Bioavailability

| Home | | Pharmacology |

Chapter: Essential pharmacology : Pharmacokinetics; Membrane Transport, Absorption And Distribution Of Drugs

Bioavailability refers to the rate and extent of absorption of a drug from a dosage form as determined by its concentrationtime curve in blood or by its excretion in urine.


BIOAVAILABILITY

 

Bioavailability refers to the rate and extent of absorption of a drug from a dosage form as determined by its concentrationtime curve in blood or by its excretion in urine (Fig. 2.6). It is a measure of the fraction (F ) of administered dose of a drug that reaches the systemic circulation in the unchanged form. Bioavailability of drug injected i.v. is 100%, but is frequently lower after oral ingestion because the drug may be incompletely absorbed. The absorbed drug may undergo first pass metabolism in the intestinal wall/liver or be excreted in bile.

 


 

Incomplete bioavailability after s.c. or i.m. injection is less common, but may occur due to local binding of the drug.

 

Oral formulations of a drug from different manufacturers or different batches from the same manufacturer may have the same amount of the drug (chemically equivalent) but may not yield the same blood levels—biologically inequivalent. Two preparations of a drug are considered bioequivalent when the rate and extent of bioavailability of the drug from them is not significantly different under suitable test conditions.

 

Before a drug administered orally in solid dosage form can be absorbed, it must break into individual particles of the active drug (disintegration). Tablets and capsules contain a number of other materials—diluents, stabilizing agents, binders, lubricants, etc. The nature of these as well as details of the manufacture process, e.g. force used in compressing the tablet, may affect disintegration. The released drug must then dissolve in the aqueous gastrointestinal contents. The rate of dissolution is governed by the inherent solubility, particle size, crystal form and other physical properties of the drug. Differences in bioavailability may arise due to variations in disintegration and dissolution rates.

 

Differences in bioavailability are seen mostly with poorly soluble and slowly absorbed drugs. Reduction in particle size increases the rate of absorption of aspirin (microfine tablets). The amount of griseofulvin and spironolactone in the tablet can be reduced to half if the drug particle is microfined. There is no need to reduce the particle size of freely water soluble drugs, e.g. paracetamol.

 

Bioavailability variation assumes practical significance for drugs with low safety margin (digoxin) or where dosage needs precise control (oral hypoglycaemics, oral anticoagulants). It may also be responsible for success or failure of an antimicrobial regimen.

 

However, in the case of a large number of drugs bioavailability differences are negligible and the risks of changing formulation have often been exaggerated.

 

Contact Us, Privacy Policy, Terms and Compliant, DMCA Policy and Compliant

TH 2019 - 2022 pharmacy180.com; Developed by Therithal info.