Comparative effects of sodium channel blockers in short term rat whole embryo culture.

This study was undertaken to examine the effect on the rat embryonic heart of two experimental drugs (AZA and AZB) which are known to block the sodium channel Nav1.5, the hERG potassium channel and the l-type calcium channel. The sodium channel blockers bupivacaine, lidocaine, and the l-type calcium channel blocker nifedipine were used as reference substances. The experimental model was the gestational day (GD) 13 rat embryo cultured in vitro. In this model the embryonic heart activity can be directly observed, recorded and analyzed using computer assisted image analysis as it responds to the addition of test drugs. The effect on the heart was studied for a range of concentrations and for a duration up to 3h. The results showed that AZA and AZB caused a concentration-dependent bradycardia of the embryonic heart and at high concentrations heart block. These effects were reversible on washout. In terms of potency to cause bradycardia the compounds were ranked AZB>bupivacaine>AZA>lidocaine>nifedipine. Comparison with results from previous studies with more specific ion channel blockers suggests that the primary effect of AZA and AZB was sodium channel blockage. The study shows that the short-term rat whole embryo culture (WEC) is a suitable system to detect substances hazardous to the embryonic heart.

Assessment of the effects of changes in body temperature on cardiac electrophysiology in anaesthetised guinea pigs.

INTRODUCTION: Anaesthetised guinea pigs are commonly used within Safety Pharmacology to evaluate drug effects on cardiac electrophysiology. However, anesthesia compromises the ability to thermoregulate, which can be further challenged when more invasive surgery is required. As anaesthetised animals are often used when screening for cardiotoxicity, thereby influencing go/no-go decisions, we wanted to quantify the impact of small temperature changes on the recorded electrophysiological parameters. METHODS: Male guinea pigs were anaesthetised by pentobarbital, placed on a pre-heated table and a rectal thermistor inserted for monitoring of body temperature. After intubation animals were vagotomised and ß-blocked, and lead II ECG needle electrodes attached. Following thoracotomy an atrial pacing electrode was attached and a suction MAP electrode positioned on the ventricular epicardium. In control animals temperature was kept constant (38.1±0.1°C) over the duration of the experiment. Animals in one group were slowly warmed to 41.9°C by a heating plate and a heating lamp, and in another group slowly cooled to 34.4°C by turning off all heating equipment. MAP duration at 90% repolarisation (MAPD90), AV conduction, ECG and body temperature were recorded during cardiac pacing every 5min up to 50min. RESULTS: No time-dependent changes were seen in the control group. In contrast, a linear correlation was found between changes in body temperature and MAPD90, AV conduction, QTc and QRS intervals. For each degree temperature fell below 38°C MAPD90 was prolonged by 6.1ms, and for each degree above 38°C MAPD90 was shortened by 5.3ms. Corresponding changes were seen for QTc interval and AV conduction time, while effects on the QRS interval were smaller. DISCUSSION: The data highlights the importance of carefully controlling body temperature when performing electrophysiological recordings in laboratory animals. A change by a single degree can affect electrophysiological parameters by 5-10%, thus increasing the risk for a false positive or negative interpretation of cardiotoxicity.

Evaluation of the guinea pig monophasic action potential (MAP) assay in predicting drug-induced delay of ventricular repolarisation using 12 clinically documented drugs.

INTRODUCTION: While the dog in vivo model is commonly employed in the later phase of discovery for assessing drug-induced QT prolongation, an early screening assay is valuable when selecting compounds for further development and when compound availability usually is low. One such screening assay is the anaesthetised guinea pig monophasic action potential (MAP) model. The aim of the present study was to evaluate the ability of this model to detect proarrhythmic properties by testing a set of reference compounds with known clinical profile. Moreover, these results were compared to data previously obtained using in vivo canine QT assays (QT PRODACT study). METHODS: Anaesthetised and ventilated male guinea pigs were vagotomised and pretreated with propranolol. After thoracotomy, a pacing electrode was clipped to the left atrial appendage and a suction MAP electrode positioned on the left ventricular epicardium. The drug or corresponding vehicle was injected intravenously in cumulative doses and MAP duration at 90% repolarisation (MAPD90) was recorded during cardiac pacing. RESULTS: The 8 drugs known to be proarrhythmic in the clinic all displayed dose-dependent prolongation of MAPD90, while the 4 drugs devoid of dysrhythmia in man had no effect. When comparing doses producing a 10% MAPD90 increase with doses reported to increase QTc by 10% in dogs a strong correlation was seen (R(2) 0.94 and 0.58 for anaesthetised and conscious dogs, respectively). DISCUSSION: The guinea pig MAP assay identified all clinically positive drugs while negative drugs were without effect on ventricular repolarisation. Furthermore, a good concurrence is shown between the guinea pig and dog models in identifying compounds with proarrhythmic properties. Overall, the study reinforces the anaesthetised guinea pig MAP model as a reliable assay predicting QT liability of new chemical entities and as a highly sensitive early screening model for cardiovascular risk.