Glossary of Pharm: Principles of Drug Action

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Define First Order kinetics
process by which a constant percentage of substrate is metabolized per unit time (i.e half life)
Define zero order kinetics
a constant amount of drug is metabolized per unit time
(i.e alcohol)
what is an agonist?
a drug that binds to and activates a receptor
what is a full agonist?
a drug that binds to a recptor and has the ability to produce 100% of the maximal response
what is a partial agonist?
drugs that bind but produce less than 100% of the maximal possible biological response no matter how high their concentration
what are antagonists?
drugs that bind to receptors or other drugs and INHIBIT a biological response
What does a competitive antagonist do?
it binds reversibly to the same active site as the agonist, shifts the dose response curve to the right, increases the ED50
How can a competitive antagonist be overcome?
by increasing the concentration of the agonist; the maximal efficacy of the drug will not change
what does a non competitive antagonist do?
It binds irreversibly to a different site on the nzm than the agonist; it cannot be overcome by increasing the concentration of the drug; lowers Emax
How does a non competitive antagonist effect the efficacy of a drug?
it reduces the maximum efficacy will be reduced
Define efficacy
the ability to produce a biological effect
Define potency
a measure of the amount of drug needed to cause a biological effect; demonstrated by the ED50
What is Kd?
the conc. of drug yeilding 50% occupancy of the receptor; dissociation constant
What is EC50?
the drug concentration that produces 50% of the maximal possible response in a graded dose-response curve
What is volume of distribution (Vd)?
the apparent volume into which a drug is able to distribute
Vd =
= total drug in the body/ plasma concentration of drug

= x/Cp
What is the significance of a large Vd?
a large Vd signifies that most of the drug is being sequestered in some organ or compartment
What is maintenance dose?
a dose of drug given to achieve a therapeutic plasma concentration over an extended period of time
Maintenance Dose =
= Clearance x desired steady state concentration
= CL x Css
What is loading dose?
In some clinical situations the desired plasma concentration must be acheived quickly. In these cases a single loading dose is injected, followed by a routine maintenance dose
Loading Dose =
= Vd x desired Cp
Define peak and trough concentrations
maximum and minimum plasma concentrations, which are observed during dosing intervals
What variables affect peak and trough concentrations?
they will fluctuate around the steady state plasma concentration
What is the steady state plasma concentraion (Css)?
the point at which the rate of drug availability is equal to the rate of drug elimination
How does frequency of dosing affect Css?
it doesn't
How many half lives are required to reach Css?
approximately 4.3
What is clearance (CL)?
volume of plasma cleared of drug per unit time
What is therapeutic index?
the ratio of a drug's toxic dose to its therapeutic dose
What do pharmacokinetics describe?
Where the drug goes and how much needed for desired effect.
What factors affect pharmacokinetics?
Absorption, distribution, metabolism, and excretion.
What affects absorption?
Rate is affected by route of drug administration and extent is determined by metabolism and lipid solubility.
What affects the distribution?
Diffusion across gradient, lipid/water partitioning, charge and pH.
Are drugs more likely to be trapped in ionized or neutral form?
Ionized, since ions don't easily cross membranes.
What is first pass metabolism?
The reduction in bioavailability in orally administered drugs because of metabolism in the liver.
What is the half-life of a drug?
half-life = 0.693 * Vd/CL
What are pharmacodynamics?
This term describes how a drug works. This is determined by 1) drug-receptor interactions; 2) duration of drug action.
What are the 2 phases in drug metabolism?
Phase I = conversion of drug to more polar or reactive compound; Phase II = increase water solubility to increase water solubility.
What are the cytochrome P-450 group of enzymes?
Enzymes located in the smooth ER which help catabolize drugs.
What is an alkanizing agent?
What is and acidifying agent?
ammonium chloride
What are some factors that affect Vd?
disease (obesity, renal disease), size (capillary permeability), lipid solubility, and charge
What are the 3 basic reactions in Phase I of drug metabolism?
oxidation, reduction, and hydrolysis
P-450 is responsible for which Phase I reaction?
What are 4 reactions in Phase II drug metabolism?
glucoronidation, sulfation, acetylation and glutathione conjugation
Which is the most important organ in drug metabolism?
Liver is most important, followed by the kidney
What are the 2 components of the cytochrome P-450 enzyme system?
oxidase and reductase
What are examples of P-450 inducers?
barbiturates, ethanol, which enhance the metabolism of warfarin (anticoagulant) and tylenol (analgesic), respectively.
What are examples of P-450 inhibitors?
cimetidine and allopurinol, which decrease the metabolism of valium and tolbutamide, respectively
what is an inverse agonist?
inhibits receptor by locking it in an inactive conformation

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