Absorption of Drugs
- Absorption of drugs is the transfer of a drug from its site of administration to the blood stream.
- Once in the bloodstream, the drug can be transported to its site of action, metabolized, and eventually eliminated from the body.
- The rate and efficacy of absorption depends on the route of administration, for Iv delivery absorption is complete, for example the total dose of the drug reaches the systemic circulation.
- other routes may result in Only partial absorption, thus slower bioavailability.
- Lipid soluble drugs willingly move over most biological membranes but water soluble drugs penetrate the cell membrane through aqueous channels.
- Mechanism of absorption of drugs from the GIT include passive diffusion, facilitated diffusion, active transport, endocytosis and endocytosis.
Factors affecting drug Absorption:
Solubility:
Drugs with elevated lipid solubility
are absorbed at a fast rate. Drugs specified in aqueous solution are more fastly
absorbed than those given an oily solution, suspension or solid form.
Degree of ionization:
It is inversely proportional to
absorption; greater the degree of ionization, Lesser the absorption. Lipid
soluble drugs are in a unionized form and are readily absorbed where as water
soluble drugs are in ionized form and can be absorbed if they have very small
molecular size.
Pharmaceutical preparations:
Solutions are better absorbed than
suspensions, smaller the molecular size of powders, the more efficient their
absorption.
Drugs that are formulated as
sustained-release or enteric-coated tablets may have delayed or altered
absorption patterns compared to immediate-release tablets.
Route of administration:
The route of administration refers to
the way in which a drug is introduced into the body. Different routes of
administration have different rates and degrees of drug absorption.
For example, drugs administered
intravenously (IV) have 100% bioavailability, meaning the entire dose reaches
the bloodstream. On the other hand, oral drugs may undergo first-pass
metabolism in the liver before reaching the bloodstream, resulting in reduced
bioavailability.
Absorption from parenteral route is
rapid than oral route and absorption from intramuscular route is rapid than from
subcutaneous route.
Surface area, concentration of drug
and local blood flow these are directly proportional to absorption of the drug.
Gastric Emptying:
The rate of gastric emptying can
affect the absorption of orally administered drugs. Drugs that are absorbed in
the small intestine may be affected by the rate at which they are emptied from
the stomach.
Expression of P-glycoprotein:
P-glycoprotein is a multidrug
transmembrane transporter protein responsible for transporting various
molecules (including drugs) across cell membrane. it is expressed throughout
the body.
Functions of P-glycoprotein:
Liver -> Transports the drugs to bile for
elimination.
Kidney ->
Pumps drugs into
urine for excretion.
Placenta
-> transport drugs
back into maternal blood thereby reducing foetal exposure to drugs.
Brain
capillaries -> Pumps
drugs back into blood thereby limiting drug access to brain.
Intestine
-> Transport drugs
into the intestine lumen and reducing drug absorption into the blood.
Physicochemical Properties of the Drug:
The physicochemical properties of a
drug can affect its solubility, permeability, and stability, which in turn
affect drug absorption. For example, drugs that are lipophilic (fat-soluble)
tend to be absorbed more readily than drugs that are hydrophilic
(water-soluble).
GI pH:
The pH of the gastrointestinal (GI)
tract can affect the solubility and stability of drugs, which in turn can
affect their absorption. For example, some drugs may be absorbed more readily
in an acidic environment, while others may require an alkaline environment.
Other factors:
• If a drug moves
through the GIT very fast as seen with severe diarrhoea is not good absorbed;
conversely anything that slow down the transport of drugs from stomach to intestine,
slow the rate of absorption.
• Presence of
food and other substances in the GIT may interfere with the absorption of drugs
Some drugs may be absorbed more readily when taken with food, while others may
be absorbed more slowly or incompletely. for example tetracyclines are
antibacterial agents (protein synthesis inhibitors) that are poorly absorbed if
given with calcium.
• Exercise
decreases the absorption of drugs taken by mouth because blood is diverted away
from the GIT to skeletal muscles.
The absorption of drugs can occur
through various routes of administration, including:
Oral Administration:
Oral administration is the most common
route of drug administration. Drugs taken orally must first pass through the
gastrointestinal tract, where they may undergo metabolism and absorption into
the bloodstream.
Injection:
Injection is a method of administering
drugs directly into the bloodstream or into a specific tissue or organ.
Injection can be performed intravenously, intramuscularly, subcutaneously, or
intra-arterially.
Inhalation:
Inhalation is a route of
administration that involves inhaling drugs in the form of a gas, vapour, or
aerosol. The lungs have a large surface area for absorption, allowing drugs to
be absorbed quickly and efficiently.
Topical Application:
Topical application involves applying
drugs directly to the skin, mucous membranes, or other body surfaces. Topically
applied drugs can be absorbed through the skin or other tissues and into the
bloodstream.
Learn more
What is absorption of drugs?
What is drug absorption?
Factors affecting drug absorption
Factors modifying drug absorption
Top 5 factors affecting drug
Absorption
Affect of solubility on drug
absorption
affect of pharmaceutical preparations
on absorption of drugs
Affect of route of administration on
absorption of drugs
Effect of GI tract pH on absorption of
drugs
Affect of physiochemical properties of
drugs on absorption of drugs
Effect of the P-glycoprotein on
absorption of drugs
Functions of P-glycoprotein
Effect of gastric emptying on absorption of drugs
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