© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Fundamentals of Pharmacology for Veterinary Technicians Chapter 4 Pharmacokinetics

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Pharmacokinetics Pharmacokinetics is the physiological movement of drugs Four steps: –Absorption –Distribution –Biotransformation (metabolism) –Excretion

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Movement of Drugs Across the Cell Membrane Pharmacokinetics includes the movement of substances across cell membranes Basic mechanisms: –Passive diffusion –Facilitated diffusion –Active transport –Pinocytosis/phagocytosis

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Movement of Drugs Across the Cell Membrane Passive diffusion: movement of particles from an area of high concentration to an area of low concentration –Good for small, lipophilic, nonionic particles Facilitated diffusion: passive diffusion that uses a special carrier molecule –Good for bigger molecules that are not lipid soluble

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Movement of Drugs Across the Cell Membrane Active transport: molecules move against the concentration gradient from areas of low concentration of molecules to areas of high concentration of molecules; involves both a carrier molecule and energy –Good for accumulation of drugs within a part of the body Pinocytosis/phagocytosis: molecules are physically taken in or engulfed. Pinocytosis is engulfing liquid; phagocytosis is engulfing solid particles –Good for bigger molecules or liquids

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Movement of Substances Across the Cell Membrane

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Absorption Drug absorption is the movement of drug from the site of administration into the fluids of the body that will carry it to its site(s) of action Drug factors include drug solubility, pH, and molecular size Patient factors include the animal’s age and health status

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Absorption Terms Bioavailability: percent of drug administered that actually enters the systemic circulation Ionization: the property of being charged –Hydrophilic = ionized –Lipophilic = nonionized Nature of the drug: pH of drug –Weakly acid drugs = hydrophilic form in alkaline environment –Weakly alkaline drugs = hydrophilic form in acid environment Ion trapping: when drugs change body compartments, they may become ionized and trapped in the new environment Drug form is important; oral drugs must have different properties than parenteral drugs

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Distribution Drug distribution is the physiological movement of drugs from the systemic circulation to the tissues Goal of distribution is for the drug to reach the target tissue or intended site of action Factors affecting drug distribution: –Membrane permeability: large molecules cannot pass through fenestrations of blood capillaries –Tissue perfusion: relative amount of blood supply to an area or body system –Protein binding: drugs bound to proteins cannot leave the capillaries due to the size of the protein –Volume of distribution: how well a drug is distributed throughout the body based on the concentration of drug in the blood

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Biotransformation Biotransformation is also called drug metabolism, drug inactivation, and drug detoxification Biotransformation is the chemical alteration of drug molecules by the body cells of patients to a metabolite that is in an activated form, an inactivated form, and/or a toxic form Primary site of biotransformation is the liver

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Interactions Affecting Drug Metabolism Drug interactions may affect metabolism: –Altered absorption: one drug may alter the absorption of other drugs –Competition for plasma proteins: drug A and drug B may both bind to plasma proteins; one may have a higher affinity than the other –Altered excretion: some drugs may act directly on the kidney and decrease the excretion of other drugs –Altered metabolism: the same enzymes may be needed for biotransformation of two drugs that are prescribed at the same time for an animal

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Interactions Affecting Drug Metabolism Drug interactions may affect metabolism (cont.): –Inducing the enzyme system: some drugs alter metabolism by causing liver enzymes to become more efficient –Liver issues: the liver’s ability to metabolize drugs may be impaired –Tolerance: the animal may develop tolerance or decreased response to a drug

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Elimination Drug elimination is removal of a drug from the body (excretion) Renal elimination of drugs involves –Glomerular filtration –Tubular secretion –Tubular reabsorption Liver elimination of drugs is also important Other elimination routes include the intestine and through milk

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Drug Elimination Terms Withdrawal time: period of time after drug administration during which the animal cannot be sent to market for slaughter and the eggs or milk must be discarded because of the potential for drug residues Half-life: time required for the amount of drug in the body to be reduced by half of its original level Steady state: point at which drug accumulation and elimination are balanced

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. How Do Drugs Work? Drugs work in a variety of ways: –Drugs alter existing cellular functions –Drugs alter the chemical composition of body fluids –Drugs can form a chemical bond with specific cell components on target cells within the animal’s body

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Receptors Receptors are three-dimensional proteins or glycoproteins –Located on the surface, in the cytoplasm, or within the nucleus of cells Affinity is the strength of binding between a drug and its receptor –High-affinity drugs bind more tightly to a receptor than do low-affinity drugs

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Receptors

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Agonist vs. Antagonist Agonist: drug that binds to a cell receptor and causes action Antagonist: drug that inhibits or blocks the response of a cell when the drug is bound to the receptors

© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation. Agonist vs. Antagonist