28 Section 1/ General Principles of PharmacologyAqueous Solutions
The aqueous solution are most quickly
absorbed. But due to their poor stability,
solubility limitation and packaging problems,
most of the drugs are not designed in aqueous
solution unless unavoidably needed in some
specific preparations.
In aqueous suspensions, the particle size
is the important factor for their stability,
dissolution and absorption of drug e.g.
sulphadimethoxine when given in
suspension form is absorbed much quickly
as compared when given in tablet form.
Penicillin V, when given in aqueous
suspension gives much higher initial blood
level as compared to the tablets or capsules.
Solid Forms – Powders, Capsules, Tablets
Powder: When drug is given in the form
of powder its absorption is largely
dependent upon particle size, interaction
with other diluents and dissolution rate.
The micronized powder gives much
higher concentration in blood as compared
to ordinary powder and tablets.
Capsule: The absorption of capsule is
also dependent on their type e.g. hard
capsule and soft capsule. The hard capsule
dissolves much more readily in
gastrointestinal tract as compared to soft
capsules.
Tablets: After swallowing, the tablet
first disintegrates into granules and
primary solid particles and then into
solution by dissolution for absorption.
There is a specific disintegration and
dissolution time for each tablet specified in
the various pharmacopoeias. The
absorption of any tablets is dependent on
their disintegration and dissolution time,
which may be affected by adding certain
pharmaceutical additive e.g. dissolution can
be increased by increasing the starch
content, binders can retard the dissolution
as they delay disintegration.BIOAVAILABILITYThe bioavailability of any drug is defined
as its rate and extent of absorption. This is
mainly used to describe the biological
availability of a drug from a preparation and
is calculated/determined in terms of
amount or rate of presence of drug in various
body fluids like blood, urine etc.
It is also used to indicate a measurement
of rate and relative amount of an
administered drug in general circulation.
Bioavailability, after determining the
rate and amount of drug absorbed, and the
duration of drug’s presence in the body fluid
gives an idea about the therapeutic
efficiency and toxicity.
The extent of absorption of a drug can
be estimated by comparing the total area
under the drug concentration in the blood
versus time curve or the total amount of
unchanged drug excreted in the urine after
administration of drug and compared to the
administration of standard (standard may
be an intravenous injection, where the
bioavailability of a drug reaches 100%).
For bioequivalence studies, the two
formulations of same drug is administered
orally as single dose. Figure 1.4.2 shows that
in the study the different parameters are
obtained. Firstly the peak height which
represents the highest concentration of the
drug reached in the blood at a particular
time i.e. time of peak concentration. The area