40 Questions
What is the primary driving force for passive diffusion of a drug?
Concentration gradient across a membrane
Which type of diffusion does not require energy?
Facilitated diffusion
What type of drugs can penetrate the cell membrane through aqueous channels or pores?
Water-soluble drugs
Which mode of drug entry involves specific carrier proteins that span the membrane?
Active transport
What type of diffusion is capable of moving drugs against a concentration gradient?
Active transport
What is the difference between facilitated diffusion and active transport?
Energy requirement
What type of drug absorption is influenced by the route of administration?
Bioavailability
What is the result of administering drugs through routes other than intravenous?
Partial absorption
What percentage of bioavailability is conferred by IV administration?
100%
What occurs when a drug is rapidly metabolized in the liver or gut wall during its initial passage?
First-pass metabolism
Why are very hydrophilic drugs poorly absorbed?
Because they are unable to cross lipid-rich cell membranes
What type of drugs are often administered via the sublingual, transdermal, or intravenous route?
Drugs with high first-pass metabolism
What determines the rate and extent to which a drug reaches the systemic circulation?
All of the above
What is the area under the curve (AUC) used to measure?
Plasma concentrations of the drug versus time
Why are drugs that are extremely lipophilic poorly absorbed?
Because they are insoluble in aqueous body fluids
What is the reason why many drugs are either weak acids or weak bases?
To ensure solubility in aqueous solutions
What is the primary characteristic of active transport systems?
They are selective and may be competitively inhibited
What is the process by which a drug is engulfed by the cell membrane and transported into the cell?
Endocytosis
Which of the following substances is transported across the gut wall by endocytosis?
Vitamin B12
What is the primary factor that determines the effective concentration of the permeable form of a drug at its absorption site?
All of the above
Which of the following is true about the permeation of weak acids through membranes?
The uncharged, protonated HA can permeate through membranes
What is the primary characteristic of weak bases that influences their permeation through membranes?
The protonated form is usually charged
Which of the following is an example of a substance that is stored in intracellular vesicles in the nerve terminal and released by exocytosis?
Norepinephrine
What is the primary difference between endocytosis and exocytosis?
Endocytosis is used for absorption, while exocytosis is used for secretion
What is the formula to calculate Vd?
Dose / C0
What is the primary route of elimination for lipophilic drugs?
Hepatic metabolism
What is the result of drug metabolism?
Production of products with increased polarity
What is the approximate volume of distribution of heparin in a 70-kg individual?
4 L
What is the primary role of the kidney in drug elimination?
Elimination of polar drugs into the urine
What is the characteristic of drugs that distribute into the extracellular fluid?
Low molecular weight and hydrophilicity
What is the rate of elimination for most drugs?
First-order kinetics
What is the approximate volume of distribution of aminoglycoside antibiotics in a 70-kg individual?
14 L
What is the characteristic of drugs that distribute into the total body water?
Low molecular weight and lipophilicity
What is the purpose of phase I and phase II reactions?
To increase the polarity of drugs
What is clearance (CL) a measure of?
The amount of drug cleared from the body per unit of time
What is the approximate volume of distribution of ethanol in a 70-kg individual?
42 L
What is the plot of drug concentrations in plasma after a single injection of drug at time = 0?
A log scale with an exponential decrease
What is the relationship between drug clearance and Vd?
Drug clearance is a first-order process that allows calculation of Vd
What is the meaning of C0 in the context of Vd?
The concentration of drug that would have been achieved if the distribution phase had occurred instantly
What does a larger Vd indicate?
Greater distribution into tissues
Study Notes
Mechanisms of Absorption
- Absorption of drugs from the GI tract depends on chemical properties and can occur through:
- Passive diffusion: driven by concentration gradient, no carrier, not saturable, low structural specificity
- Facilitated diffusion: uses carrier proteins, no energy required, saturable, inhibited by competing compounds
- Active transport: uses carrier proteins, energy-dependent, saturable, can move against concentration gradient
- Endocytosis and exocytosis: for large drugs, involves engulfment and transport across cell membrane
Factors Influencing Absorption
- pH affects drug absorption: weak acids and bases are more permeable in their uncharged form
- Ionization constant (pKa) affects the ratio of charged and uncharged forms
- Bioavailability: affected by first-pass metabolism, solubility, and chemical characteristics of the drug
- First-pass metabolism: occurs in liver and gut wall, reduces amount of unchanged drug entering systemic circulation
- Solubility: very hydrophilic or lipophilic drugs are poorly absorbed due to inability to cross cell membranes
Distribution of Drugs
- Volume of distribution (Vd): the apparent volume of fluid that the drug would have to be dissolved in to produce the observed plasma concentration
- Types of distribution:
- Plasma volume: for drugs with low Vd, approximately 4 L in a 70-kg individual
- Extracellular fluid: for hydrophilic drugs, approximately 14 L in a 70-kg individual
- Total body water: for lipophilic drugs, approximately 42 L in a 70-kg individual
Determination of Vd
- Calculation of Vd: uses the dose and extrapolated plasma concentration at time zero
- Formula: Vd = Dose / C0
Drug Clearance
- Elimination of drugs occurs through:
- Hepatic metabolism
- Biliary elimination
- Urinary elimination
- Most drugs are eliminated according to first-order kinetics
- Clearance (CL) estimates the amount of drug cleared from the body per unit of time
- Metabolism leads to production of polar products, allowing for elimination
- Lipid-soluble drugs are metabolized in the liver to increase polarity before elimination
Learn about the factors influencing drug absorption, including environment, chemical characteristics, and route of administration. Understand the mechanisms of absorption from the GI tract.
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