Biopharmaceutics and Pharmacokinetics

Q: The term "prodrug" refers to a

(a) :  chemical substance that is part of the synthesis procedure in preparing a drug
(b) :  compound that liberates an active drug in the body
(c) :  compound that may be therapeutically ent active but is still under clinical trials
(d) :  drug that has only prophylactic activity in the body
(e) :  drug that is classified as being "probably effective"
Answer:  compound that liberates an active drug in the body
Explanation:  In order to take advantage of certain desirable characteristics, some drugs are marketed as prodrugs. These are chemical modifications of biologically active drugs and are not active themselves. However, the active form of the drug, the metabolite, is liberated in the body by biotransformation. Prodrugs may possess better water solubility, be morestable, have a less objectionable taste, or give higher blood levels than the parent compound.

Q: A knowledge of Vd for a given drug is useful because it allows us to

(a) :  estimate the elimination rate constant
(b) :  determine the biological half-life
(c) :  calculate a reasonable loading dose
(d) :  determine the best dosing interval
(e) :  determine the peak plasma concentration
Answer:  calculate a reasonable loading dose
Explanation:  Because the Vd is a parameter that allows accountability for all of the drug in the body, it can be used to calculate the loading dose that would rapidly result in a desired plasmaconcentration (Cp)'. (Vd)(Cp) Loadmg dose = (S) (F) where S= portion of the salt form that is active drug and F= fraction of dose absorbed.

Q: The equation that describes the process of "passive transport" is

(a) :  Fick's law
(b) :  Henderson-Hasselbalch
(c) :  Noyes-Whitney
(d) :  Stoke's law
(e) :  Michaelis-Menten
Answer:  Fick's law
Explanation:   Fick's First Law states that the rate of diffusion is directly related to the diffusion coefficient and the surface area of the membrane and inversely proportional to the thickness of the membrane. The driving force in the equation is the concentration gradient, because the greater the difference in concentrations on each side of the membrane, the greater the amount of drug diffusing (first-order kinetics).

Q: All of the following drugs are believed to undergo significant first-pass hepatic biotrans formation EXCEPT

(a) :  lidocaine
(b) :  morphine
(c) :  nitroglycerin
(d) :  phenytoin
(e) :  propranolol
Answer:  phenytoin
Explanation:  Only 5% of phenytoin is metabolized in the liver; most of the drug is excreted unchanged. When significant amounts of a drug are metabolized by the liver immediately af- ter absorption through the GI tract wall (first-pass effect), manufacturers may compensate by increasing the dose present in oral dosage forms. For example, propranolol is availableas 40- and 80-mg tablets, whereas the parenteral form is 2-mL ampules containing 2 mg/mL.

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