Troubleshooting problems with in vitro screening of drugs for QT interval prolongation using HERG K+ channels expressed in mammalian cell lines and Xenopus oocytes.

The majority of drugs associated with QT interval prolongation share an ability to inhibit ionic currents passed by HERG potassium channels. One method of screening new chemical entities (NCEs) for QT prolonging potential is therefore to use heterologous systems expressing HERG channels. Such systems are also of value in the understanding of the function, kinetics, sorting, pharmacological sensitivities, and important molecular determinants of the HERG potassium channel. The methods for incorporating the HERG potassium channel into cells and measuring the consequent current are a mixture of techniques that are standard (for heterologous expression of most ion channels) and individualised to HERG. This review presents a selection of the most commonly used methods for examining heterologous HERG currents, as well as introducing some of the technical problems that may be encountered and their solutions. In mammalian cell lines, problems such as fragile membranes, high leak currents, inability to form a gigaseal, diminished HERG current, endogenous transient outward current, altered kinetics, and even occasional run down can interfere with measurements. In Xenopus oocytes, endogenous chloride currents, insufficient superfusate flow, diminished HERG current and HERG current 'run up' may create difficulties.