Dissociation of HERG channel inhibitors

The HERG (human ether-a-go-go-related gene) encodes a delayed rectifier K+ channel that plays a pivotal role during the repolarisation phase of the cardiac action potential. Prolongation of the duration of the ventricular action potential, due to inherited mutations or drug block may cause long QT syndrome with the risk of life threatening torsade de pointes arrhythmias (TdP) and sudden cardiac death. We have recently shown that the HERG inhibitors bepridil, terfenadine, E-4031 and domperidone are efficiently trapped in the closed channel conformation while other blockers such as amiodarone, cisapride, droperidol and haloperidol are able to dissociate from resting closed channels (Stork et al. 2007). In our previous studies we were, however, unable to detect specific drug properties (e.g. size, hydrophobicity etc.) that either facilitate or prevent drug trapping. In the present project we aim to study the dissociation pathways of trapped and non-trapped compounds from closed (resting) channels making use of specially designed propafenone- (prototype of a trapped compound) and verapamil-derivatives (prototype of a non trapped compound). To enable a description of the molecular events underlying the processes of drug trapping and dissociation the consortium (S. Hering`s group and Prof. Ecker from the University of Vienna and B. De Groot`s group at the Max Plank Institute in Göttingen) will apply molecular dynamics simulations (incl. drug docking studies and QSAR) and biophysical methods. Force probe MD simulations will facilitate analysing drug dissociation from different channel states. The proposed work flow and the combination of in silico methods, biophysical calculations and experiments will lead to a better characterization of structural and functional determinants of HERG block and recovery. A major goal of this work is to identify/characterize specific drug properties, facilitating or preventing drug trapping and dissociation.

Understanding the unintended block of the hERG (ether-a-go-go-related gene, homo sapiens) K+ channel with pharmaceuticals is a major task for pharmaceutical research. HERG encodes a delayed rectifier K+ channel that plays a pivotal role during the repolarisation phase of the cardiac action potential. Prolongation of the duration of the ventricular action potential, due to inherited mutations or drug block may cause long QT syndrome with the risk of life threatening torsade de pointes arrhythmias (TdP) and sudden cardiac death. At the beginning of this project we knew that HERG inhibitors bepridil, terfenadine, E-4031 and domperidone are efficiently trapped in the closed channel conformation while other blockers such as amiodarone, cisapride, droperidol and haloperidol are able to dissociate from resting closed channels (Stork et al. 2007). The major goal of this project concerned investigating the dissociation pathways of trapped and non-trapped compounds from closed (resting) channels making use of specially designed propafenone derivatives. A combination of experimental and computational methods enabled us to unravel the molecular mechanisms of drug trapping and dissociation. Surprisingly, our investigations revealed that one of the amino acids, essential for drug binding plays a major role in dissociation as well. Further our calculations revealed how inhibitors are trapped during channel closure. Surprisingly, our investigations revealed that inhibitors can only dissociate from fully open channels. During the characterization of structural and functional blocking and dissociation determinants, we identified a novel binding determinant, essential for channel-drug interactions.