QT prolongation and Torsades de Pointes associated with clarithromycin

Old drugs for a new indication: a review of chloroquine and analogue in COVID-19 treatment

As an innovative therapeutic strategy, drug repurposing affords old, approved, and already established drugs a chance at new indications. In the wake of the COVID-19 pandemic and the accompanied urgency for a lasting treatment, drug repurposing has come in handy to stem the debilitating effects of the disease. Among other therapeutic options currently in clinical trials, chloroquine (CQ) and the hydroxylated analogue, hydroxychloroquine (HCQ) have been frontline therapeutic options in most formal and informal clinical settings with varying degrees of efficacy against this life-threatening disease. Their status in randomized clinical trials is related to the biochemical and pharmacological profiles as validated by in vitro, in vivo and case studies. With the aim to bear a balance for their use in the long run, this review not only synopsizes findings from recent studies on the degrees of efficacy and roles of CQ/HCQ as potential anti-COVID-19 agents but also highlights our perspectives for their consideration in rational drug repositioning and use.

Duration and Life-Stage of Antibiotic Use and Risks of All-Cause and Cause-Specific Mortality: Prospective Cohort Study

Rationale: The overuse of antibiotics has been an important clinical issue, and antibiotic exposure is linked to alterations in gut microbiota, which has been related to risks of various chronic diseases such as cardiovascular disease and cancer. Also, duration of antibiotic exposure may be a risk factor of premature death. Objective: We investigated associations of life-stage and duration of antibiotic use during adulthood with risks of all-cause and cause-specific mortality. Methods and Results: This prospective cohort study included 37 516 women aged ≥60 years who were free of cardiovascular disease or cancer from the Nurses' Health Study. Participants reported a total amount of time they used antibiotics (none, <15 days, 15 days to <2 months, or ≥2 months) in the middle- (age, 40-59) and late adulthood (age, 60 or older). We estimated hazard ratios for all-cause mortality and deaths from cardiovascular disease or cancer over 10 years according to duration of antibiotic use. During 355 918 person-years of follow-up, we documented 4536 deaths from any cause (including 728 cardiovascular deaths and 1206 cancer deaths). As compared with women who did not use antibiotics, those who used them for ≥2 months in late adulthood had increased risks of all-cause mortality (hazard ratio, 1.16 [95% CI, 1.01-1.33]) and cardiovascular mortality (hazard ratio, 1.49 [95% CI, 1.04-2.13]), but not cancer mortality (hazard ratio, 0.85 [95% CI, 0.65-1.12]) after adjustment for chronic metabolic diseases, antibiotic use during middle adulthood, indication for use, demographic factors, and lifestyle/dietary factors. The association was more evident among women who also used antibiotics in middle-adulthood than among those who did not use during this life-stage. Conclusions: Long-term use of antibiotics in late adulthood may be a risk factor for all-cause and cardiovascular mortality. The unfavorable effect of antibiotic exposure for subsequent risks of deaths due to chronic diseases needs to be considered.

Adult Human Primary Cardiomyocyte-Based Model for the Simultaneous Prediction of Drug-Induced Inotropic and Pro-arrhythmia Risk

Cardiac safety remains the leading cause of drug development discontinuation. We developed a human cardiomyocyte-based model that has the potential to provide a predictive preclinical approach for simultaneously predicting drug-induced inotropic and pro-arrhythmia risk.

Methods: Adult human primary cardiomyocytes from ethically consented organ donors were used to measure contractility transients. We used measures of changes in contractility parameters as markers to infer both drug-induced inotropic effect (sarcomere shortening) and pro-arrhythmia (aftercontraction, AC); contractility escape (CE); time to 90% relaxation (TR90). We addressed the clinical relevance of this approach by evaluating the effects of 23 torsadogenic and 10 non-torsadogenic drugs. Each drug was tested separately at four multiples of the free effective therapeutic plasma concentration (fETPC).

Results: Human cardiomyocyte-based model differentiated between torsadogenic and non-torsadogenic drugs. For example, dofetilide, a torsadogenic drug, caused ACs and increased TR90 starting at 10-fold the fETPC, while CE events were observed at the highest multiple of fETPC (100-fold). Verapamil, a non-torsadogenic drug, did not change TR90 and induced no AC or CE up to the highest multiple of fETPCs tested in this study (222-fold). When drug pro-arrhythmic activity was evaluated at 10-fold of the fETPC, AC parameter had excellent assay sensitivity and specificity values of 96 and 100%, respectively. This high predictivity supports the translational safety potential of this preparation and of the selected marker. The data demonstrate that human cardiomyocytes could also identify drugs associated with inotropic effects. hERG channel blockers, like dofetilide, had no effects on sarcomere shortening, while multi-ion channel blockers, like verapamil, inhibited sarcomere shortening.

Conclusions: Isolated adult human primary cardiomyocytes can simultaneously predict risks associated with inotropic activity and pro-arrhythmia and may enable the generation of reliable and predictive data for assessing human cardiotoxicity at an early stage in drug discovery.

Macrolide antibiotics and the risk of ventricular arrhythmia in older adults

BACKGROUND

Many respiratory tract infections are treated with macrolide antibiotics. Regulatory agencies warn that these antibiotics increase the risk of ventricular arrhythmia. We examined the 30-day risk of ventricular arrhythmia and all-cause mortality associated with macrolide antibiotics relative to nonmacrolide antibiotics.

METHODS

We conducted a population-based retrospective cohort study involving older adults (age > 65 yr) with a new prescription for an oral macrolide antibiotic (azithromycin, clarithromycin or erythromycin) in Ontario from 2002 to 2013. Our primary outcome was a hospital encounter with ventricular arrhythmia within 30 days after a new prescription. Our secondary outcome was 30-day all-cause mortality. We matched patients 1:1 using propensity scores to patients prescribed nonmacrolide antibiotics (amoxicillin, cefuroxime or levofloxacin). We used conditional logistic regression to measure the association between macrolide exposure and outcomes, and repeated the analysis in 4 subgroups defined by the presence or absence of chronic kidney disease, congestive heart failure, coronary artery disease and concurrent use of a drug known to prolong the QT interval.

RESULTS

Compared with nonmacrolide antibiotics, macrolide antibiotics were not associated with a higher risk of ventricular arrhythmia (0.03% v. 0.03%; relative risk [RR] 1.06, 95% confidence interval [CI] 0.83-1.36) and were associated with a lower risk of all-cause mortality (0.62% v. 0.76%; RR 0.82, 95% CI 0.78-0.86). These associations were similar in all subgroups.

INTERPRETATION

Among older adults, macrolide antibiotics were not associated with a higher 30-day risk of ventricular arrhythmia than nonmacrolide antibiotics. These findings suggest that current warnings from the US Food and Drug Administration may be overstated.

Drug-induced proarrhythmia: risk factors and electrophysiological mechanisms

Drug-induced ventricular tachyarrhythmias can be caused by cardiovascular drugs, noncardiovascular drugs, and even nonprescription agents. They can result in arrhythmic emergencies and sudden cardiac death. If a new arrhythmia or aggravation of an existing arrhythmia develops during therapy with a drug at a concentration usually considered not to be toxic, the situation can be defined as proarrhythmia. Various cardiovascular and noncardiovascular drugs can increase the occurrence of polymorphic ventricular tachycardia of the 'torsade de pointes' type. Antiarrhythmic drugs, antimicrobial agents, and antipsychotic and antidepressant drugs are the most important groups. Age, female sex, and structural heart disease are important risk factors for the occurrence of torsade de pointes. Genetic predisposition and individual pharmacodynamic and pharmacokinetic sensitivity also have important roles in the generation of arrhythmias. An increase in spatial or temporal dispersion of repolarization and a triangular action-potential configuration have been identified as crucial predictors of proarrhythmia in experimental models. These studies emphasized that sole consideration of the QT interval is not sufficient to assess the proarrhythmic risk. In this Review, we focus on important triggers of proarrhythmia and the underlying electrophysiological mechanisms that can enhance or prevent the development of torsade de pointes.

Heterogeneous Phenotype of Long QT Syndrome Caused by the KCNH2-H562R Mutation: Importance of Familial Genetic Testing

INTRODUCTION AND OBJECTIVES

Long QT syndrome is an inherited ion channelopathy that leads to syncope and sudden death. Because of the heterogeneous phenotype of this disease, genetic testing is fundamental to detect individuals with concealed long QT syndrome. In this study, we determined the features of a family with 13 carriers of the KCNH2-H562R missense mutation, which affects the pore region of the HERG channel.

METHODS

We identified the KCNH2-H562R mutation in a 65-year-old man with a prolonged QTc interval who had experienced an episode of torsade de pointes. Subsequently, a total of 13 mutation carriers were identified in the family. Carriers (age 48 [26] years; 46% males) underwent clinical evaluation, electrocardiography and echocardiography.

RESULTS

The mean (standard deviation) QTc in carriers was 493 (42) ms (3 [23%] showed normal QTc); 6 (46%) had symptoms (4, syncope; 1, sudden death; 1, aborted sudden death [proband]). While under treatment with beta-blockers, 11 of 12 carriers (92%) remained asymptomatic at 5 years of follow-up (1 patient required left cardiac sympathectomy). The QTc shortening with beta-blockers was 50 (37) ms. There was 1 sudden death in a patient who refused treatment.

CONCLUSIONS

Family study is essential in the interpretation of a genetic testing result. This article describes the heterogeneous and variable phenotype of a large family with the KCNH2-H562R mutation and highlights the role of genetic study for the appropriate identification of at-risk individuals who would benefit from treatment.

Clinical manifestation of macrolide antibiotic toxicity in CKD and dialysis patients

Macrolide antibiotics, erythromycin, clarithromycin and azithromycin are commonly prescribed for upper respiratory infection, and their use has recently been further linked to immunomodulatory effects. With the widespread and expanded use of macrolides, special attention should be paid to their potential adverse effects. We reported two cases of end-stage renal disease (ESRD) patients who developed hallucinations such as vivid images of worms after taking clarithromycin. Similar to previous case reports of clarithromycin neurotoxicity, the visual hallucination resolved upon cessation of clarithromycin. Furthermore, we discussed the pharmacokinetic properties and other toxicities of macrolide antibiotics in patients with chronic kidney disease and ESRD.

Macrolide antibiotics and the risk of cardiac arrhythmias

Randomized, controlled trials have demonstrated that chronic therapy with macrolide antibiotics reduces the morbidity of patients with cystic fibrosis, non-cystic fibrosis bronchiectasis, chronic obstructive pulmonary disease, and nontuberculous mycobacterial infections. Lower levels of evidence indicate that chronic macrolides are also effective in treating patients with panbronchiolitis, bronchiolitis obliterans, and rejection after lung transplant. Macrolides are known to cause torsade des pointes and other ventricular arrhythmias, and a recent observational study prompted the FDA to strengthen the Warnings and Precautions section of azithromycin drug labels. This summary describes the electrophysiological effects of macrolides, reviews literature indicating that the large majority of subjects experiencing cardiac arrhythmias from macrolides have coexisting risk factors and that the incidence of arrhythmias in absence of coexisting risk factors is very low, examines recently published studies describing the relative risk of arrhythmias from macrolides, and concludes that this risk has been overestimated and suggests an approach to patient evaluation that should reduce the relative risk and the incidence of arrhythmias to the point that chronic macrolides can be used safely in the majority of subjects for whom they are recommended.

Drug-induced QT interval prolongation: mechanisms and clinical management

The prolonged QT interval is both widely seen and associated with the potentially deadly rhythm, Torsades de Pointes (TdP). While it can occur spontaneously in the congenital form, there is a wide array of drugs that have been implicated in the prolongation of the QT interval. Some of these drugs have either been restricted or withdrawn from the market due to the increased incidence of fatal polymorphic ventricular tachycardia. The list of drugs that cause QT prolongation continues to grow, and an updated list of specific drugs that prolong the QT interval can be found at www.qtdrugs.org. This review focuses on the mechanism of drug-induced QT prolongation, risk factors for TdP, culprit drugs, prevention and monitoring of prolonged drug-induced QT prolongation and treatment strategies.

Clarithromycin-Induced Torsades de Pointes

Acquired QT prolongation can be caused by cardiac and noncardiac medications. Macrolides are known to prolong QT interval. Rarely, it can be fatal. We report a case of torsades de pointes in a patient taking clarithromycin as a part of Helicobacter pylori eradication regimen.

Torsades de pointes following clarithromycin treatment

A 75-year-old woman presenting with pre-syncope, shortness of breath and nausea was admitted to the emergency department following treatment with clarithromycin. Shortly after admission she developed a prolonged QT interval leading to torsades de pointes (TdP) and cardiac arrest. She was successfully cardioverted and clarithromycin was discontinued resulting in restoration of her usual QT interval. This case is an example of acquired long QT syndrome; a disorder that can be precipitated by macrolide antibiotics such as clarithromycin. Additional risk factors present in this case include: female gender, old age, heart disease, hypokalemia and hypomagnesemia. In this manuscript we comprehensively review past cases of clarithromycin-induced long QT syndrome (LQTS) and discuss them within the context of this case.

Clarithromycin, QTc interval prolongation and torsades de pointes: the need to study case reports

BACKGROUND

The manufacturers of clarithromycin sought a drug similar in efficacy to erythromycin but with a superior side-effect profile. They generally achieved this outcome, but postmarketing findings identified a series of reports linking clarithromycin to QTc interval prolongation and torsades de pointes (TdP) ultimately leading to a Black Box Warning. We sought to clarify risk factors associated with TdP among case reports of patients receiving clarithromycin linked to QTc interval prolongation and TdP.

METHODS AND RESULTS

In a detailed literature search, we found 15 women, five men, and one boy meeting our search criteria. Among the 17 adults with reported clarithromycin dose and concurrent QTc interval measurement, we found no statistically significant relationship between clarithromycin dose and QTc interval duration. This did not change for the adults who developed TdP. Among adults, major risk factors were female sex (15), old age (11) and heart disease (17). A total of eight adult subjects had all three major risk factors and 14 of the 20 adults had at least two major risk factors. All adult subjects had at least two risk factors besides clarithromycin. A total of four of the 20 adults received cisapride and three received disopyramide. Three adults were considered to suffer from some aspect of the congenital long QT syndrome.

CONCLUSIONS

We believe that the risk factor description for this drug should be refined to emphasize the major risk factors of (1) female sex, (2) old age and (3) heart disease.

Medications for Extensively Drug-Resistant Tuberculosis: Back to the Future?

Objective:

To reexamine the existing medications for the potential treatment of extensively drug-resistant tuberculosis (XDR-TB), based on susceptibility data, and to identify potential future medications from the literature.

Data Sources:

Relevant information was identified through a search of MEDLINE (1966–November 2007), PubMed (1955–November 2007), American Search Premier (1975–November 2007), International Pharmaceutical Abstracts (1960–November 2007), Science Citation Index Expanded (1996–November 2007), Cochrane Databases (publications archived until November 2007), and various tertiary sources as listed in the references, using the terms extensively drug-resistant tuberculosis (XDR-TB), ethambutol, pyrazinamide, para-aminosalicylic acid, cycloserine, linezolid, diarylquinoline, nitroimidazopyran, fluoroquinolones, β-lactams, new treatments, and ethionamide alone or in combination regimens.

Study Selection and Data Extraction:

After identification of the relevant information, the data presented in this article were selected based on clinical relevance and value of information.

Data Synthesis:

Based on susceptibility data, pyrazinamide, ethambutol, para-aminosalicylic acid, cycloserine, and ethionamide may be used for the treatment of tuberculosis. However, due to the emergence of XDR-TB, many of these agents are no longer successful treatment regimens. We have found limited data supporting potential future use of β-lactams, clarithromycin, and linezolid in resistant TB infections. TMC207, nitroimidazopyran, and SQ109 compounds may also prove to be viable options in the near future for treatment of tuberculosis, especially in cases with resistance to mainstay medications.

Conclusions:

Extensively resistant tuberculosis appears to be a potentially catastrophic disease if allowed to spread. Due to its resistance profile, very few potentially effective agents are available, calling attention to this growing problem.

Azithromycin as a cause of QT-interval prolongation and torsade de pointes in the absence of other known precipitating factors

During treatment with azithromycin, a 55 year-old woman developed a newly prolonged QT interval and torsade de pointes in the absence of known risk factors. Female gender and acute renal failure may be considerations in patients treated with azithromycin.

Current concepts in the mechanisms and management of drug-induced QT prolongation and torsade de pointes

Drug-induced long QT syndrome is characterized by a prolonged corrected QT interval (QTc) and increased risk of a polymorphic ventricular tachycardia known as torsade de pointes (TdP). We review mechanisms, predispositions, culprit agents, and management of this potentially fatal phenomenon. Virtually all drugs that prolong QTc block the rapid component of the delayed rectifier current (I(kr)). Some drugs prolong QTc in a dose-dependent manner, others do so at any dose. Most patients that develop drug-induced TdP have underlying risk factors. Female sex is the most common. Implicated drugs include class 1A and III antiarrhythmics, macrolide antibiotics, pentamidine, antimalarials, antipsychotics, arsenic trioxide, and methadone. Treatment for TdP includes immediate defibrillation for hemodynamic instability and intravenous magnesium sulfate. Potassium levels should be maintained in the high normal range, and all QT prolonging agents must be promptly discontinued.

PRECLINICAL ELECTROPHYSIOLOGY ASSAYS OF MITEMCINAL (GM-611), A NOVEL PROKINETIC AGENT DERIVED FROM ERYTHROMYCIN

Mitemcinal (GM-611) is a novel erythromycin-derived prokinetic agent that acts as an agonist at the motilin receptor. Erythromycin has shown QT prolongation and torsades de pointes (TdP) in humans and cisapride, a second class of prokinetic agents typified by the 5-HT(4) receptor agonist, has been terminated due to TdP. In this study an extended series of safety pharmacology protocols and evaluations have been undertaken to assess the potential risk of mitemcinal on QT prolongation or proarrhythmic effects. Mitemcinal and its metabolites, GM-577 and GM-625, inhibited the human ether-a-go-go-related gene (HERG) tail current in a concentration-dependent manner with IC(50) values of 20.2, 41.7, and 55.0 microM, respectively. Administration of 10 mg/kg mitemcinal in anesthetized guinea pigs resulted in a slight prolongation of the monophasic action potential (MAP) duration during atrial pacing at the plasma concentration of mitemcinal 1.1 microM, with low maximum increases in MAPD(70) (6.6%) and MAPD(90) (4.6%) relative to vehicle. A 10-min infusion of 20 mg/kg of mitemcinal in a proarrhythmic rabbit model did not evoke TdP even when QT and corrected QT (QTc) intervals were significantly prolonged. In this study, the Cmax plasma-free concentration of mitemcinal indicates that the prolongation was more than 400-fold that of the therapeutic dose. Our findings of a wide safety margin and the absence of TdP within this margin suggest that mitemcinal may provide sufficient safety in clinical use.

Clarithromycin treatment and QT prolongation in childhood

The effect of clarithromycin on the QT interval was studied in a group of 28 children treated for respiratory tract infections. QTc was measured before and following 24 h of treatment. A modest (average 22 ms, 95% CI 14-30 ms) but significant QTc prolongation (p<0.001) was observed, with seven cases having a QTc >440 ms during treatment (including a single case with QTc >460 ms).

CONCLUSION

Serial QTc measurements are necessary for early detection of children at risk for drug-induced arrhythmias.

Antimicrobial-associated QT interval prolongation: pointes of interest

Until recently, cardiac toxicity manifesting in the form of arrhythmias related to QT interval prolongation was uncommonly appreciated within the antimicrobial class of drugs, but it was well described among antiarrhythmic agents. Antimicrobials that are associated with QT prolongation include the macrolides/ketolides, certain fluoroquinolones and antimalarials, pentamidine, and the azole antifungals. Although, in most cases, mild delays in ventricular repolarization caused by these drugs are clinically unnoticeable, they may serve to amplify the risk for torsades de pointes (TdP) when prescribed in the setting of other risk factors. Conditions or variables that influence proarrhythmic risk include sex, age, electrolyte derangements, structural heart disease, pharmacokinetic/pharmacodynamic interactions, and genetic predisposition. It is important that clinicians be knowledgeable about drugs with QT liability, as well as the risk factors that increase the probability of TdP. Additionally, because TdP remains a difficult-to-measure adverse event, we must rely upon multiple data sources to determine the risk versus the benefit for newly approved drugs.

Torsades de pointes induced by antibiotics

BACKGROUND

Several frequently used antibiotics are associated with an arrhythmia called "torsades de pointes" (TdP). This potentially fatal arrhythmia is considered unpredictable.

METHODS

In order to investigate the prevalence of risk factors for TdP prior to initiation of antibiotic therapy, we conducted a literature search for all published reports on TdP induced by antibiotics and we asked pharmaceutical companies for additional unpublished reports.

RESULTS

We studied 61 reports on 78 patients with TdP induced by antibiotics. Female gender was the most common risk factor for TdP: 66.7% (n=52) of all patients were women. Advanced heart disease and concomitant use of a QT interval-prolonging agent or an inhibitor of liver drug metabolism were also frequently present (59% and 48.7%, respectively). Most patients had at least one and 58 patients (74.3%) had two risk factors or more for TdP prior to initiation of antibiotic therapy.

CONCLUSION

Contrary to common belief, TdP induced by antibiotics may be predictable by simple history-taking and by obtaining a baseline electrocardiogram. We wish to draw attention to risk factors for TdP prior to the initiation of antibiotic therapy.

Electrophysiologic properties and antiarrhythmic actions of a novel antianginal agent

Ranolazine is a novel antianginal agent capable of producing anti-ischemic effects at plasma concentrations of 2 to 6 microM without a significant reduction of heart rate or blood pressure. This review summarizes the electrophysiologic properties of ranolazine. Ranolazine significantly blocks I(Kr) (IC(50) = 12 microM), late I(Na), late I(Ca), peak I(Ca), I(Na-Ca) (IC(50) = 5.9, 50, 296, and 91 microM, respectively) and I(Ks) (17% at 30 microM), but causes little or no inhibition of I(to) or I(K1). In left ventricular tissue and wedge preparations, ranolazine produces a concentration-dependent prolongation of action potential duration (APD) in epicardium, but abbreviation of APD of M cells, leading to either no change or a reduction in transmural dispersion of repolarization (TDR). The result is a modest prolongation of the QT interval. Prolongation of APD and QT by ranolazine is fundamentally different from that of other drugs that block I(Kr) and induce torsade de pointes in that APD prolongation is rate-independent (ie, does not display reverse rate-dependent prolongation of APD) and is not associated with early after depolarizations, triggered activity, increased spatial dispersion of repolarization, or polymorphic ventricular tachycardia. Torsade de pointes arrhythmias were not observed spontaneously nor could they be induced with programmed electrical stimulation in the presence of ranolazine at concentrations as high as 100 microM. Indeed, ranolazine was found to possess significant antiarrhythmic activity, acting to suppress the arrhythmogenic effects of other QT-prolonging drugs. Ranolazine produces ion channel effects similar to those observed after chronic exposure to amiodarone (reduced late I(Na), I(Kr), I(Ks), and I(Ca)). Ranolazine's actions to reduce TDR and suppress early after depolarization suggest that in addition to its anti-anginal actions, the drug possesses antiarrhythmic activity.

QT prolongation with antimicrobial agents: understanding the significance

Cardiac toxicity has been relatively uncommon within the antimicrobial class of drugs, but well described for antiarrhythmic agents and certain antihistamines. Macrolides, pentamidine and certain antimalarials were traditionally known to cause QT-interval prolongation, and now azole antifungals, fluoroquinolones and ketolides can be added to the list. Over time, advances in preclinical testing methods for QT-interval prolongation and a better understanding of its sequelae, most notably torsades de pointes (TdP), have occurred. This, combined with the fact that five drugs have been removed from the market over the last several years, in part because of QT-interval prolongation-related toxicity, has elevated the urgency surrounding early detection and characterisation methods for evaluating non-antiarrhythmic drug classes. With technological advances and accumulating literature regarding QT prolongation, it is currently difficult or overwhelming for the practising clinician to interpret these data for purposes of formulary review or for individual patient treatment decisions. Certain patients are susceptible to the effects of QT-prolonging drugs. For example, co-variates such as gender, age, electrolyte derangements, structural heart disease, end organ impairment and, perhaps most important, genetic predisposition, underlie most if not all cases of TdP. Between and within classes of drugs there are important differences that contribute to delayed repolarisation (e.g. intrinsic potency to inhibit certain cardiac ion currents or channels, and pharmacokinetics). To this end, a risk stratification scheme may be useful to rank and compare the potential for cardiotoxicity of each drug. It appears that in most published cases of antimicrobial-associated TdP, multiple risk factors are present. Macrolides in general are associated with a greater potential than other antimicrobials for causing TdP from both a pharmacodynamic and pharmacokinetic perspective. The azole antifungal agents also can be viewed as drugs that must be weighed carefully before use since they also have both pharmacodynamic and pharmacokinetic characteristics that may trigger TdP. The fluoroquinolones appear less likely to be associated with TdP from a pharmacokinetic perspective since they do not rely on cytochrome P450 (CYP) metabolism nor do they inhibit CYP enzyme isoforms, with the exception of grepafloxacin and ciprofloxacin. Nonetheless, patient selection must be carefully made for all of these drugs. For clinicians, certain responsibilities are assumed when prescribing antimicrobial therapy: (i) appropriate use to minimise resistance; and (ii) appropriate patient and drug selection to minimise adverse event potential. Incorporating information learned regarding QT interval-related adverse effects into the drug selection process may serve to minimise collateral iatrogenic toxicity.

Characterization of the inhibitory effects of erythromycin and clarithromycin on the HERG potassium channel

Both erythromycin and clarithromycin have been reported to cause QT prolongation and the cardiac arrhythmia torsade de pointes in humans, however direct evidence documenting that these drugs produce this effect by blocking human cardiac ion channels is lacking. The goal of this study was to test the hypothesis that these macrolide antibiotics significantly block the delayed rectifier current (IKr) encoded by HERG (the human ether-a-go-go-related gene) at drug concentrations, temperature and ionic conditions mimicking those occurring in human subjects. Potassium currents in HEK 293 cells stably transfected with HERG were recorded using a whole cell voltage clamp method. Exposure of cells to erythromycin reduced the HERG encoded potassium current in a concentration dependent manner with an IC50 of 38.9 +/- 1.2 microM and Hill Slope factor of 0.4 +/- 0.1. Clarithromycin produced a similar concentration-dependent block with an IC50 of 45.7 +/- 1.1 microM and Hill Slope factor of 1.0 +/- 0.1. Erythromycin (25-250 microM) and clarithromycin (5 or 25 microM) also produced a significant decrease in the integral of the current evoked by an action potential shaped voltage clamp protocol. The results of this study document that both erythromycin and clarithromycin significantly inhibit the HERG potassium current at clinically relevant concentrations.

Review of the predictive value of the Langendorff heart model (Screenit system) in assessing the proarrhythmic potential of drugs

Prolongation of the QTc interval of the electrocardiogram (ECG) is used as a surrogate marker for a rare, but life threatening, ventricular arrhythmia known as torsades de pointes (TdP). However, the clear link between QTc prolongation and the arrhythmogenic risk has not been demonstrated unequivocally. In the present review article, we examine (a) the current understanding of electrophysiological and pharmacological mechanisms linking changes in action potential (AP) properties with proarrhythmia and (b) the value of the isolated, paced Langendorff-perfused female rabbit heart model (Screenit system) in predicting the torsadogenic potential of drugs in man. The Screenit system records monophasic action potentials (MAPs) from which the following parameters are evaluated: action potential duration (APD), conduction, instability (indicative of beat to beat APD variability), triangulation (indicative of changes of Phase 3 repolarization), and reverse-use dependency (indicating that the APD is more prolonged at slow heart rates). So far, over 16,000 experiments have been conducted, including approximately 300 dedicated tests to evaluate, in a blinded manner, approximately 70 clinically used drugs. The drugs tested covered a wide range of compounds from various pharmacological and chemical classes with clinical torsadogenic propensity, as well as drugs without the latter effect in clinical settings. Overall, the Screenit system and its associated analysis classified the drugs based on their effects on AP morphology and conduction and additionally identified, in a qualitative manner, drugs clinically associated with TdP. Such an identification is based on the triangulation, reverse-use dependency, and instability of the AP, as well as on the direct indexes of proarrhythmia such as early afterdepolarization (EADs), ventricular tachycardia (VT), and ventricular fibrillation (VF). Overall, drugs that readily induce arrhythmia and/or EADs and/or causes triangulation, reverse-use dependency, and/or instability and/or a chaotic Poincaré plot in a range of concentrations likely to be achieved in man is likely to cause TdP in man, eventually. Only if none of these elements is present, at concentrations well exceeding the free therapeutic plasma concentration, can one expect that the drug will probably be devoid of torsadonenicity. Therefore, this in vitro model provides detailed information on the overall profile of drug-induced electrophysiological effects. In combination with other in vitro and in vivo repolarization assays and with pharmacokinetic data in man, it is a valuable tool to establish an integrated cardiovascular risk assessment of pharmaceutical compounds.

Clarithromycin reducesIsusandItocurrents in human atrial myocytes with minor repercussions on action potential duration

The macrolide antibacterial agent clarithromycin has been shown to cause QT interval prolongation on the electrocardiogram. In rabbit heart preparations clarithromycin (concentration dependently) lengthened the action potential duration and blocked the delayed rectifier current. The aim of the present study was to investigate the clarithromycin effects: (i) on the Ca2+ L-type and the main K+ repolarizing currents on human atrial myocytes, using whole-cell patch clamp recordings and (ii) on action potentials recorded from human atrial and ventricular myocardium using conventional microelectrodes. It has been found that (i) 10-30 microM clarithromycin reduced the sustained current Isus significantly and that a 100 microM concentration was needed to cause a significant reduction in the transient outward current Ito, whereas clarithomycin did not affect the calcium current and (ii) clarithromycin (10-100 microM) prolonged the action potential duration in atrial preparations but did not alter the different parameters of the ventricular action potential. It is concluded that clarithromycin exerts direct cardiac electrophysiological effects that may contribute to pro-arrythmic potential.

Torsade de pointes due to noncardiac drugs: most patients have easily identifiable risk factors

Numerous medications, including drugs prescribed for noncardiac indications, can lead to QT prolongation and trigger torsade de pointes. Although this complication occurs only rarely, it may have lethal consequences. It is therefore important to know if patients with torsade de pointes associated with noncardiac drugs have risk factors that are easy to identify. We reviewed reports of drug-induced torsade de pointes and analyzed each case of torsade de pointes associated with a noncardiac drug for the presence of risk factors for the long QT syndrome that can be easily identified from the medical history or clinical evaluation (female gender, heart disease, electrolyte disturbances, excessive dosing, drug interactions, and history of familial long QT syndrome). We identified 249 patients with torsade de pointes caused by noncardiac drugs. The most commonly identified risk factor was female gender (71%). Other risk factors were frequently present (18%-41%). Virtually all patients had at least 1 of these risk factors, and 71% of patients had 2 or more risk factors. Our study suggests that almost all patients with torsade de pointes secondary to noncardiac drugs have risk factors that can be easily identified from the medical history before the initiation of therapy with the culprit drug.

Clinical relevance and management of drug-related QT interval prolongation

Much attention recently has focused on drugs that prolong the QT interval, potentially leading to fatal cardiac dysrhythmias (e.g., torsade de pointes). We provide a detailed review of the published evidence that supports or does not support an association between drugs and their risk of QT prolongation. The mechanism of drug-induced QT prolongation is reviewed briefly, followed by an extensive evaluation of drugs associated with QT prolongation, torsade de pointes, or both. Drugs associated with QT prolongation are identified as having definite, probable, or proposed associations. The role of the clinician in the prevention and management of QT prolongation, drug-drug interactions that may occur with agents known to affect the QT interval, and the impact of this adverse effect on the regulatory process are addressed.

The incorporation of an ion channel gene mutation associated with the long QT syndrome (Q9E-hMiRP1) in a plasmid vector for site-specific arrhythmia gene therapy: in vitro and in vivo feasibility studies

The present studies investigated the cardiac potassium channel missense mutation, Q9E-hMiRP1, for potential use as a gene therapy construct for cardiac arrhythmias. This gene abnormality is one of a number of mutations that can cause the long QT syndrome (LQTS), a hereditary arrhythmia disorder that is associated with sudden death. However, individuals who carry the Q9E-hMiRP1 variant are predisposed to developing the LQTS only after clarithromycin administration. Because the electrophysiologic mechanism of action of Q9E-hMiRP1 (i.e., diminished potassium currents resulting in delayed myocardial repolarization) is comparable to that of class III antiarrhythmic agents, we examined Q9E-hMiRP1 as a candidate gene therapy construct for site-specific treatment of reentrant atrial cardiac arrhythmias. Our rationale was also based on the hypothetical safety of the atrial use of Q9E-hMiRP1 because LQTS characteristically causes ventricular but not atrial arrhythmias. Furthermore, the possible use of clarithromycin to control the conduction effects of overexpressed Q9E-hMiRP1 pharmacologically was another attractive feature. In our studies we investigated the use of two bicistronic plasmid DNA gene vectors with either hMiRP1 or Q9E-MiRP1 and green fluorescent protein (GFP), plus a C-terminus of the hMiRP1 or of the Q9E-hMiRP1 coding region for the FLAG (MDYKDDDDK) peptide. We generated two stable cell lines using HEK293 and SH-SY5Y (human cell lines), overexpressing the genes of interest, confirmed by real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blots. The expected plasma membrane localization of each overexpressed transgene was confirmed by immunofluorescent confocal fluorescent microscopy using anti-FLAG antibody. Patchclamp studies demonstrated that cells transfected with Q9E-hMiRP1 plasmid DNA exhibited significantly reduced potassium currents but only with clarithromycin administration. A novel plasmid DNA delivery system was formulated for use in our animal studies of the hMiRP1 vectors, which was composed of DNA-anti-DNA antibody-cationic lipid (DAC) heteroplexes. In vitro and in vivo studies using DAC heteroplexes containing anti-DNA antibodies with nuclear targeting capability demonstrated significantly increased transfection compared to naked DNA, and to DNA-cationic lipid complexes. Pig atrial myocardial injections of DAC heteroplexes demonstrated 16% of regional cardiac myocytes transfected using the Q9E-hMiRP1 plasmid, and 15% of cells with the hMiRP1 vector. It is concluded that the present studies support the view that site-specific gene therapy for atrial arrhythmias is feasible using plasmid vectors for overexpressing ion channel mutations that have electrophysiologic effects comparable to class III antiarrhythmic agents.

Knowledge deficits related to the QT interval could affect patient safety

BACKGROUND

Recently, some QT-prolonging, noncardiac medications were withdrawn from the U.S. drug market because of continued inappropriate use by health care practitioners despite warnings and label changes from both the drug manufacturers and the U.S. Food and Drug Administration. This led us to assess the health care practitioners' knowledge of the QT interval and medications that may prolong it.

METHODS

We surveyed health care practitioners, primarily specialists in cardiology, to identify knowledge deficits related to the QT interval.

RESULTS

From a total of 334 survey respondents, 157 (47%) were physicians; 271 (81%) stated that cardiology was their area of specialization. Most of the respondents (86%) said that they would check an ECG before and after starting QT-prolonging medications, but less than half (42%) of all respondents and only 60% of physician respondents were able to accurately measure a sample QT interval on the survey. Less than two-thirds (63%) of respondents were able to accurately identify possible QT-prolonging medications, while only about half (51%) could accurately identify medication combinations that might prolong the QT interval.

CONCLUSIONS

We identified significant knowledge deficits regarding the QT interval and QT-prolonging medications. Additional research is needed to determine the extent to which these knowledge deficits may negatively affect patient safety. We must also develop effective strategies to eliminate these deficits.

Comparative effects of clarithromycin on action potential and ionic currents from rabbit isolated atrial and ventricular myocytes

Prolongation of QT interval by several antibacterial drugs is an unwanted side effect that may be associated with development of ventricular arrhythmias. The macrolide antibacterial agent clarithromycin has been shown to cause QT prolongation. To determine the electrophysiologic basis for this arrhythmogenic potential, we investigated clarithromycin effects on (i). action potentials recorded from rabbit Purkinje fibers and atrial and ventricular myocardium using conventional microelectrodes and (ii). potassium and calcium currents recorded from rabbit atrial and ventricular isolated myocytes using whole-cell patch clamp recordings. We found that (i). clarithromycin (3-100 microM) exerted concentration-dependent lengthening effects on action potential duration in all tissues, with higher efficacy and reverse frequency-dependence in Purkinje fibers. However, clarithromycin did not cause development of early afterdepolarizations, and the parameters other than action potential duration were almost unaffected; (ii). clarithromycin (10-100 microM) reduced the delayed rectifier current. Significant blockade (approximately 30%) was found at the concentration of 30 microM. At 100 microM, it decreased significantly the maximum peak of the calcium current amplitude but failed to alter the transient outward and inwardly rectifier currents. It was concluded that these effects might be an explanation for the QT prolongation observed in some patients treated with clarithromycin.

QTc-prolonging drugs and hospitalizations for cardiac arrhythmias

Cardiac arrhythmia as an adverse effect of noncardiac drugs has been an issue of growing importance during the past few years. In this population-based study, we evaluated the risk for serious cardiac arrhythmias during the use of several noncardiac QTc-prolonging drugs in day-to-day practice, and subsequently focused on several specific groups of patients who could be extremely vulnerable for drug-induced arrhythmias. We performed a case-control study in which patients (cases), hospitalized for nonatrial cardiac arrhythmias from 1987 to 1998, were compared with their matched controls regarding current use of QTc-prolonging drugs. Odds ratios (OR) and 95% confidence intervals (CI) were calculated using multivariate conditional logistic regression, adjusting for potential confounding factors. Data were obtained from the PHARMO record linkage system. We identified 501 cases, 39 of whom used QTc-prolonging drugs. A statistically nonsignificant increased risk for arrhythmias (OR 1.2, 95% CI 0.8 to 1.9) was observed in patients who received QTc-prolonging drugs. A clearly increased risk of arrhythmias was, however, found in patients with a history of asthma (OR 9.9, 95% CI 1.0 to 100) and in patients using potassium-lowering drugs (OR 5.3, 95% CI 1.1 to. 25.9). Our data do not suggest that there is a strong overall association between the use of QTc-prolonging drugs and hospitalization for cardiac arrhythmias in the population at large. However, there appears to be clinically relevant associations of patients with a history of asthma and patients taking potassium-lowering drugs. The use of QTc-prolonging drugs should therefore be either avoided or monitored closely in these specific patients.

Divergent proarrhythmic potential of macrolide antibiotics despite similar QT prolongation: fast phase 3 repolarization prevents early afterdepolarizations and torsade de pointes

Macrolide antibiotics are known to have a different proarrhythmic potential in the presence of comparable QT prolongation in the surface ECG. Because the extent of QT prolongation has been used as a surrogate marker for cardiotoxicity, we aimed to study the different electrophysiological effects of the macrolide antibiotics erythromycin, clarithromycin, and azithromycin in a previously developed experimental model of proarrhythmia. In 37 Langendorff-perfused rabbit hearts, erythromycin (150-300 microM, n = 13) clarithromycin (150-300 microM, n = 13), and azithromycin (150-300 microM, n = 11) led to similar increases in QT interval and monophasic action potential (MAP) duration. In bradycardic (atrioventricular-blocked) hearts, eight simultaneously recorded epi- and endocardial MAPs demonstrated increased dispersion of repolarization in the presence of all three antibiotics. Erythromycin and clarithromycin led to early afterdepolarizations (EADs) and torsade de pointes (TdP) after lowering of potassium concentration. In the presence of azithromycin, no EAD or TdP occurred. Erythromycin and clarithromycin changed the MAP configuration to a triangular pattern, whereas azithromycin caused a rectangular pattern of MAP prolongation. In 13 additional hearts, 150 microM azithromycin was administered after previous treatment with 300 microM erythromycin and suppressed TdP provoked by erythromycin. In conclusion, macrolide antibiotics lead to similar prolongation of repolarization but show a different proarrhythmic potential (erythromycin > clarithromycin > azithromycin). In the presence of azithromycin, neither EAD nor TdP occur. This effect may be related to a rectangular pattern of action potential prolongation, whereas erythromycin and clarithromycin cause triangular action potential prolongation and induce TdP.

Optimizing antimicrobial dosing in the critically ill patient

"We know everything about antibiotics except how much to give," Maxwell Finland once stated. Finally, with the proliferation of pharmacodynamics as a science, we are addressing the question of how much to give. We have moved from an era of more or less arbitrary antimicrobial dosage selection toward one characterized by evidence-based optimal dosing strategies. Optimizing antimicrobial therapy in critically ill patients is more than just the selection of a suitable dose for a particular patient. Optimizing therapy also involves the selection of an appropriate single or combination antibiotic regimen that is active against the suspected or documented pathogens at the site(s) of infection. The regimen should offer the fewest potential adverse events, and the duration of therapy should be the shortest possible so as not to encourage resistance. Dosing of the chosen regimen should reflect variables that are often ignored, such as the patient's weight and age. The new continuous renal replacement therapies are commonly used in the critical care unit and must be considered. Finally, the cost of the regimen should be considered, but not only the cost to purchase the chosen antimicrobial agent but the cost to administer it (, the cost of minibags or syringes, intravenous tubing, saline flushes [all multiplied by the number of times per day the drug is given]), and, most importantly, if the patient fails to respond to therapy, the cost necessary to re-treat the patient to bring about a cure. In this review, we discuss some of the principles required to optimize antimicrobial dosing and recently obtained data regarding its application to the critically ill patient.

QT prolongation and torsades de pointes associated with concurrent use of cisapride and erythromycin

Prolongation of QT interval may lead to serious, potentially life-threatening, ventricular tachyarrhythmia, such as torsades de pointes. The cause may be an inherited or an acquired malfunction of ion channels at the myocardial cell membrane. Metabolic abnormalities, starvation, nervous system injury, and drug administration cause the much more frequent acquired long QT syndrome (LQTS). Types Ia and III antiarrhythmic drugs account for the majority of these life-threatening events, whereas a number of drugs widely used in otolaryngology, such as antibiotics and antihistamines, have been recently implicated. A case of a life-threatening ventricular tachyarrhythmia after the concurrent administration of cisapride and erythromycin is presented. Reviewed are drugs commonly prescribed in otolaryngology, as well as the associated risk factors that potentially lead to LQTS.

[Ventricular tachycardia and long QT associated with clarithromycin administration in a patient with HIV infection]

Prolongation of the QT interval is associated with a high risk of serious ventricular tachyarrhythmias, usually torsade de pointes (TdP) polymorphic ventricular tachycardia, although monomorphic ventricular tachycardia may also develop. Both congenital and acquired forms have been reported, acquired forms being much more prevalent. An association between human immunodeficiency virus (HIV) infection and a higher rate of dilated cardiomyopathy has also been recognized. The severity of immunodeficiency seems to influence both the incidence and severity of cardiomyopathy. A higher prevalence of QT prolongation has been reported among hospitalized HIV-positive patients with HIV infection, possibly related to drugs prescribed for such patients or to an acquired form of long QT syndrome arising from HIV infection. We report a case of QT prolongation and development of ventricular arrhythmia in one HIV patient that started with intravenous clarithromycin and cotrimoxazole therapy.

Blockade of human cardiac potassium channel human ether-a-go-go-related gene (HERG) by macrolide antibiotics

Several macrolides have been reported to cause QT prolongation and ventricular arrhythmias such as torsades de pointes. To clarify the underlying ionic mechanisms, we examined the effects of six macrolides on the human ether-a-go-go-related gene (HERG)-encoded potassium current stably expressed in human embryonic kidney-293 cells. All six drugs showed a concentration-dependent inhibition of the current with the following IC(50) values: clarithromycin, 32.9 microM; roxithromycin, 36.5 microM; erythromycin, 72.2 microM; josamycin, 102.4 microM; erythromycylamine, 273.9 microM; and oleandomycin, 339.6 microM. A metabolite of erythromycin, des-methyl erythromycin, was also found to inhibit HERG current with an IC(50) of 147.1 microM. These findings imply that the blockade of HERG may be a common feature of macrolides and may contribute to the QT prolongation observed clinically with some of these compounds. Mechanistic studies showed that inhibition of HERG current by clarithromycin did not require activation of the channel and was both voltage- and time-dependent. The blocking time course could be described by a first-order reaction between the drug and the channel. Both binding and unbinding processes appeared to speed up as the membrane was more depolarized, indicating that the drug-channel interaction may be affected by electrostatic responses.

Efavirenz-associated QT prolongation and Torsade de Pointes arrhythmia

OBJECTIVE

To report a case of acquired long QT syndrome that, after exclusion of all other possible causes, was probably related to therapy with efavirenz, a novel nonnucleoside reverse transcriptase inhibitor.

CASE SUMMARY

This patient presented with recurrent syncope and polymorphic ventricular tachycardia, which was treated with overdrive ventricular pacing and was eliminated by discontinuation of the offending drug.

DISCUSSION

This is the first reported case of QT prolongation and severe ventricular arrhythmia associated with the use of efavirenz. The temporal relationship between the initiation of treatment and the onset of electrocardiographic abnormalities, the absence of other apparent precipitating factors, as well as the normalization of QT interval and the resolution of the arrhythmia after discontinuation of the drug, strongly suggest a causal relationship between efavirenz and this adverse clinical event.

CONCLUSIONS

Our case shows that any new pharmaceutical compound introduced in clinical practice may potentially result in QT prolongation and life-threatening arrhythmia.

Comparative safety of the different macrolides

The macrolides are generally well tolerated when used for the treatment of acute infections. Even when given long term for prophylaxis, there are few discontinuations due to side-effects. There are isolated reports of QT(c) prolongation in patients treated with erythromycin and other 14-membered-ring macrolides. Since the 14-membered-ring macrolides are metabolized by P450 isoenzymes, there is the potential for interaction with other therapeutic agents also metabolized in this way. Pharmacokinetic studies have demonstrated interactions between either erythromycin or clarithromycin and cyclosporin, cisapride, pimozide, disopyramide, astemizole, carbamazepine, midazolam, digoxin, hydroxymethylglutaryl-coenzyme A reductase inhibitors (i.e. 'statins') and warfarin. In patients receiving such concurrent therapy, azithromycin may be superior to erythromycin and clarithromycin.

Drugs and cardiotoxicity in HIV and AIDS

The advent of potent antiretroviral drugs in recent years has had an impressive impact on mortality and disease progression in HIV-infected patients, so that issues related to long-term effects of drugs are of growing importance. Hyperlipidemia, hyperglycemia, and lipodystrophy are increasingly described adverse effects of highly active antiretroviral therapy (HAART), in particular when protease inhibitors are used. Hyperlipidemia is strikingly associated with the use of most available protease inhibitors, with an estimated prevalence of up to 50%. Because of the short observation period and the small number of cardiovascular events, epidemiological evidence for an increased risk of coronary heart disease in HIV-infected patients treated with HAART is not adequate at present; however, it is likely that shortly more data will accumulate to quantify this risk. Before starting HAART and during treatment it is reasonable to evaluate all patients for traditional coronary risk factors, including lipid profile. Among the drugs that are currently used in HIV+ patients, antibacterials, antifungals, psychotropic drugs and anti-histamines have been associated with QT prolongation or torsade de pointe, a life-threatening ventricular arrhythmia. Among the risk factors that may precipitate an asymptomatic electrocardiographic abnormality into a dangerous arrhythmia is the concomitant use of drugs that share the CYP3A metabolic pathway. Since most protease inhibitors are potent inhibitors of CYP3A, clinicians should be aware of this potentially dangerous effect of HAART. Anthracyclines are potent cytotoxic antibiotics that have been widely used for the treatment of HIV-related neoplasms. Their cardiotoxicity is well known, ranging from benign and reversible arrhythmias to progressive severe cardiomyopathy. The increased survival and quality of life of HIV+ patients emphasize the importance of a high awareness of adverse drug-related cardiac effects.

Acute canine model for drug-induced Torsades de Pointes in drug safety evaluation-influences of anesthesia and validation with quinidine and astemizole

An acute in vivo model for drug-induced torsades de pointes (TdP) for use in safety evaluation of drugs was developed using dogs with acute complete atrioventricular (AV) block. In order to study the effects of anesthetic agents on the inducibility of TdP, arrhythmias were induced by programmed electrical stimulation (PES) before and after cumulative intravenous administration of quinidine under anesthesia with sodium pentobarbital, halothane, or isoflurane. Both prolongation of the QTc and the incidence of TdP were greatest in dogs anesthetized with halothane and were smallest in those given pentobarbital, suggesting that halothane is the most suitable anesthetic for this TdP model. To further validate this model, astemizole was administered intravenously to other dogs under halothane anesthesia. Astemizole at 0.3 mg/kg caused slight prolongation of the QT interval but did not induce any arrhythmias. At 1 mg/kg, however, TdP were induced in 5 of 10 animals and in an additional 2 animals at 3 mg/kg. Single and multiple ectopic beats preceded the induction of TdP, and the ectopic beats were observed in a dose-dependent manner. The plasma concentrations of quinidine in dogs with TdP were equivalent to or less than quinidine levels in humans with TdP, while those of astemizole were higher in dogs. In conclusion, this acute canine model of TdP with halothane anesthesia, complete AV block, PES, and simultaneous measurements of plasma drug concentration would be valuable for assessing the risk of drugs, especially I(Kr) blockers, to induce TdP in humans.

Risk assessment for antimicrobial agent-induced QTc interval prolongation and torsades de pointes

Over the past several years a multitude of new pharmaceutical agents have been released to the market. Several of them were withdrawn altogether or their use severely restricted to certain indications due to unexpected adverse events, including fatalities. Progress in developing new compounds clearly has surpassed our technology, in some cases, to measure and predict certain toxicities. Prolongation of the QT interval, which may lead to potentially life-threatening ventricular arrhythmias such as torsades de pointes, is one example. Regulatory agencies such as the Food and Drug Administration are increasing standards by which drugs are evaluated for cardiac toxicity related to QT interval prolongation. It is imperative that clinicians be knowledgeable of the risk factors for QT prolongation and avoid the use of culpable agents in patients at risk for QT prolongation.

Comparative pharmacodynamic analysis of Q-T interval prolongation induced by the macrolides clarithromycin, roxithromycin, and azithromycin in rats

In order to evaluate the arrhythmogenic potency of macrolide antibiotics in a quantitative manner, we analyzed the influence of clarithromycin (CAM), roxithromycin (RXM), and azithromycin (AZM) on Q-T intervals from pharmacokinetic and pharmacodynamic points of view and in comparison with the potency of erythromycin (EM) previously reported by us for rats. Male Sprague-Dawley rats were anesthetized, and CAM (6.6, 21.6, and 43.2 mg/kg of body weight/h), RXM (20 and 40 mg/kg/h), and AZM (40 and 100 mg/kg/h) were intravenously injected for 90 min to obtain the time courses of drug concentrations in plasma and the changes in the Q-T intervals during and after the drug injections. Distinct Q-T interval prolongation of up to 10 ms was observed with CAM at its clinical concentrations. RXM and AZM evoked Q-T interval prolongation at concentrations higher than their clinical ranges. The potencies for Q-T interval prolongation, assessed as the slope of the concentration-response relationship, were 6.09, 0.536, and 0.989 ms. ml/microg for CAM, RXM, and AZM, respectively. There was hysteresis between the change in the Q-T intervals and the time course of the plasma concentration of each drug. The rank order of clinical arrhythmogenicity was estimated to be EM > CAM > RXM > AZM, as assessed from the present results and our previous report for EM. In conclusion, RXM and AZM were estimated to be less potent at provoking arrhythmia than EM and CAM. These results should be useful for making a safer choice of an appropriate agent for patients with electrocardiographic risk factors.