Toward in vivo -relevant hERG safety assessment and mitigation strategies based on relationships between non-equilibrium blocker binding, three-dimensional channel-blocker interactions, dynamic occupancy, dynamic exposure, and cellular arrhythmia

Diffusion of a single fully solvated blocker copy into a large cavity lined by the intracellular cyclic nucleotide binding homology domain (the initial capture step). Occupation of this cavity is a necessary but insufficient condition for ion current disruption.

Translocation of the captured blocker along the channel axis (the I _Kr disruption step), such that:

The head group, consisting of a quasi-linear moiety, projects into the open pore, accompanied by partial de-solvation of the binding interface.

One tail moiety packs along a kink between the S6 helix and proximal C-linker helix adjacent to the intra-cellular entrance of the pore, likewise accompanied by mutual de-solvation of the binding interface (noting that the association barrier is comprised largely of the total head + tail group de-solvation cost).

Blockers containing a highly planar moiety that projects into a putative constriction zone within the closed channel become trapped upon closing, as do blockers terminating prior to this region.

A single captured blocker molecule may associate and dissociate from the pore many times before exiting the CNBHD cavity.

Benefit/risk.

The predicted arrhythmogenic fractional hERG occupancy (based on action potential simulations of the undiseased human ventricular cardiomyocyte).

Alteration of the safety threshold due to underlying disease.

Risk of exposure escalation toward the predicted arrhythmic limit due to patient-to-patient pharmacokinetic variability, drug-drug interactions, overdose, and use for off-label indications in which the hERG safety parameters may differ from their on-label counterparts.