Drug bioavailability, as described in both Ayurveda and health sciences, refers to the rate and extent to which a drug or its active components are absorbed and become available in the body to produce a therapeutic effect. This process involves the drug entering systemic circulation or reaching its site of action. 

Drug bioavailability depends on several factors, which are solubility, permeability, and absorption properties. Innovative approaches, such as the application of nanoparticles, nanosponges, and Ayurvedic formulation methods, can enhance bioavailability and significantly impact the overall therapeutic activity of the drug.

For medical students who are preparing for the NEET PG examination, a proper understanding of the factors and clinical significance of bioavailability is crucial. Keep reading for a detailed insight into the bioavailability of drugs, their factors, and importance. 

What are the Factors Affecting Bioavailability?

Various factors affecting the bioavailability of drugs include:

1. First-Pass (Hepatic) Metabolism

When a drug is absorbed from the gastrointestinal (GI) tract, it initially enters portal circulation before entering systemic circulation. If the drug is subjected to rapid metabolism in the liver or intestinal wall by first-pass effects, the amount of unchanged drug entering systemic circulation is decreased. 

First-pass effect in the intestine or liver decreases the efficacy of most oral drugs. For instance, over 90% of nitroglycerin is metabolised first. Hence, it is administered primarily by the sublingual or transdermal route. 

Those drugs that are subjected to extensive first-pass metabolism must be administered in a way that sufficient active drugs arrive at the site of action.

2. Biological Factors

There are also certain biological factors affecting bioavailability. Insulin is degraded in the gastrointestinal tract by digestive enzymes. Tetracycline binds to food components, such as calcium in milk, and decreases its effect. Penicillin G is unstable in gastric acid. These examples show how biological interactions can affect drug absorption and effect.

3. Pharmaceutical Factors (Drug Formulation)

Drug absorption can also be altered by aspects unrelated to its chemistry. Factors like particle size, salt form, crystal structure, enteric coatings, and the use of excipients such as binders and dispersing agents can influence how easily the drug dissolves, which in turn affects the rate of absorption.

4. Solubility of the Drug

Hydrophilic drugs are less absorbed because they are unable to diffuse through cell membranes with a high lipid content. On the other hand, extremely lipophilic drugs are also poorly absorbed as they are almost entirely insoluble in the body’s aqueous fluids, making it difficult for them to reach the cell surface. 

A compound needs to be predominantly lipophilic yet not excessively soluble in aqueous solutions in order to be adequately absorbed. This explains why many drugs are weak acids or weak bases. Highly lipid-soluble drugs are transported in aqueous solutions with the aid of carrier proteins, such as albumin.

5. Other Factors

Other effects, including intestinal motility and disease conditions that might influence liver metabolism or gastrointestinal function, affect drug bioavailability.

What is the Importance of Drug Bioavailability in Drug Therapy?

Bioavailability is an essential pharmacological term that has direct implications for the therapeutic effect of a drug. It refers to the rate and extent to which an active drug enters systemic circulation and is available at the site of action. This has direct implications for clinical dosing decisions of choice, route, time, and drug form.

It is affected by intrinsic and extrinsic factors. Intrinsic factors are the metabolic activation of the drug, receptor targeting, genetic variation, route of administration, and absorption site. Extrinsic factors are food and drug interactions, and patient lifestyle components such as diet and alcohol consumption.

One such well-established example is warfarin, which binds heavily to plasma protein albumin. Only the unbound drug remains pharmacologically active. Changes in albumin levels or protein intake can impact warfarin’s bioavailability, making careful monitoring essential to prevent overdosing or underdosing.

Another example is nitroglycerin, which is usually prescribed to relieve angina. Oral nitroglycerin is extensively metabolised by the liver in its first pass, lowering its bioavailability and prolonging its effect. 

For rapid relief of chest pain, sublingual nitroglycerin is preferred because it bypasses the liver and is quickly absorbed into the bloodstream, showing results within two to three minutes. Bioavailability is also significant in dosage determination. 

The area under the curve (AUC), clearance (CL), and volume of distribution (Vd) are the most critical pharmacokinetic values utilised in the calculation of loading doses and maintenance doses by the following equations:

Loading Dose (LD) = (Css × Vd) ÷ F

Maintenance Dose (MD) = (Css × CL × DI) ÷ F

Where (F) represents the bioavailability, (Css) is the desired steady-state drug concentration, (DI) is the dosing interval, and (Vd) and (CL) are drug-specific factors.

Lack of consideration of bioavailability in deciding on treatment can result in either subtherapeutic concentrations of drugs that are ineffective in treating the disease or toxic drug concentrations. Bioavailability optimisation is thus crucial in an attempt to attain the intended therapeutic effect and rule out the risk of side effects.

FAQs About Bioavailability of Drugs

1. What does the bioavailability of a drug mean?

Bioavailability describes the degree to which a drug or substance successfully reaches its target site within the body and becomes available to produce its intended biological effect.

2. Does bioavailability depend on clearance?

Drug clearance parameters will also influence bioavailability and steady-state concentration. This is commonly observed in kidney disorders where the renal system’s ability to eliminate drugs through urine is reduced.

3. What is the optimal route of bioavailability?

Intravenous (IV) administration offers the most reliable and nearly complete bioavailability. It avoids the gastrointestinal tract, thereby bypassing issues related to oral ingestion.

4. How to enhance the bioavailability of drugs?

To enhance the oral bioavailability of drugs that have high solubility but low permeability, water-in-oil-in-water (w/o/w) double emulsions are explored. A still more recent dosage form, SDEDDSs (self-double-emulsifying drug delivery systems), has also been established by mixing hydrophilic surfactants in water-in-oil emulsions.

5. What are the components of bioavailability?

Bioavailability is a process with these steps: liberation, absorption, distribution, metabolism, and elimination, all collectively referred to as the LADME process.

Conclusion

Bioavailability of drugs is a parameter that must be known to allow proper dosing, optimal therapeutic action, and reduce adverse effects. It is the cornerstone of successful drug therapy from pharmacokinetics to clinical practice. For NEET PG aspirants, grasping this concept not only strengthens pharmacology but also aids in solving integrated clinical questions.

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