How does Bioavailability affect Drug Efficacy?

The most interesting aspect of medicinal drugs is that they can be administered through different routes by using varied delivery techniques. The common routes of drug administration include oral, inhalational, intramuscular, intravenous, sublingual, topical, and per-rectal. For a single route, there may be different delivery systems. For instance, the same drug can be administered orally in the form of tablets, syrups or capsules. After entering the human body, the drug has to reach its site of action to generate the therapeutic response. On its way, a fraction of the drug may be lost due to poor absorption or metabolism. Therefore, only a part of the administered dose reaches its site of action. This fraction of the administered drug, that reaches its site of action in active form, is called Bioavailability of drugs.

The maximum therapeutic response that can be produced by a drug is called its efficacy. In general, the response of a drug is proportional to the concentration of drug at its site of action. Evidently, the concentration of the drug will be higher if its Bioavailability is higher.

For an administered dose of a drug, its Bioavailability depends upon the following factors.

Route of Administration: Drugs administered by injection (intramuscular/intravenous) have better Bioavailability than those administered orally or rectally.

Disintegration: After oral intake, tablets or capsules have to disintegrate in the gut to be properly absorbed fully. If the vehicle of the drug disintegrates poorly, the drugs are partly absorbed, reducing the Bioavailability of drugs.

The PH of the Medium: Some drugs are absorbed better in acidic medium and some in basic medium. Different parts of the gut have different pH values. For example, the pH of the stomach is highly acidic, whereas the pH of the small intestine is more alkaline. So, it is important that the drug disintegrates in that part of the gut which has the ideal pH for its absorption. To ensure this, some tablets are coated with special polymers, which allow dissociation at the right pH. Such tablets are often called enteric-coated preparations. These preparations have greater Bioavailability than the simpler ones.

Presence of Enzyme Inhibitors: Different enzymes will destroy many drugs while it is transported to the site of action. Adding inhibitors of metabolizing enzymes to the drugs to ensure that a large fraction of the drug reaches the site of action.

When two drug preparations involving the same active ingredient demonstrate similar Bioavailability and Bioequivalence in Drug Development and therapeutic response, they are said to be bioequivalent. Bioequivalence is an important measure of the efficacy of a new drug preparation compared to an established one. Different in-vivo and in-vitro tests are available to study Bioequivalence. A Bioequivalence Study involving different preparations of the same drug molecule has shown that the efficacy of preparation depends largely on its Bioavailability.

Optimizing the route and delivery system of a drug can improve its Bioavailability. This will, in turn, facilitate the drug to reach a higher concentration at the site of action, and ensure a greater efficacy.