# 09 - D. Bioavailability

# D. Bioavailability:

© SPMM Course 
D. Bioavailability: 
Bioavailability refers to how much of an administered drug reaches its target. It is the extent to 
which the drug reaches the systemic circulation when taken by a patient orally or parenterally, 
compared with the same quantity of drug given intravenously. In other words, it is the fraction 
that circumvents the first pass effect and actually reaches the systemic circulation. 
Plotting plasma concentration against time, for a given dose, provides oral bioavailability. The 
area under the curve (AUC) after a single dose is proportional to the amount of drug in plasma 
and allows determination of the fraction of the dose absorbed-the bioavailability. The area under 
the curve obtained for orally administered drug divided by the area under the curve obtained for 
intravenous administration of the same dose gives the bioavailability fraction. It is determined by 
three factors: 
1. Absorption 
2. Distribution 
3. Elimination (metabolism and or excretion). 
When a drug is administered intravenously, the availability of the drug is 100%. In other words, 
the amount of drug that enters systemic circulation following IV administration is 100%. This is 
not the case with extravascular or non-parenteral administrations such as oral, per rectal, 
inhalational, intramuscular or subcutaneous routes. The reduction in amount reaching circulation 
is related to the degree of absorption and the effect of ‘first-pass’ metabolism, also called 
presystemic metabolism. This metabolism is prominent in the gut mucosa, liver and to some 
extent in the muscle tissue. This explains why higher doses are generally needed orally as 
compared to intramuscularly. Certain examples of drugs that can undergo a high degree of firstpass metabolism include imipramine (only 30-80% of the oral dose enters systemic circulation) 
and fluphenazine (only 10% of oral dose enters systemic circulation). 
Hepatic impairment can reduce first pass metabolism, requiring adjustment of dosages of drugs 
that are metabolized by the liver. 
Bioequivalence: It is a measure of comparability of plasma levels of two different formulations of 
the same active compound when given at same dose and the same route of administration. Two 
products are said to be bioequivalent when the graphical trace of their plasma level plot against 
time are superimposable. For this to happen, the two compounds must have the same 
bioavailability and rate of absorption. Bioequivalence is an important feature to be considered 
when changing from one brand to another brand of the same compound e.g. camcolit vs. priadel