BIO AVAILABILITY
Definition:
- Fraction of administered drug reaching systemic circulation in unchanged form.
Steps:
- On oral administration à Absorbed into portal circulation à Reaches liver.
- Drug may be metabolized (first pass metabolism/pre-systemic metabolism) à remaining drug moves to systemic circulation.
Calculation:
1. Calculated by comparing AUC (area under plasma concentration-time curve) for i.v. route & particular route administrated.
- AUC – Describes extent of absorption of the drug.
- Tmax – Describes about time to reach maximum concentration.
- Ie., Absorption rate
- Cmax – Describes maximum concentration of drug that can be obtained.
2. Also calculated by comparing excretion in urine.
Main determinants:
- Absorption.
- First pass metabolism.
Routes of administration & bioavailability:
- Oral:
- Frequently lower.
- Due to incomplete drug absorption.
- Absorbed drug may undergo first-pass metabolism in intestinal wall/liver or be excreted in bile.
- After i.v. injection:
- 100% bioavailability – Due to 100% drug absorption.
- After s.c. or i.m. injection:
- Incomplete bioavailability – Due to local binding of drug.
Exam Important
- Fraction of administered drug reaching systemic circulation in unchanged form is bioavailability.
- Drug may be metabolized (first pass metabolism/pre-systemic metabolism), hence oral drugs have lowered bioavailability.
- Bioavailability is calculated by comparing AUC (area under plasma concentration-time curve) for i.v. route & particular route administrated.
- AUC – Describes extent of absorption of the drug.
- Tmax – Describes about time to reach maximum concentration Ie., Absorption rate.
- Cmax – Describes maximum concentration of drug that can be obtained.
- Main determinants of bioavailability include absorption & First pass metabolism.
- Bioavailability of oral drugs is frequently lower.
- Bioavailability after i.v. injection of drug is 100%.
- Bioavailability after s.c. or i.m. injection of drug is incomplete bioavailability, due to local binding of drug.
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