Pharmacology Chapter on Oral Absorption

Questions and Answers

What is the most likely effect of complexation in drug absorption?

Only affects fat-soluble drugs

Decreases absorption (correct)

Increases absorption

No effect on absorption

Quinolones are effective against bacterial infections.

True

What is the primary mechanism by which cyclosporin is metabolized?

by CYP3A

The enteric-coated formulation of ASA is designed to resist _____.

stomach acid Signup and view all the answers

Match the following drugs with their application or mechanism:

Furosemide = Loop diuretic Erythromycin = Macrolide antibiotic Insulin = Hormonal therapy for diabetes Tolterodine = Muscarinic receptor antagonist for urinary incontinence Signup and view all the answers

What is the formula for calculating absolute bioavailability (F)?

$F = \frac{AUC_{oral} \times Dose_{iv}}{AUC_{iv} \times Dose_{oral}}$ Signup and view all the answers

Relative bioavailability (Frel) can be used to compare the absorption between two different drug formulations.

True Signup and view all the answers

What role does the cytochrome P450 (CYP450) enzyme play in drug metabolism?

It metabolizes drugs in the liver and gut. Signup and view all the answers

The percentage of the oral dose that does not reach the systemic circulation after administration is __________.

73% Signup and view all the answers

Match the following parameters with their descriptions:

AUC = Area Under the Curve, a measure of drug exposure Fa = Fraction of drug absorbed from the gut FG = Fraction of drug escaping gut wall metabolism FH = Fraction of drug escaping hepatic metabolism Signup and view all the answers

Study Notes

Oral Absorption

Absolute Bioavailability: Defined as the extent of drug absorption into systemic circulation; calculated using the area under the curve (AUC) for intravenous (iv) vs. oral doses.
Relative Bioavailability: Compares absorption between two formulations; for example, a bioequivalence study compares a test drug (like generic LOVRAK®) with a reference drug (ZOVIRAX®).

Factors Influencing Oral Bioavailability

Absorption affected by gut wall and hepatic first-pass effect, which includes presystemic elimination.
Enzymes/transporters such as CYP450 play a significant role in drug metabolism and absorption.
CYP450: A superfamily of enzymes involved in the metabolism of various drugs, significant for oral bioavailability.

Example: Cyclosporine

Total bioavailability formula: ( F = F_a \times F_G \times F_H ).
F values: Fa (0.86), FG (0.41), FH (0.76) yield a total absorption of merely 27% into systemic circulation.
73% of the oral dose is lost due to metabolism and transport effects.

Drug Interaction and Bioavailability

Certain drugs can affect absorption through various mechanisms, including complexation (e.g., quinolones with polyvalent cations) and efflux pumps (e.g., P-glycoprotein for cyclosporin).

Extended Release Formulations

Studies indicate mean steady-state plasma concentration profiles between immediate release (IR) and extended release (ER) formulations, using tolterodine as an example.

Enteric-Coated Formulations

Pharmacokinetics differ based on whether doses are administered on an empty stomach or after a meal; variations in lag time observed with enteric-coated aspirin.

Felodipine Case Study

Bioavailability varies significantly based on grapefruit juice intake and other factors; inhibition can drastically affect the oral bioavailability (Foral ranging from 0.15 to 0.45).

Oral Transporters

Fexofenadine: An example of a drug with absorption mediated by transporters like OATP2B1 and MRP3 across the GI tract, highlighting the importance of transport mechanisms on oral bioavailability.

Competing Reactions in GIT

Drug absorption can be competed by various reactions within the gastrointestinal tract, indicating that dissolution may not always be the rate-limiting step.

Description

This quiz focuses on the concepts of oral absorption and bioavailability of drugs. Explore key terms like absolute and relative bioavailability, as well as the significance of AUC in pharmacokinetics. Test your understanding of how drugs are absorbed into the systemic circulation.