Evaluation of antiarrhythmicdrugs

EVALUATION OF ANTIARRHYTHMIC DRUGS

Presented by:Syed KashifDepartment of PharmacologyAACP

2

CONTENTS

Introduction- Basic Understanding of Arrhythmia & ECG. Classification Of Drugs

Conclusion.

In – Vitro methods:1) Isolated guinea pig papillary muscle.2) Langendorff technique3) Acetylcholine & potassium Induced arrhythmia

In-Vivo Methods: Chemically induced

arrhythmiaElectrically induced

arrhythmiaMechanically induced

arrhythmia Exercise Induced Arrhythmia

Cardiac Arrhythmias

Arrhythmia means an Abnormal heart rhythm

Results from the abnormalities of:

Impulse generation (Rate or Site of origin)

Conduction Both

Arrhythmia /dysrhythmia: abnormality in the site of origin of impulse, rate, or

conduction

If the arrhythmia arises from the ventricles it is

called ventricular arrhythmia

If the arrhythmia arises from atria, SA node, or AV node it is called supraventricular

arrhythmia

Causes of arrhythmia

arteriosclerosis

Coronary artery spasm

Heart block

Myocardial ischemia

ARRHYTHMIA

Arrhythmia /dysrhythmia: abnormality in the site of origin of impulse, rate, or

conduction

If the arrhythmia arises from the ventricles it is

called ventricular arrhythmia

If the arrhythmia arises from atria, SA node, or AV node it is called supraventricular

arrhythmia

Causes of arrhythmia

arteriosclerosis

Coronary artery spasm

Heart block

Myocardial ischemia

Non-pacemaker action potential

Phase 0: fast upstroke

Due to Na+ influx Phase 3:

repolarizationDue to K+

efflux

Phase 4: resting membrane potential

Phase 2: plateu

Due to Ca++ influx

Phase 1: partial repolarization

Due to rapid efflux of K+

N.B. The slope of phase 0 = conduction velocityAlso the peak of phase 0 = Vmax

CLASSIFICATION OF ARRHYTHMIAS1. Characteristics:

a. Flutter – very rapid but regular contractionsb. Tachyarrhthmia – increased ratec. Bradyarrhythmia – decreased rated. Fibrillation – disorganized contractile activity

2. Sites involved:

a. Ventricularb. Atrialc. SA Noded. AV Node

Supraventricular

MECHANISMS OF CARDIAC ARRHYTHMIAS

(A) Enhanced Automaticity:

In cells which normally display spontaneous diastolic depolarization (SA Node, AV Node, His-Purkinje System)

Automatic behavior in sites that ordinarily lack pacemaker activity

A normal cardiac action potential may be interrupted or followed by an abnormal depolarization

Reaches threshold & causes secondary upstrokes

2 Major forms:

1. Early Afterdepolarization2. Late Afterdepolarization

(B) After depolarization and Triggered Automaticity

(C) Re-entrant Arrhythmia

Defined as circulation of an activation wave around an inexitable object

3 requirements for Re-entrant Arrhythmia:

1. Obstacle to conduction

2. Unidirectional block

3. CT>ERP

Unidirectional Block

Establishment of Re-entrant circuit

NORMAL HEARTBEAT AND ATRIAL ARRHYTHMIA

Normal rhythm Atrial arrhythmia

AV septum

NORMAL ECG : P wave – Atrial depolarisation. QRS wave – ventricular depolarisation. T wave – ventricular repolarisation.

Management Of

Arrhythmias

MANAGEMENT

Non Pharmacological

Acute Management Prophylaxis

Pharmacological

NON PHARMACOLOGICAL Acute1. Vagal Maneuvers2. DC Cardioversion

Prophylaxis1. Radiofrequency Ablation2. Implantable Defibrillator

PHARMACOLOGICAL APPROACH

Drugs may be antiarrhythmic by:

Suppressing the initiator mechanism Altering the re-entrant circuit

1. Terminate an ongoing arrhythmia2. Prevent an arrhythmia

VAUGHAN WILLIAMS CLASSIFICATION

Phase 4

Phase 0

Phase 1

Phase 2

Phase 3

0 mV

-80mV

II

IIII

IV

Class I: block Na+ channels Ia (quinidine, procainamide, disopyramide) (1-10s)Ib (lignocaine) (<1s)Ic (flecainide) (>10s)

Class II: ß-adrenoceptor antagonists (atenolol, sotalol)

Class III: block K+ channels (amiodarone, dofetilide,sotalol)

Class IV: Ca2+ channel antagonists (verapamil, diltiazem)

21

CLASSIFICATION OF ANTIARRHYTHMICS(BASED ON MECHANISMS OF ACTION)

Class I – Blockers of fast Na+ channels Subclass IA

Cause moderate Phase 0 depression Prolong repolarization Increased duration of action potential Includes

Quinidine – 1st antiarrhythmic used, treat both atrial and ventricular arrhythmias, increases refractory period

Procainamide - increases refractory period but side effects Disopyramide – extended duration of action, used only for

treating ventricular arrthymias

22


Subclass IB Weak phase 0 depression Shortened depolarization Decreased action potential duration Includes

Lidocane (also acts as local anesthetic) – blocks Na+ channels mostly in ventricular cells, also good for digitalis-associated arrhythmias

Mexiletine - oral lidocaine derivative, similar activity Phenytoin – anticonvulsant that also works as antiarrhythmic

similar to lidocane

23


Subclass IC Strong Phase 0 depression No effect on depolarization No effect on action potential duration

Includes Flecainide (initially developed as a local anesthetic)

Slows conduction in all parts of heart, Also inhibits abnormal automaticity

Propafenone Also slows conduction Weak β – blocker Also some Ca2+ channel blockade

24

CLASSIFICATION OF ANTIARRHYTHMICS(BASED ON MECHANISMS OF ACTION) Class II – β–adrenergic blockers

Based on two major actions1) blockade of myocardial β–adrenergic receptors2) Direct membrane-stabilizing effects related to Na+ channel blockade

Includes Propranolol

causes both myocardial β–adrenergic blockade and membrane-stabilizing effects

Slows SA node and ectopic pacemaking Can block arrhythmias induced by exercise or apprehension Other β–adrenergic blockers have similar therapeutic effect

Metoprolol Nadolol Atenolol Acebutolol Pindolol Stalol Timolol Esmolol

25


Class III – K+ channel blockers Developed because some patients negatively

sensitive to Na+ channel blockers (died!) Cause delay in repolarization and prolonged

refractory period Includes

Amiodarone – Prolongs action potential by delaying K+ efflux but many other effects characteristic of other classes

Ibutilide – slows inward movement of na+ in addition to delaying K + influx.

Bretylium – First developed to treat hypertension but found to also suppress ventricular fibrillation associated with myocardial infarction

Dofetilide - Prolongs action potential by delaying K+ efflux with no other effects

26


Class IV – Ca2+ channel blockers slow rate of AV-conduction in patients with atrial

fibrillation

Includes Verapamil – blocks Na+ channels in addition to Ca2+;

also slows SA node in tachycardia Diltiazem

BradyarrhythmiasResting heart rate of <60/min

Classified as Atrial/AV Nodal/Ventricular

Management:

• Acute→ i/v atropine

• Permanent→ Pacemakers.

28

EXPERIMENTAL EVALUATION OF ANTIARRHYTHMIC DRUG ACTION…………..

29

Evaluation of Antiarrhythmic Drug Action In-Vitro Models: 1) Isolated guinea pig papillary muscle.2) Langendorff technique3) Acetylcholine & potassium Induced arrhythmia

In-Vivo Methods:

Chemically induced arrhythmia Electrically induced arrhythmia Mechanically induced arrhythmia Exercise Induced Arrhythmia

EVALUATION OF ANTIARRHYTHMIC DRUGS

30

IN VIVO METHODS

IN-VIVO METHODS :

I. Chemically induced arrhythmiaII. Electrically induced arrhythmiaIII. Mechanically induced arrhythmiaIV. Exercise Induced arrhythmia.

I.CHEMICALLY INDUCED ARRHTHMIA A large number of chemical agents alone or

in combination are capable of inducing arrhythmia.

ACONITINE ANTAGONISM IN RATSPRINCIPLE: Aconite persistently activate

sodium channel.METHOD:

Continous infusion in saphenous vein

ACONITE 5mg/kg dissolved in 0.1 N HN03

MONITOR LEAD IIECG EVERY 30

SECONDS

Anaesthetize with Urathene

1.25 g/kg

Test compound 3 mg/kg orally

or IV 5 minutes before aconite infusion

EVALUATION The anti-arrhythmic effect of the test compound is

measured by the amount of test drug, required to precipitate aconitine/100 gm animal

Ventricular extra systoles. Ventricular tachycardia. Ventricular fibrillation and death. E.g- procainamide 5 mg/kg IV & lidocaine 5 mg/kg IV leads

to increase in lethal dose by 65%.

35

STROPHANTHIN OR OUABAIN INDUCED ARRHYTHMIA IN DOG:

Principle :Strophanthin K induces ventricular tachycardia and multifocal ventricular arrhythmia in dogs.

METHOD: Dogs of either sex are first anesthetized & artificial respiration

is given

Two peripheral veins are cannulated for the administration of the arrhythmia

inducing substance and the test compound.

For intraduodenal administration of the test drug, the duodenum is cannulated..

36

METHOD CTD..Strophanthin K administered by continuous i.v infusion at a rate of 3mg/kg/min

When the arrhythmias are stable for 10 min, the test substance is

administered i.v ( 1.0 and 5.0 mg/kg) or i.d ( 10 and 30 mg/kg)

ECG recordings are obtained at 0.5, 1, 2, 5 and 10 min following administration of test drugs and further duration increased if required

37

EVALUATION

Ajmalin , quinidine and lidocaine reestablish normal sinus rhythm at 1 mg/kg ,3 mg/kg iv and 10 mg/kg id.

Test compound is said to have an anti

arrhythmic effect if extra systoles

disappear within 15 min .

The test drug is considered to have “

NO Effect” if it does not improve strophanthin intoxication within 60

min.

Test compound is said to have an anti arrhythmic effect if the extra systoles immediately disappear . If not then the increasing doses are administered at 15 min- intervals. If the test substance does reverse the arrhythmias the next dose is administered after the reappearance of stable arrhythmias.

I.DI.V

ADRENALINE INDUCED ARRHTHMIA PRINCIPLE: Adrenaline at high dose

may precipitate arrhythmia.

METHOD:

Evaluation: A test compound is said to have antiarrhythmic effect if the extrasystole disappears immediately after drug administration.

Dogs (10-11 kg) are anaesthetized with

pentobarbitone (30-40 mg/kg ) i.p

Femoral vein is cannulated and

adrenaline is infused at a rate of 2-2.5 mg/kg via

femoral cannula

Test drug is administered 3 mins. After adrenaline infusion and

Lead II ECG Recorded.

II.ELECTRICALLY INDUCED ARHYTHMIAS

VENTRICULAR FIBRILLATION ELECTRICAL THRESHOLD

Principle :Ventricular fibrillations can be induced by various techniques of electrical stimulation like single pulse stimulation , train of pulse stimulation , continous 50 HZ stimulation.

Ventricular fibrillation threshold :The minimal current intensity of the pulse train required to induce sustained ventricular fibrillation.

Requirement: Animals – Adult dogs (8-12kg) Anesthetic – Sodium pentobarbital (35mg/kg)

PROCEDURE

↓

Sinus node is crushed and electrical stimulation is provided with Ag-AgCl stimulating electrode embedded in a

Teflon disc sutured to anterior surface of left venticle.

Chest cavity is openedHeart suspended in pericardial

cradle

Artificial respiration – Harvard respiratory pumpB.P – monitored

Body temperature – maintained by thermal blanket

Adult dogs are anaesthetized and heart is exposed.

PROCEDURE CTD…

Test drug/ std/control Drugs are administered through

the femoral vein.

A nodal const. current for 400ms is applied through the

driving electrode

ECG of lead II is recorded

EVALUATION To determine VFT :- A 0.2 to 1.8 second train of

50 Hz pulses is delivered 100 ms after every 18thbasic driving stimulus. The current intensity is increased from diastolic threshold. When ventricular fibrillation occurs, the heart is immedietely defibrilleted and allowed to recover for 15-20 mins.

VFT is determined before and after administration of test drugs at given time intervals.

Compared using student - t Test.

III.MECHANICALLY INDUCED ARRHYTHMIA Coronary artery

occlusion/reperfusion arrhythmia

CORONARY ARTERY OCCLUSION/REPERFUSION ARRHYTHMIA

Arrhythmias –induced directly by ischemia and reperfusion

• Coronary artery ligation in anesthetized dog results in:

↑ in HR ↑in heart contractility ↑ in BP Ventricular arrhythmias

REQUIREMENTSAnimals – Dogs (20-25Kg)

Anesthetic – Thiobutobarbital sodium (30mg/kg)

PROCEDURE:Animals – selected

↓Anesthetized

↓Artificial respiration – positive pressure respirator

↓Femoral artery – cannulated and connected to

pressure transducer↓

Chest cavity –openedLAD is exposed

↓

Silk suture is placed around LAD↓

After 45 min(equilibration) – test/std/control- administered through saphenous vein

↓After 20 min- ligature of coronary artery is closed

for 90 min↓

Occlusion released – reperfusion period maintained for 30 min.

• All the parameters – recorded• At the end – surviving animals are sacrificed by

an overdose of Pentobarbital sodium.

EVALUATION

Mortality Hemodynamics Arrhythmia Ventricular fibrillation % animals with VF.

IV .EXERCISE INDUCED ARRHYTHMIA

EXERCISE INDUCED VENTRICULAR FIBRILLATION PURPOSE AND RATIONALE: Tests combining coronary constriction with physical

exercise may resemble most closely the situation in coronary patients

• REQUIREMENTS:Animals – Mongrel dogs (15 -19 kg)

Anesthetic – Sodium pentobarbitone (10mg/kg) i.v.SURGICAL PREPARATION:

Animals – selected↓

Anesthetized↓

Chest cavity – opened↓

Heart suspended in pericardial cradle↓

Around left circumflex artery – Doppler flow inducer- to measure blood pressure

- Hydraulic occluder - to occlude coronary artery are placed

↓Pair of insulated silver coated wires – sutured on left and

right ventricles- measure HR (By Gould biotachometer)↓

Occlusion of LAD(Two stage)↓

Myocardial infarction↓

After 24 hrs- test drug/std/control - administered During this experiment:• Transdermal fentanyl patch (↓post operative discomfort)• Bupivacaine HCl• Antibiotic therapy(amoxicillin)

IN VITRO METHOD:

1) Isolated guinea pig papillary muscle. 2) Langendorff technique 3) Acetylcholine & potassium Induced

arrhythmia

1)ANTI-ARRHYTHMIC ACTIVITY IN THE ISOLATED RIGHT VENTRICULAR GUINEA-PIG PAPILLARY MUSCLE:

Principle: In right ventricular guinea pig papillary muscle, developed tension (DT), excitability (EX) and effective refractory period (ERP) are measured.

METHOD:The heart is removed & placed into a pre-

warmed, pre-oxygenated PSS & right ventricle is opened

The tendinous end of the papillary muscle is ligated with a silk thread and the chordae

tendinae are freed from the ventricle

The preparation is mounted in organ bath & experimental conditions are maintained

The silk thread is used to connect the muscle to forced transducer and muscles are field stimulated to contract isometrically at a

duration of 1 ms.

Pulses are delivered using constant voltage stimulator and

the developed tension is recorded using polygraph recorder.

The force frequency curve is obtained by measuring the

developed tension over a range of stimulus frequencies

(0.3,0.5,0.8,1.0,1.2 HZ)

The percentage change in post treatment versus pretreatment

developed tension at 1 HZ is used to quantitate the agents inotropic effect.

EVALUATION:

Change in Effective Refractory Period (Post treatment minus pretreatment)

Degree of shift in the strength-duration curve.i.e.(area between post and pre treatment curve)

percentage changes in post treatment developed tension. Duration of action potential. Contraction force. The results of above calculations are to classify the

compounds as class I ,III or IV antiarrhythmic agents on the basis of its effect on developed tension, excitability and effective refractory period.

EVALUATION MOA OF DRUGEXCITABILITY Na+ ChannelContraction Force Ca+2 ChannelEffective Refractory period

K + Channel

LANGENDORFF TECHNIQUE

PRINCIPLE: Heart is perfused in a retrograde direction from aorta either at constant pressure or at constant flow with oxygenated saline soln.

Retrograde perfusion closes the aortic valve , just as in situ heart during diastole .

The perfusate is displaced through the coronary artery using a canula inserted in the ascending aorta following of the coronary sinus and opened right atrium and flows out via the right ventricle and pulmonary artery.

PROCEDURE Animal – Guinea pig (300-500g)

Animal – selected

Sacrificed (stunning)

heart – removed – placed in Ringer’s solution (37 C)⁰

Aorta – located and cut – cannulated with Ringer’s solution (perfused at 40 mm Hg)

Ligature – placed around LAD

Ligature – placed around LAD

Test /std/control - administered.

Occluded for 10 minutes

reperfusion

ECG electrode – pulsatile stimulation, induction of arrhythmia

Heart rate and contractile force measured

Heart rate measured through

chronometer coupled to polygraph, Contractile force measured by force

transducer. Incidence and duration of ventricular

fibrillation or ventricular tachycardia is recorded in the control as well as test group.

3)ACETYLCHOLINE & POTASSIUM INDUCED ARRHYTHMIA New Zealand White rabbits 0.5 to 3 kg . The animals are sacrified and heart removed

immediately.Atria dissected from other tissue in Ringer’s solution.

Fibrillation is produced when atria are exposed to Acetylcholine 3x10-4 g/ml or 0.1 gm potassium chloride.

After 5 mins exposure to Ach. Or KCL the atria are stimulated with rectangular pulses of 0.75 ms duration of 10 V.

A mechanical record is taken on kymograph. Controlled arrhythmia are produced and allowed

to continue for 6 to 10 mins. After rest period of 30 mins test compound is

added to bath. Evaluation:- Test compound is found to be

effective if fibrillation disappears immediately or within 5 mins following test drug supplementation to organ bath.

67

CURRENT STATUS OF ANTIARRHYTHMIC DRUGS

Based on results of large Randomized controlled trials

Cardioversion /defibrillator devices as compared to Antiarrhythmic drugs have

proved to be superior in prolonging survival

CONCLUSION Although no animal model can accurately

resemble with human disease condition and species differences also exist, close similarities with humans suffering from or threatened by arrhythmias can be developed by selecting appropriate model and species.

Rather than a single model or experimental technique, combinations of investigations, like isolated heart (langendorff arrangement or working heart), whole hearts in anesthetized or conscious animals, excised cardiac preparations, testing the function can be used.

69

REFERENCES Vogel WH, Schölkens BA. Drug Discovery and Evaluation [Internet]. Vogel HG,

Vogel WH, Schölkens BA, Sandow J, Müller G, Vogel WF, editors. Berlin, Heidelberg: Springer Berlin Heidelberg; 2002. P.208-29.

V.Gerald ,Drug Discovery and Evaluation Pharmacological Assay; Second Edition;2002; Springer; pg no. 236- 242

Tripathy .K D ,Essitial of Medical Pharmacology; sixth Edition;2008; Jaypee Brothers Medical Publishers LTD; New Delhi; pg no 508- 520

Evaluation of antiarrhythmicdrugs

Health & Medicine

Transcript of Evaluation of antiarrhythmicdrugs