A computational model predicts adjunctive pharmacotherapy for cardiac safety via selective inhibition of the late cardiac Na current

Abstract

Background: The QT interval is a phase of the cardiac cycle that corresponds to action potential duration (APD) including cellular repolarization (T-wave). In both clinical and experimental settings, prolongation of the QT interval of the electrocardiogram (ECG) and related proarrhythmia have been so strongly associated that a prolonged QT interval is largely accepted as surrogate marker for proarrhythmia. Accordingly, drugs that prolong the QT interval are not considered for further preclinical development resulting in removal of many promising drugs from development. While reduction of drug interactions with hERG is an important goal, there are promising means to mitigate hERG block. Here, we examine one possibility and test the hypothesis that selective inhibition of the cardiac late Na current (I_NaL) by the novel compound GS-458967 can suppress proarrhythmic markers.

Methods and results: New experimental data has been used to calibrate I_NaL in the Soltis-Saucerman computationally based model of the rabbit ventricular action potential to study effects of GS-458967 on I_NaL during the rabbit ventricular AP. We have also carried out systematic in silico tests to determine if targeted block of I_NaL would suppress proarrhythmia markers in ventricular myocytes described by TRIaD: Triangulation, Reverse use dependence, beat-to-beat Instability of action potential duration, and temporal and spatial action potential duration Dispersion.

Conclusions: Our computer modeling approach based on experimental data, yields results that suggest that selective inhibition of I_NaL modifies all TRIaD related parameters arising from acquired Long-QT Syndrome, and thereby reduced arrhythmia risk. This study reveals the potential for adjunctive pharmacotherapy via targeted block of I_NaL to mitigate proarrhythmia risk for drugs with significant but unintended off-target hERG blocking effects.

Keywords: GS-458967; Late Na current; Long-QT Syndrome; Proarrhythmia.

Fig. 1

A comparison of experimentally recorded and model generated transmembrane ion currents from rabbit ventricular myocytes. (A) Experimentally measured I_Kr (upper) [55], and I_K1 (lower) [14]. (B) Simulated I_Kr (upper) and I_K1 (lower) compared. (C) Experimental action potential clamp waveform (upper) and corresponding L-type Ca²⁺ current (lower) from rabbit ventricular myocyte [56]. (D) Simulated rabbit ventricular myocyte action potential and model generated L-type Ca²⁺ current. (E) Experimentally recorded Ca²⁺ transient during the AP [15]. (F) Corresponding simulated Ca²⁺ transient.

Fig. 2

Experimental (symbols) and model optimized (lines) drug free Na⁺ current parameters in a rabbit ventricular myocyte. (A) Na⁺ current activation curve derived from data generated in response to depolarizing voltage clamp pulses [18]. (B) Steady-state inactivation [18]. (C) Recovery from inactivation at −100 mV [18]. (D) I_Na waveform in drug free conditions at low gain normalized to peak current [9]. (E) Drug free I_Na at high gain [9]. (F) Superimposition of model predicted and experimentally recorded drug free or baseline I_Na in response to a slow depolarizing ramp voltage protocol.

Fig. 3

Experimentally measured and model predicted effect of GS-458967 on I_Na in rabbit ventricle. (A) Optimized model generated concentration-dependent data for GS-458967 on late Na current compared to two separate sets of experimental data - Blue asterisks indicate experimental data set #1 (n = 4) [9], and red circles are from experimental data set #2 (For 0 μM and 0.03 μM, n = 3. For 0.1 μM and 0.3 μM, n = 4. For 1 μM, n = 6. For 3 μM, n = 2). The effect of high concentration GS-458967 on peak I_Na is indicated for experiments (blue square), and simulated I_Na peak (black triangle). (B) Behavior of a myocyte ‘population’ was simulated by randomly varying the amplitude of maximal conductances for I_Na, I_CaL, I_Ks, I_Kr, I_K1, I_to, I_NaK, I_NaCa (to within 10% of their nominal values in the rabbit ventricular myocyte model). This approach allowed for efficient analysis of 100 distinct cell action potentials. APD₉₀ was calculated at 1 Hz for each case. These simulated myocyte properties were compared to experimental data set #1 (blue asterisks) and experimental data set #2 (red circles).

Fig. 4

In silico prediction of GS-458967 induced reduction of I_NaL and concentration-dependent shortening of APD in rabbit ventricular myocytes. (A) Simulated effects of GS-458967 on rabbit ventricular myocyte AP and (B) the corresponding effects of GS-458967 on late I_Na.

Fig. 5

GS-458967 can effectively attenuate APD prolongation by ATX-II in rabbit ventricular myocytes. (A) Experimental data from two distinct data sets from rabbit ventricular myocytes showing drug free conditions (left), the effect of 3 nM ATX-II (middle) and the combination of 3 nM ATX-II with 0.3 μM GS-458967 (right). (B) Simulated effects on virtual rabbit ventricular myocyte showing drug free (left), simulated effect of ATX-II (middle) and ATX-II with co-treatment with GS-458967 0.3 μM (right).

Fig. 6

Simulations showing that GS-458967 can effectively reduce spatial APD dispersion caused by ATX-II. (A) Space-time plots of membrane potential (top) and pseudo ECGs (lower) computed from a 165-rabbit myocyte transmural cardiac preparation in the presence of ATX-II during a “short-long-short” pacing protocol. (B) 0.03 μM GS-458967 markedly diminishes QT interval prolongation and APD dispersion as indicated by reduced T-wave amplitude. (C) 0.1 μM GS-458967 further reduced QT interval prolongation and reversed the repolarization gradient as demonstrated by inversion of the T-wave.

Fig. 7

In silico pharmacological results suggesting that GS-458967 can reduce all proarrhythmia-linked parameters set out in the TRIaD approach: Triangulation, reverse use dependence, beat-to-beat instability of action potential duration, as well as temporal and spatial action potential duration dispersion. Predicted temporal action potential duration dispersion of 1000 simulated myocyte action potentials generated after incorporating physiological noise to induce beat-to-beat variability at 1 Hz in (A) the drug-free control case, (B) effects of simulated application of the I_Kr blocker Dofetilide (16 nM) and (C) predicted effects of 0.3 μM GS-458967 with Dofetilide 16 nM. Action potential triangulation as a function of APD prolongation for individual myocytes for (D) control (slope = 0.37), (E) Dofetilide 16 nM (slope = 0.52) and (F) Dofetilide 16 nM + GS-458967 0.3 μM (slope = 0.35). (G) Instability of APD was quantified as the difference between the maximum and minimum of 1000 individual myocytes in the presence of physiological noise current as a function of prolongation of APD (shown in panels A–C). (H) Simulated beat-to-beat instability of rabbit ventricular myocyte action potentials to small perturbations before and after application of drugs. Poincaré plots of sequential APD pairs indicating beat-to-beat instability are shown for each case. (I) GS-458967 improved dofetilide induced reverse use dependence: Action potential adaptation curves show APD₉₀ at various pacing frequencies in the presence or absence of drugs.

Fig. 8

GS-458967 can prevent spiral wave reentry in the setting of acquired Long-QT Syndrome. A two-dimensional simulated heterogeneous anisotropic rabbit ventricular tissue was activated using a paired stimulus (S1-S2) protocol. (A) Shows the control or drug-free case, (B) with ATX-II, (C) with ATX-II and 0.3 μM GS-4589677, (D) 16 nM Dofetilide, or (E) 16 nM Dofetilide and 0.3 μM GS-458967. Tissues (5 cm × 5 cm) were stimulated (S1) along left edge (endocardium) and this followed by a premature stimulus (S2) applied in the vulnerable window (see Methods). Six snapshots obtained following application of GS-458967, dofetilide or both at selected time points. Corresponding pseudo-ECGs are in the right panels. Membrane voltage values are indicated by the color gradient.