Drug Discovery Targets
The real challenge in developing therapeutics for ion-channel targets is the size of ion-channel families and their widespread distribution. The ongoing focus for the development of therapeutics with channel specificity is to develop subtype selectivity to improve efficacy and reduce the impact of side effects.
Within the pharmaceutical division of Devgen (www.devgen.com), the research focus is on treatment for inflammatory and metabolic diseases and arrhythmia. Petra Blom, Ph.D., section leader within the medicinal chemistry group, and Titus Kaletta, Ph.D., head of preclinical development, are working on a project targeting the Kv4.3 ion channel. The goal of their work is to develop a first-in-class oral treatment for acute and chronic atrial fibrillation.
Inhibition of Kv4.3 and Kv1.5 prolongs the action potential of the atria and the atrial refractory period. This mechanism is predicted to cardiovert patients and to keep them in sinus rhythm. Devgen identified inhibitors with excellent availability that are atrial selective and show no propensity to cause life-threatening ventricular arrhythmias in animal models, according to Dr. Blom.
Kv4.3, a voltage-gated potassium channel, functions in the early stage of the cardiac action potential. Channel activity is difficult to screen given the rapid polarization and depolarization cycle. “To solve this problem,” Dr. Kaletta noted, “the team developed an HTS assay based on the expression of the cloned human Kv4.3 channel in C. elegans.
“This whole-animal screening system combines the simplicity and convenience of C. elegans as a research tool and its close resemblance to human systems.” In the test, the action potential of the pharynx muscle, which shares comparable features to the action potential of the human heart, is used. Ingestion of fluorescently labeled food is used to measure the Kv4.3 channel activity in the pharynx of C. elegans.
“When the Kv4.3 channel is on, the pharynx pumps food poorly, generating a low fluorescent signal in the gut. When the channel is inhibited by, for example, a small molecule compound or is off, the pharynx activity changes, which results in a significant change in the fluorescent signal. The measured fluorescent signal directly correlates with ion-channel activity.”
With about 100 worms per well, the assay is performed in 96-well plates and enables the collection of 5,000 data points per day. The Devgen team obtained a 2% hit rate from the screen of its compound library using the C. elegans HTS assay. They verified selected hits in mammalian cells by means of patch-clamp electrophysiology; 83% of the hits were true human Kv4.3 inhibitors.
“Kv4.3 is a promising drug target for treatment of atrial fibrillation,” indicated Dr. Blom. “We identified two lead series that are potent human Kv4.3 blockers, which have proven efficacy in mice and dogs without hERG, Na+, or Ca2+ channel activity. With no safety issues encountered to date, we are proceeding with the preclinical development.”
Adrian Mason, Ph.D., senior scientist, and a team of scientists at Organon Laboratories (www.organon.co.uk; a part of Schering-Plough) are focusing on a different set of voltage-gated ion channels called HCN channels (hyperpolarization-activated cyclic nucleotide-gated channels). HCN channels, members of the voltage-gated cation-channel superfamily, selectively enable the flow of K+ and Na+ ions across the membrane.
Based on in situ hybridization studies, immunocytochemistry, and electrophysiological recordings, the four subtypes of HCN channel have been found to be widely distributed in humans. They are particularly plentiful in the brain, heart, and retina, but there is also evidence for expression in peripheral tissues such as kidney and testis. To a pharmaceutical company, the presence of HCN channels in many organs suggests therapeutic opportunities in a variety of human disorders but also flags the danger of side effects as a result of drugs acting at these targets.
Fortunately, there is some differential distribution of the HCN-channel subtypes that might be exploited to circumvent this problem. For example, HCN1 is highly expressed in the brain but restricted mainly to the cortex, hippocampus, and cerebellum. HCN4 is expressed prominently in the sino-atrial node of the heart, where it helps to control the cardiac rhythm, although it is also present in thalamic brain regions.
Although Organon has a successful history of developing and marketing ion-channel blocker drugs that are used during surgery to produce muscle relaxation, this current project takes the company in a new direction. “To be honest, the focus on HCN channels as a target for drug development started out of a bit of serendipity,” Dr. Mason said. “We had a drug candidate that was being developed for a psychiatric indication but found that it had a high specificity for HCN channels and blocked their activity. We followed up on this observation by testing related compounds using patch-clamp techniques on cloned HCN channels that had been expressed in HEK293 and CHO cell lines.”
Several years ago, evidence began to accumulate in the scientific literature that HCN channels might play a role in pain. Not only are the channels expressed in dorsal root ganglion cells, but knowledge of the basic biology of the channels suggested that they might have a pivotal influence on impulse activity in sensory neurons.
Building on the work of others, the Organon team showed that this was indeed the case and also demonstrated that their compounds were effective in animal models of pain. The team’s current focus is on finding therapeutic solutions to neuropathic pain.
In an attempt to exploit the well-established role of HCN4 in the heart, several companies have tried to develop HCN modulators for cardiac indications. Unfortunately, most of these compounds have failed due to side effects. Just one drug of this type has made it to market—Procoralan® (ivabradine from Servier), which is authorized in the EU for symptomatic treatment of stable angina.
“The challenge now is to find drugs acting at HCN channels that are effective in treating neuropathic pain but don’t have unwanted effects on other physiological functions,” said Dr. Mason. “It won’t be an easy task, but with our increasing knowledge of the basic biology of HCN channels together with recent developments in ion-channel screening technology, we are confident that we can give it our best shot.”