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.
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