Tendencias vibracionales en grupos electrógenos

Identification and mitigation of cardiac conduction liabilities is at AstraZeneca per- formed using multiple nonclinical in vitro and in vivo assays (Figure 2.8). In vitro assays primarily involve assessing the interaction of compounds with the cardiac ion channels that are known to control the heart beat, where conduction slowing is primarily studied by assessing effects on the sodium (Nav1.5) and L-type calcium (Cav1.2) channels. Inhibition of Nav1.5 leads to reduced Na+influx which slows ven- tricular depolarisation, causing widening of the QRS complex (Figure 2.9) and/or prolongation of the PR interval (reviewed in [19, 52]). In early discovery phases such as the lead generation and lead optimisation phases, automated or conventional func-

tional human Nav1.5 (hNav1.5) whole-cell voltage clamp assays are typically used to identify compounds that inhibit hNav1.5 and reduce the Na+fluxin vitro. Safety margins of 30-100 fold between predicted maximal exposures in humans (predicted Cmax) and IC50 measured in vitro have been suggested for hNav1.5-QRS based on collected information of in vitro potencies and clinical findings [19].

Potential mechanisms for PR prolongations are more complex, as the PR interval represents action potential propagation through the atria, the AV node, the His-Purkinje system and the bundle branches to reach the Purkinje fibres. The major mechanism for drug-induced PR prolongation is AV block through inhibi- tion of the cardiac L-type calcium (Cav1.2) channel (reviewed in [15], Figure 2.9). Cav1.2 is activated following the fast Na+ depolarisation, initiating an influx of Ca2+ that in turn triggers the release of intra-cellular Ca2+ deposits, provoking my- ocyte contraction. Cav1.2 inactivates slowly and counter-acts repolarisation of the myocytes through continued influx of Ca2+_{, and is thus also vital for maintaining} the plateau in the ventricular myocyte action potential influencing the QT inter- val. Functional human Cav1.2 (hCav1.2) assays may be used to detect potential conduction liabilities [53]. However, both conventional and automated functional hCav1.2 assays have been shown to correlate poorly [54] to anin vitro contractility assay using dog myocytes [55], showing low sensitivity with many false negative re- sponders. Alternatively, radioligand binding to rat Cav1.2 (rCav1.2) may be used to detect compounds that bind to Cav1.2. Two binding sites predictive of cardiac contractility have been identified, namely the verapamil and diltiazem sites [54]. In addition to hCav1.2 inhibition, rCav1.2 binding to these sites is therefore typically analysed at AstraZeneca. In addition, PR prolongation may be caused by hNav1.5 inhibition e.g. through reduced conduction through the His-Purkinje system [56]. Contrary to hERG and hNav1.5, safety margins have not to the authors knowledge been suggested for hCav1.2 inhibition or rCav1.2 binding.

In vivo investigations of drug-induced effects on ECG intervals may be con- ducted in anaesthetised and/or conscious rats, guinea pigs and dogs [21, 22, 23]. Rodent animal studies are typically conducted early, at the lead optimisation phase, providing the first data on cardiovascular effects in complete systems at a time of (relatively) low cost and high chemical choice. Animal studies in dogs, or sometimes minipigs or non-human primates, are conducted during pre-clinical development to confirm the safety of compounds before first time in man (FTIM) studies as part of regulatory requirements. Both conscious and unconscious animal telemetry studies are used, as conscious animal studies enable long term safety assessment, while un- conscious (open chest) animal studies enable measurement of additional variables, reduce noise and allow for higher doses. ECGs are typically generated using auto- matic systems for calculating the durations of the intervals, often followed by manual checks at pre-defined time-points around which data are averaged to reduce noise

Figure 2.8 Assays for cardiac conduction risk assessment at AstraZeneca. Cardiac con-

duction risk is mitigated during lead generation/optimisation by in vitro assays, primarily

assessing effects on Nav1.5 and Cav1.2. During lead optimisation, rodent in vivo studies

are conducted to assess cardiac safety, including QRS/PR interval prolongations in moni- tored ECGs. Cardiac safety, including QRS/PR changes, is also evaluated in a large animal telemetry study during pre-clinical development, and prior to human testing.

Figure 2.9 The ECG informs on electrophysiological changes in heart function as changes in ion fluxes cause predictable changed in the ECG. Inhibition of the Cav1.2 calcium channel slows down conduction through the AV node, leading to prolonged PR interval. Inhibition of the Nav1.5 sodium channel leads to reduced rate of depolarisation across the myocyte membrane, causing widening of the QRS complex. Figure by Matt Skinner, AstraZeneca.

and extracted for analysis. Any drug-induced effects on QRS and PR intervals are typically identified by statistical tests, e.g. pair-wise comparison to the vehicle group at each time point and evaluated considering additional information, such as drug exposure and time point of the identified effects.

2.3.3 Modelling & simulation for cardiovascular risk assessment The following section describes the history and current status for a selection of modelling approaches to assess CV safety in drug discovery and development, partly published previously in Collins, Bergenholm et al. 2015 [1].