What Is the Bioavailability of Drugs

What Is the Bioavailability of Drugs?

The term bioavailability of drugs means the level to which the nutrient present in a certain drug reaches the action site. This site of action can also sometimes indicate the biological or body fluid that provides access to the action site.

Simply put, it specifies the amount of active ingredient present in the drug that would be absorbed by the site of action.

For instance, talking about most of the orally consumed medication, gastrointestinal (GI) tract receives the active ingredient of the drug. Then, through systemic circulation, the ingredients reach the site of action. Here, the concentration of the active metabolite or ingredient in the blood becomes a marker for measuring bioavailability.

The blood concertation mentioned above not only indicates the release or absorption of the active ingredient from the drug but also includes other factors. These factors are elimination, systemic metabolism, and distribution.

Mainly, three pharmacokinetic variables are utilized for bioavailability: • The highest blood concertation – Cmax.

• The time required to reach this highest concentration – Tmax.

• The time curve versus the area covered under blood concentration of drug – AUC.

An Example of Bioavailability Consider that a drug A has 60% or 0.6 bioavailability. This is derived as: • Only 80% of the consumed drug initially reaches the GI tract. • Further, 20% is used in metabolism before the drug achieves systemic circulation.

The combined effect leaves 60% dose of the drug ingredient that reaches the site of action.

What Is Bioequivalence?

Bioequivalence refers to the drug products that are pharmaceutically equivalent in relevance to the extent of bioavailability. It only means that if you have two medicines with the same bioavailability, these drugs are bioequivalent.

For instance, we often ask for a different medicine with the same function if the chemist doesn’t have a particular drug. Here, we are actually asking for a drug with the same bioavailability as the drug prescribed by our doctor. This means that if a Drug A has 50% bioavailability, the Drug B (a similar drug given by the chemist) should also have the same or nearly similar bioavailability.

It is assumed that if the plasma concentration corresponding to the active ingredient present in the drugs is similar, then these

drugs would react similarly. This is to say that the same concentration of nutrient of the drug will reach the site of action.

However, it is interesting to note that only animal studies or in vitro test can’t determine bioequivalence. Through established measures, the drugs made for humans must be regulated in humans only.

Note: The bioequivalence cannot be decreased or increased.

Either two drugs are bioequivalent or they are not.

The Acceptance Criteria for Bioequivalence Study For the acceptance of the bioequivalence of two drugs, the criteria to be followed is given below: You have two drugs: one is generic and other belongs to some brand. For these drugs to be bioequivalent, the difference arising in the Cmax and AUC of the drugs should not be more than 20%.

Both, bioequivalence and bioavailability of drug are related to each other, but these terms are not the same. While bioavailability defines how a drug will impact the site of action, bioequivalence simply gives similarity in bioavailability of different drugs.