Pharmacology: Drug Absorption Factors

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Questions and Answers

What role does plasma protein binding have in drug distribution?

It maintains the free-drug concentration as a constant fraction of the total drug in the plasma. (correct)

It enhances the elimination of the drug from the body.

It ensures that all drugs remain in a free state within the bloodstream.

It prevents drugs from interacting with tissue membranes.

Which factor can contribute to higher drug concentrations in tissues compared to interstitial fluid and blood?

Difficulties in drug metabolism

Accumulation through binding to lipids, proteins, or nucleic acids (correct)

Rapid filtration through the kidneys

Increased binding to enzymes in the blood

Why do lipophilic drugs penetrate cell membranes more easily than hydrophilic drugs?

They can form chemical bonds with cellular proteins.

They have a higher molecular weight than hydrophilic drugs.

They are actively transported into cells against the gradient.

They dissolve in lipid membranes and penetrate the cell surface. (correct)

What does the apparent volume of distribution (Vd) signify about a drug?

It reflects the fluid volume required to contain the drug throughout the body at the same plasma concentration. Signup and view all the answers

What potential effect do tissue reservoirs have on drug action?

They can prolong the drug's action or lead to local drug toxicity. Signup and view all the answers

What is the significance of a drug being uncharged in relation to its absorption through membranes?

It allows the drug to pass through membranes more readily. Signup and view all the answers

How does blood flow to the absorption site influence drug absorption?

Increased blood flow supports faster absorption from the intestines. Signup and view all the answers

What role does the total surface area play in the absorption of drugs?

A greater surface area allows for more efficient drug absorption. Signup and view all the answers

What does bioavailability compare for determining the extent of drug absorption?

Plasma levels after oral administration and levels achieved by IV administration Signup and view all the answers

What is a primary influence on the bioavailability of drugs administered orally?

First-pass hepatic metabolism Signup and view all the answers

How does the contact time at the absorption surface affect drug absorption?

Prolonged contact time increases drug absorption. Signup and view all the answers

What function does P-glycoprotein serve in drug absorption?

It reduces drug absorption by transporting drugs from tissues into blood. Signup and view all the answers

Why are very hydrophilic drugs poorly absorbed?

They cannot cross lipid-rich cell membranes. Signup and view all the answers

What does bioavailability refer to in pharmacology?

The proportion of a drug that reaches systemic circulation unchanged. Signup and view all the answers

What effect does the first-pass metabolism have on a drug's efficacy when taken orally?

It reduces the amount of unchanged drug reaching the systemic circulation. Signup and view all the answers

Which property of a drug is crucial for its absorption related to solubility?

It should be largely lipophilic with some aqueous solubility. Signup and view all the answers

If 100 mg of a drug is administered orally and 70 mg is absorbed unchanged, what is the bioavailability of the drug?

0.7 or 70% Signup and view all the answers

What effect does the presence of food in the stomach have on drug absorption?

It dilutes the drug and slows gastric emptying. Signup and view all the answers

What aspect of the drug formulation can alter drug absorption besides its chemistry?

Particle size and enteric coatings Signup and view all the answers

What is true about bioequivalent drug formulations?

They have comparable bioavailability and similar peak blood concentrations. Signup and view all the answers

What characterizes therapeutic equivalent drug products?

They are both bioequivalent and pharmaceutically equivalent. Signup and view all the answers

Which of the following can be a contributing factor to chemical instability of a drug?

Degradative enzymes in the GI tract Signup and view all the answers

What is the process by which a drug leaves the bloodstream and enters extracellular fluid?

Distribution Signup and view all the answers

What happens during the distribution phase of an intravenously administered drug?

The drug rapidly enters the tissues. Signup and view all the answers

What factor is primarily responsible for blood flow distribution to various organs?

Blood flow Signup and view all the answers

What role does capillary permeability play in drug distribution?

It influences how effectively drugs enter non-vascular compartments. Signup and view all the answers

How must drugs enter the brain to bypass the blood-brain barrier?

By active transport or passing through endothelial cells Signup and view all the answers

What effect does binding to plasma proteins have on drug distribution?

It slows down the transfer of drugs out of the vascular compartment. Signup and view all the answers

Which plasma protein is primarily involved in drug binding?

Albumin Signup and view all the answers

What does a low volume of distribution (Vd) generally indicate?

Extensive protein binding of the drug Signup and view all the answers

How is the volume of distribution (Vd) calculated?

Vd = dose/C0 Signup and view all the answers

Given a dose of 10 mg and a plasma concentration at time zero of 1 mg/L, what is the volume of distribution?

10 L Signup and view all the answers

If a drug has low molecular weight and is hydrophilic, where is it likely to distribute?

Into the interstitial fluid Signup and view all the answers

For a patient weighing 60 kg given a 1000 μg dose of a drug with a plasma concentration of 2 μg/L, what is the volume of distribution per kg?

8.33 L/kg Signup and view all the answers

What is the significance of the drug half-life (T1/2)?

Time it takes to reduce drug concentration by half Signup and view all the answers

What is demonstrated by a larger volume of distribution (Vd)?

Greater distribution into tissues Signup and view all the answers

A drug that is extensively protein bound is likely to have what characteristic?

Low volume of distribution Signup and view all the answers

Study Notes

Factors Influencing Absorption

A drug's ability to pass through membranes is enhanced by its uncharged state.
The distribution of a drug between ionized and non-ionized forms is influenced by ambient pH and the drug's pKa.
Higher blood flow to the intestines favors drug absorption over the stomach.
The intestines have 1000 times the surface area of the stomach, making drug absorption more efficient.
Rapid GI transit (like severe diarrhea) can hinder drug absorption.
Factors delaying gastric emptying to the intestines slow down drug absorption.
Food in the stomach dilutes drugs and slows gastric emptying, resulting in slower absorption.
P-glycoprotein, a transmembrane transporter protein, is involved in drug transport across cell membranes.
This protein is found in tissues, including the liver, kidneys, placenta, intestines, and brain capillaries, and can reduce drug absorption.

Bioavailability

Bioavailability refers to the rate and extent to which an administered drug reaches systemic circulation.
It's expressed as a percentage of the administered dose that reaches the systemic circulation.
Determining bioavailability is important for calculating drug dosages for non-IV routes of administration.

Factors Influencing Bioavailability

First-pass hepatic metabolism can reduce the amount of unchanged drug entering systemic circulation.
Drugs with high first-pass metabolism require higher doses to ensure enough active drug reaches the target site.
Very hydrophilic drugs are poorly absorbed due to difficulty crossing lipid-rich cell membranes.
Extremely lipophilic drugs are also poorly absorbed because of their insolubility in aqueous body fluids.
Ideally, a drug should be predominantly lipophilic with some aqueous solubility for optimal absorption.
Some drugs are unstable in gastric pH or degraded by enzymes in the GI tract, impacting bioavailability.
Drug formulation factors, such as particle size, salt form, enteric coatings, and excipients, can influence dissolution and absorption rates.

Bioequivalence

Bioequivalent drug formulations have comparable bioavailability and similar times to reach peak blood concentrations.
Pharmaceutically equivalent formulations have the same dosage form, active ingredient strength, and administration route.
Therapeutic equivalent drug products are both bioequivalent and pharmaceutically equivalent.

Drug Distribution

Drug distribution involves the reversible movement of a drug from the bloodstream to extracellular fluid and tissues.
For IV administered drugs, the initial phase after administration represents the distribution phase, characterized by rapid transfer from circulation to tissues.

Factors Affecting Drug Distribution

Blood flow: Vessel-rich organs (brain, liver, kidney) have higher blood flow than skeletal muscles, adipose tissue, skin, and viscera.
Capillary permeability: It's determined by capillary structure and drug's chemical nature.
- Brain capillaries: These capillaries have tight junctions forming the blood-brain barrier (BBB), restricting drug access to the central nervous system.
Plasma protein binding: Reversible binding to plasma proteins, primarily albumin, sequesters drugs in a non-diffusible form, slowing transfer from the vascular compartment.
Tissue protein binding: Drugs may accumulate in tissues due to binding to lipids, proteins, or nucleic acids, often leading to higher concentrations in tissues than in interstitial fluid and blood.
Lipophilicity: Lipophilic drugs easily cross membranes, while hydrophilic drugs require passage through slit junctions.

Volume of Distribution (Vd)

Vd represents the apparent fluid volume required to contain the entire drug in the body at the same concentration as in plasma.
A higher Vd indicates greater distribution into tissues, while a lower Vd suggests confinement to plasma or extracellular fluid.
Vd calculations are important for understanding how a drug distributes within the body and its half-life.

Effect of Vd on Half-Life

Drug half-life (T1/2) is the time it takes to reduce the plasma drug concentration by half.
A larger Vd typically results in a longer half-life, as the drug is distributed to a larger volume of tissue.
A smaller Vd usually means a shorter half-life, as the drug is mostly confined to the plasma.

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Description

Explore the key factors influencing drug absorption and bioavailability in this pharmacology quiz. Understand how pH, blood flow, and membrane transport proteins affect the efficiency of drug absorption in the gastrointestinal tract.