ABSORPTION OF DRUGS
Link for video demonstration 👇
1. https://youtu.be/coxjQQHId7c
2. https://youtu.be/XPO164AFKtA
INTRODUCTION TO BASIC TERMINOLOGIES TO BIOPHARMACEUTICS & PHARMACOKINETICS
Biopharmaceutics is defined as the study of factors influencing the rate and amount of drug that reaches the systemic circulation and the use of this information to optimise the therapeutic efficacy of the drug products.
Bioavailability is defined as the rate and extent (amount) of drug absorption.
Absorption is the process of movement of drug from its site of administration to the systemic circulation.
Drug distribution is the movement of drug between one compartment and the other (generally blood and the extravascular tissues).
Elimination is defined as the process that tends to remove the drug from the body and terminate its action.
Elimination occurs by two processes—biotransformation (metabolism), which usually inactivates the drug, and excretio which is responsible for the exit of drug/metabolites from the body.
Pharmacokinetics is defined as the study of time course of drug ADME and their relationship with its therapeutic and toxic effects of the drug.
Pharmacokinetics is a study of what the body does to the drug, that relates changes in concentration of drug within the body with time after its administration.
Pharmacodynamics is a study of what the drug does to the body, that relates response to concentration of drug in the body.
INTRODUCTION TO ABSORPTION OF DRUGS
Drug absorption is defined as the process of movement of unchanged drug from the site of administration to systemic circulation.
This definition takes into account the loss of drug that occurs after oral administration due to presystemic metabolism or first-pass effect.
Not only the magnitude of drug that comes into the systemic circulation but also the rate at which it is absorbed is important. A drug that is completely but slowly absorbed may fail to show therapeutic response as the plasma concentration for desired effectis never achieved. On the contrary, a rapidly absorbed drug attains the therapeutic level easily to elicit pharmacological effect. Thus, both the rate and the extent of drug absorption are important.
ABSORPTION OF DRUG FROM COMMON ROUTES OF DRUG ADMINISTRATION (PARENTAL)
Intravenous: Complete (100%) systemic drug absorption
Advantages-
Drug is given for immediate or controlled effect, may inject large fluid volumes, suitable for irritating drugs.
Disadvantages-
Increased chance for adverse reaction, requires skill in insertion of infusion set, tissue damage at site of injection.
Intramuscular injection: Rapid absorption from aqueous solutions & Slow absorption from non-aqueous (oily) solutions.
Advantages-
Easier to inject than intravenous injection, larger volumes may be used compared to subcutaneous solution.
Disadvantges-
Irritating drugs may be very painful,variable rates absorption, depending upon muscle group injected and blood flow.
ABSORPTION OF DRUG FROM COMMON ROUTES OF DRUG ADMINISTRATION (ENTERNAL)
Buccal or sublingual (SL): Rapid absorption of lipid-soluble drugs.
Advantages-
No presystemic metabolism.
Disadvantages-
Some drug may be swallowed. Not for most drugs or drugs with high doses.
Oral (PO): Absorption may vary. Generally slower absorption rate compared to IV bolus or IM injection.
Advantages-
Safest and easiest route of drug administration.
Suitable for both immediate-release and modified-release drug products.
Disadvatages-
Some drugs are unstable in GIT, or undergo presystemic metabolism or show erratic absorption.
Rectal (PR): Absorption may vary from suppository. More reliable absorption from enema (solution).
Advantages-
Useful when patient cannot swallow medication.
Used for local and systemic effects.
Disadvantages-
Absorption may be erratic.
Suppository may migrate to different position.
Some patient discomfort.
ABSORPTION OF DRUG FROM VARIOUS OTHER ROUTES OF DRUG ADMINISTRATION
Transdermal: Slow absorption, rate may vary.
Advantages-
Increased absorption with occlusive dressings.
Transdermal delivery system (patch) is easy to use and withdraw.
Continuous release for a specified period.
Disadvantages-
Used for lipid-soluble drugs with low dose and low MW.
Low presystemic metabolism.
Some irritation by patch or drug.
Permeability of skin variable with condition, anatomic site, age, and gender.
Type of cream or ointment base affects drug release and absorption.
Inhalation: Rapid absorptio, total dose absorbed is variable.
Advantages-
May be used for local or systemic effects.
Disadvantages-
Particle size of drug determines anatomic placement in respiratory tract.
May stimulate cough reflex.
Some drug may be swallowed.
CELLULAR MEMBRANE: STRUCTURE & PHYSIOLOGY
For a drug to be absorbed and distributed into organs and tissues and eliminated from the body, it must pass through one or more biological membranes/barriers at various locations. Such a movement of drug across the membrane is called as drug transport.
The basic structure of cell membrane is shown as:
The cellular membrane consists of a double layer of amphiphilic phospholipid molecules arranged in such a fashion that their hydrocarbon chains are oriented inwards to form the hydrophobic or lipophilic phase and their polar heads oriented to form the outer and inner hydrophilic boundaries of the cellular membrane that face the surrounding aqueous environment. Globular protein molecules are associated on either side of these hydrophilic boundaries and also interspersed within the membrane structure. In short, the membrane is a mayonnaise sandwich where a bimolecular layer of lipids is contained between two parallel monomolecular layers of proteins. The hydrophobic core of the membrane is responsible for the relative impermeability of polar molecules. Aqueous filled pores or perforations of 4 to 10 Ã… in diameter are also present in the membrane structure through which inorganic ions and small organic water-soluble molecules like urea can pass. In general, the biomembrane acts like a semipermeable barrier permitting rapid and limited passage of some compounds while restricting that of others.
The GI lining constituting the absorption barrier allows most nutrients like glucose, amino acids, fatty acids, vitamins, etc. to pass rapidly through it into the systemic circulation but prevents the entry of certain toxins and medicaments. Thus, for a drug to get absorbed after oral administration, it must first pass through this barrier.
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