Bioavailability and Bioequivalence

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Bioavailability and bioequivalence are both terms used in pharmacology to describe the specific actions and properties of a drug product.

Bioavailability describes the person of an administered dose of that is in active circulation in the bloodstream once a drug is delivered orally, intravenously or any other means (e.g., rectal, sublingual, nasal, transdermal, etc.) 

By definition, a drug delivered intravenously has a bioavailability of 100% since it is delivered directly into the bloodstream.

There are a number of factors that can influence drug bioavailability. They include, among other things:

  • Physical properties of the drug, including drug half-life
  • Drug formulation (e.g., immediate release, time release)
  • Gastrointestinal health of the patient
  • Age of the patient, which can impact the speed of drug metabolism
  • Whether the drug is taken with or without food
  • Renal (kidney) and hepatic (liver) function, which impacts drug clearance

Interactions with other drugs or food can also directly interfere with bioavailability. In the treatment of HIV/AIDS, for example, the antiretroviral drug Norvir (ritonavir)—used to "boost" the serum drug concentration of other HIV drugs—can also increase the systemic bioavailability of a drug like Viagra (sildenafil citrate). In doing so, it can significantly extend the half-life of Viagra while magnifying the side effects.

Conversely, over-the-counter magnesium- and aluminium-based antacids (like Tums or Milk of Magnesia) can significantly reduce the bioavailability of many HIV drugs when taken simultaneously – by up to 74% with drugs like Tivicay (dolutegravir) – and, in doing so, potentially undermine the goals of therapy.

Bioequivalence is a term used in pharmacology to describe two different drug products that, based on their efficacy and safety, are essentially the same.

The U.S. Food and Drug Administration (FDA) defines bioequivalence as "the absence of a significant difference in the rate and extent to which the active ingredient or active moiety in pharmaceutical equivalents or pharmaceutical alternatives becomes available at the site of drug action when administered at the same molar dose under similar conditions in an appropriately designed study."

Bioavailability and bioequivalence are directly inter-related insofar as relative bioavailability (i.e., the comparative bioavailability of one drug to another) is one of the measures used to evaluate drug bioequivalency.  

For FDA approval, a generic drug must demonstrate a 90% confidence interval (CI) in both the extent and rate of bioavailability compared to that of the originating, reference drug.

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