Pharmacology: Drug Actions and Classifications

• Pharmacology is the study of drugs and their effects on the body
• It encompasses how drugs are absorbed, distributed, metabolized, and excreted (pharmacokinetics), and how they interact with the body at the cellular and molecular level (pharmacodynamics)

Drug Classifications

• Drugs are classified based on various criteria including their chemical structure, mechanism of action, and therapeutic use
• Chemical classification groups drugs with similar chemical structures
• Pharmacological classification groups drugs with similar mechanisms of action
• Therapeutic classification groups drugs based on their use in treating specific diseases or conditions

Pharmacodynamics

• Pharmacodynamics describes the effects of a drug on the body
• This includes the drug’s mechanism of action, its therapeutic effects, and its adverse effects
• Drugs typically exert their effects by interacting with specific receptors, enzymes, or ion channels
• Receptors are proteins on cell surfaces or within cells that bind to drugs, initiating a cellular response
• Agonists are drugs that bind to receptors and activate them, producing a response
• Antagonists are drugs that bind to receptors but do not activate them; instead, they block the binding of agonists, preventing a response
• Enzyme inhibitors bind to enzymes and prevent them from catalyzing reactions
• Ion channel blockers bind to and block ion channels, preventing the flow of ions across cell membranes

Pharmacokinetics

• Pharmacokinetics describes how the body affects a drug
• It involves four main processes: absorption, distribution, metabolism, and excretion
• Absorption is the process by which a drug enters the bloodstream
• The route of administration affects absorption; intravenous (IV) administration results in the most rapid and complete absorption
• Oral administration involves absorption from the gastrointestinal tract, which can be affected by factors such as gastric pH, food, and other drugs
• Distribution is the process by which a drug spreads throughout the body
• Factors affecting distribution include blood flow, tissue binding, and the drug’s ability to cross cell membranes
• The blood-brain barrier restricts the passage of many drugs into the brain
• Metabolism (or biotransformation) is the process by which the body chemically modifies a drug
• The liver is the primary site of drug metabolism, involving enzymes such as cytochrome P450 (CYP450) enzymes
• Metabolism can either activate or inactivate a drug, and can also make it easier to excrete
• Excretion is the process by which the body eliminates a drug
• The kidneys are the primary site of drug excretion, filtering drugs and metabolites from the blood into the urine
• Other routes of excretion include feces, sweat, and exhaled air
• Half-life is the time it takes for the concentration of a drug in the body to decrease by half
• It determines the frequency of drug administration needed to maintain therapeutic levels
• Steady state is the point at which the rate of drug administration equals the rate of drug elimination, resulting in a stable drug concentration in the body

Therapeutic Uses

• Therapeutic uses of drugs involve using them to prevent, diagnose, or treat diseases and conditions
• Drugs are selected based on their ability to target the underlying cause of the disease or to alleviate its symptoms
• The therapeutic index is a measure of a drug’s safety, comparing the dose required to produce a therapeutic effect to the dose that causes toxicity
• Drugs with a narrow therapeutic index require careful monitoring to avoid toxicity

Adverse Effects

• Adverse effects are unintended and undesirable effects of a drug
• They can range from mild to severe, and can include side effects, allergic reactions, and toxicities
• Side effects are predictable and often unavoidable effects that occur at therapeutic doses
• Allergic reactions are immune responses to a drug, ranging from mild rash to life-threatening anaphylaxis
• Drug interactions occur when one drug affects the activity of another drug
• These interactions can be pharmacokinetic (affecting absorption, distribution, metabolism, or excretion) or pharmacodynamic (affecting the drug’s mechanism of action)
• Contraindications are specific situations in which a drug should not be used because it could be harmful to the patient
• Monitoring is essential to detect and manage adverse effects
• This includes monitoring vital signs, laboratory values, and patient symptoms