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Questions and Answers
What does pharmacodynamics study?
Which of the following statements about drug metabolism is correct?
Which route of drug excretion is NOT typically considered main?
What is the effect of high plasma protein binding on a drug's distribution?
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Which statement about first-pass metabolism is correct?
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What factor influences the absorption rate of oral drugs?
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Which of the following statements about drug ionization is true?
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What does drug elimination encompass?
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Which statement is true regarding phase I and phase II reactions in drug metabolism?
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What is the role of prodrugs in pharmacology?
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How does renal drug excretion relate to the ionization of drugs?
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Which of the following statements is accurate concerning drug half-life?
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Which statement correctly describes the CYP enzyme system?
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What impact does urine pH have on renal drug excretion?
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Which of the following statements about area under the curve (AUC) is true?
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What can be a consequence of drug binding to plasma proteins?
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For Drugs C and D, if they are equivalent in potency, what does this imply about their therapeutic effects?
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Which statement about Drug E and Drug F is true given that Drug E is more efficacious?
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When combining Drugs E and F, what is likely to occur if their individual maximum responses do not reach the combined maximum?
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Why is the combination of morphine and buprenorphine recommended?
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What does a narrow therapeutic index indicate about a drug?
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What is the implication of co-administering a potent CYP inhibitor with a drug that has a narrow therapeutic index?
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How is absolute oral bioavailability calculated for a drug given orally?
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Relative bioavailability compares which of the following?
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Study Notes
Pharmacokinetics
- The study of what an administered drug does to the body.
- Focuses on how the body absorbs, distributes, metabolizes, and excretes drugs.
- The absorption of a drug into the bloodstream.
- The distribution of the drug throughout the body.
- Metabolism of the drug by the body.
- Excretion of the drug from the body.
Pharmacodynamics
- The study of how drugs interact with the body to produce their effects.
Absorption
- The process by which a drug enters the bloodstream.
- Absorption can occur through various routes: oral, intravenous, intramuscular, subcutaneous, topical, rectal, inhalation.
- Factors affecting absorption:
- Drug formulation: Solutions are absorbed faster than tablets.
- Route of administration
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Physiological factors:
- Gastric emptying rate and intestinal motility
- Blood flow to the absorption site.
- pH of the environment (e.g., the acidity of the stomach).
Distribution
- The process by which a drug travels from the bloodstream to the rest of the body.
- Depends on:
- Physicochemical properties of drugs (e.g., lipid solubility, size of the drug).
- Blood flow to different organs and tissues.
- Binding to plasma proteins.
- Presence of barriers such as the blood-brain barrier.
- Factors affecting distribution:
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Physicochemical properties
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Lipid solubility:
- Highly lipid-soluble drugs tend to distribute more readily into tissues.
- Drugs that are highly ionized in the plasma (carry a charge) generally do not distribute into adipose tissues.
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Size:
- Large molecules, such as monoclonal antibodies, are not easily filtered through the glomerular endothelium.
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Lipid solubility:
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Plasma protein binding:
- Drugs with high plasma protein binding tend to distribute less extensively into tissues.
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Physicochemical properties
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Volume of distribution :
- Reflects how widely the drug is distributed in the body.
- High volume of distribution indicates that a drug is distributed extensively into tissues and may have a longer half-life.
- Drugs with high volume of distribution:
- May require a higher loading dose to reach therapeutic levels.
- May have a longer half-life:
- The longer a drug remains in the body, the greater the chance of drug interactions or side effects.
- Drugs with low volume of distribution:
- May have a shorter half-life
- May require more frequent dosing.
Metabolism
- The process by which the body breaks down drugs (and other molecules) into less active forms.
- The liver is the primary site of drug metabolism, but it also occurs in the intestines, kidneys, lungs and blood plasma.
- Phase I reactions
- Often involve oxidation, reduction, or hydrolysis.
- They may result in a drug that is more active, less active, or inactive.
- Phase II reactions
- Involve conjugation of drug metabolites to an endogenous substrate (like glucuronic acid, glutathione) to form a more water-soluble metabolite (and therefore更容易排泄).
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Cytochrome P450 (CYP) enzymes
- A family of enzymes that play a major role in drug metabolism.
- CYP3A4 is the most significant CYP enzyme isoform for drug metabolism.
- Significant drug interactions can be mediated by induction and inhibition of CYP enzyme systems.
- Induction increases the activity of CYP enzymes, leading to a faster metabolism of drugs.
- Inhibition: slows down drug metabolism, potentially leading to higher drug levels in the body.
Excretion
- The process by which the body eliminates drugs and their metabolites.
- The kidneys are the major route of drug excretion, but drugs can also be excreted through:
- the biliary tract
- feces
- sweat
- breast milk
- lungs
- Factors affecting excretion:
- pH of the urine
- Plasma protein binding
- Renal function
Pharmacokinetic Parameters
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Area under the curve (AUC)
- Represents the total drug exposure to the body after a dose.
- It is proportional to the amount of drug absorbed and the rate of elimination.
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Half-life (T1/2)
- The amount of time it takes for the plasma drug concentration to decrease by half.
- Used to calculate the time to reach steady-state plasma concentrations.
- Longer half-life (T1/2) = longer time to reach steady state
- Shorter half-life (T1/2) = shorter time to reach steady state (e.g., 4-5 half-lives)
- Drug with a longer half-life:
- Can be administered at longer dosing intervals.
- Drug with a short half-life:
- May require more frequent dosing.
- May require a loading dose.
Pharmacodynamics
- Describes how drugs produce their effects.
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Agonists
- Drugs that bind to receptors and stimulate a response.
- Full agonists: produce the maximum possible response.
- Partial agonists: produce a submaximal response even at high doses.
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Antagonists
- Drugs that bind to receptors and block the agonist from binding.
- Therapeutic Index (TI)
- Ratio of the toxic dose to the therapeutic dose.
- A larger TI indicates a wider margin of safety, i.e., a drug is less likely to cause toxicity at therapeutic doses.
- A smaller TI indicates a narrower margin of safety and a drug is more likely to cause toxicity at therapeutic doses.
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Bioavailability (F):
- Is the fraction of the administered dose that reaches the systemic circulation.
- Expresses the amount of drug that is available to produce its effect in comparison to the dose given.
- Oral bioavailability:
- Is influenced by first-pass metabolism, which often refers to the metabolism of a drug by the liver after oral administration.
- For a drug with a high first-pass metabolism, oral bioavailability is low, and the oral dose must be higher than the intravenous dose.
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Absolute bioavailability (F):
- The fraction of the administered dose that reaches the systemic circulation relative to an intravenous dose.
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Relative bioavailability (Frel):
- The fraction of the administered dose that reaches the systemic circulation relative to another formulation of the same drug.
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AUC ratio:
- AUC ratio of the drug given via different routes is often used to calculate bioavailability.
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Description
This quiz covers the fundamental concepts of pharmacokinetics and pharmacodynamics, focusing on how drugs are absorbed, distributed, metabolized, and excreted by the body. It also examines the interactions between drugs and the body that result in therapeutic effects. Test your knowledge on drug absorption routes and influencing factors!
