Pharmacokinetics
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| Biopharmaceutics | study of the interrelationship of the physical and chemical properties of the drug, dosage form, and route of administration on the rate and extent of systemic drug absorption
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| What does biopharmaceutics provide? | provides the scientific basis for design and development of dosage forms
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| pharmacokinetics | study of the time course of drug absorption, distribution, metabolism, excretion--a scientific discipline that deals with mathematical description of biologic processes affecting drugs
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| Two types of PK | experimental, theoretical
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| experimental pk | developing biological sampling techniques and analytical methods for measurement of drug concentration
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| theoretical pk | development of mathematical models that predict drug disposition after drug administration
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| therapeutic window | (experimental pk) more clinically useful index of safety-- describes the dosage range between the minimum effective therapeutic dose, and the minimum toxic dose
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| therapeutic index | (experimental pk) estimates the safety of the drug--calculated ration of the LD50 to the ED50
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| three other types of pk | population, regional, clinical
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| types of theoretical pk models | compartment, physiologic pk, empirical
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| population pk | identification, quantification, and explanation of interpatient variability in developing drug dosing strategies- optimizing dosing strategies for a population or subgroup
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| what does population pk examine? | examines the relationship of demographic, genetic, pathophysiological, environmental, and other drug related factors that contribute to variability observed in safety and efficacy of the drug
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| bayesian theory | provides a quantitative tool for incorporating subjective judgment with objective data in making risk decisions, especially when complex decisions involving several variables are involved
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| what was bayesian theory originally developed to do? | to improve forecast accuracy in medicine by combining subjective prediction with newly collected laboratory data
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| equation for bayesian theory | prob (pk parameter/plasma drug concentration) = [prob(P) * (Prob(C/P)] / Prob(C)
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| regional pk | the study of pk within a given defined anatomic area of the body between specific afferent and efferent blood vessels- provides a link between systemic pk and molecular pharmacology
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| clinical pk | application of pharmacokinetic principles to the safe and effective therapeutic management of drugs in an individual patient
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| theoretical approach to developing a kinetic model (6 steps) | 1. conceptualize the system, 2. codify current facts, 3. test competing hypotheses, 4. identify controlling factors, 5. estimate inaccessible system variables, 6. predict system response under new conditions
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| clinical application of pk models | plasma, tissue, urine drug concentrations, optimum dosage regimen, interactions, concentrations correlate to therapeutic effect, bioavailability
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| basic assumption in pk | changes in plasma drug concentrations directly reflect changes in drug concentration in tissues where the disease process is being modified by the drug (may not be true for all drugs)
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| principle of kinetic homogeneity | predictable relationship between plasma drug concentration and concentration at receptor site, plasma concentration does not equal the receptor site C but indicated how it changes over time- how therapeutic C are established
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| compartment models | body can be represented as a series or system of compartments that communicate reversibly with each other- compartment represent a group of similar tissues or fluids
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| one compartment model | most frequently used in clinical practice
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| basic assumption of one compartment model | all body tissues and fluids are considered as a part of this single compartment- distribution is instantaneous
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| basic assumption- two compartment model | body is compromised of rapidly distributing tissues called central compartment, and slowly distributing tissues- peripheral compartment
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| central compartment | highly perfused organs often have similar drug distribution patterns
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| peripheral compartment | tissues or organs to which drugs distribute slowly
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| mammillary model | most common compartmental model used in pk
consists of one or more peripheral compartments- elimination occurs from the central compartment
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| caternary model | consists of compartment joined to one another like a train
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| physiologic pk model | describes drug movement and disposition in the body based on organ blood flow and organ spaces penetrated by the drug
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| two types of PPK model | blood flow-limited (perfusion) or diffusion limited (membrane-limited)
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| basic assumption of blood flow-limited model | transmembrane movement of a drug is rapid without any resistance from capillary membrane- tissue to venous drug blood concentration ratio is constant- drug doesn't bind either to plasma or tissues
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| basic assumption of diffusion-limited model | cell membrane acts as a major barrier for drug, blood flow is very rapid but drug penetration is slow thus, drug concentration gradient is established between tissue and venous blood- time lag in equilibration between blood and tissue is complicated
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| diffusion-limited model | for tissue distribution in which the tissue is subdivided into compartments representing capillary bed, interstitial fluid, and intercellular space (Q is blood flow)
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| advantages of ppk models | drug concentrations are calculated based on tissue size and blood flow, predicts pk of a drug when only animal data is available
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| limitation of ppk models | information required for adequately describing a ppk model are experimentally difficult to obtain
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| interspecies scaling- application of ppk model | allometric method that allows approximate interspecies scaling based on size, aging rate, and life span of species- uses a physiologic variable that is graphed against body weight of the species on log-log axes= linear relationship,
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| basic assumptions of interspecies scaline | physiological variables like HR, organ weight are related to the weight or body surface area of the species, all mammals use same energy source and energy transport systems across animal species
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| summary of compartment model | simple and flexible for clinical pk application, extrapolation to specifc tissue compartments are not accurate, empirical model lacks physiologic relevanct
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| summary of ppk model | realistic model that accounts for distribution, binding, metabolism, and blood flow of drug, reliable estimations based on tissue size and blood flow, complex model
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| statistical moment theory (SMT) empirical pk | study time-related changes in macroscopic events
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| macroscopic event | overall event brought about by constitutive elements involved
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| mean residence time (MRT) | describes the average time for all the drug molecules to reside in the body- uses the area under the curve
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| MRT = AUMC/AUC | AUMC- area under the first moment vs time curve from zero to infinity/ AUC- area under plasma time versus concentration curve from time zero to infinity
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| intestinal reserve length | concept interrelates physiological, physicochemical and dosage form factors that affect drug absorption
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| predictive capability of compartment pk | plasma conc vs time, population pk, variability within specific population based on fitted rate constants from in vivo experiments
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| predictive capability of reserved length | effect of drug release/dissolution, transit, and permeability on fraction of drug absorption
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| predictive capability of macroscopic mass balance approach | effect of drug release/dissolution, transit and permeability on fraction of drug absorption
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| predictive capability of compartmental absorption and transit model | plasma conc vs time based on pk and biopharmaceutic data of the compound
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| predictive capability of physiological based pk | plasma conc vs time profile based on tissue kp data that can be scaled between species
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