Chapter 5: Pharmacodynamics Learning Objectives 1. Define pharmacodynamics and its relationship to the action of drugs on the body. ● Pharmacodynamics is the study of how drugs interact with the body to produce their effects. It looks at how drugs work at the molecular, cellular, and physiological levels to impact the body. Chapter 5 Readings In Chapter 5, the basic features of the dose-response relationship are discussed, including maximal efficacy and relative potency. Dose-response curves reveal three phases: phase 1 where low doses have no measurable response, phase 2 where an increase in dose corresponds to an increase in response, and phase 3 where further increase in dose does not elicit a greater response. Pharmacodynamics is defined as the study of the biochemical and physiologic effects of drugs on the body and how they are produced. Bullet points Therapeutic Index ● ● ● ● The therapeutic index is a measure of a drug's safety It is defined as the ratio of a drug's LD50 to its ED50 A large therapeutic index indicates that a drug is relatively safe A small therapeutic index indicates that a drug is relatively unsafe Dose-Response Relationship ● ● ● Most drugs have a graded dose-response relationship The response gets more intense with increasing dosage Maximal efficacy is defined as the biggest effect a drug can produce Receptors ● ● ● A receptor is any functional macromolecule in a cell to which a drug binds to produce its effects Binding of drugs to receptors is almost always reversible The receptors through which drugs act are normal points of control for physiological processes Antagonists ● ● Antagonists have affinity for receptors but no intrinsic activity Antagonists can bind to receptors but do not produce any effects Types of drug action ● ● ● ● ● Drugs can mimic or block the action of the body's own regulatory molecules at receptors Antagonists prevent receptors from being activated Partial agonists have moderate intrinsic activity and can act as agonists or antagonists Continuous exposure to agonists can lead to receptor desensitization, while exposure to antagonists can result in hypersensitivity Some drugs act through physical or chemical interactions with small molecules instead of receptors Selectivity and nonselectivity of drug effects ● ● A drug can have selective effects if it interacts with one type of receptor that regulates a few processes A drug can have nonselective effects if it interacts with one type of receptor that regulates multiple processes, or if it interacts with multiple receptors Drug dosage and safety ● The initial dose of a drug is an approximation, and subsequent doses may need to be adjusted based on the patient's response ● ● The therapeutic index, which measures a drug's safety, is the ratio of LD50 (lethal dose) to ED50 (therapeutic dose) Drugs with a high therapeutic index are considered safe, while drugs with a low therapeutic index are not safe Agonists and antagonists ● ● Agonists activate receptors and have both affinity and high intrinsic activity Antagonists prevent receptor activation by endogenous regulatory molecules and other drugs