Inhibition of cardiac Ca2+ release channels (RyR2) determines efficacy of class I antiarrhythmic drugs in catecholaminergic polymorphic ventricular tachycardia

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 Ca(2+) 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.

Conclusions: RyR2 cardiac Ca(2+) 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.

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 (P_o), mean open time (T_o), and mean closed time (T_c) are expressed relative to values in absence of drug. D, Comparison of IC₅₀ values determined by least-squares fitting of Hill curves to the P_o and T_o concentration response data (n=7 to 13 channels per group). *P<0.05.

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.

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.

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.

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.