Absorption of Drugs
Drug absorption is defined as the process of movement of unchanged drug the
site of administration to systemic circulation. Absorption can also be defined
as the process of the movement of unchanged drug from the site of administration
to the site of measurement ie plasma.
Routs of Drug Administration
The Enteral Route:
Include per oral ie gastrointestinal , sublingual / buccal and rectal routs.
The GI rout is most common for administration of majority of drugs.
- The parentral Route
Includes all routes of administration through or under one or more layers
ofskin. While no absorption is required when the drug is administered iv, it is
necessary for exravascular parentral routes like the subcutaneous and intramuscular
routes.
- The Topical Route
Includes skin, eyes or other specific membranes. THe intranasal, inhalation,
intravaginal and transdermal routes may be considered enteral or topical
according to different definitions.
MECHANISM OF DRUG ABSORPTION
Passive diffusion
Pore transport
Facilitated Diffusion
Active transport
Ionic or electrochemical diffusion
Ion-pair transport
Endocytocis
Passive diffusion or Nonionic Diffusion
More than 90% of drugs follow this process . Here the
driving force is concentration or electrochemical gradient.
It is defined as the difference in the drug concentration on the either side
of the membrane. Drug movement is a result of the kinetic energy of the
molecule.
Explanation By
Explanations of Passive diffusion on the Basis of Ficks Law
- The drug moves down the concentration gradient indicating downhill
transport - The rate of drug transfer is directly proportional to the concentration
gradient between GI fluids and the blood compartment - Greater the area and lesser the thickness of the membrane, faster the
diffusion thus, more rapid is the rate of drug absorptioin from the
intestine than from the stomach' - Equilibrium is attached when the concentration on either side of the
membrane becomes equal - Drugs which can exist in both ionised and unionised forms approach
equilibrium primarily by the transfer of unionised species; the rate of
transfer of unionised species is 3 to 4 times the rate for ionised drugs. - Greater the membrane partition coefficient of the drug, faster the
absorption: since the membrane is lipoidal in nature a lipophilic drug
diffuses at a faster rate by solubilising in the lipid layer of the
membrane - The drug diffuses rapidly when the vol of GI fluid is low: conversly,
dilution of GI fluids decreases the drug concentration in this fluids (CGIT)
and the lower concentration gradient (CGIT - C). This phenomena
is however made use of in treating cases of oral overdose or poisening. - Passive diffusion is a energy independent and non saturable but dependent
to a lesser extend on the square root of the molecular size of the drug the
molecular w.ts of most drugs lie bw 100-400 daltons which can be effectively
absorbed passively
Pore Transport or convective Transport or Bulk flow or Filtration
The process is important in the absorption of low mol wt (less than 100) low
mol size (smaller than the diameter of the pore). And generally water soluble
drugs through narrow, aqueous filled channels or pores in the membrane
structure.( Ex: Urea Water Sugars) Chain like or linear compounds of mol wt upto
400 daltons can be absorbed by filtration. The driving force is
constituted by the Hydrostatic pressure or the osmotic differncess across the
membranes due to which bulk flow of water along with small solid molecules occurs
through aqueous chanels.
Water flux that promotes such a transport is called as solvent drag.
Facilitated Diffusion
It is a carrier mediated transport system that operate down the concentration
gradient( Down hill transport) But its a faster process than the passive
diffusion. The driving force is conc gradient hence a passive processsince no
energy expenditure is involved.
The process is not inhibited by metabolic poisons that interfere with energy
production. Facilitated diffusion is of limited importance in the
absorption of drugs.
Ex: Entry of glucose to Rbc. Intestinal absorption of VIt B1 & B2
Absorption of Vit B12 in intestine, An intrinsic factor (IF), a glycoprotien
produced by the gastric parietal cells forms a complex with vith B12 which is
then transported across the intestinal membrane by a carrier system.
Active transport
Comparison with Facilitated diffusion
- The drug is transported from a region of lower to one of higher
concentration ie against the concentration gradient or uphill transport.
with out any regard for equilibrium - Since the process is uphill, energy is required in the work done by the
carrier - As the process requires expenditure of energy it can be inhibited by
metabolic poisons that interfere with energy production like fluorids,
cyanade and dinitrophenol and lack of Oxygen, etc.
Ex : Absorption of Na, Ca, K, Fe, Glucose certain amino acids and vit like
niacin pyridoxin and ascorbic acid. Drugs having structural similarity to such
agents are absorbed actively, particularly the agents use full in cancer
chemotherapy.
Ex : absorption of 5 fluro Uracil and 5 Bromo uracil via the pyramidine
transport system absorption of Methyl dopa L dopa via L amino acid transport
system.
Enalapril via small peptide carrier system
Ionic or electrochemical diffusion
The rate of permiation of the drugs follow the order
unionicedmolecule>anions>cations
The permeation of the ionised drugs, particularly the cationic
drugs, depended on the potential difference of the electrical gradient as the
driving force across the membrane. A cationic charge is repelled due to the
positive charge on the out side of the membrane.
Ion-pair transport
Mechanism of absorption explains the absorption of the quaternary ammonium
compounds and sulfonic acids, which ionise under all pH conditions, is ion pair
transport.
Endocytocis
Minor transport mechanism which involves the engulfing extracellular
materials within a segment of the cell membrane to form a saccule or a vesicle (
hence also called as corpuscular or vesicular transport) which is then
pinched-of intracellularly
Includes 2 type process
- Phagocytosis (cell eating) : adsorptive uptake of solid particulate
- Pinocytosis (cell drinking)
: uptake of fluid solute.
Factors Influencing GI Absorption of a Drug from its Dosage
form
PHARMACEUTIC FACTORS
Physicochemical Properties of Drug Substances
Drug solubility and dissolution rate
Particle size and effective surface area
Polymorphism and amorphism
Pseudopolimorphism ( Hydrates and solvates)
Salt form of the drug
pKa of the drug and pH
Drug stability
Dosage Form Characteristics and pharmacologic
Ingredients
Disintegration time (tablets and capsules)
Dissolution time
Manufacturing Variables
Pharmaceutic Ingredients
Nature and type of dosage form
Product age and storage condition
PATIENT RELATED FACTORS
Age
Gastric emptying time
Intestinal transit time
Gastro intestinal pH
Disease states
Blood Flow to the GIT
Gastrointestinal contents
Other Drugs
Food
Fluids
Other Normal GI contents
Presystemic Metabolism
Luminal Enzymes
Gut wall enzymes
Bacterial Enzymes
Hepatic Enzymes
No comments:
Post a Comment