Ion channels represent 15 – 20% of historic drug approvals and recent drug discovery projects. Many ion channel families (Nav, Cav, TRPx and GABA) are validated as therapeutic targets based on human genetics, animal models and selective pharmacology. However, ion channels are challenging targets requiring specialist target class knowledge and screening technology such as automated patch clamp (APC) electrophysiology.
Cardiac toxicity remains the leading cause of new drug safety side-effects. Current preclinical cardiac safety assays rely on in vitro cell-based ion channel assays and ex vivo and in vivo animal models⁽¹⁾. These assays provide an indication of acute risk but they do not always predict the effect of chronic compound exposure, as recently seen with oncology drugs. Therefore, new assays are required to characterise chronic structural and functional effects in human cells earlier in drug discovery. Impedance-based technology can provide more accurate chronic cardiotoxicity measurements in an efficient manner using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs).
Ion channels play a key role in regulating resting membrane potential and cell excitability and are attractive targets for therapeutic intervention.
Thallium (Tl+) flux assays, which measure the flow of Tl+ through potassium channels, offer a high throughput method for the identification of potassium channel activators. However, these assays are a surrogate for channel function and it is important to have an appropriate panel of orthogonal and translational electrophysiology assays in place to confirm activity at the channel of interest.
The FDA’s Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative is designed to remove the over-reliance on hERG data to predict human clinical cardiac risk⁽¹⁾, with recent results suggesting that inclusion of additional cardiac ion channels and assays (e.g. peak and late Nav1.5, Cav1.2, dynamic hERG⁽²⁾) improve risk predictions of in silico action potential models⁽¹⁾. The CiPA working groups currently use a mixture of manual and automated patch clamp (APC) platform data, but future CiPA drug screening will likely rely on APC data.
Acid-sensing ion channels (ASICs) are proton-gated ion channels which are highly sensitive to extracellular acidosis and are permeable to cations1, predominantly Na+. To date, six different ASIC subunits (1a, 1b, 2a, 2b, 3 and 4) encoded by four genes have been identified.
New cardiac safety testing guidelines are being finalised, as part of the FDA’s Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative, which aim to remove the over-reliance on screening against the hERG channel by expanding the panel to include hNav1.5, hCav1.2, hKv4.3/KChiP2.2, hKir2.1 and hKv7.1/KCNE1 human cardiac ion channels. In addition, the CiPA working groups have recently identified two additional in vitro assays required for in silico models to reliably predict proarrhythmia. The first is a ‘late’ sodium current assay, as inhibition of persistent inward current can affect repolarisation and mitigate proarrhythmia (e.g. ranolazine). The second assay quantifies the degree of drug trapping in the hERG channel using the Milnes voltage protocol⁽¹⁾, which can improve the prediction of proarrhythmic risk⁽²⁾.
Metrion Biosciences’ CEO Dr Andrew Southan presents an overview of Metrion Biosciences, highlighting the company’s vision, expertise and services. December 10th, 2020.
The cardiac late Na+ current (late INa) generates persistent inward currents throughout the plateau phase of the ventricular action potential and is an important determinant of repolarisation rate, EADs and arrythmia risk. As inhibition of late INa can offset drug effects on hERG and other repolarising K+ conductances it is one of the key cardiac channels in the Comprehensive in vitro Proarrythmia Assay CiPA panel being developed by the FDA to improve human clinical arrythmia risk assessment.
Marc Rogers (Metrion CSO) presents a talk at the Nanion Virtual User Meeting 2020 entitled “Designing multiple assay protocols for ligand-gated ion channels using the stacked-tip feature on the Patchliner and SP384i platforms”. 14th October 2020.
Stevens, E. B and Wright, P. D Future Drug Discovery, 2020.