This document outlines potential new targets for anti-arrhythmic drug therapy that control spontaneous cardiac activity. It discusses ion channels such as hyperpolarization-activated non-selective cation channels and stretch-activated ion channels, as well as calcium handling mechanisms like calcium-calmodulin dependent protein kinase II, the sodium-calcium exchanger, and ryanodine receptors. Specific drugs that target these channels and proteins are also reviewed, including ivabradine for hyperpolarization-activated channels and potential inhibitors of calcium handling targets.

1. NEW TARGETS THAT
CONTROL SPONTANEOUS
ACTIVITY IN ANTI-
ARRHYTHMIC DRUG
THERAPY.
By
Ezekiel Faith Simisola

2. OUTLINE
• Introduction
• Ion channels that generate spontaneous
activity
- Hyperpolarization activated non
selective cation channels
- Stretch activated ion channels
• Targeting calcium handling mechanisms.

3. INTRODUCTION
• An arrhythmia is an irregular or abnormal heart
rhythm.
• Cardiac arrhythmias are characterized by
disturbances in the heart’s electrical activity and
contribute greatly to cardiovascular morbidity and
mortality.
• Acute myocardial infarction, heart failure,
hypokalaemia, hyperthyroidism, drugs (AADs e.g
digoxin, verapamil, amphetamines, caffeine, cocaine
are common causes.

4. • AADs are still one of the most important
therapeutic options, several challenges in their
administration, including their narrow therapeutic
window and adverse drug reactions, remain to be
solved and should be considered in future drug
discovery. They poses a significant risk of
proarrhythmia, and their side effects often
outweighs their benefits. Therefore, investigating
novel, safe and effective therapeutic options with
AA activity is unavoidable.

5. IMPULSE PROPAGATION
• For the heart to function properly, excitation and contraction of
all myocytes in the heart needs to be coordinated and balanced.

6. HYPERPOLARIZATION ACTIVATED NON SELECTIVE
CATION CHANNELS
• Hyperpolarization-activated cyclic nucleotide–gated (HCN)
channels are integral membrane proteins that serve as non
selective voltage-gated cation channels in the plasma
membranes of heart cells, initially discovered some over 20 years
ago.
• They are referred to as pacemaker channels because they help to
generate rhythmic activity within groups of heart cells.
• HCN channels are activated by membrane hyperpolarization, are
permeable to Na + and K +, and are constitutively open at voltages
near the resting membrane potential.
• HCN channels are of four isoforms (HCN 1,2,3,4).

7. • The current through HCN channels, designated If or Ih,
plays a key role in the control of cardiac rhythmicity and
is called the pacemaker current or "funny" current.
• HCN4 is the main isoform expressed in the sinoatrial
node, but low levels of HCN1 and HCN2 have also been
reported.
• The current through HCN channels, plays a key role in the
generation and modulation of cardiac rhythmicity, as
they are responsible for the spontaneous depolarization
in pacemaker action potentials in the heart and as such
interesting targets for AAD therapy.
HYPERPOLARIZATION ACTIVATED NON SELECTIVE
CATION CHANNELS

8. IVABRADINE
• Ivabradine is a heart-rate-lowering agent that acts by
selectively and specifically inhibiting the cardiac pacemaker
current (If) (a mixed sodium-potassium inward current that
controls the spontaneous diastolic depolarization in the
sinoatrial node and hence regulates the heart rate).
• Inhibition of this channel (HCN4) disrupts If ion current flow,
thereby prolonging diastolic depolarization, slowing firing in
the SA node, and ultimately reducing the heart rate.
• The cardiac effects of ivabradine are specific to the SA node,
and the drug has no effect on blood pressure, intracardiac
conduction, myocardial contractility, or ventricular
repolarization.
HCN INHIBITORS

9. IVABRADINE
HCN INHIBITORS
• After oral administration, the drug is rapidly and almost
completely absorbed from the GIT.
• Oral bioavailability of ivabradine is approximately 40%
because of the first-pass effect in the liver and intestines.
• Food delays the absorption of ivabradine by
approximately one hour.
• Ivabradine is metabolized predominantly in the liver and
intestines by the cytochrome P450 (CYP) 3A4 enzyme.

10. • The recommended starting dosage of ivabradine is
5 mg twice daily, administered with food. After two
weeks, the patient should be assessed, and dose
adjustments should be made to achieve a resting
heart rate of between 50 bpm and 60 bpm.
Thereafter, further dose adjustments (if necessary)
should be based on the patient’s resting heart rate
and tolerability.
• The maximum dosage of ivabradine is 7.5 mg twice
daily.
IVABRADINE

11. STRETCH-ACTIVATED ION CHANNELS
(SACs)
• Were discovered in 1983 in embryonic chick
skeletal myocytes by Faluni Guharay and Frederick
Sachs.
• In subsequent years, SACs have been identified in
many other cell types including cardiomyocytes.
• Stretch-activated ion channels (SAC) have been
identified as one contributor to mechanosensitive
autoregulation of the heartbeat.

12. • They also appear to play important roles in the
development of cardiac pathologies – most
notably stretch-induced arrhythmias.
• As recently discovered, some established cardiac
drugs act, in part at least, via
mechanotransduction pathways suggesting SACs
as potential therapeutic targets.
• Cardiac SACs can be either cation non-selective
(SACNS) or potassium-selective (SACK)
STRETCH-ACTIVATED ION CHANNELS
(SACs)

13. • SACs are activated rapidly (within tens of
milliseconds) and lead to increased ion transients,
which result in rapid alterations of cardiac electrical
activity.
• In cardiomyocytes, SACs activation has been shown
to result in membrane depolarization and increase
action potential duration.
• Permeable to sodium, potassium and calcium ions.
STRETCH-ACTIVATED ION CHANNELS
(SACs)

14. • Several pharmacological compounds have been
identified to modulate SACs activity and their
potential role as pharmacological tools for heart
rhythm management.
• Most of the known SAC-modulators are non-
specific inhibitors, such as Gadolinium ions,
Amiloride and Cationic antibiotics (streptomycin,
penicillin, kanamycin).
STRETCH-ACTIVATED ION CHANNELS
(SACs)

15. • SACs blockers such as Streptomycin and Gadolinium
have been shown to inhibit the intracellular
accumulation of Ca2+ ions thereby reducing contraction.
• Among the very few specific SACs inhibitors reported so
far is the peptide GsMTx-4, isolated from a spider
venom.
• The mode of action of GsMTx-4 is thought to involve
insertion into the outer membrane leaflet in the
proximity of the channel, relieving lipid stress and
favouring the closed state of SACs.
STRETCH-ACTIVATED ION CHANNELS
(SACs) INHIBITORS

16. DRUGS THAT AFFECT CALCIUM
HANDLING
New targets include;
Calmodulin-dependent protein kinase II (CaMKII),
The Na/Ca exchanger (NCX),
The Ryanodine receptor (RyR), and its associated
protein FKBP12.6 (Calstabin).
• All these proteins are related to intracellular
calcium (Ca2+) handling.

17. • Drugs that modify these targets are currently
being investigated in order to achieve clinical
applicability in cardiac arryhthmias.
• The focus is on interfering with calcium
handling of the cardiomyocytes and to
become active in the prevention or
suppression of VTs.
DRUGS THAT AFFECT CALCIUM
HANDLING

18. SODIUM-CALCIUM EXCHANGER (NCX)
• The Cardiac sodium-calcium exchanger plays
an important role in calcium homeostasis. It is
the primary mechanism of removing calcium
ions that enters the myocytes through the
LTCC on a beat to beat basis.
• NCX is implicated in the mechanism of
arrhythmias, hence NCX blockade represents
potential therapeutic strategy in the
treatment of arrhythmias.

19. • Known inhibitors of NCX are KB-R7943 and SEA-0400.
• They have limited selectivity and efficacy.
• Blocking NCX theoretically leads to Ca2+ accumulation and
increased SR Ca2+ load, which in turn could lead to adverse
effects such as Ca2+ sparks.
• Nonetheless, due to the fact that SEA-0400 probably
simultaneously inhibits LTCC, thereby inducing negative
inotropic effects, this counteraction possibly preserves
cardiac output.
SODIUM-CALCIUM EXCHANGER (NCX)
INHIBITORS

20. • Moreover, NCX has a profound role in EAD
formation. Therefore, blocking of NCX
potentially exerts antiarrhythmic effects.
• SEA-0400 has recently been shown to
effectively prevent torsades des pointes
(TdP) arrhythmia and EAD formation and, as
important, without the occurrence of the
negative inotropic effects that are typically
observed when LTCC is blocked alone
SODIUM-CALCIUM EXCHANGER (NCX)
INHIBITORS

21. RYANODINE RECEPTOR (RyR)
• Spontaneous release of Ca2+ by RyR is involved in the
generation of triggered activity and as such RyR is an
interesting target in antiarrhythmic therapy.
• This spontaneous release of Ca2+ is causative for
arrhythmias as found in a disease named
catecholaminergic polymorphic ventricular
tachycardia (CPVT).
• Mutations in RyR can enhance the susceptibility for
Ca2+ leak, especially under conditions with an
increased adrenergic drive.

22. • K201 (JTV-519) is a more recent compound that
has been tested as a potential inhibitor of RyR. It
decreased spontaneous Ca2+ release by binding to
calstabin, thereby increasing its affinity for RyR
and stabilisation of the closed conformation of
RyRs
• Although K201 has already been used in clinical
trials for treatment of atrial fibrillation (AF), the
results seem disappointing: only one has been
completed without publication of the results, and
two other trials were prematurely terminated.
RYANODINE RECEPTOR INHIBITOR

23. RYANODINE RECEPTOR INHIBITION
• Carvedilol, a registered β-blocker, also blocks RyR and
prevents spontaneous Ca2+ release at doses higher than
needed for its β-blocking activity.
• VK-II-86, a carvedilol analogue shows an enhanced
specificity regarding the RyR blocking capacities.
• It has been suggested that in combination with a potent
β-blocker, this could be a promising antiarrhythmic
approach although thus far no further studies have been
published using these compounds in order to test their
efficacy.

24. CALMODULIN PROTEIN KINASE
II(CaMKII)
• CAMKII activation is mainly by camodulin (CaM),
a small cytoplasmic protein. CaM binds to cystolic
calcium thereby activating CAMKII.
• Under pathological condition, CAMKII is activated
by reactive oxygen species (ROS).
• CaMKII activation is indirectly dependent on
[Ca2+]i, due to its capability of
autophosphorylation. It is not solely dependent
on the rise and fall in Ca2+.

25. • When phosphorylation of CaMKII has been
accomplished, the enzyme becomes persistently
active and therefore the natural beat-to-beat fall in
Ca2+ will not immediately affect the enzymes’
activity.
• CaMKII has a central role in Ca2+ handling,
influencing RyR, LTCC, and SERCA.
• By phosphorylating RyR, CaMKII increases its
sensitivity to calcium ions allowing the release of
calcium ions from the SR.
CALMODULIN KINASE II(CaMKII)

26. • LTCC phosphorylation by CaMKII promotes its
opening and slow the channel inactivation process.
• CaMKII on SERCA (via phosphorylation of PLB) leads
to an increased SR Ca2+ load.
• CaMKII activation leads to an increase in [Ca2+]i and
is therefore able to influence NCX by pushing it into
the forward mode which results in a depolarising
current.
CALMODULIN DEPENDENT PROTEIN
KINASE II(CaMKII)

27. CALMODULIN DEPENDENT PROTEIN
KINASE II(CaMKII)
• All of the above-mentioned actions are under
physiological conditions.
• During cardiac pathology CaMKII expression
and function is upregulated, which can trigger
proarrhythmia via induction of ectopic
activity. This makes it an interesting target for
antiarrhythmic intervention

28. CaMKII INHIBITION
• Two CaMKII inhibitors are known, W7 and KN-93.
• KN-93 is a compound that competes with CaM for the
binding site on CaMKII, and through this mode it inhibits
activation of CaMKII
• However, KN-93 also appears to act as a multi-channel
blocker in cardiomyocytes
• W7 is actually an inhibitor of CaM and therefore considered
to be an indirect inhibitor of CaMKII as well as of other
targets of CaM (e.g. RyR, LTCC).