Drugs & Addiction Exam #2 (Ch.4 Pharmacokinetics)

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Pharmacokinetics

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Processes involved in absorbing, distributing, biotransforming, and eliminating drugs

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Steps in the drug experience (absorption)

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Feedback at some steps complicates the process. Absorption of a drug may increase the rate of its metabolism to defend against toxic effect

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The seven steps of the drug experience

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1. A drug of a specified chemical structure is present
2. A certain quantity of this drug is measured
3. This quantity of the drug is administered in one of a number of possible ways
4. The drug is absorbed into the blood and distributed to site(s) of action
5. Some pharmacological effect is produced
6. In human's a drug's pharmacological effects may be modified depending on characteristics of the person, such as genetic constitution, gender, age, personality, and drug tolerance
7. The setting or context of drug use also may modify a drug's pharmacological effects

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Drug dose (mg drug/kg body weight)

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2 key factors: (1) Amount of the drug taken, (2) Wight or mass of the person taking the drug

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Forms and methods of taking drugs

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oral ingestion, inhalation, injection, topical application

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Routes of administration

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Oral, Injection (hypodermic needle), Subcutaneous (skin popping), Intramuscular (IM), Intravenous (IV) "mainlining"

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Drug absorption

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The rate and extent to which a drug leaves its site of administration

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Absorption influences ____?

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Bioavailability: The portion of a drug that reaches its site of action or a body fluid that takes it to its site of action

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Factors influencing absorption

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Solubility in lipid (fat), form in which the drug is administered: water solution, suspension (not dissolved), oily solution, solid (pill form, slow has to go through stomach walls to get to blood stream), acidity or alkalinity (pH) of the environment, rate of blood circulation at the site of administration

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Intravenous Injection (IV)

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bypasses these factors that retard absorption, puts drug directly into bloodstream, most effective/exact method

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Drug elimination

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Water-soluble drugs may be directly excreted by kidneys and urinary tract

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Metabolism is done by ____?

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Enzymes: in the liver, the gastrointestinal (GI) tract, and the kidneys

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Action of enzymes

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First pass effect: Orally administered drugs may be metabolized in the GI tract before circulating to the rest of the body. As a result, less of the drug reaches its site of action

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Action of metabolites

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Metabolic by-products of drugs may be pharmacologically active. This activity is responsible for some of the side effects of a drug

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Routes of excretion

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Kidneys excrete water-soluble drugs and metabolites in urine. Mother's milk excretes small amounts of drugs

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Zero-order kinetics

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The rate of elimination is unrelated to the concentration in the blood

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First-order kinetics

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The rate of elimination increases with higher levels of the drug in the blood. Most drugs show first-order kinetics

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Drug half-life

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A half-life of a drug is the amount of time needed to reduce the amount of drug in the body by one-half. Half-life relates to kinetics. Reflects the ability of the body to clear the drug

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Drug testing: Urinalysis

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more sensitive because the drug or its long-term metabolites may be detected. Blood testing reveals mainly recent use of the drug

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Validity of drug testing depends on _____?

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The size of the most recent dose, the testing method used, laboratory quality control

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Detectability of drugs depends on ____?

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Clearance rates of the drug & clearance rates of the metabolites

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Potential drug testing problems

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Two-step testing is highly accurate: Screening followed by confirmatory tests of positive screens. Screening may produce false results -- either false negatives or false positives. Only positive test results are followed by confirmatory tests

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False negative findings

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Screening test indicated no use of a drug in a person who did use it

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False positive findings

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Screening test indicated use of a drug in a person who did not use it

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False negatives come from 3 main methods...

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(1) Substitution: switch a clean urine sample for a "dirty" one
(2) Adulteration: Add compounds that interfere with the test
(3) Dilution: Drink large amounts to dilute the drug in the urine below detectable levels

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False positives

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Findings may result from use of legal substances. Some users of illegal drugs get a prescription for similar legal drugs. Positive screens result from ingesting large amounts of poppy seeds