WHAT IS 'Bioavailability'

Bioavailability refers to the portion of a nutrient that reaches the human bloodstream. When discussing a drug, bioavailability can refer to either the percentage of a dose that reaches its target or the rate at which it achieves the bloodstream. Bioavailability can be an important factor in the efficacy and commercial viability of that drug.

BREAKING DOWN 'Bioavailability'

Bioavailability is a term taken from the field of pharmacokinetics, the study of the movement of drugs through the human body. In scientific notation, it is represented by a capital or lowercase F.  The term can describe the delivery of food nutrients to the circulatory system, which can be affected by a range of physiological variables such as body weight and metabolism. The pharmacological definition, however, is designed to assign an absolute bioavailability to each drug, consistent among identical dosages of that drug, and unaffected by those physiological factors. This definition can expressed by the following formula:

This formula compares the bioavailability of a drug taken intravenously (IV), with the same drug taken orally (oral). F refers to the absolute bioavailability of an oral dose. AUC refers to area under curve of the two dosages. This can be thought of as the amount of a dose that reaches the bloodstream. Since an IV dose reaches the bloodstream directly and in its entirety, its bioavailability is always 1. Thus, the bioavailability of an oral dose is expressed as a percentage of the bioavailability of an equivalent IV dose.

Variables that Affect Bioavailability

The above calculation of absolute bioavailability is meant to express the bioavailability of a particular compound free of physiological variables mentioned above. There are several variables, however, that differentiate the bioavailability of one drug from that of another. Most important is first pass metabolism, which refers to the process by which the drug is absorbed between its entry into the body and its entry into the bloodstream. This occurs most rapidly in the liver, and drugs taken orally travel most quickly to the liver.  

Another factor is the chemical stability of the compound. Relatively unstable drugs will break down more rapidly in the gastrointestinal tract, and may be better candidates for non-oral application.

A third variable, which may in some cases lead to bioavailability variability among identical compounds with different manufacturers, is the quality control or production techniques used by those labs. Particle size or the inclusion of diluents (thinning agents) or binding agents can affect drug absorbability from brand to brand.

Finally, the form of the dosage tends to impact bioavailability. Liquids generally are more bioavailable than solids, and gases surpass liquids. Pills tend to be the most marketable form, so manufacturers seek ways to boost the bioavailability of solids whenever possible. Possible solutions range from molecule variations to enhancements to the discovery and developments stages of a drug’s lifecycle. In recent years, solubility consultancies have emerged to aid manufacturers in improving their bioavailability as a competitive advantage in the marketplace.

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