The role of the anaesthetised guinea-pig in the preclinical cardiac safety evaluation of drug candidate compounds.

Despite rigorous preclinical and clinical safety evaluation, adverse cardiac effects remain a leading cause of drug attrition and post-approval drug withdrawal. A number of cardiovascular screens exist within preclinical development. These screens do not, however, provide a thorough cardiac liability profile and, in many cases, are not preventing the progression of high risk compounds. We evaluated the suitability of the anaesthetised guinea-pig for the assessment of drug-induced changes in cardiovascular parameters. Sodium pentobarbitone anaesthetised male guinea-pigs received three 15 minute intravenous infusions of ascending doses of amoxicillin, atenolol, clonidine, dobutamine, dofetilide, flecainide, isoprenaline, levosimendan, milrinone, moxifloxacin, nifedipine, paracetamol, verapamil or vehicle, followed by a 30 minute washout. Dose levels were targeted to cover clinical exposure and above, with plasma samples obtained to evaluate effect/exposure relationships. Arterial blood pressure, heart rate, contractility function (left ventricular dP/dt(max) and QA interval) and lead II electrocardiogram were recorded throughout. In general, the expected reference compound induced effects on haemodynamic, contractility and electrocardiographic parameters were detected confirming that all three endpoints can be measured accurately and simultaneously in one small animal. Plasma exposures obtained were within, or close to the expected clinical range of therapeutic plasma levels. Concentration-effect curves were produced which allowed a more complete understanding of the margins for effects at different plasma exposures. This single in vivo screen provides a significant amount of information pertaining to the cardiovascular risk of drug candidates, ultimately strengthening strategies addressing cardiovascular-mediated compound attrition and drug withdrawal.

A gender-independent proarrhythmic action of 17beta-estradiol in anaesthetized rabbits.

Women are at increased risk of having drug-induced arrhythmias such as torsade de pointes but less susceptible to arrhythmias associated with myocardial ischaemia. We have shown previously that 17beta-estradiol had greater antiarrhythmic activity in female rats than in male rats subject to myocardial ischaemia. The aim of this work was to investigate the effects of acute administration of 17beta-estradiol in both sexes in an established in vivo model of drug-induced arrhythmias. In alpha(1)-adrenoceptor-stimulated, pentobarbital-anaesthetized rabbits, 17beta-estradiol (100, 300 or 1000 ng/kg bolus followed by 10, 30 or 100 ng/kg/min infusion) tended to increase the incidence of torsade de pointes, induced by clofilium, in both sexes: from 50% in controls to 80%, 70% and 80% in females; from 40% in controls to 60%, 70% and 80% in males with increasing doses of 17beta-estradiol (n=10 per group). The total duration of all episodes of torsade de pointes was increased significantly by the highest dose of 17beta-estradiol compared to vehicle in both female and male rabbits: from 9+/-4 s to 93+/-26 s in females; from 26+/-14 s to 96+/-20 s in males. There were no baseline differences between the sexes in heart rate, QTc interval or epicardial monophasic action potential duration. The proarrhythmic effect of acute administration of 17beta-estradiol in the alpha(1)-adrenoceptor-stimulated anaesthetized rabbit model was independent of gender. This indicates that the underlying mechanism differs from that involved in the gender-selective reduction of ischaemia-induced arrhythmias by 17beta-estradiol.

Sex differences in ventricular repolarization: from cardiac electrophysiology to Torsades de Pointes.

A number of non-cardiovascular drugs have been withdrawn from clinical use due to unacceptable adverse cardiac side-effects involving drug-induced Torsades de Pointes (TdP)--a rare, life-threatening polymorphic ventricular tachycardia associated with prolongation of the action potential duration of ventricular myocytes and, hence, prolongation of the QT interval, of the electrocardiogram (ECG), which measures the total time for activation of the ventricles and their recovery to the resting state. Research has suggested that women are more prone to develop TdP than men during administration of medicines that share the potential to prolong the QT interval, with 65-75% of drug-induced TdP occurring in women. Clinical and experimental studies show that female sex demonstrate differences in the electrocardiographic pattern of ventricular repolarization in human and other animal species and is associated with a longer rate-corrected QT (QTc) interval at baseline than males. Reports of a similar propensity towards drug-induced TdP in both premenopausal and postmenopausal women support factors in addition to those of female sex hormones eliciting sex-based differences in ventricular repolarization. However, conflicting evidence suggests sex hormones may have a role in increasing the susceptibility of women or ultimately reducing the susceptibility of men to TdP. Cyclical variations in hormone levels during the menstrual cycle have been associated with an increased and reduced risk of TdP. In contradiction to this finding, the male sex hormone is thought to be beneficial. Modulation of the ventricular repolarization by testosterone may explain why the QTc interval shortens at puberty, and might account for the tendency towards an age-dependent reduction in the incidence of drug-induced TdP in men. Mechanisms underlying these differences are not fully understood but a case for the involvement of gonadal steroids is obviously strong. Therefore, further non-clinical/clinical investigations ought to be a necessary step to elucidate any sex differences in cardiac repolarization characteristics, QT interval prolongation and susceptibility to cardiac arrhythmias. This may have implications for the development of the safest medicinal products and for the clinical management of cardiac arrhythmias.

Optimising conditions for studying the acute effects of drugs on indices of cardiac contractility and on haemodynamics in anaesthetized guinea pigs.

INTRODUCTION: Detecting adverse effects of drugs on cardiac contractility is becoming a priority in pre-clinical safety pharmacology. The aim of this work was to optimise conditions and explore the potential of using the anaesthetized guinea pig as an in vivo model. METHODS: Guinea pigs were anaesthetized with Hypnorm/Hypnovel, isoflurane, pentobarbital or fentanyl/pentobarbital. The electrocardiogram (ECG), heart rate, arterial blood pressure and indices of cardiac contractility were recorded. In further experiments in fentanyl/pentobarbital anaesthetized guinea pigs the influence of bilateral versus unilateral carotid artery occlusion on haemodynamic responses was investigated and the effects of inotropic drugs on left ventricular (LV) dP/dt(max) and the QA interval were determined. RESULTS: Pentobarbital, given alone or after fentanyl, provided suitable anaesthesia for these experiments. Bilateral carotid artery occlusion did not alter heart rate or arterial blood pressure responses to isoprenaline or angiotensin II. Isoprenaline and ouabain increased LVdP/dt(max) and decreased the QA interval whereas verapamil had opposite effects and strong inverse correlations between LVdP/dt(max) and the QA interval were found. DISCUSSION: Conditions can be optimised to allow the pentobarbital-anaesthetized guinea pig to be used for simultaneous measurement of the effects of drugs on the ECG, haemodynamics and indices of cardiac contractility. The use of this small animal model in early pre-clinical safety pharmacology should contribute to improvements in detecting unwanted actions on the heart during the drug development process.