Bioavailability is broadly defined as the degree to which or rate at which a drug or other substance is absorbed or becomes available at the spot of physiological activity after administration (principal pharmacokinetic properties of drugs). In other words Bioavailability is a fraction of administered drug that reaches the systemic circulation. For example, if 100 mg of a drug is administered orally and 70 mg of this drug is absorbed unchanged, the bioavailability is 0.7 of seventy percent.
Bioavailability is determined by comparing plasma levels of a drug after a particular route of administration (for eg., oral administration) with plasma drug levels achieved by IV injection in which all of the agent rapidly enters the circulation.
Types of Bioavailability:
Absolute bioavailability is determined by comparing the blood (plasma) concentration-time-curves of a compound after application (usually orally) of that compound to the rear of intravenous application of the identical compound (e.g. tablet or capsule or from a food vs. intravenous). So, intravenous treatment is used as the reference form of application.
Relative bioavailability is determined by comparing the plasma concentration-time-curves after administration of dualistic unlike formulations of the same compound (e.g. capsule vs. tablet vs. dissolved in water etc.).
The bioavailability of a drug when administered by a non-intravaneous route varies from person to person. It may be influenced by physiological and other features including whether the drug is taken with other drugs or with or without food, and the occurrence of a disease that influences the gastrointestinal system or liver function.
Other factors affecting or influencing a drug’s bioavailability are its physical properties, the drug formulation, such as prolonged release or instant release, an individual’s circadian rhythm, drug interfaces, food interfaces, rate of metabolism (the effect of enzyme induction or reticence by other medications and foodstuff), health of the gastrointestinal tract, age of the patient, and the disease stage.
If a drug is taken orally, it quickly reaches the stomach, dissolves and some of it is absorbed by the small intestine. From the small intestine it travels to the hepatic portal vein before it reaches the systemic circulation. Some of the features that may prevent the drug from reaching the systemic circulation are the drug’s properties and the patient’s bodily state.
Drugs which are extremely hydrophobic may not be readily absorbed because they are unsoluble in body fluids, while highly hydrophilic drugs – which have an attraction for fluids – cannot cross lipid-rich cell membranes.
First Pass Effect– The term used for hepatic metabolism of drug when absorbed and distributed through portal blood. Greater the first pass effect, reduced amounts of the drug reach the systemic circulation. Elaborately, if the drug is rapidly absorbed by the liver, the amount of unchanged drug that gains access to the systemic circulation is reduced. Many drugs, such as propranolol or lidocaine, undergo significant biotransformation in a single passage through the liver.
Future Conference “Global Summit on Pharmaceutical Science and Clinical Trials 2017″ enhances the new ideas in the stream of Drug Bioavailability, Pharmacovigilance, Pharmaceutical Nanotechnology and paves way to encounter the challenges of the same.