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PHARMACOKINETICS AND PHARMACODYNAMICS CH 3 INTRODUCTION • Pharmacokinetics: the study of how a drug moves into, through, and out of the body • How is it absorbed into the body, where is it distributed to, how it is metabolized, how is it eliminated • Think: what happens to the drug while it is in the body? • Pharmacodynamics: the study of how the drug produces its effects on the body • What tissues are effected/what changes are made because of the drug • Think: what happens to the body because of the drug? • Therapeutic range: ideal range of drug concentrations in the body. • Also called the therapeutic window • The manufacturer's dose should achieve concentrations in the therapeutic range. • Too much drug will make concentrations in the body exceed the top of the therapeutic range (maximum effective concentration) THERAPEUTIC RANGE • Insufficient drug amounts will cause the concentration of drug in the body to be below the therapeutic range (minimum effective concentration) • To maintain a drug’s therapeutic range, the amount of drug entering the body must be balanced with the rate that the drug leaves the body. THERAPEUTIC INDEX • Therapeutic index: the comparison between a drug’s ability to produce the desired effect and its tendency to produce toxic effects. • Expressed as a ratio between the LD50 and the ED50 • LD50= dose of a drug that is lethal in 50% of animals in the clinical trial • ED50= dose of a drug that is effective in 50% of animals in the clinical trial • TI= LD50/ED50 • The larger the TI, the safer the drug DOSAGE REGIMEN AND ROUTES OF ADMINISTRATION • Dose, dosage interval, and route of administration are the three components of the therapeutic administration of drugs • Dose: the amount of drug administered at one time • LOADING DOSE: amount of drug designed to raise the drug concentration to the therapeutic range in a short time. • Larger than the maintenance dose. • Administered as either a large amount once or small amounts frequently. • Used in situations where achieving the effect of the medication quickly is critical • MAINTENANCE DOSE: maintains therapeutic concentrations established by the loading dose. • Smaller than the loading dose DOSAGE REGIMEN AND ROUTES OF ADMINISTRATION • Dosage interval: time between administrations of separate drug doses • Once daily dosage interval produces a wide swing in drug concentrations (high concentration after the drug is absorbed, possibly toxic, and a low concentration right before the next dose, possibly subtherapeutic). • More frequent dosing results in less of a difference in drug concentrations throughout the day. This option would also be more likely to result in concentrations in the therapeutic range. • Total daily dose: the combined amount of drug (in mass) administered in a given day • Ex: 100 mg would be the total daily dose of a 25mg tablet was given every six hours. The total daily dose would be the same if it was a 50mg tablet given BID. DOSAGE REGIMEN AND ROUTES OF ADMINISTRATION • Route of administration: the means by which a drug is given. • Parenteral: a route that does not involve the GI tract (ex: injection) • Intravenous: drug is injected directly into a vein. It is carried toward the heart where it is diluted before being distributed throughout the body. • IV bolus- single, large volume injected at once • IV infusion- drug is slowly injected (dripped) into a vein over second, minute, or hours. Results in a steady accumulation of drug concentrations in the body until the drug concentrations plateau (steady state) • The accidental injection of an IV drug outside the vein is termed extravascular or perivascular. • Intra-arterial: drug is injected directly into an artery. It is carried toward a specific organ or tissues at high concentrations DOSAGE REGIMEN AND ROUTES OF ADMINISTRATION • Intramuscular: drug is injected directly into a muscle • Subcutaneous: drug is administered beneath the skin • Intradermal: drug is administered within the skin • Intraperitoneal: drug is administered within the abdominal cavity • Per os: drug given by mouth • Topical: drug administered on the surface of the skin • Aerosol: drug is administered in an inhaled mist or gas and absorbed into the airways • PASSIVE DIFFUSION: the random movement of drug molecules from an area of high to low concentration • Down the concentration gradient. • The majority of drug movement through tissue fluid or membranes occurs this way. Drug moves from the site of administration (high concentration) to other areas of the body (low concentration). • Once equilibrium is achieved, the drug molecules may still move, but will do so evenly throughout the tissues. MOVEMENT OF DRUG MOLECULES • No energy is used. • For a drug to diffuse across the phospholipids of a membrane, it must be able to dissolve in fat (since cell membranes are made of phospholipids). • FACILITATED DIFFUSION: diffusion across a cell membrane that involves a carrier molecule within the membrane. • Protein with receptor sites for drug molecules MOVEMENT OF DRUG MOLECULES • When a drug molecule fits into the receptor, the carrier protein changes to allow the drug to pass across the membrane (no dissolving required) • No energy needed (passive process) • Direction of movement is down concentration gradient • ACTIVE TRANSPORT: Carrier protein assists in moving the drug molecules across the membrane without the limitations of a concentration gradient. • Energy is required • Does not result in an equal amount of drug molecules on each side of the membrane MOVEMENT OF DRUG MOLECULES • Less common method of movement • PINOCYTOSIS: Part of the cell membrane forms an invagination to take in drug molecules (cell drinking) • A form of active transport that is not often used • Slow process because the cell membrane has to change its shape MOVEMENT OF DRUG MOLECULES • PHAGOCYTOSIS: Part of the cell membrane surrounds/engulfs the drug molecules (cell eating) • A form of active transport that is not often used • Slow process because the cell membrane has to change its shape MOVEMENT OF DRUG MOLECULES • Factors that affect rate of drug molecule transport: • Facilitated diffusion and active transport • Number of carrier proteins available • Diffusion • A large the concentration gradient makes drug molecules move quickly • Drug molecule size (smaller will move faster) • Lipophilic molecules will be able to dissolve within the membrane quickly. Hydrophilic molcules will require a carrier protein • The higher the temp, the faster the diffusion rate • The thicker the membrane, the slower the diffusion rate