Pharmacokinetics Flashcards
Ionisation of weak drugs are given by the equation
pH=pKa +log [conc. of ionized drug]\ [conc. of unionised drug]
Weak acids are ionized in
Alkaline medium
Weak basic drugs are ionized in
Acidic medium
Which can’t cross lipid membranes
Ionized or unionised drugs,why?
Ionized,coz of its charge (polarity)
Lipids(non polar)
Aspirin is absorbed more in gastric or intestinal medium,why?
Gastric medium=acidic=unionised=diffusion through lipid membranes
Atropine is absorbed in gastric or intestinal medium, why?
Atropine=basic drug
Intestinal medium=basic=unionised= diffusion through lipid membranes
Ion trapping
Is when a weak electrolyte crossing a membrane to encounter a ph from which it is not able to escape easily.
E.g: Aspirin –gasrtic mucosal cell damage
Weak acidic/basic drugs are ph______
Dependent
Weak acidic drugs e.g
Weak- Warfarin
A- Antiepileptic(phenobarbiton, phenytoin
C- Chlorothiazide
I -Ibuprofen & Aspirin
D- Dopamine pro drug- levodopa
Or
PAPS
Phenobarbiton
Aspirin
Penicillin
Salfadiazine
Weak basic drugs
ACE as in pokemon
(Mostly ends with -ine)
Atropine
Chloroquine
Ephedrine
Intracellular ph
7.0 -7.4
Plasma pH
7.35-7.45
Bioavailability
Refers to rate and extent of absorption of a drug from a dosage form administered by any route.
OR
It is a measure of the fraction of administered dose of a drug reaching the systematic circulation in the unchanged form
Bioavailability is determined by
Concentration-time curve in blood or by its excretion in urine
Low bioavailability of orally administered drugs
1- incomplete absorption
2-absorbed drug any undergo first pass metabolism in intestine or liver or be excreted in bile.
Bioequivalence
Two preparation of drugs are said to be bioequivalent when rate and extent of bioavailability of the active drug from them is not significantly different under suitable test conditions
Contains of tablets and capsules other than active drug
Diluents, stabilizing agents,binders, lubricants
Differences in bioavailability may rise due to
Disintegration -breaking into individual particles of active drug
Dissolution rate - in aqueous gastrointestinal contents
Practical significance of bioavailability variation
For
1- drugs with low safety margin-Digoxin
2- where dosage needs precise control - oral hypoglycemics,oral anticoagulant
3- may be responsible for success or failure of an antimicrobial regimen
Extant of distribution of a drug and its pattern of tissue distribution depends on
+lipid solubility
+Ionisation at physiological ph
+Presence of tissue specific transporter
+Difference in regional blood flow
Apparent volume of distribution V
The volume that would accommodate all the drugs in the body,if the concentration throughout was as same as in plasma
Apparent volume of distribution V, equation
V=dose administered i.v/plasma concentration
Name some lipid insoluble drugs
Streptomycin,
Gentamicin
Do lipid insoluble drugs enter cells
No
