamate chiral stationary phase. Biomed Chromatogr. 1993; 7:38–40. [PubMed: 8094305]
16. Chopra N, Laver D, Davies SS, Knollmann BC. Amitriptyline activates cardiac ryanodine channels
and causes spontaneous sarcoplasmic reticulum calcium release. Mol Pharmacol. 2009; 75:183–
195. [PubMed: 18845675]
17. Chopra N, Kannankeril PJ, Yang T, Hlaing T, Holinstat I, Ettensohn K, Pfeifer K, Akin B, Jones
LR, Franzini-Armstrong C, Knollmann BC. Modest reductions of cardiac calsequestrin increase
sarcoplasmic reticulum Ca2+ leak independent of luminal Ca2+ and trigger ventricular
arrhythmias in mice. Circ Res. 2007; 101:617–626. [PubMed: 17656677]
18. Vaughan Williams EM. The relevance of cellular to clinical electrophysiology in classifying
antiarrhythmic actions. J Cardiovasc Pharmacol. 1992; 20(Suppl)
19. Harron DW, Brogden RN. Propafenone. A review of its pharmacodynamic and pharmacokinetic
properties, and therapeutic use in the treatment of arrhythmias. Drugs. 1987; 34:617–647.
[PubMed: 3322781]
20. Stoschitzky K, Klein W, Stark G, Stark U, Zernig G, Graziadei I, Lindner W. Different
stereoselective effects of (R)- and (S)-propafenone: clinical pharmacologic, electrophysiologic,
and radioligand binding studies. Clin Pharmacol Ther. 1990; 47:740–746. [PubMed: 2162749]
21. Diaz ME, O’Neill SC, Eisner DA. Sarcoplasmic reticulum calcium content fluctuation is the key to
cardiac alternans. Circ Res. 2004; 94:650–656. [PubMed: 14752033]
22. Hasdemir C, Aydin HH, Sahin S, Wollnik B. Catecholaminergic polymorphic ventricular
tachycardia caused by a novel mutation in the cardiac ryanodine receptor. Anadolu Kardiyol Derg.
2008; 8:E35–E36. [PubMed: 18849218]
23. Terentyev D, Kubalova Z, Valle G, Nori A, Vedamoorthyrao S, Terentyeva R, Viatchenko-
Karpinski S, Bers DM, Williams SC, Volpe P, Gyorke S. Modulation of SR Ca release by luminal
Ca and calsequestrin in cardiac myocytes: effects of CASQ2 mutations linked to sudden cardiac
death. Biophys J. 2008; 95:2037–2048. [PubMed: 18469084]
24. Jiang D, Wang R, Xiao B, Kong H, Hunt DJ, Choi P, Zhang L, Chen SR. Enhanced store overload-
induced Ca2+ release and channel sensitivity to luminal Ca2+ activation are common defects of
RyR2 mutations linked to ventricular tachycardia and sudden death. Circ Res. 2005; 97:1173–
1181. [PubMed: 16239587]
25. Bers DM. Cardiac excitation-contraction coupling. Nature. 2002; 415:198–205. [PubMed:
11805843]
26. Venetucci LA, Trafford AW, O’Neill SC, Eisner DA. Na/Ca exchange: regulator of intracellular
calcium and source of arrhythmias in the heart. Ann N Y Acad Sci. 2007; 1099:315–325.
[PubMed: 17446473]
27. Marban E, Robinson SW, Wier WG. Mechanisms of arrhythmogenic delayed and early
afterdepolarizations in ferret ventricular muscle. J Clin Invest. 1986; 78:1185–1192. [PubMed:
3771791]
28. Zeiler RH, Gough WB, El-Sherif N. Electrophysiologic effects of propafenone on canine ischemic
cardiac cells. Am J Cardiol. 1984; 54:424–429. [PubMed: 6465028]
29. Xu L, Jones R, Meissner G. Effects of local anesthetics on single channel behavior of skeletal
muscle calcium release channel. J Gen Physiol. 1993; 101:207–233. [PubMed: 8384242]
30. Shoshan-Barmatz V, Zchut S. The interaction of local anesthetics with the ryanodine receptor of
the sarcoplasmic reticulum. J Membr Biol. 1993; 133:171–181. [PubMed: 8390576]
31. Honerjager P, Loibl E, Steidl I, Schonsteiner G, Ulm K. Negative inotropic effects of tetrodotoxin
and seven class 1 antiarrhythmic drugs in relation to sodium channel blockade. Naunyn
Schmiedebergs Arch Pharmacol. 1986; 332:184–195. [PubMed: 2422563]
32. Antiarrhythmic Drug Evaluation Group (A.D.E.G.). A multicentre, randomized trial on the benefit/
risk profile of amiodarone, flecainide and propafenone in patients with cardiac disease and
complex ventricular arrhythmias. Eur Heart J. 1992; 13:1251–1258. [PubMed: 1396837]
Hwang et al. Page 10
Circ Arrhythm Electrophysiol. Author manuscript; available in PMC 2013 May 29.
NIH
-PA
Auth
or M
anuscrip
tN
IH-P
A A
uth
or M
anuscrip
tN
IH-P
A A
uth
or M
anuscrip
t
33. Sumitomo N, Harada K, Nagashima M, Yasuda T, Nakamura Y, Aragaki Y, Saito A, Kurosaki K,
Jouo K, Koujiro M, Konishi S, Matsuoka S, Oono T, Hayakawa S, Miura M, Ushinohama H,
Shibata T, Niimura I. Catecholaminergic polymorphic ventricular tachycardia:
electrocardiographic characteristics and optimal therapeutic strategies to prevent sudden death.
Heart. 2003; 89:66–70. [PubMed: 12482795]
34. Rosso R, Kalman JM, Rogowski O, Diamant S, Birger A, Biner S, Belhassen B, Viskin S. Calcium
channel blockers and beta-blockers versus beta-blockers alone for preventing exercise-induced
arrhythmias in catecholaminergic polymorphic ventricular tachycardia. Heart Rhythm. 2007;
4:1149–1154. [PubMed: 17765612]
35. Wilde AA, Bhuiyan ZA, Crotti L, Facchini M, De Ferrari GM, Paul T, Ferrandi C, Koolbergen
DR, Odero A, Schwartz PJ. Left cardiac sympathetic denervation for catecholaminergic
polymorphic ventricular tachycardia. N Engl J Med. 2008; 358:2024–2029. [PubMed: 18463378]
36. Katz G, Khoury A, Kurtzwald E, Hochauser E, Porat E, Shainberg A, Seidman JG, Seidman CE,
Lorber A, Eldar M, Arad M. Optimizing CPVT therapy in calsequestrin-mutant mice. Heart
Rhythm. 2010; 7:1676–1682. [PubMed: 20620233]
37. Knollmann BC. Power and pitfalls of using transgenic mice to optimize therapy for CPVT: a need
for randomized placebo-controlled clinical trials in genetic arrhythmia disorders (editorial). Heart
Rhythm. 2010; 7:1683–1685. [PubMed: 20673814]
38. Kang G, Giovannone SF, Liu N, Liu FY, Zhang J, Priori SG, Fishman GI. Purkinje cells from
RyR2 mutant mice are highly arrhythmogenic but responsive to targeted therapy. Circ Res.
107:512–519. [PubMed: 20595652]
39. Echt DS, Liebson PR, Mitchell LB, Peters RW, Obias-Manno D, Barker AH, Arensberg D, Baker
A, Friedman L, Greene HL, Huther ML, Richardson DW, CAST Investigators. Mortality and
morbidity in patients receiving encainide, flecainide, or placebo: the Cardiac Arrhythmia
Suppression Trial. N Engl J Med. 1991; 324:781–788. [PubMed: 1900101]
40. Kroemer HK, Fromm MF, Buhl K, Terefe H, Blaschke G, Eichelbaum M. An enantiomer-
enantiomer interaction of (S)- and (R)-propafenone modifies the effect of racemic drug therapy.
Circulation. 1994; 89:2396–2400. [PubMed: 7910120]
41. Kroemer HK, Funck-Brentano C, Silberstein DJ, Wood AJ, Eichelbaum M, Woosley RL, Roden
DM. Stereoselective disposition and pharmacologic activity of propafenone enantiomers.
Circulation. 1989; 79:1068–1076. [PubMed: 2713973]
Hwang et al. Page 11
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CLINICAL PERSPECTIVE
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare familial
arrhythmia syndrome characterized by emotional or physical stress-induced polymorphic
or bidirectional ventricular tachycardia. CPVT has been linked to mutations in genes that
regulate Ca2+ release from the sarcoplasmic reticulum (eg, RYR2, CASQ2). Although β-
blockers are first-line therapy, they are not always effective, and better drug therapy is
needed. We recently discovered that the class I antiarrhythmic drug flecainide directly
targets the molecular defect in CPVT by blocking RyR2 Ca2+ release channels and
prevented CPVT in mice and humans. In the present study, we extended this work and
tested the efficacy of all Food and Drug Administration-approved class I antiarrhythmic
drugs on RyR2 channels, in isolated myocytes, and in vivo using a mouse CPVT model.
We found that only propafenone and flecainide inhibit RyR2 channels and prevent
exercise-induced CPVT in mice, whereas all other class I drugs lack RyR2 inhibitory
properties and were ineffective. This result suggests that RyR2 channel inhibition
importantly contributes to antiarrhythmic efficacy in CPVT and should be considered
when selecting drug therapy for CPVT patients. As illustrated by the CPVT case report,
propafenone may be a promising alternative to flecainide for CPVT patients whenever
flecainide is not clinically available or not tolerated. However, RyR2 channel inhibition
by propafenone is stereoselective, with R-propafenone being significantly more potent
than S-propafenone. Because propafenone is available clinically only as racemate and its
metabolism is also stereoselective, large interindividual differences in clinical response
may be expected when treating CPVT patients with propafenone.
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Figure 1.
Among class I antiarrhythmic drugs, only flecainide, propafenone, and its enantiomers (S-
and R-propafenone) inhibit RyR2 activity. A, Relative change in channel open probability
caused by study drug application at a concentration of 20 μmol/L. B, Records are
representative examples of single channel activity of RyR2 in lipid bilayers. The baseline
current during channel closures is labeled “C” (dashed lines) and channel openings
correspond to upward current transitions. Control conditions were 1 mmol/L luminal Ca2+
(trans bath), 0.1 μmol/L cytoplasmic Ca2+ plus 2 mmol/L ATP (cis bath). Bilayer potential
was 40 mV (relative to trans bath as ground). Relatively high drug concentrations (50 μmol/
L) were used to better illustrate their effects on RyR2 channel gating. Drug addition
introduced short (≈1 ms) closures to a substrate, labeled “S” (dotted lines), at ≈30% of the
full channel conductance. The full durations of the long (≈1 second) closed periods that are
present in control and drug records are not seen on this time scale. C, Concentration-
response relationship of RyR2 channel inhibition by R- and S-propafenone and flecainide.
RyR2 channel open probability (Po), mean open time (To), and mean closed time (Tc) are
expressed relative to values in absence of drug. D, Comparison of IC50 values determined by
least-squares fitting of Hill curves to the Po and To concentration response data (n=7 to 13
channels per group). *P<0.05.
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Figure 2.
Efficacy of class I antiarrhythmic drugs and other Na+ channel inhibitors for suppressing
isoproterenol-induced spontaneous Ca2+ waves (arrow). A, Representative Ca2+
fluorescence records from Casq2−/− myocytes after 30 minutes’ exposure to vehicle
(DMSO), procainamide (15 μmol/L), lidocaine (50 μmol/L), R-propafenone (6 μmol/L),
and tetrodotoxin (TTX, 6 μmol/L). After 20 seconds pacing and in presence of isoproterenol
(1 μmol/L), SCWs were recorded during 40 seconds without pacing. SR Ca2+ contents were
quantified by rapid caffeine (10 mmol/L) application at the end of recording. Average rate of
SCW (B) and SR Ca2+ content (C) are expressed relative to vehicle. Quinidine (6 μmol/L),
disopyramide (6 μmol/L), mexiletine (6 μmol/L), flecainide (6 μmol/L), and ranolazine (15
μmol/L), *P<0.01 versus vehicle, #P<0.05 versus R-propafenone; n=28 to 38 myocytes per
group.
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Figure 3.
R-propafenone prevents catecholamine-induced ventricular arrhythmia in Casq2−/− mice.
Plotted is the heart rate of an anesthetized Casq2−/− mouse in response to isoproterenol (3
mg/kg) injection 30 minutes after subcutaneous injection with vehicle (DMSO, left) or R-
propafenone (n=14, 5 mg/kg, right). Note the complete suppression of ventricular ectopy
illustrated by the representative ECG records above the heart rate plot.
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Figure 4.
Efficacy of class I antiarrhythmic drugs in preventing exercise-induced VT in conscious
Casq2−/− mice. Procainamide (n=5, 20 mg/kg), lidocaine (n=5, 20 mg/kg), R-propafenone
(n=9, 5 mg/kg), or S-propafenone (n=5 per dose, 5 mg/kg, 20 mg/kg as indicated) was
injected intraperitoneally 30 minutes before exercise. A, Representative example of
nonsustained bidirectional VT episode. B and C, Incidence of VT during treadmill exercise
(B) and during the 4-hour period after exercise (C) for each treatment group. *Versus
vehicle, P<0.05.
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Figure 5.
Effect of propafenone treatment in a 21-year-old CPVT patient (RyR2 missense mutation)
with recurrent appropriate ICD shocks despite maximal conventional drug therapy and
bilateral cardiac sympathetic denervation. Single arrow indicates the date of left cardiac
sympathetic denervation and double arrow the date of right cardiac sympathetic denervation.
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