““__IN THE NAME OF ALLAH IN THE NAME OF ALLAH THE MOST BENEFICIENT THE MOST BENEFICIENT

AND THE MOST AND THE MOST MERCIFULL_”MERCIFULL_”

ABSORPTION OF DRUGSABSORPTION OF DRUGS

Definition:-Definition:-

“ “__It is the __It is the transfer of drug from transfer of drug from its site of its site of administration into administration into the blood stream__”the blood stream__”

PROCESS OF DRUG PROCESS OF DRUG ABSORPTIONABSORPTION

Depending on the chemical Depending on the chemical properties,drugs may b absorbed by eitherproperties,drugs may b absorbed by either

ExocytosisExocytosis Carrier mediated transpot Carrier mediated transpot Endocytosis and exocytosis Endocytosis and exocytosis

PASSIVE DIFFUSION:-PASSIVE DIFFUSION:- “ “__Passage of drugs molecules by __Passage of drugs molecules by

diffusing as un-ionized moiety through diffusing as un-ionized moiety through lipid membrane__”lipid membrane__”

DEPENDS ON:-DEPENDS ON:- i. i. molecular sizemolecular size ii. molecular chargeii. molecular charge iii. lipid-water partition co-iii. lipid-water partition co-

efficientefficient iv. conc. gradientiv. conc. gradient

1.SIMPLE DIFFUSION:-

“__Driving force for absorption is the

concentration gradient across a membrane seperating two body compartments__”

Characteristics:-

Doesn’t involve carrier Process is not saturated Shows low stuctural specificity

Importance:-

Vast majority of drugs gain access to the body by this mechanism.

I.FICK’S LAW OF DIFFUSION:-

“_Passive flux(F)of unionized molecule across the lipid membrane is_” :

Directly propotional to conc.gradient(C1-C2). Area(A) across which diffusion occurs. Permeability(P) co-efficient. Inversely propotional to thickness(T) of diffusion path.

II.Diffusion of weak acids &basesII.Diffusion of weak acids &bases

Many drugs are either weak acids Many drugs are either weak acids or weak bases.or weak bases. i.Acidic drugs dissociates as;i.Acidic drugs dissociates as;

HA________H*+AHA________H*+A ii.Basic drugs dissociates as;ii.Basic drugs dissociates as;

BH*_______B(unprotonated)BH*_______B(unprotonated)+H*+H*

Diffusion stateDiffusion state

For weak acids-----Prontonated form For weak acids-----Prontonated form

For weak base-----Unprotonated form For weak base-----Unprotonated form

Determination of the ratio of charged and uncharged form

pH at the site of absorption

Strength of weak acid & weak bases(pKa)

Decrease pKa ----- stronger the acids

Increase pKa ----- stronger the bases

Determination of presence of amount of drug Determination of presence of amount of drug on either side of membrane:-on either side of membrane:-

It is represented byIt is represented by HANDERSON-HASSELBALCHHANDERSON-HASSELBALCH equation.. equation..

pH=pKA+log pH=pKA+log unprotonated speciesunprotonated species

protonated speciesprotonated speciesi.For acidsi.For acids pH=pKa+log pH=pKa+log (A)(A) (HA)(HA)ii.For basesii.For bases pH=pKa+log pH=pKa+log (B)(B)

(BH)(BH)

B.Carrier mediated diffusion:-B.Carrier mediated diffusion:-““__Movement of drug across the __Movement of drug across the membrane mediated by carrier membrane mediated by carrier protein in the membrane__”protein in the membrane__”

Characteristics:-

Saturable process Specific for chemical stucture of the drug.

TYPESTYPES1.Facilitated diffusion:-1.Facilitated diffusion:-

““_Movement is driven by _Movement is driven by concentration gradient for which no energy is concentration gradient for which no energy is required_”required_”

2.Active transport:-2.Active transport:-

“ “_Movement occur against a _Movement occur against a conc. gradient that requires ATP which is conc. gradient that requires ATP which is generated by Na-K ATPase.generated by Na-K ATPase.

C.Endocytosis:-C.Endocytosis:-

“ “_Process by which _Process by which drug molecule is engulfed by cell drug molecule is engulfed by cell membrane_”membrane_”

Exocytosis:-Exocytosis:- “ “_Reverse processis _Reverse processis responsible for secretion of many responsible for secretion of many susbatance from the cell_”susbatance from the cell_”