Implications of Dynamic Occupancy, Binding Kinetics, and Channel Gating Kinetics for hERG Blocker Safety Assessment and Mitigation.

Blockade of the hERG potassium channel prolongs the ventricular action potential (AP) and QT interval, and triggers early after depolarizations (EADs) and torsade de pointes (TdP) arrhythmia. Opinions differ as to the causal relationship between hERG blockade and TdP, the relative weighting of other contributing factors, definitive metrics of preclinical proarrhythmicity, and the true safety margin in humans. Here, we have used in silico techniques to characterize the effects of channel gating and binding kinetics on hERG occupancy, and of blockade on the human ventricular AP. Gating effects differ for compounds that are sterically compatible with closed channels (becoming trapped in deactivated channels) versus those that are incompatible with the closed/closing state, and expelled during deactivation. Occupancies of trappable blockers build to equilibrium levels, whereas those of non-trappable blockers build and decay during each AP cycle. Occupancies of ~83% (non-trappable) versus ~63% (trappable) of open/inactive channels caused EADs in our AP simulations. Overall, we conclude that hERG occupancy at therapeutic exposure levels may be tolerated for nontrappable, but not trappable blockers capable of building to the proarrhythmic occupancy level. Furthermore, the widely used Redfern safety index may be biased toward trappable blockers, overestimating the exposure-IC50 separation in nontrappable cases.

A mathematical model of electrotonic interactions between ventricular myocytes and fibroblasts.

Functional intercellular coupling has been demonstrated among networks of cardiac fibroblasts, as well as between fibroblasts and atrial or ventricular myocytes. In this study, the consequences of these interactions were examined by implementing the ten Tusscher model of the human ventricular action potential, and coupling it to our electrophysiological models for mammalian ventricular fibroblasts. Our simulations reveal significant electrophysiological consequences of coupling between 1 and 4 fibroblasts to a single ventricular myocyte. These include alterations in plateau height and/or action potential duration (APD) and changes in underlying ionic currents. Two series of simulations were carried out. First, fibroblasts were modeled as a spherical cell with a capacitance of 6.3 pF and an ohmic membrane resistance of 10.7 G Omega. When these "passive" fibroblasts were coupled to a myocyte, they caused slight prolongation of APD with no changes in the plateau, threshold for firing, or rate of initial depolarization. In contrast, when the same myocyte-fibroblast complexes were modeled after addition of the time- and voltage-gated K(+) currents that are expressed in fibroblasts, much more pronounced effects were observed: the plateau height of the action potential was reduced and the APD shortened significantly. In addition, each fibroblast exhibited significant electrotonic depolarizations in response to each myocyte action potential and the resting potential of the fibroblasts closely approximated the resting potential of the coupled ventricular myocyte.

K+ currents activated by depolarization in cardiac fibroblasts.

K(+) currents expressed in freshly dispersed rat ventricular fibroblasts have been studied using whole-cell patch-clamp recordings. Depolarizing voltage steps from a holding potential of -90 mV activated time- and voltage-dependent outward currents at membrane potentials positive to approximately -30 mV. The relatively slow activation kinetics exhibited strong dependence on the membrane potential. Selected changes in extracellular K(+) concentration ([K(+)](o)) revealed that the reversal potentials of the tail currents changed as expected for a K(+) equilibrium potential. The activation and inactivation kinetics of this K(+) current, as well as its recovery from inactivation, were well-fitted by single exponential functions. The steady-state inactivation was well described by a Boltzmann function with a half-maximal inactivation potential (V(0.5)) of -24 mV. Increasing [K(+)](o) (from 5 to 100 mM) shifted this V(0.5) in the hyperpolarizing direction by -11 mV. Inactivation was slowed by increasing [K(+)](o) to 100 mM, and the rate of recovery from inactivation was decreased after increasing [K(+)](o). Block of this K(+) current by extracellular tetraethylammonium also slowed inactivation. These [K(+)](o)-induced changes and tetraethylammonium effects suggest an important role for a C-type inactivation mechanism. This K(+) current was sensitive to dendrotoxin-I (100 nM) and rTityustoxin Kalpha (50 nM).