Proarrhythmic effects of antiarrhythmic drugs

Cardiac repolarisation and drug regulation: assessing cardiac safety 10 years after the CPMP guidance

December 2007 marks the 10-year anniversary of the first regulatory guidance for evaluation of drug-induced QT interval prolongation. A decade on, it seems surprising that this document, which was released by the Committee on Proprietary Medicinal Products, caused such acrimony in the industry. Sponsors now routinely evaluate their new drugs for an effect on cardiac electrophysiology in preclinical studies, in addition to obtaining ECGs in all phases of drug development and conducting a formal thorough QT study in humans.However, concurrently, new concerns have also emerged on broader issues related to the cardiovascular safety of drugs because of their potential to shorten the QT interval as well as to induce proischaemic, profibrotic or prothrombotic effects. Drugs may also have an indirect effect by adversely affecting one or more of the cardiovascular risk factors (e.g. through fluid retention or induction of dyslipidaemia). In addition to peroxisome proliferator-activated receptor agonists and cyclo-oxygenase 2 selective inhibitors, three other drugs, darbepoetin alfa, pergolide and tegaserod, provide a more contemporary regulatory stance on tolerance of cardiovascular risk of drugs and their benefit-risk assessment. This recent, more assertive, risk-averse stance has significant implications for future drug development. These include the routine evaluation of cardiovascular safety for certain classes of drugs. Drugs that are intended for long-term use will almost certainly require long-term clinical evaluation in studies that enrol populations that most closely resemble the ultimate target population. Novel mechanisms of action and biomarkers by themselves are no guarantee of improved safety or benefits. Even some traditional biomarkers have come to be viewed with scepticism. Requirements for more extensive and earlier postmarketing assessment of clinical benefits and rare, but serious risks associated with new medicinal products should create a new standard of evidence for industry and regulators and almost certainly result in better assessment of benefit/risk, more effective and balanced regulatory actions and better care for patients.

Critical care cardiology

Emergency management of the patient with cardiac disease is an important part of veterinary practice. Although the causes of cardiac disease may be diverse, the understanding of basic pathophysiology will enable the clinician to formulate a rational diagnostic and therapeutic plan. The veterinary clinician must be able to triage the emergency patient, assess the clinical condition, and provide appropriate therapy. Close monitoring of the critically ill patient is crucial to patient survival and will help tailor therapy.

Changes in the distribution of ventricular ectopic beats in long-term electrocardiograms

Only simple methods have been used to assess antiarrhythmic and proarrhythmic effects when comparing baseline and on-therapy Holter recordings taken from the same patient. The paper suggests a new method for definition of these effects based upon comparisons of statistical distributions of ectopic beats. The method is based on multiple random sampling of the recordings and on application of the Smirnov test to compare the samplings. The results of these multiple comparisons are subsequently evaluated using the chi-squared test. An analysis is reported of Holter recordings made on seven patients suffering from ventricular arrhythmia but with anatomically normal heart. In each patient, one baseline recording and one recording on each of three different drugs were made. The results show that the definition of proarrhythmic effects can be partly addressed in a precise mathematical way. The method can also detect a significant change in the character of arrhythmia which can neither be classified as antiarrhythmic nor as proarrhythmic.

H2-receptor antagonism is not pro-arrhythmic in a chronic canine model

Seven to 28 days after coronary artery ligation, programmed electrical stimulation was performed in conscious dogs. Groups of 6 previously inducible dogs in which no arrhythmia could presently be achieved were randomly allocated to receive quinidine, cimetidine, ranitidine or placebo. Results were assessed for the drugs' ability to induce ventricular tachycardia or fibrillation, and compared with placebo using Fisher's Exact Test. In the placebo group 4/6 dogs remained unchanged, one developed an arrhythmia, and one died. With quinidine, 3/6 dogs developed an arrhythmia (0.5 mg/kg, 4.0 mg/kg, 4.0 mg/kg) and three died (4 mg/kg, 8 mg/kg, 16 mg/kg) (p less than 0.05 compared with placebo). With cimetidine, 4/6 dogs remained unchanged, one developed an arrhythmia after 4 mg/kg, and one died after 0.5 mg/kg. After ranitidine 3/6 dogs remained unchanged and three died (1.0 mg/kg, 4.0 mg/kg, 16.0 mg/kg). PR, QTc, QRS, refractory periods, and mean systolic pressure remained unchanged after placebo, cimetidine, and ranitidine, but QTc increased (p less than 0.05) and mean systolic pressure fell (p less than 0.01) after quinidine. Heart rate did not change following placebo, but increased (p less than 0.05) after each of the three drug treatments. These results fail to show a significant arrhythmogenic effect of cimetidine or ranitidine in a model validated by the significant pro-arrhythmic effects of quinidine. The cause of death in all cases was ventricular fibrillation.

Proarrhythmic effects of antiarrhythmic drugs in patients with malignant ventricular arrhythmias evaluated by electrophysiologic testing

The provocation or worsening of arrhythmias by antiarrhythmic regimens was evaluated in patients with malignant ventricular arrhythmias undergoing electrophysiologic studies. In 314 patients with sustained or nonsustained ventricular tachycardia or ventricular fibrillation, 801 drug studies were performed using a standard protocol of programmed electrical stimulation. The criteria for proarrhythmia were: 1) initiation of sustained ventricular tachyarrhythmia in a patient in whom only nonsustained tachycardia was induced at baseline; 2) conversion of a sustained tachycardia that could be terminated by programmed electrical stimulation at baseline to one that required cardioversion for termination during drug therapy; 3) initiation of a sustained tachyarrhythmia by a less aggressive mode of stimulation than was required at baseline; and 4) development of spontaneous or incessant ventricular tachycardia. Proarrhythmia criterion 1 occurred during 20 (18%) of 118 studies and at least once in 15 (28%) of 54 patients. Criterion 2 was met during 39 (7%) of 578 studies and at least once in 29 (13%) of 220 patients. Criterion 3 was achieved during 135 (20%) of 687 studies in patients with sustained ventricular tachyarrhythmias at baseline. Criterion 4 occurred during 9 (1%) of 801 drug studies. In 40 patients in whom well tolerated ventricular tachycardia was initiated with fewer extrastimuli during drug study than at baseline, the drug was continued and the patients were followed up. The recurrence rate of tachycardia was the same in these patients as in 73 patients followed up on regimens on which the number of extrastimuli required for initiation was not reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. A model of reentry based on anisotropic discontinuous propagation

Available models of circus movement reentry in cardiac muscle and of drug action on reentrant arrhythmias are based on continuous medium theory, which depends solely on the membrane ionic conductances to alter propagation. The purpose of this study is to show that the anisotropic passive properties at a microscopic level highly determine the propagation response to modification of the sodium conductance by premature action potentials and by sodium channel-blocking drugs. In young, uniform anisotropic atrial bundles, propagation of progressively earlier premature action potentials continued as a smooth process until propagation ceased simultaneously in all directions. In older, nonuniform anisotropic bundles, however, premature action potentials produced either unidirectional longitudinal conduction block or a dissociated zigzag type of longitudinal conduction (a safer type of propagation, similar to transverse propagation). Directional differences in the velocity of premature action potentials demonstrated that anisotropic propagation was necessary for a reentrant circuit to be contained within an area of 50 mm2, even with very short refractory periods. Quinidine produced Wenckebach periodicity, which disappeared after acetylcholine shortened the action potential. Quinidine also produced use-dependent dissociated zigzag longitudinal conduction in the older, nonuniform anisotropic bundles but not in the young, uniform anisotropic bundles. The electrophysiological consequence was that propagation events differed in an age-related manner in response to the same modification of the sodium conductance. The electrical events at microscopic level showed that conditions leading to obliteration of side-to-side electrical coupling between fibers (e.g., aging and chronic hypertrophy) provide a primary mechanism for reentry to occur within very small areas (1-2 mm) due to a variety of propagation phenomena that do not occur in tissues with tight electrical coupling in all directions.

Proarrhythmic responses during electrophysiologic testing

Antiarrhythmic drugs may worsen ventricular arrhythmias in certain patients. This effect, termed proarrhythmia, aggravation or provocation of arrhythmia, can be investigated with either noninvasive or invasive techniques. Using electrophysiologic study, 160 patients with ventricular tachycardia or ventricular fibrillation were evaluated during treatment with 432 different antiarrhythmic regimens. Proarrhythmic responses were noted in 68 drug trials (16%), and at least 1 event was observed in 51 patients (32%). Nonsustained ventricular tachycardia was converted to sustained ventricular tachycardia in 17% of these studies. Hemodynamically stable ventricular tachycardia was converted to an arrhythmia that required cardioversion for termination in 5% of the studies. Ventricular tachyrhythmia was more easily induced in 12% of trials. These proarrhythmic responses were not related to changes in QRS duration, QT interval or JT interval measured at baseline or to changes produced by antiarrhythmic drugs.

BW A256C, a chemically novel class 1 antiarrhythmic agent. A comparison of in vitro and in vivo activity with other class 1 antiarrhythmic agents

BW A256C (5(3)-amino-6-(2,3-dichlorophenyl)-2,3(2,5)-dihydro-3(5)-imino-2 -isopropyl-1,2,4-triazine) is a novel class 1 antiarrhythmic agent designed to combine the features of potency with reduced central nervous system penetration. BW A256C reduced the maximum rate of depolarization of guinea-pig ventricle and dog Purkinje fibres in vitro (EC50, 2.2 X 10(-6) M and 1.8 X 10(-6) M, respectively), being significantly more potent than quinidine, lidocaine, disopyramide and flecainide. BW A256C was also more potent than these agents at inhibiting aconitine-induced arrhythmias in anaesthetized rats; however, unlike these agents, BW A256C was devoid of hypotensive activity at antiarrhythmic doses. In anaesthetized dogs, intravenous administration of BW A256C (0.25-1 mg kg-1) caused a dose-dependent suppression of ventricular arrhythmias that occurred on reperfusion of an occluded coronary artery. In conscious dogs, intravenous infusion (total dose, 1.5 mg kg-1) or oral administration of BW A256C (1.25-5 mg kg-1) caused dose-dependent suppression of the ventricular ectopic activity that occurred following 20-24 h of permanent coronary artery ligation. In the conscious dog, BW A256C was approximately 7 times more potent and was also longer acting than flecainide. Administration of BW A256C was not associated with any evidence of peripheral or CNS toxicity. However, plasma levels 3-4 times greater than the antiarrhythmic levels were associated with a proarrhythmic activity.