Pharmacokinetics Overview

Overview

This lecture covers the fundamentals of pharmacokinetics, detailing how drugs move through the body via absorption, distribution, metabolism, and excretion, and emphasizing clinical implications for nurses.

Learning Outcomes

• Explain pharmacokinetics and its application to clinical practice.
• Identify and describe the four components of pharmacokinetics.
• Discuss how drugs travel across plasma membranes.
• Recognize factors affecting drug absorption, distribution, metabolism, and excretion.
• Distinguish between loading and maintenance doses.

Introduction to Pharmacokinetics

• Pharmacokinetics is the study of drug movement throughout the body.
• It includes how the body absorbs, distributes, metabolizes, and excretes drugs.
• Understanding pharmacokinetics helps predict drug actions and side effects.

Absorption

• Absorption is the movement of a drug from the site of administration into circulating fluids.
• Oral drugs must be dissolved before absorption; dissolution rate affects bioavailability.
• Liquids are absorbed faster than solids; acidic gastric pH accelerates absorption.
• Enteric-coated drugs pass through the stomach and dissolve in the intestine; do not crush them.
• Factors affecting absorption: drug form, dosage, route, gastric pH, motility, food, and interactions.
• The first-pass effect refers to liver metabolism reducing oral drug bioavailability.

Distribution

• Distribution is the transport of drugs by the bloodstream to tissues.
• Highly perfused organs (heart, brain, liver, kidneys) receive drugs faster.
• Lipid-soluble drugs distribute more completely and cross barriers like the blood-brain barrier.
• Many drugs bind to plasma proteins (e.g., albumin); only unbound drugs are active.
• Drug-drug and drug-food interactions can alter protein binding and increase toxicity risk.

Metabolism (Biotransformation)

• Metabolism converts drugs into more water-soluble forms for excretion, mainly in the liver.
• The cytochrome P450 enzyme system is key for drug metabolism.
• Some drugs become more active after metabolism (prodrugs).
• Hepatic enzyme activity varies with age, genetics, liver disease, and other factors.
• The first-pass metabolism can inactivate oral drugs before systemic circulation.

Excretion

• Excretion is the removal of drugs from the body, primarily via the kidneys.
• Only free (unbound), water-soluble, and small drugs are filtered in the kidneys.
• Liver or kidney impairment prolongs drug action due to decreased excretion.
• Excretion routes include urine, bile, lungs, saliva, sweat, and breast milk.
• Urine pH affects excretion: acidic urine eliminates weak bases; alkaline urine eliminates weak acids.
• Enterohepatic recirculation can prolong drug presence in the body.

Drug Dosing Concepts

• Drug half-life is the time required for half of the drug amount to be eliminated.
• Short half-life drugs require more frequent dosing; long half-life drugs may need once-daily dosing.
• Steady-state concentration is achieved when drug administration equals elimination.

Key Terms & Definitions

• Pharmacokinetics — the study of drug movement throughout the body
• Pharmaco — medicine
• Absorption — movement of a drug from administration site into bloodstream.
• Distribution — transport of drugs throughout body tissues and fluids.
• Metabolism (Biotransformation) — chemical alteration of drugs, mainly in the liver.
• Excretion — removal of drugs from the body via kidneys, bile, lungs, etc.
• First-pass effect — reduction in bioavailability of oral drugs by liver metabolism.
• Bioavailability — percentage of administered drug available for activity.
• Half-life (t½) — time for plasma concentration of a drug to be reduced by half.
• Prodrug — medication that becomes active after metabolism.
• Plasma protein binding — reversible attachment of drug molecules to blood proteins.
• Cytochrome P450 system — liver enzyme system important in drug metabolism.
• Enterohepatic recirculation — recycling of drugs between intestine and liver.

Action Items / Next Steps

• Review pharmacokinetics chapter in the textbook.
• Familiarize yourself with common drug interactions affecting absorption and metabolism.
• Practice calculating half-life and steady-state drug concentrations.
• Check medication forms for enteric coating before administration.