Drug Bioavailability can be known as the active amount of drug reaches the systemic circulation to the site of action. The Bioavailability of the drug depends on the form of dosage and also its design and the manufacturing. The differences in the bioavailability between the drugs will have the clinical significance so, knowing the equivalence of the formulations is needed.
If the formulations contain the same active compound in the same quantity and also reaches the official standards they are chemically equivalent, however, inactive compounds in the formulation may differ. When the plasma concentrations of two different drugs given to the same patient in same dosage regimen are the same then it’s considered as bioequivalence. If they show the same therapeutic and adverse effects then they are therapeutically equivalent. The bioequivalent products are ought to be therapeutically equivalent. Therapeutic non-equivalence is also observed during the long term treatment of the patient who is stabilized on one formulation is given with the non-equivalent substitute.
Causes of low bioavailability
- Metabolisation of drugs before reaching adequate plasma concentrations. This usually occurs in the oral dosage forms that are poorly water-soluble and slowly absorbed.
- Insufficient time of absorption in the GI tract.
- Age, sex, physical activity, genetic phenotype, stress, disorders
- Chemical reactions that reduce the absorption. The reactions include complex formation, hydrolysis by gastric acid or digestive enzymes, adsorption to other drugs, metabolism by luminal microflora.
The assessment of the bioavailability is done by determining the area under the plasma concentration-time curve. AUC is the most reliable measure of the drug bioavailability. If plasma concentration curves are superimposable then drug products may be considered as bioequivalent in extent and rate of absorption. The drugs that are excreted primarily unchanged in urine, bioavailability will be determined by measuring the total amount of drug excreted after every single dose.