Definition:(Bio-availability): It is drfined as the fraction of unchanged drug reaching the systemic circulation following administration by any route. (Ref:Katzung-11th,P-43)
e.g. Bio-availability of digoxin is 62%.It means if a patient orally takes 250mg of digoxin.Only 155mg (62%) of drug will reach to the systemic circulation.
Significance of bio-availability:
- It indicates the route of administration of drug.
- To compare different formulation of same drug.
- To compare different formulation produced by different comparies.
- To asses drug serum concentration of individualization of doses.
Factors modifying bio-availability:
- Route of administration:
- Dosages from a drug:Liquid>solid. (Ref:Katzung-11th,P-43)
- Size of drug:Crystalliod>Colloid.
- Lipid solubility:↑ absorption-↑ Bioavailability.
- Character of excipient:It also effect bioavailability.
- Pre systemic first pass metabolism→↑Meatbolism→↓Bioavailability.
- Presence of other agents:Vit c-↑ absorption of Fe-↑ bioavailability.
- Vascularity:Increase vascular- ↑ Bioavailability.
Importance of Bioavailability:
- To compare different formulation produced by different compaines.
- To compare different formulations of same drug.
- To assess drug serum conc & individualization of doses:
- When intensity or effects is difficulty to judge cliically(Phenytoin for epilepsy).
- Low safety magrin.(Lithium).