🧬 Pharmacokinetics – “What the Body Does to the Drug”
Pharmacokinetics involves ADME:
- Absorption
- Distribution
- Metabolism
- Excretion
📥 1. Absorption
Definition: Movement of the drug from the site of administration into the bloodstream.
🛣️ Routes of Drug Administration:
- Oral (PO) – Tablet, capsule, syrup.
- Parenteral – IV, IM, SC (no absorption barrier for IV).
- Others – Sublingual, rectal, inhalation, transdermal.
🔄 Mechanisms of Absorption (Across Membranes):
- Passive Diffusion: Lipid-soluble drugs move from high to low concentration. No energy required.
- Facilitated Diffusion: Uses carrier proteins; no energy. E.g., Glucose transport.
- Active Transport: Uses carrier proteins and ATP. Moves from low to high concentration.
- Pinocytosis (Cell Drinking): Engulfing large drug molecules (e.g., Insulin).
⚙️ Factors Affecting Absorption:
(i) Pharmaceutical Factors:
- Formulation: Tablet, capsule, liquid.
- Binder & Disintegrant: Affects how quickly a tablet breaks down.
- Particle Size: Smaller particles → larger surface area → faster dissolution.
- Dissolution Rate: Critical for bioavailability.
(ii) Physicochemical Properties:
- Lipid Solubility: Lipophilic drugs cross cell membranes more easily.
- pH and Ionization:
- Weak acids (e.g., aspirin) absorb better in acidic stomach.
- Weak bases (e.g., atropine) absorb better in basic intestines.
- Only non-ionized drugs are readily absorbed.
(iii) Biological Factors:
- Surface Area: Greater in intestines than stomach → better absorption.
- Gastric Emptying Time: Faster emptying → quicker absorption.
- Presence of Food: Can delay or enhance absorption.
- Diseases: GI, liver, kidney diseases can alter absorption.
🧪 2. First-Pass Metabolism
- Drugs taken orally pass through the liver via the portal vein before entering systemic circulation.
- Liver enzymes may metabolize the drug, reducing its bioavailability.
- Injection routes bypass this.
🔄 Transport Across Membranes
🧭 Passive Transport
- Simple Diffusion – Lipid-soluble drugs.
- Filtration – Water-soluble drugs through pores.
- No energy or carrier required.
- Cross blood-brain barrier (BBB) only if lipid-soluble and non-ionized.
⚡ Active Transport
- Requires energy (ATP) and carrier proteins.
- Moves drugs against concentration gradient.
- E.g., Levodopa, Glucose, Gentamicin.
💧 Pinocytosis
- Cells engulf large molecules.
- E.g., Insulin.
⚙️ Mechanism of Drug Action (“How Drug Acts”)
1. Physical Action
- Osmotic effect: Mannitol (diuretic), magnesium sulfate (purgative).
- Surface tension reduction: Kills bacteria by disrupting membranes.
2. Chemical Action
- Neutralization: Antacids neutralize stomach acid.
- Chelation: Binds metals in poisoning (e.g., EDTA).
3. Enzyme Inhibition
- Anti-cholinesterases: Inhibit acetylcholine breakdown.
- ACE Inhibitors: Lower blood pressure.
4. Radioactive Action
- Iodine-131: Destroys thyroid in hyperthyroidism.
- Contrast Media: Iodine-based for imaging.
5. Adsorption
- Kaolin, activated charcoal: Absorbs toxins/bacteria in diarrhea.
6. Demulcent Action
- Soothes mucous membranes (e.g., throat lozenges).
7. Electrical Charge-Based
- Heparin binds to clotting factors due to its negative charge.
8. Receptor-Mediated Action
- Most drugs act via receptors:
- Agonists stimulate receptors.
- Antagonists block receptors.
📌 Comparison: Passive vs Active Transport
| Property | Passive Transport | Active Transport |
|---|---|---|
| Direction | High → Low conc. | Low → High conc. |
| Energy Required | No | Yes (ATP needed) |
| Carrier Required | No | Yes |
| PH Dependent | Yes | No |
| Lipid Soluble | Yes (better) | Not necessary |
✨ Motivational Quotes from Notes
- “Substitute your worry with your work and see miracles happen.”
- “If you can’t excel with talent, triumph with effort.”
- “Stop blaming others, take responsibility for every area of your life.”
- “Sometimes we learn more from a person’s errors than their virtues.”
- “I believe fundamental honesty is the keystone of business.”