ICH S7B Guideline
Step 2 Revision
The Nonclinical Evaluation of the Potential for Delayed Ventricular
Repolarization (QT Interval Prolongation) by Human Pharmaceuticals
The
Nonclinical Evaluation of the Potential for Delayed Ventricular
Repolarization (QT Interval Prolongation) by Human Pharmaceuticals
The assessment of the
effects of pharmaceuticals on ventricular repolarization and
proarrhythmic risk is the subject of active investigation. When
additional data (nonclinical and clinical) are accumulated in the
future, they will be evaluated and this guideline might be revised.
This guideline describes a nonclinical testing
strategy for assessing the potential of a test substance to delay
ventricular repolarization. This guideline includes information
concerning nonclinical assays and an integrated risk assessment.
The QT interval (time
from the beginning of the QRS complex to the end of the T wave) of
the electrocardiogram (ECG) is a measure of the duration of
ventricular depolarization and repolarization. QT interval
prolongation can be congenital or acquired (e.g.,
pharmaceutical-induced). When ventricular repolarization is delayed
and the QT interval is prolonged, there is an increased risk of
ventricular tachyarrhythmia, including torsade de pointes,
particularly when combined with other risk factors (e.g.,
hypokalemia, structural heart disease, bradycardia). Thus, much
emphasis has been placed on the potential proarrhythmic effects of
pharmaceuticals that are associated with QT interval prolongation.
Ventricular repolarization, determined by the
duration of the cardiac action potential, is a complex physiological
process. It is the net result of the activities of many membrane
ion channels and transporters. Under physiological conditions, the
functions of these ion channels and transporters are highly
interdependent. The activity of each ion channel or transporter is
affected by multiple factors including, but not limited to,
intracellular and extracellular ion concentrations, membrane
potential, cell-to-cell electrical coupling, heart rate, and
autonomic nervous system activity. The metabolic state (e.g.,
acid-base balance) and location and type of cardiac cell are also
important. The human
ventricular action potential consists of five sequential phases:
·
phase 0: The upstroke of the action
potential is primarily a consequence of a rapid, transient influx of
Na+ (INa) through Na+ channels.
·
phase 1: The termination of the
upstroke of the action potential and early repolarization phase
result from the inactivation of Na+ channels and the
transient efflux of K+ (Ito) through K+
channels.
·
phase 2: The plateau of the action
potential is a reflection of a balance between the influx of Ca2+
(ICa) through L-type Ca2+ channels and
outward repolarizing K+ currents.
·
phase 3: The sustained downward
stroke of the action potential and the late repolarization phase
result from the efflux of K+ (IKr and IKs)
through delayed rectifier K+ channels.
·
phase 4: The resting potential is
maintained by the inward rectifier K+ current (IK1).
Prolongation of the action
potential can result from decreased inactivation of the inward Na+
or Ca2+ currents, increased activation of the Ca2+
current, or inhibition of one or more of the outward K+
currents. The rapidly and slowly activating components of the
delayed rectifier potassium current, IKr and IKs,
seem to have the most influential role in determining the duration
of the action potential and thus the QT interval. The human
ether-a-go-go-related gene (hERG) and KvLQT1 gene encode
pore-forming proteins that are thought to represent the
a-subunits
of the human potassium channels responsible for IKr and IKs,
respectively. These
a-subunit
proteins can form hetero-oligomeric complexes with auxiliary
b-subunits
(i.e. MiRP and MinK gene products), which have been speculated to
modulate the gating properties of the channel proteins. The
most common mechanism of QT interval prolongation by pharmaceuticals
is inhibition of the delayed rectifier potassium channel that is
responsible for IKr.
This guideline extends and complements the “ICH
Guideline on Safety Pharmacology Studies for Human Pharmaceuticals”
(ICH S7A). This guideline applies to new chemical entities for human
use and marketed pharmaceuticals when appropriate (e.g., when
adverse clinical events, a new patient population, or a new route of
administration raises concerns not previously addressed).
Pharmaceuticals for which testing is not called for are described in
ICH S7A.
Principles and recommendations described in ICH
S7A also apply to the studies conducted in accordance with the
present guideline.
In vitro
and in vivo assays are complementary approaches; therefore, according
to current understanding, both assay types
should be conducted.
The investigational approach and evidence of
risk should be individualized for the test substance, depending on
its pharmacodynamic, pharmacokinetic and safety profiles.
The objectives of studies are to: 1) identify
the potential of a test substance and its metabolites to delay
ventricular repolarization, and 2) relate the extent of delayed
ventricular repolarization to the concentrations of a test substance
and its metabolites. The study results can be used to elucidate the
mechanism of action and, when considered with other information,
estimate risk for delayed ventricular repolarization and QT interval
prolongation in humans.
Nonclinical methodologies can address the
following:
·
Ionic currents measured in isolated
animal or human cardiac myocytes, cultured cardiac cell lines, or
heterologous expression systems for cloned human ion channels,
·
Action potential parameters in
isolated cardiac preparations or specific electrophysiology
parameters indicative of action potential duration in anesthetized
animals,
·
ECG parameters measured in
conscious or anesthetized animals,
·
Proarrhythmic effects measured in
isolated cardiac preparations or animals.
As indicated above, these four functional
levels can be investigated by in vitro and/or in vivo
methods. Findings from the first three functional levels listed above are
considered useful and complementary. The value of
proarrhythmia models is discussed in section 3.1.4.
In vitro electrophysiology studies can
explore potential cellular mechanisms that might not be evident from
in vivo data. Changes in
other cardiovascular parameters or effects on multiple ion channels
can complicate interpretation of data. Complementary
assessments in other systems can address this issue. Although delay
of repolarization can occur through modulation of several types of
ion channels, inhibition of IKr is the most common
mechanism responsible for pharmaceutical-induced prolongation of QT
interval in humans.
Experimental models that possess the full
complement of mechanisms can be more informative with regard to the
clinical situation. Carefully designed and conducted in vivo
studies allow evaluation of metabolites and can enable estimation of
safety margins. In vivo ECG
evaluations provide information on conduction properties and
non-cardiac influences (e.g., autonomic nervous system tone).
Studies of action potential parameters provide information on the
integrated activity of multiple ion channels in the heart.
The following sections describe a general
nonclinical testing strategy for assessing risk for delayed
ventricular repolarization and QT interval prolongation that is
pragmatic and based on currently available information.
The figure illustrates the component elements of the testing
strategy, but not specific test systems or their designs.
Results from an assay that evaluates effects on
IKr or the ionic current through a native or expressed IKr
channel protein, such as that encoded by hERG (see section 3.1.2).
Results from an in
vivo assay that measures indices of ventricular repolarization
such as QT interval (see section 3.1.3).
Consideration should
be given to whether the test substance belongs to a
chemical/pharmacological class in which some members have been shown
to induce QT interval prolongation in humans (e.g., antipsychotics,
histamine H-1 receptor antagonists, fluoroquinolones). This should,
where appropriate, influence the choice of reference compound(s) and
be included in the integrated risk assessment.
·
Additional information for the
integrated risk assessment can include results from:
·
Pharmacodynamic studies,
·
Toxicology/safety studies,
·
Pharmacokinetic studies, including
plasma levels of parent substance and metabolites (including human
data if available),
·
Drug interaction studies,
·
Tissue
distribution and accumulation studies,
·
Post-marketing
surveillance.
Follow-up studies are
intended to provide greater depth of understanding or additional
knowledge regarding the potential of test substance for delayed
ventricular repolarization and QT interval prolongation in humans.
Such studies can provide additional information concerning potency,
mechanism of action, slope of the dose-response curve, or magnitude
of the response. Follow-up studies are designed to address specific
issues, and, as a result, various in vivo or in vitro
study designs can be applicable.
In circumstances
where results among nonclinical studies are inconsistent and/or
results of clinical studies differ from those for nonclinical
studies, retrospective evaluation and follow-up nonclinical studies
can be used to understand the basis for the discrepancies. Results
from follow-up studies can be a significant component of an
integrated risk assessment.
Relevant nonclinical
and clinical information along with the following should be
considered in the selection and design of follow-up studies:
·
Use of ventricular repolarization assays that measure
action potential parameters in isolated cardiac preparations (see
section 3.1.2),
·
Use of specific electrophysiological parameters
indicative of action potential duration in anesthetized animals (see
section 3.1.3),
·
Repeated
administration of test substance,
·
Selection of
animal species and gender(s),
·
Use of metabolic inducers or
inhibitors,
·
Use of concurrent positive control
substances and reference compounds (see section 3.1.1),
·
Inhibition of other channels not
previously evaluated,
·
Measurement of
electrophysiological parameters at multiple time points,
·
Confounding
effects in conscious animals that limit
the interpretation of data such as test substance-induced effects on
heart rate or autonomic tone, or toxicities such as tremor,
convulsion, or emesis.
The integrated risk
assessment is the evaluation of non-clinical study results including
the results from follow-up studies and other relevant information.
The integrated risk assessment should be scientifically based and
individualized for the test substance. Such an assessment can
contribute to the design of clinical investigations and
interpretation of their results. The integrated risk assessment
should be provided for the Investigator’s Brochure and the
Nonclinical Overview (ICH M4). The integrated risk assessment should
also consider:
·
Potencies of test substance in S7B
assays relative to reference compound(s),
·
Safety margins from in vivo
QT assays,
·
Assay sensitivity and specificity,
·
Contribution of metabolites to QT
interval prolongation as well as metabolic differences between
humans and animals.
Evidence of risk is the overall conclusion from
the integrated risk assessment for a test substance to
delay ventricular repolarization and
prolong QT interval in humans.
Results from S7B nonclinical studies assessing
the risk for delayed ventricular
repolarization and QT interval prolongation generally do not
need to be available prior to first administration in humans.
However, these results, as part of an integrated risk assessment,
can support the planning and interpretation of subsequent clinical
studies. The early availability of these data is considered
valuable.
This section provides an overview of
methodologies currently used to assess the potential for a test
substance to delay ventricular repolarization and to prolong QT
interval. The following criteria should be considered in selecting
the most appropriate test systems:
·
Assay methodology and experimental
endpoints are scientifically valid and robust,
·
Assays and preparations are
standardized,
·
Results are reproducible,
·
Endpoints/parameters of the assays
are relevant for assessing human risk.
Positive control substances should be used to
establish the sensitivity of in vitro preparations for ion
channel and action potential duration assays. In the case of in
vivo studies, positive control substances should be used to
validate and define the sensitivity of the test system, but need not
be included in every experiment.
For test substances belonging to a
chemical/pharmacological class that is associated with QT interval
prolongation in humans, the use of concurrent reference compound(s)
(member(s) of the same class) in in vitro and in
vivo studies should be considered to facilitate ranking the
potency of the test substance in relation to its comparators.
Whether or not positive control substances or
reference compounds are used in experiments should be justified.
In vitro electrophysiology studies can
provide valuable information concerning the effect of a test
substance on action potential duration and/or cardiac ionic
currents. These assays have an important role in assessing the
potential for QT interval prolongation and elucidating cellular
mechanisms affecting repolarization. In vitro
electrophysiology studies employ either single cell (e.g.,
heterologous expression systems, disaggregated cardiomyocytes) or
multicellular (e.g., Purkinje fiber; papillary muscle; trabeculae;
perfused myocardium; intact heart) preparations. Multicellular
preparations are stable test systems to study action potential
duration. While more fragile, single cell preparations minimize
diffusional barriers to the site of action.
The analysis of parameters for each phase
of the action potential such as Vmax for phase 0 (INa),
APD30 for phase 2 (ICa) and “triangulation”
for phase 3 (IK) can be useful to investigate
the effects on specific channels responsible for these phases. In
addition, some parameters derived from the Langendorff preparation
have been reported to provide information regarding proarrhythmia.
Heterologous expression systems, where human ion channel
protein(s) are expressed in noncardiac cell lines, are used to
assess the effects of a test substance on a specific ion channel.
Disaggregated myocytes are technically more challenging than the
expression systems but have the advantage of being suitable for
assessing effects on both action potential duration and ionic
currents.
Tissue and cell preparations for in vitro
assays are obtained from different laboratory animal species
including rabbit, ferret, guinea pig, dog, swine, and occasionally
from humans. The ionic mechanisms of repolarization in adult rats
and mice differ from larger species, including humans (the primary
ion currents controlling repolarization in adult rats and mice is Ito);
therefore, use of tissues from these species is not considered
appropriate. Species differences in terms of which cardiac ion
channels contribute to cardiac repolarization and to the duration of
the action potential should be considered in selecting a test
system. When native cardiac tissues or cells are used, the
characteristics and source of the preparation should be considered
because the distribution of ion channel types varies according to
the region and type of cell.
Test substance concentrations for in vitro
studies should span a broad range, covering and exceeding the
anticipated maximal therapeutic plasma concentration. Ascending
concentrations should be tested until a concentration-response curve
has been characterized or physicochemical effects become
concentration-limiting. Ideally, the duration of exposure should be
sufficient to obtain steady-state electrophysiological effects,
unless precluded by the viability of the cell or tissue
preparation. The duration of exposure should be indicated.
Appropriate positive control substances should be used to establish
the sensitivity of the in vitro assay system as well as to
confirm that the ion channels of interest are present and stable.
Factors that can confound or limit the
interpretation of in vitro electrophysiology studies include
the following:
·
The testing of high concentrations
of the test substance can be precluded by limited solubility in
aqueous physiological salt solutions,
·
Adsorption to glass or plastic or
non-specific binding to the test matrix can reduce the concentration
of the test substance in the incubation or perfusion medium,
·
Test substance concentrations can
be limited by cytotoxic or physicochemical attributes of the test
substance that disrupt cell membrane integrity so that
electrophysiological endpoints cannot be obtained,
·
Cardiac cells and tissues have
limited capacity for drug metabolism and therefore in vitro
studies using the parent substance do not provide information on the
effects of metabolites. When in vivo nonclinical or clinical
studies reveal QT interval prolongation that is not corroborated by
in vitro studies using the parent substance, testing
metabolites in the in vitro test systems should be
considered.
High throughput potassium channel assays are
being developed. While novel ion channel activity assays can be
useful in preliminary screening of test substances to identify lead
candidates for further electrophysiological testing, more experience
will establish whether they have sufficient predictive value to be
an alternative to voltage clamp assays.
Another screening approach is the use of
competition binding protocols in which test substances are studied
for their ability to displace a radiolabeled
hERG channel blocker
from a cell line expressing
hERG. However, competition for radioligand-binding sites
provides no information on agonistic or antagonistic effects of the
test substance on IKr. Moreover, this assay will not
identify test substances that bind to
hERG at sites other than
the radioligand binding sites. Based upon these potential
limitations, this assay is not considered a substitute for voltage
clamp assays described above.
Intact animal models
allow investigation of ventricular repolarization or
associated arrhythmias where integrated effects on the full
complement of ion channel and cell types are assessed. Also,
potential neuronal and hormonal influences on the pharmacodynamic
effect of the pharmaceuticals are present in animals.
The QT interval of the ECG is the most commonly
used endpoint to gauge effects of a test substance on ventricular
repolarization. In specialized electrophysiology studies, regional
information regarding the ventricular
repolarization (e.g., monophasic action potential duration and
effective refractory period) can also be obtained from in vivo
models. Additional safety parameters of interest, including
blood pressure, heart rate, PR interval, QRS duration, the presence
of U waves, and arrhythmias, can be assessed simultaneously.
The QT interval and
heart rate have an inverse, non-linear relationship, which varies
among species, between animals, or even within the same animal at
different heart rates. Thus, a change in heart rate exerts an effect
on QT interval, which can confound the assessment of the effect of
the test substance on ventricular repolarization and the QT
interval. There are two important situations where there is
variability in heart rate among animals: one is due to difference in
autonomic tone, and the other is due to effects of test substances
on heart rate. Therefore, the interpretation of data from in vivo
test systems should take into account the effect of coincident
changes in heart rate. Ideally, QT interval data obtained after
administration of a test substance should be compared with control
and baseline data at similar heart rates. When the variability is
not due to the test substance, it can be reduced by training, or the
use of anesthetized animal models. When the effects are due to test
substances, the most common approach is to correct the QT interval
for heart rate (QTc) using formulae such as Bazett or Fridericia;
however, these corrections can yield misleading data, especially
when differences in heart rate between treatment and control are
large. An alternative approach is to maintain a constant heart rate
using cardiac pacing.
Laboratory animal species used for in vivo
electrophysiology studies include dog, monkey, swine, rabbit,
ferret, and guinea pig. The ionic mechanisms of repolarization in
adult rats and mice differ from larger species, including humans
(the primary ion currents controlling repolarization in adult rats
and mice is Ito); therefore, use of these species is not
considered appropriate. The most
appropriate in vivo test systems and species should be
selected and justified.
The dose range should
be in accord with that discussed in ICH S7A and, whenever feasible,
should include and exceed the anticipated human exposure.
The dose range can
be limited by animal
intolerance to the test substance,
e.g.,
emesis, tremor, or
hyperactivity.
For
studies designed to
relate the extent of delayed
ventricular repolarization to concentrations of the parent test
substance and its metabolites,
controlled exposure via constant intravenous infusion can be used.
Monitoring exposure to the test
substance and metabolites (see ICH S3A) provides opportunities to
interpret dose- and concentration-response data and to design
follow-up studies, if appropriate.
Factors that should
be considered in conducting studies and interpreting the results
include the following:
·
Data
acquisition and analysis methods,
·
Sensitivity and reproducibility of the test systems,
·
Dosing period and measurement points,
·
Heart rate and other cardiovascular effects that confound
interpretation of QT interval data,
·
Inter-species and gender
differences, e.g., cardiac electrophysiology, hemodynamics, or
metabolism of pharmaceuticals,
·
Pharmaceuticals
that have effects on several ion channels can
yield complex dose-response relationships that could be
difficult
to interpret.
The
precise relationship between
test substance-induced
delay of ventricular repolarization and risk of proarrhythmia is not
known.
Directly assessing the proarrhythmic risk of pharmaceuticals that
prolong the QT interval would be a logical undertaking;
however,
modeling of the clinical
condition
where pharmaceuticals elicit arrhythmia is complicated.
Indices of proarrhythmic
activity (e.g. electrical instability and temporal and/or spatial
dispersion of refractoriness, reverse use-dependency, changes in
action potential configuration) and
animal models
might have utility in assessing
proarrhythmia. Interested parties are encouraged to develop
these models and test their usefulness in predicting risk in humans.
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Date created:
September 20, 2004 |