Physicochemical Determinants for Drug Induced Blockade of HERG Potassium Channels: Effect of Charge and Charge Shielding

The data on the activities of all previously described HERG blockers and of the most widely cited IKr blockers were analyzed with respect to the effect of potential charged center(s) and its shielding by surrounding structural elements. The following model was considered: the less shielding of the charged form of the drug occurs, the easier its deprotonation will be and the less potency of the blockade of HERG / IKr channels will be. Tertiary amines which form ammonium ions shielded by two structural fragments of the drug molecule were found to be potent HERG / IKr blockers with IC50 < 1 μM (16 of 19 compounds, 84%). However, if the charged center was found at the molecular periphery as such groups as dimethylamino, N-methylpiperidino, N-methylpiperazino, N-methylpyrrolidino, pyrrolidino, imidazolo and partial periphery (diethylamino), then only moderate potency for HERG blockade with 1 μM <IC50< 10 μM (8 of 11 compounds; 73%) was observed. Similarly, 27 of 32 weak HERG blockers ( IC50 >10 μM) were found to be primary or secondary amines, or neutral or very weakly basic compounds. Ions of primary and secondary amines are susceptible to the fast deprotonation of the charged center and they, as well as non-charged compounds, have a low probability of induction of Torsades de Pointes (TdP). Conformational analysis and modeling of the interaction of the charged fragment of the drugs with acetone, a system that mimics a ketone fragment of HERG / IKr channel, supports preference of the conformation with the shielded charged center for potent HERG / IKr blockers. The absence of stereospecificity of HERG / IKr blockade observed in most of the published studies reinforces the importance of charged center shielding as a key parameter. We suggest that the introduction of a hydroxy group at position 3 relative to a tertiary ammonium charged center, or the introduction of hydroxy, alkoxy or amino groups at position 2 relative to the nitrogen center of an aromatic system, should provide easy access of a water molecule to the proton, thereby facilitating deprotonation and thus leading to a moderate or weak HERG / IKr blockade and a reduced risk of TdP.