pharmacologyConcepts12Jul06-1

pharmacologyConcepts12Jul06-1 - Concepts of Pharmacology -...

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1 Concepts of Pharmacology - Half Life Calculation - C. M. Prada, MD July 12, 2006
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2 Pharmacokinetics = availability - dosage and rate of administration - modes of transport – across biologic membranes; bound to proteins from plasma and tissues - blood flow to the site of action - extent and speed of the metabolic process of the drug - rate of the removal of the drug (and its metabolic products) Pharmacodynamics = pharmacologic effect (in relation with the plasma drug concentration) - cellular mechanisms of drug action - clinical evaluation of drug effects - biologic variability
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3 Definition : quantitative study of absorption, distribution, metabolism, and elimination of chemicals in the body, as well as the time course of these effects. Summary: - absorption - distribution - metabolism - elimination
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4 Concentration of a drug at its site of action is a fundamental determinant of its pharmacologic effects. Drugs are transported to and from their sites of action in the blood – because of that: the concentration at the active site is a function of the concentration in the blood . The change in drug concentration over time in the blood, at the site of action, and in other tissues is a result of complex interactions of multiple biologic factors with the physicochemical characteristics of the drug.
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5 medicine movement
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6 Pharmacokinetic Concepts: Rate Constants and Half-Lives Disposition of most drugs follows first-order kinetics a constant fraction of the drug is removed during a finite period of time. This fraction is equivalent to the rate constant of the process. Rate constants: k ; min -1 or h -1 The absolute amount of drug removed is proportional to the concentration of the drug In first-order kinetics , the rate of change of the concentration at any given time is proportional to the concentration present at that time. When the concentration is high, it will fall faster than when it is low. First-order kinetics apply not only to elimination, but also to absorption and distribution.
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7 Half-Lives The rapidity of pharmacokinetic processes is often described with half-lives Half-Life = the time required for the concentration to change by a factor of 2. Half-Life = the period of time required for the concentration or amount of drug in the body to be reduced to exactly one-half of a given concentration or amount. Half-Life = the time required for half the quantity of a drug or other substance deposited in a living organism to be metabolized or eliminated by normal biological processes. Also called biological half-life .
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8 Zero-Order Elimination approximately constant rate of elimination
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9
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10 First-Order Elimination Ct is concentration after time t C 0 is the initial concentration ( t =0) k is the elimination rate constant - first-order logarithmic process - that is, a constant proportion of the agent is eliminated per unit time (Birkett, 2002) Birkett DJ (2002). Pharmacokinetics Made Easy (Revised Edition). Sydney: McGraw-Hill Australia. ISBN 0-07-471072-9 .
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12
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14 First-Order Elimination (cont.) dC 1). ------ = kC dt
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pharmacologyConcepts12Jul06-1 - Concepts of Pharmacology -...

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