Drug Administration part III

matrix-type tablet,
the drug is embedded in an inert
meshwork from which it is released by
diffusion upon being moistened. In contrast
to solutions, which permit direct
absorption of drug , the use
of solid dosage forms initially requires
tablets to break up and capsules to open
(disintegration) before the drug can be
dissolved (dissolution) and pass
through the gastrointestinal mucosal
lining (absorption). Because disintegration
of the tablet and dissolution of the
drug take time, absorption will occur
mainly in the intestine . In
the case of a solution, absorption starts
in the stomach .
For acid-labile drugs, a coating of
wax or of a cellulose acetate polymer is
used to prevent disintegration of solid
dosage forms in the stomach. Accordingly,
disintegration and dissolution
will take place in the duodenum at normal
speed and drug liberation
per se is not retarded.
The liberation of drug, hence the
site and time-course of absorption, are
subject to modification by appropriate
production methods for matrix-type
tablets, coated tablets, and capsules. In
the case of the matrix tablet, the drug is
incorporated into a lattice from which it
can be slowly leached out by gastrointestinal
fluids. As the matrix tablet
undergoes enteral transit, drug liberation
and absorption proceed en route

coated tablets,
coat thickness can be designed such that
release and absorption of drug occur either
in the proximal or distal
bowel. Thus, by matching
dissolution time with small-bowel
transit time, drug release can be timed to occur
in the colon.

Drug liberation and, hence, absorption
can also be spread out when the
drug is presented in the form of a granulate
consisting of pellets coated with a
waxy film of graded thickness. Depending
on film thickness, gradual dissolution
occurs during enteral transit, releasing
drug at variable rates for absorption.
The principle illustrated for a capsule
can also be applied to tablets. In this
case, either drug pellets coated with
films of various thicknesses are compressed
into a tablet or the drug is incorporated
into a matrix-type tablet. Contrary
to timed-release capsules
, slow-release tablets have the advantage
of being dividable ad libitum;
thus, fractions of the dose contained
within the entire tablet may be administered.