Questions and Answers
What is the study of the actions, mechanisms, uses, and adverse effects of drugs?
Pharmacology
What is the risk associated with the use of any drug?
Adverse effects
What should the physician assess when deciding which drug to prescribe?
The balance of desired and adverse effects
Pharmacology can be divided into how many disciplines?
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What is the first step in the process of pharmacokinetics?
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What happens to the drug during the metabolism phase?
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What is the definition of a drug?
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What is the purpose of using drugs?
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What determines the rate of drug absorption at any given pH?
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What is the pH at which 50% of a drug is ionized and 50% is non-ionized?
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What is the significance of pKa in drug absorption from the GIT?
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How can aspirin toxicity be treated?
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What is bioavailability?
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What is the oral bioavailability of a drug?
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What does an oral bioavailability of 20% mean?
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What is the apparent volume of distribution?
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What is the main objective of pharmacokinetics?
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What is the process of drug absorption?
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What is the effect of alkalinizing the urine on the excretion of a weak acid?
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What is the effect of acidifying the urine on the excretion of a weak base?
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Which of the following routes of administration does not require drug absorption?
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What is the main factor affecting drug absorption?
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What is the rate of penicillin G excretion in urine per minute?
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What is the volume of plasma cleared from penicillin G per minute?
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What is the relationship between molecular size and drug absorption?
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What is the formula to calculate renal clearance of a drug?
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What is the effect of ionization on drug absorption?
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What is the renal clearance of penicillin G if the renal excretion rate is 1200 mg/min and the plasma concentration is 3 mg/mL?
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What is the effect of vascularity at the site of absorption on drug absorption?
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What is the renal clearance of Drug X if 600 mL of urine was collected in one hour and the concentration of Drug X in the urine was 1 mg/mL and the mid-point plasma concentration was 0.1 mg/mL?
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What is the term for the process by which the body removes drugs and their metabolites?
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What is the unit of renal clearance?
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What is the time required for complete drug elimination?
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What is the definition of steady state plasma concentration (Cpss)?
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When is the Cpss reached?
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What is the relationship between the rate of drug administration and the rate of drug elimination at Cpss?
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What is the abbreviation for elimination half-life?
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What is the significance of 5 t1/2 in pharmacology?
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What is the definition of elimination half-life (t1/2)?
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What is the relationship between the rate of drug administration and the rate of drug elimination at steady state?
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Study Notes
Pharmacokinetics
- Pharmacology is the study of the actions, mechanisms, uses, and adverse effects of drugs.
- A drug is any natural or synthetic substance that alters the physiological state of a living organism.
- Pharmacology can be divided into two disciplines: pharmacokinetics and pharmacodynamics.
Pharmacokinetics
- Pharmacokinetics is the branch of pharmacology that studies the movement and disposition of drugs within and by the body.
- It includes:
- Drug absorption
- Distribution
- Metabolism
- Excretion
Drug Absorption
- Definition: passage of drug molecules from the site of administration to the systemic circulation.
- Applies to all routes of administration, except for topical and intravenous routes.
- Requires the drug to cross one or more layers of plasma membranes.
- Factors affecting absorption:
- Molecular size: small molecules are absorbed faster than large molecules.
- Dose and concentration of the drug: absorption increases with increasing dose (up to a limit).
- Lipid solubility: ionization of the drug affects its ability to cross biological membranes.
- Route of administration: IM route is faster than oral route.
- Integrity of the absorbing surface: ischemia decreases absorption.
- Vascularity at the site of absorption: affects the rate of absorption.
pKa and Drug Ionization
- Most drugs are weak acids or bases that exist in both ionized and nonionized forms.
- Only nonionized form (nonpolar) of the drug is lipid soluble and can cross biological membranes.
- Ionized form (polar) of the drug is non-lipid soluble (water soluble) and cannot cross biological membranes.
- The pKa of a drug determines the ratio between the two forms and affects the rate of drug absorption.
- Clinical significance of pKa:
- Site of drug absorption from the GIT: aspirin is a weak acid that becomes nonionized and more absorbable in the stomach (pH = 1.5).
- Drug excretion by the kidney: aspirin toxicity can be treated by alkalinization of urine, and amphetamine toxicity can be treated by acidification of urine.
Bioavailability
- Definition: the fraction of the administered dose of a drug that reaches the systemic circulation.
- Intravenously administered drug has 100% bioavailability.
- Oral bioavailability of a drug can be calculated using the formula: (total amount of drug reaching circulation / total amount of drug administered) × 100.
Apparent Volume of Distribution (Vd)
- Definition: the volume of fluid that would be required to contain the total amount of drug in the body at the same concentration as found in the plasma.
- Drugs are distributed to organs and tissues via the circulation, diffusing from the blood to the interstitial fluid and into the cells.
Elimination Half-Life (t1/2)
- Definition: the time required for the plasma concentration of a drug to decrease by one-half.
- Steady-state plasma concentration (Cpss) is reached after 5 t1/2.
- Cpss is the steady level of drug in plasma achieved when the rate of drug administration equals the rate of drug elimination.
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