2. Introduction
• Drugs which antagonize the receptor action of
Adrenaline and other related drugs at the receptor level
• They occupy adrenergic receptors (α and β ) but do not
produce signal transduction – affinity is there but without
IA - - competitive antagonists
• Antiadrenergics Vs Adrenergic Neurone Blockers !
• For pharmacologic research, adrenoceptor antagonist
drugs have been very useful in the experimental
exploration of autonomic nervous system function
• Effects vary according to the drug's selectivity for and
receptors - α and its subtype specific or β and its
subtype specific
3. Antiadrenergic Drugs -
Background
Clinically – to modify the responses of endogenous catecholamines
(Adrenaline and Noradrenaline) - physiologic and pathophysiologic
Nonselective α-antagonists: have been used in the treatment of
pheochromocytoma (tumors that secrete catecholamines), and α-1
selective antagonists are used in primary hypertension and
benign prostatic hyperplasia (BHP)
β-receptor antagonist: hypertension, ischemic heart disease,
arrhythmias, endocrinologic and neurologic disorders, and
many other conditions
Blockade of peripheral dopamine receptors is of no recognized
clinical importance at present
In contrast, blockade of central nervous system dopamine receptors
is very important – antiemetic, antipsychotic and TCAs
7. Ergots
Natural: considered derivatives of Lysergic acid
(LSD) – cognition enhancer
Amine alkaloid: Ergometrine (ergonovine)
Amino acid alkaloids: Ergotamine and ergotoxine –
vasoconstrictor, partial agonist and antagonist at α
receptors and 5-HT receptors (1 and 2)
Semisynthetic derivatives: Dihydroergotamine
(DHE) and Dihydroergotoxine – more α-
receptor blocking property
8. General Effects of alpha Blockade
- CVS
Arteriolar and venous tone are determined to a large
extent by receptors on vascular smooth muscle
Reduction in Blood Pressure: Blockade of α-1 (also α-
2) receptor causes – pooling of blood in capacitance
vessels – reduced venous return and Cardiac output –
fall in mean BP
Postural Reflex is interfered – dizziness and Syncope on
standing
Reason: Normally 700 ml of blood is pooled to legs when a person
stands up, and therefore syncope should occur. But do not occur in
case of normal person because of Baroreceptor reflex which
stimulates VMC and sympathetic system is activated. Contraction of
veins occur via α-1 receptor. Blockade of such receptor may therefore
may lead to Postural hypotension
Vasopressor effects of Adrenaline is not found - Vasomotor
reversal of Dale
9. Effects of alpha Blockade -
Others
Reflex tachycardia due to fall in - BP and increased
NA release due to presynaptic alpha-2 blockade
Nasal stuffiness and Miosis
Increased Intestinal Motility - diarrhoea
Reduced GFR: Sodium retention and increase in blood
volume – also reflex renin release
Tone of the Bladder trigone, sphincter and prostate is
maintained by α1A sympathetic
Blockade produces increased urine flow
Inhibition of Ejaculation – due to inhibition of contraction
of vas deferens and others
10. Individual Agents -
Phenoxybenzamine
Non specific, long acting irreversible alpha antagonist
MOA: Spontaneously cyclizes in the body to give ethyleniminium
intermediate – forms a strong covalent bond with α receptors –
blockade of alpha receptor (lasts for 3 – 4 days)
Also blockade of 5-HT, histaminergic and cholinergic receptors
Clinically:
Postural hypotension: Venodilatation>arteriolar
In recumbent position, however:
Blood flow to many organ increased due to reduction in peripheral
resistance and increased venous return
Shifts blood from pulmonary to systemic circulation
Shifts blood from extravascular to vascular compartment
CNS stimulation – nausea, vomiting on IV injection but oral
doses cause depression, tiredness and lethargy
11. DRC
100% _
50% _
0.1 1 10 100
Log dose
A
A+1XC
A+10XC
A = Agonist
C = antagonist
12. Phenoxybenzamine
Pharmacokinetics:
Erratic oral absorption and painful on IM or SC injections
Most of the administered drug Excretes in urine in 24 Hrs
Small amount may remain in tissue bound covalently – leading to
accumulation in adipose tissue
ADRs: Postural hypotension, nasal stuffiness, miosis
and inhibition of ejaculation
Uses:
Phechromocytoma, Secondary shock and Peripheral vascular
disease (Raynaud`s disease)
Preparation and dosage:
20-60 mg orally
1 mg/kg IV infusion for 1 Hr.
13. Phentolamine
Non specific, short acting reversible alpha antagonist
Potent competitive antagonist at both 1 and 2 receptors
Quick acting (in minutes)
Reduction in Peripheral Resistance - blocking both α-1 and α-2
receptors - causes NA release and venodilatation more than
arteriolar
Cardiac stimulation:
Enhanced NA release due to alpha-2 blockade
Inhibits serotonin release – muscarinic agonist (?)
Uses: Pheochromocytoma, clonidine withdrawal, cheese
reaction and in extravasations of NA and Adr injection
Dose: 5 mg IV injection as and when needed
14. Prazosin
Highly selective alpha-1 blocker (1:1000)
Non-specific blockade of all subtypes - α1A, α1B and α1D
Blockade of sympathetic vasoconstriction - fall in BP
NA is not released as α-2 is not blocked (only mild tachycardia)
Dilates arterioles more than veins – Postural hypotension is less –
only 1st
dose effect (dizziness and fainting)
Also inhibits PDE – rise in smooth muscle cAMP - vasodilatation
Kinetics: effective orally (70%), metabolized in liver and half life is
6-8 Hrs
Uses:
Hypertension
Raynaud`s disease
BHP
Dose: start with 0.5 mg bed time and then 1- 4 mg tds.
15. Other alpha Blockers
Terazosin:
Similar to Prazosin but better bioavailability (90%)
Duration of action is longer – 24 Hrs
Use: Preferred in BHP – single dose and apoptosis, also in
hypertension
Similar is Doxazosin
Tamsulosin:
Uroselective (vasicoselective) - α1A and α1D but not α1B
No change in BP and HR at therapeutic doses and
Postural hypotension
Preferred drug in BHP
Only once dosing regime (MR caps)
ADRs: Retrograde ejaculation and dizziness
17. Uses of α-blockers
Pheochromocytoma:
Tumor of medullary cells of Adrenals
VMA and normetanephrine estimation is diagnostic
Phentolamine test: Injection of 5 mg IV over 1 minute
(recumbent)
35 mm (Systolic) and 25 mm (Diastolic) of Hg
Treatment:
Surgery
Phenoxybenzamine in preoperatively and intra-operative because:
To Normalize blood volume: Excess CA shifts blood from vascular to extra
vascular
To prevent outpouring of CA during surgery
To prevent unwanted hypotension due to dilatation of blood vessels
following removal of tumor
(Previously - Clonidine suppression test – Measurement of plasma
CA levels)
18. Uses of α-blockers
– contd.
Hypertension:
Not useful except Prazosin due to
Compensated cardiac stimulation
Postural hypotension, Impotence, nasal blockage etc.
Phentolamine and Phenoxybenzamine – in clonidine withdrawal and cheese
reaction
BHP:
Static component: Size of prostate (5-alpha reductase)
Dynamic component: Tone of Prostate and bladder neck (alpha-1
mediated)
Converts testosterone to active dihydrotestosterone)
Effects of α blocking – relaxation of neck and prostate structures –
reduction in obstruction
5-α reductase inhibitors like Finesteride decreases size of the prostate –
better voiding
α blockers – 2 weeks and 5-alpha reductase inhibitors – 6 months
Remember – BHP is a progressive disease
19. Other uses of α-blockers
Secondary Shock – Phenoxybenzamine
Peripheral vascular disease – beneficial in
Raynaud`s disease
Congestive Heart Failure – short term
Papaverine/Phentolamine induced penile
erection (PIPE) for impotence
22. β - adrenergic Blockers
Cardioselective:
Metoprolol, atenolol, acebutalol, bisoprolol,
esmolol, betaxolol, celiprolol, nebivolol
Nonselective (β1 and β2):
Without intrinsic sympathomimetic activity:
Propranolol (membrane stabilizing action), Sotalol
and Timolol
With intrinsic sympathomimetic activity (ISA):
Pindolol and Oxprenolol
Additional alpha blocking property: Labetolol and
Carvedilol
23. Actions –
Propranolol
Heart:
Decrease in Heart rate, decrease in cardiac output, decrease in
force of contraction
Prolongs systole - synergy of contraction disturbed in ventricles
Not prominent in Normal persons, but in presence of sympathetic
over activity (exercise, emotion)
Decreased ventricular size in normal subject – dilatation in low
cardiac reserve patients
Cardiac work and oxygen consumption - reduced
Total coronary flow reduction (aortic pressure) – subepicardial
region but not subendocardial region – benefit in angina
Delayed AV conduction
At high doses membrane stabilizing and direct depressant action
24. Propranolol –
Blood Pressure
No direct and acute action on Blood Pressure
In fact blocks vasodilatation fall in BP by Isoprenaline and enhances
rise in BP by adrenaline – re-reversal of vasomotor reversal
But beneficial in hypertensives on prolonged administration
Normally, propranolol would block CA induced vasodilatation and
cause increase in TPR and decrease in cardiac output – but
negligible change in BP
ADAPTATION: But chronic exposure will lead the resistance
vessels to adapt to chronically low CO – TPR falls (Most possible
explanation of antihypertensive effect)
Other explanations may be:
Decreased Renin release (β1)
Central reduction of sympathetic outflow
Blockade of NA release – blockade of beta recptors
26. Actions - Propranolol
Respiratory:
Bronchoconstriction due to blockade of dilator beta-2 receptors
Not considerable in normal individual - May be dangerous in
presence of asthma (avoid)
Beta-1 selective drugs are preferred
Contrary: COPD patients tolerate
Eye: Decreases IOP by reducing production of aqueous humor –
glaucoma
CNS: No considerable CNS effect except – behavioural,
forgetfulness and nightmare etc. Suppresses anxiety
Skeletal Muscle:
Reduction of Tremor
Reduction of exercise capacity: reduction in blood flow,
glycogenolysis and lipolysis
27. Propranolol Actions –
Metabolic
Lipid: Inhibits sympathetic stimulation of lipolysis and
consequent increase in free fatty acid level – triglyceride level
increased. LDL/HDL ratio increases
Carbohydrate: Inhibition of glycogenolysis in heart, muscle and
liver – β2 mediated
Recovery from insulin action delayed
Warning signs are masked
But, Glucagon is the main hormone that responses to
hypoglycaemia
Still, beta blockers should be used in caution in patients with
diabetes and low glucagon reserve patients and in
pancreatectomized patients
β1 selective are much safer
28. Pharmacokinetics (Propranolol
as prototype) - absorption
Most of the drugs are well absorbed after oral administration;
peak concentrations occur 1–3 hours after ingestion including
propranolol
Propranolol undergoes extensive hepatic (first-pass) metabolism
High oral:parenteral ratio – 40:1
Interindividiual and equieffective dose bioavailability variation
Metabolism dependent on hepatic blood flow – itself decreases
hepatic blood flow – higher bioavailability on chronic
administration – saturation of hepatic extraction mechanism
Higher bioavailability if taken with food
Dose available as 10-80 mg tabs. (40 – 160 mg /day)
Sustained-release preparations of propranolol and metoprolol are
available
29. Propranolol – ADRs and CI
1. Precipitation of CCF/Oedema –
loss of sympathetic support –
careful addition of beta-1 selective
2. Bradycardia
3. Respiratory: COAD and Bronchial
asthma (life threatening asthma)
4. Risk of Coronary Heart Disease
(triglyceride and LDL increase and
fall in HDL)
5. Tiredness and reduced exercise
capacity – beta-2 muscle
6. Variant angina exacerbation –
unopposed coronary constriction
by alpha receptor
7. Cold hands and feet – worsening
of Peripheral vascular disease
8. Withdrawal
9. Others: GIT upset, nightmare,
forgetfulness and sexual distress
30. Drug Interactions
Propranolol and insulin:
Delayed recovery of hypoglycemia by insulin
Warning signs are suppressed
Propranolol + alpha agonists: Rise in BP
NSAIDs + Propranolol: Attenuation of
antihypertensive action of beta-blockers
31. Other Drugs – beta blockers
Cardioselectivity: More selective in blocking β1 receptors than β2
Advantages:
Lower propensity to cause Bronchoconstriction
Lesser interference with carbohydrate metabolism – safer in diabetics
Lower incidence of cold hand and feet – no/less β2 block
Lesser suppression of essential tremor
Lesser impairment of exercise capacity
Intrinsic sympathomimetic activity: Pindolol, celiprolol
Advantages: Partial agonist action
Lesser bradycardia and depression of contractility – preferred in elderly, sick
sinus syndrome etc.
Favourable withdrawal
Less/no interference with lipid profile not effective in migraine prolhylaxis
32. Other beta blockers – contd.
Membrane stabilizing effect: like lidocaine
Local anaesthetic action
Typically blocks Na+ channel – antiarrhythmic
action
Lipid insolubility: Atenolol, celiprolol,
bisoprolol etc.
Less likely to produce sleep disturbances
Longer acting – incompletely absorbed, no first
pass metabolism, excreted unchanged in urine –
t1/2 – 6-2 Hrs Vs 2-6 Hrs
33. Other Beta Blockers
Metoprolol:
Prototype of cardioselective blockers - β1 selective (also inverse
agonist)
Safer in patients with bronchoconstriction and preferred in
patients with insulin
Less first pass metabolism
Slow and fast hydroxylators (CYP2D6 substrate)
Used:
In Diabetics and patients in OHs
Cold hands and feet with Propranolol
Available as tab – 25/50/100/ mg and IV injection
Atenolol:
Selective β1 and low lipid solubility, less 1st
pass metabolism
Longer duration of action – once daily dosing
No lipid profile adverse effects - Hypertension and angina
34. Other Beta Blockers
Partial beta-agonist: Pindolol and celiprolol,
carteolol, bopindolol, oxprenolol, and penbutolol:
Major CVS applications – less plasma lipid action and
bradycardia
Intrinsic sympathomimetic activity
However, doubtful clinical benefit
Esmolol: partial agonist and MSA
Ultra short acting (less than 10 minutes) - inactivated by
esterases in blood
Degree of blockade can be titrated - Steady dose can be
maintained
Given as IV infusion in SVT, AF, Atrial flutter, arrhythmia
during anaesthesia and cardiac surgery etc.
Available as injections IV: 100 – 500 mg/10 ml inj.
35. Other Beta Blockers
Celiprolol:
Selective beta-1 with additional beta-2 agonistic activity
Safe in asthmatics
Causes vasodilatation by NO production (NA receptor mediated)
– additional benefit as antihypertensive. Dose: 200-600 mg
Nebivolol:
Highly selective beta-1 blocker
Acts as NO donor - Improves endothelial function and delay of
atherosclerosis
No deleterious effects on carbohydrate, lipid metabolism
In CHF and hypertension
Dose: 2.5/5 mg
36. Uses of Beta Blockers
1. Hypertension: 1st
line of agent (JNC 7)
2. Angina pectoris – not in vasospastic
3. Myocardia infarction:
Prophylaxis: Prevent reinfarction and Prevent ventricular fibrillation
Myocardial salvage: reduction in infarct size
1. Cardiac arrhythmias: Class II type of agent – Propranolol IV
(digitalis and anaesthesia induced)
2. Congestive Heart failure
3. Phaeochromocytoma
4. Hyperthyroidism: T4 - T3 + sympathetic symptoms
5. Migraine
6. Anxiety: social phobia
7. Essential Tremor
8. Glaucoma
37. α + β blockers
Labetolol: Alpha + beta blocker (4 diastereomers)
Commercially – α1 + β1 + β2 block + β2 agonistic
Beta blockade – 1/3rd
of Propranolol and alpha – 1/10th
of Phentolamine
Beta: alpha = 1: 5
Clinically: Low dose – like propranolol and high dose like Propranolol +
Prazosin
Fall in BP (Syst + diast) + vasodilatation (beta-2)
Limb blood flow increases - propranolol does not
Also inhbition of NA uptake
Moderately potent and Used in Phaechromocytoma, clonidine withdrawal
ADRs: Postural hypotension, failure of ejaculation and others but no
change in lipid profile
Carvedilol: β1 + β2 and α1 blocker and also Ca+ channel block
Vasodilatation – α + Ca++ channel block + antioxidant
Uses: Hypertension and especially preferred in CHF as cardioprotective
38. Summary
Usefulness and actions of β blockers shall be discussed
later - in relation to their use in the particular CVS
disease conditions
Remember:
Effects of alpha blockade - Phenoxybenzamine, Phentolamine
Test (Phaechromocytoma)
Selective alpha-1 blockers and uses - Prazosin, Terazosin and
Tamsulosine
Name with selectivity of β blockers as per classification given
Pharmacological actions and ADRs of Propranolol as discussed
Overall therapeutic uses of beta-blockers – general Idea
Individual drugs - Metoprolol, atenolol, esmolol, Labetolol and
carvedilol etc.
Vmax – maximum velocity of reaction; Km – rate constant of reaction
Normally 700 ml of blood is pooled to legs when a person stands up, and threfore syncope should occur. But do not occur in case of normal person because of Baroreceptor reflex which stimulates VMC and sympathetic system is activated and contraction of veins occur via alpha-1 receptor. Blockade of such receptor may therefore may lead to Postural hypotension.
Nasal stuffiness is due to blockade of alpha receptors which normally maintains tone of blood vessels of nose. Blockade may lead to vasodilatation and stuffiness.
Na retention
CA Levels are measured after3 hours after a 0.3 mg/kg oral test dose has been given to a patient. A positive test occurs if there is no decrease in plasma levels – positive test