Pharmacokinetics – Absorption
- Topics Covered: Drug transport processes, factors affecting absorption, and bioavailability.
Introduction to Absorption
Definition:
Absorption is the process by which a drug moves from its site of administration
into the bloodstream.
Importance:
- Determines the onset of
action.
- Influences drug efficacy.
Story:
Imagine you take a painkiller for a headache. The time it takes to relieve the
pain depends on how fast the drug is absorbed into your system.
Drug Transport Processes
1. Passive Diffusion
- Drugs move from high to low
concentration without energy.
- Example: Lipophilic drugs like
aspirin cross cell membranes easily.
Story:
Think of perfume dispersing in a room; it spreads without extra effort, just
like passive diffusion.
2. Filtration
- Movement of small
water-soluble molecules through pores in the membrane.
- Example: Small ions and molecules
like urea.
Example
Activity:
Ask students to list everyday examples of filtration, like brewing tea.
3. Specialized Transport
- Active Transport: Uses energy to move drugs
against the concentration gradient.
- Example: Levodopa for Parkinson’s
disease uses active transport to cross the blood-brain barrier.
- Facilitated Diffusion: Requires a carrier protein
but no energy.
- Example: Glucose transport.
Story:
Active transport is like using a ladder to climb a hill, while facilitated
diffusion is like taking an elevator—both need assistance.
4. Pinocytosis
- The cell engulfs the drug in
vesicles.
- Example: Vitamins like fat-soluble
A, D, E, and K absorbed in the gut.
Analogy:
Pinocytosis is like a bubble wrapping around candy in water to take it inside.
Factors Affecting Absorption
|
Factor |
Effect |
|
Physicochemical
Properties |
Lipophilic
drugs absorb faster; hydrophilic drugs may need specialized transport. |
|
Formulation |
Tablets
dissolve slower than liquids. |
|
Route
of Administration |
Intravenous
bypasses absorption; oral depends on the gut environment. |
|
Gastrointestinal
Factors |
Food,
pH, and enzymes can enhance or inhibit absorption. |
|
Blood
Flow |
High
blood flow increases absorption rate (e.g., muscles absorb faster than fat
tissue). |
Story:
Imagine pouring sugar into water versus honey. Sugar dissolves faster, just
like how drug formulation affects absorption speed.
Bioavailability
Definition:
The fraction of an administered dose that reaches systemic circulation in its
active form.
Key
Concepts:
- 100% Bioavailability: Intravenous (IV) drugs
(e.g., morphine IV).
- Less Than 100%: Oral drugs due to
first-pass metabolism (e.g., propranolol).
Example:
Oral nitroglycerin has poor bioavailability and is given sublingually to bypass
the liver.
Activity:
Ask students: Why are some drugs taken sublingually or rectally? Discuss their
bypass of first-pass metabolism.
Clinical Relevance of Bioavailability
- Therapeutic Dosing: Knowing bioavailability
helps determine the correct dose.
- Example: Oral amoxicillin
has ~90% bioavailability; dosing accounts for this.
- Drug Formulation: Controlled-release forms
can improve absorption rates.
- Example: Extended-release
metformin.
Story:
During the development of oral insulin, scientists faced challenges due to poor
bioavailability. They overcame this by exploring new delivery systems like
nanoparticles.
Summary and Questions
- Key Points:
- Drug absorption depends on
transport processes and external factors.
- Bioavailability is crucial
for effective dosing and formulation.
Activity:
- Match drug types with their
primary transport process.
- Identify one factor that
affects drug absorption and explain why.
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