1. hERG Blocking
2016/01/22
Jun Min Jung

2. Overview
Certain compounds block the cardiac K+ (hERG) ion channel and induce arrhythmia
The Safety margin for hERG is IC50/Cmax, unbound >30
hERG blocking might be decreased by reducing the basicity, reducing lipophilicity, and removing oxygen H-bond acceptors.

3. Heart Structure

4. Resting membrane potential (휴지막 전위)
막 안쪽과 바깥쪽 이온의 농도차이
막에 대한 이온의 투과성 차이

5. Action potential (활동전위)

6. Action potential (활동전위)

7. Action potential, Depolarization, Repolarization, Hyperpolarization

8. Cardiac action potential at the cellular level

10. Pivotal role of hERG channel in QT prolongation
Normal conditions
Electrocardiogram (ECG)
ECG
QT-interval R
P =
QRS = Ventricular depolarization T = Ventricular re-polarization
Auricular depolarisation T P Q S
QT-interval
Ventricular myocyte action potential (AP) AP 1
0 =
1 =
depolarization
transition phase
Na+ in
2 =
slow re-polarization
Ca2+ in 3=
rapid re-polarization 2 3
K+ out K+
out (hERG)
~ 200 ms
Outward K+ flow mediated by hERG channel
K+ flow (hERG mediated)

11. Pivotal role of hERG channel in QT prolongation
Although it is not the only ion channel involved, hERG channel is the most important factor in QT prolongation events, since it plays a key role during re-polarization
ECG
Electrocardiogram (ECG) R
Increase in QT interval (long QT syndrome -> A > 10 %) T P Q S
QT-interval
Ventricular myocyte action potential (AP) AP 1
3= “not so rapid” rapid re-polarization,
due to the lack of contribution of hERG to K+ rapid efflux 2 3
~ 200 ms
Outward K+ flow mediated by hERG channel
K+ flow (hERG mediated)
Drug induced inhibition reduces hERG mediated K+ efflux

12. hERG: human Ether a-go-go Related Gene
Dr. Barry Ganetzky
hERG gene isolated by Dr. Barry Ganetzky in 1994
hERG encodes the a-subunit of human IKr channel
hERG named in relation to its counterpart in fruit flies (EAG)
EAG (ether a-go-go gene) discovered in the 60s by Dr. Kaplan
Flies with mutations in this gene start to shake their legs
when anaesthetised with ether

13. Topology of hERG channel a subunits
Turret segment
Voltage sensor
Pore domain
Out
Selectivity filter
- - +
Pore helix - +
S6 Helix S1 S4 S5 S6 +
S5 Helix
- - - + In
N-terminal
C-terminal
Single a sub unit of hERG channel
Homology model of the pore domain
K+ Channel formed by 4 identical subunits
Each subunit consists of:
Intracellular terminal domains: modulation of channel state by interaction with cytosolic messengers
6 TM domain:
S1-S4 voltage sensor domain: modulation of channel state by changes in conduction
S5-S6 Pore domain: formation of the actual pore and selectivity for K+

14. Topology of pore domain of hERG channel (closed state)
Selectivity filter S5 S6
Homology model (top view)
Homology model (side view)
Single subunit S5-S6 domain
Each subunit contributes with S5-S6 pore domain
Functional pore is a tetramer with C4 symmetry
Selectivity filters facing each other generate a ~6Å pore that mimics the hydration shell of K+ in the op en state of the channel
Homology model based on KcsA (closed state of pore domain)

15. Topology of hERG: K+ efflux in open and closed states
Repolarization K+
Depolarization K+ K+ K+ K+ K+ K
Homology model closed state (side view)
Homology model open state (side view)
Closed (inactive) state: C-terminus ends of S6 cross over limiting the movement of K+ ions
Open (active) state: Gly648 hinge bending results in an increase of aperture size K+
gain access to selectivity filter
Homology model based on KcsA (closed state of pore domain) Homology model based on KvAP (open state of pore domain)
Morais-Cabral et al, Nature,414, (2001)

16. General pharmacophore model of hERG blockers
H-bond donor/acceptor
Positive charge / Aromatic
Hydrophobic group
hERG Homology model
(2 subunits shown)

17. hErg channel blockers

18. terfenadine (antihistamine)
astemizole
(antihistamine)
terfenadine (antihistamine)
grepafloxacin (antibiotic)
sertindole (neuroleptic)

19. terfenadine (antihistamine)
astemizole
(antihistamine)
terfenadine (antihistamine)
grepafloxacin (antibiotic)
sertindole (neuroleptic)

20. “Local” pharmacophores: Amine containing blockers
Ekins’ Pharmacophore N+ N OH
Hydrophobic
Terfenadine OH
Hydrophobic
Hydrophobic
Hydrophobic
Cavalli’s Pharmacophore N+ H N N
Aromatic (C2)
Aromatic (CO) N
Astemizole F N O
Aromatic (C1)
Pearlstein’s Pharmacophore
Aromatic (“Handle”)
Arom/polar (“Tail”) N+ F N N NH N
Sertindole O
Aromatic (“Handle”) Cl
Ekins et al, J. Pharmacol.Exp. Ther., 301, (2002)
Cavalli, J. Med. Chem., 45, (2002)
Pearlstein, J. Med. Chem., 46, (2003)

21. A hERG pharmacophore Lipophilic base, usually a tertiary amine
X = 2-5 atom chain, may include rings, heteroatoms or polar groups

22. Structure Modification Strategies for hERG
Common to binding in the hERG channel

23. Thank You