Bioavailability of Drugs

It is only possible to get the therapeutic effects of any drug when the drug can be absorbed by the body, so bioavailability is the key to making an enhancement that delivers proven benefits. Bioavailability of drugs indicates the percentage, amount or concentration of drug that reaches into the systemic circulation and is available at the site of action. When the drug is administered into the body through the intravenous route or parenteral route, then bioavailability achieved by the drug is 100 percent.

Types of bioavailability are as follow:

1. Absolute Bioavailability: When the drug is administered through the intravenous route, the bioavailability of the drug achieved will be 100 percent. If the reference standard is an IV dose, it is referred to as Absolute Bioavailability.

2. Relative Bioavailability: It is the bioavailability of the drug when obtained and it is compared with a reference standard. If the reference standard is any other dosage form than the IV dose, it is referred to as Relative bioavailability.

3. Bioequivalence: When two or more than two drugs that are having the same composition, when administered inside the body they will show statistically the same desired outcomes for patients over a given period of time.

Various factors affecting Bioavailability

1. Physiochemical Properties

A) State of drug: Drugs in solution form or suspension are better absorbed compared to solid dosage form as they require time for dissolution as well as absorption.

B) Particle size: Decrease in particle size, increases surface area and hence solubility of drug increases which increase in absorption, dissolution increases and in turn bioavailability increases.

C) pKa value and state of ionization: Non-ionised drugs are better absorbed than ionized drugs depending upon the surrounding ph and pKa value of the drug.

D) Partition Coefficient: It is the ratio of solubility of the drug in a non-aqueous solvent to the ratio of solubility of the drug in an aqueous solvent. As there is an increase in the partition coefficient of drugs, there will be an increase in the bioavailability of drugs.

E) Route of administration: When the drug is administered through the parenteral route, bioavailability achieved is 100 percent. When the drug is administered through the oral route, bioavailability decreases while the drug administered through inhalation or sublingual route the bioavailability increases.

F) Plasma protein binding: When the drug is bound to the plasma protein, it will increase the therapeutic effect of the drug, i.e, duration of action of the drug will be increased, as a result, the bioavailability of the drug will last for a longer duration.

2. Dosage form: The rate of bioavailability (from higher to lower) of drugs in the different dosage forms is as follows:

Solution ˃ Suspension ˃ Powder ˃ Tablets ˃ Capsules

Solutions are better absorbed than suspension than powder than tablet than capsules. 

3. Manufacturing variables: Various additives such as Diluent, Fillers, Absorbers, Binding agents are added to make the particular dosage form. A change in concentration of the binders will change the disintegration time of the granules. As there is an increase in disintegration time, there will be a decrease in the bioavailability of drugs. The addition of diluent, i.e, Calcium Phosphate in Tetracycline capsules will change the dissolution rate and hence bioavailability of the drug will be changed.

4. Disintegration rate: By maintaining the concentration of filler inside the particular dosage form, the rate of disintegration can be maintained which in turn will affect the bioavailability.

5. Dissolution rate: Higher the dissolution rate of the drug, the more is the bioavailability of the drug.

6. Biological Factors:

A) GIT motility: When GIT motility increases, there will be a decrease in the bioavailability of the drug as well as a decrease in the rate of dissolution. 

B) First-pass metabolism: Drugs that are administered through the sublingual route (Nitroglycerine tablets). When drugs are taken orally, they will directly go to the hepatic portal system (liver) which is the site for metabolism, drugs get metabolized in the liver rapidly and hence there will be no or negligible bioavailability. 

C) GIT flora: Microorganisms that are present in our GIT, higher antibiotics absorb microorganisms that are present in GIT, as a result, the functioning of the microorganism will get disturbed and as a result bioavailability of the drug will be decreased. 

D) Disease status: If the patient is suffering from renal failure, and was previously given higher class drugs, there will be a decrease in the rate of absorption of drugs due to the resistance developed and hence decreasing the bioavailability of drugs. 

Why bioavailability studies are so important and need careful consideration?

Below listed are the few key points to make us understand the importance of bioavailability studies:
1. To determine the safety and efficacy of the drug product as required by the FDA.

2. Development of suitable dosage forms of new drug entity.

3. Determination of the influence of excipients, patient-related factors & possible interaction with other drugs on the efficiency of absorption.

4. Control of quality of a drug product during the early stages of marketing in order to determine the influence of processing factors, storage and stability on drug absorption.

5. Comparison of availability of a drug substance from different dosage forms or from the same dosage form produced by different manufacturers.

6. Bioavailability studies are required for new drug formulations to the reference formulation.

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