2.2 Pharmacokinetics and Pharmacodynamics

Drugs interact with our bodies in complex ways, affecting cells, enzymes, and ion channels. Understanding these interactions helps us grasp how medications work and why they sometimes cause side effects. Pharmacodynamics explores the intricate dance between drugs and our body's systems.

Pharmacokinetics looks at how our bodies process drugs over time. From absorption to elimination, many factors influence a drug's effectiveness. Half-life, side effects, and potential for tolerance or toxicity all play crucial roles in determining how we use medications safely and effectively.

Pharmacodynamics

Drug interactions with body cells

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• Drugs bind to specific receptors on cell surfaces or within cells
  • Agonists activate receptors to produce a response (morphine activates opioid receptors)
  • Antagonists block receptors, preventing activation (naloxone blocks opioid receptors)
  • Receptor affinity determines the strength of drug-receptor binding
• Some drugs act by inhibiting or enhancing enzyme activity
  • Enzyme inhibitors block the action of enzymes, reducing the production of specific substances (statins inhibit HMG-CoA reductase)
  • Enzyme inducers increase the activity of enzymes, enhancing the production of specific substances (rifampin induces cytochrome P450 enzymes)
• Certain drugs interact with ion channels, altering the flow of ions across cell membranes
  • Channel blockers block ion channels, reducing the flow of ions (calcium channel blockers)
  • Channel openers open ion channels, increasing the flow of ions (potassium channel openers)
• Drugs can interact with transporters, which are proteins that move substances across cell membranes
  • Transporter inhibitors block the action of transporters, reducing the movement of substances (SSRIs inhibit serotonin transporters)
  • Transporter enhancers increase the activity of transporters, enhancing the movement of substances (probenecid enhances uric acid transport)

Pharmacokinetics

Significance of drug half-life

• The time required for the concentration of a drug in the body to decrease by half
• Determines the frequency of drug administration
  • Drugs with shorter half-lives require more frequent dosing to maintain therapeutic levels (acetaminophen)
  • Drugs with longer half-lives can be administered less frequently (fluoxetine)
• Steady-state concentration achieved when the rate of drug elimination equals the rate of drug administration
  • Typically reached after 4-5 half-lives
  • Important for maintaining consistent therapeutic effects

Factors in drug effectiveness

• Absorption influenced by route of administration (oral, parenteral, topical), gastrointestinal factors (pH, motility, food intake), and drug formulation (tablets, capsules, liquids)
  • Bioavailability affects the amount of drug that reaches systemic circulation
• Distribution affected by blood flow to tissues, protein binding, tissue permeability, and blood-brain barrier
• Metabolism impacted by liver function, enzyme activity (cytochrome P450), and genetic variations in metabolic enzymes
  • First-pass effect can significantly reduce oral drug bioavailability
• Excretion influenced by renal function, urine pH, and active tubular secretion and reabsorption
• Patient factors such as age, body weight and composition, comorbidities, and concomitant medications

Side effects vs adverse effects

• Side effects are predictable, dose-dependent effects that are not the primary therapeutic goal
  • Often mild and tolerable (drowsiness, dry mouth, nausea)
• Adverse effects are unexpected, dose-independent effects that are harmful to the patient
  • Can be severe and life-threatening (anaphylaxis, liver toxicity, cardiac arrhythmias)
• Importance of monitoring to recognize and manage side effects and adverse effects
  • Adjusting dosage or switching medications when necessary
  • Reporting adverse events to improve drug safety

Drug tolerance and toxicity

• Tolerance is decreased responsiveness to a drug after repeated exposure
  • May require higher doses to achieve the same therapeutic effect
  • Can develop with various drug classes (opioids, benzodiazepines)
  • Mechanisms include receptor downregulation, enzyme induction, and compensatory physiological changes
• Toxicity refers to harmful effects resulting from excessive drug exposure
  • Can occur due to overdose, accumulation, or drug interactions
  • Dose-dependent toxicity: Severity increases with higher doses
  • Idiosyncratic toxicity: Unpredictable, not directly related to dose
  • Management involves supportive care, antidotes (when available), and discontinuation or dose adjustment of the offending drug
• Therapeutic index relates drug efficacy to toxicity, guiding safe dosing

Drug Interactions

• Drug-drug interactions can affect pharmacokinetics or pharmacodynamics
• May result in increased or decreased drug effects, toxicity, or treatment failure
• Common mechanisms include enzyme inhibition or induction, altered absorption, or competition for receptor binding
• Careful medication reconciliation and monitoring are essential to prevent adverse outcomes