Pharmacokinetics – Absorption of Drug
Pharmacokinetics is the quantitative study of drug movement in, through and out of the body. It deals with the rates of absorption, distribution, metabolism and excretion (ADME) of drugs. When the dose of the drug and blood concentrations at different time of intervals is determined, intensity of pharmacological response as a function of time can be established.
Absorption of Drugs:
Absorption is the movement of drug from its site of administration into the circulation. The absorption of a drug involves it passage across the cell membrane. As a rule lipid insoluble and water insoluble drugs are not absorbed from the gut.
Drugs can be passing through the cell membrane by two processes.
A.Passive Transfer –
- Simple Diffusion
B.Specialized Transport –
- Active Transport
- Facilitated Diffusion
A. Passive Transfer:
It is a bidirectional process. Polar water- soluble and non-polar lipid soluble substances can be transported by this process.
Only water soluble substances can be transported by this process. It involves passage through pores present in the cell membrane.
B. Specialized Transport:
It is selective process which requires energy. Also it requires a carrier and so is called as carrier transport.
It is very similar carrier transport , but it does not require energy.
Proteins and macro molecules are transported by this process. It is the process of transport across the cell in particulate form by formation of vesicles.
Factors Modifying Drug Absorption
The absorption of a drug can be influenced by the following factors-
1. Physical state: Drugs in the form of liquids are well absorbed than solid. Gases are quickly absorbed trough lungs.
2. Particle size: Smaller the particle size, better is the absorption of a drug. If the particle size is large, the drug is slowly absorbed and hence the action is delayed.
3. Solubility: An easily soluble drug is quickly absorbed. Also drug in the form of solutions are quickly absorbed than solids.
4. Concentration: Concentrated forms of drugs are quickly absorbed than dilute solutions.
5. Area of absorbing surface: Greater the area of absorbing surface, quicker is the absorption of a drug. For example lungs and peritoneal cavity are large surface areas from where drugs can be quickly absorbed.
6. Circulation to site of absorption: Increased blood flow to the area of absorption can increased the absorption of a drug. This can be achieved by massage or local application of heat. Vasoconstrictors decrease blood flow and so decrease the absorption of drug.
7. Route of administration of drug: This is very important factor which determines drug absorption. Some drugs are absorbed only on parenteral administration and they fail to get absorbed on oral administration. e.g. insulin.
Bioavailability of Drug
Bioavailability refers to the rate and extent of absorption of a drug from a dosage form as determined by its concentration-time curve in blood or by its excretion in urine. It is a measure of the fraction (F) of administered dose of a drug that reaches the systemic circulation in the unchanged form. Bioavailability of a drug injected by i.v is 100%, but is frequently lower after oral ingestion, because-a) the drug may be incompletely absorbed. b) the absorbed drug may be undergo first pass metabolism in intestinal wall / liver or be excreted in bile.
Incomplete bioavailability after s.c or i.m injection is less common, but may occur due to local binding of the drug. Differences in bioavailability may arise due to variations in disintegration and dissolution rates.
Differences in bioavailability are seen mostly with poorly soluble and slowly absorbed drugs. Reduction in particle size increases the rate of absorption of aspirin.There is no need to reduce the particle size of freely water soluble drugs.e.g paracetamol.