Bioavailability is a measurable quantity that refers to the active ingredient of drugs. The value corresponds to the percentage of an active ingredient that reaches the systemic distribution in the organism in unchanged form. Thus, bioavailability corresponds to the speed and extent to which a drug reaches absorption and can exert its effect at its destination.
What is bioavailability?
Bioavailability is a measurable quantity that relates to the active ingredient of drugs. Bioavailability is a pharmacological term that refers to the percentage of an active ingredient in a given drug dose that is available in unchanged form to the systemic and blood circulation. Thus, bioavailability corresponds to a measure of the rate and extent to which a given drug is absorbed and ultimately reaches its respective site of action. A special measure of bioavailability is absolute bioavailability. Intravenously administered drugs have, by definition, 100 percent bioavailability. Absolute bioavailability is therefore the bioavailability of a drug compared to its intravenous administration. Relative bioavailability is always referred to when one form of administration of an active ingredient is compared with another form of administration. In pharmacokinetics, bioavailability corresponds to an important parameter, especially in the context of drug approval.
Function and task
After ingestion of a particular drug, its active ingredients are not immediately available in the body. Orally given drugs, for example, must first pass through the gastrointestinal tract, where they are absorbed by the intestinal walls and only then are they absorbed into the blood and passed on to the liver. The time it takes for the substance to reach the plasma and be transported via the bloodstream to the intended site corresponds to bioavailability. Bioavailability is thus a measurable quantity and is often officially indicated on drugs. To measure the quantity, for example, after oral administration of the drug or active ingredient in question, its concentration in plasma is determined at various time intervals. The measurements usually result in a diagram with a curve-like progression, which makes the flow of the administered drug or active ingredient visible. What is under the curve is called the AUC and corresponds to a closed “area under the curve”. This area shows a proportional behavior to the respective amount of active ingredient that has reached the organism with the administration. Formulas are available to calculate the absolute bioavailability. The formula F = AUC (peroral) / AUC (intravenous) gives the absolute size. For drugs, the magnitude of bioavailability is critical in determining bioequivalence. Bioequivalence is always the term used when two drugs have the same active ingredient and are at the same time interchangeable, although they differ in the manufacturing process or in their excipients. If both drugs have the same active ingredient but different bioavailability, they are not bioequivalent and therefore cannot be interchanged. So-called bioenhancers are available to the drug industry to influence bioavailability. They increase bioavailability by increasing the absorption of certain substances in the intestine. In addition, they inhibit the degradation of the substances within the liver and improve the binding possibilities of the active ingredients at the intended binding sites. Furthermore, some bioenhancers increase the ability of active ingredients to cross the blood–brain barrier.
Diseases and ailments
Under certain circumstances, the bioavailability of certain agents or drugs may be reduced. For example, drugs and active ingredients may be broken down when the drug first passes through the liver when administered orally. This effect is known as the first-pass effect. After absorption, the active ingredient reaches the liver via the portal vein. There it is partially metabolized by the cells of the liver. In this way, only part of the active ingredient actually contained still reaches the inferior vena cava. Thus, only the remaining part of the drug can be used for systemic distribution. The first-pass effect is usually circumvented by parenteral, sublingual, rectal or buccal administration of the drug. Another possibility is the administration of so-called prodrugs.These drugs contain inactive or at least less active substances that reach an active form only after metabolization by the liver. Prodrugs are always of great importance when an actually active substance does not reach the desired site of action at all, or reaches it in a reduced or insufficiently selective manner during oral administration. The prodrug concept improves the pharmacokinetic properties of the active ingredients and, with oral absorption, also improves the bioavailability of the drugs by reducing the first-pass effect or by enabling certain drugs to pass through the blood-brain barrier. The bioavailability of a drug can vary from person to person. For each drug, for example, the proportion of active ingredients distributed systemically depends heavily on the particular function of the liver and is not influenced solely by the chemical properties of the drug. For example, bioavailability automatically increases in people with certain liver diseases. The same applies to older people, whose liver is only able to function to a reduced extent for age-related physiological reasons. In patients with liver disease, the standard dosage of a certain drug can thus lead to dangerous concentrations of the active ingredients in the plasma and in this way produce undesirable effects. Knowledge of patients’ liver values is therefore one of the most important bases for deciding on a particular drug therapy or drug management.
