Bioavailability and Bioequivalence

Theoretical Foundations

Bioequivalence studies compare how two pharmaceutical products release the active ingredient for it to be absorbed in the body. In other words, they compare the bioavailability of two pharmaceutical products. 


Why can a bioequivalence study assure the same behavior of the product?

Both pharmacokinetic studies as well as bioavailability / bioequivalence studies are kinetic models. This means that they are based on very accurate mathematical models. They allow for the interpretation of what happens with the drug when it enters the body. For a drug to be effective it has to bind to a receptor. Then, it is logical to think that the effect a pharmaceutical product causes has a close relationship with the number of molecules of drug that bind to the receptors.

Therapeutic effect associated with the number of molecules of drug in receptors.

If the same number of molecules of a drug is present in the same number of receptors, it may be inferred that the same pharmacodynamic effects will be achieved.

In the same subject the number of receptors is stable. Therefore, different therapeutic effects will be the consequence of different number of molecules of the drug released to the receptors. Systemic circulation is the one that takes the molecules of drug to the receptors. Then we may say that the number of molecules in the systemic circulation (plasma concentration) is a measure of the number of molecules present in the receptor.

Plasma concentration of the drug depends on its absorption, distribution, metabolization and elimination.

Parameters determining plasma concentration.

Distribution, metabolism and elimination of a drug depend on its chemical nature and on the genetics of the patient. When a comparative study is made these three variables remain constant. Therefore, in a comparative study of two products having the same active ingredient the diference between these two products will arise from the differences in absorption (See Figure 3). Absorption depends on the chemical nature of the drug and the characteristics of the pharmaceutical product.

Comparative Study. Only Variable: Absorption

Based on what we said previously, we may conclude that:

  • Availability of the drug may be modified at the receptor by its formulation. 

  • Absorption, distribution, metabolism and elimination are constant in the same volunteer. 

  • The differences in plasma concentration in the same subject are a result of differences in the drug's formulation. 

  • The differences of absorption, metabolism, distribution and elimination among different subjects increase the variability in plasma concentration 

And conclude that:

  • Plasma concentration of a drug determines the number of molecules reaching the receptor and consequently its therapeutic effect. 

  • Plasma concentration is determined by absorption, distribution, metabolism and elimination of the drug. 

  • The latter remains constant for the same subject. Differences in plasma concentration are the result of the differences in the amount of drug absorbed and this depends on the quantity of drug the formulation releases.


A bioequivalence study compares how two pharmaceutical products with the same active ingredient release it in the systemic circulation of the body.
"In-Vivo" studies are made to compare the products. This is made through the comparison of pharmacokinetic parameters obtained mathematically: The "area under the curve" (AUC), and maximum concentration (Cmax).

Oral Bioavailability

When bioavailability is determined, the plasma concentration is measured against time. (see Graph 1.)

The "area under the curve" (AUC) is a measure of this plasma ratio or bioavailability, which is defined between zero time, when the drug was administered, and "t" time, when the last measurement was taken. This can be mathematically extrapolated to infinite (AUC).

The time when the drug is at its highest concentration (Cmax) is called "tmax".

These paramenters give us an idea of how the drug reaches systemic circulation. 
The time the highest concentration goes down to half its value is called half-life or "t1/2 ". This value is an indicator of how the drug is eliminated.


When are bioequivalence studies made?

A bioequivalence study compares how two pharmaceutical products with the same active ingredients release it to the systemic circulation.

Furthermore, the absorption depends only on the drug's and the patient's characteristics.
Then, the pharmaceutical product is only responsible for releasing the drug in the body for it to absorb the drug.

A bioequivalence study compares how two pharmaceutical products, which are identical in terms of composition and administration, release the drug and make the drug available for the body to absorb it.

Therefore, if the drug in the pharmaceutical product is solubilized, it makes no sense to make bioequivalence studies.

If the drug is very soluble, it is not necesary to make an "in-vivo" study (in healthy volunteers), only "in-vitro" studies are made (in laboratory studies).

Summarizing, no legislation demands bioequivalence studies for parenteral products (injectable forms) in solution, because the drug already enters the body as a solution. It is important to highlight that in a lyophilized product or a sterile powder it is not necessary to state bioequivalence of any kind because, although the drug is in a solid state, the product must be dissolved in order to be injected.
In the case of products for local use, no bioequivalence studies are required because the drug does not enter the systemic circulation to be effective.

On the other hand, bioequivalence studies are required for solid oral forms such as tablets, capsules or oral suspensions. Studies are also required for other pharmaceutical forms such as inhalant powders or inhalant sprays in which the drug enters the body as a solid matter.

Although in the previous cases bioequivalence studies are required taking into account the drug's solubility and permeability characteristics, on many occasions it is enough to test bioequivalence "in-vitro".

In-vivo versus in-vitro bioequivalence studies


How is an "in-vivo” bioequivalence study made?

As mentioned previously, bioequivalence studies compare bioavailability of two pharmaceutical products. That is to say, they compare the concentration of the active ingredient in the systemic circulation along the time.

Bioequivalence Studies


For bioequivalence studies in healthy volunteers ("in-vivo") a protocol is submitted to the appropriate Health Authority stating all the details on how the study will be made. ANMAT (Argentina health authority) has this requirement, other health authorities may or may not demand it, depending on local regulations.

This protocol must then be approved before the studies start, if required by the local regulation.

As this study involves human beings, before its submission to the Health Authority the protocol must be approved by an independent Ethics Board. This requirement is international and enforceable in all countries that adhere to the Helsinki Convention, as with Argentina.

The study is made with healthy volunteers to reduce Inter-volunteer variability. To reduce this variability even further, inclusion and exclusion parameters are defined for the protocol.

More than 24 volunteers are usually used, depending on the variability of the drug.

Randomized, cross-over clinical studies are then designed. This means that each volunteer has identical probabilities to receive any of the two treatments and that the way each treatment is administered, is defined at random. The study design may also vary with the drug.

In each treatment a blood sample is taken before taking the medication, and later several samples are taken along a period of time which is 3 times the half-life of the drug. 12 samples are usually taken, but this also depends on the drug.

Between the two treatments there is a period of wash out, to enable the body to eliminate all the drug it absorbed in the first treatment.

Then the samples are processed, the drug content and its metabolites in plasma are analyzed and there is a statistical treatment to determine whether there is a relevant difference between the behavior of the two products or not.