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Late Sodium Current Block for Drug-InducedLong QT Syndrome: Results From a Prospective
Clinical Trial
L Johannesen1,2, J Vicente1,3,4, JW Mason5,6, C Erato5, C Sanabria5, K Waite-Labott5, M Hong7,
J Lin7, P Guo7, A Mutlib7, J Wang7, WJ Crumb8, K Blinova1, D Chan1, J Stohlman1, J Florian3,
M Ugander1,2, N Stockbridge3 and DG Strauss1,2
Drug-induced long QT syndrome has resulted in many drugs being withdrawn from the market. At the same time, thecurrent regulatory paradigm for screening new drugs causing long QT syndrome is preventing drugs from reaching themarket, sometimes inappropriately. In this study, we report the results of a first-of-a-kind clinical trial studying late sodium(mexiletine and lidocaine) and calcium (diltiazem) current blocking drugs to counteract the effects of hERG potassiumchannel blocking drugs (dofetilide and moxifloxacin). We demonstrate that both mexiletine and lidocaine substantiallyreduce heart-rate corrected QT (QTc) prolongation from dofetilide by 20 ms. Furthermore, all QTc shortening occurs in theheart-rate corrected J-Tpeak (J-Tpeakc) interval, the biomarker we identified as a sign of late sodium current block. This clinical
trial demonstrates that late sodium blocking drugs can substantially reduce QTc prolongation from hERG potassium channel block and assessment of J-Tpeakc may add value beyond only assessing QTc.
Study Highlights• WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC? þ hERG potassium channel blocking drugs that also
block the late sodium current (e.g., ranolazine) have a low risk of torsade de pointes. A prior clinical trial demonstrated thatranolazine prolongs QTc and Tpeak-Tend intervals, without J-Tpeakc interval prolongation.  WHAT QUESTION DOES
THIS STUDY ADDRESS? þ This study tested the hypothesis that two late sodium current blocking drugs (mexiletine andlidocaine) can shorten drug-induced QTc prolongation from dofetilide by shortening the J-Tpeakc interval.  WHAT THISSTUDY ADDS TO OUR KNOWLEDGE þ This prospective clinical trial demonstrated that late sodium current block
can substantially shorten drug-induced QTc prolongation. Furthermore, this study supports that the J-Tpeakc interval is a biomarker of the balance between hERG potassium channel block (prolongs J-Tpeakc) and late sodium current block (shortens
J-Tpeakc).  HOW THIS MIGHT CHANGE CLINICAL PHARMACOLOGY AND THERAPEUTICS þ Coadministration of late sodium current blocker can mitigate dofetilide-induced QTc prolongation and deserves further study in patients.
In addition, the absence of J-Tpeakc prolongation may be a sign of “benign” QTc prolongation.

Drug-induced QT prolongation increases the risk for torsade depointes, a potentially fatal ventricular arrhythmia.1 QT prolongation and increased risk for torsade de pointes have resulted in 14
drugs being removed from the market worldwide.2 Furthermore,
many drugs remain on the market with a known torsade depointes risk, including numerous antibiotics, antimalarial, antiviral, psychiatric, oncology, and cardiac drugs.3 At the same time,
the current regulatory paradigm for assessing drug effects on cardiac repolarization is preventing potentially effective medicinesfrom reaching the market, sometimes inappropriately.2 To
address this, the US Food and Drug Administration (FDA) andmultiple public-private partnerships are studying novel approaches

to assess the cardiac safety of new drugs with a Comprehensive in
vitro Proarrhythmia Assay and in Phase 1 clinical trials.4,5 Essential to the novel approaches is a focus on understanding mechanisms by studying the effects of drugs on multiple cardiac ionchannels, which can be either proarrhythmic or antiarrhythmic
depending on the combination.6
Almost all drugs on the market that can cause torsade depointes block the hERG potassium channel7 and prolong the
QT interval of the electrocardiogram (ECG).8 However, somedrugs block the hERG potassium channel and prolong QT witha minimal torsade de pointes risk (e.g., ranolazine,9 amiodarone10), likely because of additional block of inward currents,

1
Center for Devices and Radiological Health, US Food and Drug Administration, Silver Spring, Maryland, USA; 2Department of Clinical Physiology, Karolinska
Institutet and Karolinska University Hospital, Stockholm, Sweden; 3Center for Drug Evaluation and Research, US Food and Drug Administration, Silver Spring,n Institute for Engineering Research (I3A), IIS Arago
n, University of Zaragoza, Zaragoza, Spain; 5Spaulding Clinical, West
Maryland, USA; 4BSICoS Group, Arago
Bend, Wisconsin, USA; 6University of Utah, Salt Lake City, Utah, USA; 7Frontage Laboratories, Exton, Pennsylvania, USA; 8Zenas Technologies, Metairie,Louisiana, USA. Correspondence: DG Strauss (David.Strauss@fda.hhs.gov)

Received 14 May 2015; accepted 5 August 2015; advance online publication 11 August 2015. doi:10.1002/cpt.205
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Figure 1 Late sodium current (shaded) correlates with the plateau of theaction potential and early part of repolarization on the ECG, from J-point topeak of the T-wave.

such as the late sodium current or the L-type calcium current.11
Preclinical studies have suggested that late sodium or calcium current block can shorten hERG potassium channel block-induced

action potential and QT prolongation and prevent torsade depointes.12–15 However, species differences in cardiac ion channel
expression exist and these preclinical observations have not yet
been translated to drug-induced long QT syndrome in humans.16
In a prior retrospective analysis of 34 clinical trials and a prospective clinical trial of 4 individual drugs,17,18 we demonstrated
that hERG potassium channel block prolongs both ECG early
repolarization (J-Tpeak, or corrected J-Tpeak (J-Tpeakc) when corrected for heart rate) and late repolarization (Tpeak-Tend),
whereas additional late sodium or calcium current block shortens
early repolarization (J-Tpeakc; Figure 1). The prior studies werelimited by the assessment of individual drugs in which the effectsof combinations of drug-ion channel effects were inferred. Thus,
we designed a ﬁrst-of-a-kind Phase 1 clinical trial combined witha comprehensive preclinical assessment to assess the effects ofdrug combinations to dissect out the effects of single vs. multiple
cardiac ion channel block.
The primary objective was to test the hypothesis that latesodium current blocking drugs (mexiletine or lidocaine) can
attenuate the effect of hERG potassium current blocking drugs(dofetilide) on ventricular repolarization (QT or QTc when corrected for heart rate) by shortening J-Tpeakc. The secondary
objective was to assess the ability of a selective calcium current
blocker (diltiazem) to reduce QTc prolongation associated withhERG potassium current block (moxiﬂoxacin). In order tounderstand the mechanisms of our ﬁndings, we performed ionchannel patch clamp experiments using overexpression cell lines
and proﬁling of drug metabolites.
RESULTSClinical trial design

This Phase 1 clinical trial was designed as a pharmacokineticpharmacodynamic investigation of mexiletine combined with
dofetilide, lidocaine combined with dofetilide, and diltiazem

Figure 2 Morning, afternoon, and evening doses for each of the five treatment periods. Below the table, an illustration of the plasma drug level is shownto indicate when oral and intravenous dosing took place as well as when ECGs and plasma samples were taken (in hours after first oral dose). Dof, dofetilide; mex, mexiletine; lido, lidocaine; mox, moxifloxacin; dil, diltiazem.
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Figure 3 Changes shown in QTc for dofetilide alone (a), mexiletine combined with dofetilide (b), and lidocaine combined with dofetilide (c), while thelower part of the panels show the concentration of dofetilide (green) and mexiletine or lidocaine (blue) in their respective panels. The error bars represent
95% confidence intervals (CI). (d) Example ECGs from a subject during the evening dose of placebo, dofetilide, mexiletine combined with dofetilide, andlidocaine combined with dofetilide. The remaining bottom panels (e and f) show relative change compared to dofetilide alone for mexiletine combinedwith dofetilide (e) and lidocaine combined with dofetilide (f), where only the timepoint at the end of the loading dose is shown and points are shiftedslightly for each interval to avoid overlap. Dof, dofetilide; mex, mexiletine; lido, lidocaine. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]combined with moxiﬂoxacin. The design was a ﬁve-period,
randomized, crossover study with one week between treatment
periods. In each treatment period, the subjects were dosed threetimes during the day (Figure 2) to allow for evaluation of theeffects of low dose late sodium current block by itself, and ofincreasing levels of late sodium combined with hERG potassiumchannel block (see Figure 2 for dosing details). We were unable
to combine diltiazem with dofetilide because of a pharmacokinetic interaction. Thus, we administered high-dose moxiﬂoxacin(hERG potassium channel blocker) in the morning and afternoon doses and moxiﬂoxacin combined with diltiazem in theevening dose.
The study included 22 healthy subjects (9 women) and theirbaseline characteristics are described in Table 1. The average age
was (mean 6 SD) 26.1 6 4.9 years. The average weight was 69.9
6 9.0 kg. Twenty subjects started the placebo treatment day, 20
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the dofetilide alone day, 21 the mexiletine and dofetilide day, 19
the lidocaine and dofetilide day, and 20 the moxiﬂoxacin and diltiazem day. No serious adverse events were observed, and theadverse events occurring at a frequency of >10% (dizziness, nausea, vomiting) are shown in Supplementary Table S1 by treatment day. There were three dropouts because of adverse events(Supplementary Figure S1).
Prolongation of QTc by dofetilide

Figure 3 shows the placebo and baseline corrected changes in the
QTc interval along with plasma drug concentrations, whereas
Figure 4 shows the concentration-dependent response. Dofetilide alone (Figure 3a) prolonged the QTc interval relative to placebo by 22.4 ms (P < 0.001) after the administration of theafternoon dose. After the evening dose of dofetilide, the QTcinterval was prolonged by 39.1 ms (P < 0.001) and the plasma
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Figure 4 (a) The plasma drug concentration-dependent analysis for QTc, J-Tpeakc, and Tpeak-Tend, from left to right for dofetilide combined with mexiletine(red) or lidocaine (blue), and (b) for moxifloxacin and diltiazem. The solid line and shaded area reflects the fit of a linear model between drug concentration(dofetilide in panel a and moxifloxacin in panel b) and change in placebo and baseline-corrected change in each ECG interval. The points with error bars
(95% confidence intervals (CIs)) reflect the maximum change when mexiletine is combined with dofetilide (red) or lidocaine is combined with dofetilide(blue) for panel a and moxifloxacin combined with diltiazem (red) in panel b. Dof, dofetilide; mex, mexiletine; lido, lidocaine; mox, moxifloxacin; dil, diltiazem. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]concentration was increased. Dofetilide prolonged both J-Tpeakc
and Tpeak-Tend (P < 0.001 for all; Figure 4a). Based on our ion
channel patch clamp experiments (Figure 5a), the dofetilide
plasma concentration after the evening dose was associated with
45% hERG potassium channel block.
Shortening of the QTc interval by mexiletine and lidocaine

The ﬁrst dose of mexiletine alone shortened QTc by 10.6 ms(P < 0.001) relative to placebo (Figure 3b), whereas the ﬁrst
dose of lidocaine alone shortened QTc by 7.6 ms (P 5 0.005;
Figure 3c). Both mexiletine and lidocaine alone shortened the JTpeakc interval, but not Tpeak-Tend (Figures 3e and 3f).

The afternoon dose was either the combination of mexiletineand dofetilide, or lidocaine and dofetilide. When compared to thedofetilide alone day in the afternoon, both mexiletine (29.8 ms;
P < 0.001; Figure 3e) and lidocaine (212.5 ms; P < 0.001;
Figure 3f) shortened QTc when coadministered with dofetilide.
This QTc shortening increased further after the evening dose,
which was also associated with higher concentrations of mexiletineand lidocaine (Figure 3e and 3f). When compared to the dofetilide alone day in the evening, both mexiletine (219.8 ms; 95%
conﬁdence interval: 225.2 to 214.3; P < 0.001) and lidocaine(219.7 (225.2 to 214.1) ms; P < 0.001) signiﬁcantly shortened
QTc after the evening dose (Figures 3e and 3f), which was the

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a: Dofetilide

b: Mexiletine
100

hERG

75

relative block (%)

relative block (%)

100

50

25

0

75

Late sodium

50
Calcium

25

0
0.1

10.0

1

Dofetilide (nM)c: Lidocaine

d: Moxifloxacin and M2

100

100

75

Late sodium

relative block (%)

relative block (%)

100

Mexiletine (µM)

50

25

0

IKs

75

hERG

50

M2 hERG
Late sodium

25

0
1

100

10

Lidocaine (µM)

1000

Moxifloxacin (µM)

e: Diltiazem
100

Calcium

relative block (%)

hERG

75

50

Late sodium

25

0
100

10000

Diltiazem (nM)hERG

Late sodium

Calcium

IKs

Figure 5 The results of the patch clamp experiments for dofetilide (a), mexiletine (b), lidocaine (c), moxifloxacin and M2 metabolite (d), and diltiazem (e)for hERG (red), late sodium (green), calcium (blue), and IKs (purple). The lines in each plot correspond to a fit between the measured relative reduction incurrent and drug concentration. The error bars denote 6 SE. The vertical dashed lines in each panel correspond to the range of observed clinical plasma
concentrations, corrected for protein binding, and the solid line is the population average maximum plasma concentration in the clinical trial. [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]prespeciﬁed primary endpoint of the study. An ECG example ofthe effects of mexiletine and lidocaine on drug-induced QTc prolongation is shown in Figure 3d. Our ion channel patch clamp
experiments (Figures 5b and 5c) suggest that both mexiletine andlidocaine have 20% late sodium current block at the concentrations that caused 20 ms shortening of QTc.
For both mexiletine and lidocaine, the shortening of the QTcinterval was entirely of the J-Tpeakc interval (Figure 3e and 3f).
Speciﬁcally, after the evening dose of mexiletine and dofetilide,
the J-Tpeakc interval was shortened (223.2 ms; P < 0.001),
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whereas no change in the Tpeak-Tend interval was observed (P 5
0.56; Figure 3e). Similarly, lidocaine shortened the J-Tpeakc interval (220.5 ms; P < 0.001) without changing the Tpeak-Tendinterval (P 5 0.13; Figure 3f). Of note, the measured dofetilideconcentrations were higher in the mexiletine-dofetilide arm
compared to dofetilide alone (1.8 6 0.3 vs. 1.5 6 0.3 ng/mL;
P < 0.001), whereas the dofetilide concentrations in thelidocaine-dofetilide arm trended toward greater dofetilide concentrations (1.8 6 0.4 vs. 1.5 6 0.3 ng/mL; P 5 0.068). The
ﬁndings of the time-dependent analysis are consistent with the
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concentration-dependent analysis shown in Figure 4, where eachpanel shows the effect of increasing dofetilide concentration on
QTc, J-Tpeakc, and Tpeak-Tend alone, compared to the changes
when dofetilide was coadministered with mexiletine or lidocaine.
This is similar to the exposure-response relationship for ranolazine, a drug that blocks the hERG potassium channel and thelate sodium current. In our prior study, ranolazine caused
concentration-dependent prolongation of QTc and Tpeak-Tend,but not J-Tpeakc.17 Thus, prolongation of QTc and Tpeak-Tend
without prolongation of J-Tpeakc suggests the presence of multichannel block, where the J-Tpeakc interval is a balance of inward
and outward current.
Effects of dofetilide, mexiletine, and lidocaine on heart rate
and other ECG intervals are shown in Supplementary
Figure S2. There were no changes in PR, QRS, or heart rate afteradministration of dofetilide alone (Supplementary Figure S2).
Mexiletine and lidocaine were associated with small increases inheart rate (4.4 bpm; P 5 0.002 and 3.1 bpm; P 5 0.033, respectively). Notably, there was no effect of either drug on QRS duration, suggesting a lack of signiﬁcant peak sodium current effects
at resting heart rates.
Combined effects of moxifloxacin and diltiazem

The morning dose of moxiﬂoxacin prolonged the QTc interval
relative to placebo (20.2 ms; P < 0.001; Supplementary
Figure S3). The afternoon dose further prolonged QTc relative
to placebo (29.9 ms; P < 0.001) and was associated with a higher
moxiﬂoxacin plasma concentration, which exhibits 20% hERG
potassium channel block (Figure 5d). However, after the evening
dose, QTc remained prolonged (31.3 ms; P < 0.001) despite aslightly lower plasma concentration of moxiﬂoxacin and coadministration of diltiazem (Supplementary Figure S3), which was
associated with 35% calcium channel block (Figure 5e). Comparing the evening to the afternoon dose, QTc and Tpeak-Tend
were prolonged, whereas J-Tpeakc was shortened (Supplementary
Figure S3). To account for the higher moxiﬂoxacin plasma concentration in the afternoon compared to the evening (9.5 6
1.7 lg/mL vs. 8.0 6 1.7 lg/mL; P 5 0.008), we conducted aconcentration-dependent analysis (Figure 4b). However, this analysis did not support diltiazem shortening moxiﬂoxacin-induced QTc
prolongation. Of note, diltiazem was associated with a 20 ms PR
prolongation (Supplementary Figure S2), suggesting that theplasma concentration of diltiazem was associated with sufﬁcient calcium channel block to cause slowed conduction through the atrioventricular node. Interestingly, the results of our patch clamp
analysis also suggest that moxiﬂoxacin blocks the KvLQT1/minK
potassium channel (IKs current; Figure 5d), the effect of which
could be enhanced by the autonomic response resulting from theblood pressure drop associated with diltiazem administration.19,20 It
is possible that this contributed to the lack of QTc shortening when
diltiazem was added to moxiﬂoxacin.
hERG potassium channel blocking effects of moxifloxacin
metabolite

An alternative for the unexpected results from moxiﬂoxacindiltiazem could be due to the accumulation of a moxiﬂoxacin

metabolite that exhibits hERG potassium channel block. The
moxiﬂoxacin glucuronide metabolite (M2) has been reported toreach plasma concentrations 40% of moxiﬂoxacin and is only 5%
protein bound, as opposed to 39% protein binding of moxiﬂoxacin.21 We assessed the relative potency of the hERG potassiumchannel block of this metabolite and observed that the metabolite
blocks hERG to a degree similar to that of the parent drug (Figure 5d). We proﬁled metabolites from ﬁve subjects and determined that the relative percentage of the M2 metabolite tended
to increase during subsequent dosing (median: 14.1% vs. 20.9%;
n 5 5; P 5 0.06). This suggests that the repeat intravenous moxiﬂoxacin administration throughout the day led to an accumulation of an hERG potassium channel blocking moxiﬂoxacin
metabolite, thus confounding the diltiazem analysis.
DISCUSSION

This study highlights how a comprehensive assessment of drug
effects on multiple cardiac ion channels through translational
mechanistic-based approaches has the potential to improve thecardiac safety assessment of new drugs. In addition, this study
provides an assessment in healthy volunteers of a therapy for
drug-induced long QT syndrome and demonstrates that block of
the late sodium current by mexiletine and lidocaine can have asubstantial effect on cardiac repolarization in humans. Furthermore, this study provides substantial evidence that the J-Tpeakc
interval is a biomarker of the balance between hERG potassiumchannel block (prolongs J-Tpeakc) and late sodium current block(shortens J-Tpeakc). The combinations of dofetilide with mexiletine and dofetilide with lidocaine reproduced the ECG signatureof ranolazine: QTc and Tpeak-Tend prolongation with no net
effect on the J-Tpeakc interval. Thus, we hypothesize that this is asign of “benign” QTc prolongation because of a balance of hERG
and late sodium current block.
Interest in the late sodium current increased in the 1990s
when it was discovered that patients with congenital long QTtype 3 (LQT3) had prolonged QTc because of increased latesodium current.22 Two of the drugs initially tested were theVaughan-Williams class Ib antiarrhythmics lidocaine and mexiletine.23,24 Although mexiletine and lidocaine block the peaksodium current at very high concentrations, at clinical concentrations, they preferentially block the late sodium current.25,26 Thisis consistent with the observations in this study where they produced 20 ms of QTc shortening (evidence for late sodium current block) while having no effect on QRS duration (evidence
for absence of signiﬁcant peak sodium current block).
In addition to showing antiarrhythmic efﬁcacy in preclinicalmodels of congenital LQT3,15,22 mexiletine was also shown tohave antiarrhythmic efﬁcacy in models of LQT2 (decreasedhERG potassium current),27 which is most relevant to druginduced long QT syndrome. A clinical study of mexiletine founda statistically signiﬁcant decrease in QTc in patients with LQT3
but not in patients with LQT2; however, only six patients with
LQT2 were studied.24 The present study demonstrates that both
mexiletine and lidocaine can shorten QTc prolongation becauseof dofetilide, which is widely accepted to be a selective hERG
potassium channel blocker.

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A recent preclinical study found that chronic exposure to dofetilide for ﬁve or more hours may increase late sodium current.28
In the present study, the primary ECG analysis was performed
eight hours after the ﬁrst dofetilide dose. It is therefore possiblethat part of the QTc shortening by mexiletine and lidocaine wasfrom blocking an increase in the late sodium current induced bydofetilide. However, even before dofetilide administration, latesodium current block alone caused QTc shortening.
Although this is the ﬁrst known clinical trial combining a latesodium current blocker with a selective hERG potassium channel
blocker, small patient studies were performed in the 1980s combining mexiletine (Vaughan-Williams class Ib antiarrhythmic)
with quinidine (Vaughan-Williams class Ia antiarrhythmic).27,29,30 Quinidine is a strong hERG potassium channel
blocker, and with chronic therapy it was observed that mexiletine
shortened the quinidine-induced QTc prolongation.27 In addition, an in vivo study of chronic atrioventricular-blocked dogs
demonstrated that mexiletine reduced sotalol-induced QTc prolongation and the number of torsade de pointes events.13 Speciﬁcally, torsade de pointes occurred in six of eight dogs receiving
sotalol alone, but in only one of eight dogs receiving the combination of mexiletine and sotalol. Other preclinical studies havedemonstrated that late sodium current block can prevent dispersion of repolarization (a substrate for torsade de pointes) and prevent early after depolarizations (a trigger for torsade de pointes)
that is caused by drug-induced hERG potassium channel
block.10,15 Although these data suggest that Vaughan-Williams
class Ib antiarrhythmics, such as mexiletine or lidocaine, may
reduce the risk for torsade de pointes associated with hERG
potassium channel block, it should be noted that some preclinicalstudies suggest that lidocaine may increase conduction delay andheterogeneity in ischemic tissue.31–33
Despite no known interaction between the pharmacokineticsof dofetilide and mexiletine, an increase in the observed maximum concentration of dofetilide after coadministration with
mexiletine was observed. It is not known if this is a chance ﬁnding, as the study was not designed to evaluate this, but it highlights the importance of evaluating pharmacokinetic interactions
when using a late sodium current blocker like mexiletine to counter drug-induced QTc prolongation.
We did not observe a reduction in QTc prolongation when
diltiazem was administered with moxiﬂoxacin. The addition ofdiltiazem was associated with a small shortening of J-Tpeakc; however, there was also a small prolongation of Tpeak-Tend and QTc.
This ﬁnding was, in part, confounded by a decrease in the moxiﬂoxacin concentration in the evening, but the concentrationdependent analysis showed a similar pattern. The lack of shortening of the J-Tpeakc interval could potentially be due to the hERG
potassium channel blocking moxiﬂoxacin metabolite M2 accumulating during the day and reaching higher plasma levels when
diltiazem was coadministered. Another possible explanation forthe lack of QTc shortening with diltiazem is moxiﬂoxacininduced block of the IKs current. Although the effect of the IKs
current is minimal in the absence of autonomic stimulation,20 an
autonomic response triggered by diltiazem reducing the bloodpressure could increase the effect of IKs current block.19
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Recently, there have been two efforts aimed at improving
assessment of drug-induced proarrhythmic risk. A preclinicaleffort, the Comprehensive in vitro Proarrhythmia Assay, combines assessment of multiple cardiac ion channel currents using
patch clamp studies integrated into in silico models and conﬁrmed with experiments in induced pluripotent stem cell derived
cardiomyocytes.4 The other effort focuses on assessment of QTc
prolongation using pharmacokinetic-pharmacodynamic modelingin Phase I clinical studies.5 The ﬁndings of this clinical study,
and our previous study,17,18 show that electrocardiographic biomarkers J-Tpeakc and Tpeak-Tend could be used to conﬁrm multichannel effects, for example, prolongation of the QTc intervalwith minimal or no prolongation of J-Tpeakc is likely an indicatorof the presence of a multichannel block.
Limitations

Reducing drug-induced QTc prolongation does not necessarily
equate to preventing drug-induced torsade de pointes. However,the combination of dofetilide and mexiletine or lidocaine
recreates the ECG signature we observed in our prior clinical trial
with ranolazine,17 a QTc prolonging drug with a low torsade depointes risk. Future studies will be required to assess whether
coadministration of a late sodium blocking drug with an hERG
potassium channel blocker can reduce arrhythmic events, such as
torsade de pointes without affecting therapeutic efﬁcacy. The
similar pharmacokinetic proﬁles among oral mexiletine, dofetilide, and sotalol suggest that this could be tested. In addition,
new late sodium current blockers are in development.14
CONCLUSION

This study demonstrates that late sodium current block can
reduce QTc prolongation associated with hERG potassium channel block. These ﬁndings suggest that coadministration of a latesodium current blocker can reduce drug-induced QTc prolongation in patients. Whether the reduction of drug-induced QTc
prolongation in patients results in a lowered risk for torsade depointes remains to be studied, but the minimal-to-no risk for torsade de pointes with ranolazine suggests that it might. This study
also demonstrates that shortening of the J-Tpeakc interval is an
ECG sign of late sodium current block. J-Tpeakc is the only biomarker that represents a balance of inward and outward ionchannel currents.34 Additional biomarkers beyond QTc (e.g.,
J-Tpeakc) could be applied in Phase 1 clinical trials using
concentration-response modeling in lieu of dedicated thoroughQT studies.
METHODSThis study was approved by the FDA Research Involving Human Subjects Committee and the local institutional review board. All subjects
gave written informed consent and the study was performed at a Phase 1
clinic (Spaulding Clinical, West Bend, WI).
Clinical trial design
The design was a prospective randomized crossover study with one week
between treatment periods. It included typical inclusion and exclusion
criteria for dedicated QT studies.35 Healthy subjects between 18 and 35
years of age, weighing between 50 and 85 kg, and without any family history of cardiovascular disease or unexplained sudden cardiac death were
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eligible for participation in the study. In addition, the subjects had tohave <12 ventricular ectopic beats during a three hour continuousrecording at screening, as well as a baseline QTc of <430 ms, using Fridericia correction.36
In each of the ﬁve treatment periods, the subjects were dosed threetimes during the day (Figure 2). Dofetilide (Tikosyn, Pﬁzer, New York,NY) and mexiletine (Teva Pharmaceuticals USA, North Wales, PA)
were administered orally immediately after meals, whereas lidocaine (B.
Braun Medical, Bethlehem, PA) was administered intravenously (seeFigure 2 for dosing details). The doses of mexiletine and lidocaine werechosen to target plasma levels from prior studies.23,27 Similarly, the dofetilide dose was chosen based on prior experience.17 Moxiﬂoxacin doses
were chosen to achieve QTc prolongation of at least 20 ms, while maintaining a total daily dose within what has been studied previously. Last,
the diltiazem dose was chosen to target the highest dose in the FDA
label.
Intravenous infusions were split into a 60-minute loading dose followed by a 30-minute maintenance dose. The primary pharmacokinetic
samples were taken at the start and end of each of the three maintenancedoses during the day, as these were expected to be associated with highest
plasma concentrations of both the oral and intravenously administereddrugs. Additional pharmacokinetic samples were taken at 30 and 120
minutes poststart of intravenous dosing during each of the three dosing
periods during the day.

ECG measurement methodology
Continuous ECG recordings were performed using the Mortara Surveyor system (Mortara, Milwaukee, WI) sampled at 500 Hz with anamplitude resolution of 2.5 lV. From the continuous recording, three
ECGs were extracted before the draw of each pharmacokinetic sample,
based on heart rate stability and signal quality.37
Semi-automated measurements of the QRS, J-Tpeak, Tpeak-Tend, and
QT interval were performed using the derived vector magnitude lead, as
previously described (see Supplementary Methods).17 The QT interval
was corrected for heart rate using Fridericia’s correction (QTc 5 QT/(RR/1,000 ms)1/3, with RR in ms).36 Similarly, the J-Tpeak interval was
corrected for heart rate (J-Tpeakc 5 J-Tpeak/(RR/1000 ms)0.58, with RR
in ms).18 No correction was performed for the Tpeak-Tend interval, asprior studies have shown a lack of rate dependence at resting heart
rates.18,38 Additionally, the global PR interval was measured automatically (AMPS LLC, New York, NY).
Pharmacokinetic sample analysis
Quantiﬁcation of drug concentration in plasma was performed using avalidated liquid chromatography with tandem mass spectroscopy method
by Frontage Laboratories (Exton, PA). In addition, the relative abundance of parent moxiﬂoxacin to the M2 metabolite was determined forﬁve subjects. The pharmacokinetic sample analysis is described in further
detail in the Supplementary Methods.
Patch clamp experiments
Stably transfected hERG, Nav1.5, KvLQT1/minK (HEK-293) or
Cav1.2 cells (CHO) from Cytocentrics Biosciences (Rostock, Germany)
were used for whole-cell patch-clamp measurements to determine reduction in hERG, late sodium, L-type calcium, and IKs currents for all ﬁve
drugs (dofetilide, lidocaine, mexiletine, moxiﬂoxacin, and diltiazem); see
Supplementary Methods for a more detailed description and Supplementary Figure S4 for examples of current traces.39 In addition, theeffect of the moxiﬂoxacin M2 metabolite on hERG was assessed. All currents were elicited using a ventricular action potential waveform (pacing
rate 5 0.1 Hz) at physiologic temperature (36 6 18C).
Statistical methods
The primary endpoint of the clinical trial was the reduction of
dofetilide-induced prolonged QTc by either mexiletine or lidocaine using
a linear mixed-effects model with time, treatment, sequence, and period

Table 1 Demographics
DemographicsaAll subjects (N 5 22)Age, years

26.1 6 4.9

Female9 (41%)

Race
White

10 (45%)

Black or African American

10 (45%)

American Indian/Alaska Native

1 (4.5%)Asian

1 (4.5%)Hispanic or Latino

2 (9%)

Weight, kg

69.9 6 9.0

Vital signs
Systolic blood pressure, mm Hg

109.5 6 5.5

Diastolic blood pressure, mm Hg

60.2 6 3.5Heart rate, bpm

61.3 6 6.7

ECG
PR, ms160.8 6 19.1

QRS, ms

86.7 6 8.5

J-Tpeakc, ms

229.5 6 19.0

Tpeak-Tend, ms

81.9 6 6.4

QTc, ms

397.8 6 14.2

ECG, electrocardiogram; J-Tpeakc, heart-rate corrected J-Tpeak; QTc, heart-ratecorrected QT.
aContinuous variables are listed as mean 6 SD.

as ﬁxed effects, as well as an interaction between treatment and time, anda random intercept per subject. The analysis was performed using SAS
9.3 (SAS Institute, Cary, NC). The prespeciﬁed analysis only included
the two center time points (60 and 90 minutes poststart of infusion) forthe evening dose (highest expected plasma concentration of all drugs).
All data before withdrawals was included in the analysis. For the secondary endpoint of diltiazem combined with moxiﬂoxacin, a similar analysis
was performed, except it involved comparing the evening timepoints(moxiﬂoxacin and diltiazem) compared to the afternoon timepoints(moxiﬂoxacin alone). Adjustment for multiple comparisons was performed according to the Bonferroni method (i.e., a signiﬁcance level of
0.025 for the primary analysis). All other by-time analyses were carriedout using a similar approach, but were not further adjusted for multiplicity, and should be interpreted with an appropriate level of caution.
In addition, a linear mixed-effects model was used to explore the concentration dependency of change in QTc, J-Tpeakc, and Tpeak-Tend with concentration as a ﬁxed effect and subject as a random effect on intercept andconcentration. Differences in maximum concentrations were evaluated using
an independent t test with equal variance. Wilcoxon sign test was used toevaluate the difference between relative abundance of moxiﬂoxacin M2
metabolite relative to the parent between the morning and evening dose.
Ion channel patch clamp results are presented as percentage of reduction of current amplitude, which was measured as current reduction aftera steady-state effect had been reached in the presence of drug relative tocurrent amplitude before drug was introduced (control). Each cell served
as its own control.
Additional Supporting Information may be found in the online version ofthis article.

CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 99 NUMBER 2 | FEBRUARY 2016

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CLINICAL TRIALACKNOWLEDGMENTS
We would like to thank staff at Spaulding Clinical Research for executing
the clinical study (NCT02308748). No official support or endorsement by
the FDA is intended nor should be inferred. The mention of commercial
products, their sources, or their use in connection with material reported
herein is not to be construed as either an actual or implied endorsementof such products by the FDA.

SOURCE OF FUNDING
This project was supported by the FDA Critical Path Initiative, Office ofWomen’s Health, and appointments to the Research Participation
Programs at the Oak Ridge Institute for Science and Education throughan interagency agreement between the Department of Energy and the
FDA.
AUTHOR CONTRIBUTIONS
All authors have contributed as follows: protocol development (L.J.,
J.V., J.W.M., C.S., K.W.L., J.F., N.S., D.G.S.), data collection (C.E.,
C.S., K.W.L., M.H., J.L., P.G., A.M., J.W., W.J.C.), data analysis (L.J.,
J.V., J.F., D.G.S.), and preparing the manuscript (L.J., D.G.S.). All
authors discussed the results and implications and commented on
the manuscript.
CONFLICT OF INTEREST
J.W.M., K.W.L., C.S., and C.E. are employees of Spaulding ClinicalResearch, M.H., P.G., J.L., A.M., and J.W. are employees of Frontage
Laboratories, and W.J.C. is an employee of Zenas Technologies LLC,
which are contract research organizations.

C 2015 American Society for Clinical Pharmacology and Therapeutics
V

Published 2015. This article is a U.S. Government work and is in the publicdomain in the USA.

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