Biophysics and Molecular Biology of Cardiac Ion Channels for the Safety Pharmacologist

Cardiac safety pharmacology is a continuously evolving discipline that uses the basic principles of pharmacology in a regulatory-driven process to generate data to inform risk/benefit assessment of a new chemical entity (NCE). The aim of cardiac safety pharmacology is to characterise the pharmacodynamic/pharmacokinetic (PK/PD) relationship of a drug’s adverse effects on the heart using continuously evolving methodology. Unlike Toxicology, safety pharmacology includes within its remit a regulatory requirement to predict the risk of rare cardiotoxic (potentially lethal) events such as torsades de pointes (TdP), which is statistically associated with drug-induced changes in the QT interval of the ECG due to blockade of I Kr or K v11.1 current encoded by hERG. This gives safety pharmacology its unique character. The key issues for the safety pharmacology assessment of a drug on the heart are detection of an adverse effect liability, projection of the data into safety margin calculation and clinical safety monitoring. This chapter will briefly review the current cardiac safety pharmacology paradigm outlined in the ICH S7A and ICH S7B guidance documents and the non-clinical models and methods used in the evaluation of new chemical entities in order to define the integrated risk assessment for submission to regulatory authorities. An overview of how the present cardiac paradigm was developed will be discussed, explaining how it was based upon marketing authorisation withdrawal of many non-cardiovascular compounds due to unanticipated proarrhythmic effects. The role of related biomarkers (of cardiac repolarisation, e.g. prolongation of the QT interval of the ECG) will be considered. We will also provide an overview of the ‘non-hERG-centric’ concepts utilised in the evolving comprehensive in vitro proarrhythmia assay (CIPA) that details conduct of the proposed ion channel battery test, use of human stem cells and application of in silico models to early cardiac safety assessment. The summary of our current understanding of the triggers of TdP will include the interplay between action potential (AP) prolongation, early and delayed afterdepolarisation and substrates for re-entry arrhythmias.