Therapeutics CHANNELLING DRUG DISCOVERY current trends in ion channel drug discovery research Ion channels are proteins that span cell membranes thus forming conduits or ‘channels’ through which charged ions such as sodium and potassium can pass across a normally impermeant barrier such as the plasmalemma. By so doing, ion channels can mediate a wide variety of physiological functions from the generation of action potentials in nerve cells to immune cell function and more. By Dr David Owen lthough the pharmaceutical industry has channels are fundamental in controlling the heart and Andrew been slow to recognise the true potential of beat, sensory transduction including pain and Silverthorne Aion channels due to a combination of disbe- brain function. In non-excitable cells, ion channels lief that ion channel dysfunction could cause disease are involved in hormonal secretion, immune cell and a lack of screening technology to keep up with responsivity, cell-cycling, ion distribution and other HTS assays, ion channels are currently very more. Why is it then that only now at the begin- much in vogue. The discovery of ‘Channelopathies’ ning of the 21st century that ion channels are sud- and exciting emerging ion channel screening tech- denly creating so much interest in the pharmaceu- nologies herald a new era of intensive ion channel- tical industry? Here we review the current trends in based drug discovery. Here we review the current the ion channel drug discovery business and how a trends in the ion channel drug discovery business. convergence of research and technology develop- ments may provide the answer. Why should drug companies be Ion channels are a super-family of proteins that interested in ion channels? span cell membranes and form conduits or chan- We have known about ionic currents in nerve cells nels through which charged ions such as sodium since 19521 and been able to electrically visualise and potassium can pass across what is normally an single ion channels in real time using the patch- impermeant barrier. We tend to think of the plas- clamp technique since 19812. The first sodium ion malemma as being home to most ion channels but channel was cloned in 1984 (Noda et al) and the intracellular organelles such as mitochondria and first potassium channel in 19873. Since then we endoplasmic reticulum also have ion channels. have realised that ion channels are physiologically Broadly speaking, ion channels can be divided into important in a huge variety of functions and in all those controlled by receptors and those opened cells. In excitable cells such as nerve and muscle, and closed (gated) by changes in the voltage of the ion channels generate and shape electrical signals cell. By their very nature voltage-gated ion chan- leading to action potential propagation, neuro- nels are particularly attractive targets, but also transmitter release and muscle contraction. Ion present a challenge for the industry particularly in 48 Drug Discovery World Spring 2002 Therapeutics finding appropriate screening technology. This review will focus primarily on these so-called volt- AB age-gated ion channels. In 1984 in the first edition of his seminal book on Activation gate the ‘Ionic Channels of Excitable Membranes’, Hille4 recognised in his preface that ion channels went well Outside beyond nerve cells and were likely to be important in non-excitable cells such as ‘sperm, white blood cells and endocrine glands’. He also predicted that our Plasmalemma genome would probably code for more than 50 dif- ferent types of ion channel. Bearing in mind that the Inside first ion channel had yet to be cloned (it was later Voltage Binding sites for that same year), this was a bold statement to make. sensor small molecules He needn’t have worried though, 18 years later the K+ Inactivation gate Human Genome Project predicted that there are more than 300 human genes encoding ion channels5. Voltage-gated ion channels are turned-on and off (or gated) by movements of so-called ‘gates’ creat- ed by elements of its own protein structure. In chan- without doubt. Add to the variability of ion channel Figure 1 nels such as K+ channels, a change in the voltage of structure afforded by this heteromeric association A cartoon representation of a K+ channel showing activation the cell causes a segment of the channel (known as phenomenon, variations on the basic subunit caused and inactivation ‘gates’ and the voltage sensor) to move within the membrane by ‘splice variation’ and it easy to imagine that the possible binding sites for small thereby opening a channel through which K+ ions number of 300 could easily be increased by a factor molecules and water molecules can pass one at a time. Other of two or more. Overlayed on this variety are more B Many K+ channels are parts of the protein which make up the lining of the or less specific tissue distributions of ion channel composed of 4 subunits that together form the functional channel determine which ions can or cannot pass expression as for traditional targets such as neuro- channel through the channel and also act as receptors for transmitter receptors. Of course some ion channels small molecules and toxins which can modulate are more ubiquitous than others, but it is clear that these functions in various ways (Figure 1A). A func- there is real potential for selective modulation of ion tional voltage-gated K+ channel is composed of at channels both between tissues and within cell-types, least four subunits represented as cylinders, which an important consideration in any drug discovery assemble as a complex in the membrane (Figure context. At this point in time, we know that K+ cur- 1B). Na+ and Ca2+ channels have analogous fea- rents (that is currents carried by the flux of potassi- tures although the four subunits found in K+ chan- um ions across a membrane) can be generated by nels are contained within a single protein. one or more of around 70 different potassium-selec- tive ␣-subunits. Na+ currents arise from around 10 Ion channel cloning different genes; there are around nine voltage-gated Ever since 1984 when the first sodium channel was and another seven non-transmitter operated chlo- cloned, cloning of new ion channels has gathered ride channels and 13 voltage-gated calcium channel pace, culminating perhaps in completion of The ␣-subunits. Other channel types have significant Human Genome Project. The combined effort of the numbers of family members as well. For example, HGP and Celera parallel project indicates that we there are around 20 TRP channels, 12 Deg/ENaC can expect around 300 ion channel genes divided channels, 13 connexins and so on. between the major ion channel families. We also See also: www.gene.ucl.ac.uk/nomenclature/ know that for K+ channels, for example, which are genefamily/KCN.html,www.gene.ucl.ac.uk/ composed of tetramers of a basic pore-forming sub- nomenclature/genefamily/CACN.html and 6. unit (␣-subunit), it is also possible to get functional Although traditionally one thinks of sodium and channels from permutations of ␣-subunits. In addi- potassium channels and the generation of action tion, many pore-forming subunits associate with potentials in nerve cells, in fact all cells (as far as we auxiliary subunits which, while not necessarily pore- know) have ion channels of some type or other. The forming in themselves, can modify the properties of bewildering number of different types of ion chan- the ion channel, either biophysically (eg speed-up nels identified at a molecular level suggests that ion inactivation) or pharmacologically (eg increase sen- channels are important in a similarly large number sitivity to drugs). While many of these ␣-subunits of physiological processes. Sure enough, ion chan- are known, many more remain to be discovered nel involvement ranges from action potential Drug Discovery World Spring 2002 49 Therapeutics threshold in nerve and heart to the resting potential their propensity to block cardiac ion channels such of immune cells and sperm motility. The richness of as hERG and the potential for drug-induced QT this pool of possible therapeutic targets is surely prolongation prior to first use in humans. If there irresistible. See also: www.neuro.wustl.edu/neuro- wasn’t a demand for ion channel screening resource muscular/mother/chan.html7,8. before aLQT there certainly was afterwards. This has had a number of important consequences: Relevance of ion channels to disease G The profile and importance of the ion channel as (channelopathies) a target for drugs has been raised dramatically. For many years, ion channel modulators were seen Almost everyone in the R&D hierarchy of a drug as palliative at best. However, since the first ‘chan- company now knows about at least one voltage- nelopathy’ was identified in the cystic fibrosis activated ion channel: hERG. transmembrane regulator protein (CFTR) by G Since the CPMP note there has been a rapidly Riordan et al in 19899, this has turned into a escalating requirement for hERG and other ion growth industry. There are around 30 chan- channel screens. In some cases these have been pro- nelopathies to date including major therapeutic vided in house but in many companies these assays areas such as diabetes, cardiac disease, deafness, are out-sourced to other specialist organisations. blindness and epilepsy. All can be caused by ion G The realisation that activity at ion channels like channels that malfunction or are not expressed at hERG are best eliminated early in the drug discov- all. See also 10 and www.neuro.wustl.edu/neuro- ery process has heightened the need for screening muscular/mother/chan.html. techniques with appropriate information content Without doubt these links have reinforced the and throughput that will make this possible.