Safety of non-antiarrhythmic drugs that prolong the QT interval or induce torsade de pointes: an overview

The long and growing list of non-antiarrhythmic drugs associated with prolongation of the QT interval of the electrocardiogram has generated concern not only for regulatory interventions leading to drug withdrawal, but also for the unjustified view that QT prolongation is usually an intrinsic effect of a whole therapeutic class [e.g. histamine H(1) receptor antagonists (antihistamines)], whereas, in many cases, it is displayed only by some compounds within a given class of non-antiarrhythmic drugs because of an effect on cardiac repolarisation. We provide an overview of the different classes of non-antiarrhythmic drugs reported to prolong the QT interval (e.g. antihistamines, antipsychotics, antidepressants and macrolides) and discusses the clinical relevance of the QT prolonging effect. Drug-induced torsade de pointes are sometimes considered idiosyncratic, totally unpredictable adverse drug reactions, whereas a number of risk factors for their occurrence is now recognised. Widespread knowledge of these risk factors and implementation of a comprehensive list of QT prolonging drugs becomes an important issue. Risk factors include congenital long QT syndrome, clinically significant bradycardia or heart disease, electrolyte imbalance (especially hypokalaemia, hypomagnesaemia, hypocalcaemia), impaired hepatic/renal function, concomitant treatment with other drugs with known potential for pharmacokinetic/pharmacodynamic interactions (e.g. azole antifungals, macrolide antibacterials and class I or III antiarrhythmic agents). This review provides insight into the strategies that should be followed during a drug development program when a drug is suspected to affect the QT interval. The factors limiting the predictive value of preclinical and clinical studies are also outlined. The sensitivity of preclinical tests (i.e. their ability to label as positive those drugs with a real risk of inducing QT pronglation in humans) is sufficiently good, but their specificity (i.e. their ability to label as negative those drugs carrying no risk) is not well established. Verapamil is a notable example of a false positive: it blocks human ether-a-go-go-related (HERG) K(+) channels, but is reported to have little potential to trigger torsade de pointes. Although inhibition of HERG K(+) channels has been proposed as a primary test for screening purposes, it is important to remember that several ion currents are involved in the generation of the cardiac potential and that metabolites must be specifically tested in this in vitro test. At the present state of knowledge, no preclinical model has an absolute predictive value or can be considered as a gold standard. Therefore, the use of several models facilitates decision making and is recommended by most experts in the field.

Non-antiarrhythmic drugs prolonging the QT interval: considerable use in seven countries

AIMS

Many drugs belonging to different therapeutic classes appear to share a potentially fatal side-effect: ventricular tachyarrhythmias associated with QT prolongation. The aim of this study was to assess the relevance and the magnitude of the problem in seven countries by grouping all nonantiarrhythmic drugs according to the type of evidence on QT prolongation and analysing their sales data.

METHODS

We divided all nonantiarrhythmic QT-prolonging agents into the following categories (in increasing order of clinical relevance): group A, drugs with published clinical or preclinical evidence on QT prolongation or with relevant official warnings; group B, drugs with published clinical or preclinical evidence; group C, drugs with published clinical evidence; group D, drug with published clinical evidence on torsades de pointes or ventricular arrhythmias associated with QT prolongation; group E, drugs belonging to group D with official warnings. We retrieved 1998 sales data from community pharmacies in seven countries (Australia, Denmark, England, Germany, Greece, Italy and Sweden). Data for individual agents were expressed as defined daily doses per 1000 inhabitants per day (DDD/1000/day). Overall use in each country was calculated for each drug group. Groups D and E were considered as the most clinically relevant.

RESULTS

Among the 102 nonantiarrhythmic agents meeting at least one of the inclusion criteria, 33 drugs had sales data > or =1 DDD/1000/day and 71 drugs had a use > or =0.1 DDD/1000/day in at least one country. Among the 37 nonantiarrhythmic agents with published reports of ventricular arrhythmias associated with QT prolongation, 12 compounds had sales data > or =1 DDD/1000/day. Total consumption in each country ranged: from 51.9 to 94.7 DDD/1000/day for group A; from 51.6 to 92.7 DDD/1000/day for group B; from 37.1 to 76.6 DDD/1000/day for group C; from 12.9 to 29.1 DDD/1000/day for group D; and from 5.8 to 15.3 DDD/1000/day for group E.

CONCLUSIONS

In spite of wide variations in the sales of individual agents, the overall extent of use of nonantiarrhythmic QT-prolonging drugs was of the same order of magnitude in all countries. The significant use of drugs belonging to categories D and E should prompt careful risk/benefit assessment of each agent.

Drug development for the irritable bowel syndrome: current challenges and future perspectives

Medications are frequently used for the treatment of patients with the irritable bowel syndrome (IBS), although their actual benefit is often debated. In fact, the recent progress in our understanding of the pathophysiology of IBS, accompanied by a large number of preclinical and clinical studies of new drugs, has not been matched by a significant improvement of the armamentarium of medications available to treat IBS. The aim of this review is to outline the current challenges in drug development for IBS, taking advantage of what we have learnt through the Rome process (Rome I, Rome II, and Rome III). The key questions that will be addressed are: (a) do we still believe in the "magic bullet," i.e., a very selective drug displaying a single receptor mechanism capable of controlling IBS symptoms? (b) IBS is a "functional disorder" where complex neuroimmune and brain-gut interactions occur and minimal inflammation is often documented: do we need to target gut motility, visceral sensitivity, or minimal inflammation? (c) are there validated biomarkers (accepted by regulatory agencies) for studies of sensation and motility with experimental medications in humans? (d) do animal models have predictive and translational value? (e) in the era of personalized medicine, does pharmacogenomics applied to these medications already play a role? Finally, this review will briefly outline medications currently used or in development for IBS. It is anticipated that a more focused interaction between basic science investigators, pharmacologists, and clinicians will lead to better treatment of IBS.

Role of 5-HT1B/D receptors in canine gastric accommodation: effect of sumatriptan and 5-HT1B/D receptor antagonists

The 5-HT1B/D receptor agonist sumatriptan has been proposed to treat dyspeptic symptoms, because it facilitates gastric accommodation. It is unknown whether stimulation of 5-HT1B/D receptors is involved. Thus, in four conscious dogs, we compared the effects of sumatriptan alone or combined with N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1-biphenyl]-4-carboxamide hydrocloride (GR-127935), N-[3-[3 (dimethylamino)-ethoxy]-4-methoxyphenyl]-2'-[methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)]-[1,1-biphenyl]-4-carboxamide hydrocloride (SB-216641 hydrochloride), or 3-[4-(4-chloro-phenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol hydrochloride (BRL-15572 hydrochloride) (respectively, nonselective 5-HT1B/D, selective 5-HT1B, and selective 5-HT1D receptor antagonists) on gastric accommodation to isobaric distensions performed with a barostat. An exponential and a linear model were used to fit the pressure-volume relationship. An exponential equation fitted the data better than a linear equation. Sumatriptan (800 nmol/kg iv) induced an immediate gastric relaxation (Deltavolume: 112 +/- 44 ml, P < 0.05). After sumatriptan, the pressure-volume curve was shifted toward significantly higher volumes. This effect was fully reversed by GR-127935 or SB-216641 but not by BRL-15572. In conclusion, 5-HT1B receptors seem to play an important role in modulating gastric accommodation to a distending stimulus. An exponential model for pressure-volume curves fits well with the concept of gastric adaptive relaxation.

Variations in Gastric Tone Associated with Duodenal Motor Events After Activation of Central Emetic Mechanisms in the Dog

To define the variations in gastric tone associated with the intestinal motor correlates of vomiting, we studied the effects of intracerebroventricular pharmacological manipulation in four fasting, conscious dogs with an injection catheter implanted into the fourth cerebral ventricle. Simultaneous monitoring of gastric tone and duodenal motility allowed us to study the correlation between gastric and duodenal motor events. We compared the effects of i.c.v. morphine (0.1 to 1 μLg/kg), apomorphine (0.003 to 0.03 (μg/kg), and the α2‐adrenoceptor agonist ST91 (0.1 to 1 μg/kg). These agonists, by the i.c.v. route all induced gastric relaxation accompanied by a retrograde power contraction in the duodenum and, at the highest doses, emesis; subsequently, a burst of intense phasic pressure activity developed in the duodenum. However, at a dose that was fully effective i.c.v., they had no effect when given intravenously. Naloxone or domperidone, at doses that had no effect per se, blocked the effects of morphine and apomorphine, respectively, except for a residual gastric relaxation. Idazoxan failed to block the effects of ST91. We conclude that a common pattern of motor events is triggered in the upper gut by different drugs by activation of receptors involved in the mechanism of vomiting and probably located in the central nervous system, and that the effects of morphine and apomorphine are mediated respectively by opioid and dopamine receptors.

Methylnaltrexone Progenics

Progenics is developing methylnaltrexone (MNTX), an opioid antagonist licensed from UR Labs and the University of Chicago, for the potential treatment of the side effects of opioid pain therapy, such as constipation and post-operative bowel dysfunction. MNTX was discovered at the University of Chicago and subsequently licensed by UR Labs. In October 2001, Progenics entered into an agreement with UR Labs to obtain exclusive worldwide rights to the drug [423791]. MNTX is in phase II trials, with phase III studies expected to begin in 2002 [423791]. In October 2001, a double-blind, randomized, phase II study evaluating sc MNTX in cancer patients for the treatment of opioid-induced constipation was initiated at the University of Chicago Medical Center [423791]. In December 2001, the company stated that it was preparing to initiate phase IIb trials of the compound in opioid-induced constipation and post-operative bowel dysfunction [423791], [432507]. These trials were initiated in February 2002 [440995].

[Physiopathology of the enteric nervous system: recent advances and clinical implications]

Enteric neuropathies may significantly affect the patient's quality of life, although they have a fatal outcome only in a minority of cases. Motor derangement (often, hypomotility) is a typical feature of primary and secondary neuropathies affecting the myenteric plexus. This review addresses some pathophysiological aspects of enteric neuropathies and tries to outline the limitations of the experimental evidence obtained so far as well as some questions that can possibly receive an answer by applying new concepts and new techniques. In particular, the concept of neuronal plasticity, originally developed by investigators studying the central nervous system, is discussed and applied to the enteric microenvironment, with specific reference to adaptive phenomena occurring during intestinal inflammation. The role of neurotrophins and the potential therapeutic applications are also briefly discussed.