The therapeutic effectiveness of a drug depends upon the ability of the dosage form to deliver the medicament to its site of action at a rate and amount sufficient to elicit the desired pharmacological response.
This attribute of the dosage form is referred to as physiologic availability, biologic availability or simply bioavailability. For most drugs, the pharmacologic response can be related directly to the plasma levels.
Thus the term bioavailability is defined as the rate and extent (amount) of absorption of unchanged drug from its dosage form. It can also be defined as the rate and the extent to which the ingredients or active moiety is absorbed from the drug product and becomes available at the site of action1.
As per the definition of bioavailability, a drug with poor bioavailability is one with poor aqueous solubility, slow dissolution rate in biological fluids, poor stability of dissolved drug at physiological pH, poor permeation through biomembrane, extensive presystemic metabolism. Bioavailability of poorly water soluble drugs is a major problem.
A classic example of such drug is saquinavir, a highly potent HIV protease inhibitor, whose minimum effective concentration is only 100 ng/ml, but in order to achieve this concentration, the oral dose required is 1200 mg/day (taken as six 200-mg capsules)2.The reason behind this is the very poor oral bioavailability of saquinavir due to extensive hepatic first pass metabolism. Such a high dose leads to gastrointestinal side effects such as diarrhoea, nausea, abdominal discomfort, dyspepsia, flatulence, vomiting, and abdominal pain.
Some examples of drugs showing poor bioavailability of drugs are shown below:
Drug Aqueous solubility (mg/ml) Absolute bioavailability
Felodipine 0.01900 mg/ml 15%
Ibuprofen 0.04900 mg/ml 49-73%
Candesartan cilexetil 0.00770 mg/ml 15%
Simvastatin 0.00076 mg/ml 5%
Paclitaxel 0.00030 mg/ml 6.5%
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1. L.Shargel A.B.Yu,(1999).Applied biopharmaceutics&Pharmacokinetics (4th edition) NewYork:McGraw-Hill ,ISBN 0-8385-0278-4