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Bioavailability largely responsible for the screening of a drug candidate
In the therapeutic use of any bioactive molecule, the consideration of its bioavailability within the body is very essential. In the discovery and design of prospective, viable Drug Candidates (DCs) also, the knowledge of its molecular structure and properties are essential, which indirectly determines its oral bioavailability. The polymorphic nature of the drug metabolizing enzymes within the body causes variation in the oral bioavailability as well as the clearance of the DCs. Hence, the comparison of the oral bioavailability data of the actual drugs and the DCs helps in correlating their physical properties, which is very essential part in the drug discovery process.

Bioavailability of a drug candidate may be defined as the extent to which its active part is absorbed or released from the actual ingested molecule and is available at the site of action. From the pharmacokinetic point of view, the oral bioavailability plays a very important role in the screening of the prospective DCs as it determines the effective dose, which is essential for the proper drug action within the body. Bioavailability determines the form in which the DC must be administered within the body such that it has optimal effect within the body. The DCs can be administered in the form of tablets, coated tablets, capsules, powder, solution, suspensions, formulations with controlled release, etc. Depending on the site of action of the DCs, the form of the drug product to be administered can be determined.

The formation of the active moiety of the DC, its rate of the dissolution into the fluids present at its action site, the permeability of the DCs through the membrane of the tract into the systemic circulation are important factors that determine the bioavailability of the DCs. The inherent physical properties of the DCs such as their crystalline structure, particle size, etc also determine their bioavailability. Some physiological properties of the patients also determine the bioavailability of the DCs such as the age, sex, weight, medical history, metabolism, etc. The co-administration of different drugs also has a pronounced effect on the bioavailability of the drugs and the DCs. The type of food and type of diet of an individual determines the bioavailability of a drug or a drug candidate within his body. Although, these studies may give an idea of the bioavailability but various other factors like stress, hormone levels, exercise, the change in the inherent body fluids due to some reason, the presence of other diseases, etc may affect the bioavailability of the drugs or DCs drastically. Hence, before the administration of a drug or DC, thorough knowledge about the history of the patient is essential.

The bioavailability of a drug or a DC is known by measuring its concentration in the blood or urine. The bioavailability determination is carried out using different methods such as in-vivo studies, blood- level study, urinary excretion study, etc. In the in-vivo studies, blood or plasma sample is collected at various time intervals, which correlates with the bioavailability but the data is reliable only if a large population size is used for the study, which is not practical always. The pharmacologic effect quantification is also a method of measuring the bioavailability, whereby the response produced on the body using a particular dose of drug or DC is measured. However, this method is also difficult as the pharmacologic effect cannot be quantified in all cases. Hence, alternative methods like the blood-level tests, urinary excretion studies are used, which use a small sample size of population and are feasible. The type of dosage i.e. single and multiple dosage studies are essential for bioavailability determination. It is to be seen whether a drug or DC requires a single dose or multiple doses at regular time intervals to reach the optimum level in blood or plasma to produce the desired action. In-vitro studies have also been carried out whereby the dissolution of the drug or DC has been measured. Nevertheless, the in-vitro studies are carried out in controlled conditions, which do not correlate with the actual conditions within the body.

The Bioequivalence studies have also proved very essential while carrying out the bioavailability studies. Some DCs are the new formulations of existing drugs. These are considered fit for administration by FDA (Food and Drug Association) , when the bioavailability of the formulations of the drugs i.e. their concentration in plasma to time profiles do not have much statistical difference. Hence, the bioequivalent may be defined as the bioavailability of more than one formulation of the drugs. Thus, bioavailability studies form the backbone of drug development and discovery studies.
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Bioavailability largely responsible for the screening of a drug candidate00