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Dr Rahul Kunkulol's Power point preparations
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THERAPEUTIC INDEX :AN INDEX OF SAFETY
DR RAHULASSOCIATE PROFESSOR
PHARMACOLOGY
THERAPEUTIC INDEX• Ratio of median
lethal dose to median effective dose.
LD50/ED50
• This is the dose (mg/kg)which would kill one half of limited population of the same species & strain.
MEDIAN LETHAL DOSE
LD50-• This is the dose (mg/kg) which produces desired response in 50% of test population
MEDIAN EFFECTIVE
DOSE
ED50-
THERAPEUTIC INDEX
Margin of safety is the difference between therapeutic and lethal dose
of the drug.
IMPORTANCELarger the therapeutic index safer the drug.
ex. Penicillin has high therapeutic index while digitalis has much less.
Drugs with low TI
• Digitalis• Gentamicin• Lithium• Quinine
FEATURES
TI not reliable guide for safety.
Therapeutic index varies from species to species.
Reliable only when LD50 & ED50 is determined in same strain of same species.
ED50 is determined from DRC.
No drug produces single effect but spectrum of effect aswellas differs in selectivity.
• ex. Antihistaminic , Aspirin
FACTORS MODIFYING DRUG ACTION
Factors modifying effects of drugs
Individual differ in degree and character of drug response and therefore the optimum dose of drug that produces therapeutic effect varies from person to person.
Causes of Variability in Drug Response
Those related to the biological system
1. Body weight and size2. Age and Sex
3. Species and race4. Genetics – pharmacogenetics
5. Condition of health6. Placebo effect
Those related to the conditions of administration
7. Route of administration. 8. Drug interactions
• Pharmacokinetic drug interactions• Pharmacodynamic Drug interactions
9. Resulting from repeated administration of drug: • Drug Tolerance-Tachyphylaxis;• Drug dependence
BODY WEIGHT
• Average dose is mentioned in mg/kg or as total single dose (50-100kg)• Do not apply for:
ObeseEdemaDehydratedEmaciationIndividual dose= BW/70 X average
adult dose
AGE Neonates, infants & childrenPk factors-Gastric Emptying , Liver & Renal FunctionsPd factors-Teratogenicity , Adverse Effects
Old-Absorption , Metabolism , Excretion, Sensitivity, Half Lives Of Drugs.
SEX
FEMALES MALES• BODY SIZE SEXUAL FUNCTIONS•MENSTRUATION GYANACOMASTIA• PREGNANCY • LACTATION
SPECIES AND RACE
• SPECIES –Some drugs resistant with some species•Rats- Digitalis•Rabbits-Atropine
• RACE-•Blacks require higher and Mongols require lower conc. of Ephedrine and Atropine for pupilary dilatation.• Indians- chloramphenicol
GENETIC FACTORS
Genetically mediated variations in drug responses
Different rates of metabolismEx.•Pseudocholinestrases•G6PD defeciency•Acetylation & hydroxylation
Condition of health : Pathological states
Gi diseases- Malabsorption syndrome, Achlorhydria,…….
Liver diseasesHigh first pass metabolism,Decrease -ppb, drug metabolism, Prodrug less effective
Renal disease-Decrease clearence
PLACEBO
• Inert substance•Psychological effect•Pts. Belief, attitude, expectations
Those related to the conditions of administration
7. Route of administration. 8. Drug interactions
• Pharmacokinetic drug interactions• Pharmacodynamic Drug interactions
9. Resulting from repeated administration of drug: • Drug dependence• Drug Tolerance-Tachyphylaxis;
Causes of Variability in Drug Response• Those related to the conditions of
administration1. Route of administration.2. Resulting from repeated administration of
drug: drug tolerance-tachyphylaxis;
drug dependence3.Drug interactions GI absorption;
protein binding/distribution; metabolism (stimulation/inhibition); excretion (ph/transport processes); receptor (potentiation/antagonism); changes in pH or electrolytes.
ROUTE OF ADMINISTRATION
• Governs speed and intensity of drug response. drugs may have different uses with diff.routes• Iv doses •Smaller than oral•Quick onset of action•More chances of toxicity
DRUG DEPENDENCE
•Repeated administration of drug may induce habit and dependence.Psychic dependencePhysical dependence
Drug tolerance• Requirement of large dose of a drug to
elicit an effect ordinarily produced by normal therapeutic dose of the drug• Types- a. Pseudo tolerance b. True tolerance Natural Acquired
(repeated)1.Species 1.Tissue tolerance2.Racial 2.Cross tolerance
TACHYPYLAXIS• Decrease in pharmacological
response of a drug after repeated administration at very short interval (occurs rapidly)EphedrineTyramineAmphetamine serotonin