Absolute and Relative Bioavailability explain the extent and rate of drug absorption compared to intravenous and standard formulations.
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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
- Formula:
- $F_{\text{abs}} = \frac{AUC_{\text{extravascular}} \times \text{Dose}_{IV}}{AUC_{IV} \times \text{Dose}_{\text{extravascular}}}$
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- AUC (Area Under the Curve) = Total drug exposure in blood over time.
- Formula:
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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:
- $AF_{\text{rel}} = \frac{AUC_{\text{Formulation A}} \times \text{Dose}_{\text{Formulation B}}}{AUC_{\text{Formulation B}} \times \text{Dose}_{\text{Formulation A}}}$
- Used in drug formulation studies to compare new and existing drug versions.
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Factors Affecting Bioavailability
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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.
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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.
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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.
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