1. 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
May 25, 2019

2. What is the relationship between hERG ≈ QT prolongation ≈ TdP?
Goal 1: Define the relationship between IKr block and risk for drug-induced QT prolongation and TdP
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?
May 25, 2019

3. other currents & mechanisms
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
May 25, 2019

4. Goal 1. Problem: Quantifying drug-induced IKr block
Potency
Relative potency compared to target pharmacophore
Need to compare to positive control (compare IC50 or percent channel blockade)
Kinetics/voltage dependence
Testing system
Protocol-dependent, cell type, solutions, temperature, intrinsic drug properties, adsorption to tubing/set-up, etc
Subunit interactions
Transfected cell line properties
Protein binding
May 25, 2019

5. Goal 1. Future needs: Quantifying drug-induced IKr block
To standardize potency/effect
Standardize test systems where possible
Standardize the verbiage
May 25, 2019

6. Goal 1. IKr blockade affects on action potential
IKr 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
May 25, 2019

7. Goal 1. IKr blockade affects on action potential
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)
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
May 25, 2019

8. Goal 1. Delayed repolarization (APD/EADs) influences on QT prolongation/TdP
Unknowns:
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?
May 25, 2019

9. Goal 1. Relationship between QT prolongation and TdP
Unknowns:
In face of QT prolongation, what predicts TdP? What terminates TdP?
What percentage of patients with QT prolongation are at risk of TdP?
What percentage of patients with QT prolongation are resistant to TdP?
Concerns:
How is TdP defined clinically? Is there a distinct difference from polymorphic ventricular tachycardia and TdP (associated with long QT)?
How is TdP identified in the clinic? How many cases are not identified and end up as syncope, sudden death or possibly resolve?
May 25, 2019

10. Goal 2. Evolving tools to move to better predictors of drug-induced TdP
In silico modeling in drug safety/mechanisms
In vitro cell biology of IKr
Stem cell research and more
Genetic screening/other biomarkers
May 25, 2019

11. Goal 2. in silico modeling in drug safety/mechanisms
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.
Structural modeling of hERG and other ion channels
May 25, 2019

12. Goal 2. In vitro cell biology of IKr
Future: further understand the regulation and dynamics of the IKr channel.
Lipid/structural influences, subunits, interacting proteins, transcriptional/post-transcriptional regulation, and post-translational processing.
May 25, 2019

13. Goal 2. Consequences of modifying IKr
Altered intracellular calcium dynamics
Activation of CaM Kinase II + ?
Adrenergic changes/autonomic tone
Transcription, translation, etc.
Intracellular magnesium and potassium concentrations
May 25, 2019

14. Goal 2. Cutting Edge Science! Stem cell research and more
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
May 25, 2019

15. Goal 2. Need for Genetic screening/biomarkers
Discovering and characterizing of sequence variants in patient populations. Role of known variants:
For drug screening? (Not yet)
For screening patients for TdP susceptibility
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.
May 25, 2019