Journal of Bioequivalence & Bioavailability

A brief Note on Bioavailability and Its Significance

Jessica Statin^* Department of Medicine, University of Health and Medical Sciences, Tirana, Albania
^*Correspondence: Dr. Jessica Statin, Department of Medicine, University of Health and Medical Sciences, Tirana, Albania, Email:

Received: 18-Nov-2021 Published: 09-Dec-2021

Description

The degree to which a substance or drug becomes entirely available to its intended biological target is referred to as bioavailability(s). Bioavailability is a measure of the rate and fraction of a drug's initial dose that effectively reaches either the site of action or the physiological fluid domain from which the drug's intended targets have unrestricted access. In general, bioavailability is defined as the proportion of a drug's active form that enters systemic circulation unchanged. This definition implies that 100% of the active medication that enters systemic circulation reaches the target location. However, it should be noted that this definition excludes drugs that do not require systemic circulation to work (i.e., certain topical drugs). These medications' bioavailability is assessed using many metrics.

The concept of bioavailability is essential to the pharmacokinetics paradigm. The study of drug movement through the body is known as pharmacokinetics, and it is typically represented by the letters ABCD, which stands for administration, bioavailability, clearance, and distribution. The mode and dose of a medication are referred to as administration. Clearance is the removal of an active form of a medication from the systemic circulation. If taken orally, distribution measures how far a medicine may travel to fluid compartments of the body; this definition assumes distribution follows absorption.

A drug's Route of Administration (ROA) and dosage have a major influence on the rate and degree of bioavailability. The bioavailability of a medicine is indirectly proportional to its dose. A higher dosage is required to attain the minimal effective concentration threshold for a medication with limited bioavailability. Each route of administration has the capacity to facilitate a specific plasma drug concentration for a specific length of time. In many cases, changing the route of administration necessitates a change in the dose. For example, an oral medicine must transit via the GI system, where it is prone to intestinal absorption and hepatic first-pass metabolism. On the contrary, an intravenously administered medicine (IV drug) is supposed to enter the systemic circulation instantly. It is not necessary to consider absorption or first-pass metabolism for determining appropriate dose.

Medication clearance refers to the metabolic and excretory factors that influence the rate and degree to which an active drug departs the systemic circulation. Clearance is calculated by dividing the drug clearance rate by the plasma drug concentration. The drug elimination rate is often classified as a binary system. Either first-order or zero-order kinetics are used to eliminate a drug. A consistent quantity of a medication is removed over time in zero-order kinetics, regardless of plasma concentration. However, zero-order kinetics suggests that absorption and elimination can become saturated, potentially resulting in toxicity. In first-order kinetics, a constant proportion of the drug is eliminated over time due to the drug's intrinsic half-life. Furthermore, first-order drug clearance is proportional to plasma concentration (unlike zero-order kinetics). Physicians should be aware of which category of elimination the pharmaceuticals they prescribe fall into, as this will alter drug clearance and bioavailability.

If dosages are given too often for medications with first-order kinetics, accumulation can develop. This might have unanticipated supratherapeutic outcomes as well as adverse effects. Bioavailability and clearance can be used together to calculate a drug's steady-state concentration. The time period during which the concentration of a medication in the plasma remains constant is referred to as steady-state concentration. This occurs when the rate at which a drug enters systemic circulation equals the rate at which it leaves systemic circulation. Thus, differences in variables affecting drug absorption must be considered when evaluating treatment effectiveness. Factors influencing medication clearance will invariably influence bioavailability and steady-state concentration. This is the scenario with renal disorders, which impair the kidneys' ability to eliminate drugs in the urine. Any degree of failure to remove a drug may increase its bioavailability by retaining a higher drug plasma concentration.