INTRODUCTION
Cardiotoxicity remains a major concern during drug development, with increased proarrhythmic
potential being the main culprit. The pharmaceutical industry is challenged by the
growing cost of research and development and cannot afford drug attrition in late
phases of development or withdrawals of approved drugs. The ICH S7B non-clinical and
E14-based clinical methodologies have been successful in their intent to reduce and
eliminate drug-mediated torsades de pointes (TdP) arrhythmias as there have not been
any withdrawals of marketed drugs for torsadogenic reasons since these guidelines
were adopted (1). However, the current approach is conservative and can result in
false positives. Thus, while effective, the current paradigm may be inappropriately
assigning TdP liability to some drugs, especially in the discovery realm, so work
is ongoing to shift from one that strongly relies on QT interval prolongation (or
is “hERG-centric”) to one where proarrhythmic risk would be primarily assessed using
non-clinical in vitro human models based on solid mechanistic considerations of TdP
proarrhythmia (2) in conjunction with the current in vivo QT conscious canine and
other non-rodent models.
Problems related to this drug-induced cardiotoxicity were discussed at a symposium
entitled “A Gentle Touch on the Beating Heart: Early Discovery Prediction of Cardiotoxicity
and Its Covariates” and conducted during the AAPS 2014 Annual Meeting. The aim of
this mini-symposium was to present methods of predicting cardiotoxicity (which will
be defined throughout this entire manuscript as a synonym of a proarrhythmic effect)
from early discovery data. Three speakers from different backgrounds introduced the
audience to the current safety testing paradigm, the latest achievements within drug
safety and remaining hurdles regarding the integration of multiple sources of data
acquired both at the preclinical and clinical level, and their translation to the
human in vivo situation. Despite a reasonable knowledge of the mechanisms related
to proarrhythmic cardiotoxicity, there are still questions whether the new paradigm
of testing will be able to provide all stakeholders with a sufficient level of confidence
with the application of these novel concepts and models to drug safety.
This article gives a short overview of the discussed problems, summarizes the discussion,
and gives a flavor of a new cardiac safety testing pathway likely to be introduced
in the near future. Some of the statements in the text, like the feasibility of the
individualized risk assessment, can be contradictory what was left intentionally and
shows the line of discussion between speakers. The three leading topics include a
description of the molecular and clinical background of drug-induced proarrhythmic
effects, a brief presentation of current methods used in the early prediction of such
effects together with anticipated changes in the testing paradigm and a discussion
of the in vitro–in vivo extrapolation approach, all of which promise the possibility
of risk assessment at the individual patient level.
Molecular and Clinical Background of the Drug-Triggered Cardiac Arrhythmia
The development of TdP is a potentially life-threatening ventricular arrhythmia that
can occur as an unintended result of drug therapy of various, in many cases relatively
benign, conditions. It’s rare occurrence hinders TdP observation considerably during
the conduct of clinical trials, thus necessitating the use of surrogate markers. For
most medications known to cause torsades, TdP is a result of repolarization delay
or alterations clinically reflected by QT interval prolongation on the surface electrocardiogram
(ECG). Despite being considered an imperfect biomarker with compromised sensitivity
and specificity and lack of a straightforward correlation with TdP occurrence, QT
interval prolongation remains the most globally used biomarker to assess the proarrhythmic
propensity of a drug (3,4). QT interval prolongation is a mechanism-based effect resulting
primarily from the inhibition of rapid delayed rectifier potassium current (I
Kr) mediated by ion channels encoded by the human ether-a-go-go-related gene (hERG
or Kv11.1). It has been shown that in vitro measured I
Kr inhibition potency expressed as an IC50 value correlates with the clinically observed
arrhythmia occurrence (5,6). This fact is reflected in the regulatory requirements,
which require such in vitro study to be conducted before a first-in-human trial (7).
Despite the importance of the hERG channel in safety pharmacology, studies investigating
hERG channel exclusively might not fully reflect the drug potential in modifying cardiac
ionic currents. Since the action potential of ventricular cardiomyocytes is a result
of complex interplay between inward and outward ionic currents, drug interaction with
an inward current can alter the effect of hERG inhibition of outward current and hence
may limit effects on the QT interval and subsequent assessed proarrhythmic potential.
Indeed, there are potent hERG inhibitors devoid of TdP risk (e.g., verapamil, propafenone,
amiodarone, ranolazine) because of either a counteraction to potassium current inhibition
by either sodium or calcium channel blockade or other potential antiarrhythmic mechanisms
(8,9). On the other hand, alfuzosin is an example of a drug with negligible hERG liability
although it causes QTc interval prolongation by increasing sodium current (8–11).
Therefore, one of the proposed changes to be implemented into cardiac safety testing
is an evaluation of the drug effects on multiple cardiac ionic currents. Recently,
Kramer et al. (12) assessed whether determining concomitant block of multiple ion
channels could more accurately predict the torsadogenic potential of a compound rather
than just examining the effects on hERG alone. The application of logistic regression
models using data derived from high-throughput screening multiple ion channel electrophysiology
(MICE) methods showed that for the known 23 non-torsadogenic and 32 torsadogenic drugs
from multiple classes tested that there was a significant reduction in false positives
(i.e., type 1 errors) and false negatives (i.e., type 2 errors) when compared to assessment
using the hERG assay alone.
Currently, there are efforts to develop a Comprehensive In vitro Proarrhythmia Assay
(CIPA) that aims to modify and modernize current non-clinical, “hERG-centric” cardiac
safety screening efforts (2,13,14). Current alternative screening models and methods
under consideration for the CIPA initiative include stem cells in which hERG (Kv11.1;
I
Kr current) as well as other cardiac ion channels such as the fast sodium (Nav1.5;
I
Na current) channel, persistent sodium channel (I
Nasus), calcium (Cav1.2; I
Ca current) channel as well as potassium channels such as the inward rectifier (Kir2.1-2.4;
I
K1 current), and slow delayed rectifying (Kv7.1; I
Ks current) channel can be assessed in totality (15). Rather than examining human
ion channel isoforms heterogeneously expressed in cell lines (such as CHO or HEK)
as is current practice in drug safety, there is ongoing investigation of the applicability
of human-induced pluripotent stem cells (16). The undifferentiated human stem cell
of embryonic origin (hESC) and induced pluripotent stem cell (iPSCs) of somatic origin
continue to be evaluated for their cardiac electrophysiological potential for use
as a drug screening assay (16–19). Intracellular recordings from individual cells
and multi-electrode arrays (MEAs) enable measurement of sodium, calcium, and potassium
current from stem cells (16). Note, however, that a limitation to the use of stem
cells that are currently being evaluated is that these cells are immature with regard
to their electrophysiological properties. Stem cells, to date, do not appear to fully
express all channels at the same density and at the same proportion as occurs in human
ventricular myocytes (20).
Multi-channel interactions, hERG-independent proarrhythmia mechanisms, active metabolites,
or antiarrhythmic effects of a drug are not the only reasons for a lack of absolute
concordance of risk estimates based on surrogate markers with actual TdP risk in a
patient population. Apart from pharmacodynamic effects, TdP occurrence is influenced
by many factors that define inter- and intra-individual variability (21–23). It is
known that TdP occurrence heavily depends on concomitant risk factors including age,
gender, electrolyte imbalance, heart rate (specifically bradycardia), and presence
of structural heart disease. Also, the actual risk for an individual patient is variable
with individual circadian rhythms (i.e., heart rate, serum electrolyte levels), and
rhythms with a longer periodicity (related to the effects of sex hormones) can influence
the individual response of the heart to drugs (24,25). Mechanisms responsible for
the development of cardiac arrhythmias may be categorized according to those that
modify impulse formation (such as altered normal automaticity or triggered activity
caused by early or delayed afterdepolarizations) or conduction (such as that due to
re-entry or re-excitation of cardiac tissue).
In humans, the most common causes of arrhythmias include myocardial ischemia, myocardial
infarction, or reperfusion of a previously ischemic myocardium. These conditions can
be readily reproduced in both intact and isolated hearts in many species. While the
pathogenesis of arrhythmias may not appear to be relevant to drug safety, it is the
mechanism(s) derived from decades of arrhythmia studies that is being used to define
and explain drug-induced arrhythmogenesis. These reported mechanisms may have direct
implications in the development of the CIPA paradigm and its ability to provide additional
information regarding the “proarrhythmia potential” for a new drug. The mechanism(s)
that have been developed will be important in the interpretation of ion channel data
and the predicted changes in the action potential using suggested in silico models.
Abnormal impulse generation that may be responsible for induction of TdP arrhythmias
may arise from oscillations in the membrane potential and has been characterized as
a “triggered” rhythm (26). These triggered rhythms occur in two forms: early or late
afterdepolarizations (EAD or DAD). Early afterdepolarizations interrupt either phase
2 or 3 repolarization of the AP. If these afterdepolarizations attain sufficient thresholds,
they may produce triggered responses and induce single or multiple extrasystoles or
polymorphic VT episodes such as TdP. The EAD is an oscillatory potential that is sensitive
to frequency and often occurs at slow stimulation rates. EAD activity has been shown
in vitro using many types of isolated cardiac tissue and various cell types including
mid-myocardial cells (M-cell) (27). Induction of EAD activity can be induced by a
variety of drugs that block sodium and potassium channels. When a transient depolarization
occurs during phase 4 of the cardiac AP, it is termed a delayed afterdepolarization
(DAD) which is vitally dependent upon the rate of the preceding action potential.
Thus, the amplitudes of DADs increase with decreasing cycle lengths (28). DADs have
been observed under a variety of experimental conditions, all of which have a similar
end result—intracellular Ca2+ overload. High intracellular Ca2+ concentrations saturate
sarcoplasmic reticulum sequestration mechanisms resulting in Ca2+ oscillations due
to Ca2+-induced Ca2+ release (29). The ionic currents that contribute to this mechanism
are not known.
For many years, there has been an effort to mathematically describe the genesis of
the cardiac action potential. Primarily driven by academic research groups, there
is now ongoing integration of these in silico research methods to the CIPA initiative.
Of the many in silico models developed, it is the O’Hara-Rudy model that will likely
form the basis for the in silico component of CIPA (30). The O’Hara model is a human
ventricular cardiac AP model based upon data measured from over 100 undiseased human
hearts. Components of the model were evaluated over the human range of physiological
frequencies and include calcium versus voltage-dependent inactivation of L-type calcium
current (I
CaL); realistic kinetics for the transient outward, rapid delayed rectifier (I
Kr), and inward rectifier (I
K1) potassium currents along with the Na+/Ca2+ exchanger (I
NaCa). The authors also examined model response to rate dependence and restitution
of cardiac AP duration (APD).
Overview of Methods and Models Used in Assessment of Drug-Induced Cardiotoxicity During
Drug Development
The critical nature of cardiac liability determination and implementation of an appropriate
strategy for drug safety assessment resulted in the development of three guidelines
that outlined non-clinical (ICH S7A; ICH S7B) and clinical (ICH E14) testing strategies
(7,31–33). The current non-clinical testing strategy includes in vitro I
Kr current assessment in heterologous mammalian cell lines expressing the human ether-a-go-go-related
gene (hERG, Kv11.1) channels, which have been agreed to play a crucial role in human
cardiac electrophysiology. The preclinical guidelines propose that sponsors consider
other complementary models. These include a number of in vitro assays that have been
well characterized with utility in the safety profiling of a new chemical entity (NCE).
These assays include assessment of drug effects in the isolated rabbit Purkinje fiber
preparation, the isolated Langendorff heart, and the isolated wedge preparation. Cardiac
safety pharmacology in vivo methods primarily use conscious telemetered animals to
assess the effects of the test item on the QT interval. Variables that are usually
recorded in the dog and other non-rodent species include heart rate and the ECG. Thus,
a variety of tests are used to evaluate drug safety and include effects primarily
on blood pressure, heart rate, the ECG, repolarization (APD), the hERG (I
Kr) ion channel, and cardiac conduction.
An additional consideration to drug effects on heart rhythm involves cardiac (ventricular)
contractility. This has largely been neglected by safety scientists and has only recently
become of potential interest in the cardiovascular safety milieu (34). Contractility,
it is thought could initially represent a complementary readout to the current cardiovascular
endpoints assessed under the core-battery ICH S7A guidance, but this could change.
Several papers have recently been published that evaluate direct measures of contractility
(in vivo), and contractility variables include left ventricular pressure (LVP) and
rate of contraction and relaxation (±dP/dtmax) recorded directly via invasive catheter
implantation (34,35).
Regardless of the constituents, the resultant data, in addition to data acquired from
acute and/or chronic toxicology studies, are recommended to be analyzed using an integrated
risk assessment (IRA) (36). The IRA is a holistic evaluation of non-clinical study
results and is used because there has been no conclusion regarding which single non-clinical
model could be used to accurately address issues highlighted by the regulatory guidance
documents. However, the value ascribed to data derived from in vitro and in vivo studies
is limited by other factors which may distort the perceived safety of a drug (37,38).
These factors include differences in the pharmacokinetic and pharmacodynamic relationship
between animals and humans as well as differences between species regarding metabolism
and plasma protein binding as well as variability in species dependence of TdP susceptibility
(39). Despite such differences, the assays currently used in non-clinical safety studies
can be used to inform the planning of clinical trials. Note that the information acquired
during the conduct of human trials will always surpass that of the non-clinical studies
in terms of relevance.
Conduct of a clinical development program, prior to approval application, provides
a rigorous assessment of the drug’s propensity to prolong the QT interval in humans.
Thorough QT studies (TQT) involve quantification of the degree of the drug’s influence
on cardiac repolarization in healthy volunteers as compared to placebo and a positive
control (moxifloxacin). The aim of the TQT studies is to identify drugs that clinically
require more attention toward the potential for development of a cardiac liability
and therefore require additional ECG monitoring in subsequent clinical trials to assess
arrhythmia risk in the target patient population. Although the TQT studies are informative
and the best amongst the currently available methods, they may not be cost-effective
and suffer from a low positive predictive value (40).
The classical approach to the assessment of proarrhythmia occurs via the so called
thorough QT (TQT) study that looks for the largest excursion following a dose (a guard
against hysteresis) and checks whether the upper confidence limit excludes the effect
size deemed of potential clinical relevance. Using the ICH E14 guidance, the upper
90% confidence limit and a boundary of 10 ms defines the risk. However, hysteresis
is rarely observed in practice, so this procedure is both inefficient in the use of
available data to look for an effect and is biased toward incompletely compensating
for multiplicity by taking the largest observed value amongst several time points.
Exposure-response modeling offers advantages in efficiency by combining what is known
across all time points and several doses. It is also relatively simple because there
is usually a linear relationship between exposure and QT for the small effects that
are of interest. Indeed, such assessments have long been performed as supplements
to the classical analyses of TQT studies. However, as supplementary analyses, these
have not had full prespecification and are thus regarded as exploratory.
Recent work has taken a more systematic approach, with full prespecification of analytic
methods, including tests for linearity. Retrospective application of formal methods
of exposure-response analysis across selected studies was found to be encouraging,
but Food and Drug Administration (FDA) insisted on a prospective study, too, and named
a small set of drugs and doses with peak effects in the neighborhood of 10 ms to be
assessed (41)
The conducted study was a three-period, third-party blinded, randomized, placebo-controlled
study in 20 healthy volunteers. The design was planned to be similar to a single ascending
dose (SAD) phase 1 study with the primary objective to estimate the effect of the
drugs on the QTc interval using ER analysis. Each subject undergo three treatment
periods. An incomplete block design was used what resulted in each study drug being
administered to nine subjects and placebo to six subjects in separate periods. The
design, sample size, and statistical approach is intended to result in similar power
to exclude clinically relevant QTc effects as a standard SAD FIM study (41).
Exposure-response methods have now been established in order to provide a more efficient
assessment of the QT interval, making such an assessment potentially a part of early
phase clinical studies where the highest doses are likely to be utilized, and obviating
conduct of a dedicated, expensive TQT study. However, this methodology still makes
use of a biomarker whose correlation to proarrhythmic risk is not absolute.
To address the deeper problem, we can potentially make use of fundamental understanding
of the mechanism by which torsade-like arrhythmias are generated, and the ability
to assess in vitro drug effects on the machinery derived from humans.
Torsade occurrence requires two things. The first is a relative failure of the cardiac
myocyte to execute the repolarization phase of the action potential. Following the
upstroke and plateau phases, the myocyte should re-establish polarity, allowing the
trigger to be reset for the next action potential at its expected time. During this
time, the repolarization forces, in the form of outward currents, must be vigorous
enough to suppress any residual tendency for regenerative activity. Thus, inadequate
outward current activity during this time allows inward, depolarizing current to get
the upper hand leading to “early afterdepolarizations” (EADs), essentially, new action
potentials happening long before they are due.
As long as these EADs happen more or less throughout the myocardium in synchrony,
they are generally harmless. At least, following one bad beat, things are likely to
settle back to normal activity (42,43). This is why patients with intrinsic or drug-induced
problems with repolarization can live decades without a fatal arrhythmia. So, the
second criterion for setting up an arrhythmia is some degree of heterogeneity in the
heart. If all parts of the heart are not working in synchrony, say, because of regional
ischemic disease, scarring due to an healed infarct, etc., the errant action potential
has some place to conduct and then is able to back-propagate and re-excite cardiac
tissue into what becomes a circuitous activity (42). During such an event, the usual
muscular coordination is lost and the pumping action of the heart is interrupted.
Such effect has been widely studied and discussed from various perspectives (44).
Although we can find evidence for the electrical uncoupling in the heart and can model
its effects to gain insights into proarrhythmic mechanisms, fully characterizing a
patient’s intrinsic risk in this regard lies outside what is technically feasible
today. Judging when an arrhythmia will occur and in whom is not possible. Judging
which drugs are likely to create the conditions that give rise to EADs is, on the
other hand, quite feasible (see above). Cardiac myocytes have multiple ion channel
types (see Fig. 1), and the human channels of each type can be studied in isolation
in cells overexpressing single types. Each channel type can have the effect of a drug
assessed in vitro under voltage clamp conditions, and high-throughput systems permit
replicate experiments, exposure-response characterization, etc. to be performed at
modest cost. If you know the channel densities in human cardiac myocytes, you can
then reconstruct the human cardiac myocyte action potential, as it is influenced by
time-varying drug exposure, in silico, and look for the propensity to produce EADs.
That is, at the level of the individual human cardiac ventricular myocyte, the process
of developing an action potential is completely understood, and drug effects can be
completely characterized.
Fig. 1
A cardiac action potential that outlines the major currents involved in depolarization
and repolarization of the cell membrane. The cardiac action potential (AP) conventionally
consists of several phases (0–4) with a duration of ∼300 ms. Phase 0 corresponds to
membrane depolarization (Na+ influx thru I
Na channels) while phase 1 shows the early rapid repolarization of the membrane due
to activation of the transient outward (I
to) K current. Phase 2 is the plateau of the AP (due to a reduction in Na+ influx)
and an increase in Ca2+ influx (thru I
Ca channels) while phase 3 shows membrane repolarization resulting from the coordinated
opening and closing of many different K+ channels such as the rapid (I
Kr) and slow (I
Ks) components of the delayed rectifier K channel. Phase 4 corresponds to the resting
membrane potential and is maintained by the inward rectifier (I
K1) channel. The effects of a drug that produces a prolongation in the AP by blockade
of I
Kr is shown (drug (circle))
Because this technology assesses drug effects across all, or most, of the set of cardiac
ion channel types, it should have the ability to detect drugs with isolated adverse
effects on repolarization and to differentiate those drugs from the ones with mixed
effects on depolarizing and repolarizing forces, resulting in less net risk.
In practice, there are some uncertainties. For example, you cannot be sure that the
voltage clamp protocol you used will always capture the effect of a drug on the ion
channel, because that effect may depend on some aspect of the channel’s history that
your protocol did not account for. There can also be effects of drugs that are not
the result of direct interaction with the channel protein. It is, therefore, of interest
to augment such a channel-based assay with some information from a more integrated
(and thus less easily characterized) system. Two such candidates are being explored.
Ventricular myocytes can be obtained directly from adult human ventricles, but sufficient
availability cannot yet be guaranteed by vendor sources (45). Human embryonic or pluripotent
stem cells can be induced to form ventricular myocytes, which can be cultured in vast
numbers, and these are available from numerous commercial sources. While they do not
currently replicate the electrophysiological phenotype of myocytes from the adult
human heart, they can be used, once correctly engineered, to provide some assessment
of drug effects that might be missed by the use of the hERG channel-based assay alone.
Another approach to attaining supplementary information on drug effects is to return
to the human ECG. The upstroke of the action potential is reflected in the QRS interval
of the ECG, and repolarization is represented by the QT interval. Drugs with a variety
of known channel effects have predictable effects on the morphology of the ECG waveform,
and understanding these relationships and what is meant by their changes can help
make predictions about underlying channel effects (with potential clinical implications)
of novel drugs.
In Vitro–In Vivo Extrapolation of a Drugs’ Proarrhythmic Effect—From High-Throughput
to Rare Case Analysis
All the abovementioned methods focus on a single compound or at most in combination
with metabolites when an in vivo system is in use, and idealistic (or non-realistic)
conditions which are not complimentary with the real life situation where the drug
of interest is used. Even a cursory analysis of the literature-reported TdP cases
suspected to be drug-triggered depicts one element which seems to be overlooked, namely
the influence of external factors.
Multiple elements can be listed as external factors, and the list below does not cover
all possible components:
Concomitant drugs and the pharmacokinetic (PK) and pharmacodynamics (PD) drug–drug
interactions
Food and other environmental factors
Demographic and physiological parameters and their drug-triggered modification
Genetic factors and comorbidity
Apart from a direct influence on the drug affinity to the channels, all of the above-listed
elements can modify drug pharmacokinetics. It has been widely proven that the variability
of physiological parameters directly modify drug pharmacokinetics and exposure (46–48).
Therefore, cardiac risk should ideally be assessed at the level of the individual
patient and also account for non-drug-related parameters potentially triggering serious
health-threatening situations. Examples of such situations can be highlighted amongst
the drugs which are currently on the market or were withdrawn due to non-acceptable
cardiac risk. To support such a statement, a brief, non-exhaustive analysis and description
of the commonly known proarrhythmic drugs has been performed and discussed to draw
attention to the role of these non-drug parameters.
Cisapride is often given as an exemplary non-cardiology drug associated with the risk
of TdP (49). Cisapride, a serotonin 5-HT4 receptor agonist, was developed in 1980
as a prokinetic agent that increased gastrointestinal motility. It has also been used
for the treatment of gastroesophageal reflux disease. In vitro studies revealed that
it was a potent I
Kr current inhibitor with in vitro measured IC50 values in the range of 4.3–100 nM
depending on the study settings (50). However, it has other ion channel properties
that include late calcium (I
CaL) current inhibition with IC50 values in the micromolar range (51). According to
all known classification schemes including those developed by Redfern, Mirams, and
Crediblemeds (https://www.crediblemeds.org/), it is a drug associated with a high
propensity for proarrhythmic risk (9,52,53). This assignment of risk is based on case
reports of side effect including TdP and other types of arrhythmias, especially when
the medication was taken concomitantly with other medications or in patients with
certain underlying conditions predisposing them to arrhythmias. Wysowski et al. analyzed
the post-marketing reports of QT prolongation and ventricular arrhythmia associated
with cisapride (54). From 1993 until 1999, while being marketed in the USA, the FDA
received 341 individual patient reports of multiple heart-related conditions including
117 associated with QT prolongation, 107 with TdP, 16 with polymorphic ventricular
tachycardia, and 27 with ventricular tachycardia. Eighty (23%) of the 341 patients
died. The authors concluded that in most cases, an arrhythmia occurred in the presence
of additional, complex risk factors including the presence of other drugs and/or variant
medical conditions. Among other strictly contraindicated factors that were listed
include concomitant use of CYP3A4 enzyme inhibitors, serious heart conditions, electrolyte
disorders, and overdose. A similar situation was reported in the case study provided
by Hussain and Ghazal (55). These authors report the medical situation of a 36-year-old
woman presented to the emergency room after 3 days of abdominal pain, fever, nausea,
and vomiting preceded by a Caesarean section. She was treated by multiple drugs including
antiarrhythmic drugs and cisapride which were considered as the reasons for the developed
ventricular tachycardia which degenerated to frequent episodes of TdP. After testing
multiple factors, the authors concluded that the observed TdP might not be solely
due to drug-induced QT interval prolongation. Other factors discussed by the attending
clinicians include those caused by hypokalemia resulting from repeated vomiting and
poor nutritional intake and metabolic drug–drug interactions which altered cisapride
exposure. Cisapride was withdrawn from many global markets in 2000 but remains available
for use in some EU countries to specific patients under strict black-box restrictions.
Use by these specific patients requires that both health care providers and patients
receiving cisapride are familiar with this complication associated with use and that
all parties understand and comply with specific recommendations outlined and required
for use.
Trimebutine, a drug used to regulate motility in the gastrointestinal tract via an
agonist effect on peripheral μ-, κ-, and δ- opiate receptors and modulation of gastrointestinal
and extragastric peptide release (i.e., motilin, vasoactive intestinal peptide, gastrin,
and glucagon), is provided as an example distinctly different from that of cisapride.
Trimebutine is a weak inhibitor of I
Kr currents in guinea pig ventricular myocytes as described by Morisawa and colleagues,
with negligible effects, even at concentrations much higher than those in clinical
use (56). As may be expected, based on such studies, trimebutine is classified as
a drug without known TdP risk (53). Surprisingly, a Eudravigilance system (i.e., a
data processing network and management system for reporting and evaluating suspected
adverse drug reactions) query resulted in the finding of two records that involve
cases of TdP associated with its use (57). Both cases were reported by health care
professionals which increases their potential credibility, but interestingly, both
cases concern effects on elderly patients (65–85 years of age). Such reports do not
provide a complete set of information and obviously cannot be used as strong evidence
of potential risk but can be used in signal generation. Additionally, a quick literature
search (scholar.google.com) found one report where significant QT interval prolongation
and monomorphic ventricular tachycardia was observed with high doses of trimebutine
(58). Monomorphic ventricular tachycardia, as opposed to the polymorphic variant,
is less dangerous and more easily manageable but can degenerate to a polymorphic form
including TdP. The authors conclude that while there may be a causal association between
the occurrence of arrhythmia and the use of high doses of trimebutine, it is only
probable. However, this may be sufficient to warrant further review to quantify drug
and non-drug-related triggering factors (in this case—sex, age, and plasma electrolytes).
In most cases, the clinically observed effect is likely the consequence of the multiple
actions the drug impart on varying physiological systems. For some drugs, metabolites
contain inhibitory activity against ionic currents; therefore, knowledge about their
pharmacokinetics and pharmacodynamics allows for a better prediction of the cardiac
effects and clinical data interpretation. In 2010, after 13 years of a presence on
market, the US FDA issued a safety announcement regarding Anzemet (dolasetron mesylate—an
antiemetic 5-HT3 receptor inhibitor) use, informing patients and health professionals
that the injectable form of Anzemet should no longer be used to prevent nausea and
vomiting associated with cancer chemotherapy (CINV) in pediatric and adult patients
(59,60). Such a decision was undertaken after review of dolasetron-induced TdP cases
(61). The drug can still be used in postoperative nausea and vomiting prophylaxis
and treatment because lower doses are used for these indications. Dolasetron is rapidly
metabolized to a reduced form of hydrodolasetron (MDL 74,156) by carbonyl reductase,
an enzyme widely distributed in human tissues (62,63). In view of this manner of metabolism,
dolasetron is considered as a prodrug that is converted to hydrodolasetron, which
is believed to be responsible for the majority of clinical activity (64,65). Additionally
5′-OH and 6′-OH metabolite derivatives are considered as carrying partial activity.
When given orally, dolasetron plasma concentration is in most cases undetectable and
its pharmacological activity is negligible, although after intravenous injection both
active moieties are present at the site of action and trigger potential cardiac effects.
Orally taken, dolasetron formulations are still in use for all indications and considered
as safe (66). There was however a case study published where a massive orally taken
dolasetron dose (2000 mg, p.o.) was taken. The patient’s ECG showed first-degree heart
block along with non-specific intraventricular conduction delay and a prolonged QTc
interval (67).
The above given example indicates that the route-dependent kinetic actions of drugs
should be considered during drug safety analysis and that a combination of the active
substances (i.e., parent and metabolites) rather than a single entity (i.e., the parent
molecule) should be studied for safety purposes. This is done depending upon the levels
of the metabolites in the plasma (>10%) and whether they can be synthesized and tested
alone. Similarly, toxicology species are assessed for parent and metabolite PK parameters
to ensure that adequate exposure of parent and metabolites occurs during long-term
toxicity assessments. There are multiple examples of drug–drug interactions at the
pharmacokinetic and pharmacodynamic level which trigger potential toxic effects with
terfenadine and ketoconazole being probably the most well-known examples (68). Consequences
of the latter are relatively easy to predict as ketoconazole-driven CYP3A4 inhibition
and subsequent blockade of terfenadine metabolism resulted in substantial increases
in terfenadine blood concentrations in combination with potent inhibition of the I
Kr current resulted in QT interval prolongation and precipitation of TdP arrhythmias.
At the daily routine level, what is probably most important to assess is the non-linear
effect of the drug combination. Such a situation was described for droperidol and
ondansetron where, despite of lack of exposure modification after concomitant dosing,
the QT interval was prolonged, but the observed prolongation was not proportional
to the QT prolongation observed for two drugs given separately (69).
Considering the complexity of the above-listed phenomena, their thorough analysis
during the conduct of clinical trials would be very challenging, if not impossible.
What’s more, clinical trial characteristics, namely a relatively small cohort, homogeneity
of the included individuals, short period of drug exposure, rare drug–drug and drug–environment
interaction analysis, could contribute to relatively poor prediction of rare cases
observed in subsequent clinical studies. The solution might be the development of
a relatively tight safety margin for the analyzed TdP risk surrogate as was proposed
in the ICH E14 guideline. There is however a cost connected with that as it might,
and probably has, provoked genesis of a high percentage of false positives and thus
a high clinical attrition rate of many drug candidates (1). As was mentioned above,
this was one of the reasons for the inception of a new cardiac safety testing paradigm
discussion and likely introduction. An inevitable component of this new paradigm is
in silico methods which should became a vital element of cardiac safety testing. These
include various approaches, starting from screening methods (QSAR-based models), up
to the utilization of the biophysically detailed cardiac myocyte models (15,70). The
latter techniques vary with regard of the level of complexity of the mathematical
description of the cardiac physiology at the ion channel (Hodgkin-Huxley or Markovian
notation) and cell level (single cell up to the three-dimensional heart structure)
(71,72). Such methods offer the possibility to incorporate variability of either stochastic
or deterministic nature (73,74). This can further allow for the drug cardiac safety
analysis at the population level and quantitative assessment of the combination of
drug and non-drug-related parameters (36,75,76).
Some elements should include the need for proper exposure quantification. The effect
at the clinical level is related to the concentrations of the tested substances. However,
plasma drug concentration (which is the most common effective concentration surrogate)
can be imperfect as it may vary from that in the tissues. Therefore, more suitable
effector concentration methods should be considered whenever available, possibly in
the places where drug might meet cardiac ion channels (i.e., pericardial fluid, heart
cell extracellular matrix, and cardiomyocyte cytoplasm). Something that is impossible
in clinical practice can be potentially incorporated via the application of a physiologically
based pharmacokinetic (PBPK) modeling and simulation approach.