Drug Absorption Processes

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Questions and Answers

What does bioavailability refer to in pharmacology?

The process of drug transfer from the site of administration to systemic circulation.

The fraction of a drug that is metabolized before reaching systemic circulation.

The time during which the drug concentration in the blood is reduced to 50%.

The fraction of dose that enters the systemic circulation in a chemically unchanged form. (correct)

Which factor is NOT related to the drug's absorption process?

Lipid solubility

Route of administration

Molecular size

Potency of the drug (correct)

How does pH affect drug absorption?

It affects a drug's ionization, which in turn impacts absorption. (correct)

It modifies the molecular size of the drug.

It influences the lipid solubility of the drug only.

It determines the duration of drug action.

What is the role of the first-pass hepatic effect?

To partially metabolize a drug before it reaches systemic circulation. Signup and view all the answers

Which of the following routes of administration is likely to have the highest bioavailability?

Intravenous administration Signup and view all the answers

What effect does enterohepatic circulation have on drug action?

It decreases the initial therapeutic dose required. Signup and view all the answers

Which factor related to the absorptive membrane is critical for drug absorption?

Blood flow to the area Signup and view all the answers

What is the latent period in pharmacology?

The time between administration and the onset of action. Signup and view all the answers

Which type of transport requires a carrier protein and an input of energy?

Active transport Signup and view all the answers

What is the primary difference between passive diffusion and facilitated diffusion?

Facilitated diffusion involves a carrier protein, while passive diffusion does not Signup and view all the answers

Which of the following is an example of facilitated diffusion?

Reabsorption of glucose in renal tubules Signup and view all the answers

Which statement about pinocytosis is true?

It primarily involves the uptake of fluids and small solutes Signup and view all the answers

In which transport mechanism would you find substances moving with their concentration gradient without an energy requirement?

Both B and C Signup and view all the answers

Which factor most directly affects the bioavailability of a drug administered via the oral route?

The pH of the gastrointestinal tract Signup and view all the answers

Which of the following statements regarding absorption kinetics is correct?

Active transport mechanisms can saturate, limiting drug absorption Signup and view all the answers

Which statement accurately describes the role of lipid solubility in drug absorption?

Higher lipid solubility generally increases drug absorption across cell membranes Signup and view all the answers

What is the effect of increasing the surface area on drug absorption?

It increases absorption. Signup and view all the answers

How does the pH of the medium influence the ionization of a drug?

Acidic drugs are non-ionized in alkaline environments. Signup and view all the answers

Which factor does NOT influence the contact time of a drug with the absorption membrane?

Molecular size of the drug. Signup and view all the answers

Why do lipid-soluble drugs usually have a higher absorption rate than water-soluble drugs?

They can diffuse directly through the lipid bilayer. Signup and view all the answers

Which route of drug administration typically offers the highest bioavailability?

Intravenous. Signup and view all the answers

What effect does the presence of milk have on tetracycline absorption?

Decreases absorption through chelation. Signup and view all the answers

How does the molecular size of a drug affect its ability to cross cell membranes?

Smaller drugs cross more easily. Signup and view all the answers

Which statement regarding drug absorption and acidity is true?

Acidic drugs are not ionized in acidic environments, leading to rapid absorption. Signup and view all the answers

Study Notes

Drug Absorption

Process of drug transfer from site of administration into systemic circulation.
Factors affecting absorption are related to the absorptive membrane, drug properties, and other external factors.

Nature & Composition: Cell membrane consists of a hydrophobic lipid bilayer with hydrophilic protein layers and pores.
- Lipophilic drugs pass through lipid bilayer.
- Hydrophilic drugs pass through pores or via carrier proteins.
Blood Flow: Increased blood supply at site of administration increases absorption.
Surface Area: Larger surface area leads to enhanced absorption.
Contact Time: Longer contact time between drug and absorptive membrane promotes absorption.
Pathological State: Inflammation of the gastrointestinal (GIT) tract can disrupt absorption.
- Damage to intestinal villi reduces surface area.
- Increased permeability alters drug passage.
- Impaired transporter function affects carrier-mediated absorption.
- Altered pH disrupts ionization and absorption.

Lipid Solubility:
- Lipid-soluble (non-ionized) drugs easily cross the lipid bilayer through simple diffusion.
- Water-soluble (ionized) drugs are less permeable and rely on pores or carrier proteins.
Lipid: Water Partition Coefficient: Measures lipid solubility of a drug.
- High coefficient indicates high lipophilicity and better membrane permeability (e.g., Steroids).
- Low coefficient indicates high hydrophilicity and poor membrane permeability (e.g., Insulin).
pH & Ionization:
- Non-ionized (lipid-soluble) drugs readily cross cell membranes.
- Ionized (water-soluble) drugs have lower permeability.
- Drug ionization depends on the pH of the medium.
  - Acidic drugs (Aspirin) are not-ionized in the acidic stomach, leading to rapid absorption.
  - Alkaline drugs (Antacids) are not-ionized in the alkaline intestine, promoting absorption there.
Clinical Significance of Ionization: Manipulating pH can be used to treat drug toxicity.
- Amphetamine toxicity can be treated with ammonium chloride (acidic).
- Sulfonamide toxicity can be treated with sodium bicarbonate (alkaline).
Molecular Size: Smaller molecules readily cross cell membranes.
Solubility & Pharmaceutical Form: Gaseous forms are absorbed faster than aqueous solutions, followed by oily solutions/suspensions, and lastly solid forms (capsules, tablets).
Route of Administration:
- Parenteral routes generally result in faster absorption than oral routes.
- IV, inhalation, sublingual > IM > SC > oral > Rectal > Topical routes.
Interaction with other Drugs & Food:
- Full stomach can delay absorption, while an empty stomach can accelerate it.
- Drug interactions:
  - Milk/milk products reduce tetracycline absorption (chelation).
  - Vitamin C increases iron absorption, while tannic acid reduces it.
  - Antacids (alkaline) decrease absorption of acidic drugs (ionization).

Mechanisms of Drug Absorption

Simple Diffusion: Movement of molecules from high to low concentration without requiring a protein carrier.
Passive Diffusion: Movement down the concentration gradient, no carrier involvement, and no energy expenditure.
Facilitated Diffusion: Movement down the concentration gradient, carrier-mediated, and no energy expenditure.
Active Transport: Movement against the concentration gradient, carrier-mediated, and requires energy (ATP).

Terminology

Latent Period: Time between drug administration and the onset of action.
Duration of Action: Time between the onset of action and elimination.
Half Life (t1/2): Time during which the drug concentration in the blood is reduced by 50%.
Withdrawal Time: Time between stopping drug administration and complete elimination from the body.
Drug Residue: Amount of drug/metabolites deposited in animal tissues or products following its use.
Bioavailability: Fraction of the dose that enters the systemic circulation in an unchanged form.

First-Pass Hepatic Effect (First-Pass Metabolism)

The initial metabolism of a drug by the liver before reaching the systemic circulation.
Oral drugs are absorbed from the intestine and pass through the liver, leading to partial metabolism and reduced bioavailability.
Example: Propranolol (Inderal).
Clinical Significance:
- Requires a higher oral therapeutic dose.
- Individuals with liver disease may need lower doses than those without liver disease.

Enterohepatic Circulation (Recycling)

Drug is metabolized in the liver (conjugation), excreted via bile into the intestine, deconjugated in the intestine, reabsorbed again (secondary absorption), returns to the liver, and is ultimately excreted.
Examples: Purgatives (e.g., Phenolphthalein), NSAIDs (e.g., Phenylbutazone).
Clinical Significance:
- Prolonged duration of drug action.
- May allow for lower initial therapeutic doses.

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Description

Test your knowledge on drug absorption mechanisms, including factors affecting transfer from the site of administration into systemic circulation. Understand how the absorptive membrane's nature, blood flow, surface area, and pathological states impact drug absorption.