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13 Cards in this Set

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Every type of drug molecule can pass across cell membranes (and thus across cell layers) by?
Diffusion.
What is the most common and thus most important mechanism for transmembrane drug movement?
Diffusion
The rate of transmembrane movement of drug (molecules moving across the membrane per unit time) by diffusion can be predicted/controlled by?
altering the physico-chemical characteristics of the drug molecule.

Like other mechanisms of movement of drug molecules across cell layers, diffusion is dependent on?
surface area of the "diffusion surface", on vascularization of the "diffusion surface", and blood flow through vessels in the "diffusion surface".
Drug molecules can diffuse across the cell membrane in which way?
both directions, into and out of cells.
Drug molecules will diffuse across a membrane even if?
a concentration difference or concentration gradient of the drug across the membrane does not exist.
concentration difference or concentration gradient is?
In this situation - equilibrium - the concentration of drug on each side of the membrane is the same and movement of drug molecules in one direction occurs at the same rate as movement in the other direction. Obviously in this situation the drug concentration on each side of the membrane is maintained constant.
If a concentration difference or concentration gradient for the drug exists across the membrane, then drug movement will be?
from the side with higher concentration to the side with lower drug concentration will be greater than in the opposite direction. Unequal movement driven by the concentration difference will occur until the equilibrium condition (equal concentration of drug on each side of the membrane) is reached.
Rate of diffusion of a drug dependent on concentration difference across membrane and on what certain physical & chemical characteristics of the drug molecule & In totality, these factors are each important and each factor may influence the other factors?
Size & Compactness
Lipid Solubility
Degree of ionization
Explain the Rate of diffusion Size & Compactness
Smaller (molecular weight) molecules crosses membrane more rapidly than large molecules
More compact molecules (in terms of space occupied by each molecule) cross membrane more rapidly than less compact molecules
(These statements assume that lipid solubility and ionization of the molecules is not affected by these parameters)
Explain the Rate of diffusion Lipid Solubility
Drugs may be described as lipophilic (hydrophobic) or hydrophilic (lipophobic)
More lipophilic: higher solubility in fat than aqueous solution
More hydrophilic: more soluble in aqueous solution than in fat
Lipophilic drugs diffuse across a membrane more rapidly than hydrophilic drugs - given the same concentration gradient and assuming same size and compactness.
Explain the Rate of diffusion Degree of ionization
Drug molecules exist in solution in ionized and non-ionized forms
Extent of ionization controlled by acidity (pH) of environment
Ionized form of drug is more soluble in aqueous solution than non-ionized form
Non-Ionized form of drug is more soluble in lipid (including plasma membrane) than ionized form
Non-ionized form of drug can move across membranes by diffusion
Ionized form of drug essentially unable to diffuse across membrane
At equilibrium of diffusion, the non-ionized form of the drug is at?
the same concentration in each of two compartments separated by a membrane but the concentration of the ionized form of the drug may be different in each of these compartments (see section on Ion Trapping).
At equilibrium, movement of the non-ionized drug molecule across the membrane occurs at?
the same rate in each direction.