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med school pre Rough Review > Pharmocology Answers > Flashcards

Pharmocology Answers Flashcards

1
Q

Define bioavailability

A

Bioavailability is the fraction of administered dose of a drug that reaches the systemic circulation

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2
Q

Nitroglycerin and insulin ate ineffective after oral administration. Why?

A
  • Insulin is a protein: it is degraded by peptides in the gastrointestinal tract.
  • Nitroglycerin has a very high first pass metabolism: 90% of an oral dose is inactivated before systemic circulation is reached
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3
Q

What is the main site of drug absorption for orally administered drugs? Why?

A

The main site of drug absorption is the small intestine. This is due to:

  • larger surface area available for absorption as compared with that of the stomach.
  • Greater blood flow than that of the stomach
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4
Q

Chloroquine has a Vd of 13000 L in a 70-Kg man. Explain

A

Drugs with very high Vd have much higher concentrations in the extra vascular tissue than in the vascular compartment. A drug with such high Vd is likely to be partitioned to peripheral tissues, or partitioned to body fat

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5
Q

What is the relationship between the half-life of a drug, its volume of distribution, and its clearance?

A

The equation that relates half-life with Vd and CL is: T1/2=0.693 X Vd/CL . Therefore the half life is directly proportional to the volume of distribution and inversely proportional to the clearance

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6
Q

Define clearance

A

Clearance is the volume of plasma cleared of drug per unit time. Units are volume/time. Note: it is NOT the amount of drug cleared per unit time

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7
Q

Explain why clearance is constant for the wide majority of drugs

A

This occurs because physiological mechanisms of elimination of drugs (enzymes and transporters) are not saturated at the drug concentrations used.

The dependence of the rate of the drug elimination on drug plasma concentration is described by the equation :

Rate of elimination = (Vmax x C)/( Km+C)

Where Km represents the plasma concentration at which half of the maximal rate of drug elimination is reached, and Vmax is equal to the maximal rate of elimination. (Note that this equation is entirely analogous to the Michaelis-Mentenequation for enzyme kinetics.) When the drug concentration is very low compared to the Km (C<

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8
Q

What are the factors that affect half-life?

A

Effects on Vd-
Obesity

Pathologic fluid

Effects on CL-
Ageing

CYP induction
CYP inhibition

Cardiac failure

Liver failure

Renal failure

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9
Q

How does obesity affect half life?

A

Increase

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10
Q

How does pathologic fluid affect half life?

A

Increase

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11
Q

How does ageing affect half-life?

A

Increase

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12
Q

How does CYP induction affect half life?

A

Decrease

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13
Q

How does CYP inhibition affect half life?

A

Increase

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14
Q

How does cardiac failure affect half life ?

A

Increase

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15
Q

How does liver failure affect half-life?

A

Increase

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16
Q

How does renal failure affect half-life?

A

Increase

17
Q

Explain why some drugs exhibit zero order kinetics of elimination. Describe the main features of zero kinetics of elimination.

A

The dependence of the rate of drug elimination on drug plasma concentration is described by the equation:
Rate of elimination: (Vmax x C)/(Km + C)

When the drug concentration is significantly higher than the Km (C» Km) the equation reduces to:

Rate of elimination= (Vmax x C)/C= Vmax

For drugs eliminated by zero order kinetics the enzymes and transporters responsible for eliminating the drug are saturated when the therapeutic doses are given. The enzymes and transporters are therefore working at their maximal velocity (Vmax). The rate of elimination is therefore maximal and independent of drug concentration. In these conditions, a constant amount of drug is eliminated per unit time. (Recall that in first-order kinetics a constant fraction of drug is eliminated per unit time). This situation is observed with a small number of drugs (such as aspirin at high doses, ethanol, and phenytoin)

18
Q

List the features of non-competitive antagonism

A
  • The antagonist binds to an allosteric site on the receptor to which the agonist binds
  • The antagonism is insurmountable. The Emax is decreased

med school pre Rough Review (7 decks)

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