Bio Availability

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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