The Significance of Electrocardiographic Changes Produced by Thioridazine

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.

Study of thioridazine cardiotoxic effects by means of His bundle activity recording

The effect of thioridazine on sinusal automaticity, myocardial excitability and auriculoventricular conduction have been studied in dogs by measuring the spontaneous heart rate, the effective refractory period of the atrial contractile tissue, the time of auriculonodal and infrahisian conduction. Chloralose anaesthetized dogs were administered 10 mg/kg thioridazine hydrochloride via intravenous route. While sinusal automaticity was not altered, conduction times were prolonged in the suprahisian and mainly the infrahisian part and the atrial refractory period increased. These effects are related to quinidine-like properties and consistent with cardiotoxic effects previously reported in clinical practice.

Comparison of the electrophysiological effects of two neuroleptics, melperone and thioridazine, on isolated rat atria

The effects of the two neuroleptics, melperone (a butyrophenone) and thioridazine (a phenothiazine), were compared on the electrical and mechanical activity of isolated rat atria. Both electrically stimulated and spontaneously beating atria were used. Melperone was found to prolong the effective refractory period while the threshold for electrical stimulation i.e. the excitability, was almost unaffected. Thioridazine caused a similar prolongation of the effective refractory period, but also decreased the excitability significantly. In contrast to melperone, thioridazine had a negative inotropic effect. The spontaneous pacemaker activity was depressed and the sinus node recovery time increased to a greater extent after melperone than after thioridazine. The results taken together with other recent data support the hypothesis that melperone may be a type III anti-arrhythmic according to the classification of Vaughan Williams, in contrast to thioridazine which has a quinidine-like action (type I). The results also indicate that melperone in addition to prolonging the effective refractory period, may act as an anti-arrhythmic agent by depressing automaticity.

Effect of electroconvulsive therapy on cardiac rhythm, conduction and repolarization

Because sympathetic stimulation has been implicated in the genesis of arrhythmias, we studied the effects on arrhythmias of electroconvulsive therapy (ECT). Fifteen psychiatric patients (male: 8, female: 7, age: 19-51, mean: 29.8) without known heart disease underwent 24-hout Holter recordings before, during, and after ECT (25 episodes). All patients were taking psychotropic drugs and received atropine (0.4-1.2 mg, mean: 1.1 mg IV), methohexital, and succinylcholine prior to ECT. Following ECT, mean maximum heart rate increased (106 +/- 3.2 to 142 +/- 6.0 beats/min, p less than .001), PR interval decreased (149 +/- 3.3 to 131 +/- 3.7 msec, p less than .001) and QTc interval increased (132 +/- 6.5 to 454 +/- 9.7 msec, p less than .001) compared to values obtained after atropine administration. Mean PVC or PAC frequently immediately after ECT or per 24 hours did not change significantly (PVC per 24 hours 6.8 +/- 3.2 to 10.4 +/- 6.4, NS; PAC per 24 hours 0.4 +/- 0.3 to 0.3 +/- 0.2, NS) and no complex arrhythmias were noted. Rate and PR changes suggest adrenergic effects of ECT and QTc increase may be due to imbalanced sympathetic discharge. Autonomic stimulation produced by ECT did not induce arrhythmias in these patients without heart disease. The possible antiarrhythmic role of psychotropic agents or premedication is unknown.

Phenothiazine treatment and electrocardiographic abnormalities

We have received the literature on ECG. abnormalities found in patients treated with phenothiazines. A survey of 106 electrocardiograms of patients receiving thioridazine as the only neuroleptic has been made. Fifty-nine patients (55.6%) had ECG. abnormalities considered ‘specific' to thioridazine. No statistically significant correlation could be made on the one hand with the sex of the patient, his age, duration of treatment, dosage level, use of antiparkinsonian medication and on the other hand the presence or absence of ECG. abnormalities. It is suggested that the ECG. abnormalities represent a disturbance in many patients with a particular susceptibility to the drug which is unpredictable at present.