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 Regulation of receptor sensitivity ● ● ● ● Receptors on cells can change in number and sensitivity in response to continuous activation or inhibition Continuous exposure to agonists can lead to desensitization or down-regulation of receptors Continuous exposure to antagonists can lead to hypersensitivity or up-regulation of receptors Differences in receptor sensitivity can contribute to variability in drug responses Drug responses that do not involve receptors ● ● ● Some drugs act through simple physical or chemical interactions with molecules, rather than interacting with receptors Examples include antacids, antiseptics, saline laxatives, and chelating agents These drugs interact directly with target molecules to produce their effects Measurement of interpatient variability ● ● ● Interpatient variability can be measured by evaluating patient responses to a specific therapeutic endpoint For example, the dosage required to elevate gastric pH to 5 can be measured in a study on an acid suppressant drug There can be wide variability in the doses needed to produce the target response for different subjects ED50 and Dosage Adjustment ● ● ● The ED50 is the dose that will produce a predefined intensity of response in 50% of the population Initial doses of a drug are approximations and may need to be fine-tuned based on the patient's response Dosage adjustments should be made based on the patient's response, not solely on recommended doses in a drug reference Frequency Distribution Curve ● ● A frequency distribution curve shows the range of doses required to produce a desired response in all subjects The curve illustrates that different subjects may require different doses to achieve the therapeutic endpoint Monitoring Patient Response ● ● Patient response to medication should be monitored for both beneficial and harmful effects The dosage should be adjusted based on the individual patient's response, not solely on what is recommended in a pharmacology reference ED50 and Interpatient Variability ● ● ● ED50 is the average effective dose required to produce a defined therapeutic response in 50% of the population Interpatient variability means that different patients may have different responses to a standard dose Dosage adjustments must be made based on the patient's response and should not blindly comply with recommended dosages in a book