Bioavailability is the extent and rate at which an active moiety (a drug or any other metabolite) enters our systemic circulation; thereby makes its way in at the site of action.
Understanding the Bioavailability of Drugs
It is essential to be acquainted with certain terms in the field of bioequivalence study to comprehend the workings of the bioavailability of drugs and helps in the drug discovery process. It is equally important to understand the role they play in the mechanism of actions with Chemical equivalence, Bioequivalence, and Therapeutic equivalence.
•   Chemical equivalence stipulates and determines that the drug products contain the same composite compounds in the same quantity and meet all official standards. However, any inactive ingredient in drug products might be different.
•   Bioequivalence specifies that the drug products, when given to a patient in the same dosage, results in an equivalent concentration of drug in the plasma and body tissues.
•   Therapeutic equivalence indicates that drug products, when given to a patient in the same dosage, have the same or similar therapeutic effects. In some cases, the equivalence is achieved despite non-bioavailability of the drugs.
Determining and Assessing Bioavailability
Bioavailability of drug is usually assessed by carefully determining the area under the plasma concentration– the time curve (AUC—Representative of the plasma concentration-time relationship after a single oral dose of the hypothetical drug in question). The most dependable evaluation of a drug’s bioavailability is AUC.
Understanding AUC
AUC is directly proportional to the entire quantity of an unchanged drug that reaches the body’s systemic circulation. Drug products are bioequivalent when the extent and rate of absorption of their plasma concentration curves are superimposable.
Understanding Plasma Drug Concentration
Plasma drug concentration shows a steady increase with the extent of absorption; the maximum (or, peak) plasma concentration is achieved when the drug elimination rate is equal to the rate of absorption. Bioavailability determinations which are based on the peak plasma concentration are capable of being quite misleading because the elimination process of the drug begins right after the drug gets into the bloodstream. Peak time (that is when the maximum plasma drug concentration happens) is the most extensively used general index to determine the rate of absorption- the slower the absorption takes place, the later is the peak time.
Elimination of Drug Concentration
For the drugs that are excreted primarily unaltered in the urine, bioavailability of drugs can be worked out by measuring the overall amount of drug excreted after a single dose. Preferably, urine must be collected for 7 to 10 elimination rounds for a complete urinary recovery of the absorbed drug. After multiple dosing, the bioavailability is determined by measuring the unaltered drug recovered from urine for twenty-four hours under steady conditions.
To sum up, the bioavailability of drugs is mostly ruled out by the properties of the form of dosage, which depends partly on the designing and manufacturing process of the drug. Discrepancies in bioavailability along with the given formulations of a drug can have a clinical significance; therefore, knowing whether the drug formulations are equivalent is fundamental.
