Arthur Brown, M.D., Ph.D., president and CEO of ChanTest, summed up the sentiments of a number of speakers at the recent “SBS Conference” in Lille, France, when he said, “Ion channels are as abundant as GPCRs in the known druggable genome and have as much potential as drug targets for pain, hypertension, and inflammation. Yet they still don’t receive the time and attention they should from drug developers.”
Kim Dekermendjian, Ph.D., senior research scientist in molecular pharmacology at AstraZeneca, agreed, adding “only seven percent of current drug targets are ion channels while 30 percent are GPCRs, so ion channels are totally underexploited and this is because historically it has been so difficult to screen them in a rational way.”
Since ion channels are a diverse family of pore-forming proteins that control the voltage gradient and ionic flow across the plasma membrane, it is well established that the most accurate method of measuring a drug’s effects on an ion channel is via patch clamping.
This works by forming a patch through which intracellular access is gained to measure and control membrane voltage and current flowing through the cell membrane. In the past five years relatively low-throughput automated patch clamps such as the QPatch from Sophion Bioscience, the Patchliner from Nanion Technologies, and the PatchXpress® and IonWorks® Quattro™ systems from Molecular Devices, a MDS Analytical company, have emerged. The cost per data point, however, has made these automated methods too expensive to use in primary screening.
According to Dr. Dekermendjian, sometimes the screening costs of using these systems, even in secondary screens, is referred to at AstraZeneca as “tears per data point”. As a result, surrogate measures of ion-channel activity such as ligand displacement, ionic flux, and membrane potential are used as primary screens with equipment including the Molecular Devices FLIPRTetra®, the most commonly used.
The disadvantage of using these assays is they cannot maintain a population of ion channels in a particular conformational state due to the lack of voltage control and so need to be used as methods to weed out the best drug candidates for a confirmatory patch-clamp screen. “The surrogate measures are nonfunctional and cannot identify allosteric modulators,” noted Dr. Brown. “Ion-channel sites for allosteric modulators are enormous and relatively untapped.”
Despite the shortcomings of the screening technologies available, many pharma and biotech companies are investigating ion channels as drug targets for some interesting applications. An example of this is ChanTest’s combination of surrogate assays and patch-clamping technologies for repurposing old drugs.
“With more than 86 cell lines of HEK and CHO cells expressing ion channels screened by fluorescence assay and patch clamp we provide the largest collection of validated ion channels for screening,” explained Dr. Brown.
“Using these cells we screened a library of compounds that had been withdrawn, suspended, or discontinued at late-stage clinical trials. Many of these were potent hERG blockers, something that sends regulatory authorities into seizure mode but because they showed no adverse human cardiac events we decided these could be a useful group of compounds to test as potential treatments for, in this example, cardiac diseases.
“From primary screens using the FLIPRTetra, we produced 10,000 data points per day. Any promising compounds we screened using IonWorks Quattro to generate 2,000 data points per day, followed by a PatchXpress or QPatch screen to produce 100 data points per day,” Dr. Brown said.
“Finally, those compounds that produced the best results were screened again by manual patch clamp which, because it is so difficult and time consuming, only produces seven data points per day. From this funneled screening approach, we identified a compound we call CT-1, and its ion-channel profile showed it would be an appropriate drug candidate for atrial fibrillation.”
Dr. Brown presented data demonstrating that CT-1, in both IV and oral dosage form, is well tolerated in canine models and Phase 1 dose-escalating studies for atrial fibrillation. He also showed that in a Phase II study of 27 patients with atrial fibrillation 12/20 receiving the drug converted to having normal sinus heart (NSR) rhythms within two hours of having CT-1 treatment.
“We’re pleased with the Phase II trial results because the usual rate of patient conversion from atrial fibrillation to NSR is 30 percent and we’re achieving 60 percent,” Dr Brown concluded. “CT-1 is going to be out-licensed soon, and we’re using the same integrated screening methodology to repurpose two other drugs because we estimate it can reduce the pre-clinical development time and Phase I work by several years.”