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Pharmacodynamics and Medication Effects

• Pharmacodynamics refers to the effects of a medication on the body.
• Aims to alter a function or process to achieve a desired clinical outcome.
• Any medication with beneficial effects can also cause toxicity if administered incorrectly.
• Incorrect administration includes wrong dose, route, or malfunctioning device.
• Patient-specific factors, conditions, or the medication itself can also lead to toxicity or adverse effects.
• Medication process within the body involves four key steps: absorption, distribution, biotransformation, and elimination.
• Pharmacokinetics describes the body's actions on a medication.

Receptors and Medication Binding

• Receptor sites (proteins) are present on cells throughout the body.
• Receptors are activated by both endogenous chemicals and exogenous medications/chemicals absorbed into the body.
• When a medication binds to a receptor site, one of four outcomes can occur:
  • Opening or closing of ion channels in cell walls.
  • Activation of a biochemical messenger initiating other chemical reactions.
  • Prevention of a normal cell function.
  • Initiation of a normal or abnormal cell function.
• Exogenous chemicals in the environment can mimic medication effects, including adverse effects.
• Newer medications target specific receptor sites to minimize adverse effects.
• Older medications can affect unrelated cells and tissues.
• Medications/chemicals directly affect cell activity by binding to receptor sites, classified as either agonist or antagonist.
  • Agonist: enhance cellular activity.
  • Antagonist: block the receptor site, stopping cellular activity.

Agonist Medications

• Agonist medications bind with receptor sites, initiating or altering cell action.
• Factors determining medication concentration at the target cell receptor site include dose, route, and other variables.
• Affinity, the ability of a medication to bind with a specific receptor site, is crucial.
• Medication concentration and affinity together determine how many receptor sites are bound by a medication.
• A threshold level exists; the point at which the initiation of alteration or cellular activity starts.
• Increasing medication concentrations lead to increased effects until all receptor sites are occupied or the cell's maximum capacity is reached.

Potency and Efficacy

• Potency refers to the concentration of medication needed to initiate a specific response.
  • As potency increases, the required concentration or dose decreases.
  • Lower potency requires higher concentrations of the medication.
• Efficacy is the ability to initiate or alter cell activity beneficially.
• There is a limit to the change in activity.
• No further change is possible once all cellular receptor sites are bound with agonist medications.

Dose-Response Curve

• Illustrates the relationship between medication concentration and efficacy.
• As the dose increases, it reaches a threshold, which is a range of increasing effect with the dose.
• The maximum effect range is reached after increasing the dose, having the best effect.

Antagonist Medications

• Antagonist medications prevent cellular responses to agonist chemicals by binding with receptor sites.
• They can inhibit normal cellular activation by naturally occurring agonist chemicals or treat harmful effects of exogenous medications/chemicals

Competitive vs Non-Competitive Antagonists

• Competitive antagonists temporarily bind with cellular receptor sites, displacing agonist chemicals.
  • Efficacy is directly related to the antagonist concentration near the receptor sites and its affinity compared to the affinity of the agonist chemicals present.
• Non-competitive antagonists permanently bind with receptor sites, preventing activation by agonist chemicals.
  • Their effects continue until new receptor sites or cells are created.
  • Effects cant be overcome by increased doses of agonist chemicals.
• Partial agonist chemicals bind to the receptor site but do not initiate as much cellular activity as other agonists, lowering the efficacy of other agonist chemicals.

Alternative Mechanisms of Drug Action

• Medications can alter cell, tissue, organ, and system function without directly interacting with receptors.
• Drugs can be engineered to target microorganisms, lipids, water, or exogenous toxic substances.
• Targeting specific substances present in the cell walls of bacteria or fungi (e.g., antibiotics, antifungals).

Specific action drugs

• Chelating agents bind with heavy metals (lead, mercury, arsenic), forming a compound that can be eliminated.
• Sodium bicarbonate breaks down, producing bicarbonate ions that bind with hydrogen ions, raising pH, and decreasing acidity.
• Diuretics create osmotic changes, altering fluid and electrolyte distribution, drawing excess water away from tissues and enhancing urine excretion.
• Electrolyte-based medications change the concentration and distribution of ions in cells and fluids, affecting cell function.

Factors Affecting Response to Medications

• Choice of medication, dose, route, timing of administration, and monitoring all affect response.
• Weight-based dosing matters because medications are distributed into body substances: water, lipids (fat), protein.
• Water-soluble medications require higher weight-based doses in infants than adults/elderly.
• Fat/lipid-soluble medications require higher weight-based doses in the elderly.
• Careful titration is needed when treating pediatric or elderly patients, rather than just administering a weight-based dose.

Pediatric and Elderly Considerations

• Water-soluble and lipid-soluble meds may require increased initial doses to overcome widespread distribution.
• Altered metabolism and elimination in pediatric and elderly patients prolongs effects or results in higher tissue concentrations.
• The cytochrome P450 system affects medication metabolism in the liver and varies significantly in infants, children, and the elderly.
• Declining liver or kidney function requires decreased dosage of many medications.
• Patients at extremes of age are prone to paradoxical medication reactions (experiencing opposite clinical effects).
• Geriatric patients’ doses should likely be halved