Transcript HESI/ILSI Health and Environmental Sciences Institute

HESI/ILSI Health and Environmental Sciences
Institute
Workshop session I: Molecular and Cellular Biology
Underlying TdP
Co-Chairs: Craig January and Dan Roden
Rapportuers: Kristy Bruse and Ying Ying Zhou
Workshop participants: Blake Anson, Siham Biade, Eve Bijaoui, Albert
Defelice, Michael Nabauer, Guy Salama, Peter Siegle, Steve Sorota,
Antoniao Zaza, Ravikumar Peri, Karin Sipido
July 16, 2015
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Goal 1: Define the relationship between IKr
block and risk for drug-induced QT
prolongation and TdP
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What is the relationship between hERG ≈ QT
prolongation ≈ TdP?
Genetic influences
Cellular changes
Single channel event
APD event
Other mechanisms
Clinical event
Are there other ways to predict risk for TdP?
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Goal 1. The relationship between drug-induced IKr
block and risk for drug-induced QT prolongation/TdP
IKr blockade
other currents & mechanisms
Increased APD/EADS
cell Ca2+, ?
Increased QT interval
electrical heterogeneity, ?
TdP event
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Goal 1. Problem:
Quantifying drug-induced IKr block
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Potency
 Relative
potency compared to target
pharmacophore
 Need to compare to positive control (compare IC50
or percent channel blockade)
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Kinetics/voltage dependence
Testing system
 Protocol-dependent,
cell type, solutions,
temperature, intrinsic drug properties, adsorption to
tubing/set-up, etc
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Subunit interactions
Transfected cell line properties
Protein binding
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Goal 1. Future needs:
Quantifying drug-induced IKr block
To standardize potency/effect
 Standardize test systems where possible
 Standardize the verbiage
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Goal 1. IKr blockade affects on action potential
 IKr
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blockade  other mechanisms? 
action potential effects
Discordance: What is an EAD?
 There is not agreement as to the
electrophysiological “shape” of an EAD.
Agreement: irrespective of the shape definition,
EADs can enhance dispersion of repolarization
and/or cause triggered activity TdP
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Goal 1. IKr blockade affects on action potential
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Unknown: what are these “other mechanisms”?
 Altered
channel trafficking as an alternate mechanism to
reduce IKr, then how often? how important?
 Other inward/outward current data may be useful for
predictivity
 Could in silico assist in assimilation of other ion current
data results
 AP interpretation from In vitro data- need a positive
control reference and understanding of the
strengths/weaknesses of the assay(s)
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Future: should we evaluate other ion channels to
better predict risk for TdP? Do not over-interpret
the IKr data
 Note,
other targets may be “protective” against the IKr
blockade
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Goal 1. Delayed repolarization (APD/EADs)
influences on QT prolongation/TdP
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Unknowns:
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July 16, 2015
How do APD/EADs perturb QT duration or
morphology alterations (e.g. TdP) ?
Rate dynamicity- is this important? Does it matter
how you change the rate?
Is there an optimal heart rate correction factor for
QT interval (e.g. QTc)? Is the beat to beat variance
of QT a more relevant evaluation?
How are U-waves interpreted?
What is QT prolongation?
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Goal 1. Relationship between
QT prolongation and TdP
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Unknowns:
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In face of QT prolongation, what predicts TdP?
What terminates TdP?
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What percentage of patients with QT prolongation
are at risk of TdP?
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What percentage of patients with QT prolongation
are resistant to TdP?
Concerns:
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How is TdP defined clinically? Is there a distinct
difference from polymorphic ventricular tachycardia
and TdP (associated with long QT)?
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How is TdP identified in the clinic? How many cases
are not identified and end up as syncope, sudden
death or possibly resolve?
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Goal 2. Evolving tools to move to better
predictors of drug-induced TdP
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In silico modeling in drug safety/mechanisms
In vitro cell biology of IKr
Stem cell research and more
Genetic screening/other biomarkers
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Goal 2. in silico modeling in drug
safety/mechanisms
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In silico- hypothesis generators
 Pharmacophore (QSAR) modeling-rank ordering tool
for early development
 Purpose- to refine chemical structure design for
future in vitro testing
 Modeling for AP/whole heart
Advantages: test hypotheses not otherwise accessible
Limitations: never “proves” anything. Limited to current
knowledge.
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Structural modeling of hERG and other ion channels
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Goal 2. In vitro cell biology of IKr
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Future: further understand the regulation and
dynamics of the IKr channel.
 Lipid/structural influences, subunits,
interacting proteins, transcriptional/posttranscriptional regulation, and posttranslational processing.
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Goal 2. Consequences of modifying IKr
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Altered intracellular calcium dynamics
 Activation of CaM Kinase II + ?
Adrenergic changes/autonomic tone
Transcription, translation, etc.
Intracellular magnesium and potassium
concentrations
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Goal 2. Cutting Edge Science!
Stem cell research and more
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5+ year deliverable
Direct high throughput screening for drug effects
on action potential, Ca2+i, arrythmogenicity of
mammalian and human myocytes
This may also include transgenic non-rodent
animal models
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Goal 2. Need for Genetic
screening/biomarkers
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Discovering and characterizing of sequence
variants in patient populations. Role of known
variants:
 For
drug screening? (Not yet)
 For screening patients for TdP susceptibility
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Global effort- academia, industry, regulatory
agencies: Concerted effort to ascertain
information (e.g. DNA, serum, ECG samples)
from a large number patients with drug-induced
TdP. Platform for discovery.
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