Textbook Notes (380,860)
CA (168,244)
UTSC (19,296)
Psychology (10,044)
PSYC62H3 (333)
Chapter 3

chapter 3

9 Pages
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Department
Psychology
Course Code
PSYC62H3
Professor
Suzanne Erb

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Chapter 3
PHARMACOKINETICS
-The difference in potency can be explained on the basis of how easily or quickly these
drugs get to their sites of action, and how quickly they induce those actions.
-In order for a psychotropic drug to exert any effects on an organism, it must get to its
sites of action in the nervous system.
-The effects of a particular drug on an organism depend heavily on the rate of
accumulation and the concentration of the drug at its sites of action and the duration of
the contact at those sites.
-These are a function not only of the amount of drug administered but also of its
pharmacokinetic, which refers to the dynamic processes involved in the movement of
drugs within biological systems with respect to the drug’s absorption, distribution,
binding or localization in tissues, metabolic alteration, and excretion from the body.
-The field of pharmacokinetics is primarily concerned with what the body does to the
drug; together these determine the type, degree, and duration of a drug’s effects.
-Differences among organisms in their pharmacokinetics, due to genetic differences, the
presence of other drugs, disease, and physiological and psychological status, can result in
5- to 20-fold differences in their reactions to a given amount of drug within species, and
20-100 hold differences in their reactions across species.
Drug Absorption
-Must pass through several biological membranes
-For an orally administered drug to exert an action on the brain, it must pass though a
variety of biological membrane before it can reach the cells of the brain that are
responsible for the drug’s psychoactive properties.
-several factors, such as the salt form of the drug, the particle size of the dosage form, and
the suspending agent used, can affect the degree of absorption.
-One of the most fundamental factors affecting the passage of molecules from one side of
a membrane to the other is the relative concentration of the molecule on the two sides.
-The greater the differential concentration of the molecules on the two sides, the more
readily the molecules will diffuse from the site of higher concentration to the site of
lower concentration.
-Because biological membranes consist of lipid materials group of fats and fat like
substances, the most important factor in a drug’s ability to pass though these membranes
is its lipid-solubility =lipophilicity that is its capability of being dissolved in fat.
-Penetration into the brain may decrease if the drugs lipophylicity is too high but in
general it is increases to a optimal penetration point= the faster if can get to the brain to
exert the psychoactive effect.
-The drug must also be somewhat soluble in water to be effectively transported by the
blood stream, so it can dissolve in the plasma.
-The most effective drugs are the ones that are some what soluble in oil and water.
-The relative solubility in each medium is a major factor in a drugs ability to get into the
brain.
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-To determine this relative affinity, a drug is often added to a mixture of water and
refined olive oil.
-The mixture is shaken violently for a few seconds and then spun around toe separate the
oil and water.
-The drug concentration in the oil is divided by its concentration in the water.
-This figure is referred to as the drug’s oil/water partition coefficient.
-The greater the drugs oil/water partition coefficient, the more easily the drug molecule
can passively penetrate a lipoidal membrane, if other factors are held constant.
-From the site of administration, drugs must cross one or more lipid membranes in order
to be absorbed into the circulatory system (unless injected directly into the blood supply).
-In addition, drugs taken orally or injected into the peritoneal cavity initially pass through
the liver, where they may be structurally altered (metabolism) prior to entering the blood.
-Once in the blood supply, the free, unbound drug molecules- those not bound to plasma
protein- will be distributed to various organs and the redistributed to other organs and
tissues by the blood.
-At some point, an equilibrium is often established whereby the relative concentration of
drug in tissue and blood is maintain at a constant level (the blood-to-tissue ratio varies
depending on the tissue and whether there are specific binding sites for the drug).
-Metabolism of drugs may occur in may tissues, but most metabolisms occurs in the liver.
-From the liver, drugs or their metabolites may then renter the blood supply or may be
absorbed into the bile, which eventually ends up in the intestinal tract.
-From the intestinal tract, drugs or their metabolites may be eliminated in feces or
reabsorbed into the circulatory system
-Water-soluble drugs or metabolites enter the kidneys and are excreted in urine.
-If they are still lipid-soluble, they will be reabsorbed into the circulatory system.
-A small (generally negligible) amount of drugs may be eliminated via the lungs, sweat,
tears, saliva, or mother’s milk.
Routes of Administration
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-Most common drug administration is through the mouth (per os or P.O); passes through
the GI tract, safest, cheapest, and convenient
-Another way is rectal administration.
-Several factors can influence absorption of drugs from the G.I tract, which can greatly
alter the rate of drug accumulation, its concentration and its duration at the sites of action.
-Most drugs are thought to penetrate the G.I mucosa by a process of passive diffusion,
which in turn is limited mostly by the drug’s lipid-solubility.
-Many drugs are weak bases or weak acids, and this alkalinity or acidity often results in
their being ionized.
-Such ionization reduces their solubility in cellular membranes, because proteins
embedded in these membranes have a mixture of positive and negative charges that tend
to repel charged particles.
-To facilitate their absorption, such drugs are commonly administered in the form of a
salt, a compound formed by the combination of a base and acid.
-Drugs may exist as both the nonionized and ionized species.
-The nonionized form is more lipid-soluble than the ionized form, the proportion of non-
ionized to ionized drug molecules present in a given area is important for drug absorption
-The pH of the local area determines the ratio of ionized to nonionized rug in that area.
-Weak acids, like aspirin, are less ionized in an acid medium and are therefore more
lipid-soluble through the stomach, with a pH of less than 3.
-Alkaloids like heroin, morphine, ad cocaine are poorly absorbed from the stomach. The
small intestine is slightly alkaline which favours the absorption of weak bases.
-This explains why the greater surface area of small intestine.
-People get intoxicated faster when they drink carbonated alcoholic drinks because it’s
more alkaline and it goes faster to the small intestine and is absorbed faster.
-Some drugs are poorly absorbed from any part of the G.I tract because even in their
nonionized state they have low lipid-solubility.
-Other major factors in absorption of drugs in the G.I tract include the concentration of
the drugs, the rate of movement of the contents through the tract, G.I blood flow,
digestive secretion and G.I contents.
-The effect of the movement of the contents through the G.I tract would depend on where
the drug is best absorbed; eg. Rapid emptying of the stomach would decrease the rate of
absorption of weak acids, like aspirin.
-Rapid movement through the intestine could decrease the amount of drug absorbed
because of a shorter duration of contact (or because the drugs is a weak acid)
-Slow movement through the intestine would generally favour drug absorption, due to
better contact with the absorbing cells lining the intestine.
-Because intestinal blood carries drugs away from the intestine and maintain a
concentration gradient conducive to drug absorption.
-Digestive secretion can inactivate some drugs or alter intestinal pH and drug ionization
-The content of the G.I tract that is if there is food can bind to the drug or dilute it so that
it is slowly absorbed.
Disadvantages to oral route
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Description
Chapter 3 PHARMACOKINETICS -The difference in potency can be explained on the basis of how easily or quickly these drugs get to their sites of action, and how quickly they induce those actions. -In order for a psychotropic drug to exert any effects on an organism, it must get to its sites of action in the nervous system. -The effects of a particular drug on an organism depend heavily on the rate of accumulation and the concentration of the drug at its sites of action and the duration of the contact at those sites. -These are a function not only of the amount of drug administered but also of its pharmacokinetic, which refers to the dynamic processes involved in the movement of drugs within biological systems with respect to the drugs absorption, distribution, binding or localization in tissues, metabolic alteration, and excretion from the body. -The field of pharmacokinetics is primarily concerned with what the body does to the drug; together these determine the type, degree, and duration of a drugs effects. -Differences among organisms in their pharmacokinetics, due to genetic differences, the presence of other drugs, disease, and physiological and psychological status, can result in 5- to 20-fold differences in their reactions to a given amount of drug within species, and 20-100 hold differences in their reactions across species. Drug Absorption -Must pass through several biological membranes -For an orally administered drug to exert an action on the brain, it must pass though a variety of biological membrane before it can reach the cells of the brain that are responsible for the drugs psychoactive properties. -several factors, such as the salt form of the drug, the particle size of the dosage form, and the suspending agent used, can affect the degree of absorption. -One of the most fundamental factors affecting the passage of molecules from one side of a membrane to the other is the relative concentration of the molecule on the two sides. -The greater the differential concentration of the molecules on the two sides, the more readily the molecules will diffuse from the site of higher concentration to the site of lower concentration. -Because biological membranes consist of lipid materials group of fats and fat like substances, the most important factor in a drugs ability to pass though these membranes is its lipid-solubility =lipophilicity that is its capability of being dissolved in fat. -Penetration into the brain may decrease if the drugs lipophylicity is too high but in general it is increases to a optimal penetration point= the faster if can get to the brain to exert the psychoactive effect. -The drug must also be somewhat soluble in water to be effectively transported by the blood stream, so it can dissolve in the plasma. -The most effective drugs are the ones that are some what soluble in oil and water. -The relative solubility in each medium is a major factor in a drugs ability to get into the brain. www.notesolution.com
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