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Drug binding to the inactivated state is necessary but not sufficient for high-affinity binding to human
Mark J Perrin1, Philip W Kuchel, Terence J Campbell
1Victor Chang Cardiac Research Institute, 384 Victoria Street, Darlinghurst, NSW 2010, Australia.
Insights
Drug block of the human ether-à-go-go-related gene K(+) channel (hERG) causes acquired long QT syndrome. Researchers found that preferential binding to the inactivated state is crucial for high-affinity hERG channel block, impacting cardiac repolarization.
Area of Science:
- Cardiovascular Pharmacology
- Molecular Cardiology
- Ion Channel Physiology
Background:
- Drug block of the human ether-à-go-go-related gene K(+) channel (hERG) is a primary cause of acquired long QT syndrome.
- Acquired long QT syndrome can lead to dangerous ventricular arrhythmias and sudden cardiac death due to disrupted cardiac repolarization.
Purpose of the Study:
- To investigate the state-dependent block of hERG channels by drugs, specifically examining the open versus inactivated states.
- To determine the relationship between drug affinity and the channel's inactivation properties using specific hERG mutants.
Main Methods:
- Utilized hERG channel mutants (N588K, N588E, S620T) with altered inactivation properties to study drug binding.
- Assessed the affinity of high- and low-affinity drugs for wild-type and mutant hERG channels.
- Employed computer modeling to analyze drug block kinetics relative to channel inactivation kinetics.
Main Results:
- High-affinity drugs showed reduced affinity for inactivation-deficient hERG mutants (N588K) compared to wild-type and N588E mutants.
- Most low-affinity drugs did not exhibit state-dependent block, with dl-sotalol being an exception.
- All tested state-dependent blockers had lower affinity for the non-inactivating S620T mutant compared to the N588K mutant.
- Calculated relative affinities for the inactivated versus open states, ranging from 4- to 70-fold for tested drugs.
Conclusions:
- Preferential binding to the inactivated state of the hERG channel is a necessary, but not sufficient, condition for high-affinity drug block.
- Understanding state-dependent block mechanisms is critical for predicting and mitigating drug-induced cardiac risks.
Abstract:
Drug block of the human ether-à-go-go-related gene K(+) channel (hERG) is the most common cause of acquired long QT syndrome, a disorder of cardiac repolarization that may result in ventricular tachycardia and sudden cardiac death. We investigated the open versus inactivated state dependence of drug block by using hERG mutants N588K and N588E, which shift the voltage dependence of inactivation compared with wild-type but in which the mutated residue is remote from the drug-binding pocket in the channel pore. Four high-affinity drugs (cisapride, dofetilide, terfenadine, and astemizole) demonstrated lower affinity for the inactivation-deficient N588K mutant hERG channel compared with N588E and wild-type hERG. Three of four low-affinity drugs (erythromycin, perhexiline, and quinidine) demonstrated no preference for N588E over N588K channels, whereas dl-sotalol was an example of a low-affinity state-dependent blocker. All five state-dependent blockers showed an even lower affinity for S620T mutant hERG (no inactivation) compared with N588K mutant hERG (greatly reduced inactivation). Computer modeling indicates that the reduced affinity for S620T compared with N588K and wild-type channels can be explained by the relative kinetics of drug block and unblock compared with the kinetics of inactivation and recovery from inactivation. We were also able to calculate, for the first time, the relative affinities for the inactivated versus the open state, which for the drugs tested here ranged from 4- to 70-fold. Our results show that preferential binding to the inactivated state is necessary but not sufficient for high-affinity binding to hERG channels.
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