Chapter 2: Pharmacological Principles – Pharmacokinetics, Dynamics & Therapeutics

Pharmacological Principles – Pharmacokinetics, Dynamics & Therapeutics introduces the core principles governing how medications interact with the body, encompassing major subdisciplines like pharmacology—the broadest study of drugs—pharmaceutics (dosage form design), pharmacoeconomics, and pharmacognosy (natural drug sources). The central focus is on pharmacokinetics—what the body does to the drug, involving the phases of Absorption, Distribution, Metabolism, and Excretion (ADME)—and pharmacodynamics—what the drug does to the body, including drug–receptor interactions. Pharmacokinetic phases define a drug's timeline, incorporating concepts like half-life (the time for 50% reduction), onset of action, peak effect, and duration of action, culminating in the physiological steady state. Absorption is heavily influenced by the route of administration, with orally administered drugs subject to the first-pass effect in the liver, which reduces their overall bioavailability, whereas intravenous administration achieves 100% bioavailability. Distribution involves the transport of the drug to its site of action and addresses concepts such as protein binding (primarily to albumin), where only unbound drug molecules are pharmacologically active, posing a potential risk of toxicity in patients with low protein levels. Metabolism (biotransformation), primarily occurring in the liver, involves the cytochrome P450 enzyme system, which can be altered by drugs acting as enzyme inhibitors or inducers. Pharmacodynamics explains mechanisms of action through drug-receptor interactions, categorized by response types such as agonists (stimulate a response) and antagonists (inhibit a response), enzyme interactions, or nonselective changes. The clinical application, pharmacotherapeutics, defines the goals of drug use, categorized into therapies such as acute, maintenance, palliative, or prophylactic. Finally, the chapter addresses drug safety, defining adverse drug events (ADEs), which include preventable medication errors (MEs) and adverse drug reactions (ADRs), such as allergic or idiosyncratic reactions (often genetically determined, like G6PD deficiency). Safety monitoring requires understanding drug interactions (additive, synergistic, or antagonistic effects) and the therapeutic index (the ratio between toxic and therapeutic concentrations). The chapter concludes with toxicology, discussing serious effects like teratogenesis, mutagenesis, and carcinogenesis.