| Summary: | Background: The oral route still remains the most popular method for drug delivery as it is
convenient, cost efficient and has high patient compliance. However, for a drug to be
successfully administered via this route it must be able to cross the intestinal membranes with
a rate high enough (permeability) in order to enter the blood stream to produce the desired
reaction. So, although a new chemical entity may possess pharmacological activity, it may
not have the necessary physicochemical properties to be absorbed to such an extent that it
will be of clinical value. For this reason it is necessary to have the means to accurately
predict the in vivo permeability of drugs in vitro in a cost efficient manner. Permeability can
be predicted by: (i) physicochemical characterisation, (ii) in silico, (iii) in vitro, (iv) in situ or
(v) in vivo methods. Of these methods, in vivo studies are the most accurate, however, they
have several drawbacks such as being time consuming, have high costs and ethical approval
has to be obtained. Physicochemical characterisation and in silico methods are the easiest
and cheapest to perform, but lack the physical interactions that occur at the membrane
interface, thus lacking in reliability and close correlation with the in vivo situation. In vitro
methods using intestinal tissue provides a compromise between the physicochemical
characterisation and in vivo testing, as it is easy to perform and mimics the membrane
interface found in vivo.
Aim: The aim of this study was to determine the permeability of selected drugs using rat
intestine mounted in Sweetana-Grass diffusion chambers and compare the permeability
observed with the absorption of the same drugs in humans using the Fa parameters in order to
determine the feasibility of using this in vitro method to predict in vivo permeability. Two
different methods of intestinal tissue preparation were also investigated to optimise the
method further.
Methods: Validations of the various analytical methods used to determine the selected drugs
quantitatively were performed. Jejunal tissue was prepared in one of two ways. It was either
used as is (unstripped) or the serosal muscle layer was removed (stripped). The transport of
caffeine, furosemide, verapamil, ketoprofen, propranolol, carbamazepine, promethazine,
paracetamol, acyclovir and ranitidine was determined using a vertical diffusion chamber
system, mounted with either stripped or unstripped tissue. All the studies were done in the
apical to basolateral direction. Permeability was expressed by the calculation of the apparent
permeability co-efficient (Papp).
Results: The permeability of all drugs studied was lower than found in previous studies with
Caco-2 cells. This can be expected because of the difference in the composition of the rat
intestine and the Caco-2 cell monolayer as well as the longer path length the drugs have to
travel during the absorption process through the jejunal tissue. The data show a non-linear
relationship between Fa and Papp. For the stripped method of tissue preparation the r² value
obtained was 0.0579 indicating poor correlation. For the unstripped method of tissue
preparation the r² value was 0.2877, indicating poor but improved correlation compared to
the stripped method of tissue preparation. Graphs used to indicate a correlation between Papp
and Fa showed aciclovir to be an outlier. Removal of this drug from the equation gave an r²
of 0.4013 for the stripped tissue preparation and 0.4623 for unstripped tissue preparation
indicating a much better correlation between transport across rat intestine and fraction
absorbed in humans.
Conclusion: It was possible to determine the Papp values for ten selected drugs in this study
by using the in vitro Sweetana-Grass diffusion method. These Papp values showed a nonlinear
relationship between Fa and Papp in rat jejunum, as has been observed in other studies.
When comparing the methods of tissue preparation, the unstripped method of tissue
preparation gave a better correlation between Papp and Fa (r² = 0.0579 compared to r² =
0.2877). If the outlier aciclovir is excluded, the correlation improves significantly (stripped r²
= 0.4013 unstripped r² = 0.4623). A possible reason for the high permeability of aciclovir
observed in this study may be the fact that its transport is concentration dependant. Also, the
absorption of aciclovir varies across species, thus not making it ideal for a study comparing
transport across rat intestine to absorption in humans. This method of intestinal transport
prediction does show a correlation with fraction absorbed in humans, however a larger
number of drugs, with a wider spread of Fa values should be evaluated before it can be used
to predict in vivo absorption with confidence. === Thesis (Ph.D. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2006.
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