Pharmacodynamics MBBS Class-2

7/27/2019 Pharmacodynamics MBBS Class-2

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Pharmacodynamics-2Dr.U.P.Rathnakar

MD.DIH.PGDHM


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Potency & efficacy

Dose [potency]

Effect on pain [efficacy]

2mg 4mg

8mg

Mild

Moderate

Severe

15mg

25mg300mg

Drug A[Morphine]

Drug B[Pethidine]

E Max

Drug C

[Aspirin]

Potency-A>B Efficacy-A=BPotency & efficacy of C


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Drug potency & Efficacy

Potency- Amount of drug required to obtain a

particular response[More potent less dose]

Efficacy-The ability of a drug to elicit a maximal

response

Aspirin is less potent[300mg] & less efficacious than

morphine[10mg]

Pethidine is less potent[100mg] equally efficacious

as morphine[10mg]


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Dose-response curves

R

esponse

1000mg100mg10mg


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Dose-response curves determine how much of

a drug (X-axis) causes a particular effect, or a

side effect, in the body (Y-axis).

Dose-response [potency & effect]curve[DRC]

100%

50%

EC50

E max


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Graded dose response curves[Single subject- Continuous]

Advantages of Log-dose curve

Large no.of doses can be plotted on a small graph paper

Comparison of two or more drugs easier as middle portion is

straight line

Linear [Simple]dose response curve Log dose response curve


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Quantal dose response curve

Quantal or all or none: frequency with which any drug evokesa all or none response in a population

LD50 &

ED50

35% 35%

12%

3%

11%

4%

4%

15%

50%

85%

97% 100%

1

0

0

2

0

0

3

0

0

4

0

0

5

0

0

6

0

0

Cumulative DRC


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100%

50%

0

ED50 LD[TD]50

TI Narrow

TI WIDE

TI=10/5=2 TI=100/5=20

5mg

10mg

ED50

Therapeutic index

5mg 100mg


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Therapeutic range

Therapeutic range: Range between dose which

produces minimal Th.effect and Max.acceptable

adverse effect

Therapeutic range

Acceptable-ADE

Min.Th.Effect


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Combined effect of drugs

When 2 drugs are administered together:-

1. They may be Indifferent to each other

2. One may increase the action of the other-

Synergism

3. Action may be decreased or abolished

Antagonism


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Combined effect of drugs

Indifferent Synergism Antagonism

One facilitates the

action of other

One opposes the

action of otherAdditive

Supra-additive

[Potentiation]

Competitive

[Equilibrium &Non-equilibrium]

Non-competitive

Physical

Chemical

Physiological

Receptor


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Agonists snd antagobists


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SynergismTwo drugs One increases or facilitates the action of the other

Additive

Drug A + Drug B

= Effect of A+B [1 + 1 = 2]

Advantage:

Side effect may not add up

Dose of both can be reduced

Eg.

Aspirin + Paracetamol

Nitrous oxide +Halothane

Amlodipine+Atenolol

Supra-additive (Potentiation)

Effect of combination is

more than individual effect

of 2 drugs 1 + 1 = 3 OR 1 + 0 = 2

One drug may be inactive

Levodopa+Carbidopa[Inacti

ve alone][1 + 0 = 2] Sulfamethaoxazole+Trimeth

oprim[Both active] [1+1= 3]


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Antagonism

[Effect of one drug is decreased]

1. Physical Charcoal adsorbs alkaloids

2. Chemical KMno4 oxidizes Alkaloids

BAL chelates arsenic

Protamine neutralizes heparin

Na.Thiopentone+ Succinylcholine

3. Physiological Histamine and Adrenaline

[Functional] Glucagon and Insulin

Hydrochlorothiazide and triamterene

4. Receptor:


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Antagonism [Receptor]

One[Antagonist] blocks the receptor action of the other[Agonist]

Competitive[surmountable]

Binds with the same site on

the receptor

Resembles the agonist Surmountable by increasing

the concn. Of agonist

Rightward shift of DRC

5. Eg.

Ach atropine

Morphine - Naloxone

Non-competitive[unsurmountable]

Binds to another site or

same site covalently

No resemblance Not surmountable

Flattening of DRC

Eg.

Diazepam Bicuculline

Phenoxybenzamine-

Noradrenaline


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Competitive and non-competitive antagonism

Rightward shift

of DRC

Flattening of DRC

Competitive Non competitive


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Competitive Non-competitive


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Factors modifying fuel efficiency???

Factors modifying drug action

New

DriverMfr.defect

Bad road

Over loaded

Engine problem

Tarffic congestion

[Age?]

[M or F][Genetic?]

[Envn.]

[Obesity?]

[Patho.states]

[DI, polypharmacy]

Documents

Pharmacodynamics MBBS Class-2