Targeting Voltage-Gated Sodium Channels

Question 1

Blocking pore from outside

Answer 1

• TTX and STX
• physically occlude pore from outside
• bind 1:1
• block by electrostatic interactions
• subcutaneous TTX in trials for neuropathic pain
• D + E from DEKA motif and P2 helix residues are involved

Question 2

Brugada syndrome

Answer 2

• loss of function of Nav1.5
• ventricular tachycardia can lead to death

Question 3

Loss of function of Nav1.5

Answer 3

• Brugada syndrome
• sick sinus syndrome
• dilated cardiac myopathy
• familial atrial fibrillation
• cardiac conduction diseases

Question 4

Congenital insensitivity to pain

Answer 4

• loss of function of Nav1.7
• target for decreasing pain?

Question 5

Dravet sydrome

Answer 5

• childhood epilepsy
• loss of function of Nav1.1
• seizures due to reduced GABAergic activity

Question 6

Myotonias

Answer 6

• muscle fails to relax
• repetitive AP generation
• gain of function Nav1.4

Question 7

Paroxysmal extreme pain disorder

Answer 7

• PEPD
• mutations in Nav1.7 so opens more easily and for longer
• increased Nav activity

Question 8

Small fibre neuropathy

Answer 8

• gating impaired in Nav1.7
• neuropathic pain

Question 9

Hypokalaemic periodic paralysis

Answer 9

• weakness with low potassium levels
• VSDs become leaky
• Nav1.4 inactivated

Question 10

Pore binders

Answer 10

• CBD
• BTX and VTD (activating toxins)

Question 11

Pore binders - CBD

Answer 11

• 2 sites
• approved for treatment of childhood epilepsy
• allosterically stabilises inactive state
• shifts equilibrium

Question 12

Pore binders - activating toxins

Answer 12

• e.g. batrachotoxin (BTX) and veratridine (VTD)
• bind open Nav

Question 13

Gating-modifying toxins

Answer 13

• scorpion toxins change gating
• positions against DII-S4 and S3-S4 loop
• beta-ScTx = Nav activator
• traps VSD in up position
• ProTx2 = Nav1.7 inhibitor
• binds VSD in up and down position

Question 14

Isoform-specific inhibitors

Answer 14

• toxineering
• selectivity for Nav1.7 possible
• structure-guided drug discovery to modify toxins
• small molecule toxin mimetics (e.g. Nav1.7 selective inhibitor in phase I+II clinical trials but abandoned by Pfizer)

Question 15

Nav therapeutics

Answer 15

approved:
- local anaesthetics
- anti-arrhythmics
- anti-epileptics
- anti-manics
- anti-myotonics

in the development pipeline:
- selective targeting of Navs involved in inflammatory and neuropathic pain
- important due to dependency issues with opioids
- selectivity difficult due to high sequence conservation

Question 16

Anti-epileptics

Answer 16

• use-dependence
• non-selective
• bind overactive Navs
• preferentially block neurones that are firing repetitively
• binding site overlaps with that of local anaesthetics = shown by in silico stimulation and amino acid mutagenesis

Question 17

Local anaesthetics

Answer 17

• block inner pore
• access through lateral fenestrations

Question 18

Class I anti-arrythmics

Answer 18

• block inner pore
• similar to local anaesthetic site
• use-dependence

1A e.g. quinidine
- also block Kv channels
- prolong repolarisation

1B e.g. lidocaine
- shortened repolarisation

1C e.g. flecainide
- most Nav block
- no change in repolarisation