Introduction
Pharmacokinetics is the branch of pharmacology that studies how a drug moves within the body — from entry to elimination. Sometimes described as “what the body does to the drug.”
In this Part 1 of our Pharmacokinetics series, we focus on Drug Absorption, Distribution, and Deposition. Although these notes are based on the BAMU University syllabus, they are also highly relevant for MSc Chemistry or Pharmacy students in many universities since the core concepts (ADME) are common.
Understanding pharmacokinetics helps students and researchers because:
- It explains how fast and how much of a drug reaches the bloodstream.
- It shows how drugs are transported, stored, and used inside the body
- It helps in designing safe and effective medicines.
- It prevents toxicity and improves drug formulations.
🖇Next Must read: Pharmacodynamics notes 📗
📚 Topic cover in this blog :
1. Definition of Pharmacokinetics & ADME concept
2. Drug Absorption – factors, mechanisms, and importance
3. Drug Distribution : factors,volume of distribution (Vd), and examples
4. Drug Deposition:factors and examples
5. Summary and FAQs
Pharmacokinctics:
Definition:
Main Processes of Pharmacokinetics (ADME Concept):
Pharmacokinetics involves four major processes:
1. Absorption:
• The process by which a drug enters the bloodstream after
administration.
• Example: A tablet dissolves in the stomach and gets
absorbed into the blood through the intestines.
• Route of administration (oral, intravenous, etc.) affects
absorption rate.
2.Distribution:
The transport of the drug from the bloodstream to
different tissues and organs.
Some drugs bind to plasma proteins, while others freely
move in the blood.
• The drug may leave the circulatory system and enter
interstitial and intracellular fluids.
3. Metabolism (Biotransformation):
• The conversion of a drug into a more water-soluble form for
easier elimination.
• The liver is the main site of metabolism, where enzymes
break down drugs into active or inactive forms.
4. Excretion (Elimination):
• The removal of the drug from the body, mainly through
urine (kidneys), but also via bile, sweat, or breath.
• Example: Paracetamol is eliminated through urine after
metabolism in the liver.
Importance of Pharmacokinetics:
• Helps in determining the correct dosage and frequency of
drug administration.
• Ensures drug safety by avoiding toxicity.
• Helps understand drug interactions in the body.
• Ensures drugs achieve therapeutic effects while avoiding
harmful side effects.
Importance of Pharmacokinetics in Drug Designing:
Pharmacokinetics plays a crucial role in designing effective and
safe drugs.
It helps in:
🔸️Determining the Right Dose:
Ensures the drug reaches the
required concentration in the body without causing toxicity.
🔸️Selecting the Best Route of Administration:
Helps decide whether the drug should be given orally, intravenously, or
through another route for maximum effect.
🔸️Optimizing Drug Action Duration:
Helps in designing drugs that act quickly (for emergencies) or slowly (for long-term treatment).
🔸️Reducing Side Effects:
By studying metabolism and
excretion, drugs can be modified to minimize harmful effects.
🔸️Improving Drug Bioavailability:
Helps in making drugs that are absorbed better and reach the target site efficiently.
🔸️Developing Controlled-Release Formulations:
Used to design sustained-release tablets that maintain drug levels for a longer time.
🔸️Personalized Medicine:
Helps in designing drugs tailored to
individual patients based on genetic and metabolic
differences.
1.Drug absorption
Drug absorption is the process by which a drug moves from the site of administration into the bloodstream.
It is the first step in pharmacokinetics and determines how quickly and effectively a drug reaches its target site.
Factors Affecting Drug Absorption:
1. Physicochemical Properties of the Drug:
• Solubility: Water-soluble drugs absorb faster than poorly soluble drugs.
• Molecular Size: Smaller molecules pass through membranes easily.
• pKa and Ionization: Non-ionized (lipophilic) drugs absorb better than ionized (hydrophilic) drugs.
2. Route of Administration:
• Oral (PO): Absorbed in the stomach/ intestine; influenced by food, enzymes, and pH.
• Intravenous (IV): No absorption needed; drug enters directly into the bloodstream.
• Intramuscular (IM) & Subcutaneous (SC): Moderate absorption rate.
• Topical & Transdermal:
Slow absorption through the skin.
3. Gastrointestinal (GI) Factors:
• pH of Stomach/Intestine: Acidic drugs absorb in the
stomach, while basic drugs absorb in the intestine.
• Food and Enzymes: Food can enhance or delay drug
absorption.
4. Blood Flow to the Absorption Site:
- High blood flow (e.g., intestine) leads to faster absorption.
- Poor blood flow (e.g., diseased tissues) slows absorption.
5.Drug Formulation:
- Capsules, tablets, suspensions, and sustained-release
- formulations impact absorption rates.
Mechanisms of Drug Absorption:
1. Passive Diffusion (Most Common)
• Drugs move from high concentration to low concentration across membranes.
• Example: Lipid-soluble drugs like aspirin.
2. Facilitated Diffusion
• Requires a carrier protein but does not need energy.
• Example: Transport of glucose.
3. Active Transport
• Requires energy (ATP) to move drugs against the
concentration gradient.
• Example: Absorption of vitamins like B12.
4. Endocytosis (Pinocytosis & Phagocytosis)
• Used for large molecules like proteins and peptides.
• Example: Insulin absorption.
Clinical Importance of Drug Absorption:
• Determines onset of action (how quickly a drug starts working).
• Affects bioavailability (amount of drug reaching systemic circulation).• Helps in designing drug formulations for better therapeutic effects.
Distribution and deposition of drugs :
Distribution and Deposition of Drugs are two essential concepts in pharmacokinetics.
Both describe how drugs spread
throughout the body after they enter the bloodstream and how they are stored or accumulated in various tissues.
2. Drug Distribution:
Drug distribution refers to the process by which a drug moves from the bloodstream to various tissues and organs in the body.
This is influenced by several factors:
Factors Affecting Distribution:
• Blood Flow: Organs with higher blood flow (like the liver, kidneys, and heart) receive a larger portion of the drug compared to organs with low blood flow (like fat tissues).
• Plasma Protein Binding: Drugs bind to plasma proteins (like albumin) in the bloodstream. Only the free (unbound) drug can enter tissues and exert its effect. The bound drug typically acts as a reservoir and is inactive.
• Lipid Solubility: Lipid-soluble drugs are more likely to pass through cell membranes and distribute into fattytissues, the brain, and other organs with high lipid content. Lipid-soluble drugs usually have a larger
• Molecular Size: Larger molecules may have difficulty crossing biological membranes, slowing their distribution.
• Ionization: Drugs that are ionized (charged) at physiological pH are less likely to pass through cell membranes. Non-ionized drugs are more easily absorbed and distributed.
• Barriers: Some tissues, like the blood-brain barrier (BBB) or the placental barrier, limit drug distribution. These
barriers prevent certain drugs from reaching specific areas of the body.
Volume of Distribution (Vd):
The Volume of Distribution (Vd) is a key concept in understanding distribution. It measures how extensively a drug spreads through the body compared to its concentration in the
plasma. A high Vd indicates that the drug is widely distributed into tissues, while a low Vd suggests that the drug stays in the bloodstream.
Vd = Amount of Drug in the Body / Plasma Drug Concentration
Example of drug Distribution
1)Morphine:
Distributes widely to tissues with good blood flow, including the brain, where it relieves pain and causes sedation.
2) Warfarin:
Binds to plasma proteins (mainly albumin ), and only the unbound form is active, affecting its distribution and anticoagulant effect.
3.Drug Deposition:
Drug deposition refers to the accumulation or storage of drugs in specific tissues or organs. This process may occur in certain tissues where drugs have an affinity or binding capacity.
Factors Influencing Deposition:
• Tissue Affinity:
Some drugs tend to accumulate in
particular tissues due to their chemical properties.
• Fat:
Lipophilic (fat-soluble) drugs often accumulate in fatty tissues because they are able to cross cell membranes
more easily.
• Bone:
Some drugs, like tetracyclines or bisphosphonates, accumulate in bone tissue.
• Liver/Kidneys:
Drugs that are metabolized by the liver or excreted by the kidneys can be deposited in these organs.
• Binding to Plasma Proteins:
As mentioned earlier, drugs that bind to plasma proteins are distributed less widely, remaining in the bloodstream or in the extracellular space.
Drug Reservoirs:
Some drugs can create reservoirs in
tissues, where they accumulate and are released slowly over time, leading to prolonged effects. For example,
chloroquine and tetracycline can deposit in tissues like the liver or bones.
• Bioaccumulation:
In some cases, drugs can accumulate
over time in certain tissues, especially if the body is unable to metabolize or excrete them efficiently. This can lead to toxic effects.
Example of Drug Deposition:
• Diazepam (Valium):
This drug is highly lipophilic and has a
tendency to accumulate in fatty tissues, contributing to its long half-life.
• Chloroquine:
An antimalarial drug that deposits in tissues like the liver and spleen, and can lead to long-lasting effects.
Frequently Asked Questions (PYQs)
1. What is pharmacokinetics?
Pharmacokinetics is the study of how a drug moves through the body, from absorption to elimination. It shows what the body does to a drug.
2. What are the main processes of pharmacokinetics?
The four main processes are Absorption, Distribution, Metabolism, and Excretion (ADME).
3. What factors affect drug absorption?
Absorption is affected by:
Physicochemical properties (solubility, molecular size, ionization) Route of administration (oral, IV, IM, SC, topical)
Gastrointestinal factors (pH, food, enzymes) Blood flow to the absorption site Drug formulation (tablet, capsule, suspension, sustained-release)
4.How does plasma protein binding affect drug action?
When drugs bind to plasma proteins (like albumin), the bound drug is inactive. Only the free (unbound) drug can enter tissues and act.
5.Will these notes be available in PDF?
Yes I'm working on PDF versions but its take some time.
For now, I'm sharing the notes in article format so you can start studying immediately.
💡Summary:
Pharmacokinetics may sound like a big word, but it’s something we all experience in daily life. When you take a headache tablet, ever wondered how it knows where the pain is? That’s because of absorption, distribution, and deposition. The tablet dissolves (absorption), spreads through your blood to reach the brain (distribution), and sometimes even stores in tissues for longer effects (deposition).
☆☆☆This was just Part 1 of Pharmacokinetics… stay tuned for the next part where we’ll explore metabolism and excretion in the simplest way☆☆☆
Hey Student, This is Your Time🕙:
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