INTRODUCTION Pharmacokinetics is currently defined as the study of

INTRODUCTION

 

Pharmacokinetics
is currently defined as the study of drug absorption, distribution,
metabolism, and excretion. Clinical pharmacokinetics is
the utilization of pharmacokinetic standards to the safe administration of
medications in patient’s. Primary objectives of clinical pharmacokinetics
incorporate upgrading viability and diminishing toxicity of a patient’s treatment.
The improvement of solid connections between tranquilize fixations and their
pharmacologic reactions has empowered clinicians to apply pharmacokinetic
standards to genuine patient circumstances. The fundamental procedures
associated with pharmacokinetics are absorption, distribution, and the two
routes of drug elimination, metabolism and excretion. Together they are some of
the time known by the acronym ‘ADME’.

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·        
Absorption is the process by which drugs passes
through various biological barriers and reaches he systematic
circulation(blood). Given by any route other than intravenously, drug molecules
must cross tissue membranes (e.g. skin epithelium, subcutaneous tissue, gut
endothelium, capillary wall) to enter the blood.

 

·        
Distribution is
the process in athere is transfer of a drug
from one location to another within the body. After entering the blood,
drug molecules must cross capillary walls to enter the tissues, reach cell
membranes and enter cells.

 

·        
Metabolism is the process by which active drugs to
inactive and oblique or inactive drugs to active and they are chemically
altered to make them sufficiently water-soluble for excretion in urine or
faeces (via the biliary tract). Metabolism occurs in a variety of body organs
and tissues, but chiefly in the skin, gut wall, liver and kidney.

 

Excretion is the process by which drugs exits the
body. Lipid-soluble drugs must be modified to water-soluble metabolites before
excretion via the kidney or into the intestine via the bile and the drugs that
are sufficiently water-soluble will be excreted unchanged in the urine.

DRUG ABSORPTION

 

Absorption is
the process by which drugs pass through various biological barriers and reaches
the systematic circulation(blood). Given by any route other than intravenously,
drug molecules must cross tissue membranes (e.g. skin epithelium, subcutaneous
tissue, gut endothelium, capillary wall) to enter the blood.

 

Transport of a
drug from the gastrointestinal tract, depending on their chemical properties,
drugs may be absorbed from the gastrointestinal tract by either passive
diffusion or active transport.

 

•         
Passive
diffusion: The drug moves from a region of higher to lower
concentration (Fick’s low). Passive diffusion does not contain a
provider(carrier). The vast majority of drugs gain access to the body by this
mechanism. Lipid-soluble drugs easily pass through most of the biologic
membranes due to their solubility in the membrane bi-layers. The drugs that are
water soluble penetrate the cell membrane via pores or the aqueous channels
that are present in the cell membrane. Other agents can enter the cell via
specialized transmembrane provider(carrier) proteins that facilitate the
passage of big molecules. This system is known as facilitated diffusion.

 

•         
Active
transport: This mode of drug access involves particular provider(carrier)
proteins that pass the membrane. Active transport is energy-dependent and is
driven by the hydrolysis of adenosine tri-phosphate. It is capable of moving
drugs against a concentration gradient that is, from a region of lower to
higher drug concentration.

 

•         
Endocytosis and
exocytosis: This type of delivery transports drugs of
exceptionally large size across the cell membrane. Endocytosis involves
engulfment of a drug molecule by the cell membrane and transport into the cell
by pinching off the drug-filled vesicle. Exocytosis is the reverse of
endocytosis and is used by cells to secrete many substances by a similar
vesicle formation process.

 

 

 

 

 

 

 

 

 

 

 

 

 

Drug Distribution

 

Ø  Plasma protein binding

 

The circulatory
system can transport generally insoluble substances. These substances are
exchanged by binding to the proteins which have an extremely amphipathic
structure. The groups that are hydrophilic renders the protein solubility in
water and the compounds that are lipophilic are pulled in to the lipophilic
groups and are loosely bound to the protein molecules. The majority of the
drugs go in the plasma are partly in solution and partly bound to the plasma
protein. The bound drug is inert and the unbound drug is dynamic. The
proportion of bound to the unbound drug fluctuates. Binding of the drug is
reversible. The acidic part of the drug binds to the albumin and the basic part
to ?1 – acid glycoprotein.

 

The drug binds
to form a dynamic equilibrium with the plasma protein, because of the plasma
protein the free dug reaches the receptor. This means that once the free drug
enters the target tissue then the protein bound drug will be released to
maintain equilibrium. On the off chance that two medications tie at a similar
site of the protein and are managed together at that point there can be issues.
e.g. Warfarin and aspirin, as aspirin dislodge warfarin from its bound protein
because of which there are raised levels of warfarin in the unbound state and
this can prompt warfarin toxicity.