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Pharmacokinetics of Drugs

Pharmacokinetics of drugs including membrane transport, absorption, distribution, metabolism and excretion of drugs.
Pharmacokinetics consists of two words; “pharmacon which means drug” and “kinetics which means movement”. The response of the body towards the drug or the movement of the drug inside the body is known as pharmacokinetics. It includes-
A- Absorption
D- Distribution
M- Metabolism
E- Excretion

Membrane Transport
All the pharmacokinetic processes are having the involvement of biological membrane (semi-permeable membrane) across which transport of drug occurs. These drugs are either hydrophilic or lipophilic and biological membranes have compatibility to both types of drugs, thus biological membranes are highly dynamic structures.
Drugs are transported across the membrane via two major methods. They are-
A. Passive diffusion & filtration
B. Specialized transport

A. Passive Diffusion
  • Almost 95% of drugs follow passive diffusion.
  • Transport of drugs across the membrane occurs through diffusion towards the direction of a concentration gradient.
  • The movement of drugs occurs from higher concentration to lower concentration.
  • No energy is required.
  • It is also known as the downhill movement.
  • It follows FICK’S LAW of diffusion:-
dQ / Dt is directly proportion to Cgi - Cplasma
{ where, dQ / Dt =change in qty w.r.t time
Cgi = concentration in gastro-intestinal tract
Cplasma= concentration present in plasma}

Filtration
  • Filtration is a process in which drugs transport through the aqueous pores of the membrane or through paracellular spaces of the membrane.
  • This method can be accelerated by changing the osmotic pressure.
  • Lipid soluble drugs can pass through the membrane if the diameter of pores is smaller than the size of drugs.
  • Drugs having molecular weight>100 or 200 cannot able to pass through the membrane.


B. Specialized Transport
Specialized transport can be done through various different methods such as through carriers; which is called carrier transport. Transport occurs with the help of carriers or transporters for important ions, nutrients, metabolites, transmitters etc. Drugs can also transport across the membrane through ion channels from which ions transport. On the basis of the requirement of energy, carrier transport is of two types-
1. Facilitated Diffusion
  • No requirement of energy.
  • Rapid than passive diffusion.
  • Transport occurs in the direction of its electrochemical gradient.
  • Water-soluble drugs such as vit-B & vit-C absorbs through this phenomenon.
2. Active Transport
  • Energy is required.
  • Transport occurs against its electrochemical gradient.
  • The movement of drugs occurs from low concentration to high concentration.
  • It is also known as the uphill movement.
  • Restricted by metabolic poisons.
Active transport is also of two types-
i) Primary active transport( direct use of ATP in the form of energy).
ii) Secondary active transport(indirect use of ATP in the form of energy).

Absorption
Absorption is defined as the kinetics of drugs from the site of administration to the systemic circulation. The drug has to travel across the biological membranes.
There are various factors affecting the absorption of drug-
  • Aqueous solubility
  • Concentration
  • Surface area (more surface area, after the absorption).
  • Route of administration(oral, topical, parenteral, subcutaneous, intramuscular).
Distribution
Distribution is defined as the movement of drugs from systemic circulation (bloodstream) to the organs/tissue/cells. Distribution is a reversible phenomenon.
Distribution of the drugs depends on the –
  • Lipid solubility
  • Ionization
  • Plasma protein binding
  • Presence of specific tissue transporters.
  • The difference in blood flow at the specific region
Apparent Volume of Distribution (V) = dose administered(i.v)/plasma concentration
[Higher the V Higher the quantity of drugs present in extravascular tissues.]

Metabolism Bio-transformation
Metabolism is defined as the chemical alteration of the drug inside the body. This process is also known as bio-transformation.
  • Metabolism is a non-reversible phenomenon.
  • Primarily, drug metabolised in liver and other site of metabolism are kidney, intestine, lungs and plasma.
  • Metabolism is carried out under specific condition and with the help of various enzymes or iso-enzymes.
  • There are synthetic (oxidation & reduction) and non-synthetic reactions(conjugation, methylation, acetylation) reactions wich are responsible for the bio-transformtion.

Presysematic Metabolism
It is also known as the first-pass metabolism of the drug. When the drug is administered orally they are exposed to metabolizing enzymes in the gut and liver.

Excretion   
Nonreversible removal of drugs and their metabolites from the body or systemic circulation is known as excretion. Drugs and metabolites are excreted in-
Urine- The majority of the drugs are excreted in the urine. It is the most important channel of excretion. Drugs excreted are derived from the kidney.
Feces- drugs excreted in feces are derived from the bile. Molecules having MW>300 are eliminated from this route.
Exhaled Air- Majorly gases & volatile products are eliminated by the lungs.
Saliva and sweat- Least drugs are excreted in this way. Heavy metals such as lithium, iodide, potassium are excreted in these secretions.
Note: Metabolism and excretion are collectively known as elimination.





Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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