research focus REVIEWS DDT Vol. 4, No. 9 September 1999 1359-6446/99/$ – see front matter © Elsevier Science Ltd. All rights reserved. PII: S1359-6446(99)01383-5 431 I on channels make good drug targets because they are physiologically essential, are pharmacologically access- ible, are encoded by a variety of genes and usually operate as multimeric protein assemblies, resulting in a high degree of functional and anatomical specificity 1,2 . Through molecular cloning, heterologous expression and electrophysiological characterization by patch-clamping, it is clear that the complexity of ion-channel biology also offers multiple opportunities for small-molecule drugs to achieve a specific, desired functional effect. For example, small molecules might influence a variety of biophysical properties of ion channels, such as voltage-dependence, permeability, use-dependence, activation and inactivation. In contrast to simple blockers or openers, the discovery of modulatory compounds could allow the development of drugs that specifically act on cells or tissues exhibiting aberrant levels of ion-channel activity. To screen compounds against functional ion channels or assemblies, it is necessary to reconstitute and measure channel function in a relevant biological context, such as in a cell. It is also preferable for the assay to be compat- ible with automated high-throughput screening (HTS) of large compound libraries. These assays would ideally be sensitive and fast, allow probing of various functional states and be amenable to miniaturization to 96-well plates and beyond. The importance of, and requirements for, assay miniaturization in drug discovery have been previ- ously discussed 3,4 . Current formats for ion-channel assays have been recently summarized in an excellent review 5 and will only be discussed briefly here. The purpose of this article is to discuss the implemen- tation of ion-channel assay technologies that combine high-screening throughput with high-information content. Although there are a variety of approaches being explored, including automated patch-clamping, ion-chan- nel measurements in lipid bilayers and viability assays based on the toxicity of constitutively opened channels 6 , it is not currently possible to use these methods for ion- channel HTS. Therefore, the article will focus on platform technologies that have a broad application to HTS, and in particular, will discuss the efforts of Aurora Biosciences Corporation (San Diego, CA, USA) to apply FRET-based voltage sensing to ion-channel HTS. Cell-based assays and instrumentation for screening ion-channel targets Jesús E. González, Kahuku Oades, Yan Leychkis, Alec Harootunian and Paul A. Negulescu Jesús E. González, Kahuku Oades, Yan Leychkis, Alec Harootunian and Paul A. Negulescu*, Aurora Biosciences Corp, 11010 Torreyana Road, San Diego, CA 92121, USA. *tel: +1 619 452 5000, fax: +1 619 404 6719, e-mail: NegulescuP@aurorabio.com Ion channels are an important class of drug targets. They comprise the molecular basis for essential physio- logical functions including fluid secretion, electrolyte balance, bioenergetics and membrane excitability. High-throughput screening for ion-channel function requires sensitive, simple assays and instrumentation that will report ion channel activity in living cells. This article will review relevant assay technologies for ion channels and describe voltage-sensitive probes and instruments based on fluorescence resonance energy transfer (FRET) that enable ion-channel drug discovery.