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Aop: 433

Title

Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [AO]”. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

hERG inhibition leading to cardiac toxicity

Short name

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Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.' Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you. Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help

Click to download graphical representation template

W1siziisijiwmjevmtivmtmvbwk1c3z1nnpnx0fpuf9hcmfwagljx1rlbxbsyxrlmi5qcgcixsxbinailcj0ahvtyiisijuwmhg1mdaixv0?sha=3ca01422aaef4d09

Authors

List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help

Egemen Bilgin

Yeditepe University, Department of Pharmaceutical Toxicology, Turkey

Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate. More help

Evgeniia Kazymova (email point of contact)

Contributors

List user names of all authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact. More help

Egemen Bilgin
Shihori Tanabe
Stefan Scholz
Evgeniia Kazymova

Status

The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help

Author status	OECD status	OECD project	SAAOP status
Under development: Not open for comment. Do not cite

This AOP was last modified on May 08, 2022 11:33

The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page	Revision Date/Time
Binding to hERG channel	December 13, 2021 04:54
hERG channel biogenesis interference	December 13, 2021 04:55
Direct hERG channel blockage	December 13, 2021 04:56
Induction of hERG trafficking defects	December 13, 2021 04:58
Inhibition of Ikr	December 13, 2021 05:02
Prolongation of Action Potential	December 13, 2021 05:01
Prolongation of QT interval	December 13, 2021 05:03
Torsade de Pointes	December 13, 2021 05:03
Sudden cardiac death	December 13, 2021 05:05
Binding to hERG channel leads to Direct hERG channel blockage	December 13, 2021 05:10
hERG channel biogenesis interference leads to Induction of hERG trafficking defects	December 13, 2021 05:11
Direct hERG channel blockage leads to Inhibition of Ikr	December 13, 2021 05:12
Induction of hERG trafficking defects leads to Inhibition of Ikr	December 13, 2021 05:13
Inhibition of Ikr leads to Prolongation of Action Potential	December 13, 2021 05:14
Prolongation of Action Potential leads to Prolongation of QT interval	December 13, 2021 05:14
Prolongation of QT interval leads to Torsade de Pointes	December 13, 2021 05:15
Torsade de Pointes leads to Sudden cardiac death	December 13, 2021 05:15

Abstract

In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

Cardiotoxicity is an imperative cause of removal of compounds in preclinical and clinical stage. So far it has used various animal models for cardiotoxicity, but a precise molecular involvement for toxicity has not yet been clarified. Cardiotoxicity typically manifests itself in QT interval prolongation on the electrocardiogram (ECG) and potentially fatal ventricular arrhythmia. Abnormal cardiac electrical activity generally occurs with the result of the unexpected inhibition of human ether-à-go-go-related gene (hERG). hERG inhibition results in prolongation of the QT interval on the ECG, and this prolongation is associated with ventricular repolarization within the cardiac cycle.

Directly blocking hERG channels or inhibiting hERG channels trafficking leads to inhibition of delayed-rectifier potassium current (Ikr) whose outcome is prolongation of action potential that ends up a serious cardiac situation called long QT syndrome characterized by drug-induced QT prolongation, torsade de pointes (TdP), a potentially lethal arrhythmia, and a sudden death. This AOP may be one of the pathways induced by direct or indirect hERG channel inhibitors, which suggest the pathway networks of cardiotoxicity.

[Abbreviation]: AOP: adverse outcome pathway, ECG: electrocardiogram, Ikr: delayed-rectifier potassium current, TdP: torsade de pointes

Background (optional)

This optional subsection should be used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development. The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help

Events:

Molecular Initiating Events (MIE)

An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help

Key Events (KE)

This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page. More help

Adverse Outcomes (AO)

An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help

Sequence	Type	Event ID	Title	Short name

1	MIE	1956	Binding to hERG channel	Binding to hERG channel
2	MIE	1957	hERG channel biogenesis interference	hERG channel biogenesis interference

3	KE	1958	Direct hERG channel blockage	Direct hERG channel blockage
4	KE	1959	Induction of hERG trafficking defects	Induction of hERG trafficking defects
5	KE	1960	Inhibition of Ikr	Inhibition of Ikr
6	KE	1961	Prolongation of Action Potential	Prolongation of Action Potential
7	KE	1962	Prolongation of QT interval	Prolongation of QT interval
8	KE	1963	Torsade de Pointes	Torsade de Pointes

1964

Sudden cardiac death

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button. Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the user will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.). Click ‘Create [adjacent/non-adjacent] relationship.’ The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Title	Adjacency	Evidence	Quantitative Understanding

Binding to hERG channel leads to Direct hERG channel blockage	adjacent	High
hERG channel biogenesis interference leads to Induction of hERG trafficking defects	adjacent	High
Direct hERG channel blockage leads to Inhibition of Ikr	adjacent	High
Induction of hERG trafficking defects leads to Inhibition of Ikr	adjacent	High
Inhibition of Ikr leads to Prolongation of Action Potential	adjacent	Moderate
Prolongation of Action Potential leads to Prolongation of QT interval	adjacent	High
Prolongation of QT interval leads to Torsade de Pointes	adjacent	Low
Torsade de Pointes leads to Sudden cardiac death	adjacent	Low

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and WoE summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help

Stressors

The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help

Life stage	Evidence
All life stages	High

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help

Term	Scientific Term	Evidence	Link
human	Homo sapiens	High	NCBI

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help

Sex	Evidence
Unspecific	High

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help

Homo sapiens

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence.The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications. More help

Support for Essentiality of KEs	Defining Question	High (Strong)	Moderate	Low (Weak)
	Are downstream KEs and/or the AO prevented if an upstream KE is blocked?	Direct evidence from experimental studies illustrating essentiality for at least one of the important KEs.	Indirect evidence that sufficient modification of an expected modulating factor attenuates or augments a KE.	No or contradictory experimental evidence of the essentiality of any of the KEs.
KE1: Direct hERG Channel Blockage	Strong	The human ether-a-go-go related gene (hERG) or KCNH2 gene encodes a voltage-gated potassium channel known as the hERG channel. This channel plays a key role in cardiac action potential repolarization. Reduced function of hERG causes potential action prolongation and increases the risk for potentially fatal ventricular arrhythmia, torsades de pointes [1].
KE2: Induction of hERG trafficking defect	Strong	Prolongation of the AP can result from decreased inactivation of the inward Na+ or Ca++ currents, increased activation of the Ca++ current, inhibition of one or more of the outward K+ currents or altered potassium channel trafficking and protein synthesis [2].
KE3: Inhibition of Ikr	Strong	Consequences of IKr blockade that may combine to facilitate TdP arrhythmia.AP (action potential) prolongation is a proximate effect of IKr blockade at the cellular level and in the ECG is reflected by QT interval (QTi) prolongation [3].
K4: Prolongation of Action Potential	Strong	Inhibition of hERG channels tends to lengthen the cardiac action potential and the duration from the start of the the QRS complex to the end of the T wave in the electrocardiogram (QTinterval) [4].
KE5: Prolongation of QT interval	Strong	Drug-induced QT prolongation leading to serious ventricular arrhythmias, such as torsade de pointes (TdP), poses a major safety consideration for the development and use of new drug candidates. TdP is always associated with prolongation of the QT interval of the surface ECG [5].
KE6: Torsade de Pointes	Strong	Long QT syndrome (LQTS), an abnormality of cardiac muscle repolarization that is characterized by the prolongation of the QT interval in the electrocardiogram, was implicated as a predisposing factor for torsades de pointes, a polymorphic ventricular tachycardia that can spontaneously degenerate to ventricular fibrillation and cause sudden death [6].

Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together. Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook. More help

Support for Biological Plausibility of KERs

MIE1 to KE1

Binding to hERG channel leads to Direct hERG channel blockage

Biological Plausibility of the MIE1 => KE1 is STRONG.

Abnormal cardiac electrical activity is most often a side effect from unintended block of the promiscuous drug target the human ether-à-go-go-related gene (hERG)— the delayed rectifier K+ channel in the heart. Numerous drugs interact with the promiscuous target hERG [7].

Many drugs covering a broad spectrum of pharmaceutical classes have been withdrawn from the market or have had their usage limited due to blockage of the hERG, e.g., astemizole, terfenadine, cisapride, sertindole, terolidine, droperidol, lidoflazine, and grepafloxacin [8].

MIE2 to KE2

hERG channel biogenesis interference leads to Induction of hERG trafficking defects

Biological Plausibility of the MIE2 => KE2 is STRONG.

There are drugs such as probucol, fluoxetine, arsenic, and pentamidine,

which do not block hERG channels but are torsadogenic due to abnormal potassium channel protein synthesis or trafficking [2].

In addition to direct hERG channel block, multiple pharmacological agents can cause hERG deficiency (with hERG channel block or independently) by the inhibition of its biogenesis and trafficking [9].

Several therapeutic compounds have been identified that reduce hERG/IKr currents not by direct block but by inhibition of hERG/IKr trafficking to the cell surface [10].

KE1 to KE3

Direct hERG channel blockage leads to Inhibition of Ikr

Biological Plausibility of the KE1 => KE3 is STRONG.

Some antagonists of the H1 histamine receptor, such as astemizole and terfenadine, which belong to the second generation (i.e. are devoid of sedative effects), can block the HERG channel, causing a decrease in the IKr current [11].

The Kv11.1 channel, a voltage-gated potassium channel previously known as human ether-à-go-go related gene (hERG), encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, IKr, which contributes to phase 3 repolarization in cardiac action potentials [12].

KE2 to KE3

Induction of hERG trafficking defects leads to Inhibition of Ikr

Biological Plausibility of the KE2 => KE3 is STRONG.

There are several types of cardiac K+ channels in the heart that are responsible for different phases of the action potential. Three of them are involved in (late) repolarization. One of these channels, known as hERG (human ethera-go-go-related gene,254 or Kv11.1 or KCNH2, the latter is the name of the gene) is extremely sensitive to inhibition by many compounds. It mediates the so-called rapid component of the delayed rectifier current (IKr current). If this current is suppressed, repolarization is slowed and QT interval prolongation is observed in the ECG. Because the synthesis of the hERG channels is particularly complicated, the IKr current can be suppressed not only by direct inhibition of the hERG , but also by any interference in their synthesis and/or intracellular trafficking [13].

KE3 to KE4

Inhibition of Ikr leads to Prolongation of Action Potential

Biological Plausibility of the KE3 => KE4 is STRONG.

hERG encodes a voltage-gated potassium channel which is a key component in formation of the cardiac action potential. This channel carries delayed rectifying potassium current (IKr) which underlies repolarization of the cardiac action potential. Pharmacological blockade of the hERG channel results in a slowing of repolarization of the action potential which is reflected as a prolongation of action potential duration [14].

Rapidly activating K current (IKr) blockers prolong action potential (AP) duration (APD) in a reverse-frequency-dependent manner [15].

Virtually every case of a prolonged duration of cardiac action potential related to drug exposure (acquired LQTS) can be traced to one specific mechanism: blockade of IKr current in the heart [16].

KE4 to KE5

Prolongation of Action Potential leads to Prolongation of QT interval

Biological Plausibility of the KE4 => KE5 is STRONG.

Rationale

The acquired long QT syndrome is both a threat to public health and a major stumbling block for drug development. It is most often caused through unintended blockade of the cardiac repolarizing potassium channel, IKr, encoded by the Human Ether-a-go-go related gene (hERG). Blockade of hERG channel was found to be associated with an increased duration of ventricular repolarization and prolongation of QT interval (long QT syndrome, or LQTS) [17].

KE5 to KE6

Prolongation of QT interval leads to Torsade de Pointes

Biological Plausibility of the KE5 => KE6 is STRONG.

The human ether-a-go-go related gene (hERG) or KCNH2 gene encodes a voltage-gated potassium channel known as the hERG channel. This channel plays a key role in cardiac action potential repolarization. Reduced function of hERG causes potential action prolongation and increases the risk for potentially fatal ventricular arrhythmia, torsades de pointes [1].

The blockade of the human ether-a-go-go-related gene (HERG) channel is a major concern for QT prolongation and Torsade de Pointes risk [18].

Drug-induced QT prolongation leading to serious ventricular arrhythmias, such as torsade de pointes (TdP), poses a major safety consideration for the development and use of new drug candidates [5].

KE6 to AO

Torsade de Pointes leads to Sudden cardiac death

Biological Plausibility of the KE6 => AO is STRONG.

Human hereditary long QT syndrome (LQTS) is a heterogeneous cardiac disorder characterized by a prolonged QT interval on the surface ECG and an increased risk for sudden cardiac death due to life-threatening ‘‘torsade de pointes’’ arrhythmias [19].

The importance of hERG (human ether-a-go-go-related gene1) K channels in normal human cardiac electrical activity became strikingly obvious when inherited mutations in HERG were found to cause long QT syndrome (LQTS)2, a cardiac repolarization disorder that predisposes affected individuals to arrhythmia (rapid irregular heart beats that can lead to fainting and sudden death) [20].

Empirical Support for KERs	Defining Question	High (Strong)	Moderate	Low (Weak)
Empirical Support for KERs	Does empirical evidence support that a change in KEup leads to an appropriate change in KEdown? Does KEup occur at lower doses, earlier time points, and higher in incidence than KEdown ? Inconsistencies?	Multiple studies showing dependent change in both events following exposure to a wide range of specific stressors. No or few critical data gaps or conflicting data.	Demonstrated dependent change in both events following exposure to a small number of stressors. Some inconsistencies with expected pattern that can be explained by various factors.	Limited or no studies reporting dependent change in both events following exposure to a specific stressor; and/or significant inconsistencies in empirical support across taxa and species
MIE1 => KE1	High	Abnormal cardiac electrical activity is most often a side effect from unintended block of the promiscuous drug target the human ether-à-go-go-related gene (hERG)—the delayed rectifier K+ channel in the heart [7].
MIE2 => KE2	High	In addition to direct hERG channel block, multiple pharmacological agents can cause hERG deficiency (with hERG channel block or independently) by the inhibition of its biogenesis and trafficking [9].
KE1 & KE2 => KE3	High	There are several types of cardiac K+ channels in the heart that are responsible for different phases of the action potential. Three of them are involved in (late) repolarization. One of these channels, known as hERG (human ethera-go-go-related gene,254 or Kv11.1 or KCNH2, the latter is the name of the gene) is extremely sensitive to inhibition by many compounds. It mediates the so-called rapid component of the delayed rectifier current (IKr current). Most drugs that cause QT interval prolongation are direct inhibitors of the channel, but there are many compounds that block their synthesis/trafficking or interfere at both levels [13].
KE3 => KE4	Moderate	Since almost all compounds that produce TdP in man also inhibit the rapid form of the delayed rectifier potassium current IKr, encoded by the hERG gene, the blockade of this channel and derived electrophysiological consequences on the cellular level including prolongation of action potential duration (APD) [21]. Inhibition of the hERG channel does not always translate into APD prolongation. Martin et al. (2004) investigated the APD prolonging potential of ten hERG blockers in the canine Purkinje fiber model. Only four compounds demonstrated convincing monotonic concentration-dependent APD prolongation. Comparable levels of hERG block did not result in the same APD prolongation [22].
KE4 => KE5	High	The QT interval of the human electrocardiogram (ECG) is a marker of the duration of the cellular action potential (AP) [23]. Loss of hERG function is associated with long-QT syndrome type-2 (LQT2), characterized by impaired ventricular repolarization, extended action potential duration and increased risk of potentially fatal torsades de pointes arrhythmia [24].
KE5 => KE6	Low	Prolongation of the QT interval in telemetered dogs and primates has a high predictive value for QT interval prolongation in man (ILSI workshop ‘‘Cardiovascular Risk Assessment’’, Washington June 3–4, 2003). But accumulating evidence suggests that only a weak correlation exists between QT prolongation and TdP in humans [21]. Although QT prolongation is an essentialfirst stepin TdP, it is usually not considered sufficient toinduce TdP [25].
KE6 => AO	Low	Around 50% of patients with Torsades de Pointes are asymptomatic. The most common symptoms reported are syncope, palpitations, and dizziness. However, cardiac death is the presenting symptom in up to 10% of patients [26]. The administration of an IKr current blocking agent may significantly prolong the QT interval in these silent carriers predisposing them to TdP and sudden cardiac death [27].

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help

The WOE analysis indicates that many KEs and KERs lack especially experimental evidence, but overall the analysis supports the qualitative AOP. For sudden cardiac death, a major drawback is moving from a qualitative AOP to a quantitative AOP. The most pressing future need is an adequate and robust experimental model system for the evaluation of relationships between doses, concentrations and responses within a temporal framework of the AOP.

Considerations for Potential Applications of the AOP (optional)

At their discretion, the developer may include in this section discussion of the potential applications of an AOP to support regulatory decision-making. This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale.To edit the “Considerations for Potential Applications of the AOP” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Considerations for Potential Applications of the AOP” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page or 'Update and continue' to continue editing AOP text sections. The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help

The AOP may be useful in the risk assessment on several types molecules including drugs, as well as other types of chemicals, biocides, or pesticides. This AOP elucidating the pathway from direct and/or indirect hERG inhibition to sudden cardiac death may provide important insights into the potential toxicity of direct and/or indirect hERG inhibitors.

References

List the bibliographic references to original papers, books or other documents used to support the AOP. More help

1. Choi K-E, Balupuri A, Kang NS. The Study on the hERG Blocker Prediction Using Chemical Fingerprint Analysis. Molecules (Basel, Switzerland). 25(11). doi:10.3390/molecules25112615

2. Robert M. Lester & Joy Olbertz (2016) Early drug development: assessment of proarrhythmic risk and cardiovascular safety, Expert Review of Clinical Pharmacology, 9:12, 1611-1618, DOI: 10.1080/17512433.2016.1245142

3.Hancox JC, McPate MJ, El Harchi A, Zhang Y hong. The hERG potassium channel and hERG screening for drug-induced torsades de pointes. Pharmacology and Therapeutics. 2008;119(2):118-132. doi:10.1016/j.pharmthera.2008.05.009

4.Chen WH, Wang WY, Zhang J, Yang D, Wang YP. State-dependent blockade of human ether-a-go-go-related gene (hERG) K(+) channels by changrolin in stably transfected HEK293 cells. Acta Pharmacol Sin. 2010 Aug;31(8):915-22. doi: 10.1038/aps.2010.84. PMID: 20686516; PMCID: PMC4007811.

5.Yao X, Anderson DL, Ross SA, et al. Predicting QT prolongation in humans during early drug development using hERG inhibition and an anaesthetized guinea-pig model. Br J Pharmacol. 2008;154(7):1446-1456. doi:10.1038/bjp.2008.267

6.Aronov AM. Predictive in silico modeling for hERG channel blockers. Drug Discovery Today. 2005;10(2):149-155. doi:10.1016/S1359-6446(04)03278-7.

7.Yang, P.-C. ( 1 ) et al. (no date) ‘A Computational Pipeline to Predict Cardiotoxicity: From the Atom to the Rhythm’, Circulation Research, pp. 947–964. doi: 10.1161/CIRCRESAHA.119.316404.

8.Braga RC, Alves VM, Silva MF, Muratov E, Fourches D, Tropsha A, Andrade CH. Tuning HERG out: antitarget QSAR models for drug development. Curr Top Med Chem. 2014;14(11):1399-415. doi: 10.2174/1568026614666140506124442. PMID: 24805060; PMCID: PMC4593700.

9.Mamoshina P, Rodriguez B, Bueno-Orovio A. Toward a broader view of mechanisms of drug cardiotoxicity. Cell Reports Medicine. 2021;2(3). doi:10.1016/j.xcrm.2021.100216

10.Dennis A, Wang L, Wan X, Ficker E. hERG channel trafficking: novel targets in drug-induced long QT syndrome. Biochem Soc Trans. 2007 Nov;35(Pt 5):1060-3. doi: 10.1042/BST0351060. PMID: 17956279.

11.Calderone V, Testai L, Martinotti E, Del Tacca M, Breschi M. Drug-induced block of cardiac HERG potassium channels and development of torsade de pointes arrhythmias: the case of antipsychotics. JOURNAL OF PHARMACY AND PHARMACOLOGY. 2005;57(2):151-161. doi:10.1211/0022357055272

12.Yu Z, IJzerman AP, Heitman LH. Kv 11.1 (hERG)-induced cardiotoxicity: a molecular insight from a binding kinetics study of prototypical Kv 11.1 (hERG) inhibitors. Br J Pharmacol. 2015 Feb;172(3):940-55. doi: 10.1111/bph.12967. Epub 2014 Dec 15. PMID: 25296617; PMCID: PMC4301700.

13.Mladěnka P, Applová L, Patočka J, Costa VM, Remiao F, Pourová J, Mladěnka A, Karlíčková J, Jahodář L, Vopršalová M, Varner KJ, Štěrba M; TOX-OER and CARDIOTOX Hradec Králové Researchers and Collaborators. Comprehensive review of cardiovascular toxicity of drugs and related agents. Med Res Rev. 2018 Jul;38(4):1332-1403. doi: 10.1002/med.21476. Epub 2018 Jan 5. PMID: 29315692; PMCID: PMC6033155.

14.Jing Y, Easter A, Peters D, Kim N, Enyedy IJ. In silico prediction of hERG inhibition. Future Med Chem. 2015;7(5):571-86. doi: 10.4155/fmc.15.18. PMID: 25921399.

15.Tsujimae K, Suzuki S, Murakami S, Kurachi Y. Frequency-dependent effects of various IKr blockers on cardiac action potential duration in a human atrial model. Am J Physiol Heart Circ Physiol. 2007 Jul;293(1):H660-9. doi: 10.1152/ajpheart.01083.2006. Epub 2007 Jan 12. PMID: 17220183.

16.Aronov AM. Common pharmacophores for uncharged human ether-a-go-go-related gene (hERG) blockers. J Med Chem. 2006 Nov 16;49(23):6917-21. doi: 10.1021/jm060500o. PMID: 17154521.

17.Yu HB, Zou BY, Wang XL, Li M. Investigation of miscellaneous hERG inhibition in large diverse compound collection using automated patch-clamp assay. Acta Pharmacol Sin. 2016 Jan;37(1):111-23. doi: 10.1038/aps.2015.143. PMID: 26725739; PMCID: PMC4722980.

18.Di Veroli GY, Davies MR, Zhang H, Abi-Gerges N, Boyett MR. High-throughput screening of drug-binding dynamics to HERG improves early drug safety assessment. Am J Physiol Heart Circ Physiol. 2013 Jan 1;304(1):H104-17. doi: 10.1152/ajpheart.00511.2012. Epub 2012 Oct 26. PMID: 23103500.

19.Thomas D, Kiehn J, Katus HA, Karle CA. Defective protein trafficking in hERG-associated hereditary long QT syndrome (LQT2): molecular mechanisms and restoration of intracellular protein processing. Cardiovasc Res. 2003 Nov 1;60(2):235-41. doi: 10.1016/j.cardiores.2003.08.002. PMID: 14613852.

20.Sanguinetti MC, Tristani-Firouzi M. hERG potassium channels and cardiac arrhythmia. Nature. 2006 Mar 23;440(7083):463-9. doi: 10.1038/nature04710. PMID: 16554806.

21.Hoffmann P, Warner B. Are hERG channel inhibition and QT interval prolongation all there is in drug-induced torsadogenesis? A review of emerging trends. J Pharmacol Toxicol Methods. 2006 Mar-Apr;53(2):87-105. doi: 10.1016/j.vascn.2005.07.003. Epub 2005 Nov 11. PMID: 16289936.

22.Traebert M, Dumotier B, Meister L, Hoffmann P, Dominguez-Estevez M, Suter W. Inhibition of hERG K+ currents by antimalarial drugs in stably transfected HEK293 cells. Eur J Pharmacol. 2004 Jan 19;484(1):41-8. doi: 10.1016/j.ejphar.2003.11.003. PMID: 14729380.

23.Tse G, Chan YW, Keung W, Yan BP. Electrophysiological mechanisms of long and short QT syndromes. Int J Cardiol Heart Vasc. 2016 Nov 26;14:8-13. doi: 10.1016/j.ijcha.2016.11.006. PMID: 28382321; PMCID: PMC5368285.

24.Foo B, Williamson B, Young JC, Lukacs G, Shrier A. hERG quality control and the long QT syndrome. J Physiol. 2016 May 1;594(9):2469-81. doi: 10.1113/JP270531. Epub 2016 Feb 9. PMID: 26718903; PMCID: PMC4850197.

25.Schwartz PJ, Woosley RL. Predicting the Unpredictable: Drug-Induced QT Prolongation and Torsades de Pointes. J Am Coll Cardiol. 2016 Apr 5;67(13):1639-1650. doi: 10.1016/j.jacc.2015.12.063. PMID: 27150690.

26.Cohagan B, Brandis D. Torsade de Pointes. 2021 Aug 11. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 29083738.

27.Konstantinos P. LETSAS . 2010 . İlaca Bağlı Qt İnterval Uzaması ve Torsade de Pointes: Risk Faktörlerinin Saptanması . Balkan Medical Journal