1) Absolute Bioavailability (Fabs)
Measures how much of a drug enters the bloodstream after non-IV administration (e.g., oral, subcutaneous, transdermal) compared to IV administration.
IV drugs have 100% bioavailability (F = 1) because they enter the bloodstream directly.
Formula:
Where:
AUC (Area Under the Curve) = Total drug exposure in blood over time.
2) Relative Bioavailability (Frel)
Compares the bioavailability of a drug from two different non-IV formulations (e.g., tablet vs. capsule).
Helps determine if a new formulation is as effective as an existing one.
Formula:
Used in drug formulation studies to compare new and existing drug versions.
Factors Affecting Bioavailability
1) Drug-Related Factors
Solubility – Poor solubility reduces absorption.
Molecular Size – Larger molecules have lower absorption.
Chemical Stability – Degradation in GI tract reduces bioavailability.
Ionization State – Ionized drugs at physiological pH have lower absorption.
2) Patient-Related Factors
Age – Alters drug metabolism and absorption.
Genetics – Variations in enzymes affect bioavailability.
Disease State – Conditions impact drug absorption & metabolism.
Drug Interactions – Can enhance or inhibit drug absorption.
Food Interactions – Food can alter GI pH and drug solubility.
3) Formulation-Related Factors
Dosage Form – Tablets, capsules, liquids affect absorption.
Drug Release Rate – Affects dissolution and absorption.
Excipients – Ingredients impact drug dissolution & uptake.
Particle Size – Smaller particles dissolve faster, improving bioavailability.
Polymorphism – Different crystal forms affect solubility.
