Introduction to Pharmacology
Pharmacokinetics describes what the body does to a drug after administration. This process encompasses four fundamental phases: Absorption, Distribution, Metabolism, and Excretion (ADME). Understanding pharmacokinetics enables clinicians to optimize drug dosing regimens, predict drug interactions, and achieve therapeutic efficacy while minimizing toxicity. The interplay between these phases determines drug concentration at the site of action and duration of therapeutic effect.
📋 Abbreviations Used in This Article
- ADME: Absorption, Distribution, Metabolism, Excretion
- GI: Gastrointestinal
- pH: Potential of Hydrogen (acidity/alkalinity measure)
- CNS: Central Nervous System
- P450: Cytochrome P450 enzyme system
💊 Absorption
Drug movement from administration site into systemic circulation:
Key Factors Affecting Absorption
- Route of Administration: Oral passes through GI tract; IV delivers directly to bloodstream
- Food and pH: Gastric acidity affects drug stability and dissolution
- Formulation: Capsules, sustained-release tablets control absorption rate
- First-Pass Metabolism: Oral drugs metabolized by liver before reaching systemic circulation
Clinical Examples
- Aspirin: Better absorbed in acidic stomach environment
- Penicillin: Destroyed by stomach acid if not properly formulated
- Nitroglycerin: Sublingual route bypasses first-pass metabolism for rapid effect
🩸 Distribution
Drug transport via bloodstream to tissues and organs:
| Factor | Mechanism | Clinical Significance |
|---|---|---|
| Blood Flow | Highly perfused organs (liver, kidneys, brain) receive drugs faster | Determines onset of action in target organs |
| Protein Binding | Drugs bind to albumin; only free drug is pharmacologically active | Drug interactions can displace bound drugs, increasing free concentration |
| Blood-Brain Barrier | Restricts drug entry into CNS | Only lipophilic drugs cross readily; limits CNS drug delivery |
| Tissue Binding | Fat-soluble drugs accumulate in adipose tissue | Prolongs drug effects (e.g., diazepam) |
🎯 Key Concept: Volume of distribution (Vd) quantifies drug distribution. High Vd indicates extensive tissue binding; low Vd suggests drug remains in plasma.
⚙️ Metabolism (Biotransformation)
Enzymatic conversion of drugs to more readily eliminated forms, primarily in the liver:
Phase I Reactions
- Enzymes: Cytochrome P450 system (CYP450)
- Reactions: Oxidation, reduction, hydrolysis
- Result: Modify chemical structure, often creating more polar metabolites
- Example: Codeine converted to morphine (active metabolite)
Phase II Reactions
- Process: Conjugation with endogenous molecules
- Substrates: Glucuronic acid, sulfate, acetate, glutathione
- Result: Increased water solubility for renal excretion
- Example: Acetaminophen glucuronidation for elimination
⚠️ Clinical Implications: Enzyme inducers (rifampin, phenytoin) increase metabolism, reducing drug efficacy. Enzyme inhibitors (cimetidine, ketoconazole) decrease metabolism, increasing toxicity risk.
🚽 Excretion
Drug and metabolite elimination from the body:
| Route | Mechanism | Clinical Examples |
|---|---|---|
| Renal (Urine) | Glomerular filtration, tubular secretion, reabsorption | Most drugs; primary route for water-soluble compounds |
| Hepatobiliary (Feces) | Liver excretion into bile, then intestines | Lipophilic drugs, conjugated metabolites |
| Pulmonary (Lungs) | Exhalation of volatile substances | Anesthetic gases (sevoflurane), alcohol vapor |
| Other Routes | Sweat, saliva, breast milk | Minor routes; breast milk important for nursing mothers |
📊 Pharmacokinetic Parameters
Essential concepts for dosing optimization:
Half-Life (t½)
- Definition: Time for plasma concentration to decrease by 50%
- Clinical Use: Determines dosing frequency
- Steady State: Achieved after 4 to 5 half-lives
- Example: Drug with t½ = 6 hours requires dosing every 6 to 12 hours
Bioavailability (F)
- Definition: Fraction of administered dose reaching systemic circulation
- IV Bioavailability: 100% by definition
- Oral Bioavailability: Reduced by first-pass metabolism
- Clinical Impact: Determines oral vs IV dose adjustments
🧩 Clinical Applications
Pharmacokinetics guides therapeutic decision-making:
| Application | Pharmacokinetic Principle | Example |
|---|---|---|
| Dosing Interval | Based on half-life and therapeutic window | Once-daily dosing for long t½ drugs (levothyroxine) |
| Loading Dose | Rapidly achieve therapeutic concentration | Digoxin loading in heart failure |
| Steady State | Drug intake equals elimination | Reached after 4 to 5 half-lives of regular dosing |
| Drug Interactions | Altered metabolism or protein binding | Warfarin + antibiotics requires INR monitoring |
| Renal Adjustment | Reduced clearance in kidney disease | Dose reduction for renally cleared drugs (gentamicin) |
🎯 Steady State Principle: Consistent therapeutic levels achieved when drug administration rate equals elimination rate, typically after 4 to 5 half-lives.
🎯 Clinical Pearls
Essential high-yield principles for pharmacokinetics:
- ADME framework: Absorption, Distribution, Metabolism, Excretion
- Only free (unbound) drug is pharmacologically active
- First-pass metabolism reduces oral bioavailability; sublingual/IV routes avoid this
- Phase I metabolism (P450): modification; Phase II: conjugation for water solubility
- Half-life determines dosing interval; steady state reached after 4 to 5 half-lives
- Enzyme inducers decrease drug levels; inhibitors increase toxicity risk
- Renal dysfunction requires dose adjustment for renally excreted drugs
- Lipophilic drugs cross blood-brain barrier; hydrophilic drugs do not
- Volume of distribution: high Vd = extensive tissue binding; low Vd = plasma retention
- Monitor therapeutic drug levels for narrow therapeutic index medications
🔬 Pharmacology Study Tips:
- Remember ADME: Use the acronym systematically for every drug
- Link to pharmacodynamics: Pharmacokinetics = what body does to drug; pharmacodynamics = what drug does to body
- Focus on clinical relevance: How do PK parameters affect dosing decisions?
- Drug interactions: Know major P450 inducers and inhibitors