What is the clinical evidence from this study?

Study design: Other. Population: HERG and eag potassium channel block. Intervention: Cisapride and site-directed mutagenesis of S6 aromatic residues vs. Wild-type channels. Primary outcome: IC50 for channel block by cisapride.

September 3, 2002Open Access

Position of aromatic residues in the S6 domain, not inactivation, dictates cisapride sensitivity of HERG and eag potassium channels

JCJun ChenNingbo University GSGuiscard SeebohmElectrophysiology MSMichael C. SanguinettiElectrophysiology

Key Result

The positioning of S6 aromatic residues relative to the central cavity of the channel, rather than inactivation per se, determines the sensitivity of HERG and eag potassium channels to block by cisapride.

Structured PICO

Population

In vitro study using Xenopus laevis oocytes expressing wild-type and mutant HERG and eag potassium channels to assess sensitivity to cisapride block.

Intervention

Repositioning of aromatic residues (Tyr-652 or Phe-656) in the S6 alpha-helical domain

Comparator

Wild-type channels or channels with different inactivation states

Outcome

Sensitivity to block by cisapridesurrogate

The positioning of S6 aromatic residues relative to the central cavity, rather than channel inactivation, determines the sensitivity of HERG and eag channels to drug block.

Limitations

Repositioning of a single aromatic residue required multiple mutations that altered gating properties, making it difficult to distinguish direct from allosteric effects on drug interaction.
Mutations in eag that induce sensitivity to cisapride may not confer sensitivity to other HERG blockers like MK-499.
The experimental approach cannot distinguish between differences in gating-associated rotation of S6 and differences in residue positioning relative to the central cavity.

Abstract

Unintended block of HERG K+ channels is a side effect of many common medications and is the most common cause of acquired long QT syndrome associated with increased risk of life-threatening arrhythmias. The molecular mechanism of high-affinity HERG block by structurally diverse compounds has been attributed to pi-stacking and cation-pi interactions of a drug (e.g., cisapride) with specific aromatic amino acid residues (Tyr-652 and Phe-656) in the S6 alpha-helical domain that face the central cavity of the channel. It also has been proposed that strong C-type inactivation of HERG facilitates or is the primary determinant of high-affinity drug binding. The structurally related, but noninactivating eag channel is insensitive to HERG blockers unless inactivation is induced by specific amino acid mutations Ficker, E., Jarolimek, W. & Brown, A. M. (2001) Mol. Pharmacol. 60, 1343-1348. Here we examine the relative importance of inactivation vs. positioning of S6 aromatic residues in determining sensitivity of HERG and eag channels to block by cisapride. The repositioning of Tyr-652 or Phe-656 along the S6 alpha-helical domain of HERG reduced sensitivity of channels to block by cisapride. Moreover, independent of inactivation, repositioning of the equivalent aromatic residues in Drosophila eag channels induced sensitivity to block by cisapride. These findings suggest that positioning of S6 aromatic residues relative to the central cavity of the channel, not inactivation per se determines drug block of HERG or eag channels.

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