hERG is only really the beginning…CiPA is coming

Current practice in cardiac safety pharmacology has been successful in preventing proarrhythmia- related post approval drug withdrawal. However, many challenges remain in the assessment of cardiac effects of new drugs. One challenge is to reduce the high attrition rate of compounds observed during drug development. The failure of potentially effective drugs can be partly attributed to hERG blockade or prolonged QT interval that are detected in preclinical studies.

A significant goal of the FDA-sponsored Comprehensive in vitro Proarrhythmia Assay (CiPA) initiative is to develop a new paradigm for evaluation of the cardiac safety of drugs. More specifically, CiPA is designed to assess the proarrhythmic potential (rather than using QT interval alone as a surrogate for proarrhythmia) of novel molecules using in vitro and in silico investigative approaches. It is now recognized that testing compounds on hERG alone is not sufficient to predict their proarrhythmic liability.

The cardiac action potential (AP) results from the integrated activation and inactivation of multiple ion channels, mechanisms that are mostly voltage- and time- dependent. As part of the CiPA initiative, it is proposed to test the effects of new drugs on multiple ion channels (peak INa, Ito, ICa,L, late INa, IKr, IKs, IK1) which contribute to both the depolarization and repolarization phases of the AP.

IonsGate Preclinical Services cellular assays are not limited to cardiac ion channels, as we have access to multiple non-cardiac ion channels for your discovery projects.

Parameters obtained include:

• Standard IC50s
• Use dependence of block
• Voltage dependence of block
• Customized protocols to fit our clients’ needs

Contractility Assay

Cardiac contractility is directly proportional to the amount of Ca2+ flooding the myofibrils during the action potential, and is thus dependent upon Ca2+ processing in the myocytes. Contraction of the heart is initiated by the electrical excitation of cardiac myocytes. The wave of depolarization generated from the sino-atrial (SA) node propagates through the atrium and atrio-ventricular (AV) node to the ventricle. Depolarization of the ventricle is initiated by the opening of voltage-dependent sodium channels (upstroke of the action potential). Na+ channels inactivate rapidly, limiting excessive increase of intracellular Na+.

During depolarization, voltage-dependent L-type calcium channels open, resulting in Ca2+ influx inside the cell (inward current). L-type Ca2+ channels are located primarily at sarcolemmal-sarcoplasmic reticulum (SR) junctions where the ryanodine receptors (RyR, Ca2+ release channels) are present. Ca2+ entering the cell through L-type calcium channels locally increases intracellular Ca2+ concentrations [Ca2+]i near the RyRs, leading to release of Ca2+ from the SR (Calcium-induced calcium release). The [Ca2+]i raises , allowing Ca2+ to bind to the myofilaments and trigger contraction. For relaxation to occur, [Ca2+] must decline, allowing Ca2+ dissociation from the myofilaments.

This requires extrusion of Ca2+ out of the cytosol. Ca2+ re uptake into the SR and Ca2+ efflux out of the cell through the Na+/Ca2+ exchange are the two main mechanisms involved in relaxation. NCX can operate in the forward mode (Ca2+ efflux) or in the reverse mode (Ca2+ influx), depending on the internal and external concentrations of both [Na+]i and [Ca2+]i, as well as on the membrane potential. Under physiological conditions, NCX works almost exclusively in the forward mode. Small changes in [Na+]i and [Ca2+]i may have large impact on contractility.

Because impairment of the regulatory machinery governing excitation-contraction coupling can lead to serious cardiac conditions, it is thus important to determine the effects of test compounds on parameters of cardiac myocyte contractility. IonsGate uses a system developed by IonOptix (Milton, MA, USA) which can record real-time sarcomere length of cardiac myocytes by a video acquisition camera.

Parameters obtained include:

• Sarcomere resting length
• Fractional sarcomere shortening (% of diastolic sarcomere length)
• Rate of contraction and relaxation (±dl/dt)
• T1/2 contraction and relaxation
• Dose responses can be generated and various stimulation frequencies can be applied